Pluronic F127 Diacrylate Research Articles

In situ photocrosslinkable hydrogel treats radiation-induced skin injury by ROS elimination and inflammation regulation

The clinical management of radiation-induced skin injury (RSI) poses a significant challenge, primarily due to the acute damage caused by an overabundance of reactive oxygen species (ROS) and the ongoing inflammatory microenvironment. Here, we designed a dual-network hydrogel composed of 5 % (w/v) Pluronic F127 diacrylate and 2 % (w/v) hyaluronic acid methacryloyl, termed the FH hydrogel. To confer antioxidant and anti-inflammation properties to the hydrogel, we incorporated PVP-modified Prussian blue nanoparticles (PPBs) and resveratrol (Res) to form PHF@Res hydrogels. PHF@Res hydrogels not only exhibited multiple free radical scavenging activities (DPPH, ABTS), but also displayed multiple enzyme-like activities (POD-, catalase). Meanwhile, PHF@Res-2 hydrogels significantly suppressed intracellular ROS and promoted the migration of fibroblasts in a high-oxidative stress environment. Moreover, in the RSI mouse model, the PHF@Res-2 hydrogel regulated inflammatory factors and collagen deposition, significantly reduced epithelial hyperplasia, promoted limb regeneration and neovascularization, and accelerated wound healing, outperforming the commercial antiradiation formulation, Kangfuxin. The PHF@Res-2 hydrogel dressing shows great potential in accelerating wound healing in RSI, offering tremendous promise for clinical wound management and regeneration.

A bioactive xyloglucan polysaccharide hydrogel mechanically enhanced by Pluronic F127 micelles for promoting chronic wound healing

Chronic wounds represent a formidable global healthcare challenge due to the bacteria infections and uncontrollable inflammation responses, while developing wound healing materials capable of resolving these issues remains a challenge. In this study, we integrated xyloglucan (XG) with Pluronic F127 diacrylate (F127DA)to develop a composite hydrogel for wound healing, where the XG introduced anti-inflammation and anti-bacterial properties to the construct, and F127DA provides the photocurable properties essential for hydrogel formation and robust mechanical characteristics to achieve physical strength that matches tissue regeneration. The material characterizations suggested that XG/F127DA hydrogels had great biostability, blood compatibility and antibacterial effects, which was suitable to be used as a wound healing material. The in vitro analysis by culturing L929 fibroblasts on the hydrogel surface demonstrated that the inclusion of XG could promote the cellular proliferation rate, migration rate, and re-epithelialization-related marker expression, while downregulate the inflammation process. The XG/F127DA hydrogel was further used for the full-thickness skin wound healing test on mice, where the inclusion of XG significantly increased the wound closure rate through reducing the inflammation responses, and promote re-epithelialization and angiogenesis. These results indicated that XG/F127DA hydrogel has great potential to be used for wound healing in future clinical translation.

Enzyme induced polymer degradation as a pathway to create microbial habitats for biomanufacturing intensification

Polymer hydrogels are instrumental in cell immobilization for biomanufacturing, yet traditional hydrogel bioreactors often compromise cell viability, function, and mass transfer. This results in uncontrolled microbial proliferation and significant cell leakage, thereby impeding the continuous production of high-value products. To address these challenges, we introduce an innovative liquid microcavity technique employing a blend of Gelatin-methacryloyl (Gel-MA) and Pluronic F127diacrylate (F127-DA) for precision bioprinting, followed by selective enzymatic digestion of gelatin to create microbial habitats. Through tailoring of the hydrogel’s pore structure and integration of diverse networks, our approach facilitates controlled nutrient transfer, efficient waste removal, and stable mechanical support, all while maintaining a leak-free suspension culture of microorganisms. We validated the effectiveness of our method by fabricating yeast-infused Three-dimensional (3D) lattice constructs, successfully achieving continuous fermentation of glucose into ethanol in a flow reactor with minimal efficiency reduction (≥90 % yield) over five cycles. This research not only confronts the spatial limitations inherent in hydrogel-based systems but also demonstrates a foundational potential for revolutionary change in biomanufacturing practices.

Harnessing Mechanical Stress with Viscoelastic Biomaterials for Periodontal Ligament Regeneration.

