UC Santa Barbara

The lubricating and structural properties of different mammalian synovial fluids in thin films undergoing shear between two mica surfaces are studied in detail using a surface force apparatus (SFA). A 10-13 nm thick film of synovial components (proteins, lipids, and polymers) adsorbs on the mica surfaces in less than an hour of incubation time, and induces a strong repulsion between the surfaces that prevents them from coming into contact. Upon shearing, the structure of the confined synovial fluid changes dramatically when sheared above a "critical" shear rate of about 2 s-1 (corresponding to approximately 40 nm s-1). Above this critical shear rate and up to at least 70 μm s-1, the proteins and biopolymers in the fluid gradually aggregate to form a homogenous gel layer on each mica surface. As shearing continues, the gel layer gradually breaks up into discrete/individual gel particles that can roll in the contact keeping the sheared surfaces far apart even under high compressive loads (pressure P ≈ 20 MPa). These particles eventually become elongated and finally behave as roller bearings. This mechanism is consistently observed for three mammalian synovial fluids and two types of surfaces suggesting that it actually occurs in articular joints and prostetic implants in vivo. The implications of these findings for joints and prosthetic implants structure and lubrication are discussed; in particular the formation and function of the lamina splendens. Shear-induced aggregation of synovial fluid components is observed under boundary lubrication conditions. The aggregation process starts with the formation of a gel-like layer on the surfaces, which ultimately leads to the formation of rod-like aggregates. These aggregates provide enhanced wear protection and lubrication to the surfaces using rolling friction instead of pure sliding friction. This mechanism suggests new design criteria for future bioinspired lubricants. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.