Hydraulic fracture geometry in ultrasoft polymer networks

Determination of failure properties becomes extremely challenging when materials approach very low and toughness values. New techniques are emerging to address these challenges in order to facilitate advancement of accurate tissue phantoms and medical diagnostics. We report on needle-induced 'cavitations' within a series of silicone elastomer formulations to demonstrate the ability of hydraulically-induced fracture morphology to differentiate fracture behavior in ultrasoft solids. We show, in accordance with previous results by Lefevre et al. (Int J Fract 192:1-23. 10.1007/s10704-014-9982-0. ISSN 15732673, 2015), that cavities first grow elastically and then transition to a penny-shape with an associated ring crack. Further growth occurs predominantly at the ring crack, leading to variations in cavity anisotropy quantified via the cavity's sphericity. Sphericity differentiates between materials of identical, ultralow modulus, but distinct formulation. We find that materials for which modulus is lowered by decreasing the crosslinker to pre-polymer ratio are tougher than materials in which modulus is lowered through the addition of unreactive silicone oil. We conclude that cavity shape provides a promising avenue for fracture energy determination and that the ultrasoft silicone/diluted-silicone system studied here provides a material platform for altering fracture properties while holding modulus constant.