Stiffand Tough Hydrogels Prepared Through Integration of Ionic Cross-linking and Enzymatic Mineralization

Enzymatic mineralization has become an effective approach to enhancing the stiffness of hydrogels for bone tissue engineering, but generally with limited toughness. On the other hand, double network cross linking provides hydrogel with enhanced toughness. In this study, we integrated double cross-linking method with enzymatic mineralization to synthesize stiff and tough hydrogels. We have synthesized three kinds of sodium alginate-polyacrylamide (SA-PAM) double-network hydrogels and systematically compared the composition and structure differences, mechanical properties, and biological properties of the different hydrogels in the absence and presence of mineralization. In particular, we examined the role of specific cross-linking ions, i.e., calcium, zinc and strontium ions, in modulating the mineralization process. Synergistic effect of ionic cross-linking and enzymatic mineralization was clearly observed with dramatic increase in compressive modulus. In particular, mineralized hydrogel cross-linked with Sr2+ showed the highest compressive Young's modulus of 17.28 +/- 3.56 MPa, which was 37 times of that of the original hydrogel. In addition, it had the highest tensile Young's modulus at 2.60 +/- 0.25 MPa and 84 +/- 5.5% elongation at break. Such synergistic effect from Sr2+ was attributed to a more uniformed mineralization process due to the early initiation of a more homogeneous nucleation process and subsequent denser mineralized structure. Cellular study also suggested that cell proliferation, adhesion and osteogenic differentiation were improved as a result of enzymatic mineralization. Our results provided an effective way for the preparation of stiffand tough hydrogels with osteogenesis, and demonstrated potential in bone tissue engineering applications. Statement of significance Hydrogels with excellent stiffness, stability and biocompatibility have attracted significant attentions in the bone tissue engineering applications. Our results suggested that the synergistic effect of ionic crosslinking and enzymatic mineralization rendered more enhancement of the compressive and tensile stiffness of SA-PAM DN hydrogels, as well as the toughness, swelling stability and cellular response. In particular, mineralized hydrogel cross-linked with Sr2+ showed the highest compressive Young's modulus of 17.28 +/- 3.56MPa, which was 37 times of that of the original hydrogel. Such synergistic effect from Sr2+ was attributed to a more uniformed mineralization process. The cell proliferation, adhesion and osteogenic differentiation were greatly improved as a result of enzymatic mineralization, where the MSCs cultured on strontium ion cross-linked mineralized hydrogel showed the best performance. (c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study integrates double network cross-linking with enzymatic mineralization to synthesize stiff and tough hydrogels. The synergistic effect of ionic cross-linking and enzymatic mineralization enhances the compressive and tensile stiffness of the hydrogels, as well as the toughness, swelling stability, and cellular response. The mineralized hydrogel cross-linked with Sr2+ shows the highest compressive Young's modulus and exhibits improved cell proliferation, adhesion, and osteogenic differentiation. This research provides an effective approach for preparing hydrogels with osteogenesis and demonstrates their potential in bone tissue engineering applications.