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From conventional non-conductive foams to soft
piezoresistive pressure sensors: A low-cost
approach to large-area pressure-mapping
Manuel Reis Carneiro1,2†, Luis Rosa1†, Mahmoud Tavakoli1*
1: Institute of Systems and Robotics, Dept. of Electrical and Comp. Engineering, University of Coimbra, Coimbra, Portugal
2: Soft Machines Lab, Dept. of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
*mahmoud@isr.uc.pt
Abstract—This article presents a low-cost, and scalable method
for synthesizing large sheets of functionalized foam for soft
resistive pressure sensing that is compatible with any
commercially available non-conductive foam. The process is
illustrated by soaking Pu (Polyurethane) foam in a carbon-
based polymeric mixture resulting in a functional foam with
large pressure detection range up to 500 kPa, while retaining
its deformability and recoverability after cyclic compression.
The foam also shows unprecedentedly fast response time of
~120 ms. The foam was integrated in a wearable respiration
sensor and in a large multitouch sensing mat that allows for
precise data acquisition on the position and intensity of various
simultaneous touches without the occurrence of ghost touches.
Keywords— conductive foam; haptics; piezoresistive
materials; pressure sensing; soft electronics
I. INTRODUCTION
Pressure sensors are a ubiquitous form of mechanical
transducer in many fields including the automotive
industry[1], [2], biomedical devices[3]–[5], consumer
electronics[6], [7], industrial applications[8], [9] or
robotics[10]–[12]. Among various types of pressure
sensors, piezoresistive pressure sensors have received
considerable attention due to their high sensitivity, stability,
and compatibility with microelectronic fabrication
processes[12], [13].
Recent advances in materials science and soft
engineering have led to the development of novel
piezoresistive pressure sensors made from soft conductive
foams that offer improved flexibility, low weight, and
mechanical robustness, ideal for wearable and soft
electronics applications[14].
Efforts have focused on creating optimal piezoresistive
materials, balancing pressure mapping range, sensitivity,
response time, and repeatability. Commonly, commercially
available pre-formed soft foams[15]–[23] have been used.
In a less straightforward approach, techniques for creating
porous structures, including sugar templating[24]–[26],
freezing[27], [28], or foaming agents[29] have also been
shown. The foam structures are then filled with conductive
materials, such as graphene[17], [28], carbon
nanotubes[29], [30], graphite[31], carbon black[27],
PEDOT:PSS[18], or silver NWs[25]. However, most works
have yet to demonstrate an ideal combination of low cost,
easily replicable, and scalable fabrication method for
implementation of both single sensing cells and large multi-
cell structures.
Here we propose a simple, low cost and easily scalable
method for synthesis of large sheets of functionalized foam
and implementation of resistive pressure sensing cells and
multi-touch, large-area pressure sensing pads. The
functionalization of a commercially available PU
(Polyurethane) foam is done by soaking it in a carbon-based
polymeric mixture, using reagents, materials, and
equipment that is inexpensive and readily available.
Moreover, the success of the proposed functionalization
method doesn’t depend on the foam’s material. As such, any
commercially available foam can be turned into a
piezoresistive pressure-sensing foam, avoiding more
complex methods for growing elastomeric foams from
scratch which can be time-intensive and costly. The method
works with any foam, avoiding complex elastomeric foam
growing methods. The fabricated foam detects pressures
between 0-500 kPa, retains deformability, recovers initial
resistance and shape after 50+ compression cycles, and has
good repeatability, with a fast response time of ~120 ms
compared to similar architectures.
The functional piezoresistive foam is demonstrated in a
respiration sensor using a printed bioelectronic sticker on
the user's skin, measuring chest volume changes. As well,
we implemented a 7x7 multitouch sensing mat with
physically separated sensor nodes which are able to
precisely acquire multi-touch data without the occurrence of
false touches (ghosting) which were observed when using a
continuous functional foam medium. This eliminates need
for detection thresholds or complex algorithms to detect and
discard false positives.
II. FOAM FABRICATION AND CHARACTERIZATION
A. Carbon Foam fabrication
The accessible process fabrication uses commercially
available 5mm-thick, 19.25 kg/m³ density polyurethane open
cell foam from 'Ricarlina' or 'Uline' (model S-10017). A 20
cm x 10 cm rectangle is cut from the pristine foam to fit a
standard loaf pan (Amazon Basics baking bread loaf pan, 9.5
x 5 inch).
†M.R.C and L.R. contributed equally to this work.
