Fabrication, Structure, Mechanical Properties, and Applications of Tetra-PEG Hydrogels
A new type of polymer network with extraordinarily low degrees of spatial, topological, and connectivity inhomogeneities has been developed. The gel is called “Tetra-PEG gel” after its structure and the name of constituents. Tetra-PEG gels are made by the cross-end-coupling of four-armed poly(ethylene glycol)(PEG) macromers. Two types of Tetra-PEG macromers carrying complementary end-functional groups are coupled by mixing them in aqueous solutions, resulting in Tetra-PEG gels. Mechanical measurements, such as compression/elongation measurements and viscoelastic measurements, suggest that Tetra-PEG gels are well described by the phantom network model. The gel has a homogeneous structure and resultant high mechanical strength comparable to that of native articular cartilage. Furthermore, since Tetra-PEG gel is formed by mixing two biocompatible macromer aqueous solutions, the gelation reaction itself and the resultant gel are also biocompatible. Small-angle neutron scattering studies indicate that Tetra-PEG gels are simply described by an Ornstein–Zernike scattering function without accompanying additional scattering originated from spatial inhomogeneities. These experimental findings strongly suggest that Tetra-PEG gels are near-ideal polymer networks free from cross-linking defects and entanglements. Thus, by controlling the homogeneity and connectivity of the network structure, a high strength hydrogel has been successfully designed and fabricated. This methodology will lead to a variety of applications and increase the understanding of rubber elasticity of polymer networks by providing a universal strategy for designing high-strength gels, thus opening up the biomedical applications of hydrogels. This chapter reviews the recent advances in the study of Tetra-PEG gels and forecasts future directions in their applications.