Fig. 2. Stiffening PDMS substrates through secondary thiol-ene photocrosslinking.

(a) Schematic of the stepwise preparation of photoresponsive PDMS substrates, where cured V-PDMS via Pt-catalyzed crosslinking was placed in toluene with S-PDMS, DMPA and V-PDMS, the toluene was allowed to evaporate, and the sample was exposed to light for crosslinking. (b) Moduli of photoresponsive PDMS substrates with different concentrations of S-PDMS as a crosslinker in the presence of DMPA with UV exposure (365 nm, 15 mW/cm²) for 2 min. PDMS substrates were prepared through two steps: 1) V-PDMS and curing agent were mixed at a ratio of 65:1 to form soft substrates after curing at 37°C for 2 days; 2) PDMS substrates were incubated in toluene containing varying amounts of S-PDMS (0-30 wt%), V-PDMS (0-30 wt%) and DMPA (10 mM), with the total amount of S-PDMS and V-PDMS at 30 wt/% in toluene. (c) Moduli of photoresponsive PDMS substrates (10wt% S-PDMS) after incubation in serum-containing media (10% FBS) for different time points (1, 3, 7 days) and then stiffened. (d) Moduli of photo-stiffened PDMS (10wt% S-PDMS) incubated in serum-containing media (10% FBS) for up to 7 days. (n = 3. Data presented as mean ± s.d.). For PDMS substrates used in (c) and (d), PDMS substrates were also prepared through the two-step process as used in (a), while S-PDMS (10 wt%) and V-PDMS (20 wt %) and DMPA (10 mM) were used in the swelling step. The presence of V-PDMS (20 wt%) prevents any uncrosslinked V-PDMS from diffusing from the substrates during the swelling process.