1. Background
Rubber reinforced with rigid particles is widely used in tires, shock absorbers, belts, and hoses. Many applications require high modulus to resist excessive deformation and high fatigue thresholds to resist crack growth under cyclic loads. These particles are known to greatly increase the modulus, but not the fatigue threshold. For example, the addition of carbon particles to natural rubber increases its modulus by one or two orders of magnitude, but its fatigue threshold, whether enhanced or not, has remained around 100 J m-2 for decades. 
2. Research results
Here, the team of Prof. Zhigang Suo and Prof. Yakov Kutsovsky of Harvard University amplified the fatigue threshold of particle-reinforced rubber through multi-scale stress dispersion. They synthesized a rubber in which a long, highly entangled polymer adheres tightly to rigid particles. At the crack tip, the stress is dispersed on two length scales: first through the polymer and then through the particles. The fatigue threshold for this rubber is about 1,000 J m-2. Brackets and clamps made of this rubber withstand high loads and resist crack propagation during repeated operation. Multi-scale stress dispersion expands the space for material properties, opening the door to reducing polymer contamination and manufacturing high-performance soft machines. The related research work was published in the top journal "Nature" under the title of "Multiscale Stress Deconcentration Amplifies Fatigue Resistance of Rubber". Congratulations!
3. **Express
Figure 1The synergistic effect of long polymers, polymeric microparticles, and strong adhesion between polymers and microparticles raises the fatigue threshold of filled rubber (abbreviated as particle-reinforced elastomers or composites) consisting of a network of cross-linked polymer chains and a network of permeable rigid particles. They form a network of polymers with long polymer chains, and the number of entanglements greatly exceeds the number of cross-links. Individual particles are much larger than individual polymer segments between tangles. The polymer chains and particles are connected to each other by strong bonds. As the particle volume fraction increases, the particles aggregate and penetrate. They functionalized poly(ethyl acrylate) (PEA) and silica nanoparticles of 3-(trimethoxysilyl)propyl methacrylate (TPM) as model systems. Their data will show that long polymers, aggregate particles, and strong polymer-particle adhesion work synergistically to raise the fatigue threshold. Consider a crack that impacts a polymer chain. They chose a polymer network with low friction between the polymer chains, so that the stresses were distributed throughout the polymer chain. The breaking of a single bond on the chain consumes the energy stored in each bond on the chain. Next, consider the cracks that hit the particle clusters.
Figure 2Composites under monotonically tensile conditions The molar ratio of the crosslinker to the monomer of each composite is c, and the volume fraction of the particles is f. They monotonically stretch the composite until it breaks. Composites are initially transparent, but turn white when stretched significantly. After unloading, the sample returned to transparency. The slope of the stress-tensile curve at small stretches defines the modulus. At fixed c = 10-4, the modulus of pure PEA is 07 MPa, while the hardness of composites is much higher, at f = 045 MPa at 14. In fixed f = 0At 45, the stress-tensile curve varies greatly with c. Based on the stress-tensile curve, they measured properties at break, such as strength, maximum tensile strength, and work at break, which showed different trends with c and f. They also measured the storage modulus, loss modulus, and loss tangent as a function of elongation through dynamic mechanical analysis. The C-value of composites has the same trend as pure PEA. When c < 10-25, the modulus tends to stabilize, indicating that the entanglement greatly exceeds the cross-linking. When C 10-4, the composite does not cure. In addition, the modulus increases with f. They also plotted a function of e vs. f. 
Figure 3Composites under cyclic stretch conditions They use rigid fixtures to grip the two long sides of the composite rectangular film and circularly stretch the film between one and a moderate amplitude amp. The stress-tensile curve varies with the initial cycle, but reaches a steady state after about 1,000 cycles. Using a single membrane, they can obtain steady-state stress-tensile curves at different amps. They studied the recovery of the composite by applying a loading sequence of 5000 cycles followed by a 1 h recovery. After a sequence, the composite almost regains a stretch. 
