(Alfa Aesar), 1,5-pentanediol diacrylate (B5) (Monomer Polymer & Dajac Labs), 2-methylpentane-1,5-diamine (E4) (TCI
America), 2-(3-aminopropylamino)ethanol (E6) (Fluka), poly(ethylene glycol) methyl ether thiol (800 Da), branched 25 kDa
poly(ethylenimine) (PEI) (Sigma-Aldrich), α-Mercaptoethyl-ω-methoxy polyoxyethylene (5000 Da) (NOF America
Corporation), and cell culture media components were purchased and used as received. pEGFP-N1 (EGFP) DNA (purchased from Elim
Biopharmaceuticals and amplified by Aldevron, Fargo, ND), ganciclovir (Invivogen, San Diego, CA), Label IT-Tracker Cy3 kit (Mirus
Bio LLC), and CellTiter 96 AQueous One MTS assay (Promega, Fitchburg, WI) were obtained from commercial vendors and used per
manufacturer’s instructions. HSV-tk gene was cloned into the pcDNA3.1 vector (Life Technologies) and amplified (Aldevron,
Fargo, ND).