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VOLUME 8, ISSUE 2 

 2025 
 

RESEARCH ARTICLE 

 

Phillips C, Nagpal A, Azhari F. A biofidelic mock residual limb for prosthetic socket testing. Canadian Prosthetics & Orthotics Journal. 2025; Volume 8, Issue 

2, No. 1. Https://doi.org/10.33137/cpoj.v8i2.45759 

 

  

 

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https://doi.org/10.33137/cpoj.v8i2.45759
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1 

Phillips C, Nagpal A, Azhari F. A biofidelic mock residual limb for prosthetic socket testing. Canadian Prosthetics & Orthotics Journal. 2025; Volume 8, Issue 2, No. 
1. Https://doi.org/10.33137/cpoj.v8i2.45759 

 

 

RESEARCH ARTICLE 

 

A BIOFIDELIC MOCK RESIDUAL LIMB FOR PROSTHETIC SOCKET TESTING 

Phillips C1, Nagpal A2, Azhari F1*  

1. Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Canada. 
2. Division of Engineering Science, University of Toronto, Toronto, Canada. 
  
 

 

 

 

  

 

 

 

 

 

 

 

 

INTRODUCTION   

A prosthesis user's residual limb can fluctuate significantly 

in size due to factors like activity and diet.1,2 As a result, 

traditional rigid prosthetic sockets fail to maintain a proper 

coupling at the residuum-socket interface, which is essential 

for comfort, function, and the prevention of skin and soft 

tissue problems.3,4 

Adjustable sockets are designed to accommodate an 

expanding or contracting limb via straps, movable panels, 

or other dynamic mechanisms.5-7 To evaluate their 

performance, testing protocols must involve a volume-

fluctuating residual limb, which can either be a study 

participant’s limb or a lab-fabricated mock limb. Clinical 

testing with study participants is time- and resource-

intensive, requiring meaningful limb volume fluctuation and 

accurate measurement of that volume change within the 

socket. Thus, apart from when subjective feedback (e.g., 

comfort scores) is needed, using a biofidelic mock limb—

engineered to replicate both the material composition and 

volume fluctuations of a physiological limb—offers a more 

practical, cost-effective alternative. 

Various mock limbs are used today, each designed to meet 

specific testing goals. Rigid limbs may suit structural testing 

of sockets to failure, as described in recent reviews8-10 and 

recommended by ISO 10328,11 whereas soft, compliant 

limbs may better replicate biomechanics at the residuum-

socket interface, allowing for more precise evaluation of 

tissue strains12,13 and suspension effectiveness.14 

Most prior mock limbs for testing adjustable sockets have 

been static, necessitating different mock limb sizes to 

simulate varying volumes.6 Seo et al.2 used five mock limbs 

of different volumes (neutral, ±3%, ±7%) to test their 

adjustable socket, and Murdoch15 used three mock limbs to 

investigate socket fit across three limb volumes (neutral and 

±10%). This approach limits the ability to study how the 

socket responds to a specific rate of limb volume change in 

* CORRESPONDING AUTHOR: 
Fae Azhari, PhD 

Affiliation: Department of Mechanical and Industrial Engineering, 
University of Toronto, Toronto, Canada. 

E-Mail: fae.azhari@utoronto.ca 

ORCID ID: https://orcid.org/0000-0003-2559-8566 

 

 
OPEN  ACCESS 

ABSTRACT 

BACKGROUND: Evaluating prosthetic socket fit and function relies on accurately simulating load transfer 

between the residual limb and the socket. This limb can be either real (of a study participant) or a mock residual 

limb that simulates in vivo properties. Mock limbs minimize reliance on resource-intensive clinical trials; 

however, most are static in size, limiting their use in testing clinical outcomes like socket adjustability. 

OBJECTIVE: To design and validate a biofidelic mock limb, capable of real-time, controllable volume 

adjustments of up to ±5% limb volume. 

METHODOLOGY: Water-filled bladders were embedded within a transtibial residual limb model made of a 

dual-durometer urethane composition, mimicking deep and soft tissue. An Arduino-controlled syringe system 

was used to actuate volume adjustments. The method was validated through repeatability tests at different 

rates of volume change, cycling through expansion, holding at maximum volume, and contraction. Volume 

change was quantified by measuring interfacial pressures between the limb and a static socket.  

FINDINGS: The limb was fabricated with readily available materials for less than CAD 400. Volume change 

rate had minimal effect on interfacial pressure throughout the testing cycle, and minimal hysteresis was found 

between expansion and contraction periods. Repeatability was high, with a coefficient of variation of normalized 

pressure remaining below 10.4% over three repeated tests. 