Support for this research was provided by the Fundação para a Ciência
e a Tecnologia (Portuguese Foundation for Science and Technology)
through the CMU-Program under Grant SFRH/BD/150691/2020, the
CMU-Portugal project WoW (45913), and the European Research Council,
ERC project Liquid3D, grant 101045072.
2023 IEEE SENSORS | 979-8-3503-0387-2/23/$31.00 ©2023 IEEE | DOI: 10.1109/SENSORS56945.2023.10325315
Authorized licensed use limited to: b-on: Universidade de Coimbra. Downloaded on November 29,2023 at 23:47:06 UTC from IEEE Xplore. Restrictions apply.
The conductive solution is prepared, as depicted in Fig. 1A,
by adding 47.0 g of toluene and 1.75 g of SIS pellets
(styrene/isoprene copolymer) in a glass vial and mixing those
for 3 minutes at 2000 rpm in a planetary mixer. Then 1.31 g
of carbon black are added in and the solution is mixed again
(3 min, 2000 rpm). The precut foam (20cm x 10cm x 0.5mm)
is then placed inside one of the loaf pans and the conductive
solution is poured on top (Fig. 1Bi). A second loaf pan is then
used to squeeze the foam so that the conductive solution can
infiltrate evenly into the foam (Fig. 1Bii). Finally, the toluene
is evaporated for 2 hours at room temperature inside a fume
hood so that the SIS-CB polymer solidifies and adheres to
the pre-formed PU foam (Fig. 1Bi). The foam can then be
used as is or cut to shape using scissors or a CO2 laser cutter.
Photographs of the C-foam fabrication process are shown in
Fig. 1C.
B. Resistive foam operation
The post-processed piezoresistive foam contains
dispersed carbon black particles within the non-conductive
SIS-PU matrix. At low pressures, low conductivity results
from sparse carbon particles and foam voids (Fig. 1Di). As
pressure increases, foam pores close, and conductivity
improves (Fig. 1Dii). At higher pressures, conductivity
further increases due to enhanced carbon percolation (Fig..
1Diii). The foam's porous nature, unlike bulk piezoresistive
materials, leads to better pressure response by introducing
two distinct functional zones.
C. Foam characterization
The experimental setup for foam characterization uses a
tension/compression testing system (Instron 5943) with 3D
printed ABS parts: a sensor stand and a presser tool with a 1
cm2 contact area, converting 1 N force to 10 kPa pressure on
the foam. Resistance is measured with a multimeter (GDM-
8351, GW Instek). Fig. 2A shows the setup schematic.The
functional foam's response to normal pressure was assessed
through force-controlled compression-decompression
cycles. Nine sets of five compressions were conducted, with
maximum pressures from 10 to 500 kPa. Fig. 2B's plot shows
the foam returning to its initial resistance after
decompression, even at 500 kPa, and demonstrates the
sensor's repeatability across various loadings.The sensor's
response time was evaluated by applying 500 kPa pressure,
followed by instantaneous decompression. The time taken to
reach steady resistance was measured, with the foam's
response time found to be around 120 ms (Fig. 2C).
Hysteresis loops are observed in Figs 2D and E at both small
and large applied pressures (25 kPa vs 500 kPa), expected
due to foam's viscoelastic nature. However, no permanent
deformation occurs even at the largest loading, with
resistance returning to initial values after decompression.
The foam's cyclic loading endurance was tested with 100
compression-decompression cycles at 200 kPa after 5 low-
amplitude cycles (100 kPa). Fig. 2F shows resistance
decreasing about 10% (from 1057.7 Ω to 954.8 Ω) from the
first to the 100th cycle, but the foam recovers its initial
resistance (1057.8 Ω) after a two-minute refractory period..
III. APPLICATIONS
A. Respiration monitoring patch
To implement a fully soft transducer for respiration
monitoring, a conductive stretchable ink was prepared so that
soft conductive electrodes could be printed and attached on
each side of the foam to enable pressure transduction. The
Ag-In-Ga-SIS ink is prepared according to [32] by mixing
Silver flakes and EGaIn in a SIS-Toluene solution. This ink
is printed by direct ink writing (DIW) over a soft TPU film
substrate (250um thickness), using a digital printer (Voltera
V-One). One of the TPU layers is adhered to a double sided
skin-safe adhesive (3M152A). The stack composed of 2
electrodes on TPU substrates, the medical adhesive, and the
carbon foam, which is shown in Fig. 3A is heat pressed
together to form durable skin conformal electronic patches
as described in [32]. The foam connects to a voltage divider
circuit and an Arduino Nano analog input pin. Positioned on
the user's chest over the pectoralis major muscle (Fig. 3B, C),
the respiration sensing patch reveals clear cycles (Fig. 3D)
with an average rate of 12.49 breaths per minute (~0.21 Hz)
and a 4.8 s cycle duration, typical for a relaxed, healthy
subject.[33].
