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Advancements in Electronic Materials and Devices for Stretchable Displays

Advanced Materials Technologies

February 2023
8(20)

DOI:10.1002/admt.202201067

Authors:

Yeongjun Lee

Stanford University

Hyunbum Kang

Samsung Advanced Institute of Technology

Show all 14 authorsHide

A stretchable display would be the ultimate form factor for the next generation of displays beyond the curved and foldable configurations that have enabled the commercialization of deformable electronic applications. However, because conventional active devices are very brittle and vulnerable to mechanical deformation, appropriate strategies must be developed from the material and structural points of view to achieve the desired mechanical stretchability without compromising electrical properties. In this regard, remarkable findings and achievements in stretchable active materials, geometrical designs, and integration enabling technologies for various types of stretchable electronic elements have been actively reported. This review covers the recent developments in advanced materials and feasible strategies for the realization of stretchable electronic devices for stretchable displays. In particular, representative strain‐engineering technologies for stretchable substrates, electrodes, and active devices are introduced. Various state‐of‐the‐art stretchable active devices such as thin‐film transistors and electroluminescent devices that consist of stretchable matrix displays are also presented. Finally, the future perspectives and challenges for stretchable active displays are discussed.

Conceptual image of stretchable displays in the potable electronics and skin‐like wearables.

…

Prestrain‐induced wrinkled stretchable structure. a) Schematic process diagram of prestrain method. Reproduced with permission.[¹⁶] Copyright 2006, American Association for the Advancement of Science. b) Peak strain and membrane as a function of the pre‐strain in Si ribbon/PDMS substrate system. Reproduced with permission.[²⁵] Copyright 2007, National Academy of Sciences. c) Schematic diagrams of global and local buckling and local buckling. Reproduced with permission.[²⁷] Copyright 2008, AIP Publishing.

…

Representative island‐bridge structure. a) Stretchable LEDs with rigid islands and serpentine interconnects, along with b) their geometric parameters. Reproduced with permission.[⁴⁰] Copyright 2015, Elsevier. Various strain engineering approaches with c) lower Young's modulus materials,[⁵⁵] d) gradient Young's modulus materials,[⁶⁰] and e) pillar structures.[⁶²] Reproduced with permission.[⁵⁵] Copyright 2020, Wiley‐VCH GmbH. Reproduced with permission.[⁶⁰] Copyright 2016, Wiley‐VCH GmbH. Reproduced with permission.[⁶²] Copyright 2020, American Chemical Society. f) Stretchable micro‐LED display designed with auxetic metamaterials. Digital images of g) a stretchable micro‐LED meta‐display with Poisson's ratio of ‐1 and h) display attached to a hemisphere. Reproduced with permission.[⁶³] Copyright 2022, Wiley‐VCH GmbH.

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Representative material candidates and their applications to intrinsically stretchable electronics. Digital images of a) oxidized eGaIn‐based interconnects for stretchable wireless sensor and display system and b) high‐frequency and intrinsically stretchable diodes. c) Optical microscope image of AgNW‐elastomer composite for stretchable current collectors. Reproduced with permission.[⁷⁴] Copyright 2021, Springer Nature. d) Molecular structure of PEDOT:PSS. Reproduced with permission.[⁶⁴] Copyright 2022, Springer Nature. e) Schematic and optical microscope image of PR‐PEDOT:PSS‐based conducting polymer for stretchable electrodes. Reproduced with permission.[⁶⁴] Copyright 2022, Springer Nature. f) Schematic and g) crack onset strain with terpolymer approaches for modulating the mechanical properties of CPs. Reproduced with permission.[⁶⁵] Copyright 2021, Springer Nature. h) CP/SEBS elastomer blends for intrinsically stretchable TFTs. Reproduced with permission.[⁶⁹] Copyright 2017, American Association for the Advancement of Science. i) Digital images of intrinsically stretchable polymer light‐emitting layers composed of CP/PU elastomer blends. Reproduced with permission.[⁶⁴] Copyright 2022, Springer Nature.

…

+15

Stretchable composite electrodes. a) AgNW‐embedded stretchable electrodes. Reproduced with permission.[⁸⁰] Copyright 2020, Springer Nature. b) AgNW–elastomer composite electrodes fabricated using vacuum filtration method. Reproduced with permission.[⁸¹] Copyright 2018, Royal Society of Chemistry. c) Printable elastic electrodes with Ag flakes, fluorine rubber, and fluorine surfactant. Reproduced with permission.[⁵⁷] Copyright 2015, Springer Nature. d) AgNW‐embedded stretchable electrodes. Reproduced with permission.[⁸⁴] Copyright 2019, American Chemical Society. e) AgNW‐elastomer composite electrodes fabricated using vacuum filtration method. Reproduced with permission.[⁸⁵] Copyright 2022, American Chemical Society.

…

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Review

Advancements in Electronic Materials and Devices

for Stretchable Displays

Yeongjun Lee,* Hyeon Cho, Hyungsoo Yoon, Hyunbum Kang, Hyunjun Yoo,

Huanyu Zhou, Sujin Jeong, Gae Hwang Lee, Geonhee Kim, Gyeong-Tak Go, Jiseok Seo,

Tae-Woo Lee, Yongtaek Hong,* and Youngjun Yun*

DOI: 10.1002/admt.202201067

and exchange data with other devices

and systems over the internet. Further-

more, going beyond the connection of

devices, the “Internet of Things” (IoT)

has emerged, where everything around

us is connected.[1] Devices will evolve into

human-centric devices that require more

user-convenience, portability, and better

connectivity with their surroundings, and

people will demand wearable devicesthat

are easier to carry around.[2]

In recent years, there has been a pro-

liferation of wearable electronics such

as smart watches, clothes, and patches,

and even implantable devices to detect

physiological signals for healthcare moni-

toring. With the advent of wearable tech-

nology, the medical system is shifting

from managing a few patients in hospitals

to remotely monitoring several people who

have potential risks to their health during

daily activities. Although commercial

wristwatches are useful and widely used,

some people still do not want to wear them because they can be

uncomfortable and inaccurate. In particular, during a high level

of physical activity, they cannot obtain accurate data because of

losing skin contact or motion artifacts.[3–5] This is leading to the

development of stretchable and skin-like electronics,in which

electronic functions are introduced to the surface of the skin to

maximize a user’s comfort during daily life and ensure secure

contact with the skin to detect physiological signals.[6]

A stretchable display would be the ultimate form factor for the next generation

of displays beyond the curved and foldable conﬁgurations that have ena-

bled the commercialization of deformable electronic applications. However,

because conventional active devices are very brittle and vulnerable to mechan-

ical deformation, appropriate strategies must be developed from the material

and structural points of view to achieve the desired mechanical stretchability

without compromising electrical properties. In this regard, remarkable ﬁndings

and achievements in stretchable active materials, geometrical designs, and

integration enabling technologies for various types of stretchable electronic

elements have been actively reported. This review covers the recent devel-

opments in advanced materials and feasible strategies for the realization of

stretchable electronic devices for stretchable displays. In particular, representa-

tive strain-engineering technologies for stretchable substrates, electrodes,

and active devices are introduced. Various state-of-the-art stretchable active

devices such as thin-ﬁlm transistors and electroluminescent devices that

consist of stretchable matrix displays are also presented. Finally, the future

perspectives and challenges for stretchable active displays are discussed.

Y. Lee

Department of Chemical Engineering

Stanford University

Stanford, CA 94305, USA

E-mail: yeongjunlee@gmail.com

H. Cho, H. Yoon, H. Yoo, S. Jeong, G. Kim, J. Seo, Y. Hong

Department of Electrical and Computer Engineering

Inter University Semiconductor Research Center (ISRC)

Seoul National University

Seoul 08826, Republic of Korea

E-mail: yongtaek@snu.ac.kr

The ORCID identiﬁcation number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/admt.202201067.

H. Kang, G. H. Lee, Y. Yun

Organic Material Lab.

Samsung Advanced Institute of Technology (SAIT)

Samsung Electronics

Suwon 16678, Republic of Korea

E-mail: youngjun.yun@samsung.com

H. Zhou, G.-T. Go, T.-W. Lee

Department of Materials Science and Engineering

Seoul National University

Seoul 08826, Republic of Korea

T.-W. Lee

School of Chemical and Biological Engineering

Institute of Engineering Research

Research Institute of Advanced Materials, Soft Foundry

Seoul National University

Seoul 08826, Republic of Korea

1. Introduction

Information and communication technology (ICT) has evolved

toward higher speed, density, and resolution, with lower power

consumption, and has ushered in an era of mobile devices such

as laptops, smart phones, and tablets. Interactions between

mobile devices have proliferated with advances in device per-

formance, and various services have been provided to connect

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The display is the most important front-end interface for

communication between users and devices, and thus occupies

a fairly large area in the appearance of the device. This makes a

stretchable display essential for skin-like electronics (Figure 1).

Although the research on stretchable displays is still in the early

stage, much research is being actively reported. Display form

factor innovation is accelerating. Thus, stretchable displays are

expected to be commercialized in the near future.[7]

In the mid-2010s, ﬂat-panel displays, which are usually less

than 10 cm thick, completely replaced cathode ray tube (CRT) dis-

plays, which were introduced in 1939 by the Radio Corporation

of America (RCA). It took almost 70 years to change the form

from heavy and cumbersome to a thin, light, and ﬂat screen.

Flexible displays, which are designed to withstand being folded,

bent, and twisted, as opposed to the traditional ﬂat-panel dis-

plays used in most devices, allow innovative smartphone designs

with various forms.[8] In the late 2010s, interest in various form

factors such as multi-foldable, rollable, and stretchable displays

began to increase with the release of foldable phones in the con-

sumer market. Although this market is still in an early adopter

phase, it is expected to be a mainstream in the near future. A

foldable display is a type of ﬂexible display that can be folded and

unfolded like paper. This includes not only a single fold but also

multiple folds such as G-folding, in which it is folded inward

twice in a “G” shape, and Z-folding, in which it is folded in a “Z”

shape, with only a third of the screen exposed to the outside. A

rollable display is also a type of ﬂexible display with the ability to

expand the display area by pulling on it vertically or horizontally

so that the screen behaves like paper on a scroll.

Stretchable displays, which can be stretched in all directions

to change their shape, will be the next generation of displays

with a new form factor a step beyond that of ﬂexible displays.

They can be deformed in any direction, freely folded, and ﬁt

on any surface. Based on numerous studies of stretchable elec-

tronic materials, strain releasing geometries, stretchable back-

planes, and stretchable light-emitting diodes (LEDs), a few

prototypes of stretchable displays have been demonstrated. For

example, Samsung demonstrated a 9.1-inch stretchable active-

matrix organic light-emitting diode (AMOLED) display with 5%

stretchability at the Society for Information Display (SID) in

2017.[9] Someya etal. reported a highly elastic, 1 mm thick skin

display, which consisted of a 16 × 24 array of micro-light emit-

ting diodes (μ-LEDs) with 45% stretchability, at the American

Association for the Advancement of Science (AAAS) Annual

Meeting in Austin, Texas in 2018. In 2020, Beijing Oriental

Electronics (BOE) reported a stretchable AMOLED with a 10%

stretch ratio.[10] In 2021, Lee et al. showed the feasibility of a

skin-like display patch with a passive-matrix (PM) OLED array,[11]

and Royole demonstrated a 2.7 in. 96 × 60 pixel (42 pixels per

inch, ppi) μ-LED display with 120% stretchability at SID.[12]

Based on such interest and the research conducted in academia

and industry, it is expected that stretchable displays and skin-

like electronic devices will be realized in the near future and will

provide unprecedented types and shapes of electronic devices.

This reviewoutlines the recent progress made in the develop-

ment of deformable and stretchable displays.First, it discusses the

current technical approaches and prospects for strain engineering,

including pre-strained structures, island-bridge structures, and

intrinsically stretchable materials that could be implemented for

stretchable display arrays. Second, stretchable electrodes for the

interconnections are discussed, including intrinsically stretch-

able conductors (e.g., conductive nanomaterial composites, liquid

metals (LMs), and conductive polymers) and structural designs.

Third, the stretchable thin ﬁlm transistors (TFTs) made from var-

ious semiconducting materials that are capable of being used for

a stretchable backplane are summarized. In particular, the results

of strain-engineered inorganic stretchable TFTs and intrinsically

stretchable TFTs that exploit semiconducting carbon nanotubes

(CNTs) and polymer semiconductors are discussed. Following

this, stretchable light-emitting devices such as inorganic, organic,

and metal-halide perovskites LEDs, along with alternating current

electroluminescence (ACEL) devices, are discussed. Finally, we

report the recent progress on stretchable display arrays and future

perspectives on high-density stretchable displays.

2. Structural Strategies for Stretchable Display

Most electronic materials such as semiconductors and conduc-

tors are brittle; however, they can be made more ﬂexible by

decreasing their thickness. Downscaled materials can withstand

signiﬁcantly larger strains and stresses than their bulk states.

However, they cannot aord a tensile strain that is greater

than the bending strain. To apply these electronic materials to

stretchable devices, there have been several approaches to make

electronic materials and devices stretchable at the macro-scale

level (extrinsically stretchable geometries) or at the micro-scale

level (intrinsically stretchable materials).

This section describes representative strategies for structural

designs that are viable for stretchable displays, including geo-

metrical engineering approaches to produce 1) wrinkled struc-

tures using a pre-strain method, 2) island-bridge structures,

and 3) approaches that use intrinsically stretchable materials

that exploit nanomaterial composites, molecular blending, and

molecular chemistry.

2.1. Prestrained Structures

The prestrain method is a prevalent method for forming a wrin-

kled structure for a ﬂexible device using a pre-stretched elastomer

substrate. Wrinkled structures are ﬂattened after restretching,

which allows the ﬂexible devices to be stretched without

Figure 1. Conceptual image of stretchable displays in the potable elec-

tronics and skin-like wearables.

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mechanical failure.[13,14] The wrinkled structure is fabricated by the

deposition or lamination of a ﬂexible ﬁlm on a pre-stretched elas-

tomeric substrate.[15] The ﬁrst pre-strain-induced stretchable wrin-

kled structure was developed by the transfer of thin silicon ribbons

to a pre-strained elastomeric substrate (poly(dimethylsiloxane),

PDMS). After peeling o the silicon ribbons from the elasto-

meric substrate, they had a periodic wavy structure (Figure 2a).[16]

Because the prestrain method achieves reliable and stable

stretchable properties, various materials and devices have been

researched to make stretchable devices and systems.[17–22]

The theoretical model of a buckled structure has been sug-

gested to design the materials and geometries for stretchable

devices with the pre-strain method. The wavy conﬁguration can

be determined by various factors such as the thickness, elastic

modulus, adhesion, Poisson ratio of the materials, and geom-

etry of the structure.[15] First, the wavelength (λ0) and amplitude

(A0) of the buckling are calculated by the model with a well-

deﬁned sinusoidal wavy structure with a buckled geometry.[23,24]

In this model, the elastomeric substrate is assumed to be a very

thick ﬁlm (semi-inﬁnite solid), which was widely researched for

early wearable electronics:

pre

λπ

==−

(1)

where h is the thickness of the sti material, εc is the critical

buckling strain, and εpre is the strain of the prestrained elasto-

meric substrate. However, this buckled geometry can be formed

only when εpre is larger than the critical buckling strain. The

critical buckling strain, εc, is determined by the elastic modulus

values of the elastomeric substrate, Es, and sti ﬁlm, Ef:

2/3

=









(2)

However, delamination can occur between sti materials and

soft elastomeric substrates. In this case, the model cannot pre-

dict the buckling behavior. Therefore, a new model has been

suggested to explain the ﬁnite deformation and non-linear

strain displacement relation.[15,25,26] Using this model, wave-

length λ and amplitude A of the buckling are calculated as

follows:

pre

1/3

pre

1/21/3

λλ

εξ

() ()

=++ =

(3)

where 5(

ξεε

. In addition, the maximum strain of the

ﬁlm, which is called the peak strain, can be calculated as the

sum of the membrane strain and bending strain in the buck-

ling structure. Peak strain εpeak is calculated as follows:

peak cpre

1/2

1/3

pre

1/2

εε

()

(4)

The value of εpre is much larger than that of εpeak. Therefore,

a pre-strained structure can be eectively used for a stretchable

Figure 2. Prestrain-induced wrinkled stretchable structure. a) Schematic process diagram of prestrain method. Reproduced with permission.[16] Copy-

right 2006, American Association for the Advancement of Science. b) Peak strain and membrane as a function of the pre-strain in Si ribbon/PDMS

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device (Figure 2b).[25] In addition, the maximum pre-strain

before fracture, εpre,max, can also be calculated by this model:

4143

144

pre,max

fracture

εε













(5)

where εfracture is the fracture strain of the sti material. There-

fore, the maximum prestrain is limited by the mechanical prop-

erties of the materials.

