Flexible Pressure Mapping Platform for Mobility Monitoring

Applications

S. Cruz

, D. Dias

, J. C. Viana

and L. A. Rocha

1, 2

Institute of Polymers and Composites/I3N, Polymer Engineering Department, University of Minho,

Campus Azurém, Guimarães, Portugal

Center ALGORITMI, University of Minho, Campus Azurém, Guimarães, Portugal

Keywords: Health Monitoring Device, Flexible Pressure Mapping Systems, Flexible PCB.

Abstract: The goal of the work presented here is the development, integration and testing of an innovative

technological approach to be the basis for a new product and service for markets associated with the

“Health” vector. Our research focuses on a Physiological computing approach, where a polymeric flexible

detection system, working as the sensing element is used as an input channel, and a computing system is

responsible for the physiological signals synthesis. The proposed solution provides a simpler, lower cost and

larger scale manufacturing production of polymer based sensors, along with an electronic interface and the

software design. The sensing platform consists in a flexible PCB (Printed Circuit Boards) manufactured

using conventional technology (defining the electrical connections and the capacitors dimensions) together

with two flexible polymeric membranes (TPU) printed with conductive ink (Plexcore®) for definition of the

electrodes. A Capacitance to Digital Converter (CDC) is used to measure the capacitance of the sensors, and

a graphical interface in MATLAB allows real-time visualization of data. Current results performed on the

pressure sensors indicate the feasibility of the approach.

1 INTRODUCTION

Nowadays, humans are highly dependent on the use

of computers and therefore, they become highly

commonplace in work and leisure areas. The

availability of good information processing

capabilities and sensors is pushing for new

applications relating physical monitoring. For this

reason, the interest on Flexible Pressure Mapping

Systems (FPMS) for use on non-planar surfaces

grew tremendously (Engel J et al., 2006) (Kim &

Ho, 1998) in areas such as aerospace, automotive,

biomedical and robotics (Chiang, K. Lin, & Ju,

2007) (Cheng, Lin, & Yang, 2010) (Petriu, McMath,

Yeung, & Trif, 1992) (Pritchard, Mahfouz, Evans,

Eliza, & Haider, 2008) (Yeung, Petriu, McMath, &

Petriu, 1994).

Flexible Pressure Sensors (FPS) are transducers

that measure pressure distribution between two

contact surfaces, with the particularity of being

flexible. These sensors consist in a set of array of

sensors elements to force or pressure that are

embedded in a substrate. The force or pressure

sensors are connected to electronic devices

responsible for the signals reading sent by the

sensors (several times per second) and

communicates to the computer. Specialized software

enables reading of data in real time providing an

image in 2D or 3D graphics. Analysis tools acquire

the pressure peak, the center of pressure or force, the

output signal with respect to time and various

statistical parameters, thus visualizing the magnitude

and distribution of forces applied to the FPS.

Flexible polymer detection systems provide a

better contact area and therefore more accurate

readings thanks to its ability to fold/roll compared to

traditional hard circuits. In medical applications a

wide variety of configurations is needed and

therefore it requires the pressure sensors to be

flexible (bendable to a few degrees). Also, the

sensitive area should be as small as possible.

Depending on the spatial resolution required for the

intended application, the diameter for the sensitive

area of the sensor can range from 1 mm

to 1 cm

(Ashruf, 2002)). A high precision, reproducibility

and selectivity are other essential requirements for

the sensors. Flexible sensors have the capacity to

follow all the movements, capacity to stretch to

Cruz S., Dias D., Viana J. and Rocha L..

Flexible Pressure Mapping Platform for Mobility Monitoring Applications.

DOI: 10.5220/0004719500330039

In Proceedings of the International Conference on Physiological Computing Systems (PhyCS-2014), pages 33-39

ISBN: 978-989-758-006-2

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

some degree (to measure correctly the applied

forces) and have low thickness (thick sensors tend to

provide erroneous readings (Ashruf, 2002)) which

makes them ideal candidates.

The use of Physiological Computing systems in

health care is necessary and presents significant

advantages. For instance, FPMS offers the

possibility of obtaining pressure readings measured

in the contact surface, revealing information which

is not readable to the naked eye. This technology is

interesting since it enables reading, recording,

processing and analysing the physiological physical

states of the user and continuously monitor mobility,

without the minimum disruption or loss of

information. Data acquisition and display plays an

important role in applications where for example, it

is useful and advantageous to make an evaluation of

a signal (measurement of pressure to prevent

pressure ulcers in patients in wheelchairs or hospital

beds) (Xsensor 2013).

In addition, an acquisition system can further

contribute for health monitoring situations where is

essential to follow patients during rehabilitation with

the best action/prevention, diagnosis and treatment.