CONCLUSION: The proposed biofidelic limb was validated for its ability to mimic volume change in a transtibial 

residual limb. The design enables easy replication or customization to simulate different limb physiologies and 

anatomies. The limb allows for controllable bench-top testing during prototyping of adjustable sockets or other 

devices, thus bringing devices to clinical use sooner. 

 

 

ARTICLE INFO 

Received: July 18, 2025 

Accepted: August 21, 2025 

Published: September 10, 2025 
 

CITATION 

Phillips C, Nagpal A, Azhari F. A 

biofidelic mock residual limb for 

prosthetic socket testing. 

Canadian Prosthetics & 

Orthotics Journal. 2025; 

Volume 8, Issue 2, No. 1. 

Https://doi.org/10.33137/cpoj.v

8i2.45759 

KEYWORDS 

Prosthetic Sockets; Mock 

Residual Limb; Adjustable 

Sockets; Biofidelic; Bench-Top 

Evaluation; Limb Loss; Artificial 

Limbs; Volume Change; 

Transtibial; Rehabilitation; 

Amputation; Prosthesis. 

Journal Homepage: https://jps.library.utoronto.ca/index.php/cpoj/index 

Volume 8, Issue 2, Article No. 1. 2025 

 

 

https://doi.org/10.33137/cpoj.v8i2.45759
https://orcid.org/0000-0003-2559-8566
https://doi.org/10.33137/cpoj.v8i2.45759
https://doi.org/10.33137/cpoj.v8i2.45759
https://jps.library.utoronto.ca/index.php/cpoj/index


 

2 

Phillips C, Nagpal A, Azhari F. A biofidelic mock residual limb for prosthetic socket testing. Canadian Prosthetics & Orthotics Journal. 2025; Volume 8, Issue 2, No. 
1. Https://doi.org/10.33137/cpoj.v8i2.45759 

CANADIAN PROSTHETICS & ORTHOTICS JOURNAL 

ISSN: 2561-987X A BIOFIDELIC MOCK RESIDUAL LIMB FOR PROSTHETIC SOCKET TESTING 

Phillips et al., 2025 

real time. One exception to a static mock limb is a “residual 

limb simulator” used by Paterno et al.16 to test the 

functionality of a flexible transfemoral prosthetic socket. The 

simulator utilizes a controllable syringe pump (similar to the 

design in this paper) to increase limb volume; however, no 

description of its materials, design or fabrication is provided. 

In this paper, the design and fabrication of a ‘biofidelic’ mock 

residual limb, capable of real-time, controllable volume 

adjustability, is presented. To validate the design, trends in 

interfacial pressure changes (between the limb and a rigid 

socket) were analyzed to assess repeatability at different 

rates of volume change, and the symmetry of volume 

change between two localized regions of the biofidelic limb 

was also examined.  

METHODOLOGY 

Design Criteria and Requirements 

To determine specific and meaningful design criteria, focus 

was placed on modeling a transtibial residual limb. The 

design choices were: region of adjustability, extent of 

adjustability, rate of volume adjustment, and material 

composition. 

Residual limb volume changes typically occur radially 

(rather than axially),6 and are localized to regions with more 

interstitial fluid, thus away from bony prominences.17,18 A 

biofidelic mock limb with localized, radial volume 

adjustability was designed to reflect this. While the design 

approach allowed for bladder size and placement 

customization, for this study locations were selected based 

on literature describing pressure-tolerant areas typically 

actuated by panel-based adjustable sockets: the medial and 

lateral tibial flare regions and the posterior compartment 

distal to the popliteal.6,19,20 A circumferentially symmetric 

configuration of three adjustable regions was selected to 

simulate these anatomical areas and to facilitate symmetry 

verification during expansion and contraction. 

Socket fit is often adjusted by adding sock plies, with over 

5-ply considered clinically unacceptable,21 and 10-ply 

warranting a new socket.22 For a limb of 6 cm radius, a 5-

ply sock (1.2 mm thick) corresponds to a 4% volume 

increase. Therefore, we targeted a ±5% volume change. 

Volume change rates in a mature residual limb vary widely 

depending on factors such as activity level and diet, and 

have been shown to range from as slow as ±0.07%/min to 

±3.3%/min after various activity programs.23 To capture this 

variability, testing was conducted across three orders of 

magnitude: from ±0.01%/min to ±10%/min. 

