Yang J, Illeperuma W, Suo Z. Inelasticity increases the critical strain for the onset of creases on hydrogels. Extreme Mechanics Letters. 2020;40:100966. doi:
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.
Yang J, Steck J, Bai R, Suo Z. Topological adhesion II. Stretchable adhesion. Extreme Mechanics Letters. 2020;40:100891. doi:
When two stretchable materials (e.g., hydrogels, elastomers, and biological tissues) are adhered, the interface should be stretchable, without constraining the deformation and degrading adhesion. Here we develop methods to characterize stretchable adhesion. We do so by topological adhesion, using polyacrylamide hydrogels as adherends, chitosan as stitch polymers, and a change in the pH as a trigger. We prestretch the topohered hydrogels in several ways, and measure adhesion energy when the hydrogels are either in the un-stretched or the stretched state. Stretchable adhesion is achieved when the adhesion energy can maintain a similar level, insensitive to the prestretch. We study the mechanism of stretchable adhesion formed by the chitosan topohesive.
Yang J, Steck J, Suo Z. Gelation kinetics of alginate chains through covalent bonds. Extreme Mechanics Letters. 2020;40:100898. doi:
Gelation kinetics of polymer chains through covalent bonds depends on many variables and affects many applications. Here we study the gelation kinetics of alginate chains through crosslinkers adipic acid dihydrazide (AAD), with coupling reagents 1-ethyl-3-(3-dimethylaminopropyl) (EDC) and N-hydroxysuccinimide (NHS). We use the rate equation of the reaction to calculate the extent of reaction, and use the Flory–Stockmayer theory to calculate the gelation time. The model predicts the gelation time as a function of the concentration of alginate, crosslinkers, and coupling reagents, as well as the functionality of an alginate chain and the rate constant of the reaction. Given an aqueous solution, we conduct rheological tests to measure the storage and loss moduli as functions of time, and determine the gelation time by the time when the storage modulus equals the loss modulus. The gelation times so determined for solutions of various compositions agree well with those predicted by the model. This combination of the model and experiments provide a practical approach to the design of gelation kinetics for applications.
Steck J, Kim J, Yang J, Hassan S, Suo Z. Topological adhesion. I. Rapid and strong topohesives. Extreme Mechanics Letters. 2020;39:100803. doi:
Topological adhesion, or topohesion for brevity, links two polymer networks, to be called adherends, even when the adherend networks carry no functional groups for chemical coupling. Uncrosslinked polymers, called stitch polymers, are spread between the two adherends. In response to a trigger, the stitch polymers form a stitch network in topological entanglement with both adherend networks. It is commonly believed that topohesion always takes a long time, but this is a misconceived myth. In principle, two adherends topohere strongly even when the stitch network entangles with each adherend network by a single polymer mesh size. The shallowness of this requirement dictates that topohesion is rate-limited by the gelation of the stitch network, not by the diffusion of the stitch polymers into the adherend networks. We illustrate this concept using two pieces of polyacrylamide hydrogels as adherends, an aqueous solution of cellulose as stitch polymers, and a change in the pH in the cellulose solution as a trigger. By varying the thickness of the cellulose solution, the time to topohere is tunable from seconds to hours. For a solution of thickness of 50 microns adhesion energy of 50 Jm−2 is attained in 60 s. These experimental findings dispel the myth, and shed light on the times to topohere reported in the literature. The art and science of topohesion provide fertile grounds for fundamental discovery and practical invention to enable unusual applications.