Welcome Audience

1. Welcome Audience

2. WORLD OF BIOMIMETICS AND BIOMIMETIC MATERIALS : THE DESIGN OF NATURE Dr. K. M. GUPTA Professor, Department of Applied Mechanics, Motilal Nehru National Institute of Technology, Allahabad-211004, INDIA

3. WORLD OF BIOMIMETICS AND BIOMIMETIC MATERIALS : THE DESIGN OF NATURE • Biomimetics is a new field of materials science. It opens a new era of future of engineering. Biomimetics materials are novel materials having unique characteristics. • They possess nature graded property and are being tried in almost all fields of engineering. Due to their versatility of behaviour, their concepts are now used in aircraft construction, robot construction and industrial bearings etc. • The study and simulation of biological systems with desired properties is popularly known as biomimetics. Such an approach involves the transformation of the principles discovered in nature into manmade technology.

4. Biomimetics, already popular in the fields of materials science and engineering, is being applied in diverse areas ranging from micro/nano-electronics to structural engineering and in tribology. • Design solutions can draw inspiration from many sources, including the anatomy, physiology, and behaviour of living systems. • Industry also imitates nature. It is well-known that the Wright brothers were bird watchers and their airplane wing design was modeled after birds. • In recent times, scientists have begun to take more ideas from nature with the explosion in biotechnological progress.

5. Biomimetics is currently being used to explore a variety of design projects, including the development of different biomaterials (most notably spider silk) as well as robots based on animal models. • In 1960′s Jack Steele, a researcher of the United States, used the word Bionics, referring to copying nature or taking ideas from nature. • Biomimicry refers to the study of nature’s most successful developments and then imitating these designs and processes to solve human problems. • KEYWORDS : Spider silk, Biomimetic robot, Shark skin effect, Snake scales, Gecko effect, Lotus effect.

6. 1 Biomaterials • One major application of biomimetics is the field of biomaterials, which involves mimicking or synthesizing the natural materials and applying this to practical design. • There are many examples of materials in nature that exhibit unique useful properties. • One of the major advantages of biomaterials is that they are normally bio-degradeable. • In addition, the extreme temperatures and hazardous chemicals often used in man made construction are usually unnecessary with natural alternatives. • 2 Spider Silk • Spider silk is one of the most sought after biomaterials. This material produced by special glands in a spider’s body, has the advantage of being both light and flexible.

7. It is roughly three times stronger than steel. The tensile strength of the radial threads of spider silk is 1,154 MPa while that of steel is 400 MPa only. • For a flying insect to be caught, the spider’s web must slow its motion to a halt by absorbing kinetic energy. • The force required to stop the insect’s motion is inversely proportional to the distance over which the motion must be stopped. • The longer the distance over which the insect is slowed down, the smaller the force necessary to stop it, reducing the potential for damage to the web.

8. The incredible properties of spider silk are due to its unique molecular structure, Fig.7.2. • X-ray diffraction studies have shown that the silk is composed of long amino acid chains that form protein crystals. • The majority of silks also contain β-pleated sheet crystals that form from tandemly repeated amino acid sequences rich in small amino acid residues. • The resulting beta-sheet crystals crosslink the fibroins into a polymer network with great stiffness, strength and toughness. • This crystalline component is embedded in a rubbery component that permits extensibility, composed of amorphous network chains.

9. It is this extensibility and tensile strength combined with its light weight that enables webs to prevent damage from wind and their anchoring points from being pulled off. Fig. 7.2 The structure of a strand of silk [1]

10. Likely applications of spider silk [1] • Despite the high demand for spider silk as a building material, the difficulties surrounding its harvest have precluded large scale production. • A new biotechnology firm (in Quebec, Nexia Biotechnologies) has successfully expressed the silk genes of two spider species in the milk of a transgenic goat. • This technology could have applications in the field of medicine as a new form of strong, tough artificial tendons, ligaments and limbs. • Spider silk could also be used to help tissue repair, wound healing and to create super-thin, biodegradable sutures for eye or neurosurgery, as well as being used as a substitute for Kevlar fibre.

11. 3 Biomimetic Robot: Chemistry, Life And Applications • A second application of biomimetics is the field of robotics. Animal models are being used as the inspiration for different types of robots. • Researchers closely study the mechanics of various animals and then apply these observations to robot design. The goal is to develop a new class of biologically-inspired robots with greater performance in unstructured environments. • One example is to mimic the leg and joint structure of animals for use in robot mobility such as modelling the joint and leg structure of the cockroach for the development of a hexa-pedal running robot.

