Embedded Textile Sensing System for Pressure Mapping and

Monitoring for the Prevention of Pressure Ulcers

Susana Pereira

, Joana Fonseca

, Joana Almeida

, Ricardo Carvalho

, Pedro Pereira

and

Ricardo Simoes

Polytechnic Institute of Cávado and Ave (IPCA), Barcelos, Portugal

Centre for Nanotechnology and Smart Materials (CeNTI), V. N. Famalicão, Portugal

Keywords: Pressure Ulcers, Piezoresistive Sensor, Textile Sensing Matrix, Monitoring System, Body Pressure Mapping.

Abstract: Despite improvements in medical industry and consequent modernization of the biomedical devices and

healthcare, pressure ulcers prevalence remains high particularly in hospitalized patients that present little or

no mobility. This kind of skin injury affects the patients’ quality of life and their caregivers, and on the other

hand, increases directly or indirectly the healthcare costs. Thus, the monitoring and early identification of the

risk factors that lead to the development of pressure ulcers is important to decide what are the appropriate

preventive measures. The present work aims to present a new concept of a pressure sensing and monitoring

system, able to detect the pressure exerted on the surface. In this case, the system consists of a sensing matrix

made from a kind of commercial piezoresistive sensors embedded in a textile substrate. The solution presented

can be used together with an actuation system, which will reply in order to allow the pressure relief according

to the feedback from the pressure monitoring system.

1 INTRODUCTION

People with severe motor limitations have, in most of

the cases, a decrease of sensitivity in the body´s areas

in contact with support surfaces. In addition, their

limited mobility doesn’t allow them to frequently

change position autonomously. As a result of these

complications, the people tend to become bedridden

which, consequently, may result in development of

pressure ulcers, also known as decubitus ulcers

(Rocha et al., 2008). According to the European

Pressure Ulcer Advisory Panel (EPUAP), National

Pressure Ulcer Advisory Panel (NPUAP) and Pan

Pacific Pressure Injury Alliance (PPPIA), a pressure

ulcer is a localized injury of the skin and/or

underlying tissue, usually over a bony prominence, as

a result of pressure, or pressure in combination with

shear (National Pressure Ulcer Advisory Panel,

2007), (Superfícies de apoio na prevenção das

úlceras de pressão, no date). Although there are

several risk factors which may trigger the

development of pressure ulcers, the critical

determinants include the intensity and duration of

pressure, and the tolerance of skin and its support

structures to pressure (Murray et al., 2001), (Rocha et

al., 2006), (Lyder and Ayello, 2008). Usually, a

pressure ulcer occurs when soft tissues are

compressed between bony prominences and an

external surface for a prolonged period of time.

Pressure leads to the injury when it is higher than

blood pressure within capillaries (a threshold of

32 mmHg is widely indicated as the point at which

intracapillary pressure is overcome), resulting in

capillary collapse and, consequently, insufficient

sanguineous irrigation (Murray et al., 2001), (Rocha

et al., 2008), (Lyder and Ayello, 2008), (Dealey,

2012), (Menoita et al., 2012). The development of a

pressure ulcer can occur in a short period of time

(within 2 to 6 hours), which makes it necessary to

adopt timely preventive measures, namely, identify

patients with higher risk and/or more vulnerable to

the development of pressure ulcers (Lyder and

Ayello, 2008).

The common preventive strategies widely used in

these cases, as a fundamental complement in the

pressure ulcers treatment, include the use of pressure

reduction devices or support surfaces. Among this

kind of devices there are the static devices that

provide a constant pressure redistribution, increasing

the contact surface with the skin and reducing the

force exerted per unit of area; and dynamic devices

Pereira, S., Fonseca, J., Almeida, J., Carvalho, R., Pereira, P. and Simoes, R.

Embedded Textile Sensing System for Pressure Mapping and Monitoring for the Prevention of Pressure Ulcers.