The viscoelasticity of mechanically sensitive tissues such as periodontal ligaments (PDLs) is key in maintaining mechanical homeostasis. Unfortunately, PDLs easily lose viscoelasticity (e.g., stress relaxation) during periodontitis or dental trauma, which disrupt cell-extracellular matrix (ECM) interactions and accelerates tissue damage. Here, Pluronic F127 diacrylate (F127DA) hydrogels with PDL-matched stress relaxation rates and high elastic moduli are developed. The hydrogel viscoelasticity is modulated without chemical cross-linking by controlling precursor concentrations. Under cytomechanical loading, F127DA hydrogels with fast relaxation rates significantly improved the fibrogenic differentiation potential of PDL stem cells (PDLSCs), while cells cultured on F127DA hydrogels with various stress relaxation rates exhibited similar fibrogenic differentiation potentials with limited cell spreading and traction forces under static conditions. Mechanically, faster-relaxing F127DA hydrogels leveraged cytomechanical loading to activate PDLSC mechanotransduction by upregulating integrin-focal adhesion kinase pathway and thus cytoskeletal rearrangement, reinforcing cell-ECM interactions. In vivo experiments confirm that faster-relaxing F127DA hydrogels significantly promoted PDL repair and reduced abnormal healing (e.g., root resorption and ankyloses) in delayed replantation of avulsed teeth. This study firstly investigated how matrix nonlinear viscoelasticity influences the fibrogenesis of PDLSCs under mechanical stimuli, and it reveals the underlying mechanobiology, which suggests novel strategies for PDL regeneration.

Digital Light Processing 4D Printing of Poloxamer Micelles for Facile Fabrication of Multifunctional Biocompatible Hydrogels as Tailored Wearable Sensors.

The lack of both digital light processing (DLP) compatible and biocompatible photopolymers, along with inappropriate material properties required for wearable sensor applications, substantially hinders the employment of DLP 3D printing in the fabrication of multifunctional hydrogels. Herein, we discovered and implemented a photoreactive poloxamer derivative, Pluronic F-127 diacrylate, which overcomes these limitations and is optimized to achieve DLP 3D printed micelle-based hydrogels with high structural complexity, resolution, and precision. In addition, the dehydrated hydrogels exhibit a shape-memory effect and are conformally attached to the geometry of the detection point after rehydration, which implies the 4D printing characteristic of the fabrication process and is beneficial for the storage and application of the device. The excellent cytocompatibility and in vivo biocompatibility further strengthen the potential application of the poloxamer micelle-based hydrogels as a platform for multifunctional wearable systems. After processing them with a lithium chloride (LiCl) solution, multifunctional conductive ionic hydrogels with antifreezing and antiswelling properties along with good transparency and water retention are easily prepared. As capacitive flexible sensors, the DLP 3D printed micelle-based hydrogel devices exhibit excellent sensitivity, cycling stability, and durability in detecting multimodal deformations. Moreover, the DLP 3D printed conductive hydrogels are successfully applied as real-time human motion and tactile sensors with satisfactory sensing performances even in a -20 °C low-temperature environment.

Construction of high‐toughness and self‐healing gel‐matrix/aramid fabric soft composites with high‐performance puncture resistance

AbstractIn order to develop a soft composite with high‐performance puncture resistance and self‐healing property property, herein, we introduced Pluronic F127 diacrylate (F127DA) into titania‐based nanocomposite hydrogels (PAMT) and then compounded hydrogel and aramid fabric to form gel‐matrix soft composites (GMSCs). The result shows that, with the addition of 5 wt% TiO2 nanoparticles, the hydrogels exhibited a fracture strain of 3898%, a toughness of 12.23 MJ/m3 and a self‐healing efficiency of 99.45% after healing at 100°C for 12 h, indicating multiple recycling feature. In addition, a series of mechanical tests had shown that the resultant GMSCs exhibited incredible spike puncture resistance and knife puncture resistance, 17.66 times and 13.78 times greater than that of pure fabrics, and could achieve 33.84% spike puncture self‐healing efficiency and 26.62% knife puncture self‐healing efficiency. The synergetic reinforcement mechanism of the hydrogel and fabrics was also analyzed. This study provides a valuable reference for designing a puncture resistant soft composite with rapid self‐healing and repeated recycling.