This well-deﬁned wavy structure can only be formed on a

thick elastomeric substrate, which is considered a semi-inﬁnite

solid. However, global buckling occurs instead of periodical

local buckling when the elastomeric substrate is thin. The

mode of buckling is illustrated in Figure2c.[27] To predict the

buckling mode, a pre-strained structure can be modeled with

rigidity. The critical global buckling strain can be calculated as

follows:

11.2

c,global

Gh h



()

(6)

where

FEIL



= is the critical buckling load,

is the eec-

tive shear modulus of a composite beam, hs is the thickness of

the substrate, hf is the thickness of a sti ﬁlm, L is the length

of the composite beam, ss

Eh=+

is the eective ten-

sile rigidity, and ()4()

ffss sf

EI Eh Eh EhEh hh

=−+ +

is the eective bending rigidity.[27] When εc,global is larger than

εc, local buckling occurs. However, if εc,global is smaller than εc,

global buckling occurs. To avoid this global buckling, the mate-

rials and geometrical structure must be designed appropriately

according to the model. Moreover, the adoption of a thin layer

between the ﬁlm and substrate can eliminate global buck-

ling.[17] Using these theoretical models, the geometrical param-

eters and mechanical properties of the materials are the keys to

designing a stretchable system using pre-strain methods.[23,28]

2.2. Island-Bridge Structures

The structure engineering method of implementing a stretch-

able display is to divide it into a soft area and hard area on a

plane. The rigid electronic devices (e.g., inorganic/organic

LEDs and TFTs) are placed in island-shaped rigid pixelated

areas and connected through interconnects that are intrinsi-

cally or externally (geometrically) stretchable (Figure 3a). In

this structure, the devices in the rigid pixels are hardly aected

by external strain. Thus, this structure can impart mechanical

stretchability to well-developed rigid devices, including LEDs

and TFTs, which has the advantage of using the mature manu-

facturing process that has been established for rigid displays

over previous decades. Thus, the island–bridge structure has

been the most widely adopted method to implement stretchable

displays.[10,12,29,30]

One method of connecting rigid/ﬂexible islands is to use

metallic bridges.[31] A serpentine-shaped bridge is mainly used

with metal thin ﬁlm electrodes, where the stretching motion is

performed by 3D torsions of the ﬂexible bridges.[32] Because the

local strain distribution and resultant distortion stress under

stretching are dependent on the geometry of the island–bridge

structures, the global strain range of the stretchable display is

modulated by geometrical factors such as the ratio between

island and bridge regions, design of the bridge interconnects,

in-plane/out-of-plane distortion, and buckling.[33–36]

The island and serpentine bridge structures can be fabri-

cated on one plane by patterning a silicon on insulator (SOI)

wafer[37,38] or plastic substrate such as polyimide (PI).[34,39] A

low island/bridge ratio can increase the elongation with a wide

deformation area, but there is a limit to the increase in resolu-

tion due to a small pixel area ratio. It is possible to improve

the elongation at a high island/bridge area ratio by applying

bridges with various curvatures, horseshoes, and fractal pat-

terns (Figure 3b).[40–43] In the serpentine structures, because

the strain is mainly concentrated on the inner and outer edges

of the curves,[44,45] it is necessary to limit the elongation, which

can minimize fatigue failure due to repeated stretching.

The rigid island and bridge interconnect regions can be

separately fabricated on the elastomeric substrate, and it is

also possible to apply not only serpentine ﬂexible metal ﬁlms

but also intrinsic stretchable conductors (e.g., LMs, conductive

nanocomposites, and conducting polymers).[11,46–48] The intrin-

sically stretchable interconnect can be fabricated in a straight

shape that minimizes the area occupied by the wavy ﬂexible

interconnect, thereby improving the resolution. Furthermore,

intrinsically stretchable interconnects can be patterned in a

wavy shape to further improve the elongation and mechanical

stability.[49,50] In addition, the rigid island region can be located

above or below the elastomer or embedded therein to modulate

the strain distribution and provide more options in terms of

device structures and fabrication processes.[9,51]

The disconnection of interconnects near the edge of the

island pixel area due to localized strain can limit the stretch-

ability of the island-bridge structures.[52–54] To improve the

stretchability of the stretchable display array, various strain

engineering approaches that have applied lower Young’s mod-

ulus materials (Figure 3c),[55] sti materials with a gradient

modulus (Figure 3d),[56–61] or pillar structures (Figure 3e)[62]

have been reported. In addition, a kirigami-based design for a

stretchable display without image distortion was suggested.[63]

The μ-LEDs were transferred onto an electrical circuit board

with kirigami cutting patterns (Figure 3f). The meta-atom

expanded in the perpendicular direction to the loading with a

strain identical to the loading strain, resulting in a Poisson’s

ratio of –1 (Figure3g,h).

2.3. Intrinsically Stretchable Structures

Unlike the aforementioned structural methods for fabri-

cating stretchable electronics, intrinsically stretchable devices

would not require geometry-based strain engineering. Thus,

they would suciently increase the device stretchability while

avoiding the reduction of both the overall device density and

complexity of the device processes.[64–66] Therefore, introducing

inherently stretchable materials (e.g., conductors and semi-

conductors) into stretchable electronic devices is an important

method to develop stretchable displays. Intrinsically stretchable

electronic materials are mostly based on polymeric materials

as elastic matrices or substrates that incorporate conducting

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or semiconducting materials. Because polymer materials can

be functionalized with additional unique properties such as

self-healing, biocompatibility, and antioxidation properties,

intrinsically stretchable electronic materials could broaden the

functionality of skin-like electronics, including the develop-

ment of damage-resilient and biocompatible biometric devices

beyond those used for conventional applications.[67,68] This sec-

tion discusses recent representative approaches and their elec-

tronic device applications in terms of both inherent material

designs and physical blending approaches for improving the

stretchability while maintaining the electrical/optical properties.

Intrinsically stretchable conductors have been intensively

developed and utilized in stretchable electrodes and current

collectors to demonstrate intrinsically stretchable TFTs,[66,69,70]

LEDs,[64,71,72] photovoltaics (PVs),[73] and high-frequency

diodes.[74] Furthermore, intrinsically stretchable conductors

are used as the mechanically robust interconnects of stretch-

able device arrays. Various kinds of conducting materials such

as LMs, conducting polymers, and conductive nanomaterials

(e.g., Ag nanowires (AgNWs), Ag ﬂakes, and CNTs) have been

widely used as intrinsically stretchable conductors. For exam-

ples, Matsuhisa et al. recently demonstrated the intrinsically

stretchable display and sensor systems (Figure 4a) by inte-

grating high-frequency (HF) diodes (Figure 4b), current col-

lectors, stretchable antennas, and strain sensors.[74] The con-

ducting polymer poly(3,4-ethylenedioxythiophene):polystyrene

Figure 3. Representative island-bridge structure. a) Stretchable LEDs with rigid islands and serpentine interconnects, along with b) their geometric

display designed with auxetic metamaterials. Digital images of g) a stretchable micro-LED meta-display with Poisson’s ratio of -1 and h) display attached

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Figure 4. Representative material candidates and their applications to intrinsically stretchable electronics. Digital images of a) oxidized eGaIn-based

interconnects for stretchable wireless sensor and display system and b) high-frequency and intrinsically stretchable diodes. c) Optical microscope

tion for the Advancement of Science. i) Digital images of intrinsically stretchable polymer light-emitting layers composed of CP/PU elastomer blends.

Adv. Mater. Technol. 2023, 2201067

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sulfonate (PEDOT:PSS) with ionic additives, AgNWs/tough

thermoplastic polyurethane (T-TPU)-based elastomeric com-

posites (Figure4c), oxidized eutectic gallium-indium (oxidized

eGaIn) printed on a stretchable substrate (Figure4a), and single

wall CNTs (SWCNTs) were used for high-frequency diodes,

current collectors, stretchable antennas, and strain sensors,

respectively. In addition, Zhang et al. reported highly e-

cient and intrinsically stretchable polymer LEDs (PLEDs) that

showed stable operation with up to 100% strain by developing

highly stretchable PEDOT:PSS electrodes with a polyrotaxane

(PR)-based photopatternable crosslinker (PR-PEDOT:PSS)

(Figure 4d,e).[64] Furthermore, Jiang et al. successfully demon-

strated intrinsically stretchable and biophysical sensor arrays

based on the aforementioned PR-PEDOT:PSS systems.[75]

Intrinsically stretchable semiconductors have also been

actively developed and utilized as the active layer for intrinsi-

cally stretchable devices. Among the several material candidates

for stretchable semiconductors, conjugated polymers (CPs)

have emerged as key materials because of their excellent optical

and electrical properties, with inherently lower Young’s moduli

compared to those of silicon and inorganic semiconductors.

However, the major challenge faced in developing CPs lies in

maintaining good electrical, optical, and mechanical proper-

ties simultaneously. Because of the extended π-conjugation of

the CP backbone, which is vital for electronic properties, they

often have rigid and semicrystalline properties. To lower the

rigidity and enhance the stretchability of CPs, a signiﬁcant

structural modiﬁcation process for CPs has been developed by

incorporating dynamic bonding units, conjugation breakers,

and several ﬂexible chemical structures.[67,76–78] A random ter-

polymer approach was also eectively used to simultaneously

control the backbone rigidity and thin-ﬁlm morphology. For

example, Mun etal. developed high mobility (>1 cm2 V−1 s−1)

and stretchable (>100% strain) diketopyrrolopyrrole (DPP)-

based CPs by incorporating two dierent types of randomly

distributed co-monomer units, which reduced the overall crys-

tallinity and long-range orders while maintaining short-range

ordered aggregates (Figure4f).[65] As a result, the resulting CPs

showed improved electrical and mechanical properties com-

pared to those of a control system (polymer/polymer blend)

(≈75% strain) (Figure4g).

Similar to the approach for an elastomeric composite

system for stretchable conductors, CPs/elastomer (e.g., sty-

rene-ethylene-butylene-styrene [SEBS], PDMS, or TPU) blend

systems have also been widely used. In particular, the similar

surface energies of the CPs and SEBS elastomer ensure a

nanoscale intimated morphology, which is crucial for ecient

charge transport in electronic devices. For examples, Xu etal.

reported intrinsically stretchable TFTs based on a DPP-based

CPs/SEBS active layer (Figure4h).[69] In these blended systems,

the increased polymer chain dynamics from nanoconﬁnement

signiﬁcantly reduces the Young’s modulus of the CPs and

largely delays the crack formation under strain. Zheng et al.

also improved the chemical resistance of stretchable polymer

semiconductors by selectively crosslinking the backbones of the

elastomer (polybutadiene) and alkyl chains of DPP-based CPs to

make it feasible to photopattern stretchable polymer semicon-

ductors.[79] The fabricated TFTs showed a stable performance up

to 100% strain without aecting the charge carrier ﬁeld-eect

mobility. In addition, stretchable polymer light emitting layers

with a nanoconﬁned light emitting polymer nanoﬁber network

in a polyurethane (PU) elastomer matrix showed stable photo-

luminescent characteristics on the skin during various defor-

mations (Figure4i).[64]

3. Stretchable Electrodes

Stretchable electrodes have been intensively investigated in

response to the rising demand for stretchable displays with new

form-factors such as foldable, rollable, and stretchable archi-

tectures. These displays must be able to operate reliably even

in unexpected deformations. Under such perspectives, many

researchers have developed stretchable electrodes to achieve

robust electrical interconnection between active devices such as

light-emitting devices and TFTs, since the active materials are

generally vulnerable to external stresses. To achieve mechanical

reliability and electrical conductivity simultaneously, various

methods with material-based and structural-based engineering

for stretchable electrodes have been developed, which can

maintain their interconnection under mechanical deformations

such as folding, stretching, and twisting circumstances.

In this regard, this section begins with a discussion of rep-

resentative approaches and fabrication technologies for stretch-

able electrodes, focusing on interconnection of active devices

on stretchable substrate, classifying them into 1) intrinsically

stretchable electrodes, 2) structurally designed stretchable elec-

trodes, and 3) vertical interconnection access (VIA) for highly

integrated stretchable system with advanced functionalities.

3.1. Intrinsically Stretchable Conductors

Stretchable electrodes with intrinsic elasticity are regarded as

one of the most promising candidates for realization of stretch-

able electronic systems. By maintaining conductive percolation

paths under mechanical deformation, the electrode with supe-

rior elasticity can endure localized strain from external stress,

and then successfully interconnect functional devices. As a rep-

resentative methods, elastomeric composites of an elastomer

and conductive ﬁllers,[57,80–86] LMs,[87–93] and conductive poly-

mers[48,94–97] have been commonly used for intrinsically stretch-

able electrodes.

3.1.1. Conductive Nanomaterials Composites

As intrinsically stretchable electrodes, electrically conduc-

tive composites with combining an elastomer and conductive

ﬁllers are commonly used. Depending on target stretchability

of desired applications, various types of elastomers such as

PDMS, PU, and ﬂuorinated rubber can be used, and conduc-

tive ﬁllers such as metal NWs,[80–83] metal particles,[57] and

CNT[84–86] can be mixed with the elastomer.

By using 1D metallic ﬁllers, Lee et al. suggested AgNW-

PDMS nanocomposites as intrinsically stretchable elec-

trodes.[80] Since AgNW has 1D structure with high aspect ratio

as well as high electrical conductivity, the electrodes could

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maintain electrical percolation paths by forming entangled

structure in an elastomer. To fabricate AgNW-embedded PDMS

structures, as-coated AgNWs on a carrier substrate were trans-

ferred to uncured PDMS, and then the intrinsically stretch-

able electrodes were formed by simultaneous embedding and

curing processes. The electrodes exhibited mechanical softness

due to the fact that the AgNW layer was formed just on the

surface of the PDMS substrate (Figure 5a), which has a neg-

ligible inﬂuence on elasticity of the substrate. When a tensile

strain of 20% was applied, the AgNW electrodes could absorb

the external strain and maintain their resistance under various

mechanical deformations such as bending and stretching.

There are several strategies for fabricating AgNW-based elec-

trodes, including direct patterning method such as inkjet

printing and bar coating as well as indirect fabrication tech-

niques such as shadow masking and dry transfer patterning.

