Fundamental, mechanism and development of hydration lubrication: From bio-inspiration to artificial manufacturing.

Friction and lubrication are ubiquitous in all kinds of movements and play a vital role in the smooth operation of production machinery. Water is indispensable both in the lubrication systems of natural organisms and in hydration lubrication systems. There exists a high degree of similarity between these systems, which has driven the development of hydration lubrication from biomimetic to artificial manufacturing. In particular, significant advancements have been made in the understanding of the mechanisms of hydration lubrication over the past 30 years. This enhanced understanding has further stimulated the exploration of biomimetic inspiration from natural hydration lubrication systems, to develop novel artificial hydration lubrication systems that are cost-effective, easily transportable, and possess excellent capability. This review summarizes the recent experimental and theoretical advances in the understanding of hydration-lubrication processes. The entire paper is divided into three parts. Firstly, surface interactions relevant to hydration lubrication are discussed, encompassing topics such as hydrogen bonding, hydration layer, electric double layer force, hydration force, and Stribeck curve. The second part begins with an introduction to articular cartilage in biomaterial lubrication, discussing its compositional structure and lubrication mechanisms. Subsequently, three major categories of bio-inspired artificial manufacturing lubricating material systems are presented, including hydrogels, polymer brushes (e.g., neutral, positive, negative and zwitterionic brushes), hydration lubricant additives (e.g., nano-particles, polymers, ionic liquids), and their related lubrication mechanism is also described. Finally, the challenges and perspectives for hydration lubrication research and materials development are presented.

A Near-Infrared Photothermal-Responsive Underwater Adhesive with Tough Adhesion and Antibacterial Properties.

Developing tunable underwater adhesives that possess tough adhesion in service and easy detachment when required remains challenging. Herein, a strategy is proposed to design a near infrared (NIR) photothermal-responsive underwater adhesive by incorporating MXene (Ti3 C2 Tx )-based nanoparticles within isocyanate-modified polydimethylsiloxane (PDMS) polymer chains. The developed adhesive exhibits long-term and tough adhesion with an underwater adhesion strength reaching 5.478 MPa. Such strong adhesion is mainly attributed to the covalent bonds and hydrogen bonds at the adhesive-substrate interface. By making use of the photothermal-response of MXene-based nanoparticles and the thermal response of PDMS-based chains, the adhesive possesses photothermal-responsive performance, exhibiting sharply diminished adhesion under NIR irradiation. Such NIR-triggered tunable adhesion allows for easy and active detachment of the adhesive when needed. Moreover, the underwater adhesive exhibits photothermal antibacterial property, making it highly desirable for underwater applications. This work enhances the understanding of photothermal-responsive underwater adhesion, enabling the design of tunable underwater adhesives for biomedical and engineering applications.

An underwater stable and durable gelatin composite hydrogel coating for biomedical applications.

Developing underwater stable and durable hydrogel coatings with drag-reducing, drug release, and antibacterial properties is essential for lots of biomedical applications. However, most hydrogel coatings cannot meet the requirement of underwater stability and versatility, which severely limits their widespread use. In this work, an underwater stable, durable and substrate-independent gelatin composite hydrogel (GMP) coating is developed through covalent crosslinks, where a silane coupling agent with an unsaturated double bond is grafted onto a substrate of co-deposited polydopamine and polyethylenimine. GMP coating can be easily coated onto various medical device surfaces, such as artificial joints, catheters, tracheal tubes and titanium alloys, showing excellent structural stability and mechanical tunability under extreme conditions of ultrasonic treatment for 1 h (400 W of ultrasonic power) or underwater shearing for 14 days (400 rpm). Besides, friction experiment reveals that GMP coating exhibits good lubrication properties (coefficient of friction < 0.003). The drug-loading and bacterial inhibition ring tests show that the GMP coating has a tunable drug release ability with the final releasing ratios of 70-95% by changing the content of poly (ethylene glycol) diacrylate. This work offers a scalable approach of fabricating bio-functional and stable hydrogel coatings, which can be potentially used in biomedical applications.

