Tribo-Rheological Analysis of Size-Controllable and pH-Sensitive Protein Microgels: Delving into the Soft Ball-Bearing Mechanism of Efficient Aqueous Lubrication Effect

Aqueous lubrication is vital for biological interfaces, such as the oral cavity, joints, and eyes, which depend on lubricants like saliva, synovial fluids, and tears to minimize friction between bio-surfaces. Biopolymer microgel suspensions are soft colloidal systems that have attracted a lot of research attention due to their biocompatibility, unique rheological properties, and environmental sensitivities. The soft structure of microgels, which retains water within their micron-sized framework, coupled with the "ball-bearing lubrication" effect, positions them as promising aqueous lubricants in food and biomedical fields.  

In our lab, we developed a facile method to fabricate uniform size-controllable whey protein microgels through protein-polysaccharide segregative phase separation. The microgels are crosslinked via disulfide bonding, and their size can be precisely controlled (1, 6, and 20 μm) by modulating the protein-polysaccharide interaction. This facile microgel production does not require chemical modification or high energy input, ensuring safe use as food ingredients with significant industry interests. The microgels assembly of the shape and distribution of fat droplets in food emulsions and their suspension showed a shear thinning effect and higher rheological performance than traditional oil-in-water emulsions. The microgel suspension (50% vol) demonstrated superior lubricating performance, which was capable of exceeding the lubricity of human saliva in tribological tests using a rheometer with ring-on-plate geometry. Our findings indicate that the “soft ball-bearing” mechanism is responsible for the superlubricity of our microgel. We also illustrated how variations in size and pH impact the tribo-rheological properties of the microgel suspension. These insights can guide the design of functional bio-lubricants tailored for diverse application conditions.

These findings highlight the potential of biocompatible protein microgel aqueous lubricants in diverse food and biomedical applications. For instance, they can be utilized to craft fat-reduced food formulations with enhanced creamy mouthfeel and could be pivotal in developing dry-mouth therapies for improved health outcomes.