Considering pros and cons of each fabrication methods, Yang

et al. reported facile and highly ecient fabrication methods

for robust AgNW–elastomer composite electrodes, combining a

vacuum ﬁltration system with an improved 3D PDMS mask.[81]

The AgNW solution was successfully deposited on the target

region of membrane ﬁlter via the channels within the 3D mask.

As a result, clear edges could be achieved in both the AgNWs

deposited on the ﬁlter and transferred to the PDMS (Figure5b).

The resultant electrodes showed electrical conductivity of

≈3000 S cm−1 at high deposition density of 10 g m−2. By varying

the AgNW deposition density and the PDMS peel-o direction,

the stretchable electrodes could be stretched up to strain of

Society of Chemistry. c) Printable elastic electrodes with Ag ﬂakes, ﬂuorine rubber, and ﬂuorine surfactant. Reproduced with permission.[57] Copy-

Chemical Society.

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80%, exhibiting mechanical robustness with the LEDs turning

on even under bending conditions.

Furthermore, since the composite mixed with elastomeric

matrix is generally in ﬂuidic state before curing, various solu-

tion processes can be utilized to form stretchable electrodes

depending on target applications. In this regard, Matsuhisa

et al. reported a printable elastic electrode comprised of Ag

ﬂakes, a ﬂuorine rubber and a ﬂuorine surfactant.[57] By modi-

fying the Ag ﬂake surface from a water-based ﬂuorine sur-

factant, which is main component for formation of conductive

networks, the electrical conductivity and mechanical stretch-

ability of the printed electrodes could be enhanced, showing

738 S cm−1 at strain of 0% and 182 S cm−1 at strain of 215%.

Although the electrical conductivity was relatively low, the

electrode with superior elasticity can be stretched over strain

of 200%, maintaining percolation paths of the composites by

adding the surfactant (Figure5c).

Metal nanomaterials have high electrical conductivity

because of their metallic nature but it also brings low mechan-

ical ﬂexibility compared to carbon nanomaterials. By com-

bining those metal and carbon nanomaterials, they can have

synergetic eect on conductivity and stretchability. In this

regard, Ko etal. reported stretchable conductive interconnects

with superior electrical stability by forming composites that

consist of Ag particles with dierent sizes as 0D conductive

ﬁllers, multi-walled CNTs (MWCNTs) as 1D conductive ﬁllers

and silicone rubber matrix (Figure5d).[84] The micro-sized Ag

particles could enhance electrical conductivity of the compos-

ites, while the nano-sized Ag particles could allow dense con-

ductive network to be formed. Generally, the composite mixed

with only MWCNT networks has not enough electrical conduc-

tivity compared to counterparts with other metallic conductive

ﬁllers. However, the MWCNTs used as auxiliary ﬁllers could

enhance interactive attraction between Ag particles, enabling to

achieve high electrical conductivity and mechanical robustness

of the composites. The dierent-sized Ag particles in the sili-

cone matrix formed percolation paths and the MWCNTs were

positioned around the Ag particles. The subsequently achieved

stretchable electrodes exhibited high electrical conductivity of

6450 S cm−1 with little change under 3000 tensile cycle tests

at strain of 50%. By utilizing aforementioned methods, Hong

et al. demonstrated intrinsically stretchable and printable lith-

ium-ion battery by using nanostructure-controlled multimodal

conductive ﬁllers (Figure5e).[85] As the conductive ﬁllers, var-

ious multimodal micro-particles such as Ag particles for the

anode and Ni particles for the cathode were used. As an auxil-

iary ﬁllers, MWCNTs were also employed to form a networked

structure with conductive metal ﬁllers. The composites with Ag

particles and Ni particles shows high electrical conductivity of

3912 and 2105 S cm−1, respectively, which exhibited stable oper-

ation until tensile strain of 130%.

Conductive composites are widely used in apposite ways with

various conductive ﬁllers and elastomeric substrates according

to the target application. However, the electrical conduc-

tivity remains low in compared to other metal ﬁlm, since the

majority of components are nonconductive elastomer matrix.

In addition, the strain-sensitive electrical conductivity based on

percolation of conductive ﬁllers is still limitation for cycling sta-

bility. Therefore, it is important to enhance mechanical stability

as well as electrical conductivity for implementation of practical

applications.

3.1.2. LMs

LM is a representatively used material for an intrinsically

stretchable electrode, because it can maintain high electrical

conductivity under strain by changing its shape from ﬂuidic

property. Especially, since LM exists as liquid phase at room

temperature with ultralow Young’s modulus, an LM elec-

trode is capable of easily deformed to any direction with neg-

ligible resistance change unlike other solid-phase stretchable

electrodes.

Taking advantage of ﬂuidic property, Yang et al. used a

eutectic alloy of LM (Galinstan) that consists of Ga, In, and tin

(Sn) for intrinsically stretchable electrodes.[87] The LM could

ﬂow in a liquid form, congregating together with high surface

tension (Figure 6a). Since the electrical conductivity can be

maintained due to self-healing property as long as the inter-

connection is not completely damaged, the stretchability of

LM electrodes can be directly aected by the material proper-

ties of an elastomeric substrate such as Young’s modulus, yield

strength, and maximum yield point strain. To achieve high

stretchability over 300%, the Ecoﬂex silicone rubber was used

as an elastomeric substrate, and then the LM was injected into

empty channel in the rubber to form electrical interconnection.

The subsequently achieved LM electrodes showed superior

stretchability over 300% of strain.

Similarly, Bartlett et al. also reported LM-embedded elasto-

mers. Mixing the LM eutectic alloy that consists of Ga and In

with Ecoﬂex or PU elastomer, the intrinsically stretchable elec-

trodes were fabricated (Figure6b).[88] Various types of solution

fabrication processes including dispensing, screen printing,

stencil printing, and spray coating can be utilized for patterning

of the LM-elastomer composites. Especially, the LM electrodes

patterned by stencil printing exhibited superior stretchability

from 0% to 500% strain. To further enhance mechanical reli-

ability, Pan et al. suggested a LM-elastomer composite with

various sizes of LM droplets in an elastomeric substrate

(Figure6c).[89] The eect of LM droplet sizes to stiness of the

composites was investigated. The LM-elastomer composites

with small ﬁllers could maintain mechanical stretchability, but

counterpart composites with large ﬁllers showed a degradation

of stretchability in the strain at break. Therefore, the sizes of

LM droplets must be carefully considered depending on target

stretchability.

As another characteristic of LM electrodes, a thin oxide layer

can be naturally and rapidly formed around the LM electrodes,

enabling various architectures to be easily designed with main-

taining mechanical stability. However, there are challenges in

high resolution patterning due to its rapid oxidation with high

surface tension in air environment. To overcome this issue

and exhibit feasibility of the LM electrodes for stretchable

electronic systems with various designs, Silva et al. reported

high-resolution stretchable circuits with ﬁnely patterned LM

electrodes by controlling the oxide layer of LM (Figure6d).[90]

Polyvinyl alcohol (PVA) was coated on an elastomeric substrate,

which enables inkjet printing of conductive materials on an

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elastomeric substrate, followed by printing of Ag nanoparticles

(AgNPs). By removing oxide layer of LM through dissolving in

acetic acid solution, the LM could be selectively patterned on

the printed AgNP traces due to its dewetting property from the

PVA layer. Taking advantages of the self-patterning characteris-

tics, complicated patterns and circuits with dierent ﬁne lines

could be achieved, exhibiting outstanding electrical conductivity

and stretchability over 100% of strain.

Although LM electrode can exhibit superior elasticity with

high electrical conductivity, there are still critical issues to use it

together with other conductive metal, since Ga easily penetrates

other metals along to grain boundaries, resulting in irreversible

performance degradation such as swelling and cracking. There-

fore, speciﬁc integration methods including pattering, deposition,

and encapsulation technologies must be considered together.

3.1.3. Conductive Polymers

Conductive polymers have advantages in direct processability

and tunability of the molecular structures to determine elec-

trical and mechanical properties. Since they are capable of

being soluble in proper solvents, various solution processes

such as inkjet printing, spray coating, and screen printing can

be utilized. Among a lot of conductive polymers, PEDOT:PSS

has been intensively utilized as transparent stretchable elec-

trode for stretchable electroluminescent devices due to its high

electrical conductivity, transparency, and easy work function

modiﬁcation compared to other conductive polymers.

Oh et al. reported modiﬁcation of nanostructure and vis-

coelastic property of PEDOT:PSS by using plasticizer (Triton

X-100).[94] By optimizing weight fraction of the surfactant,

electrical conductivity and mechanical stretchability could

be enhanced simultaneously. Due to the viscoelasticity from

the addition of Triton X-100, the conductive polymer could be

molded into micro-patterns which enable implementation of

high resolution OLEDs array (Figure 7a). PEDOT:PSS elec-

trodes are generally incorporated with such plasticizers to

enhance its stretchability, but the electrical conductivity is still

low to be used for realization of high performance stretch-

able devices. To further improve stretchability and electrical

conductivity, Wang et al. reported the highly stretchable,

transparent and conductive PEDOT:PSS electrodes, which

were incorporated with ionic additives-based stretchability

and electrical conductivity (STEC) enhancers (Figure 7b).[48]

The STEC enhancers with ionic additives enabled forma-

tion of soft domains, better connectivity between PEDOT-

rich domains, and higher crystallinity in PEDOT regions,

resulting in implementation of highly conductive and stretch-

able PEDOT:PSS electrodes which exhibited 4100 S cm−1

under 100% of strain.

Similarly, Park et al. suggested transfer-printed conduc-

tive polymer electrodes for highly customizable PLEDs.[95] To

achieve better wettability on an elastomeric substrate, surface

treatments such as ultraviolet (UV) light exposure and oxygen

plasma treatment have been widely used, but these treatments

could degrade performance of organic layers. To overcome

this issue, they suggested dry transfer printing methods for

Figure 6. LM stretchable electrodes. a) A eutectic alloy of LM that consists of Ga, In, and Sn for intrinsically stretchable electrodes. Reproduced with

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PEDOT:PSS patterning to minimize degradation of organic

layers during whole fabrication processes. The PEDOT:PSS

electrodes incorporated with D-sorbitol, which was used to

enhance bonding force in a lamination processes, were suc-

cessfully transferred to a PDMS substrate. The transferred elec-

trodes exhibited outstanding transparency compared to conven-

tional ITO (Figure 7c). As a proof of concept, the matrix dis-

play were demonstrated which maintains its brightness under

bending state when the PEDOT:PSS electrodes were used as

anodes of PLEDs.

Despite its transparency and intrinsic elasticity, since

PEDOT:PSS has relatively low electrical conductivity com-

pared to metal ﬁlm, it is dicult to use it alone as an stretch-

able electrode. Therefore, multidisciplinary research should be

conducted to improve electrical and mechanical performance

through material synthesis or hybrid with other electrodes.

3.2. Structurally Designed Stretchable Interconnects

Structurally designed stretchable electrodes with various geo-

metrical patterns are one of the most promising candidates for

interconnection component of high-performance stretchable

display, taking advantages originated from well-established fab-

rication processes and material properties of metal ﬁlms with

high electrical conductivity.[98] However, since the metal ﬁlms

such as Cu, Au, and Ag have much lower tensile limit than

intrinsically stretchable electrodes, speciﬁc structural designs to

dissipate localized strain should be employed. Therefore, many

researchers have developed representative structural designs

such as serpentine[99–102] and wrinkled electrodes,[98,103–105]

which can maintain electrical conductivity by changing their

geometrical shapes.

3.2.1. Serpentine Electrodes

Serpentine electrodes are commonly used as metallic stretch-

able electrodes due to their superior electrical conductivity. The

metal electrodes are capable of being easily patterned by well-

established conventional photolithographic processes. Due to

unique structural designs with horseshoe shapes, the electrodes

could endure localized strain by changing their geometrical

shapes under various mechanical deformations (Figure 8a).[99]

Considering this characteristic, Biswas etal. reported stretch-

able active matrix display with multilayered serpentine elec-

trodes.[100] By using photolithography process, 10 µm thick

Cu electrode was deposited on carrier substrate where sacriﬁ-

cial layers of poly(methyl-methacrylate) (PMMA) and PI were

coated, and then insulation layer composed of photopatternable

PI was used to secure separation of row and column electrodes

(Figure8b). Even though there were thick parts of coated ﬁlms,

the electrodes could be stretched due to its structural design.

To demonstrate their stretchability, commercially available sur-

face mount LEDs were integrated with the electrodes, followed

by encapsulation process with Ecoﬂex elastomer with very low

Young’s modulus to achieve softness. The subsequently fabri-

cated stretchable active matrix display exhibited no signiﬁcant

performance changes under stretching at 220% of strain.

By further developing the serpentine structure designs, Li

et al. suggested 2D fractal-inspired shapes for advanced inter-

connect conﬁgurations.[101] The unique shapes could render

metal electrodes enhanced stretchability compared to conven-

tional serpentine electrodes, but the stretchability is still as low

as 11% when the electrodes were just stretched along to in-plane

direction. Therefore, two-stage solid encapsulation methods

were further suggested for formation of 3D conﬁgurations,

resulting in four times improved stretchability up to 46% strain

Figure 7. Conductive polymer-based stretchable electrodes. a) Micropatterned PEDOT:PSS electrodes blended with Triton X-100 to control viscoe-

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(Figure8c). By using these conﬁgurations, stretchable LED sys-

tems could be successfully achieved, maintaining electrical per-

formance even under 40% of biaxial strain.

3.2.2. Wrinkled Electrodes

Wrinkled electrodes can be directly formed on elastomeric

substrates using not only vacuum evaporation but also various

solution processes such as inkjet printing, screen printing,

spray coating, and electroless deposition. Among various

methods, inkjet printing is commonly used for patterning of

the metal ﬁlms due to its facile customizability and low temper-

ature processability. Cho etal. reported strain-tolerant wrinkled

electrodes for stretchable electrodes.[103] The wrinkled structure

could be introduced by releasing a pre-stretched elastomeric

substrate where conductive inks were printed by inkjet printing

method. Especially, they pointed out that the stretchability was

dominantly aected by the regularity of wrinkles since mechan-

ical strain could be concentrated at the thicker regions of the

electrode. By optimizing inkjet printing processes for imple-

mentation of thin and ﬂat metal ﬁlms, well-deﬁned wrinkles

Figure 8. Structurally designed serpentine and wrinkled stretchable electrodes. a) Horseshoe-shaped serpentine structural designs to endure localized

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on the ﬁlms could be obtained (Figure8d), and showed a little

resistance changes of 10% even under 10 000 stretching cycles

at 25% tensile strain. Similarly, Byun et al. fabricated various

types of stretchable display by using customizable inkjet

printing methods with integrating LEDs on wrinkled elec-

trodes.[104] By forming gradual wrinkles on metal ﬁlms with

embedded strain modulators, the mounted LEDs could main-

tain their electrical performance on human skin (Figure8e).

To further improve mechanical reliability, Oh etal. suggested

a corrugated SWCNT diusion barrier for the wrinkled elec-

trodes.[105] LM was used as contact materials, which could dis-

sipate concentrated mechanical strain at the boundary between

LEDs and wrinkled electrodes. However, the LM could react

with the wrinkled electrodes by penetrating grain boundaries in

metals. Considering this issue, corrugated SWCNT was intro-

duced to diusion barrier for wrinkled electrodes. Based on

the corrugated and wrinkled conﬁgurations, stretchable display

with wrinkled electrodes was demonstrated, which showed no

noticeable change in the luminance of the LEDs under 20% of

biaxial strain (Figure8f).

