Characterization of CS hydrogel films. (a) The scheme shows the gradient cross-linking density across the thickness of CS hydrogel films. (b) The cross-sectional field-emission-scanning electron microscopy (FE-SEM) images of the frozen-dried CS hydrogel films and the related magnified images of the top and bottom surfaces (scale bar: 20 μm). (c, d) Young’s modulus mapping for the top and bottom surfaces of the swollen CS hydrogel films with different thicknesses (20 μm and 80 μm). (e) Young’s modulus of both the top and bottom surfaces decreases as the hydrogel thicknesses increase (from 20 μm, 40 μm, 60 μm, to 80 μm). The degree of swelling also increases as the thickness of CS hydrogel films increases.