Hydrogels with controlled, surface erosion characteristics from self-assembly of fluoroalkyl-ended poly(ethylene glycol)

Poly(ethylene glycol) (PEG) (M_w = 6k, 10k, and 20k g/mol) terminated at both ends by hydrophobic fluoroalkyl segments (-(CH₂)₂C_nF_2n+1, n = 6, 8, or 10) was synthesized and demonstrated to self-assemble into hydrogels with phase behavior and mechanical and erosion properties that can be systematically varied by molecular design. With increasing fluoroalkyl length relative to PEG length, the phase behavior of these polymers in aqueous solution changes from the single-phase behavior of familiar associative thickeners, to sol-gel coexistence, to precipitation. For those polymers that exhibit sol-gel coexistence, the equilibrium gel concentration (or swelling ratio of the gel phase) and the modulus of the gel phase are governed by the length of the PEG midblock, whereas the relaxation time is determined by the hydrophobe length. The erosion characteristics of these hydrogels correlate with their phase behavior. The gels of sol-gel coexisting species exhibit surface erosion in an open system with a slow dissolution rate controlled by the end-group length; in contrast, hydrogels from polymers that show single-phase behavior exhibit bulk erosion that is relatively fast. Therefore, the molecular structure of this class of polymers produces hydrogels whose mechanical and erosion properties can be tailored for desired applications. Based on the established biocompatibility of PEG, the degree to which the characteristics of the gel phases can be tailored, and the surface erosion characteristics that can be achieved, these materials might have applications in implantable drug-release depots.