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3D Printing of Interdigitated Dielectric Elastomer Actuators
Alex Chortos, Ehsan Hajiesmaili, Javier Morales, David R. Clarke,* and Jennifer A. Lewis*
Dielectric elastomer actuators (DEAs) are soft electromechanical devices
that exhibit large energy densities and fast actuation rates. They are typically
produced by planar methods and, thus, expand in-plane when actuated.
Here, reported is a method for fabricating 3D interdigitated DEAs that
exhibit in-plane contractile actuation modes. First, a conductive elastomer
ink is created with the desired rheology needed for printing high-fidelity,
interdigitated electrodes. Upon curing, the electrodes are then encapsulated
in a self-healing dielectric matrix composed of a plasticized, chemically
crosslinked polyurethane acrylate. 3D DEA devices are fabricated with
tunable mechanical properties that exhibit breakdown fields of 25 V µm−1
and actuation strains of up to 9%. As exemplars, printed are prestrain-
free rotational actuators and multi-voxel DEAs with orthogonal actuation
directions in large-area, out-of-plane motifs.
DOI: 10.1002/adfm.201907375
Dr. A. Chortos, E. Hajiesmaili, J. Morales, Prof. D. R. Clarke,
Prof. J. A. Lewis
John A. Paulson School of Engineering and Applied Sciences
Harvard University
29 Oxford Street, Cambridge, MA 02138, USA
E-mail: clarke@seas.harvard.edu; jalewis@seas.harvard.edu
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201907375.
cycling and breakdown behavior[33–35]
and the presence of a rigid frame limits
the geometries that can be achieved.[24,36]
Recent attention has been directed toward
developing approaches that enable contrac-
tile displacements in prestrain-free DEAs,
including manual and automated stacking
of individual planar layers[37] or sequential
deposition of active materials via inkjet
printing[38] and spray coating.[39] The fabri-
cation of contractile actuators with vertically
oriented electrodes offers a more prom-
ising approach (Figure 1b). While arrays of
vertical electrodes can be patterned litho-
graphically, new masks must be generated
for each device design.[40–42] By contrast,
3D printing enables the rapid design and
fabrication of soft materials in nearly arbi-
trary geometries.[43–47] For example, direct ink writing (DIW), an
extrusion-based 3D printing method, has been used to pattern
soft functional materials, including sensors,[48] stretchable elec-
tronics,[49] liquid crystalline elastomers,[50] and soft robots.[51,52]
While this method has recently been used to print DEAs, they do
not exhibit an in-plane contractile response.[52–54]
Here, we create 3D DEAs composed of interdigitated vertical
electrodes that are printed, cured, and encapsulated in an
insulating dielectric matrix (Figure 1c). These prestrain-free
contractile DEAs can be produced in nearly arbitrary geom-
etries. During their actuation, the stress generated is given
by
σ
=
ε
0
ε
r(E)2, where
ε
0 is the vacuum permittivity,
ε
r is the
dielectric constant, and E is the electric field. For small strains,
the actuation strain (sz) is sz =
σ
/EY =
ε
0
ε
r(E)2/EY
, where EY is
the Young’s modulus. Their actuation performance is therefore
maximized by increasing the breakdown field and dielectric
constant, while simultaneously reducing the elastic modulus
of the matrix. Since variations in the dielectric thickness can
cause localization of the electric field that results in premature
breakdown,[39] we optimized the DEA device performance by
developing a conductive electrode ink with tailored rheological
and printing behavior and a plasticized dielectric matrix that
exhibits electrical self-healing.
2. Results and Discussion
2.1. Electrode Ink Design
We synthesized a versatile elastomer for use as the continuous
phase in our conductive electrode ink via a step growth polym-
erization of ethylene glycol-based di-ene and dithiol small
molecules (Figure 2a). Using this highly selective thiol-ene
chemistry,[55] we prepared poly(ethylene glycol ethylene sulfide)
1. Introduction
Soft actuators exhibit actuation modes that mimic the capabili-
ties and efficiencies of biological systems.[1] These active devices
rely on phase change materials,[2,3] fluidic actuation,[4] or
electrostatic attraction to achieve the desired motion of interest.
Dielectric elastomer actuators (DEAs) utilize electrostatic forces
that are generated by applying a voltage across an insulating
elastomer sandwiched between two electrodes to drive their
actuation. The force induced by attraction of opposite charges
reduces the elastomer thickness in the direction of the electric
field and leads to a concomitant expansion in orthogonal direc-
tions.[5,6] Since this external field can be applied and removed
quickly, DEAs exhibit fast actuation rates and high efficiency[3,7]
making them attractive for use in soft robotics,[8–13] tunable
optical lenses,[14] and haptic interfaces.[15–17]
Most DEAs are fabricated by planar methods, such as spin
coating[8,15,18] and sequential mechanical assembly,[19–22] and
therefore expand in-plane when actuated (Figure 1a). With further
processing, these planar structures can be transformed into
bending actuators,[9] rolled actuators,[15] or prestrained systems
using rigid frames.[10,11,23–25] For many applications of interest,
contractile actuation is advantageous.[26–30] Yet prestrained DEAs
provide contractile strains in only a small proportion of the total
device area.[23,31,32] Moreover, these devices often exhibit impaired
Adv. Funct. Mater. 2020, 30, 1907375