As described in previous examples, stretchable electrodes

with serpentine and wrinkled structure could be variously

designed depending on target stretchability of desired applica-

tions, and exhibited outstanding performance with high elec-

trical conductivity under various mechanical deformations.

However, the fabrication processes for serpentine electrodes

such as photolithography and etching are still challenging

for direct fabrication onto elastomeric substrates, and pre-

stretching process for wrinkled structures is critical bottleneck

for mass production of stretchable display. Therefore, it is

important to further establish suitable fabrication technologies

considering compatibility, processability, customizability, and

feasibility.

3.3. VIAs with Stretchable Electrodes

To implement a stretchable matrix display integrated with mul-

tifunctional layers, formation of VIAs in stretchable substrate

has been vigorously investigated in multidisciplinary research

ﬁelds. By interconnecting various active layers in stretchable

systems with high integration density, versatile operations

can be achieved. To form VIAs in stretchable substrate, mate-

rials and fabrication technologies should be carefully selected

since conductive paths along to vertical direction must be reli-

ably maintained when the systems are stretched along to hor-

izontal direction. In addition, the VIAs are generally ﬁlled in

perforated regions in an elastomeric substrate, which can lead

electrical failure and mechanical tear at the regions. There-

fore, many research groups have considered various methods

to make VIAs strain-insensitive without degrading mechanical

reliability and stretchability of stretchable substrates.

To form VIAs with serpentine electrodes, Huang et al.

reported a framework for 3D integrated stretchable systems

with VIAs, which were formed by laser ablation and ﬁlling con-

ductive materials in predeﬁned region.[106] Elastomer matrix

were selectively perforated by laser ablation with careful opti-

mizations of fabrication parameters such as laser wavelength,

pulse, and absorption rate. The geometries of VIAs such as

depth and width were controlled to secure electrical contacts

between VIA and other layer. By increasing diameter from

300 to 600 µm up to the fourth layer, enough contact in through

VIA could be ensured by ﬁlling conductive solder pastes in the

region (Figure 9a). Although the VIA was nonstretchable due to

rigid nature of ﬁlling material, it was stably interconnected by

serpentine electrodes with island-bridge conﬁgurations. Based

on this approach, highly conductive and strain-insensitive

VIAs could be achieved, and the VIAs exhibited ultralow

Figure 9. VIAs with stretchable electrodes. a) 3D integrated stretchable systems with VIAs formed by laser ablation and ﬁlling conductive materials with

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resistance about 68.7 mΩ, which enables implementation of high-

performance stretchable systems with increased integration

density.

In case of VIA with wrinkled electrodes, Byun et al. sug-

gested double-side soft electronic platform with core–shell

VIA.[107] By using printing-based method, the conductive VIA

was patterned on an elastomeric substrate. To resolve discon-

tinuity in elastic modulus between the VIA and substrate,

which could lead unexpected delamination, geometrical design

and compositions of the VIA were carefully optimized. The

conductive composites that consist of Ni particles, PDMS,

and silicone resin were printed on desired position, and then

magnetic alignment of the composites was conducted to form

gradual stiness in a single VIA (Figure9b). Due to the focused

magnetic alignment, Ni particles were converged into center

of VIA and aligned along to vertical direction, simultaneously.

The subsequently stiness-gradient shell, being relatively high

Young’s modulus than the substrate, could stably protect the

conductive vertical paths. Based on this approach, double-side

stretchable platform with wrinkled electrodes was successfully

demonstrated, which showed high-speed operation of stretch-

able computing circuits.

To implement LM-based VIA, Jiang etal. reported LM alloy

VIAs for fabricating multilayered stretchable systems.[108] To

generate microholes in an elastomeric substrate, UV laser for

ablation was used to implement vertical proﬁle with microstruc-

tures, and then the liquid alloy was deposited in the holes by

spray coating. Taking advantages in unique properties such as

liquid phase and rapid oxidation in air environment, the liquid

alloy could be coated on both sides of holes (Figure9c). After

selectively pinning at both sides, the oxide layer surrounding

the coated liquid alloy was rapidly formed, which enable that

the liquid alloy could maintain its shape against high surface

tension and gravity. The subsequently formed VIA exhibited a

resistance variation (ΔR/R0<300%) under stretching test with

tensile strain >90%, which is average strain of elongation at

break for elastomer substrate.

As aforementioned examples of VIA for stretchable system,

there are various fabrication methods and material candidates

to achieve robust interlayer interconnection and high integra-

tion in the stretchable system with multilayered structure. To

further improve integration density with maintaining electrical

performance and mechanical stability simultaneously, apposite

strategies must be selected depending on target applications

with careful consideration of the pros and cons in each method.

4. Stretchable TFTs

Stretchable TFT is a fundamental component in the backplane

of stretchable display, driving the electroluminescent devices

as well as performing signal switching and ampliﬁcation. To

implement advanced stretchable systems, there are signiﬁ-

cant eorts to fabricate mechanically reliable stretchable TFTs,

which can maintain their performances in various mechanical

deformations. Along with stretchable electrodes that intro-

duced in the previous chapter, the development of stretch-

able semiconducting active materials is critical for stretchable

TFTs. Therefore, it is important to develop apposite strategies

for implementation of stretchable TFTs depending on active

channel materials. In this regard, we discuss the various meth-

odology of forming stretchable TFTs in terms of active channel

materials, classifying into 1) inorganic, 2) CNT, and 3) organic

semiconductors.

4.1. Inorganic TFTs

Despite the inherent brittleness of inorganic active channel

materials including conventional silicon-based materials and

oxide semiconducting materials, there are various attempts

to implement stretchable inorganic TFTs for practical appli-

cations. Kim et al. suggested wrinkled structured TFTs for

silicon-based CMOS integrated circuits.[17] Although the inor-

ganic materials are generally vulnerable to external stress, the

wrinkled structures can render the fragile materials stretchable

by minimizing the deformation energy. Since inorganic TFTs

could be hardly fabricated onto the elastomeric substrate due

to the high temperature process, the TFTs were transferred

from the carrier substrate to prestretched PDMS substrate, and

then released them to initial state to achieve desired stretch-

ability. As proof of concept, stretchable CMOS integrated cir-

cuits were demonstrated, which could maintain their electrical

performances under deformations (Figure 10a). Although the

wrinkled inorganic TFTs has excellent electrical properties due

to well-deﬁned crystallinity of inorganic semiconductors, they

require specialized manufacturing procedures such as pre-

stretching and transferring. In addition, cracks are also likely

to occur when external strain applied to the device exceed limit

of prestrain, which can cause irreversible damage of electrical

behavior. Moreover, since TFTs consist of multiple layers with

electrodes, dielectric, and active channels, it is dicult to form

regular wrinkles on whole layers due to variation of their thick-

ness and Young’s moduli.

A strain-modulating structures such as an island-bridge

structure can be an ecient way to facilitate fabrication pro-

cess and to modulate strain distribution.[109–112] Since the inor-

ganic semiconducting materials have higher Young’s moduli

compared to elastomeric substrate, they can be located on the

strain-free region where the rigid parts are formed in the soft

substrate. When TFTs are fabricated on the strain-free areas

with stretchable interconnects, the deformation of TFTs can be

minimized. From this perspectives, Park et al. demonstrated

an array of stretchable oxide TFTs by transferring ZnO TFTs

onto the strain-modulated substrate with rigid island struc-

ture (Figure10b).[109] The stretchable interconnects with wrin-

kled structures could endure bidirectional strain, reducing the

mechanical stress on brittle oxide TFTs formed on strain free

regions.

By further developing material and structural designs, Park

et al. used LM for stretchable electrodes and fabricated TFTs

at the cross-point rigid island (Figure 10c).[110] The LM-based

stretchable electrodes can improve maximum strain limita-

tions due to its ﬂuidic properties with maintaining electrical

conductivity under various deformations. Moreover, LM can

be easily patterned in submicrometer feature size.[113] Also, to

improve scalability of stretchable TFTs, Cantarella et al. sug-

gested structurally engineered substrate where pillar structures

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were formed to obtain strain-free regions (Figure10d).[111] Since

the pillar structures could be freely designed according to the

geometries of designed TFTs, it could be applied to large-scale

logic circuits which consists of various types of TFTs.

Likewise, most of researches about stretchable inorganic

TFTs have generally focused on structural designs to achieve

mechanical stretchability and robustness. However, structural

improvement alone cannot overcome the intrinsic brittleness

fundamentally. In addition, conventional fabrication methods

such as photolithography and etching are not fully compatible

with elastomeric substrates, resulting in further increasing the

diculty of the whole processes. For these reasons, additional

integration techniques should be developed for realization of

more advanced stretchable systems.

4.2. CNT TFTs

CNT is one of the promising materials for implementation of

stretchable TFTs due to its superior electrical and mechanical

properties.[114–118] The 1D nanostructured CNTs can maintain

their electrical percolation paths with formation of entangled

networks.

In this regard, Chae et al. demonstrated stretchable CNT

TFTs by combining semiconducting CNT networks, graphene

electrodes, and wrinkled Al2O3 dielectric layer.[117] In contrast to

conventional inorganic materials, since the CNT networks and

graphene layers are extremely thin, well-deﬁned wrinkles could

be easily formed. By combining CNTs with corrugated structure

design of dielectric layer, the mechanical stability could be fur-

ther improved without compromising electrical characteristics

up to 20% strain in channel width direction, which showed

on/o ratio of 105 and high mobility of 40 cm2 V−1 s−1. This

approach not only showed the structural design applicability

of carbon-based materials but also exhibited their stretchability

and feasibility for intrinsically stretchable TFTs. In addition,

CNTs can be used for both electrodes and active channel mate-

rials simultaneously depending on their metallic and semicon-

ducting properties, respectively. In this regard, Cai et al. dem-

onstrated fully printed stretchable TFTs and integrated logic

circuits by using CNTs as both electrode and active channel

materials (Figure 11a).[119] To implement fully stretchable TFTs,

they fabricated stretchable dielectric composite that consist

of barium titanate (BaTiO3) NPs and PDMS matrix, which

exhibited high dielectric constant and stretchability. Due to

the intrinsic stretchability of electrodes, dielectric, and active

Figure 10. Stretchable inorganic transistors. a) Wrinkled structured CMOS circuit based on silicon active channel and its output characteristics with

used for stretchable oxide transistor array, with 5% of x- and y-axis strain. Interconnections are formed with a wrinkle to withstand the external strain.

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channels, the TFTs showed stretchability beyond 50% of strain

along direction of length or channel width, and exhibited no

signiﬁcant degradation of electrical performance during tensile

stretching cycles.

To fabricate high performance stretchable CNT TFTs with

high mobility, low voltage, and high integration density,

Huang etal. combined the advantages of solution process and

conventional photolithography methods (Figure11b).[120] They

used poly(urea-urethane) (PUU) elastomer as stretchable die-

lectric material because it has a higher permittivity compared

to other elastomeric dielectrics. However, the PUU is easily

damaged from photolithographic processes such as plasma

etching. To overcome this limitation, removal-transfer-

photolithography method was proposed. As a sacriﬁcial layer,

indium gallium zinc oxide (IGZO) was used to enhance

wettability of PDMS substrate and PUU dielectric, and

etching damage of the PUU dielectric was minimized with the

appropriate use of sacriﬁcial layers. Since CNT networks have

superior carrier mobility and the charge transportation was

signiﬁcantly improved by controlling dielectric capability, the

stretchable CNT TFTs showed high mobility (≈221 cm2 V−1 s−1)

and maintained their characteristics after 2000 stretching

cycles with 50% strain.

Another advantage of CNT to expand the scope of device

applications is transparency, which can increase aperture

ratio of display pixels or be used for fully transparent wear-

able devices. Liang etal. fabricated the intrinsically stretchable

and transparent CNT TFTs (Figure11c).[121] AgNWs and PU-co-

polyethylene glycol were used for electrodes and elastomeric

dielectric for intrinsically stretchable and transparent devices.

The fabricated TFT could be stretched up to 50% strain with

maintaining its electrical performance and exhibited optical

transmittance over 90% in the 450–1100 nm wavelength range,

driving OLED in the full brightness range. These results

implied the feasibility of a CNT-based backplane for display

devices with high transparency.

Furthermore, CNT TFTs could be integrated with stretch-

able sensors for various wearable devices. Hong et al. fabri-

cated a skin-attachable and intrinsically stretchable CNT TFT

array with highly sensitive temperature sensors.[122] By using

thermos-responsive ﬁlm, CNT TFT arrays with a thermo-

chromic property in the range of human body temperature

were implemented (Figure 11d). The fabricated devices were

easily attached to the human skin with stable operation under

skin movement such as wrist bending. As a practical healthcare

application, thermochromic display devices with CNT TFTs

were demonstrated with showing a possibility of advanced

wearable display application.

With the aforementioned unique characteristics, CNTs

showed the great possibility to be applied in stretchable driving

devices. However, in the chemical manufacturing process of

CNTs, both metallic and semiconducting CNTs are grown at

once. Therefore, additional purifying processes are needed to

improve the semiconducting property of the active channels.

Figure 11. Stretchable CNT transistors. a) Schematic and image of intrinsically stretchable CNT-based transistors fabricated using inkjet printing

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Because the remaining metallic CNTs form leakage paths in

the channel even after puriﬁcation, the o-current level can be

increased. In addition, the random network structure of CNT

channels reduce the uniformity of TFT array. Therefore, devel-

opment of puriﬁcation methods with high yield and studies of

advanced passivation and percolation structure are required to

achieve CNT TFT array with high stability and uniformity.

4.3. Organic TFTs

Organic semiconductors are also one of the promising candi-

dates for active layers of deformable TFTs based on not only

easy processability including vacuum and solution processes,

but also easy controllability of mechanical and electrical prop-

erties through molecular engineering. Similar to the inorganic

TFTs, the island-bridge structures were implemented in the

early stages of researches about stretchable organic TFTs due to

mechanical brittleness of small molecule organic semiconduc-

tors.[61,123] Sekitani etal. demonstrated a stretchable organic TFT

array utilizing PI as rigid islands (Figure 12a).[123] The brittle

channel region was located on strain-free region with PI rigid

islands to minimize the mechanical stress when the device was

stretched. As a result, the fabricated TFT array with stretchable

interconnects could be uniaxially and biaxially stretched up to

70% without any mechanical or electrical failures. However,

Figure 12. Stretchable organic transistors. a) Schematic of stretchable organic transistor array utilizing PI as rigid islands and images of fabricated

of polymer dynamics of block copolymer with amorphous and crystal chains and a self-healing process with solvent vapor treatment. Reproduced with

of stretchable organic transistor array with composite-based stretchable semiconductor and elasti layer for redistribution of applied strain. Repro-

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as mentioned in the section of inorganic and CNT transistors,

this approach utilizing rigid islands has still limitations about

process complexity due to additional processes for fabrication

of rigid regions in the elastomeric substrate and concentrated

mechanical stress at the interface.

