Creating inexpensive sensors that are easy to use is the goal of many authors and researches. Materials for implementation in similar types of devices as a sensitive medium are increasingly in demand. This medium in optical sensors needs to fulfill certain specifications: it needs to be compatible with various substrates, have an appropriate refractive index, be alterable in thickness, etc.

In the present study, the influence of deformations of a flexible substrate on an active amphiphilic PVA-Ac copolymer with a 24%-acetal-content sensing layer in a thin-film humidity sensor was studied. To improve the optical contrast, metallization of the PET substrate was used, which also aimed to increase the sensitivity. After a series of up to 1000 bends, certain characteristics of the sensor were studied: hysteresis, sensitivity, the change in the transmission coefficient, etc. The initial active layer was deposited using the spin-coating method, which is fast and easy and ensures the repeatability of the results. By using 3D profilometry, the quality of PVA-based active media thin films containing additional acetal groups was observed. It was demonstrated that the deformations had no negative effects on the sensor thin-film systems and, in fact, increased their sensitivity by more than 40%. We then present a discussion of the potential causes for this, including a decrease in the adhesion between the transparent metallized substrate and the sensing medium.