Adjustable Tribological Behavior of Glucose-Sensitive Hydrogels

Friction Memory Ionogels With Hysteretic Sticky-Slippery Transition via Thermolocking the Metastable state

Many biological organisms possess adaptive friction states and display hysteretic friction recovery, allowing them to achieve specific friction memory after environment change. However, current artificial materials have limitations in maintaining on-demand friction states upon withdrawal of external triggers due to their strong dependence on external stimulus. Here, thermally induced phase separation ionogels with friction memory are reported. The kinetic difference between the evolution of bulk condensed structure and the adsorption of surface droplets after removing external stimulus delayed the recovery of friction state transition from slippery to sticky, thus achieving responsive friction memory. In addition, the metastable intermediate state generated during phase separation could be locked through vitrification, resulting in three distinct friction states determined by the synergy of stiffness and surface properties, including sticky state (coefficient of friction of 1.8), medium state (0.3-0.5) and slippery state (0.03-0.07). As a proof-of-concept, smart devices with diverse surface friction performance are engineered to realize various functions, including recognition of thermal range for target objects, de-icing, and movement manipulation. These results provide new insights for expanding the applications of ionogels in state-of-the-art soft robotics and haptic engineering, offering fresh perspectives for further development of these fields.

Bioinspired simultaneous regulation in fluorescence of AIEgen-embedded hydrogels

The development of stimuli-responsive functional fluorescent hydrogels is of great significance for the realization of artificial intelligence. In the present work, we design and synthesize a stimulus-responsive hydrogel embedded with an aggregation-induced emission (AIE) monomer, in which the fluorescence brightness and intensity can be tuned. The hydrogel embedded with tetraphenylethene-grafted-poly[3-sulfopropyl methacrylate potassium salt] (TPE-PSPMA) as the functional element is prepared by the radical polymerization method. Among them, the TPE core exhibits adaptive fluorescence ability through the AIE effect, while the PSPMA chain provides tunable hydrophilic properties under an external stimulus. The effect of different cationic surfactants with different lengths of hydrophobic tails on the fluorescence properties of TPE-PSPMA in solution is systematically investigated. With cationic surfactants, such as cetyltrimethylammonium bromide (CTAB), the fluorescence intensity is gradually tuned from 1059 to 4623. And the fluorescence intensities increase with the growth of hydrophobic tails of surfactants, which results from hydrophobicity-induced electrostatic interactions among surfactants and polymer chains. Furthermore, an obvious tunable fluorescence feature of hydrogel copolymerized TPE-PSPMA is realized, resulting from the change of brightness and the dynamic increase of fluorescence intensity (from 1031 to 3138) for the hydrogel immersed in CTAB solution with different soaking times. Such a typical fluorescence-regulated behavior can be attributed to the AIE of the TPE-PSPMA chain and the electrostatic interaction between the surfactant and the anionic polymer chain. The designed TPE-PSPMA-based hydrogel is responsive to stimuli, inspiring the development of intelligent systems such as soft robots and smart wearables.

Low-Friction Hybrid Hydrogel with Excellent Mechanical Properties for Simulating Articular Cartilage Movement

Hydrogels, which can withstand large deformations and have stable chemical properties, are considered a potential material for cartilage repair. However, hydrogels still face some challenges regarding their mechanical properties, tribological behavior, and biocompatibility. Thus, we synthesized a hybrid hydrogel by means of chemical cross-linking and transesterification using glycerol ethoxylate (GE) and zwitterionic polysulfobetaine methacrylate (PSBMA) as raw materials. The hybrid hydrogel showed excellent compressive stress at approximately 3.50 MPa and low loss factors (0.023-0.049). Moreover, because GE has good water binding properties, helping to form a stable hydration layer and maintain low energy dissipation, a low friction coefficient (μ ≈ 0.028) was obtained with the "soft-soft contact mode" of a hydrogel hemisphere and hydrogel disc under reciprocating motion. In vitro cytotoxicity, skin sensitization, and irritation reaction tests were carried out to show good biocompatibility of the GE-PSBMA hybrid hydrogel. In this study, a hybrid hydrogel with no potential cytotoxicity, strong compressive capacity, and excellent lubricity was obtained to provide a potential alternative for developing polymer hybrids, as well as demonstrating an idea for the application of hybrid hydrogels in cartilage replacement.

