by Ali MISEREZ and Paul GUERETTE
t first sight, one might wonder how Biodiversity can spark the interest of engineers. The study by biologists or environmentalists of species and ecosystem diversity seems a far reach from the concerns of the Engineering world. But this apparent dichotomy has become narrower as previously well-delineated research domains are blended into integrative research programs that harness interdisciplinary skills from both the physical and life sciences. In recent years, there has been a rapidly growing interest in the field of “Bio-Inspired Engineering” or “Biomimetics”, which aims to study natural biomaterials and structures with unexpected properties and to gain inspiration for the creation of smart, multi-functional, and biocompatible materials. Biological structures, from butterfly wings and sea shells to gecko feet and Venus’s flower basket, to name just a few, are offering numerous insights and inspiration to engineers and scientists. Ultimately, the technological interest of biomimetics aims to create analogs of biological materials using clean chemistry synthesis.
Bio-inspiration from genetic andmolecular diversity
The study of genetic and molecular diversity provides multiple opportunities to reveal, at a very fine scale, how Nature has engineered complex structures for specific functional applications. Slight gene variations enable organisms to produce the changes necessary to adapt and survive in specific environments and niches. Because these subtle variations translate into biological materials with precisely adapted properties, studying and understanding these changes teaches us about the origins of biodiversity and provides remarkable lessons that can be converted into engineering principles. A shining example of bio-inspired materials engineering can clearly be found in the field of spider silk research. Silk has been a classic case-study of bio-inspired materials, sometimes pushed to a dogmatic extreme by popular press, with claims of its superiority over steel fibers in terms of strength/weight ratio. What is perhaps more fascinating to the bio-inspired materials engineer is the extreme range of material properties and physical states- for example gels, liquid crystals, foams, and films - that silks can exhibit. Within the past decade, researchers have developed silk-based materials for a wide range of applications, including bio-compatible nano-scaffolds to grow cells on for tissue engineering, silk based micro-spheres for drug delivery and even macroscopic fibers with mechanical properties that rival those of the best natural silks.
Most, if not all of this work depends, in some way, on the knowledge originally derived from silks produced by a small handful of insect and spider species. However, spiders have diversified dramatically since they first appeared about 400 million years ago. Today there exist close to 40,000 distinct spider species that have evolved a huge variety of silks. Silk diversity, which is a direct result of natural selection, is reflected on multiple levels. These range from the modular design of silk genes and the protein-based silk molecules they encode, to the three dimensional molecular structures of silk fibers. This molecular complexity determines the mechanical performance of the silk threads in the wild, ultimately influencing the reproductive success, evolution and diversification of spiders. Genetic mutations in silk genes, new silk-gland spinning micro-environments, and new spinning behaviors, when combined with selective pressures, can result in molecular and structural innovations of spider silks. As an outcome, spiders have developed a vast arsenal of mechanically distinct silks to deal with a variety of tasks related to their life-cycle and predatory behaviors. Orb-web weaving spiders, for example, can spin up to 7 different kinds of silk, and these range from the incredibly strong and extremely tough dragline (safety line), to the strong, yet rubber-like trapping portion of the orb-web, which is named viscid silk. Viscid silk is truly remarkable because it is stretchy, like a rubber-band, but has a breaking strength that is only about 1/3 that of Kevlar, making it, to our knowledge, the strongest rubber-like material known. The breathtaking evolutionary history of silk genes and natural silk processing strategies is only beginning to be revealed. For bio-inspired engineers each spider species and each silk represents a new frontier for investigation and an understanding of the evolutionary mechanisms that underlie these natural innovations provide a road map for the parameters that can been used to control and tailor the properties of man-made silk-based materials. In comparison, developing and tailoring high-end synthetic polymers like Kevlar fiber has required years of trial-and-error, meticulously studying variables such as chemical ingredients and processing conditions. Investigating natural materials that have evolved over hundreds of millions of years provides an effective shortcut for scientists seeking to create high-end fibers and a huge range of precisely tailored materials for novel applications.
Marine-Based Biomimetics andBiodiversity
While researchers the world are feverishly studying the genetics, structure, spinning conditions, mechanical properties and evolution of spider silks, bio-inspired engineers are also looking to the sea, realizing full well that marine organisms account for 90 % of the biodiversity on Earth. With biodiversity comes the potential for new insights into molecular design, new fabrication methods, and entirely new materials with potential for novel applications. Therefore, along with spiders, we are particularly interested in the mollusks, which are one of the most diverse groups of organisms found in the ocean, with no less than 100,000 known species. Some of the most familiar mollusks include mussels, squid and snails, which are currently under intense investigation in our lab and elsewhere in the world.