Figure 4Applications of particle-reinforced elastomers with high stiffness and fatigue resistanceMany applications require elastomer materials with complex shapes and textured surfaces. Examples include gloves, textured straps, tire treads, and stamps for soft lithography. It has long been thought that particle-reinforced elastomers can be molded into complex shapes, as particles are often much smaller than the characteristic dimensions of the final shape. Here, they made an aluminum mold with fine features by laser cutting and used it to cast the particle-reinforced elastomer. Cast molds can be greatly deformed. Many applications require materials to withstand loads over multiple cycles. Examples include shock absorbers, seals, and O-rings. For elastomeric materials, the load-bearing capacity is determined by modulus, while a high fatigue threshold allows for a large number of cycles. Here, they used a cylindrical rubber bracket with cracks to demonstrate the importance of high modulus and high fatigue thresholds. When the suspension is compressed, the crack opens. Mounts without particles are soft and either break under low stress or are excessively deformed. Mounts with particles and short chains are stiffer but break at smaller loads. In contrast, there are particles and long chains (f = 0.).45 and c = 10-4) are stiff and can withstand large loads repeatedly. For this mounting, the crack does not propagate significantly after 33,000 cycles. In soft robotics, materials need to be subjected to loads with large displacements and repetitive displacements. They demonstrated the importance of high modulus and high fatigue thresholds in a recently designed compliant gripper. They chose this application because the kirigami was designed to be cut and the jig needed to withstand cyclic deformation. The gripper is made from a square sheet with a cut parallel to a diagonal. When pulled diagonally along another diagonal, the sheet bends outwards in plane, thus gripping the object. Grippers with particles require more force to close than clamps without particles. They cycle closing and opening the gripper. f = 0.45. A gripper with C = 10-2 will break in just a few cycles. In contrast, f = 045. The permanent deformation of the fixture with C = 10-4 after n = 350,000 cycles is negligible, and the crack does not propagate significantly. Next, they grasp a sphere with a clamp and measure the force required to lift the sphere. Grippers with long polymer chains and leachate particles lift up to six times the load of grippers made of pure PEA and maintain this lifting force after multiple cycles.
4. Conclusions and prospects
For elastomers with much larger entanglements than cross-links, dense entanglements can set high modulus, while sparse cross-links can set high fatigue thresholds. However, the tangle density is limited by the molecular weight of the tangle, and the modulus of elastomers is about 1 MPa. A recent technology has made it possible to synthesize highly entangled elastomers from prepolymers, potentially being used to create particle-reinforced elastomers with high fatigue resistance. In interpenetrating polymer networks, the short-chain network provides a high modulus and the long-chain network provides a high fatigue threshold. However, a modulus of 1-4 MPa is still too low for many applications. Semi-crystalline rubbers can have both high modulus and high fatigue thresholds. For example, filled natural rubber crystallizes under strain, and when loaded under non-relaxation conditions, strain-induced crystallization greatly increases the fatigue threshold. However, strain-induced crystallization melts with each stretch cycle relaxation, resulting in a fatigue threshold of approximately 100 J m-2. Thermoplastic elastomers can achieve high modulus and high fatigue thresholds, but creep is present. Elastomers reinforced with fibers and fabrics can achieve high modulus and high thresholds, but are difficult to make complex shapes and small features. As demonstrated in this study, composites with tangled long polymer chains and leachate particles can also achieve high modulus and high thresholds, and provide more options for designing materials. The effect of rigid particles on modulus has been recognized for a century, but the synergistic effect of aggregate particles and long polymer chains on the fatigue threshold has not been found. Multi-scale stress distribution is suitable for all types of rubber. These materials can be used in high-volume applications such as tires and belts, as well as emerging applications such as soft robotics and wearables. In addition to load-bearing capacity, particulates also give rubber other functions such as electrical conductivity, optical transparency, structural color, magnetic actuation, and high dielectric constant. The high load-bearing capacity and the variety of features such as this open up a huge space for material design and application.