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Adhesion strategies for heterogeneous soft materials—A review

Adhesion is a long-term research topic of great importance attracting plenty of attention. In past few years, many remarkable advances have been made in the field of heterogeneous soft material adhesion, considerably boosting the functional diversity and viability of heterogeneous soft material assemblies. This review focuses on the status quo of this field with attention leaning towards recently active and outstanding bioinspiration and biomimetic areas. First, the fundamental mechanisms of adhesion, including chemical bonds, topological entanglement, and gradient connections, are introduced. Then influence factors on adhesion strength are derived from the adhesion mechanisms. The influence factors are classified into two categories: physical factors, including interface geometry (interface stress singularity for macroscopic and interface roughness for microscopic) and mismatch of material properties, and chemical factors, including chemical complementarities and ambient conditions. Abundant examples from recent representative research efforts are provided to demonstrate how these factors are tuned to enable or improve an adhesion. As a fabrication process dealing with multiple material interfaces, additive manufacturing (AM) is discussed separately emphasizing on its space and time discretization nature. After that, several adhesion enhancement strategies are presented, among them is one synergistically integrating covalent bonds, noncovalent complexes, polymer chains/networks, and nanoparticles. Finally, based on the current research status, it is proposed that general methodologies for adhesion strategy selection and simulation models capable of accurately predicting adhesion strength are greatly needed. Heterogeneous material adhesion is also expected to acquire insights and impetuses from fields like cell differentiation and tissue development.