Material, design, and fabrication of custom prosthetic liners for lower-extremity amputees: A review

Abstract

As a physical interface, a prosthetic liner is commonly used as a transition material between the residual limb and the stiff socket. Typically made from a compliant material such as silicone, the main function of a prosthetic liner is to protect the residual limb from injuries induced by load-bearing normal and shear stresses. Compared to conventional liners, custom prosthetic liners have been shown to better relieve stress concentrations in painful and sensitive regions of the residual limb. Although custom liners have been shown to offer clinical benefits, no review article on their design and efficacy has yet been written. To address this shortcoming in the literature, this paper provides a comprehensive survey of custom liner materials, design, and fabrication methods. First, custom liner materials and components are summarized, including a description of commercial liners and their efficacy. Subsequently, digital methods used to design and fabricate custom liners are addressed, including residual limb biomechanical modeling, finite element-based design methods, and 3-D printing techniques. We hope that this review article will inspire further research and development into the design and manufacture of custom liners.

Full text

Material, design, and fabrication of custom prosthetic liners for
lower-extremity amputees: A review
Xingbang Yang
a
,
d
,
*
, Ruoqi Zhao
a
, Dana Solav
b
, Xuan Yang
a
, Duncan R.C. Lee
d
,
Bjorn Sparrman
c
, Yubo Fan
a
, Hugh Herr
d
,
**
a
School of Biological Science and Medical Engineering, Key Laboratory of Biomechanics and Mechanobiology (Beihang University), Ministry of Education, Beijing
Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing, 100191, China
b
Faculty of Mechanical Engineering, Technion, Haifa, 3200003, Israel
c
Rapid Liquid Print, Boston, MA, 02129, USA
d
Massachusetts Institute of Technology, K. Lisa Yang Center for Bionics, Cambridge, MA, 02139, USA
ARTICLE INFO
Keywords:
Review
Custom prosthetic liner
Material
Design and fabrication method
Digital modeling
FEA-informed design
Soft/flexible material 3D printing
ABSTRACT
As a physical interface, a prosthetic liner is commonly used as a transition material between the residual limb and
the stiff socket. Typically made from a compliant material such as silicone, the main function of a prosthetic liner
is to protect the residual limb from injuries induced by load-bearing normal and shear stresses. Compared to
conventional liners, custom prosthetic lower-extremity (LE) liners have been shown to better relieve stress con-
centrations in painful and sensitive regions of the residual limb. Although custom LE liners have been shown to
offer clinical benefits, no review article on their design and efficacy has yet been written. To address this
shortcoming in the literature, this paper provides a comprehensive survey of custom LE liner materials, design,
and fabrication methods. First, custom LE liner materials and components are summarized, including a
description of commercial liners and their efficacy. Subsequently, digital methods used to design and fabricate
custom LE liners are addressed, including residual limb biomechanical modeling, finite element-based design
methods, and 3-D printing techniques. Finally, current evaluation methods of custom/commercial LE liners are
presented and discussed. We hope that this review article will inspire further research and development into the
design and manufacture of custom LE liners.
1. Introduction
The fit between the residual limb and the prosthetic socket is one of
the most critical factors that contribute to the comfort and rehabilitation
of persons with limb amputation, especially for persons with lower-
extremity (LE) amputation [1,2]. The concentrated normal stresses
induced by poor socket fit can cause dermatological problems [3–5] and
deep tissue injuries [6,7]. Induced shear stresses, detrimental to skin
tissues and thought to be a leading cause of mechanically-induced skin
breakdown [4,8–10], can lead to cell separation within the skin
epidermal layer [11], skin over-tension when combined with normal
stresses [12,13], and an exacerbation of wound healing after an adverse
event [14,15]. In addition to optimized socket geometry and appropriate
alignment, a soft and flexible prosthetic liner worn between the LE and
external socket is commonly used to reduce these interface problems
[16–19].
A prosthetic liner is often referred to as a second skin, or an “artificial
skin”, interfaced, or adhered to, the biological skin. The goals of the liner
for LE amputees are to propagate and distribute shear stresses between
the soft residual limb and the stiff socket [20], protect sensitive regions
such as bony prominences by distributing compressive stresses [21],
suspend the prosthesis on the limb [22], limit pistoning [23], facilitate
limb heat transmission [19], and accommodate the stiffness, shape and
volume change of the soft tissue [24–27]. Among all the functions that a
liner may provide for LE amputees, perhaps the most critical is its ca-
pacity to improve the normal and shear stress distribution applied to the
* Corresponding author. School of Biological Science and Medical Engineering, Key Laboratory of Biomechanics and Mechanobiology (Beihang University), Ministry
of Education, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing, 100191, China.
** Corresponding author.
E-mail addresses: yangxingbang@buaa.edu.cn (X. Yang), hherr@media.mit.edu (H. Herr).