To overcome aforementioned limitations, molecular engi-

neering methods for the implementation of intrinsically

stretchable organic semiconducting materials have been devel-

oped. There are three main strategies to develop intrinsically

stretchable organic semiconductors: copolymerization or side

chain engineering to control crystallinity,[124–130] blending addi-

tives to induce phase separation or change in crystallinity,[131,132]

and making composite of organic semiconductor and elas-

tomer.[66,69,133–135] In the ﬁrst case, Oh etal. demonstrated intrin-

sically stretchable and healable block copolymer-based organic

TFTs with conjugated repeating units and non-conjugated moi-

eties as hydrogen bonding units.[125] When strain was applied

to the polymer ﬁlms, energy dissipation occurred through

the breakage of hydrogen bonds between amorphous chains

(Figure12b). As a consequence of this energy dissipation, the

polymer semiconductor could retain high charge transport abil-

ities in stretched states. Furthermore, the mechanically dam-

aged polymer ﬁlm could be healed through solvent vapor treat-

ment and thermal annealing, recovering its morphology and

ﬁeld-eect mobility to those of an undamaged ﬁlm. Stretchable

organic TFTs fabricated with CNT electrodes and PDMS dielec-

tric layer could operate stably under 100% strain and even after

the 500 stretching cycles at 25% strain.

In the second case, as a strategy of additive to control the

crystallinity of polymers, Mun et al. blended a conjugated

carbon nanoring, cycloparaphenylenes (CPP), to the conjugated

polymer semiconductor to tune the dynamic behaviors of the

polymer.[132] As CPP was added to the polymer, long-range crys-

talline order was reduced, thereby enhancing polymer dynamic

motion (Figure 12c). In addition, CPP improved the device

performance by lowering the contact resistance and increasing

charge transportation. Subsequently fabricated stretchable TFTs

with CNT electrodes and PDMS dielectric layer could operate

without any dramatic degradation under 100% strain and

1000 stretching cycles with 25% strain.

Although the two main strategies mentioned above enable

the realization of intrinsically stretchable organic TFTs, there

are some limitations on ﬁne-tuning of stretchability and elec-

trical characteristics of organic semiconductors simultaneously

due to constraints of molecular engineering. To overcome those

limitations, researchers utilize conjugated polymer and elas-

tomer composites to give stretchability to the semiconductors.

Xu etal. reported a composite-based stretchable organic semi-

conductor that conjugated polymers in composite was unidirec-

tionally ordered by a solution-shearing process (Figure12d).[134]

A nanoﬁber structure of conjugated polymer in composite was

achieved from a nanoconﬁnement eect induced by a phase

separation of elastomer and conjugated polymer. In addition,

polymer chains in the composite were aligned and elongated

by the solution-shearing process with the intensive unidirec-

tional ﬂow generated from microtrenches of the coating blade.

Due to the multi-scale ordering and alignment of conjugated

polymers in stretchable composite, charge carrier mobility

and stretchability of stretchable semiconductors were greatly

enhanced. As a result, fabricated stretchable organic TFTs with

CNT electrodes and SEBS dielectric layer can operate without

any failures under 100% strain and 1000 stretching cycles with

50% strain. Furthermore, Wang et al. introduced patterned

elastomer layers with tunable stiness to protect composite-

based organic semiconductors from the applied strain.[135] By

locally tuning the cross-linking density of the elastomer, sti

parts of the substrate were utilized as strain-relief regions sim-

ilar to rigid island structures (Figure 12e). By combining two

approaches of strain-relief structures and composite of elas-

tomer/conjugated polymer, the fabricated intrinsically stretch-

able organic TFTs showed strain-insensitivity with the change

of performance less than 5% under 100% strain.

As described above, organic semiconductors have signiﬁ-

cant advantages of controlling their mechanical and electrical

properties by engineering molecular structures or blending

several organic materials. These organic TFTs can achieve

high stretchability without any complicated approaches such

as island-bridge structures or kirigami patterns. On the other

hand, organic semiconductors have disadvantages of vulnera-

bility to chemicals, UV irradiation, or plasma treatments. How-

ever, these limitations are also becoming overcome by selec-

tive molecular crosslinking of organic materials. Recent work

reported high-density intrinsically-stretchable transistors and

circuits with all-polymer-materials by direct optical microlithog-

raphy patterning with channel length of 2 µm and a density of

42 000 transistors cm−2.[136] The electrical properties of polymer

semiconductors are lower than inorganic- and CNT-based coun-

terparts, but the intrinsically-stretchable polymer transistor

array can be a potential candidate for wearable and electronic

skin devices.

5. Stretchable Light-Emitting Devices

Stretchable light-emitting devices stably maintain light-emis-

sion under various deformations, providing new design form-

factors for deformable electronics, wearable electronics, and soft

robotics. There are two main strategies to demonstrate stretch-

able electroluminescent devices: adapting island-bridge conﬁg-

uration to the rigid and ﬂexible electroluminescent devices and

developing intrinsically stretchable electroluminescent devices.

Based on island-bridge conﬁgurations, various light-emitting

devices are integrated with stretchable interconnects that can

accommodate external stresses. When external strain is applied,

the strain is concentrated into stretchable interconnects, while

the active devices that are positioned in the strain-free region

can be eectively protected, exhibiting improved mechanical

stability. In the case of intrinsically stretchable electrolumi-

nescent devices, approaches are similar to the development

of intrinsically stretchable TFTs, such as blending additives

to induce phase separation or making a composite of light-

emitting materials with elastomers or low glass transition

temperature polymers to give stretchability. Each strategy for

structure and material engineering to achieve desired stretch-

ability can be determined based on the target light-emitting

materials. In this section, the representative strategies for the

development of stretchable light-emitting devices will be pre-

sented with the various light-emitting materials and devices,

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including 1) inorganic, 2) polymer (organic), and 3) perovskite

light-emitting materials and LEDs, and 4) ACEL.

5.1. Inorganic LEDs

Inorganic LEDs (ILEDs) are representative electroluminescent

devices that consist of inorganic compounds of ‘III-V’ or ‘II-VI’

materials. Combinations of Ga, In, N, and P such as GaN[137]

and GaAs[138] work as a p–n junction with radiative recombina-

tion energy released in light. ILEDs generally have great light

eciency, long lifetime, and environmental stability compared

to other light-emitting devices. However, intrinsic rigidity

from inorganic materials is a critical obstacle to morphological

change, therefore, proper integration methods with polymeric

substrates are highly required to overcome a signiﬁcant mod-

ulus mismatch.

The island-bridge conﬁguration is an early method to inte-

grate ILEDs with stretchable substrates.[139–141] The epitaxially

grown ILEDs on wafer inevitably require additional processes

to transfer them to other substrates. Park et al. utilized elas-

tomeric stamp with kinetic control of adhesion in order to

transfer ILEDs array mesh to a pre-strained PDMS substrate.

The Ti/SiO2 deposited island structures successfully incorpo-

rated with the elastomeric substrate. When the external strain

was applied to the PDMS, the chemically bonded islands were

not deformed, while the serpentine electrodes deformed out-

of-plane to accommodate the strain, ensuring a reliable opera-

tion of the stretchable ILEDs array under a uniaxial strain up to

≈22%. (Figure 13a).[139] Similarly, Byun etal. also systematically

fabricated strain-engineered stretchable platforms by adopting

inkjet-printed rigid islands (PRIs). PMMA ink was formulated

by changing its molecular weight and the number of printing

layers was optimized for the robustness of the islands. After

printing the PRIs, another top PDMS was covered to increase

stability, preventing the PRIs from being fractured under a uni-

axial strain of ≈40%. Then, various inorganic chips, including

ILEDs, integrated onto strain-free, engineered PRI employing

epoxy adhesives, enabling on-skin electronics. Moreover, the

inkjet printing modiﬁes island patterns and interconnects

freely, enhancing the degree of freedom of stretchable circuit

formation. (Figure13b).[104]

The island-bridge structures eectively suppressed the

deformation around ILEDs. However, the abrupt dierence of

elastic modulus can induce signiﬁcant strain concentration at

the interfaces, which causes delamination of devices and plastic

deformation of substrates. To overcome failure issues, Biswas

etal. embedded whole circuit into Ecoﬂex with very low Young’s

modulus without additional island structure. The whole cir-

cuit components including ILEDs array, stretchable electrodes,

and VIAs were fabricated on the PI substrates, and then a low

melting point solders were coated on the contact pads, where

ILEDs array would be transferred by pick-and-place assembly

techniques. The ILEDs circuit on the PI substrate was fully

embedded by Ecoﬂex over molding, and then the PI substrate

was partially perforated by electron cyclotron resonance plasma

etching, enabling the circuits to be stretchable. The embedded

circuits could be stretched up to 260% and showed high

deformability under various 3D guided shapes (Figure13c).[100]

The conventional bonding that utilizes metal soldering

or rigid adhesive with high elastic modulus ensures robust

bonding, but they are simultaneously vulnerable to mechan-

ical deformation. Therefore, Hwang et al. fabricated intrinsi-

cally stretchable anisotropic conductive ﬁlms (S-ACF) that can

directly interconnect ILEDs and substrates. The polystyrene-

block-poly(ethylene-ran-butylene)-block-polystyrene-graft-

maleic anhydride (SEBS-g-MA) was used as a free-standing

and stretchable ﬁlm. The periodically arranged metal particles

were embedded in the S-ACF for electrical connection. The

boding of ILEDs was acquired by modifying the bottom sur-

face wettability of ILEDs with 3-aminopropyltriethoxysilane

(APTES). The MA-NH2 amide bond with SEBS-g-MA template

enabled stretchable ILEDs array with high stretchability of 70%

(Figure13d).[142]

Several integration methods have been proposed to over-

come large modulus dierence in the heterogeneous interface

between rigid ILEDs and soft substrates. Although the integra-

tion concept was proved for a few ILEDs, the successiveness

and repeatability to integrate a large number of ILEDs and

other electronic devices were not identiﬁed. Moreover, since

the size of the ILEDs has minimized, a systematic integration,

including mechanical and electrical connection, applicable to

ﬁne-pitch ILEDs should be further investigated.

5.2. Organic Light-Emitting Devices

Organic light-emitting devices composed of organic materials

with electroluminescent property are one of reliable candidates

for light-emitting components in stretchable electronics due

to its intrinsic softness with mechanical robustness[143–148] and

processability on deformable substrates with low-temperature

process.[149,150] However, the stretchable organic light-emitting

devices should simultaneously sustain multiple axial strains. In

this regard, various approaches have been reported to achieving

stretchable organic light-emitting devices: using wrinkled struc-

ture and serpentine electrodes, island-bridge structures, and

intrinsically stretchable materials.

Wrinkled structure has been introduced as a representative

method for the fabrication of stretchable devices. To implement

the wrinkled structure on elastomeric platform, the stretch-

able OLEDs were fabricated on pre-stretched elastomer such

as PDMS or PU. When pre-stretched platform was released,

directional wrinkles were implemented depending on the direc-

tion of applied prestrain. When external strain was applied

along the wrinkle direction, the structure on the display was

unfolded without physical damage. Using this structure, Yokota

et al. attached ultrathin OLEDs on pre-stretched elastomer

(Figure 14a).[151] Ultraﬂexible OLEDs with thin encapsulation

layers were fabricated on 1-µm-thick parylene ﬁlms, showing

stable operation under ambient condition. After lamination

process on pre-stretched elastomeric substrate, the device

could maintain its electroluminescent performance even

after 1000 cycles with 60% strain. The encapsulated stretch-

able display with ultrathin structure with total thickness of

3 µm could be easily laminated on human skin without

degradation, thereby applied to wearable device which can visu-

alize data extracted from human body. Yin etal. expanded the

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stretchability of devices to 2D direction with random-located

buckling networks on the surface of OLEDs (Figure 14b).[152]

After attaching ultraﬂexible OLEDs on 100% pre-stretched elas-

tomer with both 2D directions, the random networks of micro-

scale buckling structures were formed after releasing process.

Through this approach, OLEDs was operated without degrada-

tion even with 50% of 2D strain. On the other side, Jeong etal.

focused on the image distortion of stretchable OLEDs with

wrinkle structures (Figure14c).[153] Unlike the other approaches

for the stretchable OLEDs with wrinkled structures, the OLEDs

were directly fabricated on pre-stretched elastomers without

lamination of plastic substrate. After fabrication of OLEDs and

releasing process, the mean values of size of wrinkles were all

less than 8 µm and those micro-wrinkles demonstrated both

distortion-free pixels and high stretchability.

By implementing stretchable OLEDs on strain-engineered

platform with serpentine shaped stretchable electrodes, Lim

etal. reported another promising approach based on lamination

Figure 13. Stretchable ILEDs. a) Island-bridge-based stretchable ILEDs fabricated by transferring an ILED array mesh to a PDMS substrate. Reproduced

American Association for the Advancement of Science.

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of 3D island-bridge structures.[62] Micropillar arrays placed on

PDMS substrate were laminated with OLEDs on SU-8 sub-

strate, therefore pillars supported the OLED pixels (Figure14d).

In addition, OLED pixels were connected with the bridges with

serpentine electrodes. Under the external strain, the serpentine

electrodes were stretched while pillars attached with PDMS

pixels were maintained its spacing. Because the mechanical

strain was dissipated to the serpentine electrodes and PDMS

substrate, the OLED arrays were reliably stretched up to 35%

without any image distortion.

Although applying island-bridge structures on OLEDs is

an ecient method for implementation of reliable stretchable

electroluminescent devices, intrinsically stretchable OLEDs

with novel material designing have a great deal of attention.

Liu et al. suggested intrinsically stretchable AM organic light-

emitting electrochemical cell (OLEC) array (Figure 14e).[70]

AgNW-coated polyurethane acrylate (AgNW-PUA) was used

as anodes and cathodes, and optimized blend of light-emit-

ting materials was adopted for better stretchability. Fabricated

OLECs were stable at 30% strain and vertically connected with

stretchable polymer semiconductors to fabricate AMOLECs. To

further enhance stretchability, Kim etal. blended Triton X-100

surfactant with active layers (Figure 14f).[71] When the sur-

factant was blended with emissive materials, they interrupted

Figure 14. Stretchable organic light-emitting devices. a) Schematic of ultraﬂexible OLEDs with passivation layers. Reproduced with permission.[151]

e) Schematic layout and optical image of stretchable OLEC pixels driven by stretchable transistors, and images of a single OLEC pixel with dierent

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interchain reactions between emissive materials, thereby

making layers stretchable. In addition, when Triton X-100 was

mixed with PEDOT:PSS which was used as hole transport layer

(HTL), the surfactant hinders the interactions between PEDOT

and PSS, so the hole transport can be enhanced and Young’s

modulus is decreased. The fabricated intrinsically stretch-

able OLEDs maintained their performance at 40% strain, and

showed stable emission even at the strain of 80%.

With promising advantages of OLEDs such as mechan-

ical reliability and possibility of large area process, various

approaches were adopted for fabrication of stretchable OLEDs.

However, stretchable encapsulation which provides long-term

stable operations even under the mechanical strain is a chal-

lenge for commercialization. Furthermore, fabrication of large

area OLEDs arrays and linkage with stretchable driving circuit

should be next steps for stretchable OLED displays.

5.3. Metal-Halide Perovskites LEDs

Metal-halide perovskites (MHP) became promising light-emit-

ting materials since the ﬁrst development of MHP LEDs in

2014.[154] The narrow emission linewidth (<20 nm) of MHP can

achieve ultrahigh color purity with the color gamut ratio >140%.

MHP is commonly composed of ABX3 forming a cubic struc-

ture, where A is an organic ammonium or an alkali metal ion,

B is a transition metal cation and X is a halide anion.

Controlling dimensionality of MHP from 3D, 2D, 1D to

0D with dimension decreased from micrometer-scale down

to nanometer-scale is the most eective approach to conﬁne

the charge carriers and to increase the possibility of radiative

recombination. Hence, using 0D nanocrystals could achieve

highly ecient LEDs on a glass substrate with EQE boosted

>23.4%.[155] However, the rigid nature of MHP nanocrystals

makes it dicult for stretchable applications.

