Diels–Alder Mediated\nControlled Release from\na Poly(ethylene glycol) Based Hydrogel

Abstract

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