Professor Asa Barber talks limpet teeth for materials science

Materials World magazine
,
30 Mar 2016

It’s stronger than titanium and makes a mockery of Kevlar. Ledetta Asfa-Wossen talked to Professor Asa Barber of the University of Portsmouth, UK, about the exciting possibilities of limpet teeth and why spider silk is yesterday’s news. 

What makes limpet teeth so strong?

Limpet teeth are constructed from a composite structure containing a soft natural polymer that acts as a framework for the formation of hard fibres. These fibres are biominerals of a material called goethite. The forming of the goethite is critical as the biology allows the goethite to form as nanofibre crystals. 

Most materials are mechanically weak because they contain defects that lead to stress concentrations. However, the nanofibres of goethite have extremely high strength, regardless of defects.    

What is goethite and where is it currently found? 

Goethite is formed in limpet teeth in a process known as biomineralisation. It is a commonly found mineral in geology and has been used in pigmentation, but the nanofibre form of goethite in limpet teeth is distinct to the form encountered in geology, as there is more control of the mineral. 

How much goethite is in these teeth? 

The tooth structure is fairly rich in goethite. Around 80% of the tooth consists of nanofibrous goethite and this provides the high strength. 

Why is goethite such a potentially attractive engineering material?

Goethite uses a sustainable raw material (as the biology uses elements from the local environment), yet has mechanical properties that rival high-performance alloys. The concept of using materials without requiring any energy intensive manufacturing processes is exciting. It is inherently a green technology, with the added advantage of superior mechanical properties. 

Biology has shown that goethite can achieve outstanding strength in limpet teeth but there is potential to achieve similar strengths in other materials. This discovery means that the fibrous structures found in limpet teeth could be mimicked and used in high-performance engineering applications such as Formula 1 racing cars, the hulls of boats and aircraft structures.

What makes limpet teeth superior to spider silk? 

Until now, spider silk was widely regarded the most promising biological material because of its super-strength and potential applications in everything from bullet-proof vests to computer electronics. We have now discovered that limpet teeth exhibit a higher strength. Spider silk is not particularly stiff and breaks at high extensions. 

Limpet teeth are more rigid and are 10% stronger than spider silk. It also has a tensile strength of 5GPa. That would be like a single string of spaghetti holding up 3,000 half-kilogram bags of sugar.

Why is limpet teeth goethite not currently being used in composites?

The biomineralisation processes to produce goethite nanofibres is presently too sophisticated and slow for current composite manufacturing. Nanofibres such as carbon nanotubes are being used in many engineering composites but they lack the low energy and sustainability of limpet teeth goethite.   

What existing natural materials are limpet teeth most similar too? 

It would be difficult to say. Nanofibrous materials are prevalent across the natural world and many sea creatures exploit biomineralisation to produce mineral nanofibres.  

Tell me more about the initial testing and inspection of the limpet teeth. 

The goethite nanofibres in the limpet teeth have such complex organisation. Measuring the tensile strength required cutting out a small volume from the teeth where we could be sure the goethite nanofibres were all pointing in one direction. 

Limpet teeth are already very small, so we had to cut out a really small sample using ion beam techniques. With such small samples, we needed to use electron microscopy and atomic force microscopy to observe them. The combination of testing and preparation required was very demanding.  

Did you compare the limpet teeth to any manmade materials?

Yes. I’ve always been interested in how the strength of limpet teeth compares to engineering alloys, including many ferrous and titanium alloys. We found limpet teeth were stronger than most of them.

What were some of the key technical challenges during your research? 

The technical challenge was mainly how to cut out samples with widths of around one micron while holding the sample at both ends. Even the glue that holds it to the atomic force microscope can break the sample. People always focus on the limpet teeth and their strength but I tend to reflect on how important the glue was in enabling our experiments!

Another challenge was to point out some of the findings of the work that tend to get overlooked. Engineers know that materials get stronger as they are smaller but we showed that limpet teeth maintain their strength even when they get bigger. I think that’s important.

How do you foresee limpet teeth being replicated or manufactured in the future?

A possible method would be to exploit biology to grow limpet tooth-like structures in large volumes. Synthetic biology is an interesting route. I also think 3D printing has huge potential, as we have a manufacturing process that can start to approach the complexity found in natural design. Materials need to get better but there is real opportunity in improving the design of them.   

Are there any limitations to forming a limpet teeth-inspired composite?

The main limit is manufacturing speed. Biology generally makes things slowly compared with current manufacturing processes in engineering.

What other applications or industries can you see this material being used in? 

Limpets need high strength teeth to rasp over rock surfaces and remove algae for feeding when the tide is in. Limpet teeth are strong and wear resistant. High strength composites are used extensively in transport, so I would say there were many applications within vehicle manufacturing. I would also consider new tooling devices made out of limpet teeth. Limpet teeth-like materials for dental restorations is also a potential area worth exploring.

What do you have planned for the next phase of research?

I want to delve into the manufacturing aspects and find out how we can assemble these materials into more complex organisations. The University of Portsmouth is building a new facility called the Future Technology Centre, which is focused on 3D engineering, and that’s driving a lot of my future investigations. 

The centre links imaging to manufacturing and highlights the importance of organising materials in 3D for enhanced performance. 

So, when can we expect to see limpet teeth-inspired composites being used in the real world? 

I think we will see the bio-inspired aspect being used soon – in the next two-to-five years. High-fidelity ‘synthetic’ limpet teeth might be a longer term proposition but I would say in the next 10 years.