Overall Additive Manufacturing of Capacitive Sensors Integrated

into Textiles: A Preliminary Analysis on Contact Pressure Estimation

Tiziano Fapanni

, Raphael Palucci Rosa

, Edoardo Cantù

, Federica Agazzi

Nicola Francesco Lopomo

, Giuseppe Rosace

and Emilio Sardini

Department of Information Engineering, University of Brescia, Brescia, Italy

Department of Engineering and Applied Sciences, University of Bergamo, Bergamo, Italy

{tiziano.fapanni, edoardo.cantu, f.agazzi003, nicola.lopomo, emilio.sardini, }@unibs.it,

Keywords: Contact Pressure Sensors, Aerosol Jet Printing, Flash Lamp Annealing, Additive Manufacturing, 3D Printing,

Printed Electronics, Textile.

Abstract: Printed electronics approaches in deploying sensors offers several advantages over traditional methods,

including their capability to be integrated into flexible substrates, including textiles. Additionally, printed

sensors can be manufactured at relatively low cost and overall include sustainable materials, making them a

more accessible option for a wider range of applications. Utilizing additive manufacturing techniques like

stereolithography and aerosol jet printing, this work focused on creating fully printed capacitive pressure

sensors within textiles. The sensors were designed as planar capacitors with micro-structured dielectrics to

enhance linearity and measurement range. Three devices, incorporating 3D pyramidal structures, were

produced and characterized under varying loads; the dielectric part was realized by using stereolithography

and directly incorporating fabric on the top/bottom sections, whereas carbon-based ink was then deposited to

produce the conductive plates and connection pads. Results indicated primarily capacitive behavior up to 10

MHz, with tunable capacitance affected by surface areas and air/resin ratio; hysteresis was also observed,

revealing inherent non-linear behavior. These main findings provide important insights into the feasibility of

the design and the additive manufacturing process. This innovation holds promise for applications in a variety

of fields, including safety and sports.

1 INTRODUCTION

In the context promoted by Industry 4.0 but also in

sports, thanks to specific enabling technologies (i.e.,

Big Data, Internet of Things, Additive

Manufacturing, and Cloud Computing), it is possible

to constitute enhanced scenario composed by

humans, servers equipped with AI-based algorithms,

and a network of interconnected Smart Objects (SOs)

(Kortuem et al., 2010; Munirathinam, 2020). Indeed,

SOs can detect variations of specific physical

quantities (i.e., temperature, humidity, mechanical

deformations, etc.), while the presence of

microcontrollers with devoted algorithms allows

https://orcid.org/0000-0002-5164-6907

https://orcid.org/0000-0002-9744-9511

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https://orcid.org/0000-0003-0604-4453

https://orcid.org/0000-0001-8629-7316

preliminary elaboration and transmission of the

acquired data. Such a kind of device provides

important added value by unlocking new

functionalities like real-time monitoring, which can

be declined in multiple ways depending on the

context in which the SOs are placed. In the realm of

sports technology, SOs can be devoted to continually

monitor athletes’ health, improve their performance,

mitigate the risk of injury, and enhance overall

engagement, particularly for youth and individuals

with disabilities. Similarly, in the context of Industry

4.0, SOs strive to also enhance workers' safety and

well-being, ultimately reducing the likelihood of

injuries, lowering the overall risk; in this context, SOs

Fapanni, T., Rosa, R., Cantù, E., Agazzi, F., Lopomo, N., Rosace, G. and Sardini, E.

Overall Additive Manufacturing of Capacitive Sensors Integrated into Textiles: A Preliminary Analysis on Contact Pressure Estimation.

DOI: 10.5220/0012597000003657

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 17th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2024) - Volume 1, pages 195-200

ISBN: 978-989-758-688-0; ISSN: 2184-4305

195

can in fact monitor both the physiological and

environmental conditions within the working area,

highlighting variations of temperature and humidity

(Borghetti et al., 2021; Saqlain et al., 2019; Perez-

Alfaro et al., 2020).

