A biocompatible and biodegradable bioprotonic flexible sensor. (a) Schematics of a likely life cycle of a degradable natural functionalize polysaccharide film. First, functionalized polysaccharide which came from nature is made into biofilm. The biofilm can be degraded via biodegradation and sent back to nature to reduce the environmental footprint. (b) Schematics of a biodegradable and biocompatible humidity sensor attached on the skin. The inset shows the chemical structure of natural functionalized polysaccharide film used to fabricate the sensor. (c) Scanning electron microscope (SEM) image of natural functionalized polysaccharide film. (d) Optical image of the electronic structure of a flexible device. The inset shows the local enlarged image of a device. (e, g) Confocal laser scanning microscopy images of stained human skin fibroblast-HSAS1 and HUVEC that were cultured on a functionalized polysaccharide film. Scale bar: 100 μm. (f, h) Cell viability of human skin fibroblast-HSAS1 and HUVEC on 2, 4, and 6 d of in vitro culture (n = 3 measurements).