Advances on Hydrogel Lubrication Modification Under Diverse Design Strategies

Abstract

Hydrogel is a new type of lubricating material, and its lubrication performance is influenced by factors such as water content, chemical structure, cross-linking density, and friction pair materials. Currently, research on the lubrication performance of hydrogels has not formed a unified standard and system. Given that the lubricity of hydrogels mainly depends on their components and structure, as well as the chemical interactions between the polymer chains of the hydrogel network and the friction interface, this review systematically discusses the diverse design strategies of hydrogel lubricants based on the hydration and boundary lubrication mechanisms of gels, while elucidating the underlying enhancement mechanisms, as well as the corresponding tribological behavior and applications for different strategies. Finally, the main challenges and future research directions are underlined, aiming to provide theoretical and technical support for the design optimization and practical application of advanced hydrogel lubrication materials.

1. Introduction

With the rapid development in intelligent manufacturing, the innovation of lubrication technology has become one of the core technologies of the green revolution in the manufacturing industry. Therefore, the development and utilization of efficient intelligent lubrication materials have become research hotspots in the field of tribology [1,2].

As is well known, traditional lubricating materials are divided into liquid lubricants and solid lubricants. Liquid lubrication possesses the advantages of a low friction coefficient, low noise, no wear debris, and long life. However, it has drawbacks such as volatility, easy migration, the need for sealing, degradation at high temperatures, and solidification at low temperatures [3]. Compared to liquid lubricants, solid lubricants have significant advantages in terms of alternating high and low temperatures, load-bearing capacity, and environmental adaptability, which play a crucial role in the lubrication of space machinery, such as in aerospace applications. However, the high noise and limited lifespan of solid lubricants limited their practical applications. Hydrogels have a unique solid-liquid biphasic network structure that contains a large number of free water molecules without dissolving. As a lubricant, hydrogel lubricants combine the advantages of both solid and liquid lubrication, presenting a liquid-solid two-phase lubrication effect, which has attracted widespread attention from researchers [4,5]. Since the early 20th century, Wichterle and Drahoslav Lim successfully synthesized the first artificial hydrogel [6]. Subsequently, a series of hydrogel materials has been developed by varying the chemical or physical cross-linking of monomers and cross-linking agents, and have been widely used in fields such as biomedicine, flexible electronics, and environmental engineering [7,8]. In comparison, domestic research on hydrogels started relatively late but has developed rapidly. In recent years, numerous results have been achieved in the biomedical field, including drug release, wound dressings, and artificial cartilage. Figure 1 shows the development route of hydrogel lubricants and the recent research trend. However, in the field of lubrication, traditional hydrogels have issues such as insufficient mechanical strength, poor lubrication durability, and severe wear, which limit their practical application [9,10]. Recently, designing and preparing efficient and intelligent hydrogel lubricants has been a hot topic in the field of tribology.

Hydrogels, as lubricants, involve various lubrication mechanisms, among which hydration lubrication and boundary lubrication play the key roles. The so-called “hydration lubrication” is due to the hydrophilicity of the hydrogel and the dipole nature of water molecules, allowing a large number of water molecules to adsorb onto the hydrogel surface, forming a hydration layer [11,12,13,14,15]. The hydration layer not only possesses high spatial stability and load-bearing capacity, but also the water molecules within the hydration layer can quickly exchange with external free water molecules, maintaining rapid relaxation efficiency, which allows it to exhibit better fluid effects and lower friction coefficient when subjected to shear forces. Boundary lubrication focuses on the physicochemical interactions between the polymer chains within the hydrogel network and the contact surfaces, as well as the interactions between the polymer chains on different hydrogel surfaces. The lubrication method of traditional hydrogels is mainly hydration lubrication, with a single lubrication mechanism. Under sustained high-load conditions, they are prone to failure and difficult to maintain long-term stable low-friction performance. Therefore, designing different strategies to improve the lubrication performance of hydrogels is an effective means to address the current issues [16,17,18]. Currently, research on the lubrication modification of traditional hydrogels mainly focuses on material design and performance breakthroughs [19,20,21]. Although the lubrication performance of hydrogels has been significantly enhanced through methods such as double network structures and nanocomposites [22,23,24,25,26,27,28,29,30,31,32], most research remains at the fundamental level with the drawbacks of the complex preparation process and high cost, making large-scale production and application difficult [33].

Hence, to promote the development of advanced hydrogel lubricants and expand the application range of hydrogels as lubricants. This article summarizes the current advanced design strategies for hydrogel lubrication modification based on the mechanisms of hydrated lubrication and boundary lubrication. We systematically elaborate on the design concepts of hydrogel lubrication modification from three aspects: component modification, structural design, and surface and interface modification. On this basis, we conduct a detailed analysis of the principles and tribological behaviors of hydrogel lubrication modification under different strategies, providing a reference for the design and preparation of new excellent hydrogel lubricants and their practical applications.

2. Component Modification Strategy

Based on hydrated lubrication, the design concept of component modification strategies is to enhance the effect of hydrated lubrication. Since the lubrication of traditional hydrogels mainly relies on the hydrated layer formed on the surface, this lubrication method is prone to failure under sustained high-load conditions and is difficult to maintain long-term stable low-friction performance. Therefore, inspired by articular cartilage, the lubricating properties of traditional gels can be enhanced by altering the components of hydrogels. Articular cartilage tissue has a certain elasticity and is mainly composed of chondrocytes, cartilage matrix, water, amino acids, collagen, and proteoglycans, among other components. The interactions between these components provide a long-lasting and stable lubricating layer. Similar to the lubrication of articular cartilage, the lubrication process of hydrogels also involves the combination and bonding of multiple components, which synergistically regulate the water content and mechanical properties of the hydrogel network, forming a stable lubrication layer.

2.1. Physical Component Doping Modification Strategy

2.1.1. Incorporation of Liposomes

As is well known, natural liposomes mainly consist of natural phospholipids and cholesterol; however, they are prone to oxidation and hydrolysis, resulting in poor stability [34,35]. Therefore, the introduced liposomes are often derivatives of natural phospholipids and cholesterol. Introducing liposomes into hydrogels can affect their water content because liposomes are composed of vesicle-like structures formed by lipid molecules, which have hydrophilic heads and hydrophobic tails. This structure helps lipid molecules spontaneously arrange into a bilayer when they encounter water, because the hydrophilic heads facing outward can adsorb a large number of water molecules, forming a thicker hydration layer. There are two pathways for introducing liposomes: one is to incorporate liposomes into the hydrogel; the other is to introduce liposomes into the hydrogel lubrication system, capturing liposomes from the outside.

Regarding the incorporation of liposomes within hydrogels, Lin et al. [36] prepared lipid composite hydrogels via polymerizing and crosslinking low concentrations of hydrogenated soy phosphatidylcholine (HSPC) lipid or the PC lipids dimyristoylphosphatidylcholine (DMPC) with poly(hydroxyethyllmethacrylate) (PHEMA) monomers. The design concept is that the lipid vesicles in the liposomes, under the influence of load, are exposed on the gel surface. The hydrophilic heads adsorb moisture to form a molecular-thick hydrated lubrication layer, reducing friction. As the friction time increases, the lipid vesicles continuously release and rearrange to form a boundary lubrication layer, further reducing friction and wear (Figure 2a). Compared to lipid-free hydrogels, although lipid-doped hydrogels presented a lower friction coefficient (CoF), HSPC-doped hydrogels reduced friction more effectively under a higher load and physiological temperature (Figure 2b,c), which suggested that two different lipid phases result in the differences of the interplay between head group hydration and bilayer robustness. Meanwhile, the wear of the DMPC-incorporating gels is only 3 μm, even for a load 10 times as large (Figure 2d), indicating an excellent load-carrying capacity. This study is expected to provide a universal method for sustained lubrication of hydrogels, with potential in the application of article joint lubrication. Furthermore, Lei et al. [37] adjusted lipid release from “fixed” to “controllable” and developed injectable rapamycin (RAPA) hydrogel microspheres (HMS) with self-renewable hydration layers (RAPA@Lipo@HMS) via microfluidic technology and light-induced free radical polymerization. Through the continuous release of liposomes from rolling microspheres, hydration layers were sustainably generated. This mechanism yields a lower coefficient of friction (CoF) compared to hydrogels doped with lipids alone due to the rolling effect of hydrogel microspheres. However, the formation of lubrication relies on pre-loaded lipids, inevitably leading to the lipids’ consumption. This leads to the risk of lipid depletion, making durability a major challenge. Additionally, the inherent soft material properties of liposomes cause the weaker mechanical strength of hydrogel lubricants, making their structures prone to damage under external forces.

