A Self-Healing Polymer for a Sustainable Future

[ACSGCI]

Contributed by Melik C. Demirel, Ph.D., Professor of Engineering, Materials Research Institute and Huck Institutes of Life Sciences, The Pennsylvania State University

Since the dawn of civilization, natural materials have been a fundamental part of human life and environment. However, in the 20th century, due to exploitation and high cost of natural resources, synthetic materials replaced natural materials. Recent advances in biotechnology and materials science have allowed the invention of eco-friendly materials that can be produced easily from sustainable resources in a broad array of useful applications including textile, cosmetic, and medicine. Proteins are key to the creation of many new, high strength materials.

Proteins have several advantages as natural materials: their chain length, sequence and stereochemistry can be easily controlled, (ii) the molecular structure of proteins is well defined (e.g., secondary, tertiary and quaternary structures), (iii) they provide a variety of functional chemistries for conjugation to other biomolecules or polymers, and (iv) they can be designed to obtain optimum physical properties. Proteins can be roughly divided into three classes (i.e., globular, membrane, and structural) according to their environmental conditions. Globular proteins are water soluble and provide a wide range of critical biological functions, for example, the enzymatic catalysis of chemical reactions. Membrane proteins, on the other hand, reside in hydrophobic environments and help signaling processes of cells. Structural fibrous proteins form highly hydrogen-bonded regular structures. Many fibrous proteins have been extensively studied such as elastins, collagens, silks, keratins, and resilins.

Squid have teeth-like structural protein inside their suckers, which serve for holding a diverse array of objects strongly. A book published by Williams a century ago in 1910 entitled ‘‘Anatomy of Squid’’ studied the squid arms and tentacles in detail but incorrectly claimed that the SRT is a chitin-like substance⁹. Four decades ago, Nixon and Dilly studied the SRT and published an article¹ in 1977, correcting the earlier mistake, which concluded that SRT is a protein complex. Moreover, they demonstrated that SRT has submicron pores. Recently, mechanical and thermal properties of SRT have been studied to demonstrate that the porous structure is important for compliance under high forces², and rubbery thermal transition provides strong under water adhesion³.

SRT protein exhibits an unusual and reversible transition from a solid to a rubber⁴, and can be thermally shaped into any 3D geometry (e.g. fibers, colloids, and thin films). Using the tools of molecular biology, large-scale computation, and mechanics, it has been demonstrated that these proteins have excellent mechanical, structural, and optical properties, in wet and dry conditions that exceed most natural and synthetic polymers⁵. Figure 1 shows a recombinant SRT fiber that is flexible and strong. This protein is a perfect candidate in textiles as well as a myriad of other materials in high demand. These materials range from disposable medical garments to fabric scrims for composites to commodity clothing items for potential use in textile applications. The thermal recyclability of the SRT can be exploited by molding it into nanotube arrays or micron size colloids. SRT has also been adapted for use in the artificial reconstruction of a Manduca sexta wing⁶. Utilizing structural proteins to accurately imitate insect wing characteristics opens the door for next-generation biomimicry, specifically in the area of mimic flapping-winged animals. With their tunable flexibility (1-10^3 MPa) and ultra-high thermal expansion coefficient (>500 10^-6 °K-1) structural proteins are also ideal candidates for optical devices. For example, optical lenses made from SRT would have great interests due to its transparency, flexibility, biodegradability, biocompatibility and molecular level structural tunability that cannot be achieved by rigid inorganic materials⁴.

We were able to cut a SRT sample in half and repair it by applying pressure and warm water as seen in the video above. SRT’s supramolecular chemistry provides self-assembly to achieve stiff self-healing morphology with soft/hard domain separation⁷. Segmented structure of the SRT protein shows soft segments with self-healing capability and hard beta-sheet segments that crosslink the structure. The ring teeth of squid was collected around the world -- in the Mediterranean, Atlantic, near Hawaii, Argentina and the Sea of Japan -- and it was found that proteins with self-healing properties are ubiquitous⁸. The underlying mechanism for self-healing is the protein’s ability to deform and soften in water above its rubbery temperature, while maintaining the hydrogen bonds reversibly⁷. Self-healing structural proteins provide not only high strength polymeric materials but also will help discovery of novel properties for clinical applications such as orthopedic devices for repair, and biodegradable gels for wound healing in the near future.

References

1. Nixon, M.; Dilly, P. In Sucker surfaces and prey capture, Symp. Zool. Soc. Lond, 1977; pp 447-511.
2. Miserez, A.; Weaver, J. C.; Pedersen, P. B.; Schneeberk, T.; Hanlon, R. T.; Kisailus, D.; Birkedal, H., Microstructural and biochemical characterization of the nanoporous sucker rings from Dosidicus gigas. 2008.
3. Pena‐Francesch, A.; Akgun, B.; Miserez, A.; Zhu, W.; Gao, H.; Demirel, M. C., Pressure sensitive adhesion of an elastomeric protein complex extracted from squid ring teeth. Advanced Functional Materials 2014, 24 (39), 6227-6233.
4. Pena‐Francesch, A.; Florez, S.; Jung, H.; Sebastian, A.; Albert, I.; Curtis, W.; Demirel, M. C., Materials Fabrication from Native and Recombinant Thermoplastic Squid Proteins. Advanced Functional Materials 2014, 24 (47), 7401-7409.
5. Guerette, P. A.; Hoon, S.; Seow, Y.; Raida, M.; Masic, A.; Wong, F. T.; Ho, V. H.; Kong, K. W.; Demirel, M. C.; Pena-Francesch, A., Accelerating the design of biomimetic materials by integrating RNA-seq with proteomics and materials science. Nature biotechnology 2013, 31 (10), 908-915.
6. Michaels, S. C.; Moses, K. C.; Bachmann, R. J.; Hamilton, R.; Pena-Francesch, A.; Lanba, A.; Demirel, M. C.; Quinn, R. D., Biomimicry of the Manduca Sexta Forewing Using SRT Protein Complex for FWMAV Development. In Biomimetic and Biohybrid Systems, Springer International Publishing: 2015; pp 86-91.
7. Sariola, V.; Pena-Francesch, A.; Jung, H.; Çetinkaya, M.; Pacheco, C.; Sitti, M.; Demirel, M. C., Segmented molecular design of self-healing proteinaceous materials. Scientific reports 2015, 5.
8. Demirel, M. C.; Cetinkaya, M.; Pena‐Francesch, A.; Jung, H., Recent Advances in Nanoscale Bioinspired Materials. Macromolecular bioscience 2015, 15 (3), 300-311.
9. Williams LW. The Anatomy of the Common Squid. Woods Hole: Marine Biology Laboratory, Woods Hole 1910.; 1910.

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