Double-network hydrogel enhanced by SS31-loaded mesoporous polydopamine nanoparticles: Symphonic collaboration of near-infrared photothermal antibacterial effect and mitochondrial maintenance for full-thickness wound healing in diabetes mellitus.

Diabetic wound healing has become a serious healthcare challenge. The high-glucose environment leads to persistent bacterial infection and mitochondrial dysfunction, resulting in chronic inflammation, abnormal vascular function, and tissue necrosis. To solve these issues, we developed a double-network hydrogel, constructed with pluronic F127 diacrylate (F127DA) and hyaluronic acid methacrylate (HAMA), and enhanced by SS31-loaded mesoporous polydopamine nanoparticles (MPDA NPs). As components, SS31, a mitochondria-targeted peptide, maintains mitochondrial function, reduces mitochondrial reactive oxygen species (ROS) and thus regulates macrophage polarization, as well as promoting cell proliferation and migration, while MPDA NPs not only scavenge ROS and exert an anti-bacterial effect by photothermal treatment under near-infrared light irradiation, but also control release of SS31 in response to ROS. This F127DA/HAMA-MPDA@SS31 (FH-M@S) hydrogel has characteristics of adhesion, superior biocompatibility and mechanical properties which can adapt to irregular wounds at different body sites and provide sustained release of MPDA@SS31 (M@S) NPs. In addition, in a diabetic rat full thickness skin defect model, the FH-M@S hydrogel promoted macrophage M2 polarization, collagen deposition, neovascularization and wound healing. Therefore, the FH-M@S hydrogel exhibits promising therapeutic potential for skin regeneration.

A "T.E.S.T." hydrogel bioadhesive assisted by corneal cross-linking for in situ sutureless corneal repair.

Corneal transplantation is an effective clinical treatment for corneal diseases, which, however, is limited by donor corneas. It is of great clinical value to develop bioadhesive corneal patches with functions of "Transparency" and "Epithelium & Stroma generation", as well as "Suturelessness" and "Toughness". To simultaneously meet the "T.E.S.T." requirements, a light-curable hydrogel is designed based on methacryloylated gelatin (GelMA), Pluronic F127 diacrylate (F127DA) & Aldehyded Pluronic F127 (AF127) co-assembled bi-functional micelles and collagen type I (COL I), combined with clinically applied corneal cross-linking (CXL) technology for repairing damaged cornea. The patch formed after 5min of ultraviolet irradiation possesses transparent, highly tough, and strongly bio-adhesive performance. Multiple cross-linking makes the patch withstand deformation near 600% and exhibit a burst pressure larger than 400mmHg, significantly higher than normal intraocular pressure (10-21mmHg). Besides, the slower degradation than GelMA-F127DA&AF127 hydrogel without COL I makes hydrogel patch stable on stromal beds in vivo, supporting the regrowth of corneal epithelium and stroma. The hydrogel patch can replace deep corneal stromal defects and well bio-integrate into the corneal tissue in rabbit models within 4 weeks, showing great potential in surgeries for keratoconus and other corneal diseases by combining with CXL.

3D-Printed GelMA/PEGDA/F127DA Scaffolds for Bone Regeneration

Tissue-engineered scaffolds are an effective method for the treatment of bone defects, and their structure and function are essential for bone regeneration. Digital light processing (DLP) printing technology has been widely used in bone tissue engineering (BTE) due to its high printing resolution and gentle printing process. As commonly used bioinks, synthetic polymers such as polyethylene glycol diacrylate (PEGDA) and Pluronic F127 diacrylate (F127DA) have satisfactory printability and mechanical properties but usually lack sufficient adhesion to cells and tissues. Here, a compound BTE scaffold based on PEGDA, F127DA, and gelatin methacrylate (GelMA) was successfully prepared using DLP printing technology. The scaffold not only facilitated the adhesion and proliferation of cells, but also effectively promoted the osteogenic differentiation of mesenchymal stem cells in an osteoinductive environment. Moreover, the bone tissue volume/total tissue volume (BV/TV) of the GelMA/PEGDA/F127DA (GPF) scaffold in vivo was 49.75 ± 8.50%, higher than the value of 37.10 ± 7.27% for the PEGDA/F127DA (PF) scaffold and 20.43 ± 2.08% for the blank group. Therefore, the GPF scaffold prepared using DLP printing technology provides a new approach to the treatment of bone defects.

