Living organisms share the ability to grow various microstructures on their surface to achieve functions. Here we present a force stamp method to grow microstructures on the surface of hydrogels ...based on a force-triggered polymerisation mechanism of double-network hydrogels. This method allows fast spatial modulation of the morphology and chemistry of the hydrogel surface within seconds for on-demand functions. We demonstrate the oriented growth of cells and directional transportation of water droplets on the engineered hydrogel surfaces. This force-triggered method to chemically engineer the hydrogel surfaces provides a new tool in addition to the conventional methods using light or heat, and will promote the wide application of hydrogels in various fields.
The field of polymer mechanochemistry has experienced a renaissance over the past decades, primarily propelled by the rapid development of force-sensitive molecular units (
i.e.
, mechanophores) and ...principles governing the reactivity of polymer networks for mechanochemical transduction or material strengthening. In addition to fundamental guidelines for converting mechanical energy input into chemical output, there has also been increasing focus on engineering applications of polymer mechanochemistry for specific functions, mechanically adaptive material systems, and smart devices. These endeavors are made possible by multidisciplinary approaches involving the development of multifunctional mechanophores for mechanoresponsive polymer systems, mechanochemical catalysis and synthesis, three-dimensional (3D) printed mechanochromic materials, reasonable design of polymer network topology, and computational modeling. The aim of this minireview is to provide a summary of recent advancements in covalent polymer mechanochemistry. We specifically focus on productive mechanophores, mechanical remodeling of polymeric materials, and the development of theoretical concepts.
Polymer mechanochemistry has experienced a renaissance over the past decades, primarily propelled by the rapid development of mechanophores and principles governing the mechanochemical transduction or material strengthening.
Ion is one of the most common additives that can impart electrical conductivity to insulating hydrogels. The concurrent toughening effect of ions, however, is often neglected. This work reports the ...extreme toughening of hydrogels via the synergistic effect of cations and anions, without the need for specific structure design or adding other reinforcements. The strategy is to equilibrate a physical double network hydrogel consisting of both multivalent cation‐ and kosmotropic anion‐sensitive polymers in specific salt solutions that can induce cross‐linking and salting‐out simultaneously. Both effects are proven positive to boost the mechanical performance and electrical conductivity of the original weak gel, and result in a tough conductive gel with exceptional physical properties, achieving significant enhancements in fracture stress, fracture energy, and ionic conductivity by up to 530‐, 1100‐, and 4.9‐folds, respectively. The optimal fracture stress and toughness reach approximately 15 MPa and 39 kJ m–2, exceeding most state‐of‐the‐art tough conductive hydrogels. Meanwhile, a satisfactory ionic conductivity of 1.5 S m–1 is attained. The presented simple strategy is also found generalizable to other salt ions and polymers, which is expected to expand the applicability of hydrogels to conditions involving demanding mechanical durability.
Strong tough conductive hydrogels are facilely developed by equilibrating an originally weak, crack sensitive, and poorly conductive hydrogels into specific salt solutions. The key to achieving the all‐around enhancement is to use the synergy of multivalent cation‐induced cross‐linking and kosmotropic anion‐induced salting‐out. The toughened hydrogels perform well as reliable solid electrolytes in soft electronics.
Double-network (DN) hydrogels have recently been demonstrated to generate numerous radicals by the homolytic bond scission of the brittle first network under the influence of an external force. The ...mechanoradicals thus generated can be utilized to trigger polymerization inside the gels, resulting in significant mechanical and functional improvements to the material. Although the concentration of mechanoradicals in DN gels is much higher than that in single-network hydrogels, a further increase in the mechanoradical concentration in DN gels will widen their application. In the present work, we incorporate an azoalkane crosslinker into the first network of DN gels. Compared with the traditional crosslinker N,N′-methylenebis(acrylamide), the azoalkane crosslinker causes a decrease in the yield stress but significantly increases the mechanoradical concentration of DN gels after stretching. In the azoalkane-crosslinked DN gels, the concentration of mechanoradicals can reach a maximum of ∼220 μM, which is 5 times that of the traditional crosslinker. In addition, DN gels with the azoalkane crosslinker show a much higher energy efficiency for mechanoradical generation. Interestingly, DN gels crosslinked by a mixture of azoalkane crosslinker and traditional crosslinker also exhibit excellent radical generation performance. The increase in the mechanoradical concentration accelerates polymerization and can broaden the application range of force-responsive DN gels to biomedical devices and soft robots.
