Remarkable Natural Material Surfaces and Their Engineering Potential

Perhaps one of the biggest differences between most living organisms and engineered materials is the ability to adapt and heal in response to damage and degradation (Nosonovsky, Self-repairing materials. In: Bhushan B (ed) Encyclopedia of nanotechnology. Springer, Netherlands, pp 2382–2385, 2012). Because engineered materials generally lack the inherent ability to fix themselves and deteriorate over time due to degradation, engineers are researching and developing self-healing materials—materials that can recover from damage without external intervention—in hopes that this will not be the case for certain applications in which self-repairing ability would be particularly advantageous (Ghosh, Self-healing materials: fundamentals, design strategies, and applications. In: Ghosh SK (ed) Self-healing materials: fundamentals, design strategies, and applications. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, pp 1–28, 2009). Though healing mechanisms of living organisms are extremely complex, making it difficult to mimic them, nature is a source of bio-inspiration for researchers who are seeking to create self-healing materials for the future of engineering. One such source of bio-inspiration includes blood clotting, which has taken the form of encapsulation (White et al., Nature 409:794–797, 2001) and hollow glass fibers in self-healing materials (Trask et al., Bioinspir Biomim 2(1):1–12, 2007). Vascular systems are another source of bio-inspiration and have led engineers to create circulatory concepts that also enable self-healing (Andersson et al., Self healing polymers and composites. In: van der Zwaag S (ed) Self healing materials. An alternative approach to 20 centuries of materials science. Springer, Netherlands, pp 19–44, 2008). This chapter shows how and why engineers used these natural mechanisms to develop self-healing materials and delves into the ways these self-healing materials can revolutionize the field of engineering and technology with a variety of applications ranging from self-repairing glass, corrosion protection, innovative aerospace composites, and more.

Shark skin has been found to possess remarkable features that have important applications in the medical and engineering arenas. Shark skin is an active skin, meaning it is very closely connected to the animal’s thrust producing muscles that enable its undulatory, or wave-like, movement. This important organ is of a complex structure, a composite that is collagen fiber reinforced and pliant. Like any other vertebrate skin, it is composed of flesh, then dermis, and finally the outermost layer, which is called the epidermis (Naresh et al., J Biosci 22(4):431–437, 1997). Covering the whole shark are pointy placoid scales commonly referred to as dermal denticles (Gilbert, Endeavour 8(4):179–187, 1984). Though dermal denticles have a range of purposes, including reducing mechanical abrasion, reducing drag while swimming, and protecting sharks from ectoparasites and predators, a very important one is bacteria prevention. Sessile bacteria are conditioned to withstand adverse environmental situations and attach to surfaces to form a biofilm—a structured community of bacteria. Biofilms can be detrimental in many environments, particularly in the biomedical arena, as they are known to cause many infections, including osteomyelitis, native valve endocarditis, and more (Glinel et al., Acta Biomater 8(5):1670–1684, 2012). As an aquatic animal, sharks are perpetually exposed to bacteria, algae, and other forms of contamination from marine organisms (Bhushan, Beilstein J Nanotechnol 2:66–84, 2011). However, their dermal denticles prevent fouling organisms from adhering to the shark’s skin due to various reasons, which will be explored in detail in this chapter. Biomimicry of shark skin has led to the creation of Sharklet AF, the first micropatterned texture designed to prevent bacteria from colonizing and migrating (Reddy et al., J Endourol 25(9):1547–1552, 2011). Sharklet AF has been applied to healthcare products, other commercial products, and boats to successfully inhibit bacterial growth (Pogoreic, Texture of shark’s skin inspires a unique approach to bacteria control for healthcare, http://​medcitynews.​com/​2012/​12/​texture-of-sharks-skin-inspires-a-unique-approach-to-bacteria-control-for-healthcare/​, 2012).

