Collagen as a humidity sensing dielectric material

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 capacitance-humidity 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.