Within these perspectives, contact pressure

sensors can provide a fundamental function to SOs,

which can be vital in many technological applications

such as robotics or exoskeletons, but also in sports

equipment or clothing. During the last years, different

pressure transducers have been explored in the

literature. Among those, the main used are

piezoresistive, piezoelectric, capacitive, and optical

(Mannsfeld et al., 2010; Pan et al., 2014; Persano et

al., 2013; Ramuz et al., 2012). In particular,

capacitive pressure sensors are interesting due to their

high sensitivity, good repeatability, temperature

independence, low power consumption and high

spatial resolution (Chortos et al., 2016).

Thanks to its versatility, its ability of working

with any kind of substrate material, and its

customizability on possible patterns, printed

electronics (PE) technologies represent a suitable

solution for the fabrication of SOs including contact

pressure sensors.

Among all the available printed electronics

technologies that allow printing sensors over any

surface, both planar or 3D, the contactless ones are

the most attractive; among them, in particular, the

Aerosol Jet Printing (AJP) represents the most actual

state-of-the-art, since it enables to use a wide variety of

functional inks and substrates, including non-planar

and 3D ones (Almuslem et al., 2019; Gu et al., 2019;

Horst et al., 2018; Fisher et al., 2023; Gramlich et al.,

2023; Werum et al., 2022). Indeed, AJP together with

advanced post-printing thermal treatments, such as

Flash Lamp Annealing (FLA), or Intense Pulsed Light

(IPL), open up to realize printed sensors even of

sensible substrates, such as plastic and paper.

Considering this, the present work aims to

propose the realization of contact pressure sensors for

potential uses in sports and safety; the sensors were

modeled as capacitive sensors and implemented

within textiles using a fully additive manufacturing

process. In this frame both stereolithography (SLA)

and AJP and FLA were employed in order to produce

both the mechanical and the electrical components

needed to realize a hybrid combination of a polymeric

matrix surrounded by fabric and enhanced with

sensing functionality. The main hypothesis was that

capacitive sensors for contact pressure estimation can

be tuned by working on the dielectric flexible

structure, while ensuring the integration within

textiles, addressing wearable applications.

In this paper, chapter two present methods

followed during design, fabrication and performed

tests, while chapter three shows the preliminary

results obtained during the validation and

characterization of the devices; chapter four will

summarize the preliminary results achieved, with

future perspective of this application.

2 MATERIAL AND METHODS

2.1 Sensor Design

A capacitive pressure sensor can be at first modeled

as a planar capacitor whose capacitance (C) can be

estimated as in equation (1), in which ɛ

depicts the

relative permittivity of the insulating layer, ɛ

the

vacuum permittivity, A is the surface of the plates of

the device and d is the distance between them.

C = (ɛ

A)/d (1

)

This model is acceptable when referring to devices

with uniform dielectrics. However, the use of micro-

structured dielectrics is widely used in order to

improve the linearity and better tune the measurement

range of the device. In this work, a modular dielectric

approach is proposed. Each single cell can be

modeled in 3D as the combination of four 3D

pyramidal structures. In Figure 1(c) the white side

represents the air space (2 pyramids), and the

combination of the 4 light blue sides of the dielectric

also composes 2 pyramidal structures. In this

configuration, the relative permittivity can be

expressed as the weighted average of the two

materials (air and dielectric) as per equation (2),

where ɛ

and ɛ

are the relative permittivity of air and

where ꭤ and ẞ represent the percentage of the single

cell area covered by air and material, respectively.

= ɛ

ꭤ + ɛ

ẞ (2

)

In order to evaluate both the selected material and

geometries three different kinds of devices were

produced. A first set of 10 x 10 x 2 mm completely

filled (Sample A) was proposed in order to evaluate

the electrical characteristics of the selected resin,

while two different geometries (10 x 10 x 2 mm and

20 x 20 x 3 mm, both with honeycomb filling, named

respectively Sample B and Sample C) were proposed

to evaluate the effect of the different geometry.

