Superlubricity or a near frictionless regime is a long-standing goal of researchers as finding materials that exhibit coefficient of friction values less than 0.01 remains challenging. The ultimate objective in identifying materials that display superlubricity is to open up opportunities for conducting all types of operations that will maximize energy savings by reducing the generation of waste heat and minimizing emissions. This will lead to tribology facilitating the move toward sustainability. 

In a past TLT article,¹ researchers were able to achieve superlubricity by synthesizing carbon nanotubes (CNTs) through a chemical vapor deposition process on stainless steels and tool steels. Disk-on-flat reciprocating sliding tests were conducted with a CNT coated 316 stainless steel disk that led to a rapid reduction of coefficient of friction values after the first five to 150 cycles. The reason for this decline is due to the CNT coating being sacrificed in situ to form a tribofilm that creates the test conditions for superlubricity. This tribofilm remained stable for over 5,000 cycles.

Anwesha Sarkar, professor of colloids and surfaces at the University of Leeds in Leeds, UK, says, “Superlubricity has been identified by the use of polymer brushes such as poly [2-(methacryloyloxy) ethyl phosphorylcholine], and polyalkylene glycols that are produced synthetically. Ideally, finding a way to develop a material that can exhibit superlubricity by using a biopolymer is most desirable from not just a sustainability perspective but also for use in biological and biomedical applications.”

Lubricants operating in biological environments must be compatible with water. Natural materials such as synovial fluids work in the human body at near zero coefficient of friction levels to lubricate articular cartilage surfaces in healthy human joints. Finding a way to produce such an engineered material must take into consideration hydration lubrication.

Sarkar says, “Hydration lubrication involves taking materials that can impart lubricity in a water environment. Water on its own does not contribute to lubrication, so there is need to find biomaterials such as biomacromolecules that can. This work has opened up new potential strategies for synthesizing molecules using biopolymer self-assembly that can impart hydration lubrication.”

Hydrogels are one such approach that has been successful, but Sarkar points out that many of these materials are made through synthetic pathways using lipids, polyols and other synthetic polymers. She says, “A greener approach is crucial to achieve superlubricity that will involve using renewable raw materials that can provide hydration lubrication.”

Sarkar and her colleagues decided to evaluate the use of plant proteins because they generate one-third of the greenhouse gas emissions of animal proteins. Combining a structural plant protein with a biopolymeric hydrogel led to a material that exhibits superlubricity.

Potato protein-based protofilaments
The researchers demonstrated that hydrolysis of the plant protein derived from potato produces protofilament that when combined with a polysaccharide hydrogel network derived from a biopolymer (such as xanthan gum) self-assembled into a protofilament-reinforced hydrogel that displays superlubricity in an aqueous environment. Sarkar says, “Potato was selected because the protein in potato is a side-stream where the main product, i.e., starch, is consumed in various food applications.”

Macroscale tribology of the self-assembled protofilament/hydrogel was conducted using a mini-traction machine where the substrate used was polydimethylsiloxane. Coefficient of friction values in the range of 0.004 to 0.00007 were achieved under moderate-to-high contact pressures between 300 kilopascals and 3 megapascals, at the macro to microscale. The latter was conducted using surface force balance measurements on mica. 

Sarkar says, “The protofilament forms a dense, electrospun, fiber-like, filamentous network that exhibits adherence to both polydimethylsiloxane and mica leading to excellent boundary lubrication properties. The hydrogel that appears to coat the protofilament imparts the water-mesh induced hydration lubrication. At the end, what we obtained is an unusual, nearly speed-independent friction curve.”

Figure 1 shows a schematic of the protofilament/hydrogel in contact with a grey-colored ball and a rectangular slab. The protofilament is represented by the green mesh, which is partially coated by the polysaccharide hydrogel illustrated as an orange-colored filament. Hydration lubrication is demonstrated by transparent water-like spheres attached to the polysaccharide hydrogel. 


Figure 1. A schematic of the protofilament/hydrogel in contact with a grey-colored ball and a rectangular slab is shown. The protofilament is represented by the green mesh, which is partially coated by the polysaccharide hydrogel illustrated as an orange-colored filament (see image on the right). Figure courtesy of the University of Leeds.

This analysis was supported by a wide range of structural techniques including light and small angle neutron-scattering and molecular dynamics simulations.

Sarkar points out that the superlubricity exhibited by the protofilament/hydrogel is independent of pH until a value of 6.0. She says, “We produced the superlubricity material at a pH of 3.0, but we did find an increase in friction at neutral pH due to the both the protofilament and hydrogel being negatively charged and not able to interact with each other.”

The protofilament/hydrogel is readily biodegradable and seems to be well suited for use in personal care and medical device applications. Sarkar says, “We intend to evaluate the profilament/hydrogel self-assembly on other soft silicone materials with roughness that may simulate the human tongue or skin.”

This proof of concept study is leading Sarkar to seek other applications for the profilament/hydrogel. Sarkar says, “We are particularly interested in collaborating with potential industrial partners in various technological applications where this technology can act as a superlubricating building block.”

Additional information on this work can be found in a recent article² or by contacting Sarkar at a.sarkar@leeds.ac.uk. 
REFERENCES
1. Canter, N. (2023), “Superlubricity in a common bearing system,” TLT, 79 (10), pp. 18-19. Available at www.stle.org/files/TLTArchives/2023/10_October/Tech_Beat_I.aspx.
2. Pabois, O., Dong, Y., Kampf, N., Lorenz, C., Doutch, J., Sierra, A., Ramaioli, Mu, M., Message, Y., Liamas, E., Tyler, A., Klein, J. and Sarkar, A. (2024), “Self-assembly of sustainable plant protein protofilaments into a hydrogel for ultra-low friction across length scales,” Communication Materials, 5, Article Number 158.