Perhaps the introduction of all academic texts should include some grandiose claims about the importance of the contents of the book. In this case, that is easily begun. The interaction of light with biological tissue is the basis of almost all life on the planet, and the dominant way that most animals receive information about the world. The scope of the structures and materials that have evolved in the context of biological interactions with light is therefore rich indeed. It includes photochemistry: the materials that are the basis of photosynthesis, bioluminescence, pigments and retinal sensors. Bio- molecules often have very complex interactions with light: harvesting it, shifting wavelengths and using it transferring energy. The complexity of some of these interactions means that even in nature they are often used in more than one way. Chemistry, however, is only the start of the story. Physical structures have evolved in parallel which use the full range of optical effects: reflection, refraction, diffraction and interference. These effects have been used in eyes and other structures to produce colour, optimize absorption or manipulate polarization.
However, all of biological photochemistry and optics is probably too ambitious a task, even for a book as interesting as this one. We are aiming for something different. We aim to understand how we can use this huge variety of materials and structures. What can we learn from the optimization of millions of years of evolution? Can we make nature’s tricks, our tricks? This more applied field itself is extensive, and has a surprising long history. As an example, graded index lenses were apparently first discovered by James Clerk Maxwell (he of the eponymous equations) while contemplating the eye of his breakfast kipper. Indeed many of the earliest examples of optical processes such as diffraction and thin film interference were first studied in nature, perhaps because it was not trivial to make structures of the right dimension and regularity. Although those structures are now well understood, the field remains surprisingly rich. Two recent high-profile papers, for example, have shown how jumping spiders can judge depth from a defocused image (Nagata et al., 2012) and suggested how the iridescent structures found in the wing of the Morpho butterfly could be used to produce the next generation of thermal imaging sensors (Pris et al., 2012).
Optical biomaterials are, in this respect, particularly compelling. Molecules of this complexity are hard to design and synthesize from scratch. Using naturally occurring biomolecules, however, has two profound benefits. The first is simple: the materials are generally biocompatible. For example, while many synthetic dyes are carcinogenic, those found in nature are non-toxic and can be used in vivo. This is even more appealing when combined with the second advantage. In many cases the genetic coding of biomolecules is understood, and can be manipulated to change the properties of the molecule. We are only beginning to exploit the benefits this brings.
In this book we will explore several families of important biomolecules: the retinal pigment Rhodopsin, GFP (green fluorescent pigment) found in a large number of marine animals (and the basis of the 2008 Nobel Prize in Chemistry) and, finally, a material that is not obviously optical at all: silk. Sometimes materials become ‘optical materials’ when you look at them in a new light. Silk and DNA have both been explored recently as optical materials. Apart from their high transparency, the attraction is their bio- compatibility and eco-friendly processing. These allow us to envisage, for example, medical optical sensing in which the sensor can be safely integrated with biological tissue. In the chapters of this book, biological micro- structures are discussed in the context of optical security devices, non-iridescent structural colour, ultra high absorption materials, photonic crystals and the manipulation of polarization.
We hope this book will stimulate and enable discussions between biologists, chemists, and physicists; doctors and engineers. Nature is no respecter of boundaries. There are surely many more interesting ideas to be borrowed from nature about how to use light in useful ways.