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.

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.

Aqueous lubrication and wear properties of nonionic bottle-brush polymers

The usage of aqueous lubricants in eco-friendly bio-medical friction systems has attracted significant attention. Several bottle-brush polymers with generally ionic functional groups have been developed based on the structure of biological lubricant lubricin. However, hydrophilic nonionic brush polymers have attracted less attention, especially in terms of wear properties. We developed bottle-brush polymers (BP) using hydrophilic 2-hydroxyethyl methacrylate (HEMA), a highly biocompatible yet nonionic molecule. The lubrication properties of polymer films were analyzed in an aqueous state using a ball-on-disk, which revealed that BPHEMA showed a lower aqueous friction coefficient than linear poly(HEMA), even lower than hyaluronic acid (HA) and polyvinyl alcohol (PVA), which are widely used as lubricating polymers. Significantly, we discovered that the combination of HA, PVA, and BPHEMA is demonstrated to be essential in influencing the surface wear properties; the ratio of 1 : 2 (HA : BPHEMA) had the maximum wear resistance, despite a slight increase in the aqueous friction coefficient.

Hydroxypropyl cellulose enhanced ionic conductive double-network hydrogels

Ionic conductive hydrogels with both high-performance in conductivity and mechanical properties have received increasing attention due to their unique potential in artificial soft electronics. Here, a dual physically cross-linked double network (DN) hydrogel with high ionic conductivity and tensile strength was fabricated by a facile approach. Hydroxypropyl cellulose (HPC) biopolymer fibers were embedded in a poly (vinyl alcohol)‑sodium alginate (PVA/SA) hydrogel, and then the prestretched PVA-HPC/SA composite hydrogel was immersed in a CaCl₂ solution to prepare PVA-HPC_T/SA-Ca DN hydrogels. The obtained composite hydrogel has an excellent tensile strength up to 1.4 MPa. Importantly, the synergistic effect of hydroxypropyl cellulose (HPC) and prestretching reduces the migration resistance of ions in the hydrogel, and the conductivity reaches 3.49 S/ m. In addition, these composite hydrogels are noncytotoxic, and they have a low friction coefficient and an excellent wear resistance. Therefore, PVA-HPC_T/SA-Ca DN hydrogels have potential applications in nerve replacement materials and biosensors.

Triborheological Study under Physiological Conditions of PVA Hydrogel/HA Lubricant as Synthetic System for Soft Tissue Replacement

In soft tissue replacement, hydrophilic, flexible, and biocompatible materials are used to reduce wear and coefficient of friction. This study aims to develop and evaluate a solid/liquid triborheological system, polyvinyl alcohol (PVA)/hyaluronic acid (HA), to mimic conditions in human synovial joints. Hydrogel specimens prepared via the freeze–thawing technique from a 10% (w/v) PVA aqueous solution were cut into disc shapes (5 ± 0.5 mm thickness). Compression tests of PVA hydrogels presented a Young’s modulus of 2.26 ± 0.52 MPa. Friction tests were performed on a Discovery Hybrid Rheometer DHR-3 under physiological conditions using 4 mg/mL HA solution as lubricant at 37 °C. Contact force was applied between 1 and 20 N, highlighting a coefficient of friction change of 0.11 to 0.31 between lubricated and dry states at 3 N load (angular velocity: 40 rad/s). Thermal behavior was evaluated by differential scanning calorimetry (DSC) in the range of 25–250 °C (5 °C/min rate), showing an endothermic behavior with a melting temperature (Tm) around 231.15 °C. Scanning Electron Microscopy (SEM) tests showed a microporous network that enhanced water content absorption to 82.99 ± 1.5%. Hydrogel achieved solid/liquid lubrication, exhibiting a trapped lubricant pool that supported loads, keeping low coefficient of friction during lubricated tests. In dry tests, interstitial water evaporates continuously without countering sliding movement friction.

Tribomechanical Comparison between PVA Hydrogels Obtained Using Different Processing Conditions and Human Cartilage

Designing materials for cartilage replacement raises several challenges due to the complexity of the natural tissue and its unique tribomechanical properties. Poly(vinyl alcohol) (PVA) hydrogels have been explored for such purpose since they are biocompatible, present high chemical stability, and their properties may be tailored through different strategies. In this work, the influence of preparation conditions of PVA hydrogels on its morphology, water absorption capacity, thermotropic behavior, mechanical properties, and tribological performance was evaluated and compared with those of human cartilage (HC). The hydrogels were obtained by cast-drying (CD) and freeze-thawing (FT), in various conditions. It was found that the method of preparation of the PVA hydrogels critically affects their microstructure and performance. CD gels presented a denser structure, absorbed less water, were stiffer, dissipated less energy, and withstood higher loads than FT gels. Moreover, they led to friction coefficients against stainless steel comparable with those of HC. Overall, CD hydrogels had a closer performance to natural HC, when compared to FT ones.

The combined impact of tissue heterogeneity and fixed charge for models of cartilage: the one-dimensional biphasic swelling model revisited

Articular cartilage is a complex, anisotropic, stratified tissue with remarkable resilience and mechanical properties. It has been subject to extensive modelling as a multiphase medium, with many recent studies examining the impact of increasing detail in the representation of this tissue's fine scale structure. However, further investigation of simple models with minimal constitutive relations can nonetheless inform our understanding at the foundations of soft tissue simulation. Here, we focus on the impact of heterogeneity with regard to the volume fractions of solid and fluid within the cartilage. Once swelling pressure due to cartilage fixed charge is also present, we demonstrate that the multiphase modelling framework is substantially more complicated, and thus investigate this complexity, especially in the simple setting of a confined compression experiment. Our findings highlight the importance of locally, and thus heterogeneously, approaching pore compaction for load bearing in cartilage models, while emphasising that such effects can be represented by simple constitutive relations. In addition, simulation predictions are observed for the sensitivity of stress and displacement in the cartilage to variations in the initial state of the cartilage and thus the details of experimental protocol, once the tissue is heterogeneous. These findings are for the simplest models given only heterogeneity in volume fractions and swelling pressure, further emphasising that the complex behaviours associated with the interaction of volume fraction heterogeneity and swelling pressure are likely to persist for simulations of cartilage representations with more fine-grained structural detail of the tissue.

