Friction and lubrication are universal phenomena between two contacting surfaces in relative lateral motion. Generally, the friction-related wear can lead to malfunction of artificial implants, surge in energy consumption and breakdown of mechanical assemblies [1,2], and such type of challenging issues are severely detrimental to a variety of biomedical applications, manufacturing processes and industrial operations. For example, wear in bone joints can give rise to a range of pathological conditions [3], gear wear can downgrade the performance of machines, apparatuses and facilities in various industries [4], and tool wear during machining processes can shorten the lifespan of tools and lower the precisions of workpieces [5]. Moreover, the friction-induced wear can also lead to considerable energy dissipation across the engineering cases, which could account for approximately 23% of the world total energy consumption according to the relevant reports [6]. Therefore, the mechanical wear arising from surface friction has been considered as a major factor responsible for the excessive cost in both personal health management and industrial production processes.

The original effort for friction reduction can be traced back to thousands of years ago when the ancient Egyptians might have replaced sliding with rolling to transport millions of massive stone blocks during the construction of those magnificent buildings. However, simply replacing sliding with rolling is not sufficient to meet the demand for further reduction in friction and advancement in lubrication. Fortunately, the emergence and development of tribology discipline allows the systematic investigation and practical application of principles and mechanisms associated with friction, lubrication and wear. Based on the basic tribology theories, numerous lubrication strategies have been proposed and implemented at both macro- and micro- scales to alleviate friction and the consequent wear particularly since the Industrial Resolution, including oil lubrication [7], lubricant additives [8], nanoparticle lubrication [9], and hydration lubrication [10]. Generally, coefficient of friction (i.e., COF denoted by μ) is used to quantitatively evaluate the performance of lubricating materials, which is defined as the ratio of friction force to the normal force between the two contacting surfaces. In addition, lubrication film thickness, lubrication interval and lubrication region are also important factors in modulating the lubrication behaviors and mitigating the wear phenomena, which will be discussed in detail in the following sections. It is no exaggeration to say that the continuous innovation of lubrication-based techniques has been contributing significantly to the notable progress of human civilization.

In the contemporary industrial and engineering applications, petroleum-based synthetic oils are the most extensively used lubricants with COF typically ranging from 0.01 to 0.1 [11]. However, most of them are intrinsically flammable and undegradable, which can lead to severe environmental pollution if not properly treated before disposal. Water-based or hydration lubrication technology has the characteristics of clean, green and environmentally friendly characteristics, so it has become a research hotspot in the field of tribology in recent decades. It has been reported that the minimum COF of aqueous lubricants can be one order of magnitude lower than that of oil-based lubricants [12,13]. Moreover, water exhibits lower viscosity (1 mPa·s, 20 °C) as compared to oil, and its viscosity-pressure coefficient is not pressure-sensitive, which suggests that water can serve as a lubricating fluid under high contact pressures. Through the distinguished work by Helmholtz and further elaboration by following scientists, the electric double layer (EDL) theory has been established, which can explicate the colloidal interactions behind hydration lubrication. Other important concepts such as hydration force, hydration layer and Stribeck curve are also involved in the basic theories of hydration lubrication. The biological evolution has given rise to various efficient hydration lubrication systems in diverse living organisms, such as the cardiovascular system [14,15], muscle and tendon tissue [16], digestive tract [17,18], oral mucosa [19,20], and articular cartilage (AC) [21,22], as illustrated in Fig. 1. With the rise of bionics in recent years, this topic has gained growing attention from many researchers across multiple/inter-disciplinary fields including mechanics, chemistry, biophysics and material science. The abundant studies relevant to this new branch of tribology, known as biotribology, have collectively improved the fundamental understanding of the hydration lubrication mechanisms and prompted the rapid advancement from bio-inspiration to artificial manufacturing. (See Table 1, Table 2.)

