A molecularly engineered dual-crosslinked hydrogel with extraordinary mechanical properties is reported. The hydrogel network is formed with both chemical crosslinking and acrylic-Fe(III) coordination; these, respectively, impart the elasticity and enhance the mechanical properties by effectively dissipating energy. The optimal hydrogel achieves a tensile stress of ca. 6 MPa at a large elongation ratio (>7 times), a toughness of 27 MJ m(-3) , and a stiffness of ca. 2 MPa, and has good self-recovery properties.

Natural articular cartilage has ultralow friction even at high squeezing pressure. Biomimicking cartilage with soft materials has been and remains a grand challenge in the fields of materials science and engineering. Inspired by the unique structural features of the articular cartilage, as well as by its remarkable lubrication mechanisms dictated by the properties of the superficial layers, a novel archetype of cartilage-mimicking bilayer material by robustly entangling thick hydrophilic polyelectrolyte brushes into the subsurface of a stiff hydrogel substrate is developed. The topmost soft polymer layer provides effective aqueous lubrication, whereas the stiffer hydrogel layer used as a substrate delivers the load-bearing capacity. Their synergy is capable of attaining low friction coefficients (order 0.010) under heavily loaded conditions (order 10 MPa contact pressure) in water environment, a performance incredibly close to that of natural articular cartilage. The bioinspired material can maintain low friction even when subjected to 50k reciprocating cycles under high contact pressure, with almost no wear observed on the sliding track. These findings are theoretically explained and compounded by multiscale simulations used to shed light on the mechanisms responsible for this remarkable performance. This work opens innovative technology routes for developing cartilage-mimicking ultralow friction soft materials.

Despite extensive efforts to mimic the fascinating adhesion capability of geckos, the development of reversible adhesives underwater has long been lagging. The appearance of mussels-inspired dopamine chemistry has provided the feasibility to fabricate underwater adhesives; however, for such a system, imitating the reversible and fast dynamic attachment/detachment mechanism of gecko feet still remains unsolved. Here, by synthesizing a thermoresponsive copolymer of poly(dopamine methacrylamide-co-methoxyethyl-acrylate-co-N-isopropyl acrylamide) and then decorating it onto mushroom-shaped poly(dimethylsiloxane) pillar arrays, a novel underwater thermoresponsive gecko-like adhesive (TRGA) can be fabricated, yielding high adhesion during the attachment state above the lower critical solution temperature (LCST) of the copolymer, yet low adhesion during the detachment state below the LCST of the copolymer. By integrating the Fe O nanoparticles into the TRGA, TRGAs responsive to near-infrared laser radiation can be engineered, which can be successfully used for rapid and reversible remote control over adhesion so as to capture and release heavy objects underwater because of the contrast force change of both the normal adhesion force and the lateral friction force. It is also demonstrated that the material can be assembled on the tracks of an underwater mobile device to realize controllable movement. This opens up the door for developing intelligent underwater gecko-like locomotion with dynamic attachment/detachment ability.

Surface-grafting polymer brushes (SPB), which are used in a versatile technique to easily realize surface modifications, can be commonly used to change the inherent surface physical/chemical properties of materials. In particular, producing functional polymer brushes with well-defined chemical configurations, densities, architectures, and thicknesses on a material surface has become increasingly important in many fields. Achieving such goals is highly dependent on the progress of novel surface-grafting strategies, which are commonly based on surface-initiated polymerization (SIP) methods. On the other hand, practical applications have been given more attention since the SPB technique enables the engineering of materials with diverse functions. This review reports some new grafting strategies for generating polymer brush layers and then systematically summarizes research advances in the application of polymer brush-modified materials in multiple fields. Correspondingly, some necessary challenges of the SPB technique are unreservedly pointed out, with consideration given to its real applications in the future. The aim of this article is to tell readers how to engineer functional materials through SPB techniques and what can be done with polymer brushes in the future.

Integrating proteins into a hydrogel network enables its good bioactivity as an ECM environment in biorelative applications. Although extensive studies on preparing protein hydrogels have been carried out, the reported systems commonly present very low mechanical strength and weak water-rentention capacity. Learning from the astringent mouthfeel, we report here a protein engineered multinetwork physical hydrogel as TA-PVA/BSA. In a typical case, the BSA protein-integrated poly(vinyl alcohol) (PVA) solution is treated by the freeze-thaw method and forms the first hydrogel network, and tannic acid (TA) then cross-links with BSA proteins and PVA chains to form the secondary hydrogel network based on the noncovalent interaction (hydrogen bond and hydrophobic interaction). The as-prepared TA-PVA/BSA composite hydrogel is a pure physically cross-linking network and possesses ultrahigh tensile strength up to ∼9.5 MPa but is adjustable, relying on the concentration of TA and BSA. Moreover, its mechanical strength is further improved by prestretching induced anisotropy of mechanical performance. Because of its controllable and layered structure as skin, the composite hydrogel presents good water-retention capacity compared to traditional high strength hydrogels. This work demonstrates a novel method to design high mechanical strength but layered physical cross-linking hydrogels and enables us to realize their biorelative applications.