2.2 Sensor Fabrication

The devices were fabricated using a multi-step

approach. First, a textile substrate (cotton, with

BIODEVICES 2024 - 17th International Conference on Biomedical Electronics and Devices

196

dimensions of 100 mm x 50 mm x 1 mm) was

attached to the building plate of a stereolithographic

printer (Photon Mono M5s, Anycubic). The dielectric

part of the capacitive pressure sensor was printed on

the surface of the textile using a flexible resin (3D

materials SuperFlex); after printing, the excess of

resin was washed of using ethanol anhydrous (Sigma

Aldrich) and post cured inside an UV chamber for 20

min at room temperature. As further step, an

additional layer of the same textile was placed on top

of the printed structures and secured with the

polymeric resin; the devices were then cured in a UV

bath at 60 °C for 20 minutes in order to achieve proper

mechanical stability. After that, a carbon-based ink

EXP 2652-8 (Creative Materials Inc.) was deposed

by AJP system (AJ300, Optomec) on both the textile

faces in order to produce the conductive plates that

compose the capacitor, as well as to produce a set of

connection pads to ease the interconnection of the

devices to the frontend electronics. Prior to printing,

the tool path was designed by using a CAD platform

(AutoCAD 2021, Autodesk). Each layer was

composed by two crossed paths in order to obtain a

complete fill of the plate. A total number of 10

depositions per plate was performed, considering as

process parameters: a) sheath gas flow equal to 110

SCCM; b) atomizer gas flow equal to 770 SCCM; c)

exhaust gas flow equal to 750 SCCM, with a printing

speed of 3 mm/s. The positioning plate of the AJ300

was set at a temperature of 70 °C. The samples were

left to dry overnight at room temperature and then

cured using FLA solution (Pulseforge, Novacentrix),

setting voltage and pulse duration at 230 V and 1750

μs, respectively. After that, the connection pads were

reinforced via drop-casting a carbon nanotube paste

in order to improve their resistance to scratch and

wear. Figure 1(a) presents the block diagram of the

overall production process, while Figure 1(b) and

Figure 1(d) show the bare photopolymer printed onto

textile substrate and the complete capacitive sensor,

respectively.

2.3 Experimental Setup

In order to evaluate the behavior of the produced

devices a dynamometer (ESM1500, MARK 10) was

used to control the displacement applied to the plates

of the printed devices. In order to apply an even force

on all the surfaces of the plates and thus ensure a

uniform displacement, a set of 3D-printed adapters in

the form of trapezoidal prims were realized via

additive manufacturing. The devices under test

(DUTs) were then connected to an impedance

Figure 1: (a) Block diagram of the sensors production steps.

The textile substrate is represented in ocher color, the

photopolymer in light-blue, the carbon plates of the

capacitor in black, carbon nanotubes in purple. (b) Picture

of the printed photopolymer. (c) Schematic visualization of

the selected 3D structure of the honeycomb dielectric. (d)

Final layout of the printed sensors.

analyzer (HP4194A, HP) to measure their main

electrical characteristics. All the samples were

measured after a set of fixed displacements applied in

increasing/decreasing steps; in fact, after the first

compression part, the sensor was unloaded again. In

each configuration, the impedance/phase spectrum of

the impedance were estimated. Figure 2 presents the

schematic representation of the experimental setup.

Figure 2: Impedance spectrum of the three different devices

taken into consideration.

3 PRELIMINARY RESULTS

The electrical characterization of the devices started

analyzing the impedance of the three devices at no

load (Figure 3).

Overall Additive Manufacturing of Capacitive Sensors Integrated into Textiles: A Preliminary Analysis on Contact Pressure Estimation

197

Figure 3: Schematic representation of the experimental

setup.

Figure 4: Increase of the three device’s capacitance due to

the applied force. Solid circles depict the experimental data,

while the lines are the respective fitting lines.

