Synthesis and characterization of biomimetic materials inspired by sandcastle worm
Date of Issue2016
School of Materials Science and Engineering
The intertidal zone abounds of organisms who critically depend on their ability to produce water-resistant adhesives for their survival. Sandcastle worm (Phragmatopoma californica, Pc) is one of the marine organisms whose molecular-scale adhesive strategy has been revealed in the past decade. Sandcastle worm are polychaetes, which construct protective, honeycomb-like tubular structures by gathering sub-millimeter sediment particles from their surroundings that they glue together using a proteinaceous adhesive. The glue secreted from sandcastle worm can adhere to different materials under seawater environment. The major components of the secreted glue are oppositely charged proteins with highly repetitive sequences, and a key characteristic of the glue is related to its processing by the formation of complex coacervates. These characteristics make the native glue cement an intriguing mimicking model for the development of adhesive materials that could be used in wet environment. This research was aimed at developing wet-adhesive materials, with the long-term goal to be used in biomedical applications. With a strong understanding of protein structures from native glue cement of sandcastle worm, we started with a co-polypeptide synthesis that mimicked both the side-chain of the sandcastle worm glue, with an amino acid composition that matches Pc glue proteins. A series of negatively charged polypeptides containing Tyr and Ser with tunable degrees of phosphorylation were developed through ring opening polymerization (ROP) of N-Carboxyanhydrides (NCAs). Co-polypeptides with variable physico-chemical properties could be prepared, including zeta potential, hydrodynamic radii, divalent ion affinity, or charge density of the colloidal suspension. Synthetic route for positively charged polypeptide containing Gly, Lys, Di-hydroxyphenylalanine (Dopa) and Tyr were also developed and their physico-chemical properties characterized. The metal-ligand coordination ability of our synthetic co-polypeptides was assessed, as metal coordination is increasingly recognized to play a critical role in many biological materials including in adhesion, cohesion and self-assembly. Next, the coacervation capability of our oppositely charged co-polypeptides was demonstrated, as reports have shown that the adhesive of sandcastle worms is delivered through the mixture of oppositely charged proteins into a so-called complex coacervate phase. The role of different parameters on coacervation was investigated, including pH, ionic strength, polypeptide concentrations, and polypeptide molar ratios. Tunable coacervation was developed by mixing different ratios of oppositely charged co-polypeptides at different pHs. The concentrated coacervate was subsequently characterized by rheology and contact angle measurements. The successful synthesis of oppositely charged co-polypeptides that closely mimic the composition of Pc-1, Pc-2 and Pc-3 and their subsequent processing into complex coacervates represent a first key step towards the engineering of biomimetic water-resistant adhesive materials that mimick the natural glue from sandcastle worm.