The biofidelic mock limb consists of a dual-durometer 

urethane bulk, surrounding a rigid centre mandrel. This 

composition allows for an effective force transfer from a 

mechanical load frame, while also offering adequate 

compliance at the limb-socket interface to simulate the 

relative movement of the limb within the socket,24 which is 

an important outcome measure of adjustable sockets. 

Water-filled bladders were used to adjust volume. Water 

was chosen over air for its incompressibility, ease of use, 

and similar material properties to physiological interstitial 

fluid or blood.17 

Design Overview and Fabrication 

The final design comprises three water-filled bladders 

embedded in a dual-durometer urethane mock limb. 

Inspired by fabrication methods described by Quinlan et 

al.24 for a static dual-durometer urethane limb, firstly, the 

inner core of the limb was casted with VytaFlexTM 60 (shore 

A hardness, Smooth-On, Reynolds Advanced Materials, 

Chicago, IL) around an aluminum mandrel (elastic modulus 

= 70 GPa) suspended approximately 2 cm from the mold 

bottom using a clamp stand during the 24-hour casting 

(Figure 1A). Next, a custom mold assembly was designed to 

form the bladder cavities within an outer layer of VytaFlexTM 

20 (shore A hardness 20, Smooth-On, Reynolds Advanced 

Materials, Chicago, IL) (Figure 1B and 1C). The cured inner 

core was suspended within the outer socket shell 

approximately 3 cm from the mold bottom using the clamp 

stand during casting of the VytaFlexTM 20 (24 hours). Lastly, 

molds were removed, leaving the final dual-durometer 

urethane limb (Figure 1D) with a volume of 1,330 mL at its 

fully-contracted state. All molds were 3D printed in PLA, 

sealed to a smooth finish with a thin layer of epoxy, and 

coated with two layers of UniversalTM Mold Release 

(Smooth-On, Reynolds Advanced Materials, Chicago, IL). 

TPU fabric was heat-sealed to form 6 x 10 cm bladders, 

which were manually inserted into the cavities of the cured 

VytaFlexTM 20. Each bladder was heat-sealed to a 3D 

printed TPU fitting made for 6.4 mm tubing. Parafilm 

secured the seal at the tube fitting (Figure 1E). Preliminary 

testing showed each bladder could hold 60 mL of water 

without leaking. Given a fully-contracted limb volume of 

1,330 mL, full bladder inflation would theoretically yield 

±6.8% volume adjustability, meeting design requirements. 

Bladders were connected to a motorized syringe system, 

controlled using a linear actuator and Arduino 

microcontroller. The motor offered a torque of 1.9 Nm, and 

power of 36 W per rotation. From preliminary testing, each 

syringe was expected to withstand a maximum fluid 

pressure of 200-250 kPa from the bladders at full capacity. 

To reduce costs, all three syringes were controlled with a 

single motor. If desired, each syringe can be controlled by 

its own motor. The total cost for all parts is less than CAD 

400. Parametric design files of the molds, a Bill of Materials 

and Arduino code are available on GitHub: 

https://github.com/Decisionics/BiofidelicLimb. 

For performance evaluation, the limb was placed in a rigid 

3D printed PLA socket (5 mm wall thickness). The socket’s 

inner geometry matched that of the fully-contracted limb. 

Bladders were then expanded and contracted at various 

https://doi.org/10.33137/cpoj.v8i2.45759
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3 

Phillips C, Nagpal A, Azhari F. A biofidelic mock residual limb for prosthetic socket testing. Canadian Prosthetics & Orthotics Journal. 2025; Volume 8, Issue 2, No. 
1. Https://doi.org/10.33137/cpoj.v8i2.45759 

CANADIAN PROSTHETICS & ORTHOTICS JOURNAL 

ISSN: 2561-987X A BIOFIDELIC MOCK RESIDUAL LIMB FOR PROSTHETIC SOCKET TESTING 

Phillips et al., 2025 

rates. Two F-Socket™ sensors (Figure 1F) from VersaTek 

(Tekscan Inc., South Boston, MA) were placed between the 

limb and socket at two bladder locations (Figure 1G and 1H) 

to read interfacial pressure. A maximum of two sensors 

could be used simultaneously with our 2-Cuff F-Socket™ 

VersaTek system (Tekscan Inc., South Boston, MA). At full 

contraction, an average pressure on the limb of 3-5 kPa was 

read, which was deemed suitable for a loosely fitted 

socket.25 The full system set-up is shown in Figure 1I. 

