No abstract

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.

Many natural organisms use “protein rubbers” to store and release an imposed strain energy with high efficiency to make motion easier. Protein rubbers exist in a complicated environment surrounded by water and other molecules such as sugars, implying that amino acid composition and its environment are important in protein rubber behavior. Here, gelatin, the hydrolysis product of animal collagen, is hydrated or “plasticized” with water, ethylene glycol, glycerol, corn syrup, and aqueous solutions of sorbitol, glucose, and fructose. The rubber formed is “dry”, that is, is not fully immersed in liquid, and has the appearance and feel of a soft rubber band. The mechanical and thermodynamic behavior of each rubber is characterized with low strain dynamic and high strain tensile experiments with good agreement between the two. Plasticized gelatin rubbers are incompressible and follow the neo-Hookean model for rubber elasticity up to moderate extension ratios. Higher molecular weight polyols with more hydrogen bond donors and acceptors create gelatin networks with lower crosslink density. Ethylene glycol–, glycerol-, sorbitol syrup–, and fructose syrup–plasticized gelatin rubbers have similar molecular relaxation mechanisms and are the most efficient rubbers when probed in the rubbery plateau region prior to approaching the glass transition. The other plasticizers have different molecular relaxation mechanisms that detract from the efficiency of energy storage and return that is not related to network formation but perhaps the individual solvation ability of each plasticizer.

There is a growing interest in making stimuli-responsive polymer systems, particularly ones that are bio-inspired/biomimetic and could perform mechanical work. Here, a biological device made from gelatin is described that can mechanically cycle back and forth in response to solution pH or ionic strength changes. The gelatin bilayer has one layer of Type A gelatin and the other of Type B gelatin, which have 2 different isoelectric points and therefore ionization states at a given solution pH. The bilayer mechanically cycles back and forth when one layer swells more than the other layer, which occurs because of solution pH or ionic strength change. Maximum bilayer bending occurs at pH 10, when the Type B gelatin layer swells significantly more than the Type A layer. The results show the ability to use the unique properties of different sources of gelatin to design a simple purely biological machine.

Corn zein and wheat gliadin protein are compounded into synthetic cis-1,4-polyisoprene rubber (IR) and sulfur-cured in a zinc oxide (ZnO)-free system. The curing kinetics and mechanical and morphological properties are compared to a ZnO-activated or carbon black (CB)-reinforced cure system. The proteins provide reversion resistance and reinforcement to IR at filler loadings as low as 1 part per hundred rubber (phr). The zein-IR composites exhibit higher moduli, better filler-matrix adhesion, and less filler agglomeration/migration than gliadin-IR because zein is more chemically compatible with IR. The gliadin-IR composites have a lower percent set and hysteresis, indicating the formation of an elastic restoring gliadin network. Optimal properties are achieved at 2-phr gliadin and 4-phr zein. At gliadin loading >2 phr and zein loading >4 phr, the protein domain size increases and mechanical properties deteriorate. At equal filler loading, property improvements over CB-IR are observed for one or both proteins.