This would also increase the autonomy of patients

during rehabilitation, during the postoperative

period; in medical facilities or in home environment,

thereby reducing the length of hospital stay. The

costs of surgery, treatment and rehabilitation are

high, not to mention the collateral consequences at

the social and psychological level. It is of clinical

interest, as well as financial interest, to eliminate the

gap between physiological data handling and

human-computer interaction, to reach the most

reliable diagnose for better action/prevention

treatment, to increase the comfort of patients and

eliminate some of the costs. Another element that

deserves consideration is that these systems allow to

perform intervention studies that assess the progress

of treatment and rehabilitation, as well as the

effectiveness of new treatments.

The objective of this study is to develop a

physiological computing approach based on a

flexible pressure mapping system for physiological

sensing and software for Physiological data handling

(see Figure 1). It will combine textile, polymers,

electronics, and psychological studies to develop a

support surface system able to acquire record and

evaluate body pressure.

The importance of real time physiological data

signal reading for prognostic for health monitoring

devices is underlined in this paper. Experimental

results on a prototype system are presented.

The paper is divided in six sections. After a brief

introduction, the system overview, the details of the

manufacturing process, the electronic interface, the

software design and experimental results are

presented. At the end, some conclusions are drawn.

2 SYSTEM DESCRIPION

The flexible pressure mapping system being

developed has the form of a carpet with four equal

sensitive areas as shown in Figure 2. The sensing

zone is about 12x8 cm

. The capacitive sensors

(between 24 and 32 sensors) are placed in these

regions. The capacitive readout electronics are

placed next to the carpet. Each sensor element

consists on a flexible Printed Circuit Board (PCB)

for definition of the electrical connections and

capacitor dielectric dimensions and two flexible

membranes with printed electrodes. It is flexible and

easy to handle. In addition, it will be integrated in

complex atmospheres, allowing the monitoring of

the balance during physical therapy sessions,

indicating the regions of the foot that are exerting

force.

Figure 1: Block diagram of the physiological computing

approach.

3 MANUFACTURING PROCESS

Three main steps are necessary to complete the

proposed fabrication process: the fabrication of the

flexible membranes with the conductive ink, the

fabrication of the flexible PCB and an assembly

step.

3.1 Flexible Membranes with

Conductive Ink

TPU is a material that offers the elasticity of rubber,

PhyCS2014-InternationalConferenceonPhysiologicalComputingSystems

but with improved mechanical properties: good

flexibility, strength and durability, excellent wear

properties and elastic memory. Hence, polyester

based Thermoplastic polyurethane (TPU),

AVALON 65 AB grade, from Huntsman, was

selected for the flexible substrate.

Due to their excellent properties (in terms of

conductivity, inkjet printability and flexibility),

conductive inks became an emerging technology,

penetrating the sensors market and enabling new

applications. Conductive inks are a growing market,

representing a $2.86 billion market in 2012 and

forecasted to rise to $3.36 billion in 2018 (Zervos,

2012). In this work, a conductive ink, Plexcore® OC

RG-1100 grade (a Sulfonated polythiophene ink)

from Sigma Aldrich was selected for the fabrication

of the electrodes.

Figure 2: Sensorial platform scheme (the dimensions are

in cm).

Printing of patterns with conductive inks on

polymers surface enables the realization of “active

polymeric materials”. A high-definition printer

(Xennia Carnelian) was used for printing and

Figure 3: Inkjet printed flexible substrates (with

conductive inks electrodes on a TPU substrate). The upper

membrane is the mirror image of the lower membrane

(Top on the rigth and bottom on the left).

definition of the electrodes of the capacitive sensors.

Inkjet Printing technology is a simple, low cost

and large scale manufacturing production technique

making it a very interesting approach due to both

simplicity and cost of the solution. The conductive

ink was printed in two TPU flexible membranes (see

Figure 3); a substrate for the superior electrodes and

another for the inferior electrodes.

3.2 Flexible PCB’s

The flexible pressure sensors results from the

combination of flexible PCB technology and flexible

membranes with conductive inks. The flexible

PCB’s consists in a flexible substrate of polyimide

(PI), 125 µm thick with copper on both sides (35

µm). The copper is subsequently machined to define

the conductive lines, and then the substrate is open

in certain regions to define the dielectric (air) of the

capacitor. The platform has 24 capacitive pressure

sensors. Multiple geometries were draw with

increasing electrode size in the initial prototype.

This approach allows testing the several

configurations, and selecting later the one that best

suits the application. Figure 4, shows an image of

the flexible PI substrate and respective conductive

lines.