12. Researchers are using biomimicry of the cockroach, one of nature’s most successful species, to design and build sprawl-legged robots that can move very quickly, Fig. 7.3. • In addition, these robots are very good at manoeuvring in changing terrain, and can continue forward motion when encountering hip-height obstacles or uphill and downhill slopes of up to 24 degrees. • These types of small, fast robots could potentially be used for military reconnaissance, bomb diffusion and de-mining expeditions. Fig . 7.3 The cockroach leg is a prime candidate for biomimicry for a robot [1] 

13. 4 Shark Skin Effect • Friction between a solid surface and a fluid can also be considered a tribological phenomenon. • Inspiration from aquatic animals’ surface material and texture would benefit the design of surfaces that could increase efficiency in cases such as underwater navigation. • One such example is the ‘Shark Skin Effect’. Sharks’ skin has grooved scales on its entire body. The scales are directed almost parallel to the longitudinal body axis of the shark.

14. The presence of this non-smooth surface texture on the shark skin effectively reduces the drag by 5%-10%, Fig. 7.4a-b. Fig . 7.4 (a) Shark, (b) riblets and grooves found on shark skin,(c) a swimming-cloth with similar surface as a shark skin could reduce frictional resistance, (d) an aircraft coated with a plastic film that has similar microscopic texture as found on a shark skin [2].

15. Benefit of this property has been utilized in following applications. • Swimsuits with biomimetically designed surfaces that mimic the non-smooth surface texture on the shark skin, have proved to be faster than conventional suits, as they reduce drag along key areas of the body, Fig. 7.4c. • Surface texture such as those on sharks’ skin also helps to reduce the friction between a solid surface and air. A transparent plastic film with similar microscopic texture (ribs parallel to the direction of flow) reduces aircraft drag by about 8% and is effective in saving fuel by about 1.5%. • The commercial aircraft Cathay Pacific Airbus 340 [2] already has been fitted with ribbed structures on its body surface, Fig. 7.4d.

16. 5 Snake Scales • Studying the frictional surfaces of snake scales would benefit when designing the surfaces with anisotropic frictional characteristics. • Snakes have friction-modifying structures consisting of ordered double-ridge micro-fibrillar array. • The double-ridge micro-fibrillar geometry provides significant reduction in adhesive forces, thereby creating ideal conditions for sliding in forward direction with minimum adhesion. • Fig. 7.5 Frictional surfaces of snake

17. Meanwhile, the highly asymmetric profile of the micro-fibrillar ending induces frictional anisotropy, as it acts as a locking mechanism prohibiting the backward motion. • Snake skin also has micro-pores that deliver an anti-adhesive lipid mixture, which further facilitates easy motion owing to boundary lubrication.

18. 6 Gecko Effect • Creatures such as beetles, flies, spiders and lizards have the ability to attach themselves to surfaces without falling off, even when the surfaces are vertically inclined. • The presence of micro/nanostructures—small hairs called “setae” on their attachment pads—enables these creatures to attach and detach easily over any surface. • As the mass of the creature increases, the radii of the terminal attachment structures decrease while the density of the structures increase, Fig. 7.6a-d. • The gecko is the largest animal that has this kind of dry attachment system and, therefore, is the main interest for scientific research, Fig. 7.6e-f.

19. Fig. 7.6a-d Terminal elements found on the attachment pads of various insects and gecko. As the size (mass) of the creature increases, the radius of the terminal attachment structures decreases, while the density of the structures increases, (e) Gecko, (f) SEM image of gecko setae [2].

20. In functional terms, the tiny hairs found on gecko feet are able to conform to the shape of surface irregularities to which the gecko is adhering. • By mimicking the shape and geometry of gecko setae, synthetic adhesives have been made from polymeric materials. • An example is ‘Gecko Tape’, which can be used for several detachment-attachment cycles before degradation of its adhesive property. • This tape has arrays of flexible polyimide pillars fabricated using electron beam lithography and dry etching in oxygen.

21. To create a gecko adhesive, the pillars must be sufficiently flexible and placed on a soft, flexible substrate so that the individual tips can act in unison and attach to uneven surfaces all at the same time. • To demonstrate the effectiveness of the gecko tape as a dry adhesive, a Spider-man toy was attached to a glass plate through the microfabricated gecko tape.