Research on incorporating liposomes within hydrogels has found that the durability of lubrication is a major challenge. Once the low-friction lubrication layer is damaged or the lipid lubrication medium within the hydrogel is depleted, the lubrication performance of the gel decreases with it. Based on this, Wang et al. [38] developed a friction-induced interfacial self-assembly (ISA) strategy (Figure 2e) and prepared the PHEMA/ε-PL hydrogels as monomer with poly-L-lysine (ε-PL) as an initiator through a one-pot method coupled with UV irradiation at 365 nm. The mechanical properties of hydrogels containing ε-PL were obtained through rheological analysis (Figure 2f), where its storage modulus (G’) was over the loss modulus (G″) with no shear thinning compared to the pure PHEMA, indicating that ε-PL increased the density of the polymer chains. In addition, in the DMPC buffer, poly-L-lysine (ε-PL) actively captured environmental DMPC lipid via non-covalent interactions of hydrogen bonding, hydrophobic forces, and coulombic interactions, enabling self-assembly and regeneration of the lubricating layer (Figure 2e). This mechanism results in sustained low CoF (Figure 2g) and self-repair capability even after damage (Figure 2h). This system shifts from passive lipid release to active external lipid capture, realizing the self-assembly and regeneration of the lubricating layer, providing a reliable strategy for long-lasting lubrication in artificial joints.

In summary, the lubrication mechanism produced by the release of stored liposomes is primarily hydration lubrication. Comparatively, the lubrication mechanism of externally obtained liposomes is rearranged to form a boundary lubrication layer, with the main lubrication method being lipid-based boundary lubrication. The lubrication systems can almost maintain a consistently low CoF over a long period, providing strong evidence for the practical application of composite lipid-lubricated hydrogels. However, it is worth noting that This strategy usually has a significant modification effect on hydrogels with greater thickness. However, since the thickness of most hydrogel lubricating materials does not exceed 1 mm, the method of incorporating liposomes within the hydrogel is relatively rare in hydrogel lubrication research [39,40]. Based on this, doping nanoparticles into hydrogels has become a major research topic.

Figure 2. (a) Lipid storage in hydrogels and shear-induced release for lubrication [38]. The CoF values of at room temperature (b), and physiological temperatures (c) at a series of external forces [36]. (d) The Wear of lipid-free and DMPC-incorporating gels after 2 h of sliding of the steel sphere on the gels under loads of 1 N and 10 N, respectively [36]. (e) Promotion of interfacial self-assembly via supramolecular interactions, endowing the hydrogels with advanced lubricating capabilities through dynamic molecular arrangements [38]. (f) Frequency-dependent curves of G′ and G′′ for different polyelectrolyte hydrogels. (g) COFs over 19,800 continuous friction cycles using a PHEMA/ε-PL10 hydrogel in DMPC buffer [38]. (h) COFs obtained after surface excision of the hydrogel [38].

Figure 2. (a) Lipid storage in hydrogels and shear-induced release for lubrication [38]. The CoF values of at room temperature (b), and physiological temperatures (c) at a series of external forces [36]. (d) The Wear of lipid-free and DMPC-incorporating gels after 2 h of sliding of the steel sphere on the gels under loads of 1 N and 10 N, respectively [36]. (e) Promotion of interfacial self-assembly via supramolecular interactions, endowing the hydrogels with advanced lubricating capabilities through dynamic molecular arrangements [38]. (f) Frequency-dependent curves of G′ and G′′ for different polyelectrolyte hydrogels. (g) COFs over 19,800 continuous friction cycles using a PHEMA/ε-PL10 hydrogel in DMPC buffer [38]. (h) COFs obtained after surface excision of the hydrogel [38].

2.1.2. Doping of Nanoparticles

Nanoparticles, also named ultrafine particles with particle diameters ranging from 1 to 100 nm, belong to the category of colloidal particles. Adding functional nanoparticles to hydrogels not only enhances the mechanical strength of the gel but also improves the gel material’s ability to capture, stabilize, and dynamically release water molecules. Furthermore, nanoparticles can perform self-repair on the friction surface during friction, reducing adhesive wear at the interface [41]. There are two methods for doping nanoparticles: one is to fill the nanoparticles into the hydrogel, and the other is to encapsulate the nanoparticles within the hydrogel to form microspheres.

Incorporating single nanoparticles into hydrogels can increase the density of cross-linking points, enhancing the material’s hardness and toughness. Graphene oxide (GO) as a crosslinker and enhancer has been widely employed to synthesize nanoparticle-doped hydrogel lubricants due to its unique advantages of weak interlayer interactions, controllable surface properties, high specific surface area, and robust mechanical properties [42]. Hu et al. [43] introduced GO into polyvinyl alcohol/glycol (PVA/PEG) hydrogels to prepare the PVA/PEG/GO hydrogel. Experiment results showed that the addition of GO can improve the network crosslinking structure of the PVA/PEG hydrogel and mechanical strength. Meanwhile, the PVA/PEG/GO gel has good self-healing and tensile properties. The enhancement mechanism is because introducing GO increased the hydrogen-containing functional groups in the gel, enhancing the interaction force between hydrogen bonds. By altering the preparation process, Meng et al. [44] proposed using the freezing-thawing method to the fabrication of PVA/GO/PEG nanocomposite hydrogels, which also exhibited similar conclusions. Mxene nanomaterials has become a current research hotspot in the field of tribology because of their excellent mechanical qualities and capacity for self-lubrication [45,46,47]. MXene nanosheets not only provide mechanical support to the polymer network but also further enhance the hydrogel’s network structure through the hydrogen bond interactions between the hydroxyl and fluorine-rich groups on the MXene surface and the gel surface. Guo et al. [48] synthesised aramid nanofiber/Mxene (ANF/MXene)-reinforced polyelectrolyte hydrogels using chitosan as the gel matrix. The results of the mechanical tests showed that the tensile strength was gradually boosted with the content of MXene (Figure 3a). This phenomenon is because the storage modulus G′ and loss modulus G″ were enhanced with the addition of the Mxene (Figure 3b), indicating that introducing MXene facilitated to the formation of a more robust hydrogel structure. Additionally, this nanocomposited hydrogel can achieve active regulation of lubrication performances in simulated joint friction environments, making it suitable as a surface coating for artificial joint devices. Hu et al. [49] doped carbon nanotubes (CNTs) treated with high-concentration acid into a gel system to prepare CNT composite hydrogels. At low pH values, the composite hydrogels possessed a highly cross-linked network structure, while at high pH values, their structure became loose, and CoF increased with the increase in pH.

