As a strain sensor for wearable computing, the need is to have a flexible sensor, which changes shape with the body it is placed upon (torso, arm, leg, etc.), and produces a reproducible change in resistance in response to mechanical strain, ideally in a linear way for easy integration into a system design. Furthermore, the signal must be reliable over the lifecycle of the product it is used in with regards to signal stability, aging, drift, etc. after subjected repeated mechanical cyclic loads. Conventional tensile strain sensors have an operating range of a few percent, with large strain variations attaining 10% strain as with the HBM LD20 high strain gauge . In the conventional metal strain gauge design, a change in electrical conductivity in a thin wire or foil in response to mechanical elongation is measured to determine the strain on the structure being investigated .
The performance of strain sensors is generally reported by referencing the gauge factor (k) according to Equation (1):��RR=k��ll(1)where R is the resistance at a strain of zero, l is the length at a strain of zero, while DR and Dl are the changes in resistance and length due to an applied mechanical strain. The gauge factor (k) for conventional strain gauges is generally close to 2 . Beyond the mechanical strain limit of the sensor the resistance signal becomes unstable due to excessive strain of the sensing element and eventual mechanical failure of the structure occurs. In the current application, a strain of 20%�C100% is required, and for this reason traditional strain sensors could be not be employed.
Polymer-based materials would be able to fulfill the mechanical deformation requirements. Monofilament fiber strain sensors are an ideal form for wearable computing applications since they can be integrated into clothing in an unobtrusive way. Piezoresistive materials are often used in sensor designs where relationships between deformation and electrical resistance can be characterized and used in circuit design. However, research into piezoresistive sensors has GSK-3 mainly focused on the doping of silicon structures, and not on larger scale flexible sensors. Flexible elastomeric sensors can be achieved by using dielectrics and electro-active polymers (EAPs), elastomers with carbon-based electrodes have been investigated, but generally with the desire to develop flexible force actuators.
Alternatively, a conductive polymer composite (CPC) can be created by combining a conductive filler (e.g., silver, carbon nanotubes, carbon black) and a polymer matrix (elastomer, thermoplastic, etc.).1.2. Elastomer Capacitor SensorEAPs based on elastomers coated with conductive layers have been investigated for flexible force actuator applications. In a sensor configuration, highly flexible capacitive sensors can be created .