How strong can biomaterials be? Strong enough for diamond saws and aviation.
Limpets have 1, 920 teeth on their radula, the tongue-like appendage they use to scrape food off rocky surfaces. These teeth are made from common organic materials – chitin and goethite – but the compounds are arranged in ways that make the overall structure incredibly strong.
Limpet teeth are so strong, in fact, that they can only be cut using diamond saws. In 2015, this is exactly what researchers did 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 how much load they could take before breaking. They found that limpet teeth are made from one of the strongest known biomaterials.
In August 2022, limpets once again lit up the material science community. Another group of researchers reported in Nature that they had created a novel biomimetic material based on the limpet tooth’s structure.
Before building the novel material, the scientists delved into the biology that generates these unique organs. They observed the molecular processes behind limpet tooth generation and the developmental stages of the radula, the organ from which the teeth emerge. They also looked at how limpet genes govern chitin and iron processing. Borrowing from the generative processes behind organic materials is a hallmark of biomimetic design.
The new limpet biomaterial consists of electrospun chitin scaffolds mineralised with cultured radula cells. Since chitin is biodegradable, this innovation could inspire biocomposites to replace synthetic materials where extreme strength is required, such as in engineering, automotive manufacture, and construction.
Limpet teeth surpasses the load-bearing capacity of spider’s silk, the most famous super-strong natural material that has spawned many biomaterial innovations. Japanese startup Spiber is developing a synthetic version of spider silk protein for textiles. Israeli startup Seevix makes artificial spider silk for wound, injury and surgical suturing. Karig Labs has created a silks range with greater strength and flexibility. The company has combined genetically modified proteins found in both spider and silkworm silk, harvesting these hybrid threads from genetically engineered silkworms. The researchers behind the limpet teeth biomimicry hope their work will inspire similar spinoffs.
Measuring material strength
It is difficult to rank materials in terms of their strength since the property can be measured along different metrics. The limpet tooth’s claim to fame rests on its “ultimate tensile strength”, which is what most refer to when they make claims about how strong something is. The units for measuring ultimate tensile strength are megapascals (MPa) and Gigapascals (Gpa), with 1000 megapascals in one gigapascal.
MPa and Gpa is calculated by dividing the maximum load a material can bear before deforming. Limpet teeth averages 4.9 GPa. Compare this 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 which hits 1.5 GPa.
Apart from ultimate tensile strength, a material can also be assessed for its density-to-strength ratio. Generally, the denser a material is, the stronger it is. Some exceptional materials like spider silk bucks this trend. Spider silk (0.45-2.0 GPa) is arguably much stronger than some steels (which range between 0.2 GPa to 2 Gpa) as it can bear similar or greater loads despite being a sixth of the metal’s density. Materials with higher density-to-strength can be a more efficient design choice since light materials need less energy expenditure during transport and construction.
Another measure of strength is stiffness, which refers to the elasticity of a material when bearing loads. A very stiff material will regain its original form after being subjected to high forces. Stiffness can muddy the waters of any definitive strength ranking. Although spider silk may match or outperform steel on ultimate tensile strength and density-to-strength ratio, steel is much stiffer.
One of the strongest biomaterials in the world isn’t produced by animals but plants. Cellulose is the world’s most abundant organic macromolecule, found in nanofibre form inside plant cell walls. Its strength owes not just to the physical properties of the cellulose macromolecules but the way its nanofibers are arranged.
In 2018, researchers compacted natural wood cellulose nanofibers to obtain longer and wider fibres than are found in trees. Their alterations enhanced the nanocellulose’s strength. In this form, the material hits 1.57 GPa, more than 20 percent stronger than spider silk. The material also had a stiffness of 86 GPa – eight times that of spider silk.
With so many ways of evaluating material strength, any general ranking is of limited practical use. Every application requires specific functionalities, and each type of strength holds advantages. There are also often trade-offs between these strength criteria and it is the job of designers to negotiate these when creating a functional product.
Developments in aviation biocomposites
One measure for strength is especially useful for aerospace applications: fatigue resistance. While tensile strength and stiffness tests the maximum weight limit before structural flaws, fatigue resistance is a subtler concept. It involves testing the structural a material’s structural integrity when exposed to continuous, repetitive mechanical motions over long periods. Normally, the movements of interest are cyclical: the effect of turning turbine blades on metal parts, for example.
Developing biomaterials with high fatigue resistance is now top of the agenda for some researchers. There is growing demand for renewable materials from the commercial and defence aerospace industries as they begin to build sustainability and efficiency into the design of aircrafts.
Although it might seem a tall order to replace mined plane components with biocomposites, there is one thing working in favour of organic materials: their lightness. For some decades, manufacturers have been moving away from aluminium as their go-to structural material. Today, we see lighter carbon fibre reinforced polymer composites in fuselage and wings. The next step in aerospace materials innovation will be to create natural fibre reinforced green composites (NFRGCs). After two or three decades of scientific research into NFRGCs, commercial carriers have begun their own research into these materials.
Over the last four years, a consortium of companies and research labs led by Expleo Technology UK have been looking into biobased aviation composites. Under their project BAMCO, the consortium has been exploring bamboo fibre and resins as brand-new structural materials for the airline industry. Airbus, who has partnered with the consortium, has begun testing the material for strength and vibration absorption. The current pre-industrialisation 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.
Apart from the BAMCO project, Airbus was the corporate partner of the EU research project, Ecological and Multifunctional Composites for Application in Aircraft Interior and Secondary Structures. Between 2016 and 2019, the project assessed candidate materials for both interior and exterior parts. It also built biocomposite prototypes, such as an example trailing edge panel of a horizontal tail combining conventional carbon fibres and bio-based epoxy resin. Their 2021 white paper recommended further research avenues, such as the fatigue and ageing behaviours of natural fibre composites and their fire resistance. One key takeaway was that in terms of mechanical features such as tensile strength and impact resistance, bio-based resins compared favourably with current oil-based materials.
Another early mover in biobased aircraft is Berkeley-based biomaterials company Cambium. 2020 saw their new bio-based composite material tested by the US Naval Air Warfare Center in a fire-in-flight demonstration. During the test, flammable gels are placed on the wings and set alight mid-flight. Cambium’s product proved more effective at limiting the spread of flames across 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 is developing biocomposites materials for other heavy-duty aero-hardware. 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 just targeting military applications through this partnership. Their products are set to be deployed in renewable energy, oil and gas, and wildfire-fighting support.
Aerospace applications will be the most testing arena yet for biomaterials. The structural components of aircraft must display lightness, high tensile strength, and high stiffness, and high fatigue resistance. On top of these mechanical requirements are other key demands of aerospace structures. These include workability, so that they can be subjected to efficient manufacturing methods, chemical stability when exposed to water, and resistance to high intensity UV exposure.
For airline companies focusing on meeting sustainable fuel targets, moving the needle on aviation biomaterials is of low priority. It is more likely that the defence sector will drive innovations and roll out in the near term. The fact that biomaterials are now receiving attention from the military and aerospace sectors is itself significant, indicating shifting perceptions around the potential of biomaterials.