B. Multitouch pressure mapping mat
Pressure mapping films are flexible sheets that measure
pressure distribution on surfaces, used in industries like
molding, fitting and sealing (e.g., prosthetic limbs), and
biomechanics analysis (e.g., pressure distribution mapping
and gait analysis). Here we propose a 7x7 multimode
pressure mapping mat with isolated pressure sensing cells
that can simultaneously record the intensity of multiple
AB i ii iii
C i ii
iii iv
D i ii iii
No Pressure Low Pressure High Pressure
Fig. 1. A: Toluene, SIS pellets, and CB mixed. B: (i) CB-polymer mixture
poured over foam, (ii) soaked in by compression, (iii) toluene evaporated at
room temperature. C: Fabrication process photos: (i) pristine PU foam in loaf
pan, (ii) CB-polymer mix on PU, (iii) pressure applied, (iv) final dry foam.
D: Foam has 3 functional zones: (i) high resistance with no pressure, (ii)
pores close and some conductive paths form with low pressure, (iii) many
conductive paths are created, and resistance decreases with high pressure.
0100 400
10
P (kPa)
R (Ω)
F
100
1k
10k
300200 5000 5 20
10
P (kPa)
R (Ω)
E
100
1k
10k
1510 25
Hysteresis loop
0-25 kPa
Hysteresis loop
0-500 kPa
100
0-500 kPa 0-400 kPa
0-300 kPa 0-200 kPa
0-100 kPa 0-50 kPa
0-25 kPa 0-15 kPa
0-10 kPa
A
multimeter
17.8 17.9 18.2
0
R (kΩ)
4
18.118.0 18.3
5
3
2
1
6Response time (500-0 kPa)
500 kPa
0 kPa
120 ms
B
C
0100 400
10
P (kPa)
R (Ω)
1k
300200 500
10k
50
DE
Time (s)
250
Time (s)
R (Ω)
500
1k
1.5k
500 1000750
1057.7 Ω954.8 Ω1057.8 Ω
1st cycle: 100th cycle: 2 min rest:
Cyclic loading (0-200 kPa)
F
Fig. 2. A: Electromechanical test setup with compression column, 3D
printed mounts, and digital multimeter. B: Foam response to pressures (10-
500 kPa). C: Foam response time. D: Hysteresis at 25 kPa and E: 500 kPa
compression-decompression cycles. F: 100 consecutive cycles of foam
loading and unloading between 0 and 200 kPa.
Authorized licensed use limited to: b-on: Universidade de Coimbra. Downloaded on November 29,2023 at 23:47:06 UTC from IEEE Xplore. Restrictions apply.
simultaneous pressure points. The mat is fabricated by
printing conductive lines on a soft TPU substrate using Ag-
EGaIn-SIS ink, serving as electrodes (Fig. 3E). Soft printed
lines interface with flexible PCBs connected to copper wires
and an Arduino Nano. Flexible PCBs prevent stress-induced
delamination ([32], [34]). The carbon-impregnated foam is
diced into 7.7mm x 7.7mm x 0.5cm elements and PU foam
is laser-patterned. Active elements are spaced 2.3mm apart
and heat-bonded, forming the mat shown in Fig. F,G. .
The novel isolated cell structure's advantage was
demonstrated by fabricating a similar mat using continuous
C-PU foam. The 7 lines were connected to a voltage divider
and analog pins of a microcontroller, while the conductive
lines on the opposite surface were linked to digital output
pins, sequentially driving columns to 5V or 0V. Fig. 3H
displays a 3x3 portion of the continuous foam mat, where
touched cells (green) cause adjacent ghost touches (red) due
to foam deformation and signal scattering. This occurs with
single and multiple simultaneous touches, creating the need
for complex noise reduction algorithms or a detection
threshold that will limit sensitivity to low pressures.The
isolated-cells mat solves ghost touches by using insulating
foam barriers between cells, preventing signal scattering.
Fig. 3I i shows reduced false touches compared to continuous
foam. In most cases, no ghosting is observed (Fig. 3I ii,iii),
eliminating the need for complex software algorithms and
utilizing the full detection range. The carbon and non-
conductive portions are heat-bonded together, resulting in a
resilient, deformable mat with no loss of functionality or
integrity during testing.
