23rd International Colloquium Tribology - January 2022 63 Biomimetic water-based lubricant development: Nanoencapsulation with liposomes Manoj Murali Tribology Group, Imperial College London, United Kingdom Corresponding author: mm8415@ic.ac.uk Philippa Cann Tribology Group, Imperial College London, United Kingdom Marc A. Masen Tribology Group, Imperial College London, United Kingdom 1. Introduction The replacement of traditional mineral oil lubricants with water-based bio-compatible fluids has long been a desirable, if unrealised, ambition in many applications. This is particularly relevant in marine-based energy generation systems, where oil-based lubricants create a high risk of environmental pollution. The use of bio-lubricants has been explored in several previous studies [1], however no significant technological advances have been achieved. Most of the work has focused on traditional lubrication mechanisms, with bio-molecules being employed to form an adsorbed surface film which reduces friction. However, due to their inherent biological, thermal and/ or oxidative instability, bio-molecules are unsuited to long-term industrial applications. The alternative approach is to use stable, bio-friendly molecules, designed to exploit the lubrication mechanisms found in nature. These mechanisms have evolved to be far more diverse than those found in traditional “mineral oil” tribology and are, as yet, poorly understood [2,3]. Our investigation focuses on the lubrication mechanism of DSPC liposomes with additive payloads encapsulated within their core. The utilisation of liposomes was inspired by synovial fluid which is an excellent lubricating medium in human joints [4]. Synovial fluid contains liposome forming surface active phospholipids which have been theorised to be a significant contributor to its excellent lubricating performance [5,6]. The additional inclusion of payload delivery was then built upon through the observation of hagfish (myxinidae), a slime-producing marine fish that releases mucin filled vesicles as a defence mechanism to increase the surrounding local viscosity of seawater, a very effective defence mechanism against predators [7]. This biomimetic foundation led to the development of aqueous DSPC liposomal solutions (ALS) separately encapsulated with mucins, sugars and wear additives. Tribology tests were carried out on a reciprocating device, HFRR, (High Frequency Reciprocating Rig, PCS Instruments, London, UK) with ALS and hexadecane as a low-viscosity oil reference. Wear and friction were significantly reduced for the ALS compared to water alone. The test demonstrated that nanocapsules enter the contact and are ruptured by high shear stresses, allowing for the encapsulants within to be released to lubricate the contact. The research forms a foundation to explore synthetic nanocapsules such as polymersomes which provide additional benefits in chemical stability, adaptability and longevity. 2. Methods 2.1 Preparation and characterisation of ALS ALS were prepared by the thin film hydration method [8]. Briefly, DSPC was dissolved in a mixture of chloroform-ethanol ( 99: 1, v/ v) and stirred for 3 min to ensure complete dissolution. A rotary evaporator was used under vacuum at 60 °C to remove the solvent from the resulting homogeneous samples and obtain a thin lipid film. Next, the thin lipid film was hydrated with a chosen payload solution and thermostated for 1h at a temperature above the DSPC phase transition temperature of 55 °C to form multilamellar liposomes. Following this, the multilamellar liposomes were downsized to form small unilamellar liposomes by extruding using an extruder (Avanti, USA) through polycarbonate membranes (Whatman, Inc.) with a defined pore size of 100 nm (21 cycles). The temperature was maintained at 60 °C during the entire extrusion process. The DPSC: payload molar ratio was kept constant for all payload variants. The DSPC-payload liposomes prepared were dialysed in deionised water for 6 h via a dialysis tube (molecular weight cutoff: 300kDa) to remove any free payload from the solution. Hydrodynamic diameters and zeta potentials of the obtained ALS were measured with a dynamic light scattering instrument (Zetasizer Pro, Malvern Instruments, Malvern, UK) at 25 °C. Their morphologies were imaged by using a cryogenic field-emission scanning electron microscope (Tescan Myra SEM, Brno, Czechia). 64 23rd International Colloquium Tribology - January 2022 Biomimetic water-based lubricant development: Nanoencapsulation with liposomes 2.2 Friction and wear measurements Friction experiments were performed to assess the benefits of nanoencapsulation for lubrication performance by comparing the friction and wear results obtained on deionised water, hexadecane and empty DSPC liposomes with ALS, in which payloads were encapsulated within the vesicular core structure. This lubrication performance was investigated using a HFRR. Tests were performed on a 440C stainless steel sliding pair at 25 ºC, at a load of 0.5N, a frequency of 50Hz and a stroke length of 1mm. Some results are shown in Figure 1, showing the average coefficient of friction for hexadecane, DSPC liposome solution and deionised water. Wear performance was investigated using a white light interferometer (ContourGT Optical Profiler, Bruker, Massachusetts, USA). Wear scar cross sectional area at the wear track midpoint was measured and compared for the lubricants tested. The adsorption of the encapsulated additives was assessed by Raman Spectroscopy (alpha300R Raman Imaging Microscope, WITec, Ulm, Germany) of the resulting wear marks. 3. Results and discussion Figure 1: Comparison of average coefficient of friction of deionised water, hexadecane and empty DSPC liposomes A strong reduction in coefficient of friction is seen with liposomal solutions compared to deionised water and hexadecane. This has, for example, led to a reduction in friction coefficient from 0.3 in deionised water to 0.1 in the DSPC liposomal solution (Figure 1). Table 1: Comparison of average wear scar midpoint cross-sectional area Lubricant Average wear scar midpoint cross-sectional area (µm 2 ) Deionised Water 72.232 Hexadecane 1.943 DSPC Liposome 7.683 Wear performance is also improved (Table 1) through the addition of liposomes, however a low-viscosity oil such as hexadecane remains a better performer. The addition of payloads into the liposome is set to further improve the wear performance of ALS lubricants, which remains the limiting factor between water based lubricants and oil-based lubricants. The mechanisms behind the performance differences are explained through observational studies and Raman in which adsorption of additives through liposomal payload delivery at the contact can be investigated. 4. Conclusion ALS successfully demonstrates a reduction in both friction and wear compared to deionised water alone, moreover when combined with a wear reducing payloads the wear performance of a low-viscosity oil such as hexadecane can be approached through utilising such a system. This research demonstrates that the primary failings of past water-based lubricants have the potential to be overcome through the application of encapsulated liposomes. The lubrication mechanism achievable through this approach is both active and novel in its workings, and presents the viability of sustainable lubricants for industrial applications. References [1] Ahlroos T, Hakala TJ, Helle A, Linder MB, Holmberg K, Mahlberg R, et al. Biomimetic approach to water lubrication with biomolecular additives. Proc Inst Mech Eng Part J J Eng Tribol. 2011; 225(10): 1013-22. [2] Fan J, Myant CW, Underwood R, Cann PM, Hart A. 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