Incorporating various nanoparticles into hydrogels can not only enhance the material’s hardness and toughness but also reduce friction during the process by generating the “interlayer slip effect” and the “nano-ball bearing effect”. Huang et al. [50] incorporated single-walled CNTs and MXene into the construction of the hydrogel network structure. They prepared a dual-nanocomposite hydrogel under alkaline pH conditions through non-covalent hydrogen bonding interactions between polyvinylpyrrolidone (PVP) and phytic acid (PA). This gel exhibits significant advantages in both load-bearing capacity and lubrication performance. On one hand, the hydroxyl (-OH) and fluorine (-F) groups on the surface of MXene form multiple hydrogen bonds with PVP, enhancing the strength of the network; on the other hand, the interlayer structure of the nanoparticles CNTs and MXene causes them to align directionally at the friction interface, generating the “interlayer slip effect” and the “nano-bearing effect” at the friction interface, reducing adhesive wear at the interface. Although doping nanomaterials has been proven to enhance the mechanical properties of gels, excessively high modulus often prevents water from freely releasing onto the contact surface and thus fails to achieve a better lubrication performance. Therefore, the amount of nanomaterials added is significant for balancing the load-bearing capacity and lubrication performance of the gel. Miao et al. [51] prepared the MXene/polyvinyl alcohol hydrogel(PVA) (MP-MS) hydrogel by doping MXene and Zn²⁺ induced salting-out reaction in the PVA gel matrix, presenting a lower CoF compared to the PVA and the MXene/polyvinyl alcohol hydrogel without compositing Zn²⁺ (MP) (Figure 3c,d). This result is due to that the introduced Zn²⁺ facilitated the release of water from the hydrogel, enabling a low CoF in the solid-liquid composite system. Additionally, the amount of MXene can influence the CoF of MP-MS hydrogel. At the MXene concentration of 5 mg/mL, the MP hydrogel exhibited an optimal lubrication performance (Figure 3e), indicating that a appropriate amount of GO nanosheets could be uniformly distributed on the surface and played the role of interlayer sliding. Overall, the core tribology mechanism behind MXene-based hydrogels is the dynamic coordination between the solid-phase skeleton providing mechanical support and the free water forming the lubricating film.

Figure 3. Uniaxial tensile strain–stress curves (a) and rheological behaviors (b) of different MXene concentrations [48]. (c) Schematic diagram of friction test for a steel ball reciprocating sliding against a steel disk. Friction curves of PVA, MP, and MS-MP3 hydrogels (d) and MP hydrogels with different MXene concentrations (e) under a constant load of 100 N, stroke of 1 mm, and sliding frequency of 25 Hz [51].

Figure 3. Uniaxial tensile strain–stress curves (a) and rheological behaviors (b) of different MXene concentrations [48]. (c) Schematic diagram of friction test for a steel ball reciprocating sliding against a steel disk. Friction curves of PVA, MP, and MS-MP3 hydrogels (d) and MP hydrogels with different MXene concentrations (e) under a constant load of 100 N, stroke of 1 mm, and sliding frequency of 25 Hz [51].

Incorporating nanoparticles into responsive hydrogels can assist the hydrogels in achieving dynamic friction control. Under normal conditions, the hydrogel is in a low modulus, low strength, and high-friction state. However, when subjected to external stimuli, the nanoparticles catalyze the rapid contraction of the hydrogel’s polymer network, causing the modulus, hardness, and strength to increase instantly, while the friction coefficient decreases rapidly, achieving dynamic control from “soft to hard.” For instance, Zhao et al. [52] introduced MXene into chitosan(CS) to prepare a photoresponsive Hydrogel by photoinitiated polymerization of acrylic acid (AAc) with 2-hydroxyethyl methacrylate bromide (HEMA-Br) as photoinitiator, N, N’-methylenebisacrylamide (MBAA) as crosslinker. This was followed by immersion in calcium acetate to create a dense network structure and in situ grafting of poly(3-sulfopropyl methacrylate potassium) (PSPMA) polymer brushes to form the lubrication layer (Figure 4a). This is a smart lubricating hydrogel material, and its lubrication can be regulated by changing the intensity of NIR light. On the one hand, near-infrared light (NIR) irradiation induces temperature changes at the interface, causing the modulus of the P(AAc-CaAc-co-HEMA-Br) polymer to increase with rising temperature, resulting in phase separation. Near-infrared light (NIR) irradiation induced interfacial temperature changes, causing the increasing modulus of the P(AAc-CaAc-co-HEMA-Br) polymer with the rise in temperature (Figure 4b) and resulting in phase separation at 80 °C (Figure 4c). The transition from a soft state to a hard state reduced the contact area, thereby decreasing the friction coefficient. On the other hand, the PSPMA brushes were uniformly dispersed under NIR light due to the photothermal effects of MXene nanosheets, cultivating a hydrated layer as a lubricant, enabling it to achieve dynamic friction regulation (Figure 4d). Mas et al. [53] mimicked the structure of gecko toe setae to design a double-sided nano-hydrogel fiber composite film. Under alkaline conditions (pH = 12), it exhibited a low coefficient of friction (COF ≈ 0.4).

In addition to the method of directly doping nanoparticles, nanoparticles can also be encapsulated in hydrogels to form microspheres, achieving interfacial friction reduction. In terms of therapy of osteoarthritis (OA), the synergy of hydration, lubrication, and stimuli-responsive drug release is significant. Specifically, metal-organic framework nanoparticles (nanoMOFs) have significant value in drug delivery. Therefore, Wu et al. [54] fabricated MIL-101(Cr) nanoparticles coated with polyacrylamide microgel layers (PNIPAm) through a one-pot soap-free emulsion polymerization technique (Figure 5a). Compared to pure water lubrication, the nanoMOFs-based microgel hybrid as additives kept a lower and more stable CoF (Figure 5c), which was attributed to the synergistic effect of the hydrated lubrication of the coated PNIPAm nanosheets and the “ball bearing” effect of the nano-MOFs. Besides, due to the temperature sensitivity of PNIPAm, reversible swelling and behaviors were realized by tuning the temperature below and above the LCST (Figure 5b). Therefore, under high-temperature conditions, a lower CoF was achieved due to the formation of the PNIPAm microgel hydrolyzed lubricating layer. At the same time, the drug of diclofenac sodium-loaded hybrid was released owing to gel collapse (Figure 5d), which has potential in the therapy of osteoarthritis (OA).

In summary, the addition of nanoparticles can regulate the swelling rate of hydrogels. By utilizing the different hydrophilic and hydrophobic properties of various types of nanoparticles, the hydrophilicity and hydrophobicity of the hydrogel can also be adjusted, thereby affecting its water retention capacity. In applications, the swelling performance of the hydrogel can be adjusted according to factors such as the type, concentration, and size of the nanoparticles, thereby meeting different lubrication needs. However, research found that nanoparticles tend to agglomerate during the gel synthesis process, leading to uneven dispersion. During friction, hydrogels loaded with nanoparticles can collapse and form dense particles, affecting the friction-reducing lubrication effect of the hydrogel. Based on this, the design strategy of introducing chemical groups to enhance the long-lasting lubrication of hydrogels has attracted widespread attention from many scholars.

2.2. Introducing Functional Chemical Groups

Hydrogel chemical groups include hydrophilic groups (hydroxyl (-OH), carboxyl (-COOH), amino (-NH₂), sugar groups) that form stable structures by absorbing water through hydrogen bonding. These hydrophilic groups not only absorb water molecules through hydrogen bonding but can also react with other functional groups (e.g., responsive groups, zwitterionic groups) to form chemical bonds, further enhancing the stability and mechanical properties of the hydrogel [55,56]. Therefore, introducing functional chemical groups into the hydrogel allows these groups to interact with the hydrogel’s groups through intermolecular interactions, hydration states, and surface chemical properties, directly affecting the tribological behavior of the hydrogel (e.g., reduced friction coefficient, improved lubrication durability, and formation of boundary lubrication layers).

2.2.1. Introducing Zwitterionic Groups

Zwitterionic groups introduced into hydrogels can interact with water molecules under electrostatic interactions, forming a highly stable hydration layer. Moreover, under different conditions, the zwitterionic groups can automatically adjust the state of the hydration layer to adapt to various interfacial lubrication mechanisms. There are two ways to implement this strategy: one is by introducing zwitterionic monomers, and the other is by introducing polar polyelectrolytes.