IV. CONCLUSION
We introduce a simple, cost-effective method to create
functionalized foam for soft resistive pressure sensing with a
wide detection range (0-500 kPa), deformability, shape
recovery, repeatability, and fast response time (120 ms).
Demonstrated in a wearable respiration sensor and a 7x7
multitouch mat, the foam provides accurate data without
false positives or complex noise reduction algorithms. This
work enables low-cost, flexible, reliable pressure sensing
foams, expanding piezoresistive materials' applications in
wearables, soft robotics, healthcare, and consumer
electronics. The simple, scalable fabrication method and
exceptional foam performance promote adoption in small-
scale and large-area pressure sensing applications and inspire
novel functional materials and techniques for soft electronics
and pressure sensing advancements
TABLE I. PIEZORESISTIVE FOAMS
Ref
Range
(kPa)
Response
time (ms)
Conductive material
Foam origin
Applications
[15]
50
120
Graphene
Commercial foam
Detecting heart rate & blood pressure, finger bending, & swallowing
[16]
400
29
Carbon nanotubes
Commercial foam
Measuring fingertip force, finger bending, pulse, & walking pressure
[17]
30
300
Graphene oxide
Commercial foam
-
[18]
40
-
PEDOT:PSS
Commercial foam
-
[19]
10
45
Carbon black, MW CNTs
Commercial foam
Detecting speech & monitoring muscle movement and respiration
[20]
10
-
Carbonized melamine foam
Commercial foam
Tracking joint movement, mapping pressure through a sensor array
[21]
60
-
Nickel, Graphene
Commercial foam
Detecting pulse heart rate & providing force feedback in a robotic gripper
[22]
240
60
Velostat, Cond. fabric, Graphite
Commercial foam
Detecting balance using a shoe sensor
[23]
3000
100
Nickel, Graphene
Commercial foam
Detecting walking patterns, finger bending, and wrist blood pressure
[24]
10
130
Carbon nanotubes
Sugar templating
Monitoring finger pressure, swallowing, speech, heart rate and muscle movement
[25]
40
47
Silver nanowires
Salt templating
Monitoring heart rate, respiration, facial expressions and speech
[26]
500
200
Carbon clack
Sugar templating
Detecting textures, monitoring respiration, mapping pressure through a textile mat,
and detecting grasping patterns through a fully printed wearable glove
[27]
584
150
Carbon black
Freeze drying
Monitoring joint movement & pressure mapping through sensor array
[28]
0,25
100
Graphene oxide
Ultrasonic freeze drying
Using single-node sensors to track pressure and bending
[29]
400
150
Single-walled carbon nanotubes
Foaming agent
Sensing punch force and mapping plantar pressure
[30]
60
-
Carbon nanotubes
Sugar templating
-
[35]
600
60
polyvinyl-pyrrolidone NWs, Ag
Electrospinning
Monitoring respiration, heart rate, gait, & force on surgical robot
[36]
300
-
Carbon nanotubes
Sugar templating
Monitoring joint bending
[37]
17
-
Carbon powder
Sugar templating
Detection of strain and pressure through a multi-node matrix
this work
500
120
Carbon black
Commercial foam
Monitoring respiration, and mapping pressures with multi-node mat
Foam
Printed interconnects
2 4 6 8 10 12 14 16 18 20 22
0
time (s)
ADC count
(arbitrary units)
Respiration cycle
Printed
electrode
A B C D
Top electrode
Bott. electrode
Foam
Medical adhesive
Active
foam
Pristine
foam Soft conductive
traces
Flex PCB
E F G
0
100
0
100
0
100
0
100
0
100
0
100
Continuous
foam
Diced
foam
H
I
i) ii) iii)
i) ii) iii)
Real touch Ghosting
Fig. 3. A: Respiration-monitoring sensor schematic with carbon-foam and
soft printed conductive electrodes. B: Sensor placement on chest. C:
Sensor on user's skin. D: User's respiration cycle signal plot. E: Fabricated
7x7 multimode pressure-sensing mat schematic with diced carbon foam
cells and pristine foam for structural integrity. F: Pressure-sensing mat
schematic after layer bonding. G: System photo. H: Pressure tests on
continuous foam mat, showing ghost touches in adjacent cells (Vertical
axis unitless; 100=max pressure). I: Diced, separated sensory units
eliminate ghosting. Plots show a 3x3 mat section.
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