Preparation and characterization of biomimetic materials inspired by jumbo squid sucker ring teeth
Date of Issue2016
School of Materials Science and Engineering
Biological and Biomimetic Materials Laboratory
Dosidicus gigas (D. gigas, or Jumbo squid) sucker ring teeth (SRT) are newly discovered biological materials with intriguing combination of mechanical and physico-chemical properties. Notably, recent studies have shown that SRT are devoid of minerals, chitin and inter-chain covalent cross-links, and are entirely constituted of proteins called “suckerins” which form a polymer network reinforced by self-assembled nano-scale β-sheet structures. At the molecular (primary sequence) scale, suckerins display a block co-polymer sequence consisting of two tandem alternating modules. One is rich in alanine (Ala) and resembles the poly-Ala or poly(glycine (Gly)-Ala) domains of silk proteins and form β-sheets, and the other is dominated by Gly accompanied with high contents of tyrosine (Tyr) and leucine (Leu) residues. These characteristics make SRT a promising model for biomimetic materials research. In this project, therefore, we hypothesize that functional materials with β-sheet structures could be engineered with SRT proteins and these materials could be utilized in a broad range of biomedical and engineering applications that have been demonstrated for silks. Furthermore, the goal of the project is to further deepen the structure/property relationships of SRT, notably to link their thermo-mechanical response with the protein structure of suckerins, which is unknown prior to this work. We hypothesize such structure/property relationships would provide molecular scale biomimetic principles to develop a new range of protein- and peptide-based materials. Characterization of SRT by nanoindentation was conducted in different solvents that could disrupt their putative secondary structures. We found that β-sheets disruption in SRT was correlated to a concomitant decay of Young’s modulus (E), which provided the first direct evidence of hydrogen bonding localized in β-sheets as an essential contributor to SRT mechanics. This also explained the fundamental structure-property relationship of SRT. SRT and suckerins also displayed thermoplastic properties, where the crystalline β-sheets and amorphous domains exhibited different thermal stabilities and thermo-mechanical responses. These intriguing characteristics motivated us to fabricate materials with native SRT proteins, including melt-spun fibers and electro-spun nanofibers. The melt-spun fibers displayed polymer chain alignment and comparable mechanical strength to regenerated silk fibers. Electro-spun nanofibers were made by using an additive and a systematic optimization of electro-spinning parameters that led to smooth nanofibers. In order to sustainably and efficiently supply SRT proteins for materials engineering, a recombinant expression system for suckerin-19 was established. The production of two suckerin-19 variants was achieved and a facile chromatography-free purification method was developed, with promising protein yields. In order to demonstrate the hypothesis that recombinant SRT proteins were also able to produce materials, suckerin-19 was fabricated into tunable materials by a RuII mediated photo-cross-linking method. This method allowed the formation of hydrogels and films whose surface roughness, cross-link density and β-sheet content could be tailored. Importantly, the stiffness of these gels and films could be modulated over 7 orders of magnitude by varying the cross-linking conditions or by the addition of a common plasticizer. This tunability of E enables them to match the elasticity of a wide range of human tissues from soft liver to stiff bone, opening promising avenues in tissue engineering and restorative applications. Overall, this research demonstrates that suckerins’ characteristics, namely β-sheet self-assembly, modular sequence design, impressive mechanical properties and biocompatibility, provide a novel toolbox of protein-based materials to biomaterial scientists, with a broad spectrum of potential biomedical and engineering applications.