Structural Heterogeneity and Its Influence on Nonlinear Deformation and the Fracture of Ultrasoft Hydrogels

Understanding the deformation and fracture behavior of human organs exhibiting a very low stiffness (<1 kPa) is of vital importance to gain information on how these soft tissues react to the medical device during loading conditions in surgical robotics applications. We investigated the deformation and fracture behavior of model poly(vinyl alcohol) ultrasoft hydrogels by puncture tests with flat-ended indenters whose size is comparable to the elasto-capillary length, with a particular focus on the effect of structural heterogeneity. By tuning the polymer and cross-linker concentrations, gels with a modulus ranging from 56–2700 Pa were synthesized, with varied structural heterogeneity, examined by light scattering. We found that structural heterogeneity plays a key role in lowering the fracture resistance of the soft gels under large nonlinear deformations. This work provides insights into the mechanics and fracture of ultrasoft materials under extreme deformation conditions and opens the question of the interplay between elasticity and capillarity in such ultrasoft gels at small length scales.

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