DOI: 10.5220/0007690802910296

In Proceedings of the 12th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2019), pages 291-296

ISBN: 978-989-758-353-7

291

which offer a cyclically variable pressure (Rocha et

al., 2006). Static devices include mattresses, covers,

cushions, wheelchair cushions and positioning

supports made from viscoelastic materials, memory

foam, gel or water and air. In the other hand, dynamic

devices comprise alternating and low-air-loss

mattresses, air fluidized beds, air cells with

alternating insufflation, dynamic flotation systems

and continuous low pressure devices, among others

(Rocha and Miranda, 2006), (Fulton and Monro,

2009), (McInnes et al., 2011), (Call and Black, 2015).

Despite the benefits of these devices in preventing

pressure ulcers, they also feature some limitations,

operate preprogrammed and alternately, providing

the same pressure redistribution, not effectively

removing body pressure from high pressure points

and not adapt to different pressure and risk situations.

Currently, the market has been focusing on new

solutions which incorporate monitoring or sensing

systems, allowing the measurement of the pressure

exerted between the patient and contact surface.

Basically, these systems supply mapping the pressure

distribution in order to identify areas of the body that

are under elevated pressures (Sensor Products Inc., no

date), (Mattress Retail & Design | XSENSOR

Technology Corporation, no date). However, these

devices only allow the measurement or detection of

pressure, not providing the relief or redistribution of

the pressure, furthermore, its use in pressure ulcers

prevention has not been reported.

The present work is part of a research project,

designed ActiveRest, which has as main objective the

development of a new concept of a smart textile

mattress guard that through a body pressure mapping

system in combination with an intelligent actuation

system, will relieve the pressure on the user,

preventing the development of a pressure ulcer. In

addition, the ActiveRest project seeks to create a

solution that integrates three development stages, as

represented in the diagram of the Figure 1.

Figure 1: Main objectives of the ActiveRest project.

This paper presents the work carried out in the

design and development of a pressure sensing matrix,

based on the selection of flexible and adaptable

materials, in order to create a system that allows

pressure monitoring through the mapping of body

pressure.

2 FORCE SENSING RESISTORS

Currently, on the market and in scientific literature

there are numerous kinds of transducers that convert

force into an electric quantity. In this case, force

sensors can integrate sensors involving a variation of

an electrical property (resistance, capacitance, or

impedance), sensors generating a charge

displacement (piezoelectric), among others that use

different physical quantities (light, magnetic field,

etc.) (Giovanelli and Farella, 2016).

A promising type of pressure sensor (the terms

force and pressure are used as synonyms, considering

that force is pressure over a known area), and widely

used in several applications, is force sensing resistors

(FSR), sometimes called piezoresistive sensors.

Piezoresistive/force sensing resistors present some

advantages in relation to other force sensors, namely,

can be fabricated using flexible materials, which

make them able to adapt to the place where they are

inserted; are very robust against noise and the

conditioning electronics is simple. Moreover, the unit

costs are relatively low (Giovanelli and Farella,

2016), (Overview | Force Sensitive Resistor (FSR) |

Adafruit Learning System, no date).

This study is focused on a type of commercial

force sensing resistor, FSR 402 model of Interlink

Electronics (Figure 2) with 14,7 mm diameter active

area (FSR 402, no date). It is a robust polymer thick

film (PTF) sensor that exhibit a decrease in resistance

with increase in force applied to the surface of the

sensor. This sensor is fairly low cost and easy to use.

Figure 2: FSR 402 model of Interlink Electronics with

14,7 mm diameter.

2.1 FSR Study and Calibration

The experimental setup implemented to study and

characterize the accuracy of the sensor's response is

represented in Figure 3. In this case, on the sensor's

surface different weights values (150, 300, 450, 600

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and 750 g) were placed, and a bench multimeter was

used to measure the sensor's resistive value, which

changes according to the applied force. The

multimeter was connected to a computer that stores

the data that result of the pressure sensor reading. In

order to ensure that the weight was uniformly

distributed over the sensor´s sensing area, two types

of materials with different stiffness and texture were

tested, namely an acrylic circle and a felt circle, with

a slightly lower diameter than the sensor sensing area

(14 mm), as shown in Figure 4. These materials were

used as spacers between the contact area of the sensor

with the weight.

