Bioinspired Structural Composite Hydrogel with a Combination of Mechanical and Thermal Properties

Hydrogels are water-filled three-dimensionally crosslinked polymer networks with completely different characteristics compared to typical materials like liquids, metals, ceramics, and plastics. Hydrogels have been utilized as soft robot components, flexible backbones of bioelectronics, conductive membranes, and artificial biological tissues. For these applications, developing hydrogels with a desirable combination of stiffness and toughness is a critical issue.<br/>Since the development of soft yet tough double-network hydrogel, many studies have exhibited synthetic hydrogels applicable for soft tissues whose elastic moduli range from a few pascals to several kilopascals. Despite that, creating much strong, stiff, and tough hydrogels similar to load-bearing tissues, such as tendons and cartilages with elastic moduli in the MPa-to-GPa range, still remains challenging.<br/>To achieve such mechanical property, in this study, we apply a bioinspired structure and fracture mechanisms into a double-network hydrogel and develop structural composite hydrogels. A natural structural composite, nacre, which consists of ductile polymeric matrices and stiff inorganic microplatelets (microsized tablets) in a layered structure, has mechanisms of an effective load transfer from the matrix to the platelet and fracture resistance through platelet pull-out. That leads to the combination of stiffness and toughness. Indeed, we had previously developed composite materials (a kind of plastic) and demonstrated the reinforcing effect and mechanism of layered inorganic microplatelets in polymer matrices of the material.<br/>We expand this approach to a wet hydrogel system and present a bioinspired structural composite hydrogel composed of tough double-network polymer matrices and stiff alumina microplatelets assembled in a layered structure. To fabricate such hydrogel, we suggest a straightforward method, which is a reconstruction process comprising “drying-induced unidirectional shrinkage of a pre-gel” and “additional crosslinking with rehydration in an ionic solution”. During unidirectional drying/shrinkage, the pre-gel of the random compound becomes a thin film of layer-by-layer assembled microplatelets within highly densified polymer matrices; and then, subsequent crosslinking/rehydration fastens the densified layered structure with fully crosslinked polymer networks. During the process, the polymer–platelet interactions are also enhanced.<br/>As a result, the structural composite hydrogel exhibited highly enhanced mechanical performance, withstanding more load and dissipating more energy along with a crack deflection and platelet pull-out. The resulting tensile strengths, elastic moduli (on the order of several MPa), and high fracture energies (up to 2 kJ m^-2) are comparable to those of the stiff load-bearing tissues such as the ligament and tendon. Such mechanically robust hydrogel with the densified polymer matrix and strong polymer–platelet interactions also exhibited high ionic conductivity. The conductive hydrogel containing cations, such as Li⁺ and Zn⁺, was utilizable as a stable gel electrolyte almost without compromising mechanical performance. Further, the layered alumina platelets allowed the hydrogel to possess outstanding in-plane thermal diffusivity. When the hydrogel is introduced to serve as a gel electrolyte membrane, the hydrogel is expected to prevent rigid electronic components (electrodes) from inducing heat localization and thermal damage as well as to avoid a mechanical mismatch.