Biomimetics is on everyone’s lips and it is now difficult to imagine a future where it does not play a key role in the development of our society. The development of new materials is not unconcerned with this new discipline, though we must be aware of what we can obtain (and what we cannot) from imitating Nature’s strategies.
Living in a material world
The history of humanity begins with the development of civilisations that today we group into technological phases defined by the material that at any given time attained the highest degree of development (Stone Age, Copper Age, Bronze Age, Iron Age). Ever since, the development of the human being has been closely linked to his relationship with the materials that surround him: how to extract them, how to transform them, how to use them, how to synthesise them, how to recycle them… right from the earliest materials that man extracted from nature (timber, clay, stone, etc) to the application of heat treatments to the revolution in nanotechnology and nanomaterials.
Vapour-grown carbon nanofibres (s-VGCF) by Grupo Antolín. Photo: Xavi Padrós.
We are currently living in the silicon era, a new revolution that has propitiated the development of electronics and information and communication technologies. The technological challenges are the greatest ever faced by man in all his history. Despite having perfected the extraction of raw materials, dominated the synthesis of new materials, developed processing and manufacturing technologies and used different sources of energy for our activities, we have barely taken into account the consequences that all these phases had on our surroundings. Today we know that the environmental vector cannot be neglected in our activities; beyond greenwashing and the regulations, it has to be considered as a factor of maximum importance in any development.
In this context, recent decades have seen the emergence of a new discipline called bionics or biomimetics. These terms became popular as the result of the publication of the book Biomimicry: Innovation Inspired by Nature (1997) by Janine Beynus, which deals with “a new science that studies models from nature and imitates or is inspired in these designs and processes to address human problems”.
Biomimetics = Sustainability?
Science and Engineering have always had nature as a model and have used it to prosper; however, in recent times this natural study has become systematised, coherently involving professionals from different disciplines (biologists, designers, physicists, engineers, chemists, etc) to maximise the benefits extracted from the knowledge of Nature. While currently it still contains secrets that we cannot decipher, there is no doubt that the mimicry of natural processes, materials and solutions will be one of society’s routes to development and innovation.
At this point we have to stop and reflect: is biomimetics the universal solution to our environmental problems? The answer is no. Biomimetics is a tool under development and a source of innovation; a “new” (insofar as it refers to systematisation) starting point and approach to the search for occasional solutions to the challenges set by technological development. And we cannot always obtain the sought-for answer from Nature; at this point, as researchers well know, we need to change the model and continue to probe. But there is still a tendency to directly associate biomimetics with sustainability, as if the former unequivocally involved the latter. There is no doubt that Nature can teach us much about how to protect life and resources (she has been doing it for millions of years), but knowing how to properly channel the information she provides towards developments that represent an environmental advance depends on us only insofar as it helps in “limiting the damage to the environment”.
Bio-inspiration
Returning to the central theme, the research into and development of new materials represents one of the fields with the greatest potential for biomimetics. At a first level we can directly imitate a natural solution, as in the well-known case of Velcro. In a second, deeper phase, we can extract Nature’s productive processes and imitate them in order to optimise the production of materials, for example developing low-energy processes or those based on self-assembly (something like what Ecovative has achieved with Ecocradle).
Ecocradle. Planstarter from EcovativeLastly, the highest degree of application of biomimetics will be one that succeeds in interconnecting all the other processes, imitating the holistic workings of ecosystems. At this point we may find a parallel with the Cradle to Cradle philosophy.
There are hundreds of examples of how materials science and engineering have used this tool to suggest (and in many cases attain) new routes to development (you just have to look at websites such as asknature.org), and the lines of research in this regard are increasingly numerous. The technical challenge posed by many such investigations is evident, but perhaps the true challenge lies in knowing how to identify any strategies from Nature that are susceptible of setting out on the road towards a new advance.