Zwitterionic groups enhance interface lubrication properties by constructing a molecular-level hydration barrier relying on the synergistic effect of positive and negative charges in zwitterionic monomers. Utilizing this characteristic, Wang et al. [57] synthesized P(MPC-co-SBMA) copolymer hydrogels by performing a radical polymerization reaction with different components of methylacrylic acid choline (MPC) and sulfonated methacrylic acid ester (SBMA) monomers through a physical blending method (Figure 6a). Experiment found that the water content and compressive strength of the gel gradually increase as the content of MPC increases (Figure 6b,c), indicating that the incorporation of MPC increased the crosslink density of the hydrogels, which enhanced hydration capacity of hydrogel network structure. Meanwhile, the CoF of the gel continuously decrease with the increase of MPC, and even achieved superlubrication when the mass ratio of MPC and SBMA is 1:10 (Figure 6d). The Microscopic lubrication mechanism of above results is that the molecular structure of sulfobetaine methacrylate (SBMA) contains negatively charged sulfonic acid groups (-SO₃H) and positively charged quaternary ammonium groups (-N⁺(CH₃)₂), which can capture free water molecules through ion-dipole interactions, forming a dynamic hydrated lubrication layer. The superlubricity mechanism is mainly attributed to the hydration effect of zwitterionic MPC and SBMA polymer chains, which form a uniform hydration layer on the hydrogel surface and enable strong water molecule adsorption on the sapphire surface to provide additional stabilized hydration layers. This strategy has shown significant advantages in the biomedical field. In artificial joint cartilage coatings for curing osteoarthritis (OA), the hydrated lubrication can further achieved a balance between lubrication properties and biocompatibility through synergistic effects with biomolecules such as hyaluronic acid (HAMA) [58].

Polyelectrolytes are high molecular weight polymers that can form ions in solution, with structural units containing ionizable groups. According to the carrying charge, polyelectrolytes can be categorized into cationic polyelectrolytes, anionic polyelectrolytes, and zwitterionic polyelectrolytes. Among them, zwitterionic polyelectrolytes are neutral themselves, but can carry positive or negative charges under different conditions [59]. Incorporating polar polyelectrolytes into hydrogels can utilize strong electrostatic interactions to attract a large number of water molecules to the surface of the hydrogel, forming a stable hydration layer, thereby reducing the CoF of the hydrogel surface [60]. Wang et al. [61] used surface immersion of N-hydroxysuccinimide acrylate (NAS) and in situ photoinitiation to graft the methylacryloyloxyethyl sulfonic acid betaine (SBMA) onto the polyethyleneimine/polyacrylic acid hydrogel coating (PAE), forming a zwitterionic layer (PSV) with lubricating functions. Compared to the unmodified zwitterionic polyelectrolyte hydrogel coating, the friction coefficient of PAE-PSV achieved a good lubrication effect and anti-biofouling property, which is a promising candidate for improving the surface biocompatibility of the implantable and indwelling medical devices. Li et al. [62] polymerized SBMA with hydrogel monomers containing amide groups and modified the surface of polyvinyl chloride (PVC) catheters, preparing an artificial vascular endothelial coating (AVEC) with excellent anticoagulant properties and good lubrication performance (CoF~0.0017). This modification method is relatively simple and usually achieves a significant lubricating effect, which has positive implications for the widespread use of lubricating hydrogel coatings in medical devices. Moreover, zwitterionic polyelectrolytes also exhibit thermosensitive properties. By adjusting the ratio of cationic and anionic monomers, the electrostatic interactions between molecular chains and the competitive hydrogen bonding interactions between the hydrogel molecular chains and water molecules can be regulated, endowing the zwitterionic polyelectrolyte hydrogel with a “controllable switch” for the UCST-LCST transition [63]. This characteristic will further enhance the adaptability of the hydrogel under different operating conditions.

However, the monomer polymerization efficiency of zwitterionic hydrogels is affected by their excessive hydrophilicity. To address this issue, Li et al. [64] developed a hydration shield-assisted self-catalyzed rapid polymerization (HS-A-RP). Using polyvinyl alcohol as a zwitterionic monomer surrounding the hydrated barrier, rapid gel cross-linking and efficient polymerization was realized under Ag⁺-S₂O₈²⁻ redox catalysis (Figure 6e), significantly shortening the manufacture time of zwitterionic hydrogels (Figure 6f). This method reduces the polymerization temperature, decreases the monomer concentration requirements, and simplifies the preparation process. Compared to traditional thermally or UV-induced hydrogels, the hydrogels made HS-A-RP method has a denser network, better compression and rebound performance, supporting applications in biomedicine and soft robotics, and laying the foundation for their large-scale application.

Figure 6. (a) Synthesis processes of P(MPC-co-SBMA) copolymer hydrogel [57]. The water content (b) and compressive stress (c) for copolymer hydrogels with different mass ratios of MPC and SBMA. (d) COF-MPC amount plots of various copolymer hydrogels and PSBMA hydrogel at a sliding velocity of 48 mm s⁻¹ and a normal load of 4 N; test duration is approximately 15,000 s [57]. (e) The formation mechanism of the dehydration-aggregated zwitterionic monomers cluster is raised by the hydration shielding effect [64]. (f) The effect of incorporated hydration shielding agent on gelation time, blue dots referred to the SBMA monomer precursor with hydration shielding agent, and brown dots referred to the SBMA monomer precursor without hydration shielding agent [64].

Figure 6. (a) Synthesis processes of P(MPC-co-SBMA) copolymer hydrogel [57]. The water content (b) and compressive stress (c) for copolymer hydrogels with different mass ratios of MPC and SBMA. (d) COF-MPC amount plots of various copolymer hydrogels and PSBMA hydrogel at a sliding velocity of 48 mm s⁻¹ and a normal load of 4 N; test duration is approximately 15,000 s [57]. (e) The formation mechanism of the dehydration-aggregated zwitterionic monomers cluster is raised by the hydration shielding effect [64]. (f) The effect of incorporated hydration shielding agent on gelation time, blue dots referred to the SBMA monomer precursor with hydration shielding agent, and brown dots referred to the SBMA monomer precursor without hydration shielding agent [64].

2.2.2. Introducing Dynamic Chemical Bonds

The design concept of the introduction of dynamic covalent bonds is to take advantage of the reversible reorganization mechanism of dynamic covalent bonds to achieve the special functions of lubricating hydrogels, such as self-repair, shape memory, and stimulus response. Dynamic covalent bonding is a class of chemical bonds that can be reversibly formed and broken under specific conditions, and such reversible formation and breaking changes will trigger the conformational changes of the polymer chain of the hydrogel (stretching or curling), which will lead to the changes of the entire cross-linking network, which is macroscopically manifested as the swelling or shrinking of the volume of the hydrogel. The core mechanism of lubrication modification by introducing dynamic covalent bonding lies in the reversible reconfiguration of the dynamic crosslinked network and the capture of environmental lubricants.

