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It's biocompatible, biodegradable, and micromoldable. It is, say its inventors, twice as strong as nylon or polylactic acid (PLA), while exhibiting the strength of an aluminum alloy at half its weight. In short, Shrilk is a new bio-inspired, sci-fi sounding insectoid material promising low cost and high performance that was developed by researchers taking a leaf from nature's own book.

Karen Laird

December 21, 2011

4 Min Read
Green Matter: Creepy crawlers inspire new biomaterial

It's biocompatible, biodegradable, and micromoldable. It is, say its inventors, twice as strong as nylon or polylactic acid (PLA), while exhibiting the strength of an aluminum alloy at half its weight. In short, Shrilk is a new bio-inspired, sci-fi sounding insectoid material promising low cost and high performance that was developed by researchers taking a leaf from nature's own book. (Read our Medical Channel's article on possible implications of Shrilk for that sector).

Natural designs have been tried and tested throughout the eons of time it has taken for these to evolve. Poor designs fall by the wayside; sustainable innovations get passed on to future generations, to be used and improved on. Today, one of the fastest growing fields in science is called "biomimicry", in which researchers look to the natural world to find inspiration for new products, and to learn how to build in ways that are more efficient, lower-cost, and environmentally friendly.

First coined by a natural sciences writer called Janine Benyus, biomimicry is all about "replicating nature's blueprint". And these days it's even being taught at school: a new interdisciplinary course at UC Berkeley is called  "How Would Nature Do That?"

Which is exactly what researchers Javier G. Fernandez, Ph.D. and Donald Ingber, M.D., Ph.D. at Harvard University's Wyss Institute for Biologically Inspired Engineering looked at when studying the challenge of developing a material that could provide protection without adding weight or bulk. They saw that natural insect cuticle, such as that found in the rigid exoskeleton of arthropods -invertebrates with segmented bodies and jointed limbs, such as insects and crustaceans - not only provided protection, it also provided structure for the insect's muscles and wings. While so light that it doesn't inhibit flight, it is also so thin that it allows flexibility.

In the past, researchers have attempted to engineer materials resembling insect cuticle, using combinations of chitin, the second most abundant polymer on earth, and proteins. These attempts proved unsuccessful, yielding materials with highly unsatisfactory properties. Now, instead of merely combining materials, the Harvard researchers attempted to re-create the actual structure of insect cuticle.

Harvard released a statement that said that "insect cuticle is a composite material consisting of layers of chitin, a polysaccharide polymer, and protein organized in a laminar, plywood-like structure. Mechanical and chemical interactions between these materials provide the cuticle with its unique mechanical and chemical properties. By studying these complex interactions and recreating this unique chemistry and laminar design in the lab, Fernandez and Ingber were able to engineer a thin, clear film that has the same composition and structure as insect cuticle."

They named the material "shrilk" because chitosan is commonly isolated from shrimp shells, while the protein used, called fibroin - comes from silk.

silk_fibroin.gif

Fibroin

Silkworm silk consists of two main proteins, sericin and fibroin, fibroin being the structural center of the silk, and serecin being the sticky material surrounding it. Fibroin is largely made up of the amino acids Gly-Ser-Gly-Ala-Gly-Ala and forms beta pleated sheets, β-keratin. Image from The Department of Chemistry, University of the West Indies.

One of the most interesting aspects of  the living insect cuticle is that it has a range of material properties that can vary from very elastic to very hard, apparently depending on water content. Remarkably, shrilk exhibits similar versatility and it can be reversibly transformed between rigid and highly flexible states by altering water content alone.

In their article, published in the online issue of Advanced Materials, Fernandez and Ingber write that, its outstanding strength and versatility, as well as its low cost and density, make shrilk an excellent candidate as a biodegradable plastic that could have great value as a replacement for existing non-degradable plastics in a wide range of consumer product application areas, including disposable bottles, trash bags, packing materials, and diapers that currently pile up in waste sites around the planet. Because chitosan and fibroin are both biocompatible, shrilk on its own or in combination with other materials or crosslinking agents may be valuable for certain medical applications, such as wound dressings and scaffolds for regenerative medicine. Finally, due to the biological origin, wide availability, and low cost of its components - shrimp shells are a waste material -  shrilk represents an abundant and sustainable material that can be seamlessly integrated into the environment within several ecological cycles.

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