Photochemically deoxygenating micelles for protecting TTA-UC against oxygen quenching.

Pluronic F127, P123 and cross-linked F127 diacrylate micelles are photochemically deoxygenating nanocapsules in which oxygen could be removed by photochemical reaction with a surfactant and efficient triplet-triplet annihilation photon upconversion (TTA-UC) can be achieved in air. The efficiency of TTA-UC under air is comparable to that under deoxygenated conditions.

Mechanoresponsive Drug Loading System with Tunable Host-Guest Interactions for Ocular Disease Treatment.

Conventional administration of eye drops often requires high dosages and/or repetitive treatments to achieve therapeutic efficacy. This is inefficient and may result in side effects or even toxicity. Although many delivery systems of ophthalmic drugs have been reported, most of them work in a fixed format in which both the type and dose of the loaded drugs cannot be changed upon demand. To overcome this limitation, a hybrid double network hydrogel system composed of methacryloyl gelatin, pluronic F127 diacrylate, and β-cyclodextrin-modified oxidized dextran was developed. The hydrogels presented good mechanical strength and biocompatibility. In vitro assessments demonstrated that the hydrogels loaded with commonly used ophthalmic drugs could sustain the drug release for more than 21 days. This hydrogel system exhibited features of mechanoresponsive drug loading, and the capacity of drug loading could be significantly enhanced by macroscopically mechanical compression. Further in vivo evaluation of the drug delivery capacity showed that a dexamethasone-loaded hydrogel as a fornix insert effectively suppressed upregulation of proangiogenic factors and suture-induced corneal neovascularization in rats. This novel hydrogel system represents a promising drug delivery platform, which could potentially improve the treatments of ocular surface and other diseases.

Long‐lasting moisture and anti‐freezing tough organohydrogels based on multi‐functional nano‐micelles for flexible dual‐response sensors

AbstractConductive hydrogels have attracted widespread attention in diverse applications. However, the fragile nature of conducting polymers hinders the long‐time stability of the hydrogels. What's more, hydrogels will lose elasticity and conductivity at room temperature or sub‐zero temperature because of the high water content. Herein, we proposed a long‐term stable and anti‐freezing organohydrogel with high stretchability (up to 1140%), which consist of multi‐functional nano‐micelles (Pluronic F127 diacrylate [F127DA]), Polyacrylamide, and Poly(3,4‐ethylenedioxythiophene): polystryrene sulfonate (PEDOT:PSS) in a water/glycerin binary solvent system. The gel is thermal tolerant to maintain all the properties under extreme wide temperature spectrum (−40°C or 60°C). More importantly, the organohydrogel possesses excellent recognition capability to multiple stimuli. It is anticipated that this organohydrogel will have great potential in sensing applications.

A low-swelling and toughened adhesive hydrogel with anti-microbial and hemostatic capacities for wound healing.

Hydrogel-based wound dressings with tissue adhesion abilities are widely used for wound closure. However, currently developed hydrogel adhesives are still poor at continuing to seal wounds while bleeding is ongoing. Herein, we demonstrate an antibacterial and hemostatic hydrogel adhesive with low-swelling properties and toughness for wound healing. The hydrogel was composed of Pluronic F127 diacrylate, quaternized chitosan diacrylate, silk fibroin, and tannic acid, and it was not only able to maintain good tissue adhesion abilities in a moist environment but it also showed guaranteed tissue adhesion and mechanical strength after absorbing water due to its low-swelling and toughness properties. Furthermore, in vitro and in vivo tests demonstrated that the hydrogel also had antibacterial, antioxidant, and hemostatic properties, which could promote tissue regeneration. All these findings demonstrate that this hydrogel with multifunctional properties is a promising material for clinical wound healing applications.