Protocol for Evaluating Adjustability 

The bladders theoretically allow for a ±6.8% volume 

change; however, this may not directly reflect total limb 

volume change since bladders may, to some extent, 

compress surrounding urethane without significantly 

increasing overall limb size. To quantify the relation 

between bladder volume and limb volume, limb volume was 

measured at bladder volumes from 0-180 mL using 

circumferential measurements taken at 2.5 cm intervals 

along the length of the limb. As shown in Figure 2A, bladders 

must be fully expanded to achieve the desired ±5% limb 

volume change. 

The biofidelic limb was subjected to four different volume 

change rates inside the socket: 1) a SLOW rate over 24 

hours, 2) MODERATE rate over 3 hours, 3) FAST rate over 

30 minutes, and 4) VERY FAST rate over 3 minutes. Tests 

started with the limb at its fully contracted size, then volume 

increased to its fully-expanded size, was held, then 

contracted back to its original volume (Figure 2B). The limb’s 

behaviour (detected by measuring interfacial pressures) 

across different total volume change amounts was 

assessed by conducting the SLOW (0.01%/min), 

 

 

Figure 1: Design overview. A) Step one: casting the core with VytaFlexTM 60 around an aluminum mandrel. B) Mold set-up for casting the 

outer layer. The bladder cavity molds are held in place by clipping into the lid. The wall molds were separated to ease mold removal post-

curing. C) Step two: casting the outer layer with VytaFlexTM 20. D) Final casted limb. E) TPU bladder. F) F-socket sensor, 0.15 mm thin. 

G) Sensor placement on the limb before donning the rigid socket. H) Top view schematic of sensor placement on the limb. I) Full system 

with the biofidelic mock limb, rigid socket, and motorized syringe system. Only one F-socket sensor is shown for clarity. 

Bladder 

cavity 

molds

VytaFlex 

60 core

PLA 

outer 

wall 

molds

Lid

Lid connections 

to bladder 

cavity molds

6 cm

10 cm

Parafilm 

at TPU 

fitting

Tubing

PLA 

inner 

mold

VytaFlex 

60

Clamp

Aluminum 

mandrel 

(∅ = 30mm)

VytaFlex 

20

Bladder 

cavities

VytaFlex 

60

A B D E

Sensor 2Sensor 1
Rigid socket

Bladders

Bladder Sensor 1 Sensor 2

20 

cm

8 cm

F

Motorized syringe system

Arduino & 

motor driver

Tubing

F-socket 

sensor

Rigid 

socket

Computer

IG H

VytaFlex 

20 outer 

layer

VytaFlex 

60 core

C

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4 

Phillips C, Nagpal A, Azhari F. A biofidelic mock residual limb for prosthetic socket testing. Canadian Prosthetics & Orthotics Journal. 2025; Volume 8, Issue 2, No. 
1. Https://doi.org/10.33137/cpoj.v8i2.45759 

CANADIAN PROSTHETICS & ORTHOTICS JOURNAL 

ISSN: 2561-987X A BIOFIDELIC MOCK RESIDUAL LIMB FOR PROSTHETIC SOCKET TESTING 

Phillips et al., 2025 

MODERATE (0.08%/min), FAST (0.5%/min) and VERY 

FAST (5%/min) tests for volume changes between -2.5% 

and +2.5%. Volume changes of ±5% were also tested; 

however, due to resource constraints of the study, this was 

limited to FAST and VERY FAST rates conducted at double 

speed (1%/min and 10%/min, respectively). Furthermore, a 

variable-rate protocol was tested that cycled the limb 

through different volume change rates (Figure 2B). This 

assessed the system’s ability to vary both the rate and 

direction of syringe motion in a non-sequential manner, 

simulating potential complex volume change patterns that 

might occur over periods of time involving a combination of 

activities such as walking, sitting and standing.1,19,23 Tests 

were repeated three times. Coefficient of variation (CV = 

standard deviation/mean × 100%) was calculated to 

quantify repeatability among volume change rates, among 

repeat tests at the same volume change rate, and between 

the two sensors to quantify symmetry of volume change at 

two bladder locations. 

Interpreting the F-Socket Pressure Readings 

The F-Socket sensors are marketed for one-time use, and 

experience significant drift over a period of 20 minutes.26-28 

However, due to the high cost of each sensor, only one set 

of sensors was used for all tests, in which the sensors were 

loaded over three days. Precise pressure magnitudes were 

not necessary in this study; instead, analyzing patterns of 

pressure changes sufficed to evaluate the functionality of 

the limb through metrics like repeatability, hysteresis, and 

symmetry. Therefore, results are normalized to the 

maximum pressure reached in each test to allow for 

comparison between tests. Each pressure recording is an 

average reading across all sensels of the sensor in contact 

with the bladder. 