Manufacturing and post-engineering strategies of hydrogel actuators and sensors: From materials to interfaces.

Living bodies are made of numerous bio-sensors and actuators for perceiving external stimuli and making movement. Hydrogels have been considered as ideal candidates for manufacturing bio-sensors and actuators because of their excellent biocompatibility, similar mechanical and electrical properties to that of living organs. The key point of manufacturing hydrogel sensors/actuators is that the materials should not only possess excellent mechanical and electrical properties but also form effective interfacial connections with various substrates. Traditional hydrogel normally shows high electrical resistance (~ MΩ•cm) with limited mechanical strength (<1 MPa), and it is prone to fatigue fracture during continuous loading-unloading cycles. Just like iron should be toughened and hardened into steel, manufacturing and post-treatment processes are necessary for modifying hydrogels. Besides, advanced design and manufacturing strategies can build effective interfaces between sensors/actuators and other substrates, thus enhancing the desired mechanical and electrical performances. Although various literatures have reviewed the manufacture or modification of hydrogels, the summary regarding the post-treatment strategies and the creation of effective electrical and mechanically sustainable interfaces are still lacking. This paper aims at providing an overview of the following topics: (i) the manufacturing and post-engineering treatment of hydrogel sensors and actuators; (ii) the processes of creating sensor(actuator)-substrate interfaces; (iii) the development and innovation of hydrogel manufacturing and interface creation. In the first section, the manufacturing processes and the principles for post-engineering treatments are discussed, and some typical examples are also presented. In the second section, the studies of interfaces between hydrogels and various substrates are reviewed. Lastly, we summarize the current manufacturing processes of hydrogels, and provide potential perspectives for hydrogel manufacturing and post-treatment methods.

Facile Synthesis of Water-Soluble Rhodamine-Based Polymeric Chemosensors via Schiff Base Reaction for Fe3+ Detection and Living Cell Imaging.

Quantitative and accurate determination of iron ions play a vital role in maintaining environment and human health, but very few polymeric chemosensors were available for the detection of Fe3+ in aqueous solutions. Herein, a water-soluble rhodamine-poly (ethylene glycol) conjugate (DRF-PEG), as a dual responsive colorimetric and fluorescent polymeric sensor for Fe3+ detection with high biocompatibility, was first synthesized through Schiff base reaction between rhodamine 6G hydrazide and benzaldehyde-functionalized polyethylene glycol. As expected, the introduction of PEG segment in DRF-PEG significantly improved the water solubility of rhodamine derivatives and resulted in a good biosensing performance. The detection limit of DRF-PEG for Fe3+ in pure water is 1.00 µM as a fluorescent sensor and 3.16 µM as a colorimetric sensor at pH 6.5. The specific sensing mechanism of DRF-PEG toward Fe3+ is proposed based on the intramolecular charge transfer (ICT) mechanism, in which the O and N atoms in rhodamine moiety, together with the benzene groups from benzaldehyde-modified PEG segment, participate in coordination with Fe3+. Furthermore, DRF-PEG was applied for the ratiometric imaging of Fe3+ in HeLa cells and showed the potential for quantitative determination of Fe3+ in fetal bovine serum samples. This work provides insights for the design of water-soluble chemosensors, which can be implemented in iron-related biological sensing and clinical diagnosis.

Long Noncoding RNA ZFAS1 Promotes Progression of Oral Squamous Cell Carcinoma Through Targeting miR-6499-3p/CCL5 Axis.