The behavior of all the devices is mainly

capacitive, up to 10 MHz. At higher frequencies, in

Sample C, other phenomena related to different

effects, such as parasitic ones, are evident. From those

measurements, it is possible to underline how the

biggest value of capacitance corresponded to Sample

C, which in fact presented the bigger surface area. On

the other hand Sample A presented a bigger

capacitance value with respect to Sample B thanks to

the higher ratio of resin in the dielectric part of the

device. In Figure 4, the increase of the device

capacitance related to the applied force is underlined;

in fact, in this figure, it is possible to observe the huge

difference in terms of sensitivity introduced by the

geometrical form factor. Again, as expected, Sample

A presented a higher value of sensitivity with respect

to Sample C, while normalizing for the surface area

of the plates thanks to the bigger permittivity of the

resin compared to the one of air. The achieved

coefficient of determination is around 0.9. This may

be explained by considering the non-linear behavior

as highlighted by the hysteresis (Figure 5); the main

differences between the compression and the

releasing parts are related to the viscoelasticity of the

used dielectric resin as well as to the effect of

compression in the internal microstructure of the

dielectric part that introduces non-linearity in the

behavior of the overall device.

Figure 5: An example of the achieved data where the

phenomenon of hysteresis is underlined. In blue it is shown

the compression part of the experiment, while the red points

were the ones achieved while releasing the force in steps.

4 DISCUSSION AND

CONCLUSIONS

In this work, a set of pressure sensors on textiles were

produced using only the possibilities of additive

manufacturing such as stereolithography and aerosol

jet printing. Three different devices were produced to

underline the main differences introduced by the

dielectric structure both in terms of internal filling

(honeycomb versus filled structures) and plates’ size

(10 x 10 mm versus 20 x 20 mm). The achieved

preliminary results, in terms of variation in

capacitance and sensitivity, showed an increased

sensitivity due to the increase of the plate size (6.998

pF / kgf for Sample C) as well as considering the filled

structure (1.1278 pF / kgf for Sample A) versus the

honeycomb (0.814 pF / kgf); these findings allowed

us to verify the design assumptions as well as to

assess the feasibility of the proposed approach.

Comparing the proposed approach with respect to

the most recent research on flexible capacitive

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pressure sensors, we found several studies, which

were specifically focused on the fabrication,

characterization, and sensitivity enhancement of this

kind of sensor. In fact, Zhao et al. presented an

interesting approach concerning the use rapid

prototyping of flexible capacitive pressure sensors

based on porous electrodes (Zhao et al., 2023)

whereas He et al. and Yang et al. described a

capacitive pressure sensor with enhanced sensitivity

and fast response to dynamic interaction (He et al.,

2018) . Interestingly, Ye et al. reported the possibility

of realizing all-fabric-based flexible capacitive

sensors, underling the tremendous interest for

healthcare monitoring, soft robotics, and

human−computer interface (Ye et al., 2022).

Furthermore, the sensitivity-optimized flexible

capacitive pressure sensor microstructured dielectrics

represented a promising approach in the optimization

of the range of measurement and overall sensitivity

optimization (Hua et al., 2023; Li et al., 2021; Ma et

al., 2023; Pignanelli et al., 2019) . Indeed, these

studies emphasized aspects such as sensitivity, range

of measurement, response time, and novel fabrication

techniques.

In general, the results of our study align with the

ongoing research efforts to enhance the overall

characteristics of flexible capacitive pressure sensors.

By comparing our findings with the existing

literature, we can further validate the significance of

our research and identify potential areas for future

development and improvement. However, further

work is still needed in order to tune the device

mechanical and electrical characteristics in order to

improve their performance, such as increasing the

maximum working range, reduce hysteresis and adapt

them to the specific application field.

ACKNOWLEDGEMENTS

This study was carried out within the MICS (Made in

Italy – Circular and Sustainable) Extended

Partnership and received funding from the European

Union Next-GenerationEU (PIANO NAZIONALE

DI RIPRESA E RESILIENZA (PNRR) – MISSIONE

4 COMPONENTE 2, INVESTIMENTO 1.3 – D.D.

1551.11-10-2022, PE00000004). This manuscript

reflects only the authors’ views and opinions, neither

the European Union nor the European Commission

can be considered responsible for them.

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