Crystallite orientation map of a mature molar. Red parts have well oriented crystallites, blue parts have lower orientation. Adapted from Simmons et al.Biomimetics is a creative science where inspiration comes from nature to generate biologically inspired materials and processes. Sometimes creativity comes in the form of transplanting an idea from its biological context into a novel use. For example in architecture: the Eastgate Centre, a mid-rise office complex in Harare, Zimbabwe, takes inspiration from internal termite mound structures to stay cool without air conditioning, despite outside air temperatures reaching over 40 °C. As such this building uses only 10% of the energy of a conventional building its size [1].
Other times, biomimetics is used in more literal contexts to mimic human biological processes and structures to produce medical and dental materials for clinical benefit. In these case creativity comes in the form of finding innovative ways to design experiments to understand the complex processes and structures in as much detail as possible at as close as possible to physiological conditions, in order to replicate or replace them synthetically.
Creativity is essential if we are to solve one of the greatest challenges in the field of dentistry: Can we grow a new, functioning replacement biological tooth? How do we uncover the dynamic processes involved in biomineralization when the protein building blocks that guided the hierarchical structure are absent in the finished product?
Dental enamel is the most extreme case of a dynamic biomineralization process, where at the start of the process there is 0% mineral, 100% protein and water, and by the end its 96% mineral (bone is 60–70%) and as such is the most highly mineralized tissue in the human body. If lost through tooth decay or acid erosion, it cannot replace itself. Unlike most biological materials, enamel has no living cells so it cannot repair itself, and has no nerves so cannot feel and respond to acid attack or bacterial decay. In terms of biomimetics, our hope is to construct a similar inorganic material through synthetic routes.
One way of being creative in this field is to use animal models to study developing tissues that are difficult to obtain from living humans due to ethical considerations (since permanent developing teeth sit in the jawbones of children aged 1–13 years old). This approach has led to recent success in synthetic biological tooth replacement in mice [2]. However, it has limitations because it is known that each species has enamel specialized to their diet, habitat and function, and rodent enamel has a growth rate and ultrastructure quite different to human enamel [3].
An alternative route finds creative ways to solve the puzzle of human enamel development using two crucial tools. The first is collaboration with archaeologists who have access to human dental enamel at different stages of development from burial sites where children died whilst their teeth were still developing. This removes the compromise of using animal models.
The second is using the advanced structural characterization technique of synchrotron X-ray diffraction and X-ray diffraction tomography (XRD-CT). This allows us to quantify the crystallographic, nano- and micro-scale structures of this crystalline material, in order to replicate them.
Using synchrotron X-ray diffraction and XRD-CT at beamlines ID15 and XMaS (BM28) at the European Synchrotron Radiation Facility we have studied human teeth at several different stages of enamel development. This has revealed that, in human enamel, mineralization does not occur evenly across the whole tooth surface. Instead, it is focused initially at the interface with the underlying dentine and inside the cusps (tips of the teeth). It then spreads into the bulk of the enamel as you get older [4]. The crystallites of enamel start uniform in shape and size across the whole tooth crown. They gradually become large and needle-like at the biting regions of the tooth, and remain small and less elongated on the sides of the tooth away from biting areas when the tooth is fully mature [5]. These detailed insights help us understand the precise timing and spatial development of mineralization in enamel in a way that no other route could tell us. Understanding this complex natural biomineralization process sparks imaginative ideas to achieve similar structures synthetically.
And so, it is important to think creatively when studying complex biological systems for the benefit of medical and dental materials science. By being creative and imaginative in experimental design we can continue to push the boundaries of our knowledge of complex biological materials and processes which we can then learn to biomimetically replicate and replace.
Further reading
1. http://www.architectsforpeace.org/mickprofile.php
2. E. Ikeda, et al., Fully functional bioengineered tooth replacement as an organ replacement therapy. Proc. Natl. Acad. Sci. U.S.A., 106 (2009), pp. 13475–13480
3. A. Boyde, D.J. Chadwick, G. Cardew (Eds.), Dental Enamel, Proceedings of the Ciba Foundation Symposium 205, John Wiley & Sons, Chichester (1997), pp. 18–31
4. L. Simmons, et al., Mapping the spatial and temporal progression of human dental enamel biomineralization using synchrotron X-ray diffraction. Arch. Oral Biol., 58 (11) (2013), pp. 1726–1734
5. M. Al-Jawad, et al., Biomaterials, 28 (2007), pp. 2908–2914