Rheological Characterization of Polysaccharide−Poly(ethylene glycol) Star Copolymer Hydrogels

Binding interactions between low molecular weight heparin (LMWH) and heparin-binding peptides (HBP) have been applied as a strategy for the assembly of hydrogels that are capable of sequestering growth factors and delivering them in a controlled manner. In this work, the assembly of four-arm star poly(ethylene glycol) (PEG)−LMWH conjugate with PEG−HBP conjugates has been investigated. The interactions between LMWH and the heparin-binding regions of antithrombin III (ATIII) or the heparin interacting protein (HIP) have been characterized via heparin affinity chromatography and surface plasmon resonance (SPR); results indicate that the two peptides have slightly different affinities for heparin and LMWH, and bind LMWH with micromolar affinity. Solutions of the PEG−LMWH and of mixtures of the PEG−LMWH and PEG−HBP were characterized via both bulk rheology and laser tweezer microrheology. Interestingly, solutions of PEG−LMWH (2.5 wt % in PBS) form hydrogels in the absence of PEG−ATIII or PEG−HIP, with storage moduli, determined via bulk rheological measurements, in excess of the loss moduli over frequencies of 0.1−100 Hz. The addition of PEG−ATIII or PEG−HIP increases the moduli in direct proportion to the number of cross-links introduced. Characterization of the hydrogels via microrheology shows the gel microstructure is composed of polymer-rich fibrillar structures surrounded by polymer-depleted buffer. Potential applications of these hydrogels are discussed.