A temperature sensitive fabric using homemade electrospun nylon 6 (PA6) or Tecoplastª polyurethane (PU) nonwoven nanofiber membrane materials, commercial multiwalled carbon nanotubes (MWCNTs), and chemical vapor deposited (CVD) polypyrrole (PPy) was developed1. A simple sensor construction procedure facilitates reproducibly producing the fabric sensors using simple laboratory procedures. This advantageous procedure is completed without needing toxic semiconductor device construction environments. The work illustrates the viability of using polymers/carbon allotrope composites as functional materials for sensor applications. This thermal sensor provides useful data using Ohm's law and the overall equation for the temperature coefficient of resistance, or α = 1/R0*dR/dT, which has been well defined previously and is common for fitting the temperature dependence of the electrical resistance of materials2. Sensor response is exactly linear and negative between 25C and 45C, which is a useful range for epidermal temperature measurement3,4, and displays a high resolution, approximately equal to 0.1C to 0.5C, being that it is a mostly polymer, fabric thermal sensor. Fig. 1 illustrates the general behavior of the sensor material that classifies it as a thermistor.
Figure 1: Schematic illustrating the thermal sensor response.
This thermal sensor was designed for biomedical issues in measuring temperature of the epidermis in load bearing situations like prosthetic sockets or shoes, and could be applied in military and sports applications. Research shows that greater than a 4C difference between the same points for each foot of a diabetic, can mean they are at a greater risk for foot ulceration, or could be an indication to more severe conditions such as neuropathy if the feet do not maintain relatively consistent temperatures regularly5-9. To measure the same points on each foot is cumbersome, as one imagines. Diabetics remove their shoes and socks, taking the temperature of each point on each foot using an infrared gun or even worse, a basic home use thermometer, and recording and reporting the information manually to their physician. This provides a potential for inaccuraties. A thin fabric temperature sensor could accommodate continuously monitoring the temperature between points on each foot, making it easier for diabetics to tell if they need to maintain foot temperatures. This would make it more efficient for reporting the results to their physician if the sensor is integrated to work wirelessly with a computer at home to log the data and possibly communicate directly with their physician's equipment. In addition, amputees could benefit from measuring temperature inside their prosthetic sockets to maintain a comfortable and hygienic stump socket environmen4,10-14. Surveys show that heat and sweat are the major contributors to whether an amputee is happy with their prosthesis, which can determine whether the amputee will even wear the prosthesis regularly3,15,16. Currently, temperature-measuring devices are constructed using rigid metallic and semiconductor materials2, so it would be beneficial in some of these load-bearing situations where pressure points could be problematic, to have thermal sensors constructed from thin and soft nanocomposite materials. Even applications, where pressure points are not a main concern, like smart textiles and sports clothing could benefit from these fabric thermal sensors integrated into their increasingly more complex systems to obtain useful physiological data during periods of exercise and extreme environmental stresses and situations.
The procedure to construct these sensors starts by electrospinning PA6 or PU sheets onto a homemade rotating drum. Commercial MWCNTs are vacuum filtered onto the polymer membranes from aqueous Triton X-114 stabilized nanotube suspensions. PPy is polymerized from pyrrole in a CVD chamber at Å 100 kilopascals of vacuum and ambient temperatures for two days. The PPy overcoats on and throughout the MWCNTs on the polymer membranes, using iron(III)chloride, or ammonium persulfate as the oxidant, making a nanostructured composite material that is electrically sensitive to temperature changes. The sensors are then cut from the membrane in 1-cm2 geometries and tested in humidity and temperature controlled environment using 2, 3, and 4 electrode configurations17-20. Fig. 2 depicts the 4 electrode setup.
Figure 2: Design for sensor testing.
This material has unique electrical characteristics. The polymers generally have electrical surface resistivities in the 1010 and 1013 to 15 Ohm-cm ranges for PA6 and PU, respectively. The application of MWCNTs and PPy to the polymer membranes has reduced those surface resistivities over seven orders of magnitude to give resistivities in the kOhm range and temperature sensitive super abilities. The material's electrical resistance responds linearly and reproducibly to temperature changes between 25C and 45C with a negative temperature coefficient of resistance ranging from -0.56%/C to -0.14%/C, a 0.1C to 0.5C thermal resolution, and sensitivities ranging from 2 Ohms/¼C to nearly 30 Ohms/¼C. Fig. 3 represents the steady state calibration.
Figure 3: Steady state calibration curve showing the actual steady state temperature and resistance versus time data.
Bulk DC resistivity, and hence bulk resistance, or R = ρl/a, where R is the resistance in Ohms, r is the resistivity in Ohm/cm, l is the length travelled by the current, and a is the cross sectional area perpendicular to the flow of current, is used as a model to determine temperature. This does not really elucidate the true sensing mechanism at the nano, or atomic, scale of the sensor material. Nor does it provide any useful information regarding the nature of the interface between the sensing material, the outside environment, and the sensor's electrical connections.
Impedance spectroscopy (IS) is an important scientific tool that can provide extremely useful information about the interface between a sensor and its electrical contacts, chemi- and physi-sorption as well as any reactions and their kinetics at the material surface and within21,22. Ultimately IS elucidates a material's dielectric structure under electrical stress, which also translates into mechanical, stress, thereby providing a window into electronic transport through a material and its interfaces23,24
This presentation will focus on the value and constraints of IS in determining interfacial phenomena between this constructed temperature sensitive fabric, its electrical contacts, and the surrounding environment. An expanded equivalent circuit model for three and four electrode configurations will be proposed. In addition a simplified circuit equivalent will be shared. A short discussion on the differences in measuring techniques between solid-state devices and electrochemical cells will finish the talk, while emphasizing the use of common circuit analysis to determine and simplify "dry" and "wet" sensor models.
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