The sizes were selected to have a rest

capacitance between 5-10 pF (5 pF for the smaller

dimensions and 10 pF for the larger). The sensors

are expected to have high sensitivity due to the low

Young Modulus (as compared to silicon based

pressure sensors) of the TPU. No mechanical

reliability issues are expected since TPU has

excellent mechanical properties.

Since the readout electronics requires an

excitation AC signal, all the capacitors are

connected to the bus excitation. Figure 4 (bottom

side) shows the bottom of the flexible substrate. In

Figure 4: The top side (on the left) and bottom side (on the

right) of the flexible polyimide substrate and the

respective dielectric areas. The PCB size is 124 x 95 mm

FlexiblePressureMappingPlatformforMobilityMonitoringApplications

this case, the capacitors have an independent link

which will be multiplexed for sequential reading of

the capacitors. Noteworthy, is the ring around the

dielectrics for electrical connection of the printed

conductive electrodes. These rings enable the

electrical connections to the capacitive electrodes.

3.3 Prototype Sensors Assembly

Finally, the flexible TPU flexible membranes were

bonded with ELECOLIT 414 (a polyester-based,

electrically conductive adhesive) in parts of the

substrate to the flexible substrate of polyimide in

order to manufacture the capacitive sensors (see

Figure 5).

Figure 5: Sensor elements manufacturing process in

which: a) the geometry of copper conductors and the

capacitor dielectric are defined using a PCB flexible

process; b) the electrodes of the capacitors are inkjet

printed using conductive inks and c) the flexible substrates

with electrodes are glued to the printed circuit board.

4 ELECTRONICS INTERFACE

The reading circuit of the capacitive sensors has

been designed considering the sensors and its

manufacturing technology. Regarding the

electronics, the capacitor sensor reading is

performed using a 12-bit Capacitance to Digital

Converter (CDC) AD7150 for each sensory zone

allowing direct interface with the capacitive sensor.

The converter consists of a second-order sigma-delta

(Σ-Δ) modulator.

This low-power IC, 100 μA, has 2 input

channels, with a conversion time of approximately

10 ms and 1fF resolution, for a dynamic range of 4

pF. The use of two multiplexers enables sequential

reading of the 24 sensors comprising each sensory

area with the two multiplexers connected to the two

input channels of the converter. A microcontroller

feeds and controls the multiplexers. When testing a

particular sensor, the sensor is connected between

the excitation bus and the modulator input of the

CDC. The excitation signal is applied during the

Figure 6: Capacitive sensors reading system block

diagram.

conversion and the modulator continuously acquires

the load through the sensor. The digital filter

processes the output of the modulator and the data

can be read by the I

C serial interface established

with the microcontroller. The microcontroller allows

the reading of the corresponding digital capacity

value for each sensor, as well as configures the

CDC. The CDC is connected to a computer, where

the obtained values are recorded by a graphical

interface implemented in MATLAB. This software

provides real time readings and allows data

manipulation.

5 SOFTWARE DESIGN

The software for data acquisition and display is

implemented in the MATLAB graphical

environment. At the current stage of this work, it is a

very simple program providing:

- Serial communication with the microcontroller and

setting of the converter parameters such as the

PhyCS2014-InternationalConferenceonPhysiologicalComputingSystems

conversion mode, the converter sensitivity, the input

dynamic range and others;

- Reading of the sensors capacity througth CDC;

- Real time values display, on a 2D graph, of the

capacity variation as a function of the pressure for

each of the sensors in each sensory area;

- Recording data on file for further detailed analysis.

The software allows testing all sensorial

platform, however, at this time one sensory area of

the prototype was tested, as depicted in figure 7 .

Figure 7: MATLAB graphical environment: Real-time

prototype sensors acquisition.

6 EXPERIMENTAL PROTOTYPE

The assembled prototype (Figure 8) was placed

together with the readout electronic PCB (Figure 9)

inside a pressure chamber (Figure 10) and positive

pressures were applied (ranging between 0kPa-

100kPa). The pressure reading inside the pressure

chamber was preformed through a reference

pressure sensor (TECSIS P3297). Results are shown

in Figure 11.

The assembled prototype sensor flexibility

(bendable to a few degrees) is demonstrated in

Figure 8. The sensing platform has the ability to

bend, and it can be stretched to some degree (to

measure correctly the forces applied).

Figure 8: Assembled Prototype Sensors.