A specialized connective tissue in synovial joints of mammals, known as AC, is a notable representative among the vast source of inspiration from nature. In this fabulous biomass lubrication system, a pair of optimally smooth and slippery AC surfaces stand face to face with the confined cavity filled by the stringy synovial fluid, which can enable a nearly frictionless motion of the adjoining bones with an extremely low COF over decades [23]. The unique ability of AC to function effectively under a wide range of normal loads, shear distances and shear rates has become an appealing subject of considerable research attention. In order to unravel the inherent mechanisms behind this superior lubrication performance, many researchers have been sparing no effort to study the composition, structure and operation of AC by virtue of biochemical analysis, macro/nano-scale imaging, motion capturing and other characterization techniques [[24], [25], [26]]. It has been reported that the intact AC surface is coated by a gelatinous layer with a thickness of hundreds of micrometers, within which the complex and sophisticated network of various biomolecules such as lipids, collagen fibrils, polysaccharides (e.g., hyaluronic acid (HA)) and glycoproteins (e.g., lubricin) is hydrated and surrounded by a large amount of water molecules [27]. In the synovial fluid, there are also countless dissociative HA, lubricin and phospholipids that can help hold and regulate the water molecules in the lubrication system. The resulting boundary lubrication is capable of greatly reducing the mechanical friction between the ends of opposing bones under different working conditions. At present, a growing number of studies have been conducted to explore the multi-leveled synergistic effects involving different chemical compounds, tissue structures and motion patterns in the lubrication process [24]. All these explorations would provide comprehensive insights into the principles and mechanisms of AC lubrication systems, and more importantly, offer reliable theoretical guidance for the design and development of novel bio-inspired hydration lubrication techniques.

Within the realm of bio-inspired materials for hydration lubrication, hydrogels constitute a primary category of intense research, featuring three-dimensional (3D) networks of chemically or physically cross-linked hydrophilic polymers with high water content [28]. Benefiting from the exceptional similarity in macromolecular structure and chemical composition between AC matrixes and hydrogels, a series of AC-mimicking hydrogel materials have been prepared to acquire excellent lubrication performance [[29], [30], [31], [32], [33]]. These bio-inspired hydrogels can be potentially used to repair or even replace the worn AC surfaces, offering new opportunities for the treatment of severe osteoarthritis (OA). It has also been found that some specialized lubricating hydrogels can exhibit responsive behaviors to environmental factors including normal load [34], sliding velocity [35] and water content [36], which could strengthen their practical capability in biomedical applications. Moreover, some hygroscopic components introduced into the hydrogel networks can further enhance the lubrication ability by assisting in the retention and regulation of water molecules [37]. It has been well recognized that dense assemblies of end-tethered hydrophilic polymer chains prevailingly exist on the AC surface and construct polymer brushes with superior competence in adsorbing and holding water molecules, which also inspires the advancement in hydration lubrication techniques [[38], [39], [40]]. In recent years, the use of hydrophilic polymer brushes to reduce friction between sliding surfaces in aqueous media has attracted considerable attention [[41], [42], [43]]. Generally, apart from the polymer networks and brushes in the solid matrix, various dissociative hydrophilic or amphiphilic substances contained in the fluid phase of hydration lubrication systems in living organisms are also critical in the lubrication process. By mimicking their functionalities, a variety of additives have been adopted to improve the property of the artificially manufactured lubrication systems [44,45]. Nowadays, an increasing number of researchers have been engaged in designing and preparing novel artificial lubricating materials whose performance can be comparable or even superior to that of their models in nature.

In brief, it is necessary to gain an in-depth understanding of the physical mechanisms and bionic principles of hydration lubrication, and guide the artificial preparation of novel bio-inspired hydration lubrication systems for a wide range of industrial and engineering applications. However, there is still a lack of comprehensive review and discussion on the fundamental theories, practical applications and recent progress of the bio-inspired materials and techniques for hydration lubrication. As schematically illustrated in Fig. 2, in this review, firstly, we briefly introduced several important colloidal concepts associated with hydration lubrication, including hydrogen bonding, hydration layers, EDL forces, hydration forces and Stribeck curve. We also discussed the compositional structure and lubrication mechanisms of the typical AC system. Subsequently, we reviewed the recent development of bio-inspired hydration lubricating materials including polymer hydrogels, polyelectrolyte brushes, and hydration lubrication additives. Finally, we summarized the remaining issues and challenges in the field of hydration lubrication and prospected the future directions and focuses of the bio-tribology studies.