The noncovalent assembly of polymeric materials via specific molecular recognition interactions has become increasingly prominent in the production of responsive, reversible, and injectable hydrogels. While protein-protein interactions predominantly have been used for such assembly, 1-4 the use of extracellular matrix (ECM)-derived polysaccharide-peptide interactions has been recently emerging as an assembly approach. 5-9 The interactions of proteins and glycosaminoglycans are critical in mediating the multiple responses of the ECM, and materials in which assembly and mechanical properties are controlled by such molecular recognition events may be useful for biologically directed targeted delivery. Accordingly, the ability of heparin and related glycosaminoglycans to sequester and stabilize growth factors has been exploited in the production of surfaces and covalently crosslinked hydrogels that can mediate cell proliferation and migration, angiogenesis, and wound healing. 10-14 In particular, the controlled delivery of VEGF (vascular endothelial growth factor) and select other growth factors from polymeric matrices has shown potential for use in vascular therapies. 12,15,16 Noncovalently assembled matrices also have similar potential use in this regard. 5-9Here, we report the assembly, rheological properties, and targeted delivery/erosion profiles of noncovalently associated hydrogel networks produced via the interaction of a low-molecularweight heparin-modified star polymer (PEG-LMWH) and a dimeric, heparin-binding growth factor (VEGF). 17 Given that overexpression of VEGF and other growth factor receptors plays a key role in both normal healing and pathological conditions, 18-20 we reasoned that hydrogels assembled via such strategies may provide unique opportunities for stimuli-responsive delivery and erosion via biologically relevant, ligand-exchange mechanisms. A schematic of the assembly and erosion strategy is illustrated in Figure 1. We have focused on LMWH in these investigations in order to maximize intermolecular dimeric growth factor/LMWH binding over intramolecular binding (see Supporting Information). The PEGLMWH employed in these investigations was produced via Michael addition of thiol-terminated four-arm star PEG to maleimidefunctionalized LMWH. 7 NMR characterization of the purified product indicated at least 75% functionalization of PEG-LMWH, indicating that it is of sufficiently high functionality (f > 2) to permit formation of crosslinked networks upon interaction with VEGF (f = 2). The VEGF was expressed from E. coli and purified via heparin-affinity chromatography as previously described (Supporting Information). 16Correspondence to: Kristi L. Kiick. Hydrogels were formed via the mixing of homogeneous, lowviscosity solutions of each component in phosphate buffered saline (PBS). The PEG-LMWH solution was vortexed to ensure homogeneity. Addition of a solution of VEGF (5 μL, 2 mg/mL) to a solution of PEG-LMWH (5 μL, 80 mg/mL) immediately resulted in the formation of a self-supporting, viscoel...

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.