2.1. IPN hydrogel fabrication

PEGdA/Gel-PEG-Cys hydrogel was synthesized following previously published procedures with some modulations (Burmania et al., 2003; Xu et al., 2012; Guerra et al., 2017). The IPN prepolymer solution was made of 1% (w/v) Iragcure 2959 photoinitiator, 20% w/v PEGdA and 20% w/v Gel-PEG-Cys dissolved in 1× PBS solution (pH 4.0) with or without the addition of cisplatin (1 mg/mL, pH 4.0). Such crosslinked IPN hydrogels exhibited higher maximum weight swelling ratio, higher water content, significantly lower cumulative gelatin dissolution up to 7 days, and lower gel stiffness compared to physically incorporated gelatin hydrogels (Fu et al., 2012; Xu et al., 2013).

We first prepared various formulations (Table 1) of Gel-PEG-Cys plus PEGdA in order to have a quickly formed IPN hydrogel. 10% or 20% w/v PEGdA solution and 10% or 20% w/v Gel-PEG-Cys solution were prepared in pH 4.0 1× PBS at 37 °C. Then Gel-PEG-Cys solution was mixed with PEGdA solution and 1% w/v Iragcure 2959 photoinitiator via vortexing. 200 μL of hydrogel precursor solution was dropped onto the flat glass slide and subjected to long-wavelength UV (λ_max = 365 nm, intensity at 100 mW/cm²) light until it became a solid gel. As shown in Table 1, higher concentrations of PEGdA solution and Gel-PEG-Cys solution could take a shorter time to form a gel. It took within 2 minutes with 20% w/v PEGdA solution and 20% w/v Gel-PEG-Cys solution, which was fit to the following in vivo study.

Formulation table.