Mollusca are a phylum of animals composed of at least 50,000 living species (Bunje, The Mollusca, http://​www.​ucmp.​berkeley.​edu/​taxa/​inverts/​mollusca/​mollusca.​php, 2003). They encompass ancient life forms that first appeared on Earth about 545 million years ago at the beginning of the Cambrian Period, including conch shells, abalone, clams, mussels, and oysters, among many more. While the bodies of Mollusca are soft, they are usually covered by hard shells that accomplish many important functions (Espinosa et al., Prog Mater Sci 54(8):1059–1100, 2009). As the primary barrier between a Mollusc’s soft body and the outer environment, these shells must provide protection to damage from predators, remain intact without shattering under tidal waves, and resist pressures around hydrothermal vents found in the deep ocean where many Molluscs reside, among many other functions (Kaplan, Curr Opin Solid State Mater Sci 3(3):232–236, 1998). Over time, numerous types of shells have developed—foliated and cross lamellar, prismatic, and columnar and sheet nacreous structures (Espinosa et al., Prog Mater Sci 54(8):1059–1100, 2009)—but due to the high strength of nacreous structures, they have become very popular among researchers. Many studies have confirmed the outstanding mechanical properties of the material, and this chapter highlights these properties while exploring the structural reasons for such excellence. Nacreous shells consist of a hierarchical structure that features an armor system on one level and brick-and-mortar architecture on another. The layered structure of tablets and soft protein enhances the mechanical properties of nacre by allowing sliding, which contributes to nacre’s high toughness (Denkena et al., J Mater Proc Technol 210(14):1827–1837, 2010). Tablet waviness is another important mechanism found in nacreous shells that distributes inelastic deformations so as to prevent failure (Espinosa et al., Prog Mater Sci 54(8):1059–1100, 2009). Finally, interlocking mechanisms between tablets encourage deformation and progressive failure, increasing toughness and reducing the chance of catastrophic failure (Katti and Katti, Mat Sci Eng C 26(8):1317–1324, 2006). Engineers have been inspired to create novel nano-composites that mimic the structure and mechanisms of nacreous shells in order to achieve superior mechanical properties (Luz and Mano, Philos Trans Math Phys Eng Sci 367(1893):1587–1605, 2009). For example, Tang et al. succeeded in creating a nano-scale version of nacre using organic and inorganic layers consisting of polyelectrolytes and clays (Tang et al., Nat Mater 2:413–418, 2003). In addition, Zhu and Barthelat created a prototype of a nacre-like material composed of poly-methyl-methacrylate (PMMA) tablets (Zhu and Barthelat, A novel biomimetic material duplicating the structure and mechanics of natural nacre. In: Proulx T (ed) Mechanics of biological systems and materials, vol 2. Springer, New York, 2011). Engineering applications also include using nacre itself as bone implants due to its biocompatibility (Denkena et al., J Mater Proc Technol 210(14):1827–1837, 2010). Based on the sheer amount of useful applications and innovations that nacre has bioinspired, it truly stands out as an engineering gem.

The Earth’s waters contain hundreds of thousands of different species of beautiful, microscopic glass crystals called diatoms. Among every single one of its many species, diatoms exhibit exquisite architecture of their shells, and their applications in engineering are growing ever more known in the scientific community. Diatoms are one of the most common microaquatic single celled algae. They are eukaryotic and photosynthetic and have an estimated 200,000 species and 250 living genera (Gordon et al., Trends Biotechnol 27(2):116–127, 2008). Diatoms exist in any body of water that has enough nutrients—oceans, lakes, rivers, ponds, and even in household aquariums. They exist in various forms: planktonic or free-floating, colonial or solitary, or attached to objects such as sea ice, rocks, or other algae (Leventer, Diatoms. In: Gornitz V (ed) Encyclopedia of paleoclimatology and ancient environments. Springer, Netherlands, pp 279–280, 2009). To the naked eye, diatoms take on the appearance of scum at the top of the ocean, lake, or even the back of a whale. The slimy brown patches present on rocks in rivers are actually layers of diatoms. While they look like muck, the cells are actually surrounded by beautiful glass silica shells called frustules, and these frustules have a hierarchical structure (Dimas and Buehler, Hierarchical mechanics of diatom algae: from atoms to organism and weakness to strength, http://​imechanica.​org/​node/​11366, 2011). In addition to the astonishing beauty of diatom shells that has captivated artists, scientists, and researchers, diatoms have a wide variety of current uses including DNA purification, liquid absorbents, and matting agents. Researchers are also looking at using diatoms in the development of nano-scale biosensors (Marshall et al., PLoS One 7(3):e33771, 2012), drug delivery vehicles (Gordon et al., Trends Biotechnol 27(2):116–127, 2008), and solar panels with optimal absorption of solar energy (Norwegian University of Science and Technology, A bright future—with algae: diatoms as templates for tomorrow’s solar cells, http://​www.​sciencedaily.​com/​releases/​2012/​07/​120717100117.​htm, 2012).

