Biologically controlled gelatin actuators

Gelatin is a natural and environment-friendly polymer that can be cross-linked to various degrees with glutaraldehyde (GTA) and swollen with water to form a hydrogel. As the gelatin cross-link density increases, gel stiffness increases and swelling in water decreases with the cross-link density saturating at about 0.01 mol of GTA. Forming gelatin hydrogel bilayers with layers of differing cross-link densities results in a gelatin actuator that bends toward the layer of higher cross-link density when swollen in water. The extent of bilayer bending is dependent on the difference between the volume swelling ratio of the layers, which is directly determined by the cross-link density difference. When the gelatin bilayers are close in swell ratio or when the swell ratios are significantly different, no bending occurs. The highest bending is found when the higher cross-linked passive layer swells to about 60% of the swelling of the lower cross-linked active layer. Filling the active layer with pre-gelatinized starch increases the bilayer curvature, but it returns to its original shape after hydrolyzing the starch with α-amylase, demonstrating shape change in response to a biochemical reaction. The results show that simple gelatin bilayers can serve as actuators when stimulated with water or sugar and enzyme.