RESULTS 

Effect of Volume Flow Rate on Pressure Distribution 

Volume flow rate had little effect on interfacial pressure 

distribution and rate of pressure change across all testing 

cycles. Figure 3A shows similar trends in normalized 

pressure for ±2.5% volume change, regardless of whether 

the change occurred at a SLOW, MODERATE, FAST or 

VERY FAST rate (average (SD) and maximum CV among 

rates: 5.8% (3.2%) and 12.7%). Although pressure 

magnitudes may vary due to sensor drift, heat maps in 

Figure 3B show similar pressure distributions among test 

rates at various stages in the testing cycle. The measured 

pressure values during the VERY FAST test (which was 

performed first, and therefore should have accurate 

pressure readings) are within the range of values reported 

in the literature on pressures in tight-fitted sockets,29 

showing local pressures of approximately 200 kPa. 

However, long-term sensor drift prevented direct, 

quantitative comparison of pressure magnitudes in this 

study and those in the literature. The extent of drift over a 

24-hour period can be estimated by comparing peak 

pressures in Figure 3B between tests: the VERY FAST and 

MODERATE tests were performed 24 hours apart (VERY 

FAST and FAST were performed on the same day), as were 

the MODERATE and SLOW tests. In both cases, the 

observed drift was approximately 20% over 24 hours, 

underscoring its significance and the need to normalize 

pressure values. 

Figure 2: A) Relation between limb volume [mL] and bladder volume [mL] to measure the bladder volume required to obtain a ±5% limb volume 

change. B) Testing protocol (limb volume change [%] vs. time) for constant (top) and variable (bottom) rates of change test procedures.  

1320

1340

1360

1380

1400

1420

1440

1460

1480

0 45 90 135 180

L
im

b
 v

o
lu

m
e

 [
m

L
]

Bladder volume [mL]

0% volume change

PeriodTest

8 hrsSLOW

1 hrMODERATE

10 minsFAST

1 minVERY FAST

HOLD

INCREASE

DECREASE

Period

Constant rate test procedure:

Time 

(min):

Fully 

Expanded
Fully 

Contracted

Neutral

-2.5% or -5%

0%

+2.5% or 5%

Fully 

Contracted

Neutral

0%

-2.5%

+2.5%

+1.5%

0%

-1.75%

0 10 20 2111 13 23 25

Variable rate test procedure: Volume:

+5%

-5%

A B

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5 

Phillips C, Nagpal A, Azhari F. A biofidelic mock residual limb for prosthetic socket testing. Canadian Prosthetics & Orthotics Journal. 2025; Volume 8, Issue 2, No. 
1. Https://doi.org/10.33137/cpoj.v8i2.45759 

CANADIAN PROSTHETICS & ORTHOTICS JOURNAL 

ISSN: 2561-987X A BIOFIDELIC MOCK RESIDUAL LIMB FOR PROSTHETIC SOCKET TESTING 

Phillips et al., 2025 

Repeatability and Hysteresis in Volume Adjustability 

Tests were repeatable at all flow rates (SLOW was not 

repeated due to long test times required) and the variable 

cycle test (Figure 4A-D). The largest CV of 10.4% (variable 

test, Figure 4D) corresponds to a pressure magnitude of 

approximately 6 kPa. For the variable test, pressure change 

was only detectable when approximately +1.5% volume 

change was achieved, therefore only the peak at 20 

seconds and the hold at 21 seconds are visible in  

Figure 4D. Sensor malfunction occurred during the final test 

at the FAST rate for ±5% volume change, which is depicted 

in Figure 4B. Pressure measurements in Figure 4A and 4B 

are normalized to the maximum pressure in the ±5% test. 

Figure 4E and 4F show similar hysteresis curves between 

±2.5% and ±5% tests, verifying that limb volume changed in 

a similar fashion regardless of start and end volumes. The 

slight variation in pressure readings between ±2.5% and 

±5% tests could be attributed to sensor drift or a minor 

difference in limb placement in the socket between tests. 

The relatively small hysteresis at each test rate (Figure 4E-

4H) is confirmed by similar pressure distributions between 

points 1 and 5, and points 2 and 4 in Figure 3B. Any 

hysteresis could stem from hysteresis in the urethane, TPU 

bladder elasticity, or sensor drift. 

Symmetry Between Two Adjustable Areas 

Volume change symmetry was analyzed to identify potential 

manufacturing variances among the cavities or bladders. 

Figure 5 shows the pressure over time for both sensor 

locations, illustrating very similar trends and pressure 

distributions. Small variations seen are expected due to the 

manual nature of the manufacturing process. 