BACKGROUND: Oral squamous cell carcinoma (OSCC) ranks sixth among malignancies in the world, and there are 200,000 new cases annually, rendering OSCC a significant global public health issue that has caused great burdens on patients, society, and the economy. Despite great progress in diagnosis and treatment methods, patient survival has not been greatly enhanced. Hence, there is an urgent need to identify novel targets that can serve as early diagnostic and therapeutic biomarkers for OSCC. Long non-coding RNAs (lncRNAs) participate in several cancer types, including OSCC. This work identified the competing endogenous RNA network related to lncRNA zinc finger nuclear transcription factor, X-box binding 1-type containing 1 antisense RNA 1 (ZFAS1) in OSCC, as well as the corresponding downstream targets. MATERIALS AND METHODS: Firstly, we identified the lncRNA ZFAS1 levels in OSCC cells and tissues and confirmed its relationship to tumor progression. Secondly, we identified a lncRNA-miRNA-mRNA network, which was closely associated with OSCC development using bioinformatics methods. Next, our hypothesis that lncRNA ZFAS1 modulates OSCC progression was verified with in vitro and in vivo experiments. RESULTS: Firstly, we found lncRNA ZFAS1 expression increased within OSCC cells and tissues and was positively associated with tumor progression. Secondly, its lncRNA-miRNA-mRNA network was determined, and the target of ZFAS1 was identified as miR-6499-3p/C-C motif chemokine ligand 5 (CCL5). Mechanistically, we found that ZFAS1 up-regulated CCL5 by competitively sponging miR-6499-3p. Further studies demonstrated that ZFAS1 promoted tumor progression in vivo and in vitro. CONCLUSION: Our results indicate that ZFAS1 serves as a crucial oncogenic factor in OSCC occurrence and development and may therefore serve as a possible therapeutic target for OSCC.

Multi-functional rhodamine-based chitosan hydrogels as colorimetric Hg2+ adsorbents and pH-triggered biosensors.

HYPOTHESIS: Water contamination from heavy metal ions is a major global environmental concern. Adsorbents based on biomaterials have been demonstrated to possess remarkable removal efficiency for metal ions, but the adsorption model of biosorbents is not clear and much efforts should be devoted to study the adsorption behaviors and understand the adsorption mechanism. EXPERIMENTS: The multifunctional rhodamine-modified chitosan (RMC) hydrogel for Hg2+ adsorption with fluorescent turn-ON properties was fabricated through grafting the rhodamine-modified poly (ethylene glycol) benzaldehyde (RM-PEG) onto the hydrogel network serving as the fluorescence/colorimetric sensing receptor. The adsorption behaviors and colorimetric sensing mechanism of RMC hydrogel towards Hg2+ were investigated in detail. FINDINGS: RMC hydrogel can remove more than 96.5% of Hg2+ from aqueous solution with significant fluorescence response and colorimetric change. The high adsorption selectivity and colorimetric sensing mechanism of RMC hydrogel towards Hg2+ can be explained by the hard and soft acid/base (HSAB) theory. The O atom in hydroxyl and carbonyl groups together with the N atom in amine/imine groups of RMC hydrogel play a vital role in the adsorption of Hg2+, while the colorimetric response and fluorescence enhancement of the hydrogel after adsorption are attributed to the specific spiro-lactam structure of rhodamine moieties. The adsorption isotherms and kinetics were investigated and well described by Freundlich isotherm and pseudo-second-order kinetic model. Furthermore, RM-PEG showed low cytotoxicity towards mouse embryonic fibroblast cells and RMC hydrogel can be used as a fluorescent pH indicator from 4.2 to 7.4, demonstrating the potential applications of RMC hydrogel in biological diagnosis.

circVAPA promotes the proliferation, migration and invasion of oral cancer cells through the miR-132/HOXA7 axis.