Realizing tough adhesion between hydrogels and solid surfaces is crucial for developing emerging soft–rigid hybrid devices with a high level of complexity. However, this is extremely challenging for ...numerous non-adhesive hydrogels due to their weak interfacial interaction with solid surfaces and negligible mechanical dissipation. Here, we report a phase-separation strategy to transform traditionally non-adhesive hydrogels to tough glues for diverse solid surfaces, without the need for chemical treatment. Equilibrating a hydrogel in a mixture of both good and poor solvents induces phase separation, resulting in a significant increment of polymer volume fraction at the surface and in the bulk of the gel. The high-density polymer chains at the gel surface facilitate the formation of dense arrays of noncovalent bonds with the solid surfaces, improving the intrinsic work of adhesion and favoring the force transmission from the crack front to the bulk gel. Meanwhile, the phase-separated structure in the bulk gel allows significant mechanical dissipation upon interfacial separation. Such a synergy contributes to a high interfacial toughness. The tough adhesion, reaching over 1000 J m −2 , is instant and repeatable, with inappreciable loss in the interfacial toughness after over 100 attach/detach cycles. This facile, repeatable phase-separation approach is also universal, and can be induced by various mixed solvents and applies to multiple types of common non-adhesive hydrogels for tough yet detachable gel–solid adhesion.
Abstract Mineralized bio‐tissues achieve exceptional mechanical properties through the assembly of rigid inorganic minerals and soft organic matrices, providing abundant inspiration for synthetic ...materials. Hydrogels, serving as an ideal candidate to mimic the organic matrix in bio‐tissues, can be strengthened by the direct introduction of minerals. However, this enhancement often comes at the expense of toughness due to interfacial mismatch. This study reveals that extreme toughening of hydrogels can be realized through simultaneous in situ mineralization and salting‐out, without the need for special chemical modification or additional reinforcements. The key to this strategy lies in harnessing the kosmotropic and precipitation behavior of specific anions as they penetrate a hydrogel system containing both anion‐sensitive polymers and multivalent cations. The resulting mineralized hydrogels demonstrate significant improvements in fracture stress, fracture energy, and fatigue threshold due to a multiscale energy dissipation mechanism, with optimal values reaching 12 MPa, 49 kJ m −2 , and 2.98 kJ m −2 . This simple strategy also proves to be generalizable to other anions, resulting in tough hydrogels with osteoconductivity for promoting in vitro mineralization of human adipose‐derived mesenchymal stem cells. This work introduces a universal route to toughen hydrogels without compromising other parameters, holding promise for biological applications demanding integrated mechanical properties.
Rechargeable aqueous zinc-ion batteries (RAZIBs) offer low cost, high energy density, and safety but struggle with anode corrosion and dendrite formation. Gel polymer electrolytes (GPEs) with both ...high mechanical properties and excellent electrochemical properties are a powerful tool to aid the practical application of RAZIBs. In this work, guided by a machine learning (ML) model constructed based on experimental data, polyacrylamide (PAM) with a highly entangled structure was chosen to prepare GPEs for obtaining high-performance RAZIBs. By controlling the swelling degree of the PAM, the obtained GPEs effectively suppressed the growth of Zn dendrites and alleviated the corrosion of Zn metal caused by water molecules, thus improving the cycling lifespan of the Zn anode. These results indicate that using ML models based on experimental data can effectively help screen battery materials, while highly entangled PAMs are excellent GPEs capable of balancing mechanical and electrochemical properties.