Contents lists available at ScienceDirect
Medicine in Novel Technology and Devices
journal homepage: www.journals.elsevier.com/medicine-in-novel-technology-and-devices/
https://doi.org/10.1016/j.medntd.2022.100197
Received 19 September 2022; Received in revised form 30 November 2022; Accepted 2 December 2022
2590-0935/©2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
Medicine in Novel Technology and Devices 17 (2023) 100197

amputated residuum [28]. It is not surprising therefore that liners have
been increasingly popular and are now used in approximately 85% of
clinical prostheses [29].
To date, there are more than 70 commercial liner products for
transtibial amputation on the market, demonstrating different materials,
design methods, and fabrication techniques [28]. Many studies have
been conducted to understand existing commercial LE liners [22],
covering topics such as material properties under cyclic compression,
shear abrasion, loading rate differences, and frictional loading [20,
30–32]. In addition, heat and moisture transfer between the lower re-
sidual limb and the liner-socket system has been studied [33,34], as well
as liner performance with human subjects [35,36] and efficacy research
related to prosthetic liners [37–40]. The majority of commercial LE liners
on the market are manufactured using batch production methods.
However, these liners are not patient-specific and are not optimized to
meet specific patient requirements. Custom patient-specific LE liners, on
the other hand, are designed to improve load distribution and relieve
concentrated pressures and shear stresses from painful neuromas and
other sensitive regions. In addition, custom LE liners may provide an
improved fit for patients with unusual, odd-shaped residual limbs, for
which off-the-shelf liners usually provide a poor fit. Therefore, more and
more individuals with LE amputation prefer to use custom prosthetic
liners over mass-produced, non-custom liners [41].
To the best of our knowledge, only a few review articles have been
published on prosthetic LE liners [22,40], but none specifically addressed
custom LE liners. To this end, this paper focuses on custom LE liners,
including liner materials, components, current commercial liner prod-
ucts, and liner product efficacies. We conclude the paper with a review of
digital methods employed in designing and manufacturing custom LE
liners, including residual limb model construction, digital liner design
methods, and fabrication methods. We hope that this review article will
attract more attention to the field, and promote further research and
development for the benefit of persons with major limb amputation.
2. Materials and components of commercial/custom LE liners
The prosthetic LE liner system consists of the liner itself, the sus-
pension system, and optional accessories. The liner provides a soft
interface for the user whereas the suspension system acts as a firm
attachment between the socket and the residual limb. Fig. 1 shows the
main types of liner suspension systems. Optional accessories can provide
increased comfort and convenience to users [42–44]. This section pro-
vides an overview of these components.
The materials of prosthetic LE liners can be categorized as stiff foams
and elastomeric materials [45]. Previously, medium-density poly-
ethylene foam called Pe-Lite has been the most popular foam due to its
durability and accessibility [46]. However, LE liners made by Pe-Lite are
stiff and cannot be rolled onto a residual limb (see Fig. 1(a)), and they are
mainly used with patellar-tendon bearing (PTB) sockets. In comparison,
LE liners made from elastomeric materials such as thermoplastic elasto-
mers (TPE) gels, silicones, and polyurethanes (PU) can be rolled onto the
lower residual limb, as shown in Fig. 1(b), and have been widely used
with total-surface bearing (TSB) sockets [47]. The elastomeric materials
are softer and more compliant than the polyethylene foam and can
accommodate different lower limb shapes. Furthermore, the elastomeric
materials can deform under pressure to effectively flatten the pressure
distribution applied to an amputated residuum [48].
Cagle et al. proposed standardized material testing protocols for
elastomeric LE liners [49], defining six properties to characterize liner
performance: compressive elasticity, shear elasticity, tensile elasticity,
coefficient of friction, volumetric elasticity, and thermal conductivity. In
2018, they conducted comprehensive experiments to compare 24
different liner materials [20], as summarized in Fig. 2. Their results
denote that silicone has the highest compressive, shear, and tensile
elasticities and PU has higher elasticities than TPE except for tensile
elasticity. While elastomeric LE liners provide more comfort and less
fabrication time, they are more expensive than foam liners with inferior
durability [50]. Recently, improvements in materials and evaluation
systems have been reported. Low-density ethylene-vinyl acetate (EVA)
shows great potential to further improve foam liners. Sasaki et al. pro-
posed an affordable low-density EVA roll-on liner, which can be rolled
onto a residual limb like an elastomeric liner but at a much lower
fabrication cost [50]. Lutfiet al. proposed a sandwich-like foam structure
by combining EVA and polyurethane, which demonstrates a higher ten-
sile elasticity compared to Pe-Lite [51]. Rezvanifar et al. proposed Phase
Change Materials (PCM) liners to remove the heat between the ampu-
tated residuum and socket [52]. Experimental results suggest that PCM
liners can prolong the duration of desirable residual limb skin tempera-
ture by nearly 60%. Lee et al. proposed a proof-of-concept design of an LE
liner that can dynamically adjust the fit between the residuum and the
socket using on-chip valves [53].
