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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 magnitude1,2,3, but its fatigue threshold, reinforced or not, has remained approximately 100 J m−2 for decades4,5,6,7. 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. 

Traditional polymer processing breaks polymer chains. The resulting networks of short chains have a low fatigue threshold. This paper shows that a low-intensity process preserves long chains, leading to a network of an increased fatigue threshold. 

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. 

This paper studies a polymer network in which crosslinks are degradable but polymer chains are not. We show that entanglements markedly enhance the mechanical properties of the polymer network before degradation and slow down degradation. We synthesize polyacrylamide hydrogels with disulfide crosslinks. In a precursor of a low water-to-monomer molar ratio and low crosslinker-to-monomer molar ratio, the monomers are crowded and the resulting polymer chains are long, so that the entanglements greatly outnumber crosslinks. The as-synthesized hydrogels are submerged in pure water to swell to equilibrium. We show that entanglements enhance the swell resistance of the hydrogel, as well as stiffen and toughen the hydrogel. We further show that entanglements slow down degradation when the hydrogel is submerged in an aqueous solution of cysteine. This work demonstrates that entanglements substantially expand the properties space of degradable polymers. 

ynthesis-property relation is fundamental to materials science, but many aspects of the relation are not well understood for many materials. Impetus for this paper comes from our recent appreciation for the distinct roles of entanglements and crosslinks in a polymer network. Here we study the synthesis-property relation of polyacrylamide hydrogels prepared by free radical polymerization. Some of the as-prepared hydrogels are further submerged in water to swell either to equilibrium or to a certain polymer content. The synthesis parameters include the composition of a precursor, as well as the polymer content of a hydrogel. Series of hydrogels are prepared along several paths in the space of synthesis parameters. For each hydrogel, the stress-stretch curve is measured, giving four properties: modulus, strength, stretchability, and work of fracture. We interpret the experimentally measured synthesis-property relation in terms of entropic polymer networks of covalent bonds. When the precursor has a low crosslinker-to-monomer molar ratio, the resulting polymer network has on average long polymer segments. When the precursor has a low water-to-monomer molar ratio, the resulting polymer network has on average many entanglements per polymer segment. We show that crosslinks lower strength, but entanglements do not. By contrast, both crosslinks and entanglements increase modulus. A network of highly entangled long polymer segments exhibits high swell resistance, modulus, and strength. 

When a poly mer net workis stretched, so me poly mer strands do not bearloads. Exa mplesinclude looped strands, dangling strands, and extre mely long strands. When the poly mer net work is sub mergedin asolvent, ho wever, allstrands mix withsolvent molecules. This distinction bet ween strandsthat bearloads andstrandsthat do notleads usto modifythe Flory-Rehner model. The modified modelhasthreeparameters:thedensityofload-bearingstrands, N,thedensityofall strands,M , andtheinteraction para meter,χ. For a poly mer net worksub mergedin areservoir of solvent and bearingtriaxialstresses,the modified model provides equations ofstate,relatingthe stressestostretches, as well asthe che mical potential ofsolvent molecules andte mperatureinthe reservoir. We synthesize polyacryla mide hydrogels using precursors of various concentrations of mono mer, crosslinker, and transfer agent. We deter mine the three para meters N, M , and χ by fittingthe modified modeltoseveral experi ments,includingfrees welling,fasttension, andstress relaxation. In all sa mples tested, M is several ti mes N , whereas χ is nearly constant. This work de monstrates the consequences of the notion that so me poly mer strands in a poly mer net work b e ar l o a ds, b ut all str a n ds c o ntri b ut e t o s w elli n g. 

