Takuro Matsunaga

As a new class of high-strength hydrogels, we designed a tetra-PEG gel by combining two symmetrical tetrahedron-like macromonomers of the same size. Because the nanostructural unit of the gel network was defined by the length of the tetrahedral PEG arm, the gel had a homogeneous structure and resultant high mechanical strength comparable to that of native articular cartilage. Furthermore, since the gel was formed by mixing two biocompatible macromonomer solutions, the gelation reaction itself and the resultant gel were also biocompatible. The breaking strength had local maxima at the overlap concentration of the macromonomers (C*) and at 2C*. Dynamic light scattering measurement indicated the near absence of inhomogeneities in the network at C*. Thus, we successfully designed and fabricated a high-strength hydrogel by controlling the homogeneity of network structure for the first time, which will lead to multiplied effects, i.e., contributing to the understanding of ideal networks, providing a universal strategy for designing high-strength gels, and opening up the biomedical application of hydrogels.

The structure of Tetra-PEG gel, a new class of biocompatible, easy-made, and high-strength hydrogel consisting of a four-arm polyethylene glycol (PEG) network, has been investigated by means of small-angle neutron scattering (SANS). Since the Tetra-PEG gel is prepared by cross-end-coupling two kinds of four-arm PEG macromers having different functional groups at the ends, i.e., amine group and succinimidyl ester group respectively, the coupling reaction occurs exclusively between PEG chains carrying different functional groups. SANS results showed that the four-arm PEG macromer aqueous solutions and Tetra-PEG gels were successfully described by the theoretical scattering function for multiarm Gaussian chains and the Ornstein−Zernike function, respectively. Surprisingly, no noticeable excess scattering that originated from cross-linking was observed in Tetra-PEG gels, suggesting that its network structure is extremely uniform. Investigations on nonstoichiometric Tetra-PEG gels showed weakening of the mechanical properties as well as an increase of dangling chains (defects) in the network. It is concluded that Tetra-PEG gels have an extremely uniform network structure, probably mimicking a diamond-like structure, and this is one of the reasons for the advanced mechanical properties of Tetra-PEG gels.

A series of model networks consisting of polyethylene glycol (PEG), tetra-PEG gels, have been prepared and their structure and dynamics have been investigated by small-angle neutron scattering (SANS) and static light scattering (SLS). The Tetra-PEG gels were prepared by cross-end coupling of two types of tetra-arm PEG macromers with molecular weights, Mw, of (5 to 40) × 103 g/mol. In the SANS regime, the structure factors of both as-prepared and swollen gels can be represented by Ornstein−Zernike-type scattering functions and superimposed to single master curves with the reduced variables, ξq and I(q)/ϕ0ξ2, irrespective of the molecular weight of tetra-PEG, where q, ξ, I(q), and ϕ0 are the magnitude of the scattering vector, the correlation length, the scattering intensity, and the polymer volume fraction at preparation, respectively. In the SLS regime, however, a power-law-type upturn was observed, indicating the presence of PEG chain clusters. Interestingly, these inhomogeneities disappear by swelling. It is concluded that Tetra-PEG gels can be an “ideal polymer network” with a self-similar structure with respect to Mw without significant entanglements and/or defects. This explains why Tetra-PEG gels have high mechanical strength as reported elsewhere (Macromolecules 2008, 41, 5379).

After decades of efforts by many researchers, we have succeeded in realizing a near-ideal polymer network. This network, the Tetra network, is made by cross-end-coupling of tetra-arm polymer modules. The mechanical energy dissipation was extremely low (tan δ ≈ 10(-4) ). The macroscopic stress-strain relationship of the Tetra network was in good agreement with that of microscopic elastic blobs. The maximum breaking strength was extremely high (≥27 MPa). These results indicate that the Tetra network is closer to an ideal polymer network than any other conventional model networks. Because the Tetra network can be treated as uniformly packed elastic blobs, it should help apply the knowledge of single polymer chains seamlessly to the design of polymer materials and help further develop the theory of rubber elasticity.