Diels-Alder mediated controlled release from a poly(ethylene glycol) based hydrogel.

A synthetic amino acid bearing a furan functionality was developed and incorporated into peptide sequences using solid phase synthesis. Peptides expressing the furan moiety were attached to poly(ethylene glycol) (PEG) hydrogels through a thermally reversible covalent bond formed by a Diels-Alder reaction. Reactions of thiol and maleimide PEG macromers in an off-stoichiometric Michael addition were performed, such that the maleimide moiety was in excess, to create hydrogel networks with pendant Diels-Alder compatible tethering sites, that is, the maleimide. By making use of the Diels-Alder reaction, it was possible to control the release rate of reversibly bound moieties from the hydrogel by changing the temperature; higher temperatures favor a faster retro-Diels-Alder reaction and, therefore, a faster release from the polymer network. This concept was demonstrated by incorporating a fluorescently labeled furan-RGDS sequence into a hydrogel possessing excess maleimide functionalities and monitoring the subsequent liberation of RGDS at various temperatures, illustrating a Diels-Alder mediated release mechanism. The release profile was quantified at temperatures ranging from physiological (37 °C) to 80 °C. By changing the temperature, varying extents of release were attained over the time course of several days, ranging from 40% release for lower temperatures to complete release for the highest temperature considered. Further confirmation of a reaction-diffusion controlled release mechanism was obtained through comparison of experimental release data to a reaction-diffusion model of the release. In addition to thermal modulation, tuning of the release rate was accomplished by altering the number of possible Diels-Alder tethering sites present in the hydrogel. Increasing the amount of free maleimide and, therefore, the number of potential Diels-Alder reaction sites, effectively slowed the release of peptide from the polymer. For instance, doubling the amount of maleimide sites present in the hydrogel system decreased the amount of peptide released from approximately 60% to about 40% in the same span of time.