DISCUSSION 

The biofidelic limb enables a controlled, bench-top testing 

method for real-time evaluation of prosthetic sockets fitted 

to residual limbs experiencing volume fluctuations of up to 

±5%. The value of bench-top testing has also been 

recognized by McGrath et al.,14 who developed a mock limb 

capable of simulating residual limb perspiration. 

Constructed from readily available materials for less than 

CAD 400, the biofidelic limb presented in this study offers 

many potential adaptations and alternative applications, as 

outlined in this Discussion. While the design met all 

predetermined design criteria, key study limitations are also 

addressed below.  

Potential Design Adaptations 

In this study, specific materials and geometries were 

selected, but these can be adapted based on user needs. 

For example, a transfemoral limb model could have a 

greater diameter and height, with a thicker outer layer of the 

soft VytaFlexTM 20 urethane (or with urethane having a 

lower hardness and/or modulus) to better resemble the 

softer tissue often found in a transfemoral vs. transtibial 

residual limb. Additionally, a network of smaller bladder 

channels could be used to more closely resemble the flow 

of interstitial fluid within the limb. To promote replication and 

adaption, design files are provided on GitHub: 

https://github.com/Decisionics/BiofidelicLimb. 

 
Figure 3: A) Normalized pressure [%] vs. cycle time [% full cycle] for different volume flow rates, measured from one F-socket sensor.  

B) Pressure distribution maps shown at (1) -2.5% and (2) 0% during expansion, (3) +2.5%, then (4) 0% and (5) -2.5% during contraction.  

 

kPa
208
192
176
160
144
128
112
96
80
64
48
32
16
≥ 0

FAST

1 2 3 4 5

MODERATE

SLOW% Full cycle

INCREASE HOLD DECREASE

%
 M

a
x

im
u

m
 p

re
s

s
u

re

3

2

1

4

5

SLOW

MODERATE

FAST

VERY FAST

A B

VERY FAST

100

80

60

40

20

0
0 20 40 60 80 100

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6 

Phillips C, Nagpal A, Azhari F. A biofidelic mock residual limb for prosthetic socket testing. Canadian Prosthetics & Orthotics Journal. 2025; Volume 8, Issue 2, No. 
1. Https://doi.org/10.33137/cpoj.v8i2.45759 

CANADIAN PROSTHETICS & ORTHOTICS JOURNAL 

ISSN: 2561-987X A BIOFIDELIC MOCK RESIDUAL LIMB FOR PROSTHETIC SOCKET TESTING 

Phillips et al., 2025 

The deformation of the soft urethane (VytaFlexTM 20)                                                                                            

surrounding the bladders is complex and was not 

investigated in this study. Indentation tests of the soft 

material used for the limb would help quantify its behaviour, 

as done by Rankin et al.,12 enabling a material-driven and 

performance-based design approach to optimize bladder 

placement and size, given the intended anatomical shape 

of the limb. Nonetheless, we expect that placing the 

bladders closer to the surface would produce a stiffer limb 

with a steeper pressure response, especially at smaller 

volumes.  

The mandrel serves to mimic bone, but can also apply load 

for any load-bearing testing, which would, in general, 

increase interfacial pressure. For example, understanding 

effects on suspension from a fluctuating limb under load 

may be of interest, as it would expand on work by Quinlan 

et al.24 who previously evaluated the effects of textured 

socket surfaces on suspension using a static mock limb 

under load. McGrath et al.14 also evaluated their mock limb 

under loads that simulated the stance and swing phases of 

gait. Additionally, the rigid socket restricted limb expansion, 

and the stiffness of this socket affects the measured 

pressure distribution. Therefore, it is important to note the 

socket stiffness and applied load, especially if a rigid socket 

is used as a control comparison to an adjustable socket. 

Study Limitations 

Our bladders were initially designed to accommodate a 

±6.8% limb volume change, but achieved only ±5% due to 

nonlinear limb-bladder volume scaling, causing very little 

overall volume change with initial bladder expansion. This 

is likely due to the difference in compressibility between the 

soft outer urethane and the water-filled bladders. As the 

bladders expand from a small initial volume, the 

surrounding outer urethane is compressed. Then, once a 

certain bladder volume is reached, the surrounding 

urethane has stiffened enough that overall limb volume 

increases more proportionally with further bladder 

expansion. Further work is required to verify this hypothesis, 

including indentation tests to verify how the urethane 

stiffness varies as a function of bladder volume.  