Figure 3: Experimental setup used to study of the FSR

sensor response.

Figure 4: Acrylic and felt circles, respectively, used as

spacers between the FSR sensor and the weight.

2.1.1 Results Analysis

Initially the sensor’s response was analysed in regards

to the sensibility and response time, using the acrylic

spacer. Figure 5 shows the graphical representation of

the results obtained in the sensitivity study. The graph

represents the relationship between the sensor

conductance as a function of the pressure applied

when placed on the sensor the different weights

mentioned above.

As regards the study of the sensor’s response time,

it is represented by the graph in Figure 6. In this case,

a defined weight was used and the sensor´s response

was measured over a period of time, verifying if the

sensor reading remains constant on each sampling.

Figure 5: Sensitivity study of FSR sensor using an acrylic

spacer between the sensor and the weight.

Figure 6: Response time study of FSR sensor using an

acrylic spacer between the sensor and the weight.

According to the results obtained in both studies,

for the acrylic spacer, it can be observed some

inaccuracy in the results, so that the sensor´s response

does not vary uniformly. As an alternative to acrylic,

another material with lower stiffness was analysed,

that is the felt spacer. Figures 7 and 8 represent the

graphs with the results obtained using this material,

for the study of the sensitivity and the response time

of the sensor, respectively.

Figure 7: Sensitivity study of FSR sensor using a felt spacer

between the sensor and the weight.

Embedded Textile Sensing System for Pressure Mapping and Monitoring for the Prevention of Pressure Ulcers

293

Figure 8: Response time study of FSR sensor using a felt

spacer between the sensor and the weight.

Regarding the results obtained using a felt spacer,

the sensor showed better results and greater

sensitivity. From the graphic analysis, it was possible

to observe that commercial piezoresistive sensors are

quite accurate and have a short response time, which

is about 1 minute. This improvement can be related

with a more uniform and adequate distribution of the

force over the sensor’s contact area, minimizing some

errors associated with inconsistencies in the force

distribution, which can arise from substrate stiffness.

In order to analyse the consistency and stability of

the sensor readings, as well as the variability of the

implemented measurement system, the sensor’s

repeatability study was also performed. To this end,

different measurement series were carried out, in a

total of three repetitions. The results obtained with

this procedure are shown in graphical representation

of the Figure 9. From the graph it is observed that the

sensor’s response varied slightly throughout the tests,

however the different is minimal, which shows good

results in the repeatability of the sensor’s response.

Figure 9: Repeatability study of FSR sensor over three tests,

using a felt spacer.

3 APPLICATION OF FSR IN

REAL SOLUTIONS FOR

PRESSURE MONITORING

3.1 Implemented System Overview

The behaviour of piezoresistive commercial sensors

on pressure monitoring applications, when used

together with a commercial dynamic pressure

mattress (INVACARE LIBER L803/ESKAL L839),

was studied. For this analysis a calibrated weight of

10 kg was used and placed onto a surface of wood in

order to distribute the force applied through several

air cells (Figure 10). The FSR sensor was fixed and

centred to an air cell at the mattress bottom

(Figure 11). For this analysis two tests were

performed, first using the felt spacers between the

sensor and the air cell, and then without de felt spacer.

Each test was repeated 2 times and lasted

approximately 30 minutes to ensure that both

operating cycles (filling and emptying) of the

mattress were studied.

Figure 10: Calibrated weight of 10 kg placed on the air cells

of the dynamic mattress.

Figure 11: FSR sensor fixed to an air cell of the mattress.

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3.1.1 Results Analysis

The results obtained with and without the felt spacer

represented in the graph of the Figure 12. Analysing

the different curves, it is observed that the sensor

presents better results when the felt spacer is used,

presented a greater sensitivity. Furthermore, it is also

possible to verify the sensor´s capability to detect the

pressure exerted by the weight placed on the mattress

taking into account the alternation of the filling

cycles, featuring a similar behaviour over the time.