Mussel-inspired self-adhesive hydrogels by conducting free radical polymerization in both aqueous phase and micelle phase and their applications in flexible sensors

Polydopamine (PDA)-based self-adhesive hydrogel sensors are extensively explored but it is still a challenge to construct PDA-based hydrogels by free radical polymerization. Herein, a new approach to construct self-adhesive hydrogels by conducting free radical polymerization in both aqueous phase and micelle phase is developed. The following two-phase polymerization processes account for the formation of the self-adhesive hydrogels. The first one is the polymerization of acrylamide (AM) and dopamine (DA) in aqueous phase to form adhesive component PAM-PDA (PAM, polyacrylamide; PDA, polydopamine). The second one is the polymerization of hydrophobic monomer 2-methoxyethyl acrylate (MEA) in micelles of an amphiphilic block copolymer Pluronic F127 diacrylate (F127DA). The poly(2-methoxyethyl acrylate) (PMEA) networks help to maintain the high robustness of the hydrogel. Because PMEA and PDA form in relatively separated phases, the inhibition effect of PDA on the free radical polymerization process of PMEA is weakened. Based on this mechanism, mechanically strong and adhesive hydrogels are achieved. The introduced ions during preparation process, such as Na+, OH– and K+, endow the resulting hydrogels ionic conductivity. Resistive strain sensor of the hydrogel achieves a high gauge factor (GF) of 5.26, a response time of 0.25 s and high sensing stability. Because of the adhesiveness, such hydrogel sensor can be applied as wearable sensors in monitoring various human motions. To further address the freezing and drying problems of the hydrogels, organohydrogels are constructed in glycerol-water mixed solvent. The organohydrogels exhibit outstanding anti-freezing property and moisture retention ability, and their adhesiveness is well maintained in subzero conditions. Capacitive pressure sensors of the organohydrogels possessing a GF of 2.05 kPa−1, high sensing stability and reversibility, are demonstrated and explored in monitoring diverse human motions.

Hydrogel with high toughness and strength for fabricating high performance stab-resistant aramid composite fabric

In order to prepare a new high-performance and flexibility stab-resistant composite material, one type of high-toughness and high-strength hydrogel is combined with aramid fabric. In this paper, the nanocomposite (NC) hydrogels are prepared using Pluronic F127 Diacrylate (PF127DA) as the crosslinking agent, acrylamide as the monomer and nano-silica as nano additive. The influence of the addition amount of nano-silica on the toughness and strength of nano-composite hydrogels is discussed. The strength and toughness of the NC hydrogels with nano silica are higher than that of the acrylamide hydrogel without nano particles. The NC hydrogel with the best strength is made as a composite with aramid fabric and the effect of nanocomposite hydrogel on the stab resistance of the stab-resistant composite material is studied. The research shows that the stab resistance performance of the composite material prepared by aramid fabric and nanocomposite hydrogel is effectively improved in comparison with aramid fabric, and the yarn pulling force is also improved. The composite material with high stab resistance can be used as the stab-resistant layer of armor, cut-resistant gloves and stab-resistant base material of shoes. This work provided a possibility for preparing a kind of lightweight composite material with high stab resistance.

Highly Sensitive Pressure and Strain Sensors Based on Stretchable and Recoverable Ion-Conductive Physically Cross-Linked Double-Network Hydrogels.

Ion-conductive hydrogel sensors have attracted great research interests for applications in wearable devices, electronic skins, and implantable sensors, but most such sensors are fragile, with low conductivity and sensitivity. This study reports on novel ion-conductive double network hydrogels with a cross-linked helical structure, hydrophobic association, and metal-ion coordination. The helical κ-carrageenan first network and the second network cross-linked by Pluronic F127 diacrylate micelles and tridentate Fe3+-COO- coordination work synergistically to show the tensile strength of 2.7 MPa, fracture strain of 1400%, and tensile toughness of 9.82 MJ m-3 and fatigue resistance against cyclic loadings with high strains. The hydrogels show an ion conductivity of 1.15 S m-1, a strain sensitivity of up to 2.8, and a pressure sensitivity of 0.33 kPa-1. Sensor arrays fabricated from the conductive hydrogels provide an in-plane detection of pressures less than 200 Pa. Such hydrogel sensors have potential applications to electron skins and implantable sensors.

Integration of Macro-Cross-Linker and Metal Coordination: A Super Stretchable Hydrogel with High Toughness.