A key limitation of the study was that most tests were limited 

to ±2.5% volume change due to uncertainty in the heat-seal 

strength of the bladders at full expansion. While this volume 

range partially meets the target design criteria, the limited 

±5% tests conducted still verified the biofidelic limb’s 

functionality. Since results at ±2.5% and ±5% were 

comparable for the FAST and VERY FAST tests, it is 

reasonable to expect that the ±2.5% tests for SLOW and 

MODERATE can be generalized to the ±5% range. 

However, future designs should include a greater bladder 

capacity to ensure a safety margin, and the long-term 

bladder strength at full expansion, especially at heat-sealed 

joints, is a critical area for future evaluation.  

The long-term use of the sensing system meant pressure 

magnitudes could not be reported or compared between 

tests; thus, pressures were normalized in the study to 

address this limitation. Furthermore, the sensor drift 

prevented any direct comparison of pressure magnitudes in 

this study to those in the literature. Despite this, using 

normalized pressures met the validation goals of the study 

because the trends, repeatability, and relative changes in 

 

Figure 4: Repeatability of normalized pressure [%] vs. time [min], for A) VERY FAST, B) FAST, C) MODERATE and D) variable tests. Shaded regions 

show variation among three trials. CVs are reported as: average CV (standard deviation), maximum CV over the cycle, in [%]. Sample hysteresis curves 

of normalized pressure [%] vs. change in limb volume [%], for E) VERY FAST, F) FAST, G) MODERATE and H) SLOW tests. * Final test omitted in CV 

calculation. 

0

25

50

75

100

0 60 120 180

%
 M

a
x
im

u
m

 p
re

s
s
u

re

Time [s]

100

75

50

25

%
 M

a
x
im

u
m

 p
re

s
s
u

re

0 1 2 3

Time [min]

0

VERY FAST

±5%, 10%/min
CV: 2.8 (1.1), max 6.8

±2.5%, 5%/min
CV: 0.4 (0.2), max 2.7

0

25

50

75

100

0 600 1200 1800

100

75

50

25

%
 M

a
x
im

u
m

 p
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s
s
u

re

0 10 20 30

Time [min]

0

FAST

±5%, 1%/min
CV: 4.1 (2.4), max 7.8 *

±2.5%, 0.5%/min
CV: 5.7 (3.4), max 7.7

Final test

0

100

-5 0 5

%
 M

a
x
im

u
m

 p
re

s
s
u

re

Limb volume [%]

0

25

50

75

100

-5 -2.5 0 2.5 5

%
 M

a
x
im

u
m

 p
re

s
s
u

re

Change in limb volume [%]

FAST

±5%, 1%/min

±2.5%, 0.5%/min

0

100

-5 0 5

%
 M

a
x
im

u
m

 p
re

s
s
u

re

Limb volume [%]

0

25

50

75

100

-5 -2.5 0 2.5 5

%
 M

a
x
im

u
m

 p
re

s
s

u
re

Change in limb volume [%]

VERY FAST

±5%, 10%/min

±2.5%, 5%/min

%
 M

a
x
im

u
m

 p
re

s
s
u

re

%
 M

a
x
im

u
m

 p
re

s
s
u

re

2.5-2.5 2.51.25

Change in limb volume [%]

100

75

50

25

0 60 120 180

Time [min]

0

100

75

50

25

0 5 15 20

Time [min]

0
10 25

%
 M

a
x
im

u
m

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re

s
s
u

re

%
 M

a
x
im

u
m

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re

s
s
u

re

A B

±2.5%, 0.08%/min
CV: 3.9 (2.0), max 8.7

MODERATE

C

CV: 4.2 (2.2), max 10.4

VARIABLE

D

E F G H

100

75

50

25

0
-1.25 0

±2.5%, 0.08%/min

MODERATE

-2.5 -1.25 0 1.25

Change in limb volume [%]

±2.5%, 0.01%/min

SLOW100

75

50

25

0

https://doi.org/10.33137/cpoj.v8i2.45759


 

7 

Phillips C, Nagpal A, Azhari F. A biofidelic mock residual limb for prosthetic socket testing. Canadian Prosthetics & Orthotics Journal. 2025; Volume 8, Issue 2, No. 
1. Https://doi.org/10.33137/cpoj.v8i2.45759 

CANADIAN PROSTHETICS & ORTHOTICS JOURNAL 

ISSN: 2561-987X A BIOFIDELIC MOCK RESIDUAL LIMB FOR PROSTHETIC SOCKET TESTING 

Phillips et al., 2025 

pressure over the volume-change cycles remained 

consistent across repeated trials. This indicates that, even 

without pressure magnitudes, the system reliably captured 

the relation between changes in limb volume and interfacial 

pressure, which was an essential requirement for evaluating 

the limb’s performance. For applications requiring absolute 

pressure, sensors should be calibrated before each test or 

replaced with new sensors following every test.  

