Janus gels for biomedical applications: Progress and future prospective

The "Janus" feature/structure inspired by the ancient Roman double-sided protector is prominent in the field of materials science due to its unique "asymmetric" concept and flexible and adjustable characteristics. The emergence of numerous biomaterials based on Janus properties/structures provides a different approach to material design for complex biomedical scenarios. Gel materials with excellent water absorption, flexibility and biocompatibility in various biomedical applications have greatly increased, and the structural design and functional integration of gels have reached some bottleneck. The Janus properties/structures completely subvert the traditional concept of "homogeneous gel" and break the limitation of "two-sided consistency" in biomedical gels. The concept of "two-sided asymmetry" led by "Adhesion-antiadhesion properties" and "hydrophilic-hydrophobic properties" has emerged and expanded the broad biomedical application prospects of Janus gels. In this review, we first summarize the various structural characteristics of Janus gel materials and the preparation technology of these gels, and explore the secret behind Janus structures from the raw materials and design concepts. Secondly, different kinds of asymmetries, including “hydrophilic-hydrophobic properties”, “Adhesion-antiadhesion properties”, structural heterogeneity and other unusual asymmetry, are discussed to show the relationship between Janus characteristics and structure. The applications of advanced Janus gels in biomedical fields such as tissue repair, anti-adhesion, substance delivery, hemostasis and human activity sensing are emphatically reviewed. In addition, the latest challenges and possible future direction of Janus gel are proposed.