Bonding of hydrogels to elastomer to formulate hybrid samples for tensile testing. The pre-cut silicones or PCL samples were painted 3 times with a solution containing photoinitiator (Irg651 in DCM for silicone samples, Irgacure 651 in a mixture of DCM and Ethanol (2:3) for PCL samples) using a cotton bud. Two elastomer strips were placed into Teflon moulds, with a spacing of 5 mm. Hydrogel precursor solutions were poured into the gap defined by the two elastomer strips (Supplementary Figure ). The samples were then irradiated with UV light for 2 min at an intensity of 94 mW/cm 2 . Once cured, the resulting elastomer-hydrogel-elastomer hybrid structures were carefully removed. Finally, the samples were placed onto glass slides and kept hydrated until characterisation.