Hydrogels with ultrafast response to environmental stimuli, possessing robust structural integrity and rapid self-recovery, have been considered as promising platforms for numerous applications, for ...example, in biomimetic materials and nanomedicine. Inspired by the bundled fibrous structure of actin, we developed a robust and ultrafast thermoresponsive fibrous hydrogel (TFH) by fully utilizing the weak noncovalent bonds and strong covalently cross-linked semiflexible electrospun fibrous nets. The TFH exhibits an ultrafast response (within 10 s), rapid self-recovery rate (74% within 10 s), tunable tensile strength (3–380 kPa), and high toughness (∼1560 J/m2) toward temperature. A multiscale orientation is considered to play a key role in the excellent mechanical properties at the fibrous mesh, fiber, and molecular scales. Furthermore, to take advantage of this TFH adequately, a novel kind of noodle-like hydrogel for thermo-controlled protein sorption based on the TFH is prepared, which exhibits high stability and ultrafast sorption properties. The bioinspired platforms hold promise as artificial skins and “smart” sorption membrane carriers, which provide a unique bioactive environment for tissue engineering and nanomedicine.
Realizing tough adhesion between hydrogels and solid surfaces is crucial for developing emerging soft-rigid hybrid devices with a high level of complexity. However, this is extremely challenging for ...numerous non-adhesive hydrogels due to their weak interfacial interaction with solid surfaces and negligible mechanical dissipation. Here, we report a phase-separation strategy to transform traditionally non-adhesive hydrogels to tough glues for diverse solid surfaces, without the need for chemical treatment. Equilibrating a hydrogel in a mixture of both good and poor solvents induces phase separation, resulting in a significant increment of polymer volume fraction at the surface and in the bulk of the gel. The high-density polymer chains at the gel surface facilitate the formation of dense arrays of noncovalent bonds with the solid surfaces, improving the intrinsic work of adhesion and favoring the force transmission from the crack front to the bulk gel. Meanwhile, the phase-separated structure in the bulk gel allows significant mechanical dissipation upon interfacial separation. Such a synergy contributes to a high interfacial toughness. The tough adhesion, reaching over 1000 J m
−2
, is instant and repeatable, with inappreciable loss in the interfacial toughness after over 100 attach/detach cycles. This facile, repeatable phase-separation approach is also universal, and can be induced by various mixed solvents and applies to multiple types of common non-adhesive hydrogels for tough yet detachable gel-solid adhesion.
Non-adhesive hydrogels are tailored to show tough adhesion to various solid surfaces by a universal phase-separation method. This method opens the possibility of utilizing non-adhesive hydrogels for emerging soft-rigid hybrid devices.
Electrospinning provides a facile and versatile method for generating nanofibers from a large variety of starting materials, including polymers, ceramic, composites, and micro-/nanocolloids. In ...particular, incorporating functional nanoparticles (NPs) with polymeric materials endows the electrospun fibers/sheets with novel or better performance. This work evaluates the spinnability of polyacrylamide (PAAm) solution containing thermoresponsive poly(N-isopropylacrylamide-co-tert-butyl acrylate) microgel nanospheres (PNTs) prepared by colloid electrospinning. In the presence of a suitable weight ratio (1:4) of PAAm and PNTs, the in-fiber arrangements of PNTs-electrospun fibers will evolve into chain-like arrays and beads-on-string structures by confining of PAAm nanofibers, and then the free amide groups of PAAm can bind amide moieties on the surfaces of PNTs, resulting in the assembling of PNTs in the cores of PAAm fibers. The present work serves as a reference in the fabrication of novel thermoresponsive hybrid fibers involving functional nanospheres via electrospun packing. The prepared nanofibers with chain-like and thermoresponsive colloid arrays in the cores are expected to have potential application in various fields.
