The natural world displays marvelous examples of light manipulation, processing, and light-driven actuation. This editorial presents a brief outlook of the field that we will broadly define as “bioinspired photonics,” that the editors of APL Photonics believe to be rich in opportunity for contemporary solutions that are, indeed, inspired by what surrounds us.
Color, iridescence, and dynamic coloration systems have arguably been early drivers of curiosity and discovery in the field, from the first writings in Lucretius’ De Rerum Naturae, to Newton’s descriptions of optical interference in 1730, to imaging-enabled observations of structural iridescences in peacock feathers and in butterflies from the mid-1920s to the mid-1940s. The synergy between advanced imaging techniques, micro- and nanofabrication tools, including colloidal assembly and electromagnetic wave predictive tools has been enabling to the replication of these naturally occurring structures with impressive demonstrations of photonic crystal lattices, aberration-free imaging systems, curvilinear optics, implantable and/or injectable micro- and nanoscale optical interfaces, among many others. Multiple examples of bioinspired and biomimetic optical systems exist in the literature, including several books on the subject.
In spite of its maturity, the field is replete with opportunity that can be identified by recognizing the context in which naturally occurring systems are generated and how this relates to the contemporary technological domains that are actively researched today.
Broadly speaking, contemporary photonics technology (and technology in general) commonly relies on a broad set of materials that are precisely synthesized and dosed, highly controlled, and manufactured with strict design rules that impose purity, repeatability, and control with resulting outcomes that adhere to performance metrics and reliability. The fabrication environments that accompany both fundamental and applied research in photonics are orderly environments that relied on sophisticated top-down manufacturing techniques with exquisite control of scale and interfaces.
Natural systems push against this paradigm and generate optical systems starting from a relatively limited set of naturally derived materials that are grown in variable conditions, adapting their composition reactively to their surrounding environment to optimize a particular function related to their livelihood and resulting in outcomes that are plurifunctional and adaptive. One of the many functional examples, for instance, is the tapetum lucidum of certain animals such as the Arctic reindeer (Rangifer tarandus) that increases visual acuity across a particular spectral range and adapts to the seasons to vary its spectral response through controlled variation of the intraocular pressure in response to low visibility conditions encountered by the animal in the winter – a pair of tunable sunglasses behind the retina based on a biological photonic crystal.
Perhaps one of the most daunting technical challenges to emulate lies in Nature’s ability to form materials with dimensional control over multiple orders of magnitude that span from the nano-to the macroscale. These systems have very elegant hierarchical geometries that are mostly formed in bottom-up fashion, through directed assembly. Of special interest are optical structures that are dynamic and adaptable, embedding active biochemical cues that allow them to react either physiologically or mechanically.
An approach of recent interest involves co-opting the same materials used by Nature to construct photonic systems while studying the assembly processes that lead to the formation of these structures. Some examples include photonic structures based on structural proteins such as keratins or silks, polysaccharides such as cellulose or starch (e.g., PLA, chitosan, or zein), or structures that assemble within living organisms such as lipids within cells, collagens, guanine crystals, and so on.
Besides the beauty of the structures themselves, exemplified by birefringent fibrous assemblies, iridescent plants both marine and terrestrial, reflectors and diffusers, and highly iridescent cellular layers, the main promise offered in this context is the opportunity to merge bottom-up synthesis driven by the assembly of these natural materials, with top-down transformation techniques that the photonics world (and the technological world in general) is accustomed to. Additionally, the broad biological compatibility of these materials affords the direct integration of photonic formats within living systems.
The natural drive for survival, mating, resource harvesting, energy management, and environmental adaptation imposes functional constraints to naturally assembled structures which, more often than not, present multiple functions in a single geometry. The simple familiar example of butterfly wing iridescence provides not only a marvelous example of growth of a photonic crystal—it is accompanied by a light, multilayered, mechanically robust structure suitable for flight, which also uses its surface structure to repel dust and water, while assisting in the thermal regulation of the animal. Multifunctional optical systems that are inspired by these paradigms are a direction of particular opportunity. Such convergence of function has started to develop in disciplines such as optofluidics, where photons and volumetric flow in macro- or microfluidic channels come together to play off of one another to add degrees of freedom for light modulation and light manipulation. The connection to natural systems expands such degrees of freedom further: systems such as a plant leaves have the capacity to actuate in response to photon flux, reorient, and manage their nutrient transport, respiration, and metabolism, providing a multiscale fluidic network that is surrounded by a light responsive, adaptable environment. The fluidic network of plants has been permeated with organic electronics devices, opening the door for systems that leverage the sophistication of biological signaling and communication for next-generation energy harvesters, environmental sensors, or optical device templates.
As mentioned above, the departure from inorganic materials in favor of naturally derived materials not only introduces the paradigm of bottom-up fabrication, but also new modalities of bulk-doping and controlled interactions between photons and biological matter in modalities that are not trivial to achieve otherwise. These opportunities are enabled by the benign environments in which these materials are assembled that, for the most part, avoid noxious solvents, high temperatures, pressures, and vacuum environments. Such opportunities are not without challenges, given that the systems are often generated in wet environments or through wet processing and require control of multiscale geometries under these conditions.
However, the inclusion of biological compounds directly within optical constructs enables a symbiosis between optical and biological function with unusual types of interfaces between photons and living matter. Initial examples have included active biological components within the bulk of biomaterials reshaped into periodic and aperiodic lattices, waveguides, diffractive optics, resonators, and lasers, to name a few. As an example, resonances in random lasers or high-Q microresonators are affected by the biological activity of the molecules that are included in the material constituents, offering unprecedented mechanisms for opto-biological transduction. To establish these approaches, managing specificity and sensitivity of biological interfaces is key, and resorting to optically resonant structures could open interesting options.
One of the fundamental biological tenets is to grow materials rather than build them. While biomimicry has led to the development of several naturally inspired structures, such as curved imaging sensors or compound lens arrays, the opportunity to “grow” optics is looming on the horizon. With access and availability to new biotechnology tools paired with the existing micro- and nanoscale fabrication backbone, an expanded kit of materials of optical relevance can be obtained through genetic synthesis and modification. An example is DNA origami that templates molecules into desired structures with nanoscale precision. While the use of genetic editing tools such CRISPR-Cas9 are widely covered in the literature for their medical promise, it is quite plausible that the same tools can be used to enhance functional outcomes of technical relevance such as increased luminescence or charge transport, for instance. A recent demonstration of this explored the manipulation of structural color in bacterial colonies through the isolation of the genetic code in control of their nanoscale assembly, exemplifying the possibility of “growing color.”
These themes are just a small sample of what lies ahead at the interface of technology and biology, exemplifying some of the transdisciplinary themes that APLP encourages and welcomes. Using living components as enablers for photonics opens the door to many of the functional behaviors found in natural systems and vice versa, with opportunities in sensing, energy harvesting, photoconversion and photocatalysis, along with optically based transduction systems that can empower new distributed sensing approaches that will allow for deeper knowledge of complex natural systems while offering new strategies for photonics discovery, technological development, and manufacturing.