Bio-Inspired Adhesion, Friction and Lubrication
Biological systems have developed elegant adaptations during its evolution to survive and perform its functions efficiently under specific environmental constrains with enormous physical demands. In this dissertation, I make an effort to understand tribological phenomena in biology and translate them into a synthetic system for engineering applications. I emphasize on adhesion, friction and lubrication in three different biologically inspired soft condensed matter as described below.
Dopa (3,4-dihydroxyphenylalanine), a post-translational modification from tyrosine (Tyr), features prominently in the mussel foot proteins (mfps), ranging from less than 5 mol % in mfp-4 to 30 mol % in mfp-5. The binding ability of the mfps to different substrates has been mostly attributed to the Dopa functionality in the protein and the role of the other peptide residues in the adhesive properties of the protein remains elusive. Here we have discovered that the adhesion between mfp-1 decapeptide films ([AKPSYPPTYK]2) and mica remained unchanged with or without the Dopa residue. This is a paradigm shift in our understanding of the molecular mechanisms underlying adhesive properties of the mfps and calls for further inquiry into the effects of peptide residues beyond Dopa chemistry. We also developed a systematic body of work linking the adhesive performance to lengths and architectures of peptides. Dopa in a peptide sequence does not necessarily lead to the formation of cross-links between peptide films through metal chelation, and the length of the peptide is a crucial parameter for enabling metal ion mediated bridging between surfaces. More recently, we have been working on designing and characterizing small molecules that mimic the properties of the adhesive mussel foot proteins. The wet adhesion and coacervation of an adhesive protein (mfp-5) was recapitulated in an order of magnitude smaller length scale which shows cohesive properties superior to the mfps. We believe that the resulting insights into the molecular structure-function relationships will enable rational design of synthetic bio-inspired adhesives that would enable de novo (suture less) sealants for injuries and surgeries and nano-scale-adhesive applications in the semiconductor industry.
Geckos can attach and detach their toes reversible in matters of milliseconds from most surfaces regardless of its roughness due to the hierarchical structure of their foot-pads. Micro-flaps mimicking the function of the micron sized setae on the gecko foot pad were fabricated and investigated for its adhesion and frictional properties in a modified surface forces apparatus (SFA). A Johnson-Kendall-Roberts (JKR) model with an effective stiffness and adhesion energy parameters quantitatively described the `contact mechanics' of the tilted micro-flaps against a smooth silica surface at the macro and micro-scales. Constant attachments and detachments occurred between the surfaces during shearing and were described by an Avalanche mechanism. These results demonstrate the significance of preload, shearing velocity, shearing distances, commensurability, and shearing direction of gecko-mimetic adhesives and provide a simple model for analyzing and/or designing such systems.
Biolubrication systems show ultralow friction coefficients, remarkable wear resistance properties and are far superior to any artificial system designed to date. In this work, the role of proteins (e.g., Lubricin, Lub) and polysaccharides (e.g., Hyaluronic acid, HA) found in articular joints, and mfp-1 inspired coacervates were investigated to determine the lubrication and wear protection mechanisms conferred by the naturally occurring polymers to a mica surface. We find that Lub penetrates into a chemically bound HA on mica to form a visco-elastic gel that reduces the coefficient of friction as well as boosts the strength of the surface against abrasive wear, however, physically adsorbed HA-Lub complex were poor at conferring wear protection to mica even though it showed low friction coefficients. Similarly, coacervated mfp-1/HA rescues mica from shear induced damage only when the protein is modified with Dopa, which is responsible for attaching the coacervate to the surface. Absence of Dopa resulted in severe abrasive wear to the surfaces even under low loads (< 10 mN) during shearing. These results show that strong anchoring of polymers is crucial to protect surfaces from shear induced damage. We also demonstrate that friction coefficient is not correlated to wear.