Advances in materials, designs, and controls are propelling the field of soft robotics at an incredible rate; however, current methods for prototyping soft robots remain cumbersome and struggle to incorporate desirable geometric complexity. Herein, a vat photopolymerizable self-healing elastomer system capable of extreme elongations up to 1000% is presented. The material is formed from a combination of thiol/acrylate mixed chain/step-growth polymerizations and uses a combination of physical processes and dynamic-bond exchange via thioethers to achieve full self-healing capacity over multiple damage/healing cycles. These elastomers can be three dimensional (3D) printed with modular designs capable of healing together to form highly complex and large functional soft robots. Additionally, these materials show reprogrammable resting shapes and compatibility with self-healing liquid metal electronics. Using these capabilities, subcomponents with multiple internal channel systems were printed, healed together, and combined with functional liquid metals to form a high-wattage pneumatic switch and a humanoid-scale soft robotic gripper. The combination of 3D printing and self-healing elastomeric materials allows for facile production of support-free parts with extreme complexity, resulting in a paradigm shift for the construction of modular soft robotics.

Polymer materials containing dynamic bonds have many potential applications including adhesives, elastomers, and coatings with long lifetimes. Interpenetrated networks (IPNs) were studied, where one network had covalent linkers, and the other network had dynamic quadruple-hydrogen-bonded 2-ureido-4­[1H]-pyrimidinone (UPy) linkers. IPNs typically have superior mechanical properties to each component network. IPNs had either nonpolar poly­(ethyl acrylate) (PEA) or hydrogen-bond-rich poly­(2-hydroxyethyl acrylate) (PHEA) matrixes. Although the PHEA materials have more hydrogen bonds, the self-healing, toughness, and fracture energies were poorer than the PEA systems. This suggests that strong and dynamic hydrogen bonds, even at the potential expense of total hydrogen bonds, should be chosen for applications that require toughness such as high-performance coatings, sealants, or elastomers.

Dynamic bonds introduce unique properties such as self-healing, recyclability, shape memory, and malleability to polymers. Significant efforts have been made to synthesize a variety of dynamic linkers, creating a diverse library of materials. In addition to the development of new dynamic chemistries, fine-tuning of dynamic bonds has emerged as a technique to modulate dynamic properties. This Review highlights approaches for controlling the timescales of dynamic bonds in polymers. Particularly, eight dynamic bonds are considered, including urea/urethanes, boronic esters, Thiol-Michael exchange, Diels-Alder adducts, transesterification, imine bonds, coordination bonds, and hydrogen bonding. This Review emphasizes how structural modifications and external factors have been used as tools to tune the dynamic character of materials. Finally, this Review proposes strategies for tailoring the timescales of dynamic bonds in polymer materials through both kinetic effects and modulating bond thermodynamics.

Dynamic covalent Diels-Alder chemistry was combined with multiwalled carbon nanotube (CNT) reinforcement to make strong, tough and conductive dynamic materials. Unlike other approaches to functionalizing CNTs, this approach uses Diels-Alder...

The mechanical properties of nonequilibrium polymer hydrogels obtained from the transient cross-linking of polymer chains by a chemical fuel were investigated. Aqueous polymers featuring pendant carboxylic acids were treated with a carbodiimide to give anhydride-cross-linked gels. The anhydrides spontaneously hydrolyze back to the polymer solution, and the cycle can be repeated multiple times. Oscillatory rheology was employed to study the effects of temperature, fuel concentration, chain length, and polymer composition on the storage and loss moduli of the polymeric materials as well as the time taken for the polymers to undergo decross-linking. Regardless of the temperature used, at constant carbodiimide concentration, degelation times are more sensitive to experimental temperature than are the peak storage moduli. Decross-linking times decrease with increasing temperature. As carbodiimide concentration decreases, there is a decrease in moduli and decross-linking times. Within the scope of materials studied, the polymer structure was found to have a relatively small impact on the transient properties of gel networks compared to the fuel concentration and temperature. These findings facilitate the design of tunable on-demand networks and gels.