Suspension systems can be categorized as either having a direct me-
chanical connection or a negative pressure-based suspension. Stiff foam
LE liners are mainly used with direct mechanical connection suspension,
while elastomeric LE liners are used with both suspension types.
Numerous types of mechanical connections exist, including a distal pin
with a shuttle lock (Fig. 1(c)) [54], a belt [55], and a cuff [56]. Negative
pressure suspension can be achieved using both passive and active stra-
tegies using suspension sleeves that roll onto the residual limbs and seal
the prosthetic socket to the limb [57]. Negative pressure can be created
passively by implementing a valve built into the distal end of the socket
with an outer sleeve to seal the connection, such as the Alpha®Suction
Pro (WillowWood Global LLC, OH, USA), or by using a rubber ring called
a hypobaric sealing membrane to create a suction seal with the socket,
such as the Seal-in®(
€
Ossur, Iceland), as shown in Fig. 1(d). In both
methods, the air between the socket and the LE is pressed out after the
Fig. 1. Illustrations of some basic elements of different LE liners and suspension systems. (a) Stiff liners made from foam. (b) Elastomeric liners made from gel or
silicone. (c) Mechanical suspension system with pin and lock. (d) One type of negative pressure suspension system called Seal-in. (e) Sectional drawing of an elevated
vacuum system.
X. Yang et al. Medicine in Novel Technology and Devices 17 (2023) 100197
2

user dons the liner. Another method termed elevated vacuum (Alps South
LLC, FL, USA) employs an integrated pump to actively draw out the air
between the socket and liner, as shown in Fig. 1(e). Both subjective
questionnaire results and objective measurements of kinetics indicate
that negative pressure suspension provides more comfort than mechan-
ical connection [58,59]. Recent suspension system-related research
included the analysis of elevated vacuum liners and the proposal of novel
suspension technologies [60–63]. Youngblood et al. fabricated a domed,
carbon-fiber layup as an LE socket model, and placed different liners
underneath to simulate elevated vacuum suspension. This model could
help understand the physiological effects of elevated vacuum and guide
its implementation [60]. Xu et al. found that the vacuum level had a
significant effect on comfort level and concluded that low levels of vac-
uum should be avoided [61]. Gholizadeh et al. proposed a novel sus-
pension system for silicone LE liners by using hook and loop fabric as the
suspension system, called HOLO [62]. Clinical research conducted by
Osman et al. demonstrated the safety of HOLO and reduced traction at
the distal end of the residual limb [63].
Several optional accessories for liners exist. Among them, fabric
backing is commonly used for elastomeric LE liners. It has been shown
that attaching orthotropic fabric to the outside of the liner can make it
easier to roll the liner onto the lower residuum and slide the liner into the
socket [42]. Some liners are also equipped with softer materials at the
anterior-distal aspects of the residual limb to further reduce discomfort
[43]. Additionally, embedding sensors within the liner to measure
interface pressures may benefit the user by allowing the prosthetist to
have an indication of the interface pressure distribution [44].
The optimal LE liner depends on the user's specific requirements.
Compared to custom liners, standard commercial liners are not adapted
to the user's residual limb to accommodate user-specific surface geome-
tries, musculoskeletal profiles, tissue mechanical properties, and pain
regions. Due to the lack of personalization of mass-produced conven-
tional liners and their resulting performance imitations, more and more
custom liners are being sought after by prosthetic users.
3. Products and studies of LE custom liners
Today, several companies provide design and fabrication services for
custom prosthetic LE liners (e.g., ALPS, Custom Silicone Fabrications,
€
Ossur, Ottobock, and WillowWood) [64]. ALPS introduced a new method
for custom liner design based on a thermoformable gel, called Thermo-
liner. The gel enables the liner to be individually shaped and molded
within the prosthetic facility. The customization entails simply applying
the Thermoliner over a positive mold, and then increasing the temper-
ature to 90 C for 30 min. According to their description, this technique is
especially useful for creating customized LE liners for patients having
difficult residual limb shapes and sizes, such as Symes or knee disartic-
ulation cases [65]. Another company, Custom Silicone Fabrications of
San Francisco, offers a custom LE liner service, which uses injection
molding to design and fabricate customized liners with patient-specific
thicknesses and durometers around specific anatomical regions of the
residuum. Currently, they only fabricate liners based on a cast of a patient
residuum and not from computer-aided design (CAD) files or digital
scanning files [66,67].
