Impedance Spectroscopy of a Nanocomposite Fabric Thermistor to Determine Its Dielectric Sensing Structure

A nanocomposite temperature sensitive fabric has been developed using electrospun nylon 6 nonwoven nanofiber membrane materials, multiwalled carbon nanotubes (MWCNTs), and conductive polypyrrole (PPy). This material acts as a linear resistor between -3V and 3V, and has a temperature dependent electrical resistance. A general sensor construction procedure was developed to easily and reproducibly produce the fabric temperature sensors using simple laboratory procedures without the need for complex and toxic semiconductor device construction environments. This extremely thin temperature sensitive fabric provides useful temperature data using Ohm’s law and the general equation for the temperature coefficient of resistance, or α, which for this material is calculated for a useful temperature range between 25ºC and 45ºC.

            This functional fabric was designed in order to combat biomedical issues with measuring temperature near the surface of the skin in tight, load bearing situations like prosthetic sockets or shoes. Diabetic patients could benefit from a thin fabric sensor to measure the temperature inside their shoes. Research has shown that if there is a greater than 4ºC difference between the same points on each foot for a diabetic, they are at a greater risk for foot ulceration, which can also point to signs and severity of neuropathy if this is a frequent occurrence. Currently, diabetics would have to remove their shoes and take the temperature of each point on each foot using an infrared gun or even worse, a thermometer. This can be cumbersome and provides the potential for inaccurate results, because their foot temperature fluctuates in different environments and depends on the blood circulation to that foot. Thin fabric temperature sensors inside of a diabetic’s shoes could accommodate continuously monitoring the temperature between points on each foot, making it easier for them to tell if they need to maintain their feet temperatures. In addition, amputees could benefit from measuring the temperature inside of their prosthetic sockets to maintain a comfortable stump socket environment. Surveys have shown that heat and sweat largely contribute to whether an amputee is happy with the comfort of their prosthesis. Smart textiles and clothing are other areas to benefit from a fabric thermal sensor.

            Sensor construction starts by electrospinning nylon 6 nanofibers into sheets on a rotating drum. Pristine MWCNTs are applied to the nylon 6 membranes through vacuum filtration from an aqueous Triton X-114 stabilized solution. PPy is oxidatively polymerized from pyrrole in the vapor phase, which overcoats the nylon 6 membrane and MWCNTs, using iron(III)chloride, or ammonium persulfate as the oxidant. Sensors are cut from the membrane and tested in humidity and temperature controlled environment using 2, 3, and 4 electrode configurations.

            This material is unique because of its electrical characteristics. Nylon 6 generally has an electrical surface resistivity in the 10¹⁰ Ohm-cm range. The application of MWCNTs and PPy to the nylon 6 nanofiber membrane has reduced that surface resistivity over seven orders of magnitude to give resistivities in the kOhm-cm range. The material’s electrical resistance responds linearly and reproducibly to temperature changes between 25ºC and 45ºC with a 0.1ºC to 0.5ºC resolution, a negative temperature coefficient of resistance ranging from -0.56%/ºC to -0.14%/ºC, and sensor sensitivities range from 2 Ohms/ºC to almost 30 Ohms/ºC.

            This material is used to measure temperature using a bulk DC resistive response, but this does not provide much information regarding the true sensing mechanism of the sensor material, nor does it provide any useful information regarding the nature of the interface between the sensing material, the environment, and electrical contacts. Impedance spectroscopy (IS) is an important scientific tool that can provide powerful information about an interface between a sensor material and its electrical contacts, diffusion and adsorption of chemical or physical components at the sensor surface, as well as reactions and their kinetics inside the material and at the material surface. Ultimately IS provides insight into a material’s dielectric structure under electric field stresses and injected current transport, thereby providing a window into the nature of electronic transport through a material and its interfaces.

            This talk will highlight the importance of impedance spectroscopy in determining interfacial phenomena between the constructed temperature sensitive fabric, its electrical contacts, and the surrounding environment. It will propose an expanded equivalent circuit model for three and four electrode configurations, as well as a final simplified model. It will discuss the differences in measuring techniques between solid-state devices and electrochemical cells and emphasize the use of common circuit analysis to determine and simplify sensor models.