OBJECTIVE: To study the relationship between the circular RNA vesicle-associated membrane protein-associated protein A (circVAPA) and the pathogenesis of oral squamous cell carcinoma. METHODS: The expression of circVAPA was detected by RT-qPCR. In vitro loss-of-function experiments were performed in Cal-27 cells. The malignant phenotype of cells was evaluated by cell counting kit-8, clone formation and transwell assays. Luciferase reporter assays were used to assess the circVAPA/miR-132/homeobox A (HOXA) regulatory axis. RESULTS: circVAPA expression was significantly increased in oral cancer tissues and cells. The overall survival and progression-free survival of patients with oral cancer who exhibited high circVAPA expression were significantly shorter compared with those with low expression. circVAPA expression was closely related to tumor size, TNM stage and distant metastasis. circVAPA knockdown reduced the proliferation, invasion and migration of Cal-27 cells. MiR-132 was identified as a target of circVAPA in Cal-27 cells. Cotransfection with si-circVAPA and miR-132 inhibitor reversed the inhibitory effect of circVAPA knockdown on cell malignant phenotypes. HOXA7 was further identified as a downstream target of miR-132. CONCLUSION: circVAPA is highly expressed in oral cancer, and its abnormal expression might affect the proliferation, invasion and migration of oral cancer cells by modulating the miR-132/HOXA7 signaling axis.

An ultra-stretchable glycerol-ionic hybrid hydrogel with reversible gelid adhesion.

Functional hydrogels have attracted enormous interest as wet adhesives for biomedical research and engineering applications. However, reversible hydrogel adhesives that can be used for gelid conditions were rarely reported. In this work, we have developed a freezing-tolerant (freezing temperature < -50 °C), ultra-stretchable (stretch strain > 30000% at 25 °C) glycerol-ionic hydrogel via the ultraviolet curing of acrylamide monomer and hyper-branched polyethylenimine polymer in CaCl2-water-glycerol solution. The fabricated hydrogel exhibited reversible gelid adhesion, rapid self-healing (recover in 3 s) and weight-retaining (>2 weeks) properties. The hydrogel allows two iron substrates to adhere together at -40 °C with the lap-shear adhesion strength as high as ~1 MPa. Such strong adhesion measured was reversible, specifically achieving ~100% of initial adhesion strength at 25 °C and ~36% at -40 °C. Additionally, decreasing the testing temperature significantly improved the tensile strength but decreased the fracture strain of the hydrogel. Interestingly, lap-shear adhesion tests suggested that the gelid adhesion strength was enhanced by 130 times as the testing temperature decreased from 25 °C to -40 °C, which was mainly attributed to the enhanced mechanical strength of the bulk hydrogel as well as the increased surface interaction at gel-substrate interfaces. More importantly, the adhesion failure gradually changed from cohesive failure to adhesive failure as the temperature decreased. This work provides new practical and fundamental insights into developing multifunctional freezing-tolerant hydrogel adhesive for gelid conditions.

Antifouling and pH-Responsive Poly(Carboxybetaine)-Based Nanoparticles for Tumor Cell Targeting.

Nanocarriers with responsibility and surface functionality of targeting molecules have been widely used to improve therapeutic efficiency. Hence, we report the assembly of pH-responsive and targeted polymer nanoparticles (NPs) composed of poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) as the core and poly(carboxybetaine methacrylate) (PCBMA) as the shell, functionalized with cyclic peptides containing Arginine-Glycine-Aspartic acid-D-Phenylalanine-Lysine (RGD). The resulting polymer NPs (PDPA@PCBMA-RGD NPs) can maintain the pH-responsivity of PDPA (pKa ~6.5) and low-fouling property of PCBMA that significantly resist non-specific interactions with RAW 264.7 and HeLa cells. Meanwhile, PDPA@PCBMA-RGD NPs could specifically target αvß3 integrin-expressed human glioblastoma (U87) cells. The pH-responsiveness and low-fouling properties of PDPA@PCBMA NPs are comparable to PDPA@poly(ethylene glycol) (PDPA@PEG) NPs, which indicates that PCBMA is an alternative to PEG for low-fouling coatings. The advantage of PDPA@PCBMA NPs lies in the presence of carboxyl groups on their surfaces for further modification (e.g., RGD functionalization for cell targeting). The reported polymer NPs represent a new carrier that have the potential for targeted therapeutic delivery.