€
Ossur provides custom LE liners in their Evolution
Truly Custom Liners series. These liners are designed based on the
anatomical geometry of a positive or negative mold, or a digital model, as
well as the limb length and a series of measured limb circumferences at
specific heights [68,69]. Ottobock provides two custom LE liner series,
made of polyurethane (Skeo Unique) and silicone (Uneo Unique). The
polyurethane liners are designed with varying wall thicknesses, complex
features for improved knee flexion, a bulging, eccentric, or concave
residual-limb end, and large volumn for an extra-large residuum (length
>50 cm, circumference >50.5 cm). The silicone liners are hand-crafted
for lower limbs with unique shapes and are offered in multiple durom-
eters, undercuts, lengths, thicknesses, and colors. Both liner series can be
produced based on either a plaster cast made from the collected
anatomical data of the residuum (Fig. 3(a) and Fig. 3(b)), a digital file, or
a test socket. For each of these options, the measurement of lower limb
length and a series of measured limb circumferences at specific heights
are also required [70–73]. WillowWood offers a custom-designed, gel--
based LE liner with regions around the proximal edge and behind the
knee that are as thin as 3 mm, while regions around the bony promi-
nences and sensitive areas can be as thick as 25 mm. The liners are
designed by hand from a positive mold of the residual limb or using the
OMEGA®Software based on a digital scan (Fig. 3(c)) [74,75].
Additional research concerning novel design and fabrication methods
for custom LE liners has been carried out in academic institutions. Re-
searchers from the University of Bath proposed a new method based on
digital scanning, modification in CAD software, and manufacturing using
cryogenic CNC machining. Neoprene foam was chosen because it is
suitable for contact with human skin and its glass transition temperature
enables successful CNC machining. This is a fluff-on LE liner that cannot
be rolled on the limb. Therefore, an external suspension sleeve is required
[76]. Researchers at the University of Pittsburgh designed and fabricated
a perforated, double-layer, silicone custom LE liner using a 3D-printing
assisted casting method. The liner was designed based on a uniform
plaster cone shape and with evenly distributed and patterned holes that
facilitate the removal of sweat. The fabrication employs a novel 3D
printed mold with conical protrusions to generate holes in the liner [77,
78].
Due to improved load distribution and pain relief [79], custom liner
products are becoming more and more popular among persons with
amputation [41,79]. Some newly proposed methods, such as anatomical
geometry acquisition, digital design, and novel fabrication techniques,
are being developed to enhance customization and comfort, as discussed
in the next section.
Fig. 2. Several properties of different materials used in prosthetic LE liners: (a) Polyurethane. (b) Silicone. (c) TPE. The original data was taken from Table 1 in Cagle
et al. [20] and represents 4 Polyurethane liners, 10 Silicone liners, and 11 TPE liners. The value of each property is normalized by dividing by the maximum value of
the corresponding property (The maximum values of each property are: compress elasticity - 458 kPa; shear elasticity - 104.6 kPa; tensile elasticity - 3450 kPa;
volumetric elasticity - 208,000 kPa; Poisson ratio - 0.4998; coefficient of friction - 3; thermal conductivity - 0.183 W/mk).
X. Yang et al. Medicine in Novel Technology and Devices 17 (2023) 100197
3

4. Residual limb models for custom LE liner design
All current methods for custom LE liner design, which are based on
patient-specific anatomical geometry, use either a positive or negative
mold from a wrapping cast of the residuum (Fig. 4(a)) [65,67,69,70,75,
77], or a digital model reconstructed from laser-scanned data (Fig. 4(b))
[69,70,75]. Although these methods may result in acceptable liners, their
production is time-consuming [66], they exhibit low accuracy due to
experience-based wrap casting [80], affected by motion artifacts [69,70,
75], or demand more manual labor from the prosthetist and more
involvement from the patient compared to those of data-driven digital
design method [81]. To make a custom roll-on liner that can generate
appropriate pre-load donning-induced pressure to keep sufficient
coupling between the lower residuum and the liner, the original geo-
metric model may need to be modified such that the liner is smaller than
the limb [82]. However, currently, the modification method used is
based on the prosthetist's experience and is improved through multiple
iterative trials, without employing quantitative data or evidence-based
optimization. Moreover, even when a digital model is used, it contains
only the skin surface geometry [83], excluding the muscles, bones, ten-
dons, and other internal tissues. The mechanical interaction between
these tissues and the liner plays an important role in the liner's
post-donning shape and donning-induced pressure, further influencing
the fit and comfort of the liner and socket [84]. Therefore, there is a need
for a high-resolution residuum model reconstruction method that in-
cludes information on the internal tissue structures. To this end, medical
image acquisition and 3-D reconstruction can be employed.