The development of multifunctional hydrogels with high strength and stretchability is one of the most important topics in soft-matter research owing to their potential applications in various fields. In this work, a dual physically cross-linked network was designed for the fabrication of ultrastretchable tough hydrogels. The hydrogels were prepared through in situ polymerization of acrylic acid and acrylamide in the presence of positively charged quaternary poly(ethylene imine) (Q-PEI) and micelle-forming Pluronic F127 diacrylate, thus introducing electrostatic interactions between the positively charged Q-PEI and negatively charged poly(acrylic acid-co-acrylamide). For further mechanical reinforcement, Ca2+ and Cu2+ ions were introduced into the hydrogel network to construct coordination bonds, significantly enhancing tensile strength as well as stretchability. The hydrogel prepared with Ca2+ ion coordination bonds was found to be stretchable to 108 times its original length and exhibited a maximum toughness of 177 MJ·m-3, representing one of the most robust systems with both extraordinary toughness and superstretchability prepared to date. The hydrogels also exhibited excellent recovery of dimensions and reproducibility in terms of mechanical properties, providing a promising ultrastretchable soft-matter system with outstanding mechanical strength.

High strength zwitterionic nano-micelle hydrogels with superior self-healing, adhesive and ion conductive properties

Zwitterionic nano-micelle hydrogels were prepared by in situ co-polymerization of zwitterionic monomer- [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA) and nonionic monomer-hydroxyethyl methacrylate (HEMA) for the first time using Pluronic F127 di-acrylate (F127DA) as macro-crosslinkers. This novel hydrogel featured tough mechanical properties (sustained a strain at 98% without fracture, and a tensile fracture strain at about 1000%), adhesiveness, as well as good conductive properties (two times than the traditional chemical cross-linked zwitterionic hydrogels). Due to strong electrostatic attraction constructed by the positively and negatively charged groups in the zwitterionic hydrogels, and the reversible dissociation-association of the micelle cross-linked zones via hydrophobic associations in the gel network, the prepared micelle-based hydrogels also exhibited excellent self-healable properties, the self-healed hydrogels exhibited almost the same stretching behavior compared with the original hydrogels. This superior zwitterionic nano-micelle hydrogel could therefore have great potential in applications like wearable sensors, human-health monitors, artificial skin and axon.

Acid-responsive composite hydrogel platform with space-controllable stiffness and calcium supply for enhanced bone regeneration

Many hydrogel scaffolds for bone regeneration do not satisfy clinical needs due to uncontrollable mechanical properties and calcium supply and a lack of outstanding osteogenic functions. Herein, a novel acid-responsive composite hydrogel scaffold platform was developed to provide controllable stiffness and calcium supply for enhanced bone regeneration. For this purpose, pluronic F127 diacrylate (F127-DA) was selected as the model scaffold matrix material due to its good biocompatibility and easy formulation capability. Nano-CaCO3 was chosen as the composite material to provide an acid-responsive property and controlled calcium supply. The acid-responsive nano-CaCO3 may lead to space-controlled distribution of nano-CaCO3 in a hydrogel under acid treatment, leading to effective regulation of the mechanical properties of the hydrogel scaffold. Furthermore, the hydrogel may efficiently modulate the expression of Col1, BMP2 and OPN, which are beneficial for osteogenesis. In vivo studies in a rabbit skull bone defect model indicated that this F127-DA/nano-CaCO3 composite hydrogel platform performed better in bone regeneration than a pure F127-DA hydrogel. Therefore, this responsive composite hydrogel platform provides a simple and robust strategy for developing advanced composite hydrogel scaffolds for effective bone repair.

Super tough, ultra-stretchable, and fast recoverable double network hydrogels physically crosslinked by triple non-covalent interactions

Incorporating sacrificial bonds into hydrogels networks has been demonstrated an efficient way to obtain ultrahigh strength, but is usually at the expense of flexibility and recoverability. This study demonstrates novel triple physically crosslinked hydrogels with very high stretchability, toughness and rapid self-recovery at ambient conditions. Physically crosslinked κ-carrageenan acts as the first network to host free radical copolymerization of polymers with Pluronic F127 diacrylate micelles. Hydrophobic association of micelles in the second network contributes to outstanding stretchability (over 1100%) and toughness (10.83 MJ/m3). Further crosslinking of the second network through tridentate Fe3+–COO− coordination significantly enhances the fracture strength up to 2.7 MPa, and elastic modulus 1171 kPa. Systematic investigations demonstrate the key roles played by non-covalent crosslinking on energy dissipation during loadings and the damaged non-covalent networks could recover 40% toughness in 1 min at ambient conditions. These hydrogels are promising for applications as biocompatible load-bearing materials.