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.