Medical imaging methods such as ultrasound [85], CT [86], and MRI
[87] are commonly used to provide information on internal organs and
tissues [88] and have been used in the past to reconstruct residuum
models with the aforementioned details (Fig. 4(c)), as described in the
review by Sanders et al. [25]. Many of these imaging procedures require
contact between the skin and the measurement device or the bed, which
implies that the reconstructed geometry of the soft tissues is loaded, and
does not represent the natural, unloaded shape of the residuum. In
addition, imaging procedures that require the patient to remain still do
not allow the measurement of dynamic skin surface deformations. To
address these shortcomings, researchers from the Biomechatronics group
at MIT developed a multi-camera imaging system for residual lower
limbs with an open-source three-dimensional digital image correlation
(3D-DIC) toolbox for model reconstruction, enabling the measurement of
full-field deformation and strain maps [89,90]. Since 3D-DIC alone can
only capture the skin surface geometry without the internal tissues and
structures, new experimental setups for model registration have been
developed to obtain the geometry of bones and soft tissues and the spatial
relationship between these internal structures and the external skin
surface [91]. The proposed methods describe the combination of MRI,
CT, or ultrasound with 3D-DIC data using registration markers and a
dedicated registration algorithm. This procedure results in a digital
model with accurate skin surface geometry that includes, in addition to
the internal structures, the full-field deformations and strains [90]. This
framework has been utilized for designing and fabricating a
variable-thickness custom liner, in which the liner thickness is inversely
proportional to the skin strain measured during skin flexion [92]. Ther-
mal camera measurements indicated that the variable thickness liner
helped reduce skin irritation and thermal output in regions of high skin
strains around the knee, compared with a uniform thickness liner [92].
5. Design methods of custom LE liners
Two methods are employed in the design of all commercially-
available custom LE liners: handmade design [65,67,69,70,75] and
CAD software-based design (or digital design) [69,70,75,77](Fig. 5(a)).
In the handmade design approach, the manufacturer performs many
labor-intensive tasks such as acquiring the liner anatomical geometry by
casting [65,67,69,70,75], changing the thickness around specific regions
by adding/reducing mold material from a positive or negative mold, or
changing the liner stiffness locally by cutting the liner material around
target regions from a ready-made custom liner and replacing it with
materials with a different stiffness [65,67,70]. These processes are
typically artisanal and slow. In CAD software-based design, similar steps
are achieved by manipulating the corresponding model virtually, which
requires less manual labor [69,70,75,77]. Both of these design methods
can provide custom LE liners with patient-specific anatomical geometry,
multiple durometers, and spatially varying thickness. Yet, both processes
are mainly experience-based and are insufficiently data-driven. To ach-
ieve a good fit between the liner and the residuum, patients need to try
many iterations of the fabricated liner to provide feedback. Furthermore,
Fig. 3. Lower limb geometry acquisition methods. (a) Ottobock custom LE liner order form. Amputees need to provide their limb's geometry information, i.e., cir-
cumferences of transverse sections perpendicular to the longitudinal axis of the leg, 5 cm per interval from the bottom to the top end [72]. Copyright 2021 by
Ottobock. (b) Ottobock method of casting for the custom LE liner. First, pull a thin casting sock over the limb and mark the mid-patella tendon and top end positions on
the sock. Second, cast the limb with a plaster bandage, apply casting socks and a casting bag, and extend the casting bag up to the thigh. Finally, flex the limb as
required and turn on the casting pump until the cast has set, if needed, lightly support the droop distal soft tissue. Copyright 2021 by Ottobock [72]. (c) Modeling
method of the lower limb using OMEGA Scanner 3D by WillowWood. Use Scanner 3D to scan the residual limb and reconstruct a digital model. Copyright 2022 by
WillowWood [75].
X. Yang et al. Medicine in Novel Technology and Devices 17 (2023) 100197
4

the liner's post-donning pressure and shape are not considered quanti-
tatively. The post-donning pressure is a critical attribute that keeps the
liner attached to the residuum before the socket is donned and needs to
be limited in order to reduce the discomfort that may be caused if the
liner is too tight. The liner post-donning shape is also an important factor
that may influence the goodness of fit with the custom LE socket. These
two variables contribute to the comfort of the final liner-socket system.
Therefore, a key goal is to develop a design framework that iteratively
designs a custom liner according to patient-specific requirements (ge-
ometry, durometer, thickness) and subsequently predicts and optimizes
the parameters that contribute to patient comfort. To this end, finite
element analysis (FEA) has been widely used to design, evaluate, and
optimize prosthetic sockets [91–100]. FEA provides a constructive
evaluation of the design and helps reduce unnecessary iterations in
manufacturing, testing, and modification.
