Collagen as a Humidity Sensing Dielectric Material
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Collagen as a Humidity Sensing Dielectric Material
Mathew A. Hudspeth1 and Tolga Kaya1,2* 1
Science of Advanced Materials, ET 228, Mount Pleasant, MI 48859, U.S.A.
2
School of Engineering and Technology, ET 130G, Mount Pleasant, MI 48859, U.S.A. *Corresponding Author: [email protected]
ABSTRACT The motivating principle behind this research is the development of small, wearable devices that would use humidity and temperature measurements as metrics for health monitoring. If it is to be useful as a health monitoring tool, the device needs to respond quickly and predictably to changes in humidity. Collagen is shown to be a viable humidity sensing material for use in capacitive relative humidity (RH) sensors. As a natural by-product of meat and leather industries, collagen presents itself as an interesting and inexpensive alternative to polyimide dielectric sensing materials. We used gelatin, a partially hydrolyzed form of collagen, to allow for easier spin coating. We have successfully fabricated devices by depositing a collagen thin film (1.2 µm) via spin coating, followed by Au/Pd electrodes (60 nm) via sputter coating. A plastic mask made from a rapid prototyping machine was used during physical vapor deposition (PVD) to pattern electrodes. This simple method eliminates the need for the use of more complicated photolithography processes. Interdigitated electrodes (rather than parallel plate electrodes) form a 6 mm wide, planar capacitor structure that has little dependence on dielectric thickness and is not affected by dielectric swelling. Initial findings indicate that these devices very closely match the results of the commercial relative humidity sensor used for reference. The capacitancehumidity relationship is shown to be non-linear, with an average change of 3 fF for every 1% change in RH around 60% RH, and an average change of 7 fF for every 1% change in RH around 80% RH. In this work, we present the fabrication and characterization of these novel collagen-based relative humidity sensors.
INTRODUCTION Humidity sensors are used for a wide variety of applications from automotive and appliance sensors to home comfort management. Different sensing methods are used based on the application, such as capacitive, mass sensitive, optical, and resistive [1]. This work focuses on capacitive relative humidity sensing. The main activity of this type of sensor lies in the difference of dielectric constant between water vapor and the dielectric material. Various materials such as metal oxides [2, 3], carbon nanotubes [4], and polyimides [5-7] have been used; each with benefits and drawbacks (e.g. oxides and nanotubes are more expensive than organic counterparts, but provide for a more durable device). The overarching goal of this research is the development of a sensor package; small enough to be incorporated into clothing
for the monitoring of health indicators such as sweating (humidity), fever (temperature), and sudden drops (accelerations). Some of the first advances in capacitive humidity sensing were to create
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