Probing the Self-Assembly and Nonlinear Friction Behavior of Confined Gold Nano-Particles.

For the wide application of nanoparticles (NPs) (e.g., in nanotribology), it is of fundamental and practical importance to understand the self-assembly and lubrication behavior of confined NPs. In this work, a systematic study was conducted to probe the assembly and associated surface forces of spherical gold nanoparticles (Au NPs, diameter ∼5 nm) confined between pairs of mica (negatively charged) and (3-aminopropyl)triethoxysilane modified mica (APTES-mica, positively charged) surfaces using a surface forces apparatus (SFA) under aqueous conditions. It is observed that Au NPs were squeezed out of the confined gap between two mica surfaces during the loading process, resulting from the repulsive electric-double layer force. In contrast, multilayers of Au NPs were confined between two APTES-mica surfaces because of the attractive double-layer force between oppositely charged Au NPs and APTES-mica. Interestingly, the interaction between Au NPs and APTES-mica is stronger than the interactions between Au NPs, resulting in the rearrangement of the confined Au NPs under shearing. Importantly, a large friction coefficient (µ > 0.7) with unexpected nonlinear stick-slip friction was observed when sliding two APTES-mica surfaces with thin layers of Au NPs (∼20 nm) confined in between. The observed stick-slip motion could be explained by the velocity-dependent friction model where a critical shear velocity was required for transiting from stick-slip to smooth sliding. Our study provides useful information on the assembly and interaction forces of confined nanoparticles on charged surfaces, with implications for predicting the behaviors of NPs under confinement in various engineering applications.

Probing the Molecular Interactions and Lubrication Mechanisms of Purified Full-Length Recombinant Human Proteoglycan 4 (rhPRG4) and Hyaluronic Acid (HA).

Probing the adsorption and lubrication behavior of lubricin, also known as proteoglycan 4 (PRG4), is important for understanding the ultralow friction of cartilage lubrication. Most previous research has focused on native lubricin either purified from synovial fluid or articular cartilage explant culture media. In this work, the adsorption behavior and lubrication mechanism of full-length recombinant human PRG4 (rhPRG4) on mica as well as the effect of adding hyaluronic acid (HA, a polysaccharide) were systematically investigated using a surface forces apparatus (SFA) technique. A low friction coefficient (µ ∼ 0.04) was measured when multilayer rhPRG4 (∼31 nm) was confined in between mica surfaces, even when the load increased to ∼1.2 MPa. Intriguingly, a previously unreported ultralow friction coefficient (µ < 0.005) was observed at a low sliding velocity ( v = 0.14 µm/s) with the applied load P reaching ∼3.6 MPa when a diluted rhPRG4 solution (∼90 µg/mL) was used. The distinct friction behavior is likely due to the smooth and more close-packed lubricin coating, as made evident by the atomic force microscope imaging. Adding HA onto multilayer rhPRG4-coated mica increased the friction coefficient µ to ∼0.1; however, the load bearing property increased, indicating potential synergistic effect between rhPRG4 and HA, which was further demonstrated by the weak adhesion observed when separating rhPRG4-coated mica and HA-coated aminopropyltriethoxysilane-mica (APTES-mica). Alternatively, adding premixed rhPRG4-HA on mica had a friction coefficient (µ ∼ 0.1) close to that of injecting concentrated rhPRG4 (∼450 µg/mL) with lower load sustainability. Our results provide fundamental insights into the adsorption and lubrication behavior of lubricin and its interaction with HA, with useful implications for the underlying mechanism of ultralow friction provided by synovial fluid.

Robust polymer nanofilms with bioengineering and environmental applications via facile and highly efficient covalent layer-by-layer assembly.

