However, the resulting glass-gel-glass hybrids were too weak to enable tensile testing. This may partly be ascribed to the heavier weight of the glass substrates, compared to the silicone elastomers, but is also suggesting that partial interpenetration of the hydrogel network within the elastomer network occurs at the interface, as is known to reinforce adhesion in other systems . Therefore, gels were sandwiched between two functionalised glass slides, placed face to face. The surface density of thiol moieties, controlled by the solution concentration of mercaptopropyl trimethoxysilane had a strong impact on the adhesion strength and failure mode of resulting samples (Figure ). At low thiol concentration, samples failed at the interface and adhesion strength was low (near 5 kPa), whereas it increased to 15 kPa at higher thiol concentrations and resulted in the cohesive failure of samples. The adhesion strength measured at higher thiol concentration is comparable to the adhesion strength observed in the PEGDA based hybrids with similar hydrogel concentrations. Therefore, the data obtained for PEGDA hybrids formed with three different types of silicones (thiolene PDMS and Sylgard with and without treatment with thiolated PDMS) indicate that coupling via thiols does occur, but is less effective in strengthening corresponding interfaces and ensuring cohesive failure than direct bonding via radical coupling between the acrylate network and vinyl residues of the silicone phase.