Scheme 1.

PEG hydrogels were formed through Michael addition of cysteine-containing matrix metalloproteinase sensitive peptides (MMPxl) with 4-armed PEG-vinyl sulfone pre-modified with cell adhesion peptides (PEG-RGD). Polyplexes were encapsulated into hydrogel matrix by mixing with the precursor solution prior to gelation. Cells were seeded as single cells or a cluster of cells (shown) inside the hydrogel matrix. As the cells infiltrate the scaffold, they encounter polyplexes and are transfected.