Introducing the buckling structure to the device with thin

substrates will signiﬁcantly reduce the strain experienced by

the device, as the magnitude of the strain was proportional to

the thickness of the substrate.[156] The AgNW percolation net-

works were embedded in the PI substrate (1–2 µm) to form an

ultrathin transparent conductive electrode with excellent chem-

ical stability against organic solvents.[157] After multiple succes-

sive solution processes and Al electrode deposition, the sub-

strate was conformably attached to pre-strained 3M-VHB tape

to create a buckling structure with a small bending radius of

70 µm (Figure 15a). The stretchable MHP-based LEDs showed

a low turn-on voltage (Von) of 3.2 V (Figure15b), the maximum

luminance of 3187 cd m−2 at 9 V, and can retain the device per-

formance with tensile strain up to 50% (Figure15c).

As an alternative to rigid MHP nanocrystals, methylammo-

nium lead bromide (MAPbBr3) polycrystalline precursors are

mixed with an ion-conducting polymer with viscoelastic prop-

erties such as poly(ethylene oxide) (PEO) to reduce Young’s

modulus.[158] By mixing the conducting polymer PEDOT:PSS

with 33 wt% of PEO, the highest conductivity of 35600 S m−1

was achieved and showed no signiﬁcant change in conduc-

tivity under 20% of strain. The cathode was deposited on the

composite emitter using eGaIn (Figure 15d). The stretchable

MHP LEDs showed Von= 2.4 V (Figure15e), and the maximum

luminance of 15960 cd m−2 at 8.5 V (Figure 15f ). A similar

approach has been made using CsPbBr3 with the addition of

PEO and poly(vinylpyrrolidone) (PVP).[159] The morphological

analysis on the failure mechanism of the stretchable MHP

LEDs showed that crack initiated at the perovskite layer and

prorogated to the bottom electrode.[160] Therefore, improving

the stretchability of the perovskite layer is the key for stretch-

able MHP LEDs.

Alternatively, an air-stable stretchable display has been

achieved by the integration of the perovskite stretchable color

conversion layer (SCCL) and the stretchable light-emitting

devices.[161] Perovskite nanocrystals were dispersed in the elas-

tomer due to the high compatibility between the ligand and

the polymer matrix. The SCCL down-converts the blue emis-

sion from stretchable light-emitting devices and reemits it as

green (Figure 15g). The PL intensity decreased by 29.8% at

100% strain and recovered to the initial state after releasing

(Figure15h). The stretchable light-emitting devices with SCCL

show the stretchability of 180% (Figure15i).

Despite approaches have been made to apply organic-

inorganic hybrid perovskites for stretchable applications, more

eorts are needed to increase both stretchability and device

light-emitting eciency with innovations in materials aspect.

5.4. ACEL Devices

ACEL devices are normally operated based on hot-electron

impact on the light-emitting phosphors under a high electric

ﬁeld across the thick dielectric layer >10 µm. Phosphors are

embedded in the stretchable dielectric layer with a high dielec-

tric constant to reduce potential drop, and the dielectric layer is

sandwiched between two stretchable electrodes. As there is no

direct charge injection from the electrode to the phosphors, the

conductivity of the electrode for the ACEL devices is lower than

that of other LEDs.

To increase the stretchability of the ACEL devices, an ionic

conductive hydrogel is incorporated as a stretchable electrode.

With the application of voltage, charges in the electrode are

separated at the contact/hydrogel interface over nanometers,

which can create large capacitance on the order of 10−1 F m−2.[162]

Hygroscopic lithium chloride was adopted as the ionic con-

ductor with high conductivity of 10 S m−1, while stretchable

polyacrylamide was used as the elastomer matrix.[163] The low

elastic modulus of the hydrogel allows the free stretching of

the device without delamination at the interface (maximum

strain ≈ 480%). The device can be operated at 700 Hz under

≈25 kV cm−1 with a luminous ecacy of 43.2 mlm W−1. The

matrix with an 8 × 8 array of 4-mm pixels has been fabricated

using the replica molding technique, which can also undergo

stretching, rolling, folding, and wrapping.

As an alternative to the molding technique, a stretchable

multicolor display can be prepared using the transfer printing

method. Phosphors mixed with inks can be photopatterned

ﬁrst and then dry transferred to the dielectric layer using a

thermal release tape. Multiple cycles of transfer printing allow

the assembly of multicolor pixels on the large dielectric.[164]

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The ionic conductive hydrogel can also be constructed in

the ﬁber shape to further boost the stretchability up to 800%

(Figure 16a).[165] Two ellipse-shaped ﬁbers wrapped by ZnS

phosphors and elastomer composite to display programmable

pattern. The electroluminescent layer also serves as the pro-

tecting layer for the hydrogel electrode. The stretchable light-

emitting ﬁber was woven into a stretchable textile display. Also,

owing to the high stretchability of hydrogel that can sustain

repetitive stretching to 700% for 1000 cycles, another ACEL

device can undergo tensile strain up to 700% with maximum

luminance = 95 cd m−2 (Figure16b).[166]

Dierent from the monotonic increase in light emission of

ionic conductive hydrogel-based ACEL under stretching, the

percolation AgNW-based electrode showed a slight increase

in emission intensity of 30% followed by a monotonic

decrease in intensity with further stretching (Figure 16c).[167]

The electric ﬁeld concentrated on the AgNW decreased with

a larger open area created between AgNWs under stretching,

resulting in a decrease in the light intensity.[166] Hence, main-

taining the high electric ﬁeld under large deformation is greatly

challenging especially for conventional transparent conductive

electrodes based on low-dimensional materials and conducting

polymers. Incorporation of dielectric particles such as BaTiO3

can eectively enhance the electric ﬁeld inside the stretch-

able light-emitting layer, which substantially increases the

electroluminescent intensity (Figure16d).[168]

In addition, the high operating voltage of stretchable ACEL

devices is another issue that limits practical applications; this

is caused by the low permittivity of the elastomer. Hence,

approaches have been made to increase the permittivity of the

Figure 15. Stretchable perovskite LEDs. a) Device structure, b) current density (I)–voltage (V)–luminance (L), and c) digital images of geometrically

images of stretchable LEDs before and after integration with perovskite stretchable color conversion layer (SCCL). h) PL spectra of the SCCL under

uniaxial tensile strain. i) Digital images of the stretchable LED integrated with the SCCL under tensile strains of up to 180%. Reproduced with permis-

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dielectrics. Poly(vinylidene ﬂuoride)-based ﬂuoroelastomer has

also been developed as the dielectric materials that can main-

tain high permittivity and self-healing properties with the addi-

tion of non-ionic additives (Figure16e). The ACEL has achieved

a high brightness of 1460 cd m−2 at 2.5 V µm−1 with a maximum

stretchability of 800%.[169] Besides adding BaTiO3 particles into

the light-emitting layer will also lead to higher permittivity and

brightness (Figure16f).[170] With above merits, stretchable ACEL

devices have been successfully demonstrated for the electronic

skin that is composed of strain and electrocardiogram sensors.

Given the fact that the goal of the stretchable ACEL display is

the wearable application, both maximum luminance and device

eciency need to be improved to meet the requirement of low

power consumption. More innovations in stretchable light-

emitting materials are needed to further boost the eciency.

6. Stretchable LED Matrix Arrays

In the display, multiple LED units that compose a matrix array

are driven individually. Stretchable LED arrays in which geo-

metrically or intrinsically stretchable structures, materials, and

electronic devices (transistors and LEDs) technologies are inte-

grated are manufactured and driven in PM or AM structures.

In this chapter, the structure and characteristics of stretch-

able displays according to 1) PM and 2) AM operations, and 3)

high-resolution full-color displays at the prototype level will be

discussed.

6.1. PM Arrays

PM LED array, consisting of driver integrated circuit chip and

light-emitting units, is operated by turning on the light-emitting

units line by line, where each pixel is deﬁned at the overlapped

points of driving electrodes. Thus, PM structure can be obtained

by simple manufacturing process without complicated circuit

design. However, PM structure has limitations in increasing

resolution or panel size due to large power consumption.[171]

So, it has been utilized in limited areas, such as low-end and

small-sized mobile applications. In this regard, stretchable PM

array has been demonstrated to show 1) a novel unit technology

for integrated stretchable systems, including stretchable light-

emitting units, interconnects, and driving circuit chips,[30,172–175]

Figure 16. Stretchable ACEL devices. a) Digital images of a stretchable light-emitting ﬁber being stretched from 0% to 800%. Reproduced with permis-

structure of a ﬂuoroelastomer with a non-ionic ﬂuorinated surfactant for a healable and low-ﬁeld illuminating optoelectronic stretchable device. Repro-

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or 2) the concept of next-generation wearable displays, such as

skin-attachable patch-type display and textile display integrated

with health monitoring sensors.[11,104,176–178]

To demonstrate stretchable PM LED array, island-bridge

structure with stretchable interconnects is a practical approach.

In island-bridge structure, light-emitting units are located at

rigid-island region, where the strain is suppressed by structural

design, and they are interconnected by stretchable electrodes in

bridge region, where the strain is released. Therefore, inherent

stretchability of light-emitting unit become less essential in the

island-bridge structure, enabling ILED to be utilized in stretch-

able PM LED array.[30,172–176] Thus, several studies have been

implemented ILED-based PM array to demonstrate the feasi-

bility of novel structural designs such as serpentines,[172] helical

structures,[174] and intrinsically stretchable materials.[173]

Meanwhile, some research focused on ﬁll factor of stretch-

able display beyond the stretchability itself.[30,175] Increasing

stretchability has trade-o relationship between the ﬁll factor

of LEDs, deteriorating the resolution of display. Therefore, a

double-layered modular design separating LED unit substrates

and stretchable interconnectors (Figure 17a)[30] or 3D assembly

with hidden pixel structure[175] has been suggested. Meanwhile,

Byun etal. demonstrated a PM LED array on inkjet printing-

based stretchable platform, including printed rigid island

and wrinkled Ag interconnects in a fully-customizable way

(Figure 17b).[104] By integrating ILED chips, microcontroller

units, and other surface-mount device components on a rigid

island-embedded stretchable platform, a 2-bit digital multiplier

and temperature monitoring system were demonstrated on

skin.

Stretchable PM LED array with island-bridge structures can

also be made with thin-ﬁlm-based LEDs such as OLED and

quantum dot LED (QLED) to make them more ﬂexible and con-

formal.[11,177] Therefore, real-time health monitoring patch that

provided biological information on human skin in intuitive way

has been reported. Lee etal. demonstrated integrated system of

health monitoring patch with stretchable OLED array, photop-

lethysmography (PPG) heart rate sensor, processing modules,

and batteries (Figure17c).[11] Stretchable OLED array could be

realized by modulus-engineered strain relief layer and micro-

cracked interconnects, making compatible with lithography

processes. Lee et al. also demonstrated a sensor-integrated

tion for the Advancement of Science. d) Stretchable QLED array for monitoring body movements and temperature. Reproduced with permission.[177]

Springer Nature.

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stretchable QLED array to monitor body movement and skin

temperature (Figure 17d).[177] QLED array deposited on NOA

63 islands were operated by stretchable LM interconnects.

Beyond island-bridge structure, fully stretchable PM array

were demonstrated by using intrinsically stretchable ACEL

device (Figure17e).[178] By weaving stretchable conductive ﬁber

and ZnS-coated conductive ﬁber, each electroluminescent

pixel could be deﬁned at contact points. While they require AC

operation with high voltage, the feasibility of textile electronics

integrated with displays and other electronic components has

proven to broaden the applicable areas of stretchable elec-

tronics and more intuitive human-machine interfaces. Various

next-generation stretchable technologies are expected to con-

tinuously be demonstrated through PM driving scheme, sug-

gesting novel concept of wearable display.

6.2. AM Arrays

PM has a simpler structure and more straightforward operation

than AM by applying voltage to each data and scan line sequen-

tially to emit light at intersecting pixels. PM divides the scan

time by the number of lines and sequentially applies voltage

only for a short period of time. During one frame scan, only

pixels to which voltages of the scan line and the data line are

applied simultaneously emit light. Therefore, a high voltage is

required to ensure high brightness during a short light emis-

sion time, which shortens the lifespan of the materials. In addi-

tion, as the number of lines increases, more time division is

required, so there is a limitation in increasing resolution and

size.

AM has transistors and storage capacitors in each pixel, so

it is possible to store voltage for a certain period of time and

maintain light emission for one frame. Also pixels are individu-

ally controlled using transistors of the backplane. Therefore, it

is suitable for high-resolution large-area displays with advan-

tages such as low-power operation, minimization of line cross-

talk and ﬂicker, fast response, and capable of compensation

circuit conﬁguration. The AM stretchable display is more chal-

lenging than the PM stretchable display because the mechan-

ical stretchability of backplane circuits and their components

(e.g., transistors and capacitors) should be improved. The devel-

opment of AM stretchable display is still in the early stages and

many breakthrough studies are required, but it is essential to

produce a future high-resolution large-area stretchable display.

A few stretchable AM LED arrays have been reported with the

extrinsically stretchable geometries and rigid inorganic/organic

LEDs or the intrinsically stretchable geometries and polymer

LECs. Also, the mature high-mobility silicon transistors or

the next-generation transistors that exploit oxide-, CNT-, or

polymer–semiconductors have been used for driving AM LED

arrays.

A 32 × 32 stretchable AM ILED array (13 ppi) was fabricated

on an island-bridge patterned PI substrate with Cu serpentine

interconnects.[179] A 2T-1C structure backplane composed of

amorphous IGZO TFTs with a mobility ≈10 cm2 V−1 s−1 drives

rigid ILED pixels. An LED pixel array was connected to a pix-

elated backplane with isotropic conductive adhesive and pick-

and-place technique, then encapsulated with a thin TPU ﬁlm.

The LED chips array had uniform brightness, and showed ﬂex-

ibility and conformability on the complex surfaces without sig-

niﬁcant loss in the uniformity.

μ-LED refers to an ultrasmall LED with a size of less than

100 µm, and is attracting attention as a next-generation display

based on high brightness, low power, high resolution, inﬁ-

nite contrast, fast response, wide color gamut, long lifespan,

and stability in air and moisture. Also, fast-response and low-

power single crystal Si-TFT with the high ﬁeld-eect mobility

(≈700 cm2 V−1 s−1) which is a desirable candidate for large-area

and high-resolution display was used to drive μ-LED array.

Stretchable AM array composed of 8 × 8 μ-LED and Si-TFT

array was fabricated on the PDMS substrate using roll transfer

printing (Figure 18a).[141] High mobility Si-TFTs and μ-LEDs

were fabricated separately and sequentially transferred to a tem-

porary glass substrate with a high yield and precise alignment.

Then the Si-TFTs and μ-LEDs are connected by serpentine

interconnects and transferred on a rubber substrate to demon-

strate stretchable AM μ-LED array. The operation of μ-LED by

Si TFT was controlled with high on-o current ratio (≈107). The

rigid island part was ﬁrmly ﬁxed on the PDMS substrate, and

the serpentine interconnect at the neutral mechanical plane

composed of metal layer sandwiched by polymer was slid on

the PDMS surface to maintain the light emitting properties

without deterioration even at 40% tensile strain (Figure18b,c).