The lotus flower is famous around the world across many cultures and religions. For example, it is considered to be a sacred flower by Buddhists, symbolizing cosmic harmony and spiritual illumination. It is even believed to have birthed the sun, an Egyptian myth inspired by the fact that the flower opens in the morning and closes by evening (The Flower Expert, Lotus flowers, http://​www.​theflowerexpert.​com/​content/​aboutflowers/​exoticflowers/​lotus, 2005). What many people do not know, however, is that its counterpart—the wide, flat leaf of the lotus plant—albeit not as bright and delicate as the flower, is a hidden beauty all its own. The lotus leaf has been the subject of great interest in recent scientific research because of its superhydrophobicity, which is otherwise known as the Lotus Effect (Forbes, Self-cleaning materials: lotus leaf-inspired nanotechnology, http://​insurftech.​com/​docs/​links/​Related-Papers/​Article-1-Scientific-American-Self-Cleaning-Materals-Lotus-Effect.​pdf, 2008). The lotus leaf’s superhydrophobic, self-cleaning surface is seen as an evolutionary advantage that benefits the life and longevity of the plant in a variety of ways, such as decreasing the leaching of nutrients and reducing dust and contaminating particles that can inhibit photosynthesis (Solga et al., Bioinsp Biomim 2:S126–S134, 2007). The wettability of the lotus leaf surface is one mechanism that contributes to this intriguing characteristic. Wettability is determined by measuring the contact angle between the liquid and the surface, and contact angle is governed by Young’s law (McHale et al., Beilstein J Nanotechnol 2:145–151, 2011). The surface roughness of lotus leaves also plays an important role in the leaves’ superhydrophobicity and is characterized by a hierarchical structure consisting of papillae and nano-scale, tubule-like asperities that minimize contact area (Bhushan and Jung, Prog Mater Sci 56(1):1–108, 2011). The wonders of the Lotus Effect have been taken advantage of in a wide variety of markets, ranging from self-cleaning windows to stain-resistant clothing. Self-cleaning paint has also been developed based on the Lotus Effect, which helps prevent the growth of numerous fungi, algae, and bacteria that attach to and colonize building structures, diminishing the need for harmful biocides (Solga et al., Bioinsp Biomim 2:S126–S134, 2007). As is evident in this chapter, despite being the seemingly humble counterpart to the elegant lotus flower, the lotus leaf boasts many remarkable characteristics that engineers have been inspired by and have tried to mimic.

