Rational integration of molecular and surface interactions for low friction - high wear resistance of soft surfaces

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The development of efficient and long lasting machines and devices requires a careful control of surface properties such as friction and wear. Soft surfaces are particularly vulnerable to wear and the lack of technology able to protect such surfaces from frictional degradation has hampered the development of numerous technologies such as artificial cartilage. The main goal of this research program is to fill this technological gap by developing a platform of polymeric lubricants and soft hydrogel coatings that can act synergistically to provide enhanced lubrication and wear protection. Our approach is to design such synergistic interactions using multiple molecular mechanisms recently discovered in articular joints. These mechanisms include the directional (anisotropic) flow of synovial fluid in the cartilage pores to the surfaces under normal compression, the local trapping of anti-wear macromolecules in and at the cartilage surface during compression and finally the strong interaction between highly ramified lubricating proteins such as lubricin (LUB) and glycoaminoglycans (GAGs) with the cartilage surface and hyaluronic acid which provides low adhesion and high hydration to the surfaces. To reach our goal, we propose a research program divided into three complementary aims. Aim 1. To develop bio-inspired and efficient polymeric lubricants A key element in the development of a lubricant is the molecular architecture which defines the conformation, lubricating efficiency and load bearing capacity of the molecule. We will develop a library of multiblock polymers exhibiting the bottle-brush architecture which is extensively found in natural lubricating proteins. Two families of polymers mimicking lubricin and GAGs will be studied for their load bearing capacity, lubricating properties and wear protection ability on model flat surfaces before increasing the complexity of the system to hydrogel coatings. These measurements will be performed using the Surface Forces Apparatus (SFA) in order to establish structure-properties relationships of our polymers at the molecular level and to identify key structural parameters responsible for good lubrication and anti-wear properties. Aim 2. To engineer micro/nano inclusions into hydrogel coatings to provide superior wear resistance The poor wear resistance of hydrogel materials has hampered their biotechnological use. Nature uses abundantly soft porous collagen materials to protect hard bones from abrasion and wear. The ultrastructure of cartilage is thought to be responsible for its good anti-wear properties. We will explore this hypothesis by fabricating soft hydrogel coatings known to exhibit poor wear resistance and will modify them to incorporate certain key features of cartilage structure such as the pore structure, the pore distribution or the surface roughness. Wear resistance of the different families of coatings will be assessed by SFA and imaging in order to elucidate which structure is able to provide a significant improvement in the tribological properties. Aim 3. To formulate synergistic interactions between "bottle-brush" lubricants and structured hydrogel coatings for the enhancement of their tribological properties We will determine if combinations of bio-inspired structured hydrogel coatings and lubricating polymers can act synergistically to generate enhanced wear resistance and lubrication. Using the SFA we will elucidate the origin of such synergistic interactions by measuring how lubricant molecules adsorption, conformation, distribution and lubricating properties are controlled by the coating structure.