Especially, the strain applied to the rigid island area was less

than 0.1%, so the active components maintained the stable

characteristics even after 200 stretching cycles at 40% strain.[141]

An island-bridge structure using intrinsically stretchable

and straight interconnects instead of the serpentine intercon-

nects can minimize the space occupied by the interconnects

and thereby potentially increase the pixel density and aperture

ratio. LM that exhibits small resistance change under strain

can be a possible candidate instead of serpentine patterned

ﬂexible metal interconnects. SWCNT transistors and μ-LED

arrays were fabricated on rigid polyethylene terephthalate (PET)

islands embedded in Ecoﬂex substrates, and each island was

connected by intrinsically stretchable and straight LM intercon-

nects (Figure18d).[180] In bending or stretching, the strain was

concentrated in the Ecoﬂex region with low modulus (69 kPa),

and the negligible strain was applied in the rigid PET island

region with high modulus (2.0–2.7 GPa). For example, at a

biaxial strain of 30%, Ecoﬂex is subjected to 240% strain, while

PET is subjected to near 0% strain. Therefore, the 5 × 5 array

SWCNT TFTs fabricated in the PET region shows a drain

current change less than 4% at 30% biaxial strain. As a result,

5 × 5 AM μ-LED array fabricated with 10 μ-LEDs on each PET

island did not show any deterioration in brightness under 30%

biaxial strain and even after 1000 bending cycles (Figure18e).

Compared to the geometrical strain engineering

approaches, intrinsically stretchable electronic components

can increase stretchability of AM LED array by reducing the

portion of nonstretchable area and might be better for the

electronic skin devices with smaller mechanical constraints on

the skin. AMOLEC array integrated with intrinsically stretch-

able organic TFTs and intrinsically stretchable OLECs main-

tained brightness without deterioration at 30% tensile strain

(Figure 18f).[70] In stretchable organic TFTs, the high mod-

ulus (>2.25 GPa) of the isoindigo moiety-containing polymer

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semiconductor, which cracks at 15% tensile strain was low-

ered to <0.75 GPa by blending an azide functionalized PDMS

crosslinker. The crosslinked semiconductor ﬁlm did not show

cracks even at 100% tensile strain. In addition, crosslinkable

stretchable insulators that have the modiﬁed hydroxyl groups

of perﬂuoropolyether diols with dimethacrylate groups showed

very negligible change in dielectric constant at 100% strain.

With CNT stretchable gate, source and drain electrodes, the

5 × 5 transistor array showed less than one order of magnitude

decrease in both the mobility and on-current, and minimal

changes in leakage current even after 1000 repeated stretching

at 100% strain.

In stretchable OLECs, the AgNWs-PUA electrode and

PEDOT:PSS hole injection layer sandwiched the light emission

layer that is composed of a blend of a light-emitting polymer

(Super Yellow), an ion-conducting polymer (ethoxylated tri-

methylopropane triacrylate), and lithium triﬂuoromethane sul-

fonate, which enables formation of PIN junction in the light

emission layer. The stretchable OLECs did not showed delami-

nation, crack formation or degradation of current density under

30% strain. An intrinsically stretchable 2 × 3 AMOLEC array

showed stable operation in bending, twisting, and stretching

states (Figure18g).[70]

Based on such stretchable PM and AM LED arrays intro-

duced in this chapter, a large-area, high-resolution, and full-

color stretchable display at a level closer to commercialization

will be introduced in the next chapter.

6.3. High-Resolution Full-Color Displays

The development of high-resolution, high-density and full-

color stretchable displays is mainly conducted by display

industry because of the technological manufacturing maturity

and capacity of fabrication facilities. Although many prom-

ising intrinsically stretchable light emitting devices intro-

duced in the previous chapters are being actively developed,

important factors for manufacturers and users such as pro-

cessing yield, reliability, uniformity, eciency, lifespan, and

outdoor visibility must be considered to develop commercial

products. Therefore, in the current stage, the island-bridge

structure with established non-stretchable LEDs and transis-

tors is a viable approach for developing a prototype stretchable

displays.

PM stretchable LED array with 10 × 10 RGB LED package

chips and serpentine copper interconnects on the PI substrate

was developed in 2015 and demonstrated stable operation

at 10% strain.[172] It proved the scalability by demonstrating

80 × 45 array PM LED display with 8.5 ppi. Also recently, a 2.7 in.

and 42 ppi (96 × 60) stretchable display with 90 × 150 µm RGB

LED chips was reported.[12] The LED chips are transferred by

pick-and-place, stamp, and self-assembly techniques on the

island-bridge-patterned ﬂexible backplane (Figure 19a). Then,

assembled backplane was subsequently transferred to and

encapsulated by low modulus elastomer that are crosslinkable

at low temperature and highly transparent.

Figure 18. Stretchable AM LED array. a) Schematic and digital image of stretchable AM μ-LED array with single crystal Si-TFT backplane. b) Digital

and c) optical images of stretchable AM μ-LED array with serpentine interconnects under strains of 0% and 40%. Reproduced with permission.[141]

f) Schematic and g) digital images of stretchable AMOLEC array that incorporates intrinsically stretchable OTFT and OLEC arrays. Reproduced with

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The stretchability of the panel was simulated in terms of

the ratio between island and bridge parts, the mechanical

modulus of elastomer, and the width of interconnect and ﬂex-

ible substrate. To achieve high stretchability of panel, smaller

island/bridge ratio, smaller width of interconnect, and lower

modulus elastomer are preferred. The thickness of intercon-

nects and ﬂexible substrate have negligible eects. With 600

µm pixel pitch, the fabricated 42 ppi stretchable display panel

demonstrated stable operation with biaxial stretching, free-from

twisting, and convex deformation by poking (Figure 19b-d).

This approach has a potential to be extended to 120 ppi stretch-

able display by employing 200 µm pixel pitch and μ-LED with

smaller feature size of 20 µm.

AM A 9.1 in. full-color stretchable AMOLED display was

demonstrated with island-bridge structure (Figure 19e).[9] The

AM display exploits an OLED array which can be directly fab-

ricate on the ﬂexible substrate, and a ﬂexible low-temperature

polycrystalline silicon (LTPS) backplane that exhibits excellent

properties for the high resolution and high frame rate display

with higher carrier mobility (50–100 cm2 V−1 s−1) than a-Si

(1 cm2 V−1 s−1), organic (1 cm2 V−1 s−1), and metal oxide semi-

conductors (10 cm2 V−1 s−1). The electrical stability of LTPS TFTs

is usually suered from mechanical deformations because

both interface state density and grain boundary state density

increase under bending and compression states. The induced

instability of leakage current and threshold voltage changes

charges stored in the capacitor, resulting in nonuniform bright-

ness of pixel array. The strain applied to the LTPS TFTs was

minimized by the geometrical approach of rigid island and ser-

pentine bridge structure under 5% tensile strain. Therefore, the

Figure 19. Stretchable high-density full color display. a) Schematics of fabrication process for a stretchable PM ILED display. Digital images of a stretch-

able PM ILED display under various deformations, including b) biaxial stretching, c) twisting, and d) poking. Reproduced with permission.[12] Copyright

2021, Wiley-VCH. Digital images of a 9.1 in. stretchable AM OLED display e) without deformation and with f) convex and g) concave deformations.

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TFTs maintained stable electrical characteristics of which the

threshold voltage shift was lower than 0.1 V and the leakage

current was reduced due to aging eect.

The mechanical modulus of the ﬂexible backplane on the

patterned PI substrate was 40 MPa which is in the similar range

of soft rubber and is much lower than the modulus of PI ﬁlm

(4.2 GPa). Convex and concave shaped displays with 10 mm ver-

tical displacement are demonstrated through thermoforming

process by attaching the deformed displays on the patterned

mold with thermoplastic sheet and adhesive (Figure 19f,g).

Moreover, a 14.1 in. full-color stretchable AMOLED display

was developed and it demonstrated no signiﬁcant degradation

under deformation with 45 mm of vertical height and even after

10000 stretching cycles with 5% strain (Figure19h).[29]

7. Conclusion and Outlook

Stretchable displays, which can be freely bent in any direction

and ﬁt on any shape, are almost the end of the form factor

innovation beyond ﬂexible displays. Stretchable displays will

maximize the portability of users’ devices with multidirectional

folding, enhance bodily comfort with skin-level moduli and

softness, and enhance versatility through multi-curvature defor-

mation. In the case of ﬂexible displays, repeated deformation

only occurs in limited bent areas and typically involves unidi-

rectional folding. Moreover, ﬂexible displays with bendable,

foldable, and rollable form factors can minimize the actual

elongation of active devices by placing them in a mechanically

neutral plane. In contrast, stretchable displays involve elonga-

tion in length, which leads to the development of stretchable

geometries and soft materials with stable electrical and optical

properties under elongation to give displays a free-form factor.

This paper reviewed the current state of the research on

structural designs and elastic materials with important features,

including the conductors for interconnections, semiconductors

for TFTs, and light-emitting materials for stretchable displays

(Figure 20). Furthermore, stretchable display prototypes that

have already been demonstrated based on the aforementioned

technologies were discussed.

The various approaches that are actively being studied to

develop a stretchable display all have their pros and cons. For

example, as mentioned above, geometrically stretchable struc-

tures such as buckled structures and island-bridge structures,

which are close to ﬂexible, have the advantage that the existing

semiconductor/display devices and manufacturing processes

can be used. However, buckled structures are limited to pro-

ducing highly integrated arrays with high stretchability, and

island-bridge structures, which only increase the gap between

pixels, can produce severe image distortion. Intrinsically

stretchable structures can control the deformation of the pixel

area by adjusting the Young’s modulus. Thus, they can pro-

duce more natural image deformation and minimize the het-

erogeneity on the skin, but the development of materials with

high performance, long-term stability, and high reliability is

required.

For the commercialization of stretchable displays, the required

speciﬁcations can be varied according to their target applications

such as IoT devices, wearables, and automobiles. Nonetheless,

three key design parameters have been identiﬁed. First, the

stretchability of the display should be higher than 30%, which

approximately corresponds to a bending radius of 0.06 mm.

Stretchable displays attached to human skin should be mechani-

cally robust and operate stably under at least 30% strain, taking

into consideration the strain variation on the skin.[6,11,181,182]

Second, the resolution of the display should be higher than

200 ppi, accounting for the viewing distance and size of the

display. Third, the display should have a high brightness at low

operation voltages for outdoor visibility (e.g., 1000 cd m−2 at 5 V).

Consequently, for high-resolution stretchable displays, the

conductivity of the stretchable conductor should be higher

than 5 × 106 S m−1 to prevent a voltage drop across the narrow

interconnect lines, which causes nonuniformity of the dis-

play. Considering the conductivity and the change in resist-

ance under strain, metal-based stretchable conductors such as

LMs, metal thin ﬁlms, and metal nanomaterials are promising

stretchable interconnects. LM has excellent deformability and

decent conductivity (≈3.5 × 106 S m−1),[183–185] but it is neces-

sary to completely control the stability and ﬂowability in the

subsequent lamination process. In addition, stability must be

ensured when the electronic devices are exposed to tempera-

tures below the melting points (e.g., 15.5°C for eGaIn, –19 °C

for Galinstan) of LMs.[183–185] For metal thin ﬁlms, deformability

can be achieved by introducing a serpentine structure in addi-

tion to having excellent conductivity. However, serpentine inter-

connects are expected to be much longer and narrower than

straight interconnects, and metals with higher conductivities

than those of conventional interconnects are required.[186] Metal

nanomaterials such as AgNWs are promising next-generation

stretchable interconnects because of their excellent conductivity

and deformability. However, to apply them to mass production,

it is necessary to achieve high uniformity over a large area (e.g.,

6th generation mother glass, 1500 × 1850 mm).

Figure 20. Strategies to develop materials and devices toward realization

of stretchable display.

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For the operation of OLED pixels, driving and switching

transistors require high charge carrier mobility >10 cm2 V−1

s−1, low o-current <10 pA, large on/o current ratio (>106),

and low hysteresis of threshold voltage <0.1 V.[187,188] In addi-

tion, it may be necessary to analyze the distribution of strain

required in the device in order to optimize the arrangement

of the stretchable and rigid parts, so that the stretchable dis-

play and the rigid driving circuit and power source can be

integrated. For example, the strain on skin near the wrist is

approximately 30%, which is much higher than the strain on

skin further from the wrist (<7%).[6,11] In addition, rigid parts

are required for the protection and handling of devices in daily

life, and rigid circuits and batteries can be integrated into hard

parts. Flexible and stretchable driving circuits and batteries are

also being actively researched.[85,102,194,106,135,136,189–193] This will

enable the realization of all-parts-ﬂexible/stretchable electronic

devices. Additionally, the user should ideally be able to control

the deformation of the devices in the desired situation. For this

purpose, controlling the mechanical properties of the substrate

using materials such as shape-morphing materials can be a

suitable approach.[195–200]

Despite many research achievements, the speciﬁc form and

applications of stretchable displays are still relatively vague.

Recent studies developed deformable displays that could over-

come the design constraints and be applied to the A-pillar

and dashboard of an automobile, a health monitoring device

attached to the skin, and smart ﬁber-based clothing. Thus, inno-

vation activities to support various user scenarios and overcome

technical obstacles are becoming more common. Further work

still remains, including work to improve the display resolution,

stretchability, and reliability, but it is expected that they will be

on the market in the near future as next-generation form-factor

displays.

Acknowledgements

Y.L. and H.C. contributed equally to this work. This work was supported

by the Industry technology R&D program (20010427) funded By the

Ministry of Trade, Industry & Energy (MOTIE, Korea). This work was also

supported by the National Research Foundation of Korea (NRF) grant

funded by the Korea government (Ministry of Science and ICT) (NRF-

2016R1A3B1908431) and the Creative-Pioneering Researchers Program

through Seoul National University (SNU). This work was also supported

by Basic Science Research Program through the National Research

Foundation of Korea (NRF) funded by the Ministry of Education

(NRF-2021R1A6A3A03038934).

Conﬂict of Interest

The authors declare no conﬂict of interest.

Keywords

ﬂexible displays, stretchable electronics, stretchable LEDs, stretchable

materials, stretchable transistors, wearable displays

Received: July 1, 2022

Revised: October 4, 2022

Published online:

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Yeongjun Lee is a postdoctoral researcher in the Department of Chemical Engineering at Stanford

University, USA. He received his B.S. in the Department of Materials Science and Engineering

(MSE) from Hanyang University, South Korea in 2012. He received his Ph.D. in the MSE from

Pohang University of Science and Technology (POSTECH), South Korea in 2018. He joined the

MSE at Seoul National University, as a postdoctoral researcher and worked at Organic Material

Lab in Samsung Advanced Institute of Technology as a sta researcher (2019-2021). His research

interests include printed electronics, stretchable electronics, neuromorphic electronics, and

bioelectronics.

Hyeon Cho is a Ph.D. candidate in the Department of Electrical and Computer Engineering,

Seoul National University, Republic of Korea. He received his B.S. degree from the Department

of Electrical and Computer Engineering, Seoul National University in 2017. His current research

interests are stretchable electronic skin and soft energy generators for self-powered wearable

electronics.

Hyungsoo Yoon is a Ph.D. candidate in the Department of Electrical and Computer Engineering,

Seoul National University, Republic of Korea. He received his B.S. degree from the Department

of Electrical and Computer Engineering, Seoul National University in 2016. His current research

interests are methodologies for the integration of microdevices onto stretchable and ﬂexible plat-

forms for conformable microelectronics.

Adv. Mater. Technol. 2023, 2201067

www.advancedsciencenews.com

2201067 (35 of 37)

www.advmattechnol.de

Hyunbum Kang is currently senior researcher at Samsung Advanced Institute of Technology,

Samsung Electronics. He obtained his Ph.D. degree in the Department of Chemical and

Biomolecular Engineering at Korea Advanced Institute of Science and Technology, Republic of

Korea. His current interests include stretchable electronic materials and devices.