In this regard, “On Growth and Form” by D’Arcy Thompson (published in 1917) is an excellent starting point for the researcher, designer, architect, etc who wishes to examine the secrets of Nature in depth, especially at a formal and structural level. One of the most beautiful examples featured in this volume refers to what for us is one of the most intimate parts of nature: the design of the bones that hold up our body. In 1866 the engineer Carl Culmann discovered a replica of the design of his new crane in the section of a human body that was being analysed by an anatomist. Observable in this section was a grid that exactly followed the diagram of the stress lines, or stress and compression directions, in the loaded structure of the crane.
Crane head (left) and femur (right). As per Culmann and J.Wolf (reproduced from D’Arcy W. Thompson’s “On Growth and Form” (1961), Cambridge University Press)
In human bones, and in any that must withstand weight in general, the internal cavity is occupied by the bone marrow, blood vessels and other tissue; and in the midst of these living tissues we find the “spongy tissue” formed from small interlinked bone “trabeculae”. In a longitudinal section of the femur we see how the trabeculae are arranged in curved lines from the head to the hollow shaft of the bone, crossing each other orthogonally insofar as possible. As D’Arcy Thompson states, “the anatomical arrangement of the trabeculae exactly follows the mechanical distribution of compressive and stress forces or, in other words, it is exactly consistent with the theoretical load diagram of the crane”. As the bone grows, the areas exposed to greater stress develop a larger amount of mass.
Researchers at the International Development Center at Adam Opel GmbH developed a 3D optimisation software that imitates this biological growth. Eliminating superfluous components permits obtaining an optimised structure with a minimum amount of material. Though originally it was designed for the components of a car body, the software has been applied by the designer Joris Laarman in the “Bone Furniture” project. Just as trees add material wherever it is required to secure the structure, and the bones make use of the trabeculae to achieve an efficient and secure design, the different pieces in the “Bone furniture” series attain an optimised structure through the use of the Opel software. The “Bone Furniture” series comprises 7 pieces in different materials and finishes. The “Bone chair”, in polyurethane, is manufactured in a single piece from a ceramic mould generated through 3D printing.
A spiders web
If after this first look at our own nature we take another step forward, we encounter the spectacular diversity constituted by those beings with which we share a major part of our DNA. The animal kingdom, in the eyes of biomimetics, is an infinitely rich overview of the testing ground that is evolution. Among them, spiders have fascinated human society since the times of Antiquity. Today, their silk is one of the greatest exponents of our interest in imitating the materials produced by nature.
The spider web, an example of an efficient structure.
Its capacity to withstand high loads while experiencing major deformations represents an exceptional landmark in both natural and artificial materials. The mechanical properties of silk fibre have not yet been surpassed by those of artificial fibres. This combination of properties places spider silk in an exceptional position with regard to metallic and ceramic fibres (which can withstand high loads before fracture occurs, but whose deformation capacity is reduced) and elastomers (which undergo major deformations before fracture, but do not withstand high loads).
Manuel Elices, professor of Materials Science at the Universidad Politécnica of Madrid (UPM), has devoted considerable effort to acquiring in-depth knowledge of spider silk and has gained a few key insights that bring us closer to understanding this extraordinary material.
The silk threads of the major gland (the strongest ones), in comparison with artificial fibres, have resistance to breakage in the same order of magnitude (1.2 GPa in Argiope trifasciata and up to 4 GPa in other species) against the 3, 3.6 and 5.8 GPa of steel, Kevlar and PBO (Zylon) respectively. Where the difference does appear significant is in the deformation under breakage, which can be as high as 30% in spider silk, far higher than the percentage in other fibres. The combination of these two properties means that the energy stored before breakage is exceptionally high: between 100 and 130 MJ/m3.
In turn, the viscid thread (the outer rim thread that serves to capture prey) boosts deformability over strength. It permits an elongation of around 300%, which means lower strength requirements.
