How are biomimetics and clam shells related?

What is Biomimetics? Biomimetics is “the imitation of the models, systems, and elements of nature for the purpose of solving complex human problem”. Typically biomimetics starts with a problem and looks to nature to solve said problem. Biomimetics takes advantage of millions of years of evolution – true innovation – and applies it to a humancentric problem. Through the process of evolution and natural selection, organisms are optimised for their environment. Typically organisms have to optimise for multiple different problems at once and so co-optimise to reach multifunctional solutions. 


So what does biomimetics have to do with my PhD? On January 9^th 2015 I attended the Cambridge Philosophical Society one day meeting titled "BIOMIMETICS: from nature to applications” to find out. The meeting covered a huge range of themes, from mimicking viruses for improved drug delivery to robotic ecosystems, but here I will focus on the biomineralisation talks and attempt to answer the question… How are biomimetics and clam shells related?


Bio-Inspired Photonics Materials: By Prof. Mathias Kolle


Department of Mechanical Engineering. Massachusetts Institute of Technology, Cambridge USA

Prof. Kolle and colleagues study blue-rayed limpets (Patella pellucida) and are interested in the materials which give the limpet its shiny rays. Using a combination of material characterisation techniques, Kolle’s group have discovered a biomineralised photonic structure. The bright blue stripes in the limpet shell consist of layers of crystals with tiny gaps in-between. Importantly the layers are corrugated, rather than flat, with a colloidal absorbent layer underneath which absorbs red light and increases contrast for the blue hue.

Blue-rayed limpet and structure of stripes. Figure taken from Li Ling's thesis:

Biomineralized structural materials with functional optical properties,
Massachusetts Institute of Technology. Department of Materials Science and Engineering

 

Prof. Kolle presented the bio-inspired photonics work from a strong material science perspective. Research up to now has looked at what the limpet does, which is often enough for an application, but has yet to answer HOW it works. For me, the immediate next question is how the limpet controls the growth of the stripes, which are very different in structure to the rest of the shell. My first reaction (obviously) was to find out if there is a mantle transcriptome available for the species – there isn’t (surprise surprise). But it did get me thinking that perhaps the blue-rayed limpet could be an interesting model for functional biomineralisation genetics…

Breaking the Rules: Nature's Crystal Methods: By Prof. Fiona Meldrum 


School of Chemistry, University of Leeds UK


Prof. Meldrum is a chemist at Leeds University who studies crystals, and specifically her group are interested in calcium carbonate crystals. The presentation started with some useful thoughts on crystal structure and the revelation that, in nature, crystal geometry is not ultra regular all the time – as a biologist this seemed fairly obvious to me. Biology is really good at making organic materials at ambient conditions; specifically, biology makes crystals with no high pressures or temperatures.

A single complex crystal. Figure taken from Prof. Meldrum's website.

Prof. Meldrum and colleagues acknowledge the complexity of biological systems and then attempt replicate them in order to understand more about crystal structure and growth. They’ve found that it’s fairly easy to grow a very complex single crystal using the right template. In addition, they’ve also been able to embed additives within crystals, something biology does all the time to increase the strength of crystal structures. The ability to embed molecules inside crystals could have useful human applications such as non-toxic, non-fading paint.

Similar to the way Prof. Kolle takes a material science perspective, Prof. Meldrum tackles her questions from a pure chemistry background. The work presented is challenging traditional ideas about crystals and crystal properties. It’s easy to change crystal morphology with the right template and additive, the question for me is, how do molluscs make and lay-down the additives to control shell growth?


How Nature Makes Materials: By Prof. Ulrich Steiner

Adolphe Merkle Institute, Friburg, Switzerland

Prof. Steiner built on the work presented by Prof. Meldrum and showed that by looking very carefully at how molluscs make crystals (specifically nacre) we can create similar materials.

Replicated nacre. Figure taken from Prof Steiner's recent paper:

Finnemore, A., P. Cunha, T. Shean, S. Vignolini, S. Guldin, M. Oyen, and U. Steiner, 2012, Biomimetic layer-by-layer assembly of artificial nacre: 

Nature Communications, v. 3.

By mimicking five basic stages of shell growth (1: stabilization of amorphous calcium carbonate in solution, 2: specific aggregation and continuous film formation on organic surfaces, 3: deposition of a porous, thin organic film on a previously formed mineral layer, 4: crystalliza­tion of the formed amorphous calcium carbonate layers to aragonite or calcite and 5: Cyclical iteration of steps 1–4) Prof. Steiner’s group, for the first time, were able to successfully replicate nacre. Replication can be the first step for understanding complex biominerals, and potentially paves the way for new applications such as tough coatings fabricated from cheap abundant materials.


Biomimetic mineralization: developing a new model of bone material: By Dr. Melinda Duer

Chemistry Department, University of Cambridge

Dr. Duer started by asking the question “why make stuff?” and went on to explain that if we can make stuff, then we understand how it works. This is the kind of perspective I can get on board with. Dr. Duer reminded us of the important point that crystal formation results in heat dissipation, and the bigger crystals are, the more heat is dissipated. Thermodynamic equilibrium and the loss of energy during crystal formation must be important in biology and the evolution of biominerals.

Heat dissipation during crystalisation.

Dr. Deur and colleagues use solid-state nuclear magnetic resonance (NMR) spectroscopy to identify the atomic finger print of bone extracellular matrices, in animal tissues and in in vitro tissues. Results presented suggested that sugars, and specifically the glycosylation of collagen fibrils by PolyADPribose, are structurally important in bone. These results fundamentally change the traditional bone biomineralisation model and have huge clinical implications.

NMR atomic finger printing is the key technique which progressed science in this field – I wonder if anybody is doing this with shell matrices… 

A section from Dr. Deur’s webpage which nicely answers my question about what biomimetics has to do with my research. “On an atomic and nanoscopic lengthscale, the extracellular matrix provides a communication systems between the cells in the tissue; on a microscopic lengthscale, it provides the scaffold that supports those cells, i.e. provides a “home” for the tissue’s cells; and at the macroscopic lengthscale, the extracellular matrix is the material which forms the structures in our organs, our blood vessels, bones, tendons, intestines, etc.  Understanding how all these functions can arise in a single material is not only essential for understanding the biology of tissues, it can give us clues on how to design new smart materials ourselves.”

How are biomimetics and clam shells related? I’m interested in how organisms work, specifically I’m interested in how clams build their shells. It turns out there is a huge group of people in biomimetics who are is interested in almost the exact same thing, just coming from different perspectives in order to suit the future application. 

A word of warning… Mimicking or copying something is a step towards understanding how it works, as demonstrated by Dr. Deur, BUT it is only the first step and biological systems are complex.