Moisture-responsive polymer films on flexible substrates for optical sensing of humidity

Rapid advances in material science and processing methods over the past decade accelerated the development of flexible and stretchable sensors for various innovative applications in healthcare, medicine, biology and environment. Such devices have the potential to be applied to soft surfaces of different shape like textile fabrics or human skin. Flexible substrates allow continuous roll processing, thus providing the potential for efficient and low cost mass production.

In this paper a flexible humidity sensor is prepared by spin-coating of moisture-sensitive polymer on three types of substrates: poly(ethylene terephthalate) (PET), polylactide (PLA) and composite polysiloxane. The optical properties, surface morphology and roughness of the substrates are studied by transmittance measurements and surface profiling, respectively. Thin spin-coated polymer films of amphiphilic copolymer obtained by partial acetalization of poly(vinyl alcohol) are used as humidity sensitive media. In order to increase the sensitivity a thin metal layer of Au-Pd is incorporated between the polymer film and substrate. The thickness of metal layer is optimized in order the transmittance level of the sensor to be in the range 40-50%. The sensing properties are probed through transmittance measurements at different levels of relative humidity (RH). The influence of substrate type is studied by comparing hysteresis and sensitivity of flexible sensors with those deposited on glass substrate. The successful application of the designed flexible humidity sensors is demonstrated and discussed.