The most resistant biomaterials in the world

How resistant can biomaterials be? Strong enough for diamond and aviation saws.

Limpets have 1,920 teeth on their radula, the tongue-like appendage they use to scrape food off rock surfaces. These teeth are made from common organic materials – chitin and goethite – but the compounds are arranged in such a way as to make the overall structure incredibly strong.

The limpet teeth are so strong, in fact, that they can only be cut with diamond saws. In 2015, researchers did just that to test the physical limits of the material. They cut small pieces of the mollusc’s teeth to exclude the effects of its curvature and observed the load they could withstand before breaking. They discovered that the teeth of limpets are made from one of the strongest biomaterials known.

In August 2022, limpets once again lit up the materials science community. Another group of researchers reported in Nature that they had created a new biomimetic material based on the structure of the limpet tooth.

Before building the new material, scientists looked at the biology that generates these unique organs. They observed the molecular processes behind the generation of limpet teeth and the developmental stages of the radula, the organ from which teeth emerge. They also looked at how genes in the limpet regulate chitin and iron processing. Borrowing from the generative processes behind organic materials is a hallmark of biomimetic design.

The novel limpet biomaterial consists of electrospun chitin scaffolds mineralized with cultured radula cells. Since chitin is biodegradable, this innovation could prompt biocomposites to replace synthetic materials where extreme strength is required, such as in engineering, automotive manufacturing, and construction.

Limpet teeth surpass the carrying capacity of spider silk, the most famous super strong natural material that has spawned many biomaterial innovations. Japanese start-up Spiber is developing a synthetic version of spider silk protein for textiles. Israeli start-up Seevix makes artificial spider silk for wounds, wounds and surgical sutures. Karig Labs has created a line of bristles with more strength and flexibility. The company combined genetically modified proteins found in spider and silkworm silk, harvesting these hybrid threads from genetically modified silkworms. The researchers behind the biomimicry of limpet teeth hope their work will inspire similar spinoffs.

Measurement of the resistance of materials

It is difficult to classify materials according to their strength because the property can be measured by different measures. The limpet tooth’s fame is based on its “ultimate tensile strength”, which is what most refer to when claiming how strong something is. The units of measurement for ultimate tensile strength are megapascals (MPa) and gigapascals (Gpa), with 1000 megapascals in one gigapascal.

MPa and Gpa are calculated by dividing the maximum load a material can withstand before deforming. Limpet teeth average 4.9 GPa. Compare that to A36 steel, a type of steel alloy commonly used in construction, which has an MPa of 550 (0.55 Gpa). Tungsten, a metal used to make bullets that hits 1.5 GPa.

Besides ultimate tensile strength, a material can also be rated for its density to strength ratio. Generally, the denser a material, the stronger it is. Some exceptional materials such as spider silk go against this trend. Spider silk (0.45-2.0 GPa) is arguably much stronger than some steels (which vary between 0.2 GPa and 2 Gpa) as it can withstand similar or greater loads although it represents one-sixth the density of the metal. Materials with a higher density-to-strength ratio can be a more efficient design choice because lightweight materials require less energy expenditure during transportation and construction.

Another measure of strength is stiffness, which refers to how elastic a material is when supporting loads. Very stiff material will spring back to its original shape after being subjected to high forces. Stiffness can muddy the waters of any definitive strength ranking. Although spider silk can match or surpass steel in ultimate tensile strength and density to strength ratio, steel is much stiffer.

One of the strongest biomaterials in the world is not produced by animals but by plants. Cellulose is the most abundant organic macromolecule in the world, present in the form of nanofibers inside the walls of plant cells. Its strength owes not only to the physical properties of cellulose macromolecules, but to the way its nanofibers are arranged.

In 2018, researchers compacted cellulose nanofibers from natural wood to obtain longer and wider fibers than those found in trees. Their alterations improved the strength of the nanocellulose. In this form, the material reaches 1.57 GPa, more than 20% stronger than spider silk. The material also had a stiffness of 86 GPa, eight times that of spider silk.

