Design and Development of Textile Based Strain Sensor for Medical Textiles Applications
Authors: Uzub Muhammet 
Published: July 2019

Integrating strain sensors into textiles opens new applications. For example when attaching the sensors in the knee or elbow region, the bending angle of the joints can be measured. Such a measurement can be used in sports (e.g. to measure the number of steps and the speed while jogging) or in rehabilitation to give the patient an online feedback whether he practices the injured joint in the appropriate range. Measurement of fabric strain flexion of human joints caused an elongation of the affixed skin up to 45% in the region of the knee and 30% in the region of the lower back. Consequently, strain sensitive materials that react to elongation were integrated into tight ﬁtting garments for a reconstruction of the joint angle. One could even think of a whole body posture measurement which enables a quality and quantity measurement of exercise conduction in ﬁtness training and rehabilitation.

Keywords

Medical textiles; Conductive yarns; Flexible strain sensors; knitted structures; Rehabilitation

Introduction

The fabrication of electronic systems onto substrates represents a breakthrough in many areas of application, such as virtual reality, tele-operation, tele-presence, ergonomics and rehabilitation engineering. The need of the hour is to develop such an integrated effective system for medical textile applications which is not only flexible but also conformable to the human body, for such type of flexible strain sensors, one should focus on some crucial parameters like development of optimized composite conductive yarns used for the substrate and flexible strain gauges based on composite conductive yarns. Measuring strain in textile is a problem addressed by different research groups. One of the ﬁrst groups who mentioned this problem was Tognetti et al. They built a knitted strain sensor which was integrated into a jacket and was used to measure upper body movements [1]. Gibbs et al. designed a textile potential divider to measure joint movements. In a thin layer of poly-pyrrole (using chemical vapor deposition) was applied on the fabric substrate at low temperature. With this conﬁguration, a measurement range of up to 50% strain and a strain sensitivity of 80 was achieved [2]. Mattmann et al. investigated ethylene-vinyl acetate (EVA) and ethylene-propylenediene rubber (EPDM) composites for sensor applications. Such carbon composite materials show high relaxation behavior and creep which means that the change in resistivity is influenced by the strain rate [3]. An elastomer/carbon black-composite (CE) was used by Tognetti et al. to measure arm and ﬁnger movements. This sensor showed high relaxation behavior too. Carbon black/elastomer- and rubber-composites need to be cured after compounding and shaping [4]. In contrast, when using thermoplastic elastomer (TPE) based composites, curing is not necessary and simple thermoplastic processing technology can be used for shaping. Therefore, such polymers are interesting when developing strain sensors with large strain. Yamada et al. presented a sensor of a thermoplastic elastomer and ﬁlled with carbon black (27.6 by %vol.). They focused on noncyclic strain sensing and looked at influences of temperature and humidity on the resistance. The sensor showed a dependence of the resistance on the humidity but not on the temperature [5]. In this paper we use a similar composition (50wt–%/32vol–% carbon black) but focus on the characterization of the sensor's dynamic behavior, as sensors integrated into textiles are exposed to repeated strain cycles. The conductive fibers can be incorporated into the yarn (Figure1) and then subsequently incorporated into fabric. Integrating strain sensors into textiles opens new applications in medical textiles, for example, when attaching the sensors in the knee or elbow region, the bending angle of the joints can be measured. Such a measurement can be used in rehabilitation to give the patients an online feedback whether they practiced the injured joint in the appropriate range or not. One could even think of a whole body posture measurement which enables a quality and quantity measurement of exercise conduction in ﬁtness training and rehabilitation. Measuring the posture using strain sensors enables an unobtrusive integration into textile currently not possible with other sensors (e.g. accelerometer, gyroscope, and magnetometer). This calls for a combined effort on one hand from the materials side in terms of developing functionalized fabrics and tackling integration issues, and also from the computer applications side to provide user-friendly interfaces. Through characterization of their response and assess functionality for specific applications. These are not only valuable to the athlete who wish get the most from their training regime and assess their progress but also for different patient who need rehabilitation. This project will focus on the development of optimized composite conductive yarns and strain sensors which measures strain in textiles. Furthermore, these strain sensors will be integrated in to braces which could be used for many applications such as patient rehabilitation and monitoring and controlling of athlete movements. The scope of this work is to develop a device capable of measuring strain, based on conductive textile yarns and using them as strain sensors. Furthermore, these sensors can also be incorporated into intelligent braces which can be used for endless medical textile applications.

Material

The conductive yarns will be prepared by TPE (thermoplastic elastomer) based composites and for shaping simple thermoplastic processing technology will be used. The TPE material such as SEBS-Block copolymer (THERMOLAST Kr (FD-Series)), Compound No. TF7- ATL produced by KRAIBURG TPE GmbH, Germany may be used. Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers, are a class of copolymers or a physical mix of polymers (usually a plastic and a rubber) which consist of materials with both thermoplastic and elastomeric properties. While most elastomers are thermosets, thermoplastics are in contrast relatively easy to use in manufacturing. The carbon black powder (ENSACO 250 produced by TIMCAL, Belgium) may be used and will be added in TPE during fiber manufacturing.

Method for producing filament

For the ﬁbre manufacturing we will use extrusion and drawing method. The TPE pellets will be ﬁlled in an electrically heated torque rheometer with roller blade conﬁguration. After melting the thermoplastic part of the TPE, carbon black powder will be added and subsequently homogenized and dispersed into the polymer. The rotation speed may be constant (10 rpm) during the whole procedure. After compounding, the ﬁbre will be produced by using a capillary rheometer, and an extrusion die. The composite material will be preheated and compacted in the cylinder of the rheometer. By using the furnace (proposed) furnace we can produce activated carbon through stabilization and carbonization of different materials like polyamide and PAN (Polyacrylonitrile) in the presence of inert atmosphere. Then through ball milling machine (proposed) we can create nano particles of this activated carbon. The coating of these nano particles will give us good results regarding electrical and thermal conductivity. Strain sensor Preparation: Textile Stretch Sensor will be manufactured by Flat Hand Knitting through incorporate conductive yarns into fabric. With the help of weft knitted machine this sensor will be prepared. For the effectiveness of sensors, we may change different parameters such as Loose knitted with and without lycra yarn Tight knitted with and without lycra yarn Single Plied knitting Double plied knitting Testing: The Strain sensors will be characterized in terms of quasi-static and dynamic electromechanical transduction properties. Thermal and aging properties of the sensing fabrics will also be determined through cyclic loading on UTM machine. Conductive properties of the Textile Stretch Sensor (TSS) will be tested on ohm meter in BUITEMS Textile lab. All measurements will be performed on Electronic Extensometer (proposed). The resistance will be measured in parallel with a multi meter. These strain measurements will be done at a speed of 200 mm/min which corresponds to a strain rate of 16% = sec (sensor length 2 cm) which is achieved in typical body movements. Brace Performance Testing Following is the picture of the preliminary developed sensor. The target will be the improvement of reproducibility for the accurate results. We may also miniature the circuits involved to make it easy to wear.