Fracture and fatigue

Hydrogels are subject to consistent deformation due to body movement and thus are susceptible to fracture and fatigue. The cause is often the intricate and combined result of bond dissociation, polymer scission, and solvent migration. By using various polymer synthesis techniques, such as free radical polymerization, atom transfer radical polymerization, and polymer backbone modification, we design such polymer architectures and network topologies that can de-concentrate stress over a large volume to amplify the fracture and fatigue thresholds. Methods under investigation involve long-chain entanglement, bottlebrush polymers, and composites.

Stress-assisted hydrolytic degradation

Water can hydrolyze certain chemical bonds ubiquitous in hydrogels, such as ester, urethane, and siloxane bonds. Especially in the presence of stress, stress can assist the hydrolytic kinetics to accelerate degradation. We use a fracture mechanics approach to study how stress-assisted hydrolysis leads to crack growth, in which a big crack is made in the material and a constant stretch is applied, and the crack length is monitored over time. We have tested this in PDMS and showed that it suffers stress-assisted hydrolytic crack growth, and the growth velocity is orders of magnitude faster than that in a dry environment. We have also tested this in PLA plastics and showed that the crack velocity is insensitive to load but is sensitive to humidity and pH. We further investigate this in other implanted materials, such as urethane bonds in polyurethane, peptide bonds in adhesion, and ester bonds at the linkage of polymer brush anchored hydrogels.

Mechanical instabilities

Mechanical instabilities, such as wrinkles and creases, are often observed on the surface of hydrogels. Under compression, the material initially is flat but starts to lose homogeneity beyond a critical value, and the first-order mode of instability can evolve to subordinate modes. On-demand control of the appearance, evolution, and disappearance of instabilities is desirable. For example, various modes of instabilities can be harnessed to create surface morphologies and change surface properties useful in some applications, while instabilities need to be suppressed to prevent unwanted inhomogeneity or material damage in other applications. We employ experiments, modelling, and simulation to understand how material properties, geometries, and loading conditions control the formation of instabilities, how they evolve, and how they can be inhibited.

Relevant papers on this topic
Bai, R., Yang, J., Morelle, X.P., Yang, C. and Suo, Z., 2018. Fatigue Fracture of Self-Recovery Hydrogels. ACS Macro Letters, 7(3), pp.312-317
Bai, R., Yang, J. and Suo, Z., 2019. Fatigue of hydrogels. European Journal of Mechanics-A/Solids, 74, pp.337-370
Bai, R., Chen, B., Yang, J. and Suo, Z., 2019. Tearing a hydrogel of complex rheology. Journal of the Mechanics and Physics of Solids, 125, pp.749-761.
Bai, R., Yang, J., Morelle, X.P. and Suo, Z., 2019. Flaw-Insensitive Hydrogels under Static and Cyclic Loads. Macromolecular Rapid Communications, 40(8), p.1800883.
Mu, R., Yang, J., Wang, Y., Wang, Z., Chen, P., Sheng, H. and Suo, Z., 2020. Polymer-filled macroporous hydrogel for low friction. Extreme Mechanics Letters, p.100742.
Chu, C.K., Joseph, A.J., Limjoco, M.D., Yang, J., Bose, S., Thapa, L.S., Langer, R., and Anderson, D.G., 2020. Chemical Tuning of Fibers Drawn from Extensible Hyaluronic Acid Networks. Journal of the American Chemical Society, 142(46), pp.19715-19721.
Yang, X.#, Yang, J.#, Chen, L. and Suo, Z., 2019. Hydrolytic crack in a rubbery network. Extreme Mechanics Letters, p.100531.
Yang, X., Steck, J., Yang, J., Wang, Y. and Suo, Z., 2021. Degradable plastics are vulnerable to cracks. Engineering, 7(5), pp.624-629.
Yang, J., Illeperuma, W. and Suo, Z., 2020. Inelasticity increases the critical strain for the onset of creases on hydrogels. Extreme Mechanics Letters, p.100966.
Yang, J., Jin, L., Hutchinson, J.W. and Suo, Z., 2019. Plasticity retards the formation of creases. Journal of the Mechanics and
Physics of Solids, 123, pp.305-314.
Ouchi, T.#, Yang, J.#, Suo, Z. and Hayward, R.C., 2018. Effects of Stiff Film Pattern Geometry on Surface Buckling Instabilities of Elastic Bilayers. ACS Applied Materials & Interfaces, 10(27), pp.23406-23413
Auguste, A., Yang, J., Jin, L., Chen, D., Suo, Z. and Hayward, R.C., 2018. Formation of high aspect ratio wrinkles and ridges on elastic bilayers with small thickness contrast. Soft Matter, 14, 8545-8551
Huang, J.#, Yang, J.#, Jin, L., Clarke, D.R. and Suo, Z., 2016. Pattern Formation in Plastic Liquid Films on Elastomers by Ratcheting. Soft Matter, 12(16), pp.3820-3827. 
Yang, J. and Nie, G., 2014. Analysis of Sinusoidal Interfacial Wrinkling of an Anisotropic Film Sandwiched Between Two Compliant Layers. Journal of Applied Mechanics, 81(9), p.091013.