With so many ways to assess the strength of materials, any general rating is of limited practical use. Each application requires specific functionality, and each type of force has advantages. There are also often trade-offs between these strength criteria and it is the designers job to negotiate these when creating a functional product.

Developments in aeronautical biocomposites

One measure of resistance is particularly useful for aerospace applications: fatigue resistance. While tensile strength and stiffness test the maximum weight limit before structural defects, fatigue strength is a more subtle concept. It involves testing the structural integrity of a material when exposed to continuous and repetitive mechanical movement over long periods of time. Normally, the motions of interest are cyclical: the effect of rotating turbine blades on metal parts, for example.

Developing biomaterials with high fatigue resistance is now a priority for some researchers. There is a growing demand for renewable materials from the commercial aerospace and defense industries as they begin to integrate sustainability and efficiency into aircraft design.

Although it may seem difficult to replace mined aircraft components with biocomposites, there is one thing that works in favor of organic materials: their lightness. For several decades, manufacturers have moved away from aluminum as the structural material of choice. Today we see lighter carbon fiber reinforced polymer composites in the fuselage and wings. The next step in aerospace materials innovation will be to create natural fiber reinforced green composites (NFRGCs). After two or three decades of scientific research into NFRGCs, commercial carriers began their own research into these materials.

Over the past four years, a consortium of companies and research labs led by Expleo Technology UK have been looking into bio-based aerospace composites. As part of their BAMCO project, the consortium explored bamboo fiber and resins as an entirely new structural material for the airline industry. Airbus, which has joined the consortium, has begun testing the material for strength and vibration absorption. The current pre-industrialization phase began in 2018 and is expected to end in 2022. Phase II of the project will refine material performance and manufacture the first prototype aircraft parts.

Besides the BAMCO project, Airbus was the commercial partner of the EU research project, Ecological and Multifunctional Composites for Application in Aircraft Interior and Secondary Structures. Between 2016 and 2019, the project evaluated candidate materials for interior and exterior rooms. He has also built biocomposite prototypes, such as an example of a horizontal tailplane trailing edge panel combining conventional carbon fibers and bio-based epoxy resin. Their white paper 2021 recommended other avenues of research, such as the fatigue and aging behavior of natural fiber composites and their resistance to fire. One of the key takeaways was that in terms of mechanical characteristics such as tensile strength and impact resistance, bio-based resins compared favorably to current oil-based materials.

Another forerunner in the field of bio-based aircraft is Berkeley-based biomaterials company Cambium. In 2020, their new bio-based composite material was tested by the US Naval Air Warfare Center during an in-flight firing demonstration. During the test, flammable gels are placed on the wings and ignited in midair. Cambium’s product was found to be more effective at limiting flame spread through the wing compared to a BPA-based material. If Cambium’s biomaterials are adopted by the US Air Force, these innovations could quickly find their way into commercial aircraft.

Cambium develops biocomposite materials for other heavy-duty aerospace materials. In 2022, Cambium and Applied Aeronautics signed a commercial and product development agreement to design, manufacture and market drones made from biocomposites. The company is not only targeting military applications through this partnership. Their products are expected to be deployed in renewable energy, oil and gas, and support for wildfire suppression.

Aerospace applications will be the most challenging arena yet for biomaterials. Aircraft structural components must have light weight, high tensile strength, high rigidity and high fatigue resistance. In addition to these mechanical requirements, there are other key requirements of aerospace structures. These include workability, so they can be subjected to efficient manufacturing methods, chemical stability when exposed to water, and resistance to high intensity UV exposure.

For airlines focused on achieving sustainable fuel goals, advancing aviation biomaterials is not a priority. The defense sector is more likely to drive innovation and deployment in the near term. The fact that biomaterials are now receiving attention from the military and aerospace sectors is in itself significant, indicating a shift in perception around the potential of biomaterials.

About Dianne Stinson

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