Therefore, the fracture profile and toughness of hybrids was examined next (Figure ). The ultimate tensile stress and tensile toughness of hybrids generally increased as a function of polymer concentrations (Figures and). However, at the highest concentrations tested, the tensile toughness reached a plateau, as a result of the lower ultimate tensile strains typically observed. This was particularly pronounced for CMC-Allyl hybrids, which displayed comparable tensile toughness at the two concentrations tested, despite a clear increase in apparent modulus, as a result of a sharp decrease of the ultimate tensile strain. The impact of the elastomer chemistry on the mechanics and bonding to hydrogels was investigated next. Hybrids were generated from Sylgard PDMS, displaying comparable vinyl chemistry to thiol-ene silicones, but a crosslinking mechanisms relying on hydrosilylation. To promote bonding between the hydrogel network and the Sylgard silicone, a solution of PDMS thiol was applied to its surface prior to bonding. The impact of this change in elastomer chemistry on the apparent modulus of corresponding hybrids was more pronounced in the case of PEGDA hydrogels, compared to PDMAEMA thiol-ene hydrogels. Indeed, whilst apparent Young's moduli were comparable in the case of hybrids formed with PDMAEMA thiol-ene hydrogels, when comparing hybrids generated with thiol-ene PDMS and Sylgard, the apparent Young's moduli of PEGDA-based hybrids generated with Sylgard were systematically higher than those of hybrids generated with thiol-ene PDMS (Figure ). This different behaviour could be partly explained by differences in bonding between the different phases in these hybrid systems, perhaps indicating improved coupling between PEGDA and the thiolated PDMS applied at the surface of Sylgard PDMS. Interfacial mechanics is not expected to impact on the apparent Young's modulus significantly, although it may affect stress concentration at the interface. It is also possible that differences in oxygen concentration within Sylgard and thiol-ene PDMS differ, resulting in changes in crosslinking of the PEGDA phase. In contrast, the reduced sensitivity of thiol-ene coupling to oxygen concentrations results in a reduced impact on the apparent modulus of corresponding hybrids.