In [95,101], for example, an FEA-based iterative design method for
transtibial prosthetic liners and sockets was presented, as shown in
Fig. 5(b). This method was used to design and fabricate a custom LE liner
and its corresponding socket. The mechanical response of the limb, liner,
and socket in FEA simulations depend on the constitutive parameters
assigned to all the materials in the model, including the soft tissues and
the liner. In the past, most studies used linear elastic models to describe
soft tissues [94]. To increase the prediction accuracy, nonlinear (typically
hyperelastic) material models are now more commonly used, which
more accurately account for the large deformation experienced by the
tissues [94,100,102]. The parameters of the nonlinear model were pre-
viously obtained from cadaveric or animal data, which are not
patient-specific and do not represent the in-vivo mechanical behavior of
the target regions of the limb. To address this problem, several studies
employed in-vivo indentation. Notable early efforts have been reported
by Refs. [103,104], and [105]. Sengeh et al. developed a non-invasive
indentation method with multiple indenters to characterize spatially
Fig. 4. Three different methods to acquire the LE residuum models. (a) Wrapping cast on the limb to construct a handmade positive mold [66]. Copyright 2015 by
Smooth-On Inc. (b) Optical scanning to construct a digital surface geometry of the residuum. The positive mold for anatomical shape acquisition is a copy of the
residuum made by using wrapping cast method [83]. Copyright 2017 by PLoS One. (c) Reconstructing the lower limb model with internal tissues using MRI images
[101]. Copyright 2016 by Engrxiv.
Fig. 5. Different ways of personalized LE liner design. (a) Design the liner by hand or CAD. Change the shape of handmade positive mold by hand (add or cut mold
material) or the shape of the liner in CAD software (expand or shrink the 3D model) where the amputee feels overtight, overloose, or the region is sensitive. (b) Design
the liner based on FEA optimization. Change the shape of liner where the normal/shear stress is excessively high or the region is sensitive. Adapted with permission
from Moerman et al. [101]. Copyright 2016 by Engrxiv.
X. Yang et al. Medicine in Novel Technology and Devices 17 (2023) 100197
5

varying mechanical properties of the residuum tissues, as shown in
Fig. 6(a) [87]. Iivarine et al. proposed a manual indentation device to
measure the stiffness of soft tissue by measuring the thickness of tissue
layers using a B-mode ultrasound image and created a hyper-elastic finite
element model based on the result, as shown in Fig. 6(b) [106]. Huang
et al. developed a non-contact indentation system with an air jet as an
indenter and optical coherence tomography (OTC) to measure the
deformation of tissue, as shown in Fig. 6(c) [107], which is promising for
characterizing the mechanical properties of the lower limb. Fougeron
et al. combined a freehand ultrasound probe and force sensor to assess
the material properties of soft tissues [108]. They also measured the
hyperelastic parameters of the thigh in relaxed and contracted muscle
configurations, finding that the configurations of muscles significantly
influence the shear modulus.
For the LE liner, most previous studies have assumed linear elasticity
for characterizing liner materials [94], mainly hyperelastic materials
with nonlinear properties [109], such as thermoplastic elastomer (TPE),
silicone, and urethane [110,111]. More recently, hyperelastic nonlinear
models have been employed to represent the mechanical behavior of the
liner in the socket-liner-residuum system [48,100,102,112]. However,
they use parameters from either equivalent estimation based on linear
models [102] or from the literature [100]. Measurements of displace-
ment typically use long-travel extensometers, and sample dimensions are
measured using calipers. Even if the parameters were computed based on
specific measurement data [48,112], they may not be accurate enough
due to systematic or intrinsic properties such as deformation measure-
ment error induced by eliminating the measurement of extraneous
compliance in the loading system, gauge length measurement error
caused by crosshead extension measure, contact point slippage as
knife-edges become dull over time, and so on [113]. Recently, DIC-based
full-field displacement measurements are employed, to improve the ac-
curacy of the strain data and to evaluate heterogeneities and anisotropic
behavior. Together with force sensors, DIC may provide richer data to
more accurately characterize the mechanical properties of liner materials
[114–117]. One of the effective methods by which parameters for
hyperelastic non-linear models are identified is based on an inverse FEA
approach. Most current hyperelastic non-linear model-based methods
assume incompressibility, but the volumetric changes increase with
increasing stretch ratios, leading to variation in mechanical behavior
compared to that simulated with incompressible models under large
stretch [118]. Therefore, to better describe the response of liner materials
and characterize material parameters, appropriate assumptions on
incompressibility or compressibility should be made according to actual
application conditions and bench test results, especially when large de-
formations are expected [118].
6. Fabrication methods of custom LE liners
Three main techniques are utilized to fabricate custom LE liners
(Fig. 7(a)-(c)): injection molding [67,69,70,75,77], thermoforming [65],
and cryogenic CNC machining [76]. Thermoforming and cryogenic CNC
machining can only use materials with special physical characteristics
such as thermoformability and appropriate glass transition temperature,
which causes limitations in providing a fully customized solution [65,
76]. Injection molding is the most commonly used method for LE liner
fabrication and provides a wide range of liners using molds obtained by
wrap casting [66], CNC machining [65,67,69,70,75], or 3-D printing
[77]. Injection molding is time-consuming, labor-intensive, demands
elaborate design for demolding for complex geometries, and requires
repeated remolding work when multiple durometer materials are needed
[66,119,120]. Using all three methods, the original size and shape of the
residuum model (cast or digital) cannot be used to fabricate the liner,
because it would result in insufficient post-donning pressure to keep the
liner reliably donned on the residuum, as mentioned in Section 4.