Applications 

The biofidelic limb can be used to evaluate sockets (both 

static and adjustable) fitted to residual limbs with volume 

fluctuations of up to ±5% in real time. Clinical outcomes of 

sockets (e.g., pistoning, suspension, and interfacial 

pressures) can be evaluated by adjusting the size of the 

limb, rather than swapping out sockets or limbs of different 

sizes.2,25 Furthermore, the detailed design description 

provided in this study expands on the work of Paterno et 

al.,16 who developed a similar limb but did not provide 

design details. The methods presented here could be 

combined with those from McGrath et al.14 to develop a 

biofidelic limb capable of both volume fluctuation and 

perspiration. Beyond prosthetics, the biofidelic limb may 

support testing the fit of footwear, compression garments or 

orthotics.30,31 Load-bearing testing is also facilitated via the 

internal mandrel. 

CONCLUSION 

This work presents the design and fabrication procedure of 

a biofidelic limb used for testing prosthetic sockets, with 

adjustable water-filled bladders that enable controllable, 

repeatable volume changes of up to ±5% of limb volume. 

The biofidelic limb demonstrated high repeatability (CV < 

10.4%) and adequate symmetry (CV < 11.2%) between two 

bladders placed on two areas of the limb. Despite its 

limitations, the sensing tool effectively confirmed the 

biofidelic limb’s functionality.  

The proposed design can be tailored for different 

anatomies, volume change ranges, materials, and is easily 

manufactured with readily available materials. Furthermore, 

it allows for controllable bench-top testing during 

prototyping of devices designed for volume adjustment, 

such as adjustable prosthetic sockets, eliminating the need 

for resource-intensive clinical trials. 

ACKNOWLEDGEMENTS 

The authors would like to thank The War Amps for their funding 

support. 

DECLARATION OF CONFLICTING INTERESTS 

The authors report no conflict of interest. 
 

AUTHORS’ CONTRIBUTION 

• Clara Phillips: Conceptualization, Methodology, Validation, 

Data Curation, Writing- Original Draft, Visualization. 

 

• Aaryan Nagpal: Methodology, Software, Data Curation, 

Writing – Original Draft. 

 

• Fae Azhari: Conceptualization, Writing – Review & Editing, 

Supervision. 

 

 

Figure 5: Normalized pressure [%] vs. cycle time [% full cycle] for sample VERY FAST and FAST tests (over ±5% volume change) measured at two 

bladder locations with two F-Socket sensors. CVs are reported as the average (standard deviation), maximum CV over the cycle, in [%]. 

0

20

40

60

80

100

0 20 40 60 80 100

%
 M

a
x

im
u

m
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re
s

s
u

re

% Full cycle

FAST

VERY FAST

Sensor 1 (±5%)

Sensor 2 (±5%)

CV: 3.1 (1.6), 5.3

Sensor 1 (±5%)

Sensor 2 (±5%)

CV: 6.0 (3.9), 11.2

https://doi.org/10.33137/cpoj.v8i2.45759
https://www.waramps.ca/home/


 

8 

Phillips C, Nagpal A, Azhari F. A biofidelic mock residual limb for prosthetic socket testing. Canadian Prosthetics & Orthotics Journal. 2025; Volume 8, Issue 2, No. 
1. Https://doi.org/10.33137/cpoj.v8i2.45759 

CANADIAN PROSTHETICS & ORTHOTICS JOURNAL 

ISSN: 2561-987X A BIOFIDELIC MOCK RESIDUAL LIMB FOR PROSTHETIC SOCKET TESTING 

Phillips et al., 2025 

SOURCES OF SUPPORT 

This work was supported by The War Amps Amputation Research 

Grant. 

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https://patents.google.com/patent/US20020099450A1/en


 

9 

Phillips C, Nagpal A, Azhari F. A biofidelic mock residual limb for prosthetic socket testing. Canadian Prosthetics & Orthotics Journal. 2025; Volume 8, Issue 2, No. 
1. Https://doi.org/10.33137/cpoj.v8i2.45759 

CANADIAN PROSTHETICS & ORTHOTICS JOURNAL 

ISSN: 2561-987X A BIOFIDELIC MOCK RESIDUAL LIMB FOR PROSTHETIC SOCKET TESTING 

Phillips et al., 2025 

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