Nature has successfully combined soft matter and hydration lubrication to achieve ultra-low friction even at relatively high contact pressure (e.g. articular cartilage). Inspired by this, scientists have used hydrogels to mimic natural aqueous lubricating systems. However, hydrogels usually cannot bear high load because of solvation in water environments and are, therefore, not adopted in real applications. In this work, we developed a novel composite surface of ordered hydrogel nanofiber arrays confined in anodic aluminum oxide (AAO) nanoporous template based on a soft/hard combination strategy. The synergy between the soft hydrogel fibers, which provide excellent aqueous lubrication, and the hard phase AAO, which gives high load bearing capacity, is shown to be capable of attaining very low coefficient of friction (< 0.01) under heavy load (with mean contact pressures in the 2 MPa range). Interestingly, the composite synthetic material was very stable and could not be peeled off during sliding and exhibited the desirable regenerative (self-healing) properties, which can assure long term resistance to wear. Moreover, the crosslinked polymethylacrylic acid (PMAA) hydrogels were shown to be able to promptly switch between high friction (> 0.3) and superlubrication (∼10 −3 ) when their state was changed from contracted to swollen by means of acidbasic actuation. The mechanisms governing ultra-low and tunable friction are theoretically explained via an in-depth study of the chemo-mechanical interactions responsible for the behavior of these substrate-infiltrated hydrogels. These findings open a promising route for the design of ultra-slippery and smart surface/interface materials.

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antiwear/lubrication, [2] antifouling, [3,4] drag-reduction, [5] and bioactivity of medical materials. [6,7] The development of facile and versatile strategies for surface chemical modification to all materials is of immense scientific interest. To date, a number of methods have been developed for the functional modification of material surfaces including surface selfassembly, [8][9][10] surface chemical grafting based on coupling reaction, [11][12][13] surfaceinitiated radical polymerization, [14][15][16][17][18][19][20] etc. However, they all have their limitations. In typical case, surface self-assembly needs that the grafted surfaces are chemically reactive. Surface chemical grafting is generally not a straightforward process, for which it needs to preferentially introduce reactive groups on the grafted surfaces. Surface-initiated polymerizations also require attaching initiator onto the target surface and then graft polymers, which are generally sophisticated (inert atmosphere, UV, or heating aid). Importantly, it is still challenging to develop universal approach to modify a wide range of materials. Russell and co-workers reported a generalized approach to modify materials by spinning copolymers onto solid surfaces, [21] including metal, metal oxide, semiconductor, and polymers. The modification process is complex and under harsh condition, which needs a spinning temperature of 250 °C and a post-crosslinked treatment. Thus, the development of versatile strategy for surface modification of a wide range of materials is essential for practical applications. Inspired by excellent attachment capability of natural mussel adhesive proteins, [22] scientists discovered polyphenolic chemistry for universal surface modification. [23][24][25][26] For instance, polydopamine coating technology has been applied to a wide range of materials with any size, shape, and structure, [27,28] including noble metals, metal oxides, ceramics, and synthetic polymers, showing many potential applications, [27] especially for wet adhesion. [29] The approach is limited to the modification with polyphenolic compounds. Hydrogel is one kind of wet materials with 3D crosslinking network that can be used in wet and biological environment. Both of its strength, [30,31] toughness/ elasticity, [32,33] and water content [34] can be well controlled. Due to their remarkable features including hydrophilicity, elasticity, The development of versatile generalized strategies for easy surface modification is of immense scientific interest. Herein, a novel mechanism to form functional hydrogel coatings on a wide variety of substrate materials including polymers, polymeric resins, ceramics, and intermetallic compounds, enabling easy change of the surface wettability and lubrication property, is reported. In situ polymerization and hydrogel coating formation is initiated by free radicals generated through the redox reaction between Fe 2+ and S 2 O 8 2− at the solid-liquid interface, which shows controllable growth kinetics. Hydrogel modification is fast, co...