22. 7 The Tread Effect • Another of man’s creations analogous to a principle found in nature, namely the distinctive patterns on a tree frog’s foot, is the design of treads on automobile tires. • Tree frogs such as Amolops sp. possess large disc-like pads at the tip of their toes that assist them in attaching to surfaces such as leaves, Fig. 7.7a-c. Fig. 7.7 (a) a tree frog, (b) schematic of flat-topped cuboidal columnar cells separated bycanal-like spaces in tree frogs (c) a close-up view of treads on a car tire [2].

23. Automobile tires have treads on them. • The pads consist of flat-topped cuboidal columnar cells, which are separated from each other by canal-like spaces. • During climbing, water gets squeezed out from the contact through the channels between the foot and the surfaces on which they climb, making a perfect van der Waals contact. • While driving on wet roads, water flows out through the channels found between the treads, giving rise to intimate contact between the treads and the road, thereby creating sufficient grip during motion. • This effect is known as the ‘Tire Tread Effect’.

24. 8 Wear - Resistant Surfaces • Wear-Resistance of Sandfish in Desert • Man has begun to learn from nature to design materials and textures that have superior wear-resistance characteristics. • For example, a sandfish, living in the Sahara desert moves rapidly over the desert sand and the scales on its body have excellent sand erosion wear resistance. Fig. 7.8a Sand Fish [Source : ds-lands.com]

25. Erosion experiments conducted using sand on the sandfish’s scales, glass and soft steel for 10 hours showed that the wear trace on the sandfish’s skin was the smallest, which suggests that its wear resistance is comparatively much higher than that of glass and soft steel. • The biomaterial comprising the sandfish’s scales and their surface texture together contribute toward its high wear resistance.

26. Erosion-Resistance of Mollusc (Conch) Shells on Sandy Beach [2] • Mollusc (conch) shells experience water-sand erosion on sandy beaches. • Their anti-wear mechanism arises from the combination of their bio-tissues and their unique shape, which together prevent abrasion. • The biomaterial of mollusc shells is a bio-ceramic composite that has a complex microstructure, the result of a billion years of evolution. • Understanding the formation and microstructure of these bio-ceramic composites will help in the design of ceramics with superior mechanical properties such as toughness.

27. Fig. 7.8b Molluscs: Bivalves [Source : www.thecanadianencyclopedia.com]

28. Current methods used to fabricate ceramics still are unable to control parameters such as crystal density, orientation and morphological uniformity to the degree of perfection, the nature has achieved in mollusc shells. • Studies on the microstructure of bivalve shells are expected to provide guidelines in developing biomimetic composite materials with better tri-bological properties. • Pangolin Scales [2] • A pangolin is a soil-burrowing animal with a layer of scales covering its body. • As the pangolin burrows into soil, its scales are subjected to wear. A study on the chemical constitution of pangolin scales revealed the presence of 18 amino acids.

29. The protein in the scales mainly consists of α-keratin and β-keratin. • The specific elongation of pangolin scales is about 15% due to the presence of the proteins. • Therefore, the plasticity of the pangolin scales is low, resulting in anti-abrasive characteristics. • Studies on biomaterials that exhibit remarkable friction and wear characteristics would provide insights toward enhancing the performance of soil-engaging engineering components such as those in agricultural machinery and earth moving machinery.

30. 9 Lotus (Or Self-cleaning) Effect • A number of plants have water-repellent leaves, which exhibit super-hydrophobic property. • Lotus (Nelumbonucifera) is the most popular example (water contact angle ~ 162 degrees). • The unique ability of Lotus leaf surface to avoid wetting, popularly known as the “Lotus Effect,” is mainly due to the presence of micro-scale protuberances covered with waxy nano-crystals on their surface. • Colocasia (Colocasiaesculenta) is another example of a plant whose leaves are super-hydrophobic in nature (water contact angle ~ 164 degrees), due to the presence of micron-sized protuberances and wax crystals, Fig. 7.9a-f.

