Synthetic Mucin Lubricates Interfaces at Multiple Length Scales by Stress-Induced Nanoscale Tribofilm Formation

The tribological response of mucus is critical for organisms and in emerging applications and depends on the properties of mucins, understood to be the primary functional protein in mucus. The complexity of natural mucus makes correlating mucin structure to mucus function challenging. To address this, we study pure solutions of a synthetic mucin homopolymer, poly(Gal-Thr)22 which mimics the structure of natural mucins’ glycosylated bottlebrush domains. This permits directly correlating specific nanostructured molecular features with lubricity. For a macroscale PDMS–SiO2 contact, the boundary friction coefficient, μ, is reduced from ∼1 in pure water to <0.1 at 50 mg/mL poly(Gal-Thr)22, with more modest lubrication at lower concentrations. For a microscale PDMS–SiO2 contact measured using colloidal atomic force microscopy, which directly probes nanoscale contact mechanics, friction is also concentration-dependent: at 1 mg/mL, friction reduction vs water of 40% is attributed to reduced adhesion; at 10 and 50 mg/mL, friction reduction by 80% and 98%, respectively, is attributed to a lubricious nanoscale tribofilm that forms on the SiO2 tip. For a hard Al2O3–SiO2 microscale contact, friction falls below detection limits due to tribofilm formation under high nanoscale contact stresses. Infrared spectroscopy shows the tribofilm is a compacted structure rich in hydrogen bonds. While passively adsorbed mucin-rich films or pellicles are generally understood to provide lubricity in nature, little is known about their response to stress. Solutions of our synthetic mucin lubricate modestly when passively adsorbed, but at sufficient concentrations, with continued sliding a denser, highly lubricious tribofilm forms. Its durability permits the interface to retain lubricity when sliding in mucin-free solution. This demonstrates that a short, glycosylated homopolymer without specific surface anchoring groups substantially lubricates through stress-induced formation of an intrinsically lubricious tribofilm. This motivates further interrogation of the structure of mucins subjected to stress since until now, surface characterization in mucin lubrication studies has focused on the passively adsorbed pellicle.