Imparting conformational memory for material adhesion.

Adhesion between similar and dissimilar materials is essential to many biological systems and synthetic materials, devices, and machines. Since the inception of adhesion science more than five decades ago, adhesion to a surface has long been recognized as beyond two-dimensional. Similarly, molecular conformation - the three-dimensional arrangement of atoms in a molecule - is ubiquitous in biology and fundamental to the binding of biomolecules. However, the connection between these concepts, which could link molecular conformation in biology to micro- and macroscopic adhesion in materials science, remains elusive. Herein, we examine this connection by manipulating the molecular conformation of a mussel-inspired universal coating, which imparts a memory for recognizing different hydrogels. This approach leads to significantly (several fold) increased interfacial adhesion between the coating and hydrogels across a broad range of length scales, from molecular to macroscopic. Furthermore, we demonstrate that imparting memory is a general and facile noncovalent approach for enhancing interfacial adhesion that, with suitable energy dissipation, can be used for the bonding of materials.

Catechol-functionalized hydrogels: biomimetic design, adhesion mechanism, and biomedical applications.

Hydrogels are a unique class of polymeric materials that possess an interconnected porous network across various length scales from nano- to macroscopic dimensions and exhibit remarkable structure-derived properties, including high surface area, an accommodating matrix, inherent flexibility, controllable mechanical strength, and excellent biocompatibility. Strong and robust adhesion between hydrogels and substrates is highly desirable for their integration into and subsequent performance in biomedical devices and systems. However, the adhesive behavior of hydrogels is severely weakened by the large amount of water that interacts with the adhesive groups reducing the interfacial interactions. The challenges of developing tough hydrogel-solid interfaces and robust bonding in wet conditions are analogous to the adhesion problems solved by marine organisms. Inspired by mussel adhesion, a variety of catechol-functionalized adhesive hydrogels have been developed, opening a door for the design of multi-functional platforms. This review is structured to give a comprehensive overview of adhesive hydrogels starting with the fundamental challenges of underwater adhesion, followed by synthetic approaches and fabrication techniques, as well as characterization methods, and finally their practical applications in tissue repair and regeneration, antifouling and antimicrobial applications, drug delivery, and cell encapsulation and delivery. Insights on these topics will provide rational guidelines for using nature's blueprints to develop hydrogel materials with advanced functionalities and uncompromised adhesive properties.

A hybrid material that reversibly switches between two stable solid states.

Most types of solid matter have a single stable solid state for a particular set of conditions. Nonetheless, materials with distinct, interchangeable solid states would be advantageous for several technological applications. Here, we describe a material composed of a polymer impregnated with a supercooled salt solution, termed as sal-gel, that assumes two distinct but stable and reversible solid states under the same conditions for a range of temperatures (-90 to 58 °C) and pressure. On transient stimulation of nucleation, the material switches from a clear and soft solid to a white and hard state, which can be 104 times stiffer than the original (15 kPa versus 385 MPa). This hard solid becomes soft again by transient heating, demonstrating the reversibility of the transition. This concept, exploiting the robust physical metastability of a liquid state, is extended to sugar alcohols, resulting in a stimuli-responsive and non-evaporating sug-gel. These 'two-in-one' solid materials may find potential uses in soft robotics and adhesive applications.