Therefore, the manufacturers need to iteratively modify the geometry
during fabrication and fitting process to allow sufficient but not excessive
donning pressure.
Recently, 3D printing technology has seen dramatic development due
to its inherent advantages such as freedom of design, mass customization,
waste minimization, the ability to manufacture complex structures, and
fast prototyping [121–123]. This technique has been widely used in the
biomedical field [124–128], and specifically in prosthetics [129–131].
With the rapid development of soft robotics-related technologies in
recent years [132,133], soft material 3D-printing techniques have been
developed [134–137], with promising applications in the biomedical and
rehabilitation fields, such as flexible and wearable electronics [138,139]
and prosthetics [140]. Due to good biocompatibility and thermal sta-
bility, as well as close adhesion to the residuum for better liner fitting,
soft silicone gel is one of the most widely-used materials for prosthetic
liners [38]. Although silicone 3D printing technologies have been
explored and developed in many applications [141–144], it has not been
exploited for fabricating prosthetic liners until now. One main reason
may be that these approaches have been limited to small-scale objects
with a focus on biomedical applications, which are not applicable to
large-scale prosthetic liner design. A large-scale rapid liquid printing
(RLP) technique has been developed [145], which enables the large-scale
Fig. 6. Different ways of measuring the mechanical properties of soft tissues. (a) A non-invasive indentation method with multiple indenters [87]. Copyright 2016 by
ELSEVIER. (b) Measuring the thickness of tissue layers using B-mode ultrasound image and creating a hyper-elastic finite element model [106]. Copyright 2011 by
ELSEVIER. (c) A non-contact indentation system with an air jet as indenter and optical coherence tomography (OTC) to measure the deformation of tissue[107].
Copyright 2009 by IOP.
X. Yang et al. Medicine in Novel Technology and Devices 17 (2023) 100197
6

fabrication of high-quality flexible silicones in granular gel support baths,
as shown in Fig. 7(d). RLP allows for faster print speeds, free-form
structures, and stronger bonds by avoiding cold joints. Digital printing
technology such as RLP may prove beneficial to future custom liner
manufacturing.
7. Evaluation methods of custom/commercial LE liners
The principle of liner evaluation is to evaluate whether amputees
have a positive experience after donning the liner. The current evaluation
methods can be classified as subjective methods and objective methods,
which are summarized in Table 1.
The subjective evaluation is carried out using questionnaires to
evaluate amputees' satisfaction with their prostheses. Examples include
the Orthotics and Prosthetics User's Survey (OPUS) [146], the Trinity
Amputation and Prosthesis Experience Scales (TAPES) [147], and the
Prosthetic Evaluation Questionnaire (PEQ) [148]. As for LE liner evalu-
ation, the most popular method is a self-designed questionnaire based on
PEQ, which includes a prosthesis function scale, mobility scale, psycho-
social scale, and well-being scale [149]. These self-designed question-
naires require amputees to rate their satisfaction regarding aspects such
as fitting, donning, and walking.
Objective evaluation usually focuses on at least one property of the
LE-liner interface. Examples include: 1) Measurement of interface
pressure between the residual limb and the liner. Ali et al. used pressure
sensor arrays to measure the peak mean pressure to evaluate the per-
formance between two commercial liners [150]. Cagle et al. proposed an
FEA method to calculate the interface pressure with a 6 mm uniform LE
liner and the results agree with the literature [48]. 2) Measurement of the
pistoning, which refers to the vertical displacement of the liner-donned
lower limb within the socket [23]. Osman et al. utilized cameras to
measure the pistoning with different liners [63]. 3) Measurement of heat
dissipation. Williams et al. tested the heat dissipation of scaled-down
liners with thermistors and their results suggest that passive solutions
may not be sufficient [151]. 4) Measurement of the liner deformation.
Lenz et al. placed marks on limbs and measured the distance variations
between points using a digital caliper and a Vicon motion caption system
[152]. 5) Measurement of the radial displacement between the socket
wall and the liner. Henrikson et al. proposed an elastomeric liner with
embedded iron particles, which can provide quantitative indicators of the
relative motion between the limb and the socket [153].
Although there are many objective metrics to evaluate the liner, using
them to indicate the satisfaction of amputees is insufficient. Metrics that
work well for some subjects may fail to indicate comfort levels for others.
Further research may explore score-based evaluation metrics integrating
subjective satisfaction and objective evaluation data, using weight fac-
tors to indicate patient's personalized preference on different character-
istics of LE liners.
