Multiscale stress deconcentration amplifies fatigue resistance of rubber.

Rubbers reinforced with rigid particles are used in high-volume applications, including tyres, dampers, belts and hoses1. Many applications require high modulus to resist excessive deformation and high fatigue threshold to resist crack growth under cyclic load. The particles are known to greatly increase modulus but not fatigue threshold. For example, adding carbon particles to natural rubber increases its modulus by one to two orders of magnitude1-3, but its fatigue threshold, reinforced or not, has remained approximately 100 J m-2 for decades4-7. Here we amplify the fatigue threshold of particle-reinforced rubbers by multiscale stress deconcentration. We synthesize a rubber in which highly entangled long polymers strongly adhere with rigid particles. At a crack tip, stress deconcentrates across two length scales: first through polymers and then through particles. This rubber achieves a fatigue threshold of approximately 1,000 J m-2. Mounts and grippers made of this rubber bear high loads and resist crack growth over repeated operation. Multiscale stress deconcentration expands the space of materials properties, opening doors to curtailing polymer pollution and building high-performance soft machines.

Hydrogels of arrested phase separation simultaneously achieve high strength and low hysteresis.

Hydrogels are being developed to bear loads. Applications include artificial tendons and muscles, which require high strength to bear loads and low hysteresis to reduce energy loss. However, simultaneously achieving high strength and low hysteresis has been challenging. This challenge is met here by synthesizing hydrogels of arrested phase separation. Such a hydrogel has interpenetrating hydrophilic and hydrophobic networks, which separate into a water-rich phase and a water-poor phase. The two phases arrest at the microscale. The soft hydrophilic phase deconcentrates stress in the strong hydrophobic phase, leading to high strength. The two phases are elastic and adhere through topological entanglements, leading to low hysteresis. For example, a hydrogel of 76 weight % water, made of poly(ethyl acrylate) and poly(acrylic acid), achieves a tensile strength of 6.9 megapascals and a hysteresis of 16.6%. This combination of properties has not been realized among previously existing hydrogels.

Covalent Topological Adhesion.

Tough adhesion between wet materials (i.e., synthetic hydrogels and biological tissues) is undergoing intense development, but methods reported so far either require functional groups from the wet materials, involve toxic chemicals, or result in unstable adhesion. Here, we present a method to achieve biocompatible, covalent adhesion, without requiring any functional groups from the wet materials. We use two hydrogels as model adherends that have covalent polymer networks, but have no functional groups for adhesion. We use an aqueous solution of biopolymers and bioconjugate agents as a model adhesive. When the solution is spread at the interface of the hydrogels, the biopolymers diffuse into both hydrogels and cross-link into a covalent network in situ, in topological entanglement with the two polymer networks of the hydrogels. We characterize the chemistry and mechanics of the covalent topological adhesion. In a physiological fluid, the covalent topological adhesion is stable, but a noncovalent topological adhesion separates. Covalent topological adhesion presents immediate opportunities to advance the art of adhesion in diverse and complex environments.