Abstract A hydrogel is often fabricated from preexisting polymer chains by covalently crosslinking them into a polymer network. The crosslinks make the hydrogel swell‐resistant but brittle. This conflict is resolved here by making a hydrogel from a dough. The dough is formed by mixing long polymer chains with a small amount of water and photoinitiator. The dough is then homogenized by kneading and annealing at elevated temperatures, during which the crowded polymer chains densely entangle. The polymer chains are then sparsely crosslinked into a polymer network under an ultraviolet lamp, and submerged in water to swell to equilibrium. The resulting hydrogel is both swell‐resistant and tough. The hydrogel also has near‐perfect elasticity, high strength, high fatigue resistance, and low friction. The method is demonstrated with two widely used polymers, poly(ethylene glycol) and cellulose. These hydrogels have never been made swell‐resistant, elastic, and tough before. The method is generally applicable to synthetic and natural polymers, and is compatible with industrial processing technologies, opening doors to the development of sustainable, high‐performance hydrogels. 

Electronic devicesforrecording neuralactivityinthe nervoussyste m needto bescalableacrosslargespatialandte mporalscales whilealso providing millisecondandsingle-cellspatiote mporalresolution. H o w e v e r, e xi s ti n g hi g h- r e s ol u ti o n n e u r al r e c o r di n g d e vi c e s c a n n o t achievesi multaneousscalability on bothspatialandte mporallevels due toatrade-offbetweensensordensityand mechanicalflexibility. Here weintroduceathree-di mensional(3D)stackingi mplantableelectronic platfor m,basedonperfluorinateddielectricelasto mersandtissue-levelsoft multilayerelectrodes,thatenablesspatiote mporallyscalablesingle-cell neuralelectrophysiologyinthenervoussyste m. Ourelasto mersexhibit stable dielectric perfor mancefor overayearin physiologicalsolutions andare10,000ti messofterthanconventional plastic dielectrics. By leveragingthese uniquecharacteristics we developthe packaging of lithographednano metre-thickelectrodearraysina3Dconfiguration with across-sectionaldensityof7.6electrodesper100μ m2.Theresulting3D integrated multilayersoftelectrodearrayretainstissue-levelflexibility, reducingchronici m muneresponsesin mouse neuraltissues,and de monstratestheabilitytoreliablytrackelectricalactivityinthe mouse brain orspinalcord over months without disruptingani mal behaviour. 

Biological tissues, such as cartilage, tendon, ligament, skin, and plant cell wall, simultaneously achieve high water content and high load-bearing capacity. The high water content enables the transport of nutrients and wastes, and the high load-bearing capacity provides structural support for the organisms. These functions are achieved through nanostructures. This biological fact has inspired synthetic mimics, but simultaneously achieving both functions has been challenging. The main difficulty is to construct nanostructures of high load-bearing capacity, characterized by multiple properties, including elastic modulus, strength, toughness, and fatigue threshold. Here we develop a process that self-assembles a nanocomposite using a hydrogel-forming polymer and a glass-forming polymer. The process separates the polymers into a hydrogel phase and a glass phase. The two phases arrest at the nanoscale and are bicontinuous. Submerged in water, the nanocomposite maintains the structure and resists further swelling. We demonstrate the process using commercial polymers, achieving high water content, as well as load-bearing capacity comparable to that of polyethylene. During the process, a rubbery stage exists, enabling us to fabricate objects of complex shapes and fine features. We conduct further experiments to discuss likely molecular origins of arrested phase separation, swell resistance, and ductility. Potential applications of the nanocomposites include artificial tissues, high-pressure filters, low-friction coatings, and solid electrolytes. 

A pneumatic soft robot can be made autonomous by carrying a liquid chemical fuel. In the existing design, to transmit the fuel, the pressure of the fuel tank must exceed that of the actuator. Consequently, the fuel tank must be sufficiently stiff, which hardens the robot. Herein, inspired by pit membranes in trees, a chemical pump is developed, which is consisting of a nanoporous membrane between the fuel tank and the actuator, and coated with a catalyst on the side of the actuator. The fuel in the fuel tank migrates across the membrane and, on meeting the catalyst, decomposes into a pressurized gas and inflates the actuator. The chemical pump is driven by the free energy of reaction, against the difference in pressure. The pores in the membrane are large enough for the fuel molecules to migrate through, but small enough to block the pressurized gas to tunnel back. In a demonstration, the fuel tank has ambient pressure, and the actuator has a pressure of 350 kPa, comparable to the pressure in a car tire. The chemical pump enables pneumatic robots to be autonomous, powerful, and soft.