Over history, dragonflies have been found across the globe, allowing a rich multitude of culture and symbolism to be developed around these four-winged creatures. For example, in Native American history, dragonflies were symbols of activity and swiftness and were often associated with horses. In Japanese history, these insects were considered to serve as winged mounts for the Hotoke-Sama, or August Spirits of the Ancestors. Among Buddhists, the Hotoke-Sama were thought to return on August 15th, riding dragonflies into their old homes to be reunited with their families (Mitchell and Lasswell, A dazzle of dragonflies. Texas A&M University Press, College Station, 2005). Though such folklore may have diffused over time as scientific research dedicated to dragonflies began, fascination with their flying and maneuvering capabilities has not, perhaps even increasing in recent years. One look at the dragonfly’s impressive flying abilities can convince that this attention is well afforded: they can fly sideways, forwards and backwards, hover in midair and reverse directions instantaneously, accelerate rapidly, and fly as fast as 50 km/h (Rajabi et al., J Bionic Eng 8:165–173, 2011). Although dragonfly wings account for less than 2 % of the total body mass, they are the main enablers of such diverse flight behavior. The membrane of dragonfly wings is thin, transparent, and film-like, supported by a framework of veins (Sun and Bhushan, CR Mecanique 340:3–17, 2012). It is also layered and superhydrophobic (Song et al., Mat Sci Eng A 457(1–2):254–260, 2007). Wing corrugation increases strength and stiffness and its ability to absorb stress against bending in the spanwise direction (Sun and Bhushan, CR Mecanique 340:3–17, 2012). Wing vein structure is hierarchical, consisting of a sandwich structure on the primary level and a multilayered chitinous shell and protein fibril structure on the secondary level (Chen et al., J Bionic Eng 9:185–191, 2012). Finally, micro- and nano-scale ripple morphologies reduce pressure drag during flight (Shelton, Probing question: how do dimples make golf balls travel farther? http://​news.​psu.​edu/​story/​141235/​2007/​06/​18/​research/​probing-question-how-do-dimples-make-golf-balls-travel-farther, 2007), while vein-joints contribute to wing flexibility (Donoughe et al., J Morphol 272(12):1409–1421, 2011). All of these properties make dragonfly wings an optimal source of bioinspiration for micro-air-vehicles (MAVs) compared to other animals such as hummingbirds and butterflies. Novel designs of MAVs have already been developed based on research of the dragonfly (Ratti and Vachtsevanos, J Intell Robot Syst 65:437–455, 2012).

Moths have compound eyes, meaning they are faceted and consist of many repeated, anatomically identical unites called ommatidia (Stavenga et al., Proc Biol Sci 273(1587):661–667, 2005). Every ommatidium detects signals that are neurologically processed to form a whole image and is composed of retinula cells, a rhabdom, a crystalline cone, and a corneal lens covered in nano-scale structures called corneal nipples (Lee and Erb, Beilstein J Nanotechnol 4:292–299, 2013). Corneal nipples are covered in protuberances that are responsible for the antireflective property of moth eyes (Parker, Am Sci 87(3):248–255, 1999). This optical mechanism relies on the fact that the wavelength range of incident light is less than the dimensions of the corneal nipples. The antireflective surface of moth eyes is critical to their survival, because they allow moths to camouflage with their surroundings at night and impart superhydrophobic properties, lending anti-adhesive, anti-fogging, and self-cleaning abilities (Boden and Bagnall, Moth-eye antireflective structures. In: Bhushan B (ed) Encyclopedia of nanotechnology. Springer, Netherlands, pp 1467–1477, 2012). Moth-eye antireflective (AR) structures have been produced by engineers to enhance the surfaces of materials such as glass and silicon to an extent that surpasses the antireflective properties provided by traditional antireflection coatings, or ARCs (Boden, Biomimetic nanostructured surfaces for antireflection in photovoltaics. PhD thesis, University of Southampton, School of Electronics & Computer Science, p 18, 2009). Researchers in the solar energy field have taken particular interest in the antireflective properties of moth eyes in hopes of applying them to photovoltaic (PV) systems to increase light-efficiency and minimize energy waste (Yamada et al., Prog Photovolt Res Appl 19(2):134–140, 2010). Perhaps one of the most exciting, up-and-coming applications of the moth-eye nano-structure is an X-ray enhanced with a film of protuberances modeled after corneal nipples. Using a moth-eye based film in addition to a traditional scintillator could allow a decrease in radiation dosage and offer higher-resolution imaging—both things that would be a huge step forward for medical care and, best of all, patient treatment (The Optical Society, Insects inspire X-ray improvements: nanostructures modeled after moth eyes may enhance medical imaging, http://​www.​osa.​org/​en-us/​about_​osa/​newsroom/​newsreleases/​2012/​insects_​inspire_​x-ray_​improvements_​nanostructures/​, 2012). Though tiny, moth eyes have the potential to contribute enormously to engineering endeavors, serving as a reminder of the infinite amount of knowledge hidden even among nature’s smallest creations.

