Precise design of microwrinkles through the independent regulation of elasticity on the surface and in the bulk of soft hydrogels

Abstract: It is still difficult to precisely control microscopic wrinkles on the surface of functional materials, especially biomimetic soft hydrogels with an elastic modulus lower than 100 kPa. This is due to the difficulty in realizing the systematic and independent regulation of elasticity on the top surface and in the bulk of hydrogels, which is essential for the generation of surface microwrinkles. To overcome this problem, using a two-step photocrosslinking process with VIS and UV irradiation of a photocurable gelatinous sol, we developed a method for independently regulating the elastic modulus on the surface and in the bulk to obtain wrinkles on a biomimetic soft gel. Photocurable gelatin was first irradiated and crosslinked with VIS light in the presence of the water-soluble radical generator sulfonyl camphorquinone, which is effective in forming thick bulk gels. Next, the top surface of these precrosslinked gels was irradiated with UV light in the presence of surface-coated water-insoluble camphorquinone. As a result, this two-step photocrosslinking process enabled to independently control the elastic moduli of the surface and the bulk lower than 100 kPa and to generate several-micron-scale wrinkles on the soft hydrogel surface.