Biological soft materials in nature have complex structures that allow them to function in demanding scenarios. For example, the balanced combination of hydrophilic and hydrophobic segments in elastic biopolymers, such as in resilin, led to remarkable mechanical properties, including high stretchability and resilience, which are exploited in nature by many species for feeding and defense purposes. Developing a synthetic gel with resilin-like properties requires high stretchability to store elastic energy, low hysteresis for high energy conversion, and high retraction velocity when released from a stretched state for power amplification. We have synthesized gels capable of mimicking some of these properties through a single-step free-radical polymerization of hydrophilic and hydrophobic monomers. The effects of gel compositions and microstructure on their mechanical properties will be presented. Gels composed of acrylic acid (AAc), alkyl-acrylamide, e.g., methacrylamide (MAM), butyl acrylamide (BAM), and poly(propylene glycol diacrylate) (PPGDA) were synthesized. Gels with MAM and PPGDA displayed increasing elastic moduli with increasing PPGDA concentrations. When released from a stretched state, these gels retracted rapidly, and the retraction velocity also increased with PPGDA concentration. PPGDA is moderately hydrophobic, but additional hydrophobicity in the gels was incorporated by replacing MAM with hydrophobic BAM. The effects of increased hydrophobicity on the tensile properties, retraction velocity and acceleration will be presented. The structure of these gels has been investigated using small-angle neutron scattering. This study broadens our understanding of the structure-property relationships for stretchable hydrogels essential for their applications in numerous areas, including prosthetic devices, artificial skin, electronic devices, and soft robotics.