In the reversible repair of gel networks induced by dynamic bond, common dynamic covalent bonds and non-covalent interactions include imine bonds [65], hydrazone bonds [66], disulfide bonds [67], boronated ester bonds [68], Diels-Alder reactions [69], electrostatic interactions [70], host-guest interactions [71], hydrogen bonding [72], hydrophilic-hydrophobic interactions [73]. as shown in Figure 7. Scientists have developed a series of long-lasting self-lubricating hydrogel lubricants based on different dynamic chemical bonds. Sun et al. [74] prepared a self-healing hydrogel (Figure 8a) based on poly(N-isopropylacrylamide) modified with hydrazone bonds and sodium alginate (OSA) with exposed aldehyde groups through radical copolymerization. By adjusting the ratio of hydrazone bonds to aldehyde groups, the mechanical properties of the hydrogel can be optimized, and the presence of arylhydrazone bonds imparts self-healing capabilities to the hydrogel. The results of the continuous step stress measurement from the rheological test indicated that when switching from high stress of 700 Pa to low stress of 10 Pa, G′ immediately returned to its original value without any loss, and even the stress increased to 1060 Pa or 2000 Pa (Figure 8b), which indicated that the gel had rapid recovery characteristics, showing great potential for applications as a biomaterial in tissue engineering. Based on the specific recognition characteristics of host-guest interactions, Wang et al. [75] innovatively utilized supramolecular β-cyclodextrin (β-CD’) and adamantane (Ad) host-guest interactions to construct a self-repairing hydrated lubricating surface. First, β-cyclodextrin was covalently grafted onto the lubricating polymer (PMPC) to form the main chain (β-CD’-PMPC). Then, adamantyl groups were modified on the substrate surface, and a lubricating layer was formed via host-guest self-assembly (Figure 8c). When friction caused localized damage, mechanical force triggered the reversible dissociation of the host-guest complex, releasing lubricating polymer molecules that rebind to the exposed adamant Experiments confirmed that this system can quickly restore ultralow CoF of 0.024 in physiological fluid environments (MFP), and the repair efficiency is positively correlated with polymer concentration and contact time (Figure 8d), providing a biomimetic solution for long-life lubrication in fields such as artificial joints and precision machinery. Inspired by the reversibility and selectivity of click chemistry, Xiang et al. [76] manipulated two types of dynamic covalent bonds, boronated ester bonds and schiff base bonds, to selectively bind the lubrication micelles of F127 triblock copolymers with boronic acid (F127-B) and aldehyde groups (F127-CHO) as end groups to their paired surfaces, constructing two lubrication coatings for the friction pair (Figure 8e). In the absence of damage, the friction surface can exhibit superlubricity (μ~0.002) under physiological high pressure, and it can also recover its superlubricity after repeated damage (Figure 8f). This study provides new paradigms for developing repairable, durable lubricating surfaces in biomedicine.

Interestingly, the study found that introducing shear-responsive molecules can also achieve interfacial self-healing effects. For instance, Zhang et al. [77] also developed a shear-responsive FT-PAAm/PVA supramolecular lubricating hydrogel, which was obtained by combining the non-covalent thixotropic N-formylmethoxycarbonyl-L-tryptophan (FT) supramolecular network with the covalent polyacrylamide/polyvinyl alcohol (PAAm/PVA) interpenetrating network. FT-PAAm/PVA hydrogels also exhibit excellent in-situ self-healing effects similar to other self-healing surfaces. Its self-healing mechanism lies in the fact that shear force triggers the disintegration of the FT supramolecular network, thereby forming a hydrogel lubrication layer (Figure 9a), achieving an ultra-low friction coefficient of 0.003 (Figure 9b). At the same time, this biomimetic hydrogel also exhibits excellent shear force response characteristics. The rheological test results in Figure 9c indicate that as the FT component increases, the material’s sensitivity to shear force enhances, further reflecting its excellent lubrication performance. However, this material is difficult to achieve self-healing of the gel body in a dynamic shear environment, and once damaged, self-healing cannot be achieved. To this end, Zhang et al. [78] further introduced a polyhydroxyethyl acrylamide (PHEAA) network with intermolecular hydrogen bonds, forming a triple interpenetrating structure with the FT supramolecular network and the PVA covalent network. Under dynamic shear, the disassembled FT sol flows into the scratch gap and reassembles via π-π interactions, while the hydrogen bonds between PHEAA chains reform, achieving a synergistic self-repair of lubrication and mechanical properties. This innovation addresses the challenge of self-repair in biomimetic cartilage hydrogels within dynamic environments.

Lubricant capture driven by dynamic covalent bonds refers to the use of the reversibility and stimulus responsiveness of dynamic covalent bonds to directionally fix hydrogel lubricants at specific interfaces, thereby achieving precise adjustment and long-term maintenance of lubrication performance. The core mechanism involves the breaking and reformation of dynamic covalent bonds under external mechanical force, pH, or temperature stimuli, allowing the lubricant to be released or captured in situ. In the application of artificial joints, this strategy not only provides intelligent and adaptive lubrication but also enables controlled drug release simultaneously. Wu et al. [79] synthesized a dual-responsive hydrogel, responsive to pH and temperature, by integrating sodium methacrylate (NaMA) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) into PNIPAAm hydrogel. Experiments demonstrated that modifying the pH level and temperature of the hydrogel can alter the friction coefficient. In neutral conditions and beneath the LCST, the polymer chains undergo expansion, yielding an exceptionally low friction coefficient of approximately 0.05. As the pH value diminishes and the solution gets increasingly acidic, the pNaMA segments of the gel undergo collapse. At the same time, when the temperature increases, both polymer sections of the hydrogel break simultaneously, causing the friction coefficient to suddenly increase to 1.0, and this state is unstable. Wan et al. [80] utilized inverse emulsion polymerization, incorporating itaconic acid (IA) as the pH-responsive monomer, N-isopropylacrylamide (NIPAM) as the temperature-responsive monomer, and acrylamide (AM) as the hydrophilic nonionic monomer to synthesize a hydrogel exhibiting dual responsiveness to pH and temperature. The dynamic covalent network within the hydrogel experiences reversible phase transitions when subjected to shear force, resulting in the release of lubricating molecules (e.g., MoS₂ nanosheets) that establish a low-friction boundary layer. Under elevated stress, it demonstrated enhanced viscoelasticity, allowing it to adjust to frictional behavior in dynamic loading conditions.

Overall, introducing functional chemical groups into hydrogels can directly affect the friction behavior of the hydrogels by regulating intermolecular interactions, hydration states, and surface chemical properties. Through the precise design of dynamic covalent bonds, the capture and release of lubricants can be made efficient, controllable, and intelligent, making them suitable for extreme conditions or scenarios with high biocompatibility requirements.

Figure 9. (a) Schematic diagram of bionic shear-responsive hydrogel lubrication mechanism. (b) Variation of friction force with shear cycle times. (c) The rheological properties of gels with different amounts of FT [77].

Figure 9. (a) Schematic diagram of bionic shear-responsive hydrogel lubrication mechanism. (b) Variation of friction force with shear cycle times. (c) The rheological properties of gels with different amounts of FT [77].

In summary, the water content of hydrogels is a crucial metric to evaluate their lubricating efficacy. A greater water content facilitates the formation of a liquid lubrication coating during friction. The water content correlates with the cross-linking density and mesh size of the hydrogel. A reduced cross-linking density results in an increased mesh size and enhanced water content. Nevertheless, the water content, elevated strength, and superior wear resistance of conventional hydrogels cannot be harmonized.

3. Multi-Scale Structure Regulation Strategy

Traditional hydrogel structures are mostly single networks, formed by chemical or physical cross-linking. Chemical cross-linked networks have high mechanical strength but lack self-healing properties, while physical cross-linked networks are unstable. Therefore, modifying the hydrogel structure to ensure the lubricating properties of the hydrogel surface while improving its mechanical properties and wear resistance is necessary.

The design strategy for modifying hydrogel structures is also inspired by articular cartilage. The structure of articular cartilage exhibits a layered gradient distribution, with the superficial layer, transitional layer, radial layer, and calcified layer from the outside to the inside [81,82]. The hardness distribution is related to the orientation distribution of collagen fibers in each layer. The collagen fibers in the outermost layer are arranged parallel to the surface, and the chondrocytes are oriented in a sagittal plane, possessing a high water content, which provides excellent lubrication and wear resistance [83,84]; in the transitional layer, the collagen fibers are loosely and randomly arranged, and the chondrocytes are round, capable of withstanding certain pressure while retaining elasticity, allowing the cartilage to have cushioning ability under load; the calcified layer contains a large amount of calcified collagen fibers, which have high hardness and low elasticity, anchoring the cartilage to the subchondral bone [85,86]. The gradient elastic modulus distribution structure of articular cartilage not only ensures the lubrication performance of the surface but also guarantees the overall compressive and shear resistance. In mimicking the structure of articular cartilage, the design strategy for regulating hydrogel structures involves gradient design or multi-level integration of traditional hydrogel structures to regulate the mechanical distribution gradient in three-dimensional space, enhancing the load-bearing capacity of the gel and reducing friction. Typical design strategies include dual-network structure design and gradient/heterogeneous structure design.

