A rapidly forming polymer matrix with affinity-based controlled release properties was developed based upon interactions between heparin-binding peptides and heparin. Dynamic mechanical testing of 10% (w/v) compositions consisting of a 3:1 molar ratio of poly(ethylene glycol)-co-peptide (similar to18 000 g/mol) to heparin (similar to18 000 g/mol) revealed a viscoelastic profile similar to that of concentrated, large molecular weight polymer solutions and melts. In addition, the biopolymer mixtures recovered quickly following thermal denaturation and mechanical insult. These gel-like materials were able to sequester exogenous heparin-binding peptides and could release these peptides over several days at rates dependent on relative heparin affinity. The initial release rates ranged from 3.3% per hour for a peptide with low heparin affinity to 0.025% per hour for a peptide with strong heparin affinity. By altering the affinity of peptides to heparin, a series of peptides can be developed to yield a range of release profiles useful for controlled in vivo delivery of therapeutics.