Rice leaves repel water so efficiently that they are considered to be one of the few superhydrophobic materials (Feng et al., Adv Mater 14(24):1857–1860, 2002). These properties are very useful in a variety of applications, which is why scientists have been trying to mimic the surface of rice leaves to create water repellent materials for a wide range of uses. In this chapter, the various properties of superhydrophobic surfaces are explored and a comparison of surface structure is drawn between rice leaves and lotus leaves, which have anisotropic and isotropic properties, respectively. When we examine rice leaves, we can see how the roll-off angle—the critical angle at which the droplet begins to roll—is significantly lower in the longitudinal direction of the leaf than in the perpendicular direction of the leaf. This anisotropy originates from the multi-scale roughness of the surface (Lee et al., Adv Funct Mater 23(5):547–553, 2012). Though both surfaces are covered in a series of micropapillae that secrete epicuticular wax crystals that cause effective water repellency, the arrangement of the papillae on the lotus and rice leaves differ (Barthlott and Neinhuis, Planta 202(1):1–8, 1997). However, the hierarchical structures of papillae on both have been found to be almost identical (Woodward et al., Langmuir 16(6):2957–2961, 2000). The engineering applications of the superhydrophobic and anisotropic properties exhibited by rice leaves includes coatings on microwave and radio antennas (Antonini et al., Cold Reg Sci Technol 67(1–2):58–67, 2011), as well as self-cleaning glass developed using physical techniques such as ion etching and chemical techniques such as plasma-chemical roughening (Park et al., ACS Nano 6(5):3789–3799, 2012), which can be used in buildings as well as in the encapsulation of solar cells. Future applications are also being explored, such as in the way drugs are delivered inside the human body and in the field of micro-machines.

Despite subjecting their scales to constant friction and contact with surfaces, snakes’ skin remains remarkably intact. This resistance to wear and tear is a subject of many researchers’ interest. This chapter’s focus will be on the tribological properties of snake scales, in particular the ventral scales on the underside of the snake, as they are the most critical to movement and display an important characteristic that scientists call frictional anisotropy. Frictional anisotropy can be thought of as directional friction, or varying coefficients of friction based on orientation (Vogel, Slithering snakes: research shows snakes use friction and weight redistribution to glide on flat terrain, http://​www.​gtresearchnews.​gatech.​edu/​snakes/​, 2009). The two general strata that make up snake skin are called the dermis and epidermis. The outermost layer is called the Oberhautchen and is in direct contact with the environment (Abdel-Aal 2011). Frictional anisotropy can in part be attributed to the wide overlapping of the ventral scales on the Oberhautchen layer; these overlaps snag on asperities—or roughness and unevenness—on the surface in the direction of the scale, creating varying coefficients of friction in certain directions (Hu et al., Proc Natl Acad Sci U S A 106(25):10081–10085, 2009). In addition to the overlapping of scales, micro-hairs known as microfibrils and their orientation and upward slope are also responsible for frictional anisotropy (Hazel et al., J Biomech 32(5):477–484, 1999). Snake scales are microscopically adapted to resist wear, which can be seen in the double-ridge design of their microfibrils (Abdel-Aal, J Mech Behav Biomed Mater 22:115–135, 2013). Furthermore, a system of evenly distributed micropits reduces friction forces up to ten times, enhancing wear resistance (Hazel et al., J Biomech 32(5):477–484, 1999). Snake skin, with its natural ability to endure such high levels of abrasion, sliding, and rubbing, is naturally a source of inspiration for researchers aiming to design cylinder-piston systems for internal combustion engines (ICEs) that are textured in a way to optimize efficiency and performance. Plateau honing is one such process that may optimize a cylinder and piston’s sliding performance (Abdel-Aal and El Mansori, Reptilian skin as a biomimetic analogue for the design of deterministic tribosurfaces. In: Gruber P, Bruckner D, Hellmich C, Schmiedmayer H, Stachelberger H, Gebeshuber I (eds) Biomimetics—materials, structures, and processes. Springer, Heidelberg, pp 51–79, 2011).

