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Printed Electronics World
Posted on November 20, 2007 by  & 

Printed strain and stress sensors could be used in sports medicine

US - Smart textiles are expected to become an important family of products predicts researchers at the US National Textile Centre, University of Massachusetts Dartmouth.
The properties of conducting polymers deposited onto textiles were studied over 10 years ago by workers at Los Alamos and Milliken who also tested these materials as gas sensors. More recently, workers at Wollongong have demonstrated that elastic textiles impregnated with conducting polymers, by in situ chemical polymerization, can act as strain sensors that can be used to track the motion of human joints. A similar approach can be used to make pressure-sensing foams. Others have studied the strain sensing mechanism in more detail and have shown that two effects are important.
The researchers at NTC aimed to print stress and strain sensors on textiles to provide information about the actions of the body for the purposes of controlling and monitoring muscle action. They envisaged printing arrays of conductive piezoresistive sensors onto fabrics and using these to provide semi-quantitative information about the motion of a piece of clothing or other textile. The conducting polymer ink is a nanoparticulate suspension which dries to a conducting film. The conductivity is thought to involve quantum tunneling between particles.
The following findings were reported by Prabir K.Patra, leader; Paul D. Calvert, Chi Hau Chen, Qinguo Fan (UMass Dartmouth):

Formation of sensors and connectors

"We have successfully inkjet printed both silver and con-ducting polymer lines onto textiles using a home-built printer capable of repeatedly printing over the same area to build up thick ink lines. Conducting leads were formed in two steps://
(1) inkjet print seed layers on fabric and
(2) convert these seed layers into metallic lines by electroless plating.
We printed a suspension of 1.3 % by weight poly- (3,4- ethylenedioxythiophene)-poly-(4-styrenesulfonate) PEDOT-PSS onto mercerized plain, twill and sateen cot-ton fabrics, each with four different orientations. The printed lines were about 5 cm long by less than 1 mm wide.~

Resistance and strain in printed fabrics

The resistance of silver is very low, so these lines serve as connectors. The resistances of the conducting polymer lines are in the range of kilo-ohms per cm, and this resistance changes with strain in the fabric. Hence the conducting polymer can be used as a local strain sensor. The conductivity of the PEDOT in the coatings was about 25 S per cm. The resistance decreases as more conducting polymer ink is deposited. The first few strain cycles showed rapid incremental increases in resistance because cracks form on the surface layer of the printed sensor. On the other hand, the polymer embedded in the fabric flexes without cracking. After the initial period, the resistance value decreased with stretching and increased with relaxation (opposite to the other available strain sensor).
We continue to research this peculiar behavior. We also investigated the effect of fabric weave and orientation on the gauge factor (ratio of fractional change in resistance to fractional strain) and the effect of cyclic strains. All fabrics tested displayed a high gauge factor of 5 or more; whereas, metallic strain sensors have gauge factors in the range of 1.5 to 2.

Monitoring human motions

An assembly of sensors and connectors was attached with tape on a human knee and wrist. We then measured the resistance caused by bending the knee and twisting the wrist at both slow and fast speeds. From our original digital signal data, we reduced noise by employing the mean filter, a simple sliding-window spatial filter that replaces the first value in the window with the average (mean) of all the data values in the window.
The cyclic knee bending resistance data was displayed as a pseudo-sinusoidal wave of increases and decreases in resistance. For the slow knee bending experiment, approximately 7 pseudo-sinusoidal cycles occurred in 50 sec. For slow wrist twisting, approximately 12 pseudo-sinusoidal cycles occurred in 50 sec.
We also graphed the power spectral density (PSD) function which measures the distribution of power with the frequency of the random process. The frequency peaks from the two trials are not fixed from trial to trial, probably caused by the human motion data collection. However from 4 trials of slow knee bending motion, there were three major frequency peaks in the PSD, all concentrated below the normalized frequency of 1.0. Most of the PSD was concentrated below a normalized frequency of 0.5."
Printed conducting polymer piezoresistive strain sensors on fabrics can give a high negative gauge factor and so high sensitivity to small strains. Good performance derives from the polymer embedded within the yarn, while the surface layer cracks and becomes ineffective after one cycle of strain. The details of the sensing mechanism are unclear but it depends on improved fiber-to-fiber contact during tensile strain of the twisted yarns.
These sensors could be used in medical rehabilitation and sports medicine.
Top image from NTC shows sensors and connectors placed on knee.
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