Fig. 7. Different fabrication methods for custom LE liners. (a) Thermoforming. The thermo-material is heated to a pliable temperature and then formed onto the limb
mold. (b) Injection molding. The liquid elastomeric material is injected into a lower limb mold and is then solidified. (c) Cryogenic CNC machining. The material is
frozen below its glass transition temperature and then machined using a CNC machine. (d) 3D-printing. A liquid material is deposited into the granular gel to form a 3D
structure and is then solidified.
Table 1
Subjective and objective methods for liner evaluation.
Author Aspect Method Subject #Participants Results
Ali [150] Interface Pressure Pressure sensor arrays Dermo liner
vs
Seal-in X5
9 Peak mean pressure with Seal-in X5 is higher
Comfort PEQ Comfort with Dermo is higher
Cagle [48] Interface Pressure FEA 6 mm uniform liner –Simulation results agree with literature's reports
Osman [63] Pistoning Digital photo Dermo liner
vs
Looped liner
10 Pistoning of dermo liner is lower than looped liner
Comfort PEQ Looped liner has less distal traction
Gholizadeh
[23]
Pistoning Digital photo Dermo liner
vs
Seal-in X5
6 Pistoning decreased by 71% when using Seal-in X5
Williams [151] Heat dissipation Thermistors Scaled down liner –Passive solutions are not enough, active methods
should be explored
Lenz [152] Deformation Motion capture system (Vicon) and
digital caliper
Marked gel liner with clear
socket
–Proposed method can capture the deformation of
gel liner
Henrikson
[153]
Radial
displacement
Inductive sensing system Ferrous elastomeric liner 4 Sensors provided insight into limb-socket positions
and displacements
Note: Table 1 presents brief information on some typical liner evaluation tests. Abbreviation: PEQ- Prosthetic Evaluation Questionnaire, FEA-Finite Element Analysis.
X. Yang et al. Medicine in Novel Technology and Devices 17 (2023) 100197
7

8. Conclusions
Prosthetic liners play an important role in the distribution of body
loads and suspension between the residual limb soft tissues and the stiff
prosthetic socket. Although conventional commercial liners improve
load distribution and reduce pain, custom liners exhibit greater potential
due to their patient-specific characteristics. Potential advantages of
custom liners include personalized anatomical shape, spatially varying
durometer, spatially varying thickness, and more. Several commercial
companies have developed custom liner design processes and offer
custom liner fabrication services, and notable research and development
are carried out in academia and industry.
For patient-specific residual limb model reconstruction, all current
commercial custom liner design methods use either a wrapped cast
positive/negative mold or a scanned digital model of the skin surface.
These approaches provide relatively accurate surface geometry of the
residuum but exclude internal structures such as bones, muscles, and
tendons. Digital reconstruction approaches using ultrasound, MRI, or CT
imaging, in combination with optical scans or 3D-DIC-based skin data
comprise promising future directions for accurate residuum modeling.
Current commercial liner design methods are neither automated nor
simulation-driven, and heavily depend on the prosthetist's experience.
FEA-informed design, combined with realistic material constitutive
models and accurate mechanical property characterization protocols
could make the liner design process more scientific and automated.
For fabrication, injection molding, thermoforming, and cryogenic
CNC machining are the three most common fabrication techniques.
However, they are relatively time-consuming, labor-intensive, and de-
mand multiple working procedures. An additive manufacturing process
that can fabricate large-scale flexible materials is desired for the accurate
and efficient manufacturing of custom liners. For evaluation, subjective
methods based on the patient's satisfaction rated by self-designed ques-
tionnaires and objective methods based on quantitative data such as
interface pressure, pistoning, heat dissipation, liner deformation, radial
displacement, etc., are proposed to evaluate the performance of the LE
liners. As patients' preferences on aspects of the liner performance are
diverse, score-based evaluation metrics using weight factors to indicate
patient's personalized predilection on different characteristics of LE liners
need to be explored.
Patient consent
The authors declare that there are no patient consent is needed as no
human subject tests are conducted.
Ethical approval and informed consent
The authors declare that there are no human subjects or animal ex-
periments involved in this manuscript.
Funding sources
This work was supported by the Fundamental Research Funds for the
Central Universities (Grant number JKF-YG-22-B010). Additional sup-
port comes from the National Institutes of Health (Grant number
5R01EB024531-03).
Author contributions
Xingbang Yang: Conceptualization, Formal analysis, Funding acqui-
sition, Investigation, Project administration, Supervision, Visualization,
Writing –original draft, Writing –review &editing. Ruoqi Zhao: Formal
analysis, Investigation, Visualization, Writing –original draft, Writing –
review &editing. Dana Solav: Formal analysis, Visualization, Writing –
review &editing. Xuan Yang: Investigation, Writing –original draft.
Duncan R.C. Lee: Writing –review &editing. Bjorn Sparrman: Writing –
review &editing. Yubo Fan: Resources, Supervision, Writing –review &
editing. Hugh Herr: Conceptualization, Funding acquisition, Project
administration, Resources, Supervision, Writing –review &editing.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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