On the other hand, due to technical difficulties in

reproducibility of the experimental conditions, it was

not possible to draw any conclusions about sensor’s

accuracy.

Figure 12: Sensitivity study of FSR sensor placed on a

commercial dynamic mattress with a fixed weight of 10 kg.

The tests were performed with and without the felt spacer.

4 PRESSURE SENSING MATRIX

4.1 Textile Sensing Matrix Design

In order to create a system capable to detect and

measure the pressure exerted on a surface, for

example the body pressure exerted between the

patient and the mattress, a matrix composed of 96

piezoresistive sensors was constructed. Each sensor

was previously calibrated and characterized based on

the experimental procedure as described before. From

the sensor’s calibration process were determined the

calibration parameters by linear regression. As

support for the sensing matrix, a textile substrate was

used, on which the 96 sensors were arranged in 12

rows and 8 columns and fixed by a sewing process.

The felt spacers were fixed to the sensor’s contact

area using an adhesive tape and the connections

between the sensors were made using coated

conductor wire (0.7 mm diameter, 50 torsions per

meter, 0.694 Ω electric resistance and electric

insulation of PFA). Figure 13 shows the constructed

textile matrix, and its respective connections between

the piezoresistive sensors.

Figure 13: Sensing matrix (12x8) from commercial

pressure sensors of Interlink Electronics.

4.2 Body Pressure Mapping

Tests with the sensing matrix were carried out using

the dynamic mattress of INVACARE. The matrix

was positioned under the mattress, so that each sensor

corresponds to an air cell. The pressure exerted on the

dynamic mattress surface resulted from the

bodyweight distribution of a person with 79 kg laid

along the mattress. This experimental procedure was

adopted in order to measure the pressure variation

detected by each sensor during the operation cycles

of the alternating pressure device. The acquisition and

display software was developed using Labview, and

the implemented program provides the serial

communication with the data acquisition board, and

the display of the pressure values mapped to a color

scale. This procedure can be observed in Figure 14.

Figure 14: Body pressure mapping using the piezoresistive

sensor matrix developed and the acquisition software.

Embedded Textile Sensing System for Pressure Mapping and Monitoring for the Prevention of Pressure Ulcers

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5 CONCLUSIONS

This paper presents the work carried out in the design

and development of piezoresistive sensor matrix used

to detect and monitor the pressure. The

characterization and calibration procedures of the

FSR sensor was presented, as well as, tests in pressure

measurement applications, where the influence of the

use of different spacers material (acrylic and felt)

with different stiffness was analysed. In this case, it

was observed that the use of a spacer material, with

low rigidity, namely the felt spacer, coupled to the

commercial FSR sensor was found to greatly improve

the accuracy of pressure measurement. The sensor’s

response using the felt spacers showed greater

sensitivity.

From the graph of Figure 12 it is possible to see

that the sensor starts to respond from a conductivity

of approximately 0,05 mS which, by data from

graphic representation of Figure 7, correspond to a

pressure values below 5 kPa. With this, it is observed

that the piezoresistive sensor can detect in the

pressure range where the value of the capillary

tension inside the tissues (32 mmHg, corresponding

to about 4 kPa) is inserted, from which the pressure

begins to cause lesions on the skin, especially in

regions of vulnerable bone prominence. Thus,

according to the study developed around the use of

commercial piezoresistive sensors in pressure

detection and mapping systems, it was found that this

kind of sensors allows the pressure measurement,

including when used with other pressure ulcer

prevention systems. The developed sensing matrix

was able to read pressure variations, presenting a

satisfactory performance, which proves its usefulness

in body pressure monitoring applications.

ACKNOWLEDGEMENTS

The authors acknowledge Graça Bonifácio, Sandra

Ventura, José Casquilho, Miguel Ribeiro for their

contributions. This research is supported by FEDER

funds through the COMPETE 2020 Programme

under project ActiveRest (project 18011 of the

33/SI/2015 call).

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