Environmentally Responsive Hydrogels and Composites Containing Hydrogels as Water-Based Lubricants

Abstract

Both biosystems and engineering fields demand advanced friction-reducing and lubricating materials. Due to their hydrophilicity and tissue-mimicking properties, hydrogels are ideal candidates for use as lubricants in water-based environments. They are particularly well-suited for applications involving biocompatibility or interactions with intelligent devices such as soft robots. However, external environments, whether within the human body or in engineering applications, often present a wide range of dynamic conditions, including variations in shear stress, temperature, light, pH, and electric fields. Additionally, hydrogels inherently possess low mechanical strength, and their dimensional stability can be compromised by changes during hydration. This review focuses on recent advancements in using environmentally responsive hydrogels as lubricants. It explores strategies involving physical or structural modifications, as well as the incorporation of smart chemical functional groups into hydrogel polymer chains, which enable diverse responsive mechanisms. Drawing on both the existing literature and our own research, we also examine how composite friction materials where hydrogels serve as water-based lubricants offer promising solutions for demanding engineering environments, such as bearing systems in marine vessels.

Keywords: composite; environmentally responsive; friction pair; lubrication; water-based hydrogels.

Figure 2

Schematics of shear-responsive lubricating hydrogels. (a) Shear-responsive lubrication mechanism: Under shear force, the interaction between the segments is destroyed and a lubricating layer is formed. (b) CLX@Lipo@HA gel can expose the internal liposome micro reservoirs on the outer surface through shear-induced structural rearrangement to form a boundary layer, figure redrawn from Lei et al. [27], under CC BY license.

Figure 3

Schematics of temperature-responsive lubricating hydrogel. (a) The variation of the water temperature against the water depth in ocean (Seawater temperature varies with region and depth), data from [30]. (b) Temperature-responsive lubrication mechanism: e.g., PNIPAAm hydrogel shrinks above LCST. (c) Stick-slip switching mechanism of DMCS hydrogel and the relationship between temperature and adhesion strength or friction of DMCS hydrogel, figure adapted from Zhang et al. [32] under CC BY license. (d) Design concept of MALH, figure adapted from Zhang et al. [33] under CC BY license. (e) Preparation of polymer-brush grafted hydrogel surface and COF of hydrogel, figure adapted from Zhang et al. [41] under CC-BY-NC-ND license.

Figure 4

Schematics of photothermal responsive lubricating hydrogel. (a) Photothermal responsive lubrication mechanism: Contraction of PNIPAAm photothermal sensitive hydrogel. (b) Esophagus-inspired tubular soft actuator, figure adapted from Liu et al. [45] under CC BY license. (c) Schematic diagram of PTMGs for interfacial friction control & infrared thermal image of microgel coating and the change of COFs of microgel coating with load at different temperatures, figure adapted from [48,49] with the journal’s permission. (d) Composition of hydrogel and the change of volume and COF in photothermal response, figure adapted from Wu et al. from [50] under CC BY license.

Figure 5

The schematics of pH-responsive lubricating hydrogel. (a) Spatial distribution of surface seawater pH values every five years, redrawn from [53]. (b) Swelling/deswelling degree of cationic and anionic hydrogels under different pH conditions. (c) pH-responsive lubrication mechanism: swelling caused by deprotonation of polymer chains (top); or formation of lubricating layer after chain disassembly or bond breaking (bottom).

Figure 6

Schematics of chemical signal-responsive & Electric field responsive lubricating hydrogel. (a) Electro-Responsive lubrication mechanism: the directional movement of charged chains causes the change of surface charge of the hydrogel (top); Electro-Responsive disassembly forms a lubricating layer (bottom). (b) DN gel friction change mechanism (protons move to the cathode side under the action of electricity) and optical photos of the robot walking in sequence, figure adapted from Selvamuthu et al. [74,75] under BY-NC-ND 4.0 license.

Figure 1

Schematics of lubrication type. (a) Stribeck curve; (b) solid lubrication; (c) fluid lubrication; (d) solid–liquid mixed lubrication. W: load; v: fluid flow rate; H: fluid thickness; P: pressure; A: area.

Figure 7

Schematic illustration of friction behavior in the rubbing process of the fracture pair: (a) UHMWPE, (b) PAAm microsphere/UHMWPE composite, and (c) DN microsphere//UHMWPE composite, figure adapted from Wang et al. [82] under CC BY license.

Figure 8

Schematics of the wear and lubrication mechanisms and processes of the ANF-reinforced hydrogel/UHMWPE composites, figure adapted from Li et al. [88] under CC BY license. (a) the control group, UHMWPE; (b) the composites with hydrogel I, II; (c) the composites with hydrogel III, IV. The thin red arrows present the pressure generated from copper ball; the short black arrows indicate the carried force.