3.1. Dual-Network Structure Design

The double network (DN) structure combines the extensibility of the flexible network with the energy dissipation of the rigid network, playing to their strengths to achieve a more stable network structure. Compared to single-type networks, the double network structure can produce higher swelling capacity and mechanical strength, exhibiting better mechanical performance in lubrication [87]. Common types of DN structures include interpenetrating (IPN) network structures, physical-chemical double crosslinked networks, and physical-physical double crosslinked networks.

Inspired by the nested network structure found in bone tissue and reticular connective tissue, Wang et al. prepared an interpenetrating DN hydrogel by forming a poly(hydroxyethyl methacrylate) (PHEMA) network within the porous structure of alginate hydrogels. This hydrogel exhibited good mechanical strength and elastic deformation capability while also possessing lubrication performance as good as commercial lubricants (K-Y Jelly). Liu et al. [88] firstly prepared physically crosslinked polyvinyl alcohol (PVA) hydrogels using a freeze-thaw method, and then immersed them in polyacrylic acid (PAA) solution to form PVA/PAA double network hydrogels. The carboxyl groups of PAA formed a dense double network structure with the PVA molecular chains through hydrogen bonds and covalent bonds, significantly enhancing the mechanical strength and environmental responsiveness of the material. Meanwhile, the carboxyl groups within the PVA/PAA double-network hydrogel can adsorb metal ions or graft functional molecules, which helps enhance the hydrogel’s environmental adaptability and lubricity. In this study, the crystalline regions of PVA induced by freezing act as physical crosslinking sites. The combination of hydrogen bonding and covalent interactions between the carboxyl groups of PAA and PVA significantly enhances the mechanical properties of the hydrogels. However, the traditional DN hydrogels are usually chemically cross-linked and non-recoverable after being destroyed.

Utilizing physical cross-linking to construct the first network can benefit energy dissipation. Physical networks typically act as sacrificial bonds, breaking under load to dissipate energy while allowing partial recovery of internal damage when the DN gel relaxes. Chen et al. [89] provided an example utilizing agar as the primary network and hydrophobically associating polyacrylamide (HPAAm) as the secondary network to fabricate full physical cross-linked double-network gels, which exhibited excellent fatigue resistance and self-healing properties. The agar first network exhibits a reversible sol-gel transition at elevated or ambient temperatures, respectively. Conversely, the second network possesses the capability for self-recovery or healing through dynamic hydrophobic interactions. Consequently, the resultant totally physically cross-linked hydrogels can achieve a healing effectiveness of up to 40% following damage. Additionally, adding nanofillers also plays a role similar to the first network in the DN hydrogel. Graphene oxide (GO) as nano-enhancement was introduced into polyvinyl alcohol/polyacrylic acid (PVA/PAA) hydrogel by freezing-thawing and annealing treatment to prepare high-strength lubricating hydrogel PVA/PAA/GO [90] (Figure 10a). The tribological test results showed that PVA/PAA/GO hydrogel appreared the lowest CoF of 0.039 under a high load of 40 N compared to other hydrogel system (Figure 10b) and remained at a low level after 500,000 times of friction, which benefited from the the formation of a large number of microcrystalline areas after freezing-thawing and annealing treatment and the hydrogen bond interaction of GO as reinforcing phases and PVA. Interestingly, nanoparticles can act as cross-linking points, achieving further enhancement of the modulus. Shi et al. [91] used vinyl hybrid SiO₂ nanoparticles (VSNPs) as core crosslinking points and successfully constructed a ternarily crosslinked nanocomposite physical hydrogels (TC-NCP gels) with a single network structure by in-situ copolymerization of acrylamide (AM) and hydrophobic monomer methacrylate (C18) on the surface of VSNPs (Figure 10c), whose toughness is an order of magnitude higher than dual-network hydrogels (Figure 10d).

On the basis of ensuring mechanical stability, further enhancing the lubrication performance of the gel is necessary. Based on the hydration lubrication mechanism, Zhao et al. [92] prepared a high-performance superlubrication layer via the interfacial network semi-interpenetration method in the polyvinyl alcohol (PVA)-chitosan (CS) hydrogel system. Its performance enhancement relies on the electrostatic cross-linking crystalline regions in situ salting-out combined with surface hydration lubrication. Benefiting from the polyphosphate acting as an in-situ salting-out barrier to protect the electrostatic cross-linking within the network, the coating exhibited strong anti-swelling and mechanical properties. At the same time, the surface hydration lubrication mechanism and the stress delocalisation effect of the interpenetrating network can effectively reduce the shear stress at the contact surface. The crystalline regions of the cross-linked substrate can also passivate cracks, further inhibiting the propagation of friction cracks in the substrate layer and thereby ensuring the material’s extended service life.

Figure 10. (a) Preparation process diagram of PVA/PAA/GO hydrogel and crosslinking diagram of different hydrogels [90]. (b) Curves of friction coefficients over time of different hydrogels under a load of 40 N with 10 mm/s of speed [90]. (c) The fabrication process and the toughness compared with other samples of the TC-NCP gel [91]. (d) The force-stretch ratio curves of the notched gel samples [91].

Figure 10. (a) Preparation process diagram of PVA/PAA/GO hydrogel and crosslinking diagram of different hydrogels [90]. (b) Curves of friction coefficients over time of different hydrogels under a load of 40 N with 10 mm/s of speed [90]. (c) The fabrication process and the toughness compared with other samples of the TC-NCP gel [91]. (d) The force-stretch ratio curves of the notched gel samples [91].

3.2. Gradient Anisotropic/Heterogeneous Structure Design

The natural layered structure of articular cartilage exhibits distinct gradient characteristics. As shown in Figure 11a, from the superficial to the deep layer, cell morphology, collagen fiber alignment, and matrix composition undergo ordered changes: superficial collagen fibers are dense and parallel to the surface, serving primary lubrication [81,93,94,95]; the intermediate layer features interwoven fibers for structural support; and deep-layer fibers insert vertically into subchondral bone to enhance connection stability [96]. Natural materials can integrate these mechanical properties primarily due to their finely tuned hierarchical orientation and heterogeneous structures. This natural hierarchy provides critical inspiration for designing biomimetic layered gel lubricants. Unlike micro-design strategies based on multi-network architectures, anisotropic/heterogeneous structures refer to the local differentiation of gels in network direction, composition, and cross-linking density to meet the different performance requirements of various regions of the same material, achieving a synergistic enhancement of lubrication performance and load-bearing capacity [97]. Compared to traditional hydrogels, it can rapidly shrink through a catalyzed hydrogel polymer network, with modulus, hardness, and strength instantly increasing, and the friction coefficient quickly decreasing, achieving dynamic adjustment from “soft to hard”.