This work reports novel, robust, multifunctional covalent-bonded polymer nanofilms based on highly efficient spin-assisted layer-by-layer (LbL) assembly, with a wide range of engineering and bioengineering applications. An active-ester block copolymer, polypentafluorophenylacrylate-block-polystyrene (PPFPA-b-PS), and an amine-rich polymer, branched polyethyleneimine (PEI), were chosen as model polymers. The as-prepared nanofilms show switchable hydrophobicity with controllable thickness. Nanomechanical tests demonstrate that the surface adhesion between the PPFPA-b-PS and PEI layers facilitates excellent film stability under different solvent conditions. Robust and centimeter-scale freestanding nanofilms with good transparency and excellent flexibility could be readily obtained by peeling the films from their silicon substrates without using a sacrifice layer. More importantly, the multi-layer nanofilms containing free pentafluorophenol groups or amine groups can be readily functionalized further to tailor the properties of the films for various applications. As a proof of concept, a multi-layer nanofilm with free ester groups was modified with rhodamine-6G hydrazone and tested as a H+ sensor. The free amine groups in the polymer nanofilms show strong interactions with perfluorooctanoic acid for achieving high surface hydrophobicity. The polymer nanofilms also show exceptional tunable cell-attachment behavior by switching the outmost layer between PPFPA-b-PS and PEI, demonstrating great potential for biomedical applications.

Octadecyltrichlorosilane Deposition on Mica Surfaces: Insights into the Interface Interaction Mechanism.

Surface functionalization by alkylsilanes has been widely used in many engineering applications. In this work, a systematic investigation was conducted on the deposition behaviors of octadecyltrichlorosilane (OTCS) on freshly cleaved mica surfaces that possess a low density of reactive sites (i.e., silanol groups) by a vapor-deposition method. The deposition of OTCS molecules on mica was found to follow a two-stage process, as monitored by measuring the surface morphology using an atomic force microscope and wettability of the samples obtained at different deposition times. The contact mechanics behaviors and interaction forces of the as-obtained OTCS surfaces were characterized using a surface forces apparatus. The contact mechanics tests demonstrate that the OTCS coatings can significantly reduce the surface adhesion and adhesion hysteresis in air. The force-distance profiles of the OTCS surfaces obtained via a shorter deposition time (e.g., 2, 8 h) in aqueous solutions could be reasonably described by the classical Derjaguin-Landau-Verwey-Overbeek theory. However, for the OTCS surfaces obtained by longer deposition times (e.g., 48 h), hydrophobic interaction and steric interaction play an important role due to the enhanced surface hydrophobicity and roughness. Our results provide useful insights into the physicochemical characteristics of alkylsilane deposition and the surface interaction mechanisms of deposited alkylsilanes at solid-air and solid-water interfaces.

Ratiometric fluorescent nanosensors for copper(II) based on bis(rhodamine)-derived PMOs with J-type aggregates.

By using pentyl-linked bis(rhodamine)-derived tetra-siloxane (PRh-Si4 ) as the organosilica precursor, highly ordered PRh-bridged periodic mesoporous organosilicas (PRhPMOs) were prepared. When excited at λ=500 nm, the PRhPMO suspension that contained metal ions showed two separate emission peaks at λ=550 and 623 nm. The first peak, located at λ=550 nm, was due to ring-opening of the spiro structure in the rhodamine moiety and the second, located at λ=623 nm, originated from fluorescent aggregates of the PRh units embedded in the silica framework of the PRhPMO. By using the different intensity ratios of the two fluorescence signals (FI550/623), PRhPMOs could be used as turn-ON type fluorescent ratiometric chemosensors for Cu(2+). Furthermore, based on the single-exciton theory, it was deduced that the fluorescent aggregates formed were of the J-type and had a coplanar configuration. Consequently, PRhPMOs display a longer fluorescence lifetime and greater fluorescent quantum yield than the respective monomers dissolved in solution, which is consistent with the experimental results.