Hyunjun Yoo is a Ph.D. candidate in the Department of Electrical and Computer Engineering,

Seoul National University, Republic of Korea. He received his B.S. degree from the Department

of Electrical and Computer Engineering, Seoul National University in 2017. His current research

interests are various stress mechanisms of carbon nanotube thin ﬁlm transistors for soft

electronics.

Huanyu Zhou is currently a postdoctoral researcher in Materials Science and Engineering at

Seoul National University, Korea. He received his B.S. in 2015 and M.S. in 2016 from Yonsei

University, and his Ph.D. from Seoul National University in 2022. His current research is mainly

focused on ﬂexible and stretchable devices based on organic and organic–inorganic hybrid

materials.

Sujin Jeong is a Ph.D. candidate at the Department of Electrical and Computer Engineering,

Seoul National University, Republic of Korea. She received her B.S. degree from the Department

of Electrical and Computer Engineering, Seoul National University in 2017. Her current research

interests are polymer-based optoelectronics and its geometrical engineering for stretchable

electronics.

Adv. Mater. Technol. 2023, 2201067

www.advancedsciencenews.com

2201067 (36 of 37)

www.advmattechnol.de

Gae Hwang Lee is currently principal researcher at Samsung Advanced Institute of Technology,

Samsung Electronics. He obtained his Ph.D. degree in the Department of Physics from KAIST,

South Korea in 2009. He worked for the technologies of organic photodiodes/transistors related

to next generation electronics including stacked image sensors, stretchable display. His current

interests include stretchable semiconducting materials and wearable sensors for healthcare

monitoring.

Geonhee Kim is a Ph.D. candidate in the Department of Electrical and Computer Engineering,

Seoul National University, Republic of Korea. He received his B.S. degree from the Department

of Electrical and Computer Engineering, Seoul National University in 2016. His current research

interests are solution-processed deformable transparent electrodes and electrohydrodynamic

printing for high-resolution electronics.

Gyeong-Tak Go is a Ph.D. candidate in the Department of Materials Science and Engineering of

Seoul National University, Republic of Korea. He received his B.S. degree from the Department

of Materials Science and Engineering, Seoul National University in 2018. His research interests

include the stretchable electronics and neuromorphic electronics that exploit artiﬁcial synapses.

Jiseok Seo is a Ph.D. candidate in the Department of Electrical and Computer Engineering,

Seoul National University, Republic of Korea. He received his B.S. degree from the Department

of Electrical and Computer Engineering, Seoul National University in 2016. His current research

interests are stretchable electronic skin and solution-processed TFT.

Adv. Mater. Technol. 2023, 2201067

www.advancedsciencenews.com

2201067 (37 of 37)

www.advmattechnol.de

Tae-Woo Lee is a professor in the Department of MSE at Seoul National University, South Korea.

He received his Ph.D. in Chemical Engineering from KAIST, South Korea in 2002. He joined Bell

Laboratories, USA, as a postdoctoral researcher and worked at Samsung Advanced Institute

of Technology as research sta (2003–2008). He was an associate professor in MSE at Pohang

University of Science and Technology (POSTECH), South Korea, until August 2016. His research

focuses on printed or soft electronics that use organic and organic-inorganic hybrid materials for

ﬂexible/stretchable displays, solid-state lighting, solar energy conversion devices, and bioinspired

neuromorphic devices.

Yongtaek Hong received B.S. and M.S. in Electronics Engineering, Seoul National University

(SNU), Seoul, Korea, and Ph.D. in EE, Univ. Mich., Ann Arbor, MI, USA. He was a senior

research scientist at Display Science & Technology Center of Eastman Kodak Company,

Rochester, NY, USA (2003~2006). Since 2006, he has worked at ECE, SNU as a professor and

now is Head of SNU Entrepreneurship Center, and Vice-Head of SNU R&DB Foundation in

Business Aairs. He was a visiting professor at Chem. Eng., Stanford University (2012~2013).

His research interests include printed/ﬂexible/stretchable thin-ﬁlm devices, displays, and sen-

sors for wearable and electronic skin applications.

Youngjun Yun is currently principal researcher at Samsung Advanced Institute of Technology,

Samsung Electronics. He obtained his Ph.D. degree in School of Engineering from Durham

University, UK in 2011. He worked for the technologies of organic/oxide transistors related to

next generation display including reﬂective color displays, holographic displays, ultra-high-deﬁni-

tion displays, and foldable displays. His current interests include ﬂexible and stretchable displays

and skin-like sensors for healthcare monitoring.

Adv. Mater. Technol. 2023, 2201067

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Skin-mountable electronic materials are being intensively evaluated for use in bio-integrated devices that can mutually interact with the human body. Over the past decade, functional electronic materials inspired by the skin are developed with new functionalities to address the limitations of traditional electronic materials for bio-integrated devices. Herein, the recent progress in skin-mountable functional electronic materials for skin-like electronics is introduced with a focus on five perspectives that entail essential functionalities: stretchability, self-healing ability, biocompatibility, breathability, and biodegradability. All functionalities are advanced with each strategy through rational material designs. The skin-mountable functional materials enable the fabrication of bio-integrated electronic devices, which can lead to new paradigms of electronics combining with the human body.

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Stretchable electronics have attracted tremendous attention amongst academic and industrial communities due to their prospective applications in personal healthcare, human‐activity monitoring, artificial skins, wearable displays, human‐machine interfaces, etc. Other than mechanical robustness, stable performances under complex strains in these devices that are not for strain sensing are equally important for practical applications. Here, a comprehensive summarization of recent advances in stretchable electronics with strain‐resistive performance is presented. First, detailed overviews of intrinsically strain‐resistive stretchable materials, including conductors, semiconductors, and insulators, are given. Then, systematic representations of advanced structures, including helical, serpentine, meshy, wrinkled, and kirigami‐based structures, for strain‐resistive performance are summarized. Next, stretchable arrays and circuits with strain‐resistive performance, that integrate multiple functionalities and enable complex behaviors, are introduced. This review presents a detailed overview of recent progress in stretchable electronics with strain‐resistive performances and provides a guideline for the future development of stretchable electronics.

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Dynamic shape-morphing soft materials systems are ubiquitous in living organisms; they are also of rapidly increasing relevance to emerging technologies in soft machines1–3, flexible electronics4,5 and smart medicines⁶. Soft matter equipped with responsive components can switch between designed shapes or structures, but cannot support the types of dynamic morphing capabilities needed to reproduce natural, continuous processes of interest for many applications7–24. Challenges lie in the development of schemes to reprogram target shapes after fabrication, especially when complexities associated with the operating physics and disturbances from the environment can stop the use of deterministic theoretical models to guide inverse design and control strategies25–30. Here we present a mechanical metasurface constructed from a matrix of filamentary metal traces, driven by reprogrammable, distributed Lorentz forces that follow from the passage of electrical currents in the presence of a static magnetic field. The resulting system demonstrates complex, dynamic morphing capabilities with response times within 0.1 second. Implementing an in situ stereo-imaging feedback strategy with a digitally controlled actuation scheme guided by an optimization algorithm yields surfaces that can follow a self-evolving inverse design to morph into a wide range of three-dimensional target shapes with high precision, including an ability to morph against extrinsic or intrinsic perturbations. These concepts support a data-driven approach to the design of dynamic soft matter, with many unique characteristics.

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Intrinsically stretchable organic light‐emitting diodes (ISOLEDs) are becoming essential components of wearable electronics. However, the efficiencies of the ISOLEDs have been highly inferior to their rigid counterparts, which is due to the lack of ideal stretchable electrode materials that can overcome the poor charge injection at one‐dimensional metallic nanowire/organic interfaces. We demonstrate highly‐efficient ISOLEDs that use graphene‐based two‐dimensional‐contact stretchable electrodes (TCSEs) that incorporate a graphene layer on top of embedded metallic nanowires. The graphene layer modifies the work function, promotes charge spreading, and impedes inward diffusion of oxygen and moisture. The work function (WF) of 3.57 eV is achieved by forming a strong interfacial dipole after deposition of a newly‐designed conjugated polyelectrolyte with crown ether and anionic sulfonate groups on TCSE; this is the lowest value ever reported among ISOLEDs, which overcomes the existing problem of very poor electron injection in ISOLEDs. Subsequent pressure‐controlled lamination yielded a highly efficient fluorescent ISOLED with an unprecedently high current efficiency of 20.3 cd/A, which even exceeds that of an otherwise‐identical rigid counterpart. Lastly, a three‐inch five‐by‐five passive matrix ISOLED was demonstrated using convex stretching. This work can provide a rational protocol for designing intrinsically stretchable high‐efficiency optoelectronic devices with favorable interfacial electronic structures. This article is protected by copyright. All rights reserved

Geometrically engineered rigid island array for stretchable electronics capable of withstanding various deformation modes

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Integration of rigid components in soft polymer matrix is considered as the most feasible architecture to enable stretchable electronics. However, a method of suppressing cracks at the interface between soft and rigid materials due to excessive and repetitive deformations of various types remains a formidable challenge. Here, we geometrically engineered Ferris wheel-shaped islands (FWIs) capable of effectively suppressing crack propagation at the interface under various deformation modes (stretching, twisting, poking, and crumpling). The optimized FWIs have notable increased strain at failure and fatigue life compared with conventional circle- and square-shaped islands. Stretchable electronics composed of various rigid components (LED and coin cell) were demonstrated using intrinsically stretchable printed electrodes. Furthermore, electronic skin capable of differentiating various tactile stimuli without interference was demonstrated. Our method enables stretchable electronics that can be used under various geometrical forms with notable enhanced durability, enabling stretchable electronics to withstand potentially harsh conditions of everyday usage.

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Skin-attachable sensors, which represent the ultimate form of wearable electronic devices that ensure conformal contact with skin, suffer from motion artifact limitations owing to relative changes in position between the sensor and skin during physical activities. In this study, a polarization-selective structure of a skin-conformable photoplethysmographic (PPG) sensor was developed to decrease the amount of scattered light from the epidermis, which is the main cause of motion artifacts. The motion artifacts were suppressed more than 10-fold in comparison with those of rigid sensors. The developed sensor-with two orthogonal polarizers-facilitated successful PPG signal monitoring during wrist angle movements corresponding to high levels of physical activity, enabling continuous monitoring of daily activities, even while exercising for personal health care.

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Intrinsically stretchable bioelectronic devices based on soft and conducting organic materials have been regarded as the ideal interface for seamless and biocompatible integration with the human body. A remaining challenge is to combine high mechanical robustness with good electrical conduction, especially when patterned at small feature sizes. We develop a molecular engineering strategy based on a topological supramolecular network, which allows for the decoupling of competing effects from multiple molecular building blocks to meet complex requirements. We obtained simultaneously high conductivity and crack-onset strain in a physiological environment, with direct photopatternability down to the cellular scale. We further collected stable electromyography signals on soft and malleable octopus and performed localized neuromodulation down to single-nucleus precision for controlling organ-specific activities through the delicate brainstem.

High-brightness all-polymer stretchable LED with charge-trapping dilution

Article

Full-text available

Mar 2022
NATURE

Next-generation light-emitting displays on skin should be soft, stretchable and bright1–7. Previously reported stretchable light-emitting devices were mostly based on inorganic nanomaterials, such as light-emitting capacitors, quantum dots or perovskites6–11. They either require high operating voltage or have limited stretchability and brightness, resolution or robustness under strain. On the other hand, intrinsically stretchable polymer materials hold the promise of good strain tolerance12,13. However, realizing high brightness remains a grand challenge for intrinsically stretchable light-emitting diodes. Here we report a material design strategy and fabrication processes to achieve stretchable all-polymer-based light-emitting diodes with high brightness (about 7,450 candela per square metre), current efficiency (about 5.3 candela per ampere) and stretchability (about 100 per cent strain). We fabricate stretchable all-polymer light-emitting diodes coloured red, green and blue, achieving both on-skin wireless powering and real-time displaying of pulse signals. This work signifies a considerable advancement towards high-performance stretchable displays. A material design strategy and fabrication process is described to produce all-polymer light-emitting diodes with high brightness, current efficiency and good mechanical stability, with applications in skin electronics and human–machine interfaces.

Island Effect in Stretchable Inorganic Electronics

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Full-text available

Mar 2022
SMALL

Island‐bridge architectures represent a widely used structural design in stretchable inorganic electronics, where deformable interconnects that form the bridge provide system stretchability, and functional components that reside on the islands undergo negligible deformations. These device systems usually experience a common strain concentration phenomenon, i.e., “island effect”, because of the modulus mismatch between the soft elastomer substrate and its on‐top rigid components. Such an island effect can significantly raise the surrounding local strain, therefore increasing the risk of material failure for the interconnects in the vicinity of the islands. In this work, a systematic study of such an island effect through combined theoretical analysis, numerical simulations and experimental measurements is presented. To relieve the island effect, a buffer layer strategy is proposed as a generic route to enhanced stretchabilities of deformable interconnects. Both experimental and numerical results illustrate the applicability of this strategy to 2D serpentine and 3D helical interconnects, as evidenced by the increased stretchabilities (e.g., by 1.5 times with a simple buffer layer, and 2 times with a ring buffer layer, both for serpentine interconnects). The application of the patterned buffer layer strategy in a stretchable light emitting diodes system suggests promising potentials for uses in other functional device systems.

Auxetic Meta‐Display: Stretchable Display without Image Distortion

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Full-text available

Feb 2022
ADV FUNCT MATER

Mechanical metamaterials possess unusual mechanical properties that cannot be found in nature. Auxetic metamaterials have negative Poisson's ratios and tend to expand in a direction perpendicular to the axial extension direction. When the Poisson's ratio of a display circuit board is forced to be −1 by adopting an auxetic metamaterial, a display can be stretchable without image distortion, and this display is called a meta‐display in this study. The critical obstacles to implementing a stretchable display are large stretchability, high deformation uniformity, and low image distortion. The meta‐display overcomes these obstacles by incorporating micro‐LEDs and a kirigami‐based auxetic circuit board. An auxetic meta‐display with a stretchability of 24.5%, Poisson's ratio of −1, and no image distortion under uniaxial stretching is demonstrated. Finally, the roll transfer process enabled the scaling‐up of a 3‐inch meta‐display attachable to surfaces with non‐zero Gaussian curvatures. This conformity to the non‐zero Gaussian curvature helps realize biomedical applications such as wearable display, phototherapy, and skincare.

Morphable 3D structure for stretchable display

Article

Mar 2022
MATER TODAY

Displays that can be reversibly stretched in their geometrical layout are highly important in various applications. Stretchable displays are commonly demonstrated using two-dimensional (2D) geometries with stretchable rubber substrates and elastic interconnects. However, mechanical stretch induces deterioration of the resolution per unit area and blurs the displayed image, consequently lowering the display quality. In this study, we demonstrate a morphable 3D structure inspired by origami/kirigami to produce stretchable displays that can preserve the original image quality by maintaining the display pixel density under stretching. The morphable 3D display consists of a 7 × 7 micro-light-emitting diode (LED) pixel array integrated on a transparent epoxy structural frame, which can be stretched up to 100% without affecting the performance of the display. The functional 3D system can create important unconventional opportunities for optoelectronic devices.

Molecular Design of Stretchable Polymer Semiconductors: Current Progress and Future Directions

Article

Mar 2022

Advancements in Electronic Materials and Devices for Stretchable Displays

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