Programmable Mechanically Active Hydrogel-Based Materials

Programmable mechanically active materials (MAMs) are defined as materials that can sense and transduce external stimuli into mechanical outputs or conversely that can detect mechanical stimuli and respond through an optical change or other change in the appearance of the material. Programmable MAMs are a subset of responsive materials and offer potential in next generation robotics and smart systems. This review specifically focuses on hydrogel-based MAMs because of their mechanical compliance, programmability, biocompatibility, and cost-efficiency. First, the composition of hydrogel MAMs along with the top-down and bottom-up approaches used for programming these materials are discussed. Next, the fundamental principles for engineering responsivity in MAMS, which includes optical, thermal, magnetic, electrical, chemical, and mechanical stimuli, are considered. Some advantages and disadvantages of different responsivities are compared. Then, to conclude, the emerging applications of hydrogel-based MAMs from recently published literature, as well as the future outlook of MAM studies, are summarized.

Expansion tomography for large volume tissue imaging with nanoscale resolution

Expansion microscopy enables conventional diffraction limit microscopy to achieve super-resolution imaging. However, the enlarged tissue lacks an objective lens with sufficient working distance that can image tissues with whole-brain-scale coverage. Here, we present expansion tomography (ExT) to solve this problem. We have established a modified super-absorbent hydrogel (ExT gel) that possesses high mechanical strength and enables serial sectioning. ExT gel enables tissue and cell imaging and is compatible with various fluorescent labeling strategies. Combining with the high-throughput light-sheet tomography (HLTP) system, we have shown the capability of large volume imaging with nanoscale resolution of mouse brain intact neuronal circuits. The ExT method would allow image samples to support super-resolution imaging of intact tissues with virtually unlimited axial extensions.

Cartilage Mimics Adaptive Lubrication

The natural cartilage layer exhibits excellent interface low friction and good load-bearing properties based on the mechanically controlled adaptive lubrication mechanism. Understanding and imitating such a mechanism is important for developing high-load-bearing water-lubrication materials. Here, we report the successful preparation of thermoresponsive layered materials by grafting a poly(3-sulfopropyl methacrylate potassium salt) (PSPMA) polyelectrolyte brush onto the subsurface of an initiator-embedded high strength hydrogel [poly(N-isopropylacrylamide-co-acrylic acid-co-initiator/Fe³⁺)] [P(NIPAAm-AA-iBr/Fe³⁺)]. The top soft hydrogel/brush composite layer provides aqueous lubrication, while the bottom thermoresponsive hydrogel layer exhibits adaptive load-bearing capacity that shows tunable stiff or modulus in response to the temperature above and below the lower critical solution temperature (LCST, 32.5 °C). An obvious friction-reduction feature is realized above the LCST, resulting from the dynamic increase of the bottom layer mechanical modulus. Furthermore, in situ lubrication-improvement behavior is achieved upon applying a near-infrared (NIR) laser onto the surface of Fe₃O₄ nanoparticle (NP)-integrated layered materials. Such a typical lubrication-regulated behavior can be attributed to the synergy effect of the improved load-bearing capacity of the bottom layer and the enhanced lubrication behavior of the top layer with an increase in the polyelectrolyte brush chain density, which is similar to the mechanically controlled adaptive lubrication mechanism of the natural cartilage layer. Current research results provide an inspiration for developing novel biomimetic lubrication materials with considerable load-bearing capacity and also propose a strategy for designing intelligent/stable friction-actuation devices.

Zwitterionic Hydrogel Incorporated Graphene Oxide Nanosheets with Improved Strength and Lubricity

Graphene oxide (GO) has been evaluated as a multifunctional cross-linker or reinforcement agent in composite hydrogels. In this study, a nanocomposite hydrogel consisting of GO nanosheets and zwitterionic poly(sulfobetaine methacrylate) (PSBMA) was synthesized in an aqueous system via chemical and physical cross-linking effects. GO nanosheets were well dispersed in the hydrogels and effectively cross-linked into the sulfobetaine methacrylate (SBMA) polymer chains through the electrostatic interactions. The PSBMA hydrogel exhibited a significant enhancement in the compressive stress (close to a 5-fold increase) and a remarkable reduction in the coefficient of friction (COF) (corresponding to a decline of 52-76%) after the embedding of GO nanosheets. These improvements indicate the existence of synergetic interaction and good compatibility between GO nanosheets and the PSBMA hydrogel matrix, which results in an intertwined network structure with higher load-bearing capacity and better lubrication properties. This study provides potential in the development of new graphene-polymer composites, which is beneficial for cartilage replacement with high mechanical properties and excellent lubrication characteristics.