Even though we can characterise some of its mechanical properties relatively well, it is still difficult to find the correlation between the microstructure of the fibre and its properties. The major gland (which makes the draglines and spokes, the outer rim and the lifeline) has been the most widely studied: it generates silk comprising two proteins of which the DNA sequence that generates them is partially known. The superstructure formed from these fibres (the spider web) is also a marvel of engineering, as it is optimised to spread out the forces to which it is subjected in order to trap prey and be tolerant to damage. In this structure 4 types of thread are distinguishable, formed in turn from sets of strands: the draglines, the outer rim, the spokes and the capturing lines. The strength of a spider web resides in its configuration: each component has a different degree of stiffness (the draglines and outer rim are more rigid) so that all of them withstand a similar degree of stress. Furthermore, spiders periodically tighten the threads to better spread out the tensions.
(sxc.hu)
When receiving the impact of prey, the spider web uses two strategies to absorb and dissipate part of the impact energy. On one hand, this energy is partially invested in deforming the silk fibres, which exhibit a viscoelastic performance. At the same time, thanks to the friction of the fibres with the air, the web succeeds in dissipating another part of the energy. When the web suffers localised damage, its capacity to redistribute the tensions that the threads have to withstand allows the effects to be minimal. But it is not only the mechanical properties of spider silk that have attracted the attention of researchers. Xinwei Wang, associate professor of Mechanical Engineering at Iowa State University and his team recently established their extraordinary thermal conductivity (λ). The registered value was 416 W/(K·m); for copper, for example, it is 401 W/(K·m). If we compare these results with other biological materials, we observe that spider silk conducts heat 1,000 times better than silkworm silk and 800 times better than other organic fabrics. This is not the only surprising phenomenon that has been observed; in addition, the silk’s thermal conductivity increases when the fibre is stretched. According to Wang’s studies, an elongation of 20% also produces an increase in λ of 20%. The majority of materials lose thermal conductivity when they are stretched. We find the response to all these properties in the molecular structure of spider silk when free of defects, as well as in the proteins and structures that keep them joined together. For Wang, however, further research would be needed to completely understand the thermal properties of spider silk.
As with other examples of biomimetics, the great challenge of research consists in being able to generate these fibres industrially. For this it is firstly necessary to identify sequences of amino acids linked to the composition of the silk threads to be able to express them with a bacterium such as E.Coli. Once this is done, it will be necessary to spin the protein solution and manufacture the fibre. Major companies have sought to overcome this challenge, though without success for now.
The lotus effect
We end this short overview by turning to the Plantae kingdom, where we find one of the classic examples of application of biomimetics: the lotus effect.
(Lotus SXC.hu)
It may be a cliché but no less interesting for it, though we ought to deal with it from a merely scientific perspective (leaving aside certain almost “mystical” approaches that we often encounter). Controlling the wettability of solid surfaces is an aspect that is increasingly engaging the interest of researchers and industry. Wettability is measured by the contact angle (CA) of a drop of water on a solid surface. Depending on this angle, we classify the surfaces according to special characteristics such as super-hydrophobic (CA > 150º), hydrophobic (150º < CA < 90) and hydrophilic (CA < 30º).
Super-hydrophobic surfaces can allow water and even oil to slide over them without leaving any residue or even while trapping any surface contamination (self-cleaning properties). This property does not depend only on the CA (which is a “static” measure of wettability) but also on the capacity of the drop to slide over the surface (“dynamic” wettability). A drop of water on a hydrophobic surface can adopt one of two states: a ‘Wenzel’ state in which the drop enters into intimate contact with the peaks and troughs of a rough surface, or a ‘Cassie’ state in which the drop remains in contact only with the peaks on the surface. Even if the CA is high, it is only in this last state where the drop is free to roll along the surface, dragging dirt with it, as water and dirt have more affinity between them than either of them with the surface.
Dynamic wettability: (a) Wenzel state; (b) Cassie state
The best-known example of a self-cleaning surface is the lotus flower, whose effect was discovered in the late 1970s by Wilhelm Barthlott, although as far back as 1945 textiles had already been developed with greater hydrophobic properties than the lotus flower. Barthlott, attracted by the possibilities of the scanning electron microscope (SEM), observed that some plants barely needed prior cleaning to be observed with this technique. Outstanding among them was the lotus flower. His discoveries translated into commercial applications for self-cleaning surfaces, and he himself registered the “Lotus-Effect®” trademark.