The main methods for inducing gel-layered anisotropic/heterogeneous structures include magnetic field induction, light induction, chemical dissociation, and laser etching. For instance, Chen et al. [98] generated a bilayer gel structure by synthesizing horizontally oriented polyvinyl alcohol/acrylic acid copolymer (PVA/PDA-Fe₃O₄-MMT/PAA) and vertically oriented PVA/PDA-Fe₃O₄-carbon fiber/PAA hydrogel surfaces through magnetic field induction (Figure 11b). This structure presented a high mechanical strength with a compressive strength of 5.21 ± 0.45 MPa. After tens of thousands of in-situ friction tests under soft/hard contact similar to articular cartilage conditions, the CoF was maintained at 0.032 without significant wear, which was comparable to natural joint cartilage. This provided a novel strategy for constructing high-strength, low-friction cartilage-mimetic materials. Similarly, Zhang et al. [99] successfully prepared a double-gradient hydrogel (DGH) with robust lubrication performance through light-induced network dissociation and polymer self-growth strategies (Figure 11c). Specifically, in the vertical direction, a gradient tubular structure was constructed through controllable light-induced dissociation, which exhibits a gradual transition from a dense structure at the bottom to a more porous structure at the top in the gradient direction. The horizontal surface is composed of a rigid porous gel framework filled with a lubricating phase. This DGH can achieve an ultra-low friction coefficient (COF) of 0.0036 under a high load of 50 N. This strategy provides a new approach for developing gel materials that combine high load-bearing capacity with excellent lubrication performance. Qu et al. [100] developed a bionic hierarchical hydrogel mimicking articular cartilage, consisting of a robust polyacrylic acid-Fe/polyacrylamide (PAA-Fe/PAM) hydrogel as the load-bearing bottom layer and an alkali-etched soft sponge-like structure as the top layer (Figure 11d). Under a 3N load, it exhibited an ultra-low CoF of 0.009, demonstrating excellent anti-friction and anti-wear properties through the synergy of surface lubrication and bottom-layer load-bearing. Ma et al. [101] constructed a multi-level structured super-lubricating hydrogel system through a combination design of layered gradient structures formed by chemical dissociation and horizontal surface textures formed by laser etching. This combination of anisotropic design of layered gradients and surface textures achieves controllable release of the lubricant through a “self-pumping” mechanism during dynamic shear, while enhancing stability through “self-sealing” modification. Based on the multi-level synergistic mechanism, the hydrogel system exhibited super-lubrication properties under high contact pressure.

As mentioned in the previous discussion, nanoparticles can respond to external stimuli, achieving the soft-to-hard phase transition and assisting hydrogels in dynamic friction regulation [52]. Therefore, in the future, responsive materials can be incorporated into the preparation of gradient hydrogels to achieve active regulation of the soft-hard phases of the gel, thereby enhancing the gel’s load-bearing capacity and intelligent lubrication synergistically.

Figure 11. (a) Natural joint cartilage tissue structure [81]. (b) Schematic diagram of the bilayer structure design of magneticity-induced anisotropic hydrogel [98]. (c) Design scheme of dual gradient hydrogel (DGH) [99]. (d) The preparation process of the layered hydrogels by an alkali-induced network dissociation strategy [100].

Figure 11. (a) Natural joint cartilage tissue structure [81]. (b) Schematic diagram of the bilayer structure design of magneticity-induced anisotropic hydrogel [98]. (c) Design scheme of dual gradient hydrogel (DGH) [99]. (d) The preparation process of the layered hydrogels by an alkali-induced network dissociation strategy [100].

4. Surface and Interface Modification Strategy

The bonding strength between hydrogels and friction pairs is an important indicator for evaluating the lubrication performance of hydrogels. For traditional hydrogels, the bonding with the substrate is generally achieved through chemical cross-linking or reversible physical cross-linking. The bonding strength of physical cross-linking is relatively poor due to the non-covalent bond cross-linking method, and a large amount of moisture can weaken the activity of hydrophilic groups. In contrast, chemical cross-linking generally relies on the covalent bonding between the polymer chains on the gel surface and the substrate surface, resulting in stronger adhesion. Surface and interface modification strategies involve physical or chemical methods to achieve adhesion and differentiation between the hydrogel and the substrate [102].

4.1. Hydrogels Surface Modification (Hydrogel Paints)

The modification of the surface of hydrogels is based on the mechanism of interfacial covalent bonding. By introducing specific functional groups to the hydrogel, interfacial interconnection is promoted to achieve bond enhancement. This strategy is similar to previous gel lubrication modification strategies, but the design concept is different. Its purpose is to enhance the interaction between the gel and the interface to improve its debonding resistance strength. Yuk et al. [103] utilized the silane coupling agent 3-(Trimethoxysilyl) Propyl Methacrylate (TMSPMA) to covalently bond hydrogels to non-porous surfaces, achieving interfacial toughness exceeding 1000 J/m² (Figure 12a), which because that the silane coupling agent has organic groups and silicon elements, which can act on both hydrogel and metal oxide layer to form covalent bridging, significantly improving the adhesion and material toughness [104]. Based on this, Yao et al. [105] decoupled the preparation into three steps involving monomer copolymerization, coupling agent-mediated crosslinking, and substrate bonding via siloxane (Si-O-Si) networks (Figure 13a). This approach decoupled synthesis and assembly through physical spraying with controlled crosslinking, thereby streamlining the coating preparation workflow while enabling precise control over crosslinking degrees. Li et al. [106] designed a linking molecule (Linker-1) containing carboxylic acid groups and methacrylic acid groups (Figure 12b). Through simple dipping, brushing, and drop-casting processes, bonding with metal surfaces can be achieved within minutes, with a bond strength of up to 1000 J/m². Imbia et al. [107] first deposited polydopamine (PDA) on the substrate, and then covalently grafted zwitterionic and cationic copolymers containing aldehyde groups onto the polydopamine surface, preparing a stable hydrogel coating. Gao et al. [108] developed a stitch-bonding strategy by combining chemical bonding and topological entanglement methods. Firstly, a polymeric binder containing catechol was coated on the surface of the hydrogel with carboxyl groups, and then the substrate was coated using NaIO₄ oxidant as an intermolecular cross-linking trigger, which triggered the cross-linking to form an entangled network (Figure 12c). This hydrogel can achieve rapid adhesion on any substrate, with a substrate adhesion energy of approximately 900 J·m⁻², close to the fracture toughness of biological cartilage (≈1000 J·m⁻²). This is attributed to the synergistic effect of physical topological entanglement and chemical bonding, which improves the interfacial mechanical properties and fracture energy dissipation.

The above strategies all require modification of the substrate to achieve reliable adhesion of the gel to the substrate under specific cross-linking conditions. Inspired by the coating, Yang et al. [109] proposed a spray-assisted assembly method: dehydrated hydrogel particles are adhesively anchored to substrates and rehydrated to form uniform rehydrated hydrogel paint (RHP) coatings (Figure 13b). This strategy achieves large-area fabrication (≥2.0 m²) across diverse substrates, with structural integrity retained under compressive stresses up to 50 kPa. Additionally, the “primer adhesive” method refers to adding adhesives to the gel to achieve strong adhesion between the gel and the substrate. Commonly used adhesives include hydrophilic polyurethane, polydimethylsiloxane, phenolic epoxy resin and catechol [110,111,112,113]. This approach eliminates the need for functional group modifications of the gel and substrate, relying solely on the primer’s strong adhesion ability with the friction substrate, playing the role of “commercial super glue”. It is a user-friendly strategy with broad applications, currently being applied in marine antifouling, implant devices, lubrication [114,115,116,117,118,119].

Figure 13. Coating technology: (a) Principle of hydrogel coating [105]. (b) Preparation method of reinforced hydrogel [109].

Figure 13. Coating technology: (a) Principle of hydrogel coating [105]. (b) Preparation method of reinforced hydrogel [109].

4.2. Modification of the Friction Interfaces (Hydrogel Layers)

The design strategy for lubrication modification of the hydrogel and friction interface is mainly reflected in the preparation of hydrogel lubrication coatings. The traditional methods and structure of hydrogel formation make it difficult to achieve stable and strong adhesion with the substrate. Therefore, lubrication modification of the hydrogel and friction interface is crucial for the further development of hydrogel lubrication coatings.

