The pursuit of superlubricity—near-zero friction—is a significant engineering goal, promising substantial energy savings and advancements in biomedical applications. Hydration lubrication, mimicking biological systems like synovial fluid in joints, presents a promising pathway to achieve oil-free superlubricity at biologically relevant pressures. However, currently available aqueous lubricants often rely on synthetic chemicals. This research aims to address this sustainability gap by exploring the potential of plant proteins as building blocks for sustainable, high-performance aqueous lubricants. While plant proteins offer abundance and low carbon footprints, their complex structure and limited solubility present challenges. Previous work has shown success in transforming plant proteins into functional materials, but their application as superlubricants remains undemonstrated. This study pioneers the use of self-assembled plant protein protofilaments within a biopolymeric hydrogel network to create a novel, sustainable aqueous lubricant with superior lubricating properties.

Existing research on hydration lubrication highlights the potential of aqueous lubricants for achieving ultra-low friction. However, most current superlubricant formulations utilize synthetic components such as unilamellar vesicles, solid lubricants, poly-zwitterionic brushes, or amphiphilic surfactants. While hydrogels offer an alternative route to achieve ultra-low friction with improved sustainability, many hydrogel-based lubricants still rely on synthetic polymers. The use of plant-derived proteins as an alternative, sustainable source of building blocks for aqueous lubricants offers a compelling solution to this challenge. Although research exists on utilizing plant proteins in various functional materials such as microgels, films, and amyloid fibrils, creating a superlubricant from plant proteins remains an unexplored area.

This research employed potato protein, a readily available and biocompatible by-product from the starch industry, to create protofilaments through a facile physical crosslinking method. These protofilaments were then electrostatically assembled with highly hydrating biopolymeric hydrogels (xanthan gum (XGH) or x-carrageenan (KCH)). A comprehensive suite of techniques were used to characterize the molecular structure and lubrication performance across multiple length scales. These included: * **Dynamic Light Scattering (DLS):** Determined particle size distribution. * **Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM):** Visualized the morphology of the protofilaments and hydrogel network at the nano-scale. * **Small-Angle Neutron Scattering (SANS):** Analyzed the nanostructure of the self-assembled materials in the bulk. * **Molecular Dynamics (MD) Simulations:** Provided atomistic-level insights into the interactions between the protofilaments and the hydrogel, as well as their interactions with a hydrophobic surface (PDMS). * **Rotational and Extensional Rheometry:** Measured the shear and extensional viscosities to assess the viscoelastic properties of the lubricant. * **Macroscale Tribology (Mini Traction Machine):** Evaluated the friction-reducing capability under moderate contact pressures (300 kPa). * **Nanoscale Tribology (Surface Force Balance, SFB):** Measured friction at the nanoscale under much higher contact pressures (3 MPa). * **Quartz-Crystal Microbalance with Dissipation Monitoring (QCM-D):** Assessed the adsorption behavior of the lubricant onto a hydrophobic surface.

The study demonstrated the successful self-assembly of potato protein protofilaments (PoPF) with XGH or KCH hydrogels. Characterizations revealed a unique 'patchy architecture' where the hydrogel coated the protofilaments unevenly, leaving some areas exposed. MD simulations revealed that the exposed areas of the protofilaments interacted hydrophobically with the surface, while the hydrogel layer provided hydration lubrication. The self-assembled material exhibited exceptionally low friction coefficients: * **Macroscale:** Friction coefficients as low as 0.004 were achieved under moderate contact pressures (300 kPa), significantly outperforming individual components and the control (citrate buffer). * **Nanoscale:** Friction coefficients in the order of 10⁻⁴ to 10⁻⁵ were observed under high contact pressures (3 MPa). The friction remained ultra-low even at the highest loads tested. The self-assembled material showed strong adsorption to the surface, contributing to its superior lubrication performance. Rheological studies indicated that the hydrogel component primarily influenced the viscosity, while the superlubricity resulted from the synergistic interaction of the protofilaments and the hydrogel, combining surface anchoring with hydration lubrication. The lubrication properties remained stable across a range of pH values below the isoelectric point of potato protein.

The findings strongly support the hypothesis that a synergistic interaction between plant protein protofilaments and a biopolymeric hydrogel can create a sustainable superlubricant. The patchy architecture and the combined effects of hydrophobic anchoring and hydration lubrication are crucial for the observed ultra-low friction across multiple length scales and contact pressures. The results demonstrate the potential of utilizing abundant and sustainable plant protein materials to design high-performance lubricants for various applications. The performance of the lubricant closely mimics that of natural biological lubricants, suggesting potential for use in biomedical applications.

This study successfully demonstrated the creation of a sustainable, high-performance aqueous lubricant using self-assembled potato protein protofilaments and a polysaccharide hydrogel. This bio-based lubricant exhibits superlubricity across length scales and under a wide range of pressures, offering a promising alternative to synthetic lubricants. Future work should explore the application of this approach to other plant protein systems and investigate lubrication performance on surfaces with varying hydrophobicity.

The study primarily used smooth, hydrophobic PDMS surfaces for macroscale tribology, QCM-D, and MD simulations, while negatively charged, hydrophilic mica surfaces were used for AFM and SFB experiments. While superlubricity was observed across both types of surfaces, future work could explore how surface properties influence the lubricant's performance. Additionally, the long-term stability of the self-assembled structure under continuous shear and the effect of environmental factors on the lubricating properties require further investigation.