Covalent Attachment of PEGDA Hydrogels on Glass

Polyethylene glycol diacrylate (PEGDA) hydrogel posts were covalently attached to glass substrates using methacrylate-terminated silane monolayer formation using standard methods.^{66 ,71 (link)} The plain glass microscope slides (Thermo Fisher) were cut into desired dimensions using a diamond scribe (Ted Pella) and running and nipping pliers (Fletcher-Terry) and stored under vacuum until usage. Polydimethylsiloxane (PDMS, Sylgard® 184, Dow Corning) molds^{85 (link)} were made using standard soft-lithography protocols by mixing elastomer base and curing agent in a 10:1 ratio and cured on a SU-8 (MicroChem) master that was prepared using standard photolithography protocols. Molds were designed to create 1 × 1 cm arrays of wells with diameters of 300 μm and 30 μm and depths of 35 μm and 38.6 μΜ, respectively (Fig. S1^†). The arrays contained indexing marks in place of some wells. The individual 1 × 1 cm PDMS molds were cut using a scalpel and had 1.5 mm inlets punched out (Biopsy Punch, Miltex). Norland Optical Adhesive 81 (NOA81, Thorlabs) well arrays were formed on the acrylated glass slides by filling the PDMS molds using degas-driven flow,^{86 (link)} UV curing the NOA81, and removing the PDMS molds (see ESI,f Fig. S2).