Take the common mussel. You might not think much of the delicious dishes enjoyed by many seafood aficionados, but materials scientists and biochemists studying the attachment around fibers (byssal threads) and adhesives by which the mussels Figure 2. The Mussel attachment holdfast (byssus) is made of various functional and structural proteins and is a marvel of multifunctional engineering. (a) Picture of the green mussel (Perna viridis) common to the shores of Singapore, which can stick to virtually any hard substrate. (b) Cartoon of common mussel illustrating multiple threads and the foot, which is the secreting organ for thread formation. (c) Detailed view of one attachment fiber. The central core is a mixture of collagen and silk-like proteins and is protected by a hard coating. It sticks to structures using specific adhesive proteins. (d) and (e) are close-up Transmission Electronic Microscope (TEM) views of coatings from different species (Mytulis galloprovincialis and californianus, respectively). The coating is made of specific cuticle proteins with highly repetitive amino-acid sequences and is hardened by complexation with metal ions. The two species have slight differences in their molecular repeat, which has been adapted for their environmental niches, and this results in distinctions in micro-structure structure and extensibility (TEM in (d) and (e) courtesy of Dr. N. Holten-Andersen, University of Chicago).anchor themselves to solid substrates have gained unique insights by comparing the fibers from different species residing in various environments. The byssal attachment holdfast is far more complex than a simple visual inspection might suggest and a marvel of multifunctional engineering. Within a few minutes, mussels are able to secrete a complete array of functional and structural proteins to securely anchor themselves to the surface and, ultimately, ensure their survival. These proteins include a collagenous thread-core, an adhesive pad and glue to strongly stick to substrates, and even a protective coating around the fibers. To add to this accomplishment, the fibers feature a mechanical gradient to minimize load concentration at the attachment site and the whole process is, of course, done underwater ! All of the functional and structural proteins that constitute the byssus feature remarkable diversity. Let’s look at the coating layer in more detail. This protective coating (the cuticle) is 5-fold harder than the core fiber and protects it from abrasion and microbial attack. But the fibers must also maintain high extensibility in order to mitigate mechanical shocks from the wave impacts in tidal zones. While hardness usually comes at the expense of extensibility, the mussel fiber coating is both hard and able to sustain large reversible extension, a truly remarkable feat. Investigating the coating further, researchers have found that species living in the heavily wave swept seashores (the intertidal-zone) were able to make more extensible coatings than species residing in calmer (subtidal) waters. By then looking at the molecular composition of the coatings, which are made of hardened proteins, they found that only subtle differences existed between the protein sequences produced by various mussel species. Yet these differences were enough to result in coatings with distinct properties. To the bioengineer seeking to synthesize coatings this has a priceless significance because it provides direct guidance on how to design genetic engineering strategies that will allow for the production of materials with diverse and precisely tailored properties.
Research into materials produced by squid and snails is also yielding novel insights and inspiration. For example we have shown that squid beaks, which are used to voraciously attack and pierce unsuspecting prey, are built from a uniquely cross-linked protein/polysaccharide composite that is harder than any synthetic polymer at its tip, yet very soft at its attachment site. This results in a material with unmatched graded structural properties that is tailored to prevent abrasion damage. By studying how the animal synthesizes and assembles these materials during development, we have the very real potential to reveal entirely novel materials manufacturing strategies. As another example, our research into the marine snail’s protective egg case has shown that it is a biomaterial that is not only mechanically robust, but that it maintains unique and unrivalled shape-memory properties that are based in the molecular design of its constituent helical proteins. We easily envision that as we move towards a comparative approach on these lesser-known systems, that is to say when we look at related species of squids or snails, we will reveal deeper insights into the origins of mollusk diversity. Moreover this approach will also provide new paradigms for materials manufacturing that will be based on genetic and molecular diversity.
While bio-inspired materials engineering offers tremendous potential, there remains, unfortunately, a very sobering reality. Human activity is directly responsible for the sharp decline in biodiversity worldwide. Prof. Herb Waite, a researcher at the University of California, Santa Barbara (UCSB) and an undisputed authority in the field of biomimetics, warns that “studying animals like marine snails, clams, or others among the thousands species from oceans may be our last chance to study vanishing species”. In the coming years, “40 percent of all marine species might become extinct because of man-made reasons,” said Waite in a recent interview with Discovery Channel. “A lot of people are focused on these animals only because they are a treasure trove of applications, but those applications won't be around if these species vanish,” he added.
While the past decades have ushered in a veritable explosion of new knowledge and technologies, our enthusiasm should be tempered. The dramatic loss of biodiversity is also resulting in the destruction of vast libraries of invaluable biological information that is directly applicable to materials engineering. It is our hope that cross-fertilization of ideas between biologists, ecologists, and engineers will bring a certain level of awareness to this worrying issue.
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