Gecko adhesion is strong, sufficient enough for geckos to scale vertical walls and stick to surfaces upside down. In fact, a gecko weighing about 40–50 g is capable of generating a force that is 100 times greater than its weight (Niewiarowski et al., PLoS One 3(5):e2192, 2008). One of the main features of gecko adhesion that scientists have been trying to replicate but still find elusive is its good performance over the course of many cycles of attachment and detachment (Lee et al., Nature 448:338–342, 2007). Though there were many hypotheses for the mechanism behind this ability, van der Waals is recognized as the main contributor to gecko adhesion, with capillary forces potentially at play when water vapor exists (Zhou et al., Friction 1(2):114–129, 2013). Geckos’ adhesive ability comes from the hierarchical morphology of their toe pads, which feature lamellae, setae, and spatulae (Bhushan, Gecko effect. In: Bhushan B (ed) Encyclopedia of nanotechnology. Springer, Netherlands, pp 943–951, 2012)—all of which are described in detail in this chapter. Geckos’ dry adhesion mechanism is made possible because of the splitting of setae into hundreds and even up to a thousand spatulae (Zhou et al., Friction 1(2):114–129, 2013). Furthermore, due to the flexibility and softness of lamellar skin, it can be seen as a spring with a weak spring constant as opposed to stiff skin. This weak springiness ensures adhesion to surfaces over a wider range of normal compression displacement by preventing large deformations among the setae (Tian et al., Sci Rep 3:1382, 2013). Successful gecko-inspired adhesives would boast an array of outstanding properties, such as the ability to adhere and detach repetitively without breakage. Uses of such adhesives include sports equipment and robots that climb walls, among a variety of others (Zhou et al., Friction 1(2):114–129, 2013). Two general fabrication methods—polymer-based and carbon nanotube (CNT)-based dry adhesives—have been developed to create gecko-inspired adhesives (Jeong and Suh, Nano Today 4(4):335–346, 2009).

The butterfly amazes people not just for its beautiful, kaleidoscope-like appearance but also its potential for mimicry, which is evident in its high adaptability in various kinds of environments. Each butterfly has a particular wing structure and corresponding living habit. Although their iridescent colors attract the most attention, other characteristics like their anti-wetting property and heat dissipation due to their special surface structure are also interesting and inspiring. On the micro-scale, we see that the wings of butterflies are not as smooth as they seem; SEM observations show that the wing surface is covered by a large number of quadrate scales that overlap each other and are orderly arranged along the radial outward (RO) direction. Once magnified to the nano-scale level, research revealed that each scale consisted of ridging stripes that could be further split into multiple layers separated by air (Zheng et al.. Soft Matter 3:178–182, 2007). The iridescent effect observed in many species of butterflies is the product of the periodical multi-layers of their scales, which give rise to multiple internal reflection, refraction, and interference events (Herring, Comp Biochem Physiol A Physiol 109(3):513–546, 1994). Like lotus and rice leaves, butterfly wings are superhydrophobic, which enable them to keep their wings clean and dry (Zheng et al., Soft Matter 3:178–182, 2007). Butterfly wing replication has been explored by the scientific community in order to reproduce these outstanding properties. Dai et al. proposed a procedural texture generation approach using traits of iteration behavior to achieve the surface texture of butterfly wings (Dai et al., Visual Comput 11(4):177–187, 1995). SiO₂ inverse structure replicas using butterfly wings as templates in a sol-gel process have been attempted as well (Xu et al., Nano Res 4(8):737–745, 2011). Finally, Kang et al. used a molding lithography technique to fabricate a polydimethylsiloxane (PDMS) replica of the multi-layered scales on the upper surface of a Morpho butterfly (Kang et al., Curr Appl Phys 10(2):625–630, 2010).

