Indeed, the oxygenation level of hydrophobic substrate is known to impact on hydrogel mechanics, in particular close to the contact interface between the hydrogel precursor solution and hydrophobic substrate . This would imply a higher oxygen content in thiol-ene PDMS, compared to Sylgard, which we were not able to confirm experimentally. To explore the impact of the silicone elastomer chemistry on hydrogel bonding more directly, the fracture behaviour and toughness of corresponding hybrids was examined (Figure ). In addition, to study the impact of thiol-ene bonding on hybrid mechanics and fracture further, we introduced another set of samples in which Sylgard PDMS was not coated with the solution of PDMS-SH. The PEGDA hybrids formed with higher concentrations of PEGDA all failed adhesively, possibly due to the overall relatively stiff and tougher character of corresponding hydrogel phases, resulting in stress accumulation at the interface dominating the fracture behaviour. In agreement with this notion, comparison of the ultimate tensile strength of these hybrids indicated a plateau above an apparent Young's modulus of 200 kPa (Figure ). Therefore, these observations suggest that above such stiffness, the toughness of the gel phase and stress transfer concentrates at the interface, resulting in failure of the hybrids. In addition, in the plateau region, the ultimate tensile strength was higher in the case of uncoated Sylgard hybrid, compared to Sylgard hybrids with thiolated PDMS (Sylgard-SH) coating or thiol-ene PDMS. Therefore, at high hydrogel concentrations, the apparent Young's modulus was sensitive to the surface chemistry of the Sylgard PDMS, perhaps reflecting the impact of interfacial stress concentration on the overall deformation of the sample and suggesting that direct coupling of acrylates to Sylgard PDMS is more effective than coupling via thiol-ene bonding. In agreement with this notion, failure was systematically cohesive in these samples (Supplementary Figures S4). At lower gel concentrations, mechanical properties of the gel phase dominated stress-strain profiles and the sensitivity of associated Apparent Young's moduli to the different silicones and interfaces explored for bonding was reduced. The bonding chemistry with the silicone had less impact on ultimate tensile strength, perhaps reflecting that failure occurred more at the interface in Sylgard, both unmodified and treated with thiolated PDMS (Supplementary Figure ). Overall, PEGDA adhesion to Sylgard silicones appeared enhanced at higher hydrogel concentrations, based on the failure mechanism observed and the increased ultimate tensile strength observed compared to thiol-ene PDMS. In the case of hybrids formed with PDMAEMA-Pent, the surface chemistry of the elastomer had little impact on the apparent modulus of corresponding hybrids (Figures and), in agreement with improved coupling between the two phases and potentially also the reduced sensitivity of thiol-ene chemistry to oxygen. The chemistry of the PDMS elastomer had an important impact on the ultimate tensile strength of corresponding hybrids, varying the strength of these materials by nearly 1 order of magnitude. As this phenomenon was not associated with a switch from cohesive to adhesive failure (Supplementary Figure ), the origin of this variation remains unclear. It could reflect a change in stress transfer at the interface, differences in partial network interpenetration, or changes in ultimate strain at break. Nevertheless, our data clearly indicate that bonding and toughness is enhanced when thiol-ene hydrogels are bonded to thiol-ene PDMS elastomers with complementary imbalance of thiol moieties vs. alkene residues (excess thiol in thiol-ene PDMS and excess alkene in thiol-ene hydrogel).