31. Fig. 11 (a) Lotus (Nelumbonucifera), (b) protuberances on the surface of a Lotus leaf, (c) a micro-electro mechanical system that has six gear chains, (d) the same MEMS device in comparison with the size of a dust mite, (e) nano-patterns that mimic the protuberances of a Lotus leaf, (f) Lotus-like (fresh) surface, (g) Colocasia-like (fresh) surface, (h) Colocasia-like (dry) surface [2]

32. Both the protuberances and the wax crystals make the surfaces of Lotus and Colocasia leaves super-hydrophobic in nature, which means water droplets easily roll over leaf surfaces taking contaminants and dust particles with them. • This phenomenon is popularly known as the ‘Self-Cleaning Effect’. • The self-cleaning property is highly important for water plants. In their habitats, these plants observe the presence of free water which supports pathogenic organisms.

33. These plants protect themselves from water-borne infections by hindering any adhesion of water necessary for the germination of pathogens. • Another reason for water-repellency is the fact that CO2 diffuses 10% times slower in water than in air. • The presence of water-repellent surface ensures that sufficient intake of CO2 is observed for photosynthesis.

34. Reducing adhesion and friction in engineering applications • By studying the surface morphologies of water-repellent leaves, tribologists design and create hydrophobic surfaces to reduce adhesion between surfaces at small scales, which arise due to water condensation. • Scientists and engineers are aware of the fact that surface forces such as adhesion and friction significantly oppose easy motion between tiny elements in miniaturized devices such as micro/nano-electromechanical systems (MEMs/NEMs). • Minimizing the surface forces such as adhesion and friction, and also the occurrence of wear in miniaturized devices is a real challenge, as the size of these devices is extremely small (usually their sizes are smaller than insects such as dust mites).

35. Among the various attractive forces that contribute to adhesion at small-scales, the capillary force that arises due to the condensation of water from the environment is the strongest. • Hence, there arises a need to modify surfaces at nano/microscale in order to achieve increased hydrophobicity that would drastically reduce adhesion due to capillary force, which in turn would also reduce friction.

36. 10 Biomimetic Human Joints • The design of lubricated bearings also can benefit greatly from the knowledge of mechanisms at work in biological joints. • Biological joints are much different than the conventional industrial bearings. • Friction, wear and lubrication of surfaces greatly affect the reliability and efficiency of the various joints in the human anatomy. • When these joints fail, such as with severe osteoarthritis, the joints may require surgical repair or replacement. • However, the materials (metals, ceramics and polymers) used in most artificial joints are much different than the actual biomaterials in a human joint and are more closely related to industrial bearings.

37. A typical joint usually has a softer layer of cartilage and other materials separating the hard bone. • The natural joints use soft material much more liberally than hard material, and perhaps it is to their advantage. • More recent approaches use softer materials that are more similar to the original tissue such as hydrogels and actual living cells to replace joint surfaces. • A large number of works have numerically modeled the elasto-hydrodynamic lubrication of natural joints.

38. Even then, the fundamental mechanical models used are essentially the same as are used to consider the industrial hydrodynamic bearings in rolling element bearings. • Recently, numerical modeling has been used to design prototype surfaces designed to be soft and deformable like biological joints. • These biomimetic self-adapting surfaces change their surface profiles at the nano and micro-scale to improve performance. • The mechanism is similar to that seen in gas foil bearings.

39. 11 Conclusions • The biomaterials will be very much useful in bio-mimeting the robots, the swimsuits, tyre tread design, wear-resistant surfaces, self-cleaning systems, etc. • The study of bio-mimetics will also help the tribologists in designing the systems with reduced adhesion and friction. • The biomimetic MEMs and NEMs and biomimetic tribological surfaces are also the likely outcome. • Finally, the human joints can also be bio-mimetized. Thus, the already popular in materials science and engineering, bio-mimetics now is emerging in tribiology. • Its very fine application is found in the outer surface of the Cathay Pacific Airbus 340, modelled after the grooved scales of shark skin, reduces drag and improves fuel efficiency.

40. REFERENCES Sean Kennedy, ‘The Science Creative Quaterly, Issue 6, Biomimicry/Biomimetics : GeneralPrinciples and Practical Examples’, August 2004. Dr. R. Arvind Singh, Dr.Eui-Sung Yoon, Dr. Robert L. Jackson; ‘Science of ImitiatingNature, Tribology and Lubrication Technology’, February 2009, pp. 41-47, www.stle.org. Tom Mueller, Photograph by Robert Clark, 'Biomimetics – National Geographic magazine', http://ngm.nationalgeographic.com/2008/04/biomimetics/tom-mueller