Inelasticity increases the critical strain for the onset of creases on hydrogels

When a soft material (gel, elastomer, or biological tissue) is compressed beyond a critical strain, its smooth surface forms creases. During this process, a material particle near a crease undergoes a path of load and unload. If the material is elastic, the load and unload do not dissipate energy, and the crease sets in at a critical strain about 0.35. If the material is inelastic, however, the load and unload dissipate energy, and the crease is expected to set in at a higher critical strain. Here we study the effect of inelasticity on the onset of creases using alginate-polyacrylamide hydrogels. Such a hydrogel consists of two interpenetrating polymer networks: a polyacrylamide network of covalent crosslinks, and an alginate network of ionic crosslinks. The former is stretchy and elastic, while the latter unzips at a small stretch and causes inelasticity. We prepare alginate-polyacrylamide hydrogels with various degrees of inelasticity by varying the concentration of alginate. Our experiment confirms that larger degree of inelasticity leads to larger critical strain for crease. This study shows an example that the chemistry of materials affects the mechanics of creases.