The sensors response to loads was also

performed. The test consisted on placing weights on

top of the sensors. For this purpose an acrylic plate

with an area of 5.10 cm x 7 cm was used, as an

interface surface between the load and the sensors,

to ensure an equal distribution of the weight on all

sensors. The capacity variation was measured for a

weight range between 0Kg-19.2Kg. The sensor

output was acquired at a sampling rate of 100 Hz

and 1000 samples for each sensor. Several sensors

were tested, and the average response of 5 sensors

with circular geometry and different areas is present

in Figure 12.

Figure 9: Assembly of the reading system.

Figure 10: Capacitive sensor test setup.

7 RESULTS AND DISCUSSION

Figure 11 shows the static response of five sensors

with circular geometry with diferent areas. The

sensores capacity increases with the increasing

pressure, however, contrary to what would be

expected, higher sensor area (with 0.7 cm radius)

had the lowest capacity variations (1.68pF) when

compared to the smaller sensor areas (with 0.5 cm

radius) with a capacity variation of 2.955pF. This

unexpected behaviour is due to cross-coupling

between the sensors, i.e., when one of the sensors is

FlexiblePressureMappingPlatformforMobilityMonitoringApplications

actuated, the surrounding sensors are also changing

(with much less variation). Since the smaller sensors

are the ones that have higher cross-coupling (these

sensors are the ones with longer electrical

connections and therefore with larger coupling

capacitances to the neighbouring sensors), they end

up showing a higher variation than the larger

sensors. This same behaviour was observed on the

loading tests (Figure 12) and this problem is already

being addressed in a future redesign of the system

(sensors plus electronics circuits).

Figure 12 depict the variation of sensor

capacitation with load. It can be observed that the

sensors capacitance increases with the load, but once

again, the sensor with higher area didn’t present

more capacitance variations. The 0.7 cm radius

sensor area showed the lowest capacity variation

(1.88pF). The largest variation was seen on sensors

whose area is between the maximum area and the

minimum area, with the sensor electrodes radius -

0.65cm the range of higher capacity (3.015pF).

Figure 11: Pressure sensors static response.

Results for the rectangular sensors were not possible

due to high parallel capacitances that increase the

total sensor capacitance and the system is not

capable of handling these high values (it saturates).

The actual electronic readout and assembly process

(glue used for contact) increases the parallel

capacitance increasing all sensors in general, but,

since rectangular sensors have larger areas, they

show higher parallel capacity influence.

Regarding the system response time, tests results

have demonstrated that the developed reading

system is capable to follow the expected physical

movements (average response time of 0.5 seconds)

during therapy sessions. This acquisition time (0.5s)

is insufficient for some type of exercises (especially

if fast movements are involved), but for simple

equilibrium exercises it might be sufficient. The

acquisition time is expected to be improved in the

next prototype.

The developed software has shown the ability to

read, record, process and analyse the physiological

physical states of the user and continuously monitor

mobility, without the minimum disruption or loss of

information. Overall, the measured response is in

according to what would be expected (increasing

pressure results in a capacitance increase).

This study had some limitations: at this phase,

the capacitive results are higher than it was predict,

which implies improvements in the electronic

readout circuit in order to eliminate the parallel

parasitic capacitance. In the next phase, the

multiplexing strategy will be improved, in order to

reduce the cross-coupling influence. The process

control will be essential to achieve high

reproducibility and desirable sensor specifications.

Nevertheless, pressure is not the most

representative and the most appropriate for testing

validation. In the future, the experimental testing

will be performed with the sensor platform in

contact with the human body and its performance

will be compared to the existing pressure mapping

platforms (Novel 2013)(PPS

2013) (Xsensor 2013).

Figure 12: Sensor response to loads.

8 CONCLUSIONS

This article described the design, fabrication and

experimental results of a flexible pressure mapping

system and its readout electronics interface and

software. Development of a better and efficient

sensor, capable of being integrated into complex

atmospheres, overcoming some of the difficulties

found in Physical Rehabilitation, and able to

measure balance during physical therapy is essential.

PhyCS2014-InternationalConferenceonPhysiologicalComputingSystems

At this phase, the obtained results are very

promising and encouraging. The solution presented

here enables a high density of capacitive flexible

sensors with a simple and inexpensive process.

Nevertheless, there are some issues that must be

improved in the next prototype.

These devices can help the ubiquitous

management of the state of health of a patient, thus

enabling a better performance/prevention, diagnosis

and treatment. The developed sensors can help

humans to benefit from the interactions between

physiological data and computer. In the near future,

with the development of science and technology, we

will the development of new technologies with the

ability to make the connection between humans and

computers.

ACKNOWLEDGEMENTS

This work was co-financed by FEDER, Operational

Programme Competitiveness Factors through project

TICE-Healthy – QREN SI&IDT Mobilizing

Projects.

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FlexiblePressureMappingPlatformforMobilityMonitoringApplications