The sacred lotus (Nelumbo nucifera) has traditionally been a powerful symbol as the representation of purity in the religions of Ancient Egypt, India and China owing to the fact that its leaves emerge clean from the muddy water in which it grows. The rain sweeps dirt from the surface of its leaves more easily than from other plants.
What Barthlott discovered in the lotus leaf was the combination of two properties on its surface: on one hand, the roughness on a micrometric scale that covers the entire leaf; on the other, a coating of epicuticular wax crystalloids (in themselves hydrophobic) on these protuberances. Thanks to these two properties, the leaf exhibits a super-hydrophobic character: when the drops enter into contact with the plant, they retain an almost spherical shape with a contact angle of more than 150º. This occurs thanks to the air trapped in the surface roughness, as the drops are deposited in contact with the crests of the protuberances.
Although the case of the natural self-cleaning surface is that of the lotus leaf, there are other similar cases such as the taro leaf or the rice leaf. In these, similar microstructures are observable, though with small differences and with the CA being of the same order in all of them. Various studies reflect the possibility of replicating the above-mentioned microstructures by using the surface of the lotus leaf as a template. Studies have been published in parallel that demonstrate that in order to obtain a super-hydrophobic surface it is not required to replicate the hierarchical structures we have seen: uniform structures have been developed that can produce better performance than hierarchized structures (with CA of up to 175º).
The applications of the lotus effect are well known in industry and are among the most successful cases of commercial application in biomimetics. StoCoat® Lotusan® is an acrylic water-based paint that generates a super-hydrophobic surface, imitating the self-cleaning properties of the lotus. The product was developed from William Barthlott’s patent and registered trademark (Lotus-Effect®) and is being commercialised since 1999. It is applied on vertical concrete, masonry or plaster walls.
Surface coated in StoCoat® Lotusan® surface (photo by Materfad)
The composition of StoCoat® Lotusan® is 1-5% Trietoxi(2,4,4-Trimetilpentil) Silane, 15-40% Cristobalite (SiO2) and 10-30% TiO2 [22]. The surface microstructure generated produces a contact angle of 140º and is therefore considered a hydrophobic surface. It must be pointed out that the presence of TiO2 in the composition is not down to its photocatalytic properties but to its capacity to provide roughness and control the forming of a hydrophobic surface.
A Spanish company, Tecnan S.L., markets Tecnadis PRS®, a hydrophobising composition based on nanoparticles with high water repellence that can be applied on porous substrates such as natural stone, brick, roof tile, concrete, wood, etc. The treatment is superficial and is recommended for facades, vertical or sloping surfaces. The composition of Tecnadis PRS® is a dispersion of nanoparticles (with an average size of 15 nm) treated with active agents and other additives in a basic solvent without VOC (volatile organic compounds).
Application of the Tecnadis PRS® hydrophobising composition on a ceramic surface (photo by Pablo Axpe)
It presents the major advantage over traditional water-repelling agents (based on siloxanes, polysiloxanes or silicones) that it does not form a film or barrier on the substrate, thus allowing the material to fully breathe while preventing water penetration (from the effect of rain, splashing, etc) and preventing humidity and condensation.
Nor does it react with the material on which it is applied or change its original colour or texture. Owing to its composition, the drops slide off the surface very easily, dragging with them any dust and dirt that may be present on it and allowing the surface to remain clean for much longer. It is totally compatible with any construction material, can be re-applicable and have a durability of the protective effect against water of more than 10 years. One of the recent applications of this product has permitted the protection of problematic areas on the outer surface of the cathedral of Santiago de Compostela.
Despite these efforts, the real applications of the lotus effect are still limited because of the ease with which they can contaminate the surfaces. A minimal reduction in hydrophobicity or in the depth of the surface structure (for example through the effect of a monolayer of an oil) can destroy the super-hydrophobic effect. The lotus flower can address this through growth and self-repair, but for our artificial surfaces this is still a challenge.