Microfractures and hydrogel scaffolds in the treatment of osteochondral knee defects: A clinical and histological evaluation

BACKGROUND

Osteochondral knee defects (OCD) are often symptomatic, causing pain and functional impairment even in young and active patients. Regenerative surgical options, aiming to stimulate natural cartilage healing, have been recently used as a first line treatment. In this study, a new hydrogel is investigated in its capacity to regenerate the ultra-structural quality of hyaline cartilage when combined with a classical microfracture technique.

MATERIAL AND METHODS

Forty-six patients, affected by grade III and IV knee chondropathies, were consecutively treated between 2013 and 2015 with microfractures followed by application of a modern hydrogel in the lesion site. All patients underwent clinical evaluation (WOMAC) pre-operatively, at 6,12 and at 24 months postoperatively: the results were compared with a subsequent, consecutive, matched, control group of 23 patients treated with microfractures alone. In a parallel and separate in-vitro histological study, adipose derived mesenchymal stem cells (ADMSCs) were encapsulated in the hydrogel scaffold, induced to differentiation into chondrocytes, and observed for a 3 weeks period.

RESULTS

The initial WOMAC score of 58.6 ± 11.0 in the study group was reduced by 88% at 6 months (7.1 ± 9.2) and 95% at 24 months (2.9 ± 5.9). The "in-vitro" study revealed a histological characterization typical of hyaline cartilage in study group. Separate biopsies performed at 12 months post-op in the study group also revealed type 2 collagen and hyaline-like cartilage in the regenerated tissue.

CONCLUSION

Our study demonstrated high patient satisfaction rates after microfractures combined with a modern hydrogel scaffold; histologic evaluation supported the hypothesis of creating an enhanced chondrogenic environment. Microfracture "augmentation" using modern acellular biomaterials, like hydrogels, might improve the clinical outcomes of this classical bone marrow stimulating procedure.

Polymer grafted graphitic carbon nitrides as precursors for reinforced lubricant hydrogels

Carbon nitride-based hydrogels are formed in a two-step procedure and feature significant toughness, compressibility and lubricant properties.

Anomalous Friction between Agar Gels under Accelerated Motion

Understanding the friction phenomena on a gel surface under accelerated conditions is important for the designing of functional materials. However, there are few reports on friction under such conditions. In the present study, the effects of velocity, normal force, and gel hardness on the friction force were evaluated between two agar gels under sinusoidal motion. We found a friction phenomenon with an extremely low friction coefficient on the gel surfaces: the friction coefficient became less than 0.02 when sliding velocity increased. In addition, the profile of the friction coefficient was different between outward and homeward processes in the reciprocating sliding motion. In the outward direction, the low friction coefficient was maintained even if the sliding velocity decreased. On the other hand, the friction coefficient increased with sliding velocity in the homeward direction. This characteristic friction profile is caused by a long relaxation time on the gel surfaces. When the gel substrate is rubbed for a shorter time than the relaxation time, the morphology of the gel surface becomes unstable. Under such conditions, the formation and extinction of a thick liquid film can induce a super lubrication state and the asymmetric friction phenomena. These findings are useful not only for developing functional materials but also for understanding nonequilibrium phenomena in soft biological systems.

Propagation of Fatigue Cracks in Friction of Brittle Hydrogels

In order to understand fatigue crack propagation behavior in the friction of brittle hydrogels, we conducted reciprocating friction experiments between a hemi-cylindrical indenter and an agarose hydrogel block. We found that the fatigue life is greatly affected by the applied normal load as well as adhesion strength at the bottom of the gel⁻substrate interface. On the basis of in situ visualizations of the contact areas and observations of the fracture surfaces after the friction experiments, we suggest that the mechanical condition altered by the delamination of the hydrogel from the bottom substrate plays an essential role in determining the fatigue life of the hydrogel.

Hydrogels as a Replacement Material for Damaged Articular Hyaline Cartilage

Hyaline cartilage is a strong durable material that lubricates joint movement. Due to its avascular structure, cartilage has a poor self-healing ability, thus, a challenge in joint recovery. When severely damaged, cartilage may need to be replaced. However, currently we are unable to replicate the hyaline cartilage, and as such, alternative materials with considerably different properties are used. This results in undesirable side effects, including inadequate lubrication, wear debris, wear of the opposing articular cartilage, and weakening of the surrounding tissue. With the number of surgeries for cartilage repair increasing, a need for materials that can better mimic cartilage, and support the surrounding material in its typical function, is becoming evident. Here, we present a brief overview of the structure and properties of the hyaline cartilage and the current methods for cartilage repair. We then highlight some of the alternative materials under development as potential methods of repair; this is followed by an overview of the development of tough hydrogels. In particular, double network (DN) hydrogels are a promising replacement material, with continually improving physical properties. These hydrogels are coming closer to replicating the strength and toughness of the hyaline cartilage, while offering excellent lubrication. We conclude by highlighting several different methods of integrating replacement materials with the native joint to ensure stability and optimal behaviour.