Frog skin’s surface structure, color variations, and stickiness are protective features that have been widely explored by researchers for engineering applications. Many frogs have a thin film of water formed on their skin, and it is necessary for them to retain it in order to breathe through their skin (Mackean, Frogs—an introduction, http://​www.​biology-resources.​com/​frog.​html, 2004). Some frogs are adapted to secrete a thick mucous that prevents water from escaping and their skin from drying out (Science Score, Did you know that frogs breathe through their skin? http://​blog.​sciencescore.​com/​fun-facts-for-kids/​did-you-know-that-frogs-breathe-through-their-skin, 2012). Another function of the moist film is the lubrication effect it imparts. Color variation is one of the powerful defensive tactics of frogs that help them blend in with their natural surroundings. Some frogs such as dart frogs often display their bright skin with specific colored patterns to warn predators that they are highly toxic (Tesler, The amazing adaptable frog, http://​www.​exploratorium.​edu/​frogs/​mainstory/​index.​html, 1999). Despite the great variation in types of frogs, all frogs have independently evolved to have adhesive pads on their toes, signifying that the adhesive pad is an optimal evolutionary design for frogs (Barnes, Adhesion in wet environments: frogs. In: Bhushan B (ed) Encyclopedia of nanotechnology. Springer, Netherlands, pp 70–83, 2012). The surface morphology of these toe pads consists of hexagonal disk-like pads with channel-like spaces, as well as fine microstructure pegs (Federle et al., J R Soc Interface 3(10):689–697, 2006). Toe pad adhesion is made possible by van der Waals forces and capillary pressure forces (Lau and Messersmith, Wet performance of biomimetic fibrillar adhesives. In: von Byern J, Grunwald I (eds) Biological adhesive systems. Springer, Vienna, pp 285–294, 2010). Advances in biomimetics have been made in which toe pad replicas have been produced. For example, an early attempt at creating a replica featured polydimethylsiloxane (PDMS), and a later replica featured hierarchical micro- and nano-structures of epithelial cells (Barnes, Adhesion in wet environments: frogs. In: Bhushan B (ed) Encyclopedia of nanotechnology. Springer, Netherlands, pp 70–83, 2012).

This chapter takes a look at the structural and surface properties of various types of spider silk that enable such remarkable performance as a fiber, as well as its past and potential uses in engineering. Though there are various types of spider webs such as tangle and sheet webs, the orb web is the most common and is spun and constructed by orb-weaving spiders, Araneus and Nephila. Orb-weaving spiders are capable of producing seven different types of silk threads, each with distinct combinations of amino acid composition, function, and gland and spinneret used in production (Saravanan, J Textile Apparel Technol Manag 5(1):1–20, 2006). Dragline silk, known to be the toughest of all types of silk, serves as the framework of the spider’s web, as well as its lifeline (Heim et al., Angew Chem Int Ed 48:3584–3596, 2009). Despite inevitable variations in the material properties of dragline silk across different species of spiders, it is still found to be tougher than most biological fibers and even man-made fibers (Swanson et al., Evolution 60(12):2539–2551, 2007). While spiders use dragline silk for web framework and as a lifeline, they also spin threads to capture their prey, which are called capture threads (Vollrath, Rev Mol Biotechnol 74(2):67–83, 2000). The adhesive properties of capture threads are made possible by an interlocking mechanism (Hawthorn, Biol J Linn Soc 77(1):1–8, 2002) and van der Waals and capillary forces (Sahni et al., J Adhes 87:595–614, 2011). Spider silk has numerous future engineering applications. New biopolymer materials to be used in the medical field are a possibility with the manipulation of spider silk proteins (Römer and Scheibel, Prion 2(4):154–161, 2008). Also, spider silk is being applied to a diverse array of applications within the military arena, including, but not limited to, being used as an underwater anchoring adhesive (Military Times, Navy bets on spider silk research with USU funding, http://​www.​militarytimes.​com/​article/​20130807/​NEWS04/​308070038/​Navy-bets-spider-silk-research-USU-funding, 2013).