4.2.1. Interfacial Chemical Modification

Interfacial chemical modification refers to introducing free radical active sites on the substrate surface, and then, under external stimulation, initiating the free polymerization of monomers on the substrate surface to form a stable hydrogel lubrication coating.

For metal substrates, representative techniques include atom transfer radical polymerization (ATRP), surface-catalyzed initiated radical polymerization (SCIRP), and UV-assisted SCIRP (UV-SCIRP). To elaborate on this, Wang et al. [75] combined β-cyclodextrin with an ATRP initiator via click chemistry, using ATRP to in situ grow phosphorylcholine-based polymer brushes (PMPC) on substrates (Figure 14a). Through host-guest interactions between β-cyclodextrin and adamantane, lubrication units were continuously introduced, reducing the titanium alloy surface CoF by over 50% (Figure 14b). Building on tribological “in-situ friction polymerization” concepts, Ma et al. [120] proposed SCIR (Figure 14c), eliminating the need for harsh external stimuli. Xu et al. [121] extended the SCIP strategy to universal substrates and proposed the method of UV-SCIRP: a PDA/CA-Fe³⁺ adhesive layer is first deposited, then UV light reduces Fe³⁺ to Fe²⁺, triggering interfacial polymerization of monomer solutions into hydrogel coatings at room temperature (Figure 14d), suitable for medical devices.

Figure 14. (a) Schematic diagram of dynamic self-healing of ATRP-based in-situ polymer brushes [75]. (b) The CoF curves of polystyrene-microsphere AFM tips coated with PMPC on different substrate surfaces [75]. (c) schematic diagram of preparation of iron-based composite in-situ grafted hydrogel based on SCIRP [120]. (d) schematic diagram of hydrogel coating growth using SIL@UV-SCIRP method [121].

Figure 14. (a) Schematic diagram of dynamic self-healing of ATRP-based in-situ polymer brushes [75]. (b) The CoF curves of polystyrene-microsphere AFM tips coated with PMPC on different substrate surfaces [75]. (c) schematic diagram of preparation of iron-based composite in-situ grafted hydrogel based on SCIRP [120]. (d) schematic diagram of hydrogel coating growth using SIL@UV-SCIRP method [121].

For polymer substrates, surface-initiated strategies focus on mild, substrate-compatible polymerization to avoid damaging the soft matrix. Key methods include surface-initiated atom transfer radical polymerization (SI-ATRP) [122] for controlling chain growth, reversible addition-fragmentation chain transfer polymerization (RAFT) for uniform coating thickness [123], and enzyme-catalyzed in situ crosslinking (EISCC) [124] for biocompatible systems. These approaches leverage the polymer’s inherent reactivity (e.g., pendant functional groups) to achieve stable hydrogel adhesion, complementing the metal-substrate strategies by prioritizing “soft-soft interface compatibility”.

4.2.2. Interfacial Physical Modification

Using physical methods (e.g., embossing, laser etching) to create specific microstructures (e.g., grooves, holes, protrusions) on the substrate to enhance interfacial adhesion and form a strong gel lubrication coating.

Surface texturing, a composite strategy integrating surface modification with structural functional enhancement, fabricates ordered microstructures (e.g., grooves, pores, protrusions) on material surfaces via physical/chemical techniques (laser etching, imprinting). When combined with hydrogel systems, it enhances interfacial lubrication through two synergistic mechanisms: (1) improved substrate-gel adhesion via mechanical interlocking between textures and gel networks; (2) creation of lubricant reservoirs that sustain boundary film formation during sliding. Based on this, Yu et al. [125] validated this strategy by laser-etching LST steel to form microporous arrays, which were infiltrated with supramolecular gel lubricants (Figure 15a). This composite achieved a CoF of 0.04 under a 100 N load at 25 °C (Figure 15b), attributed to polar groups in the gel forming a 20~30 nm thick boundary film at the contact interface (Figure 15c), directly verifying the “reservoir-mediated lubrication” mechanism. Complementarily, Guo et al. [126] optimized pore density on AISI 52,100 steel via laser surface texturing (LST), then infiltrated it with PCEC/BSA hydrogel (Figure 15d). When the pitch/diameter ratio of textured surfaces was optimized to 1.2, the CoF was reduced by 50% compared to non-textured substrates (Figure 15e), driven by BSA-mediated sustained lubricant release that synergizes with mechanical interlocking (Figure 15f).

Notably, surface texturing enhances lubrication primarily through geometric optimization rather than chemical modification, representing a paradigm shift from traditional additive-dependent approaches. This strategy offers scalability for high-performance coatings, with particular promise in biotribology applications (e.g., artificial joints, medical devices) where structural compatibility is critical.

Figure 15. (a) Schematic diagram of the preparation process of LST steel impregnated with supramolecular gel lubricant [125]. (b) Evolution of CoF/time of disks with PAO10 and supramolecular gel lubricant [125]. (c) Schematic diagrams of the contact interface between LST steel and ball under high sliding velocity and high contact pressure [125]. (d) Schematic diagram of the friction test on the surface of the metal fabric coated with PCEC/BSA gel [126]. (e) The CoFs of un-textured surface and textured surface (P/Dratios = 1.2) on the conditions of 1.64 Hz, 3.6 MPa [126]. (f) Lubrication principle of textured surface-filled PCEC hydrogel [126].

Figure 15. (a) Schematic diagram of the preparation process of LST steel impregnated with supramolecular gel lubricant [125]. (b) Evolution of CoF/time of disks with PAO10 and supramolecular gel lubricant [125]. (c) Schematic diagrams of the contact interface between LST steel and ball under high sliding velocity and high contact pressure [125]. (d) Schematic diagram of the friction test on the surface of the metal fabric coated with PCEC/BSA gel [126]. (e) The CoFs of un-textured surface and textured surface (P/Dratios = 1.2) on the conditions of 1.64 Hz, 3.6 MPa [126]. (f) Lubrication principle of textured surface-filled PCEC hydrogel [126].

5. Summary and Outlook

This article systematically elaborates on the modification strategies for hydrogel lubrication based on recent research on hydrogel lubricants. Based on the characteristics of hydrogels and lubrication mechanisms, we comprehensively and systematically analyze the design concepts and principles of gel lubrication modifications from three aspects: hydrogel component modification, structural regulation, and friction interface modification. The design concept for the lubrication modification of hydrogels is to balance water content, mechanical strength, and wear resistance to form a stable lubrication layer. The core mechanism of lubrication modification in hydrogels is the regulation of hydrated lubrication and boundary lubrication. By altering the degree of cross-linking, the type of polymer, and the water content, the hardness, swelling, and hydration capacity of the gel can be controlled. For example, adding liposomes and stimuli-responsive groups can dynamically regulate lubrication, forming a stable lubrication layer on the surface. Notably, although the design strategies differ, there are similar modification methods. The three strategies in the text are self-contained, and each part is indispensable for achieving reliable lubrication of the gel. However, to expand the practical application of gels, the following issues still urgently need to be addressed:

(1)

The research lacks unified standard guidance and has not formed a systematic theoretical framework. Current research focuses on experiments with limited theoretical foundations, making it difficult to quantitatively predict friction characteristics based on hydrogel microstructures and tribological conditions.

(2)

The preparation of gels is difficult to industrialize. The preparation of high-quality hydrogel lubricants may involve complex cross-linking processes and precise condition control, which could increase production costs, limiting large-scale production.

(3)

The conducted research is mostly based on overly idealized laboratory conditions. Although the lubrication performance can be significantly improved by regulating the components and structure, issues related to long-term stability, environmental tolerance, and eco-friendliness are selectively ignored. Many studies may fail rapidly under real and complex working conditions, resulting in a low rate of successful application.

In the future, research on hydrogel lubricating materials should be problem-oriented, with design strategies as the focus. The modification methods of the gel should be optimized based on specific application scenarios and needs, developing high-end intelligent hydrogel lubricating materials that benefit multiple fields.