Symposium SM9 : Structure and Properties of Biological Materials and Bioinspired Designs

Most biological composite materials achieve higher toughness without sacrificing stiffness and strength. Interrogating how Nature employs these strategies and decoding the structure-function relationship of these materials is a challenging task that requires knowledge about the actual loading and environmental conditions of the material in their natural habitat, as well as a complete characterization of their constituents and hierarchical ultrastructure through the use of modern tools such as in-situ electron microscopy, small-scale mechanical testing capabilities, additive manufacturing, and advanced multiscale numerical models. In turn, this provides the necessary tools for the design and fabrication of biomimetic materials with remarkable properties. I will particularly focus my talk on the smashing stomatopod, Odontodactylus scyllarus. This species has been known for its heavily smashing blow creating the velocity and acceleration equivalent to a .22 caliber bullet with the forces up to 1.5 kilonewtons and the load-bearing part of its raptorial appendages, so-called dactyl club, having a capability of withstanding such tremendous forces. The focus of this research is on its extraordinary damage tolerant dactyl club. The dactyl club is mainly characterized by mineralized fiber layers resembled as a helicoidal arrangement, so-called Bouligand structure. The biomimetic helicoidal composite has shown great improvement in through-thickness damage resistance. In this study, a combined computational and experimental approach is carried out to investigate the structure-function relationship of the helicoidal composite based on fracture mechanics. In the experimental part, we 3D print prototypes of helicoidal composite for three-point bending experiments in which the toughening behavior is observed during fracture. The crack is found to propagate through the matrix of helicoidal arrangement and results in the twisted pattern. The crack propagation and toughening mechanism is further examined using analytical and numerical models. Additionally, the twisted crack pattern is investigated based on linear elastic fracture mechanics point of view to explain such toughening mechanism.

Insects of the order Embioptera, known as embiids or webspinners, spin sheets of very thin silk fibers from their forelimbs to build silken shelters in leaf litter, underground, or on bark where the climate is warm and humid. Their silk shelters are lightweight but tough and strong to keep out predators, and also hydrophobic to keep out rain. We have studied the molecular level structural properties of the silk material produced by the tropical embiid Antipaluria urichi using solid state nuclear magnetic resonance (NMR), wide angle X-ray diffraction (WAXD) and Fourier-transform infrared spectroscopy (FT-IR). These techniques have shown that the embiid silk protein secondary structure is dominated by β-sheet crystalline domains interspersed in a random coil matrix, similar to spider and silkworm silk fibers. Embiid silk differs from other silks in that approximately 70% of the silk material is comprised of β-sheet crystallites formed by serine-rich repetitive regions that are well-aligned with respect to the fiber axis. Embiid silk shows the highest crystalline fraction yet reported for any silk material. Using scanning and transmission electron microscopy, we have characterized the silk at the nanoscale, showing that the fibers are 90-100 nm in diameter—the thinnest diameter observed for silk produced by any arthropod. Using the aforementioned techniques, we have observed that each silk fiber is coated in an outer layer of lipid material, which gas-chromatography mass spectrometry (GC-MS) revealed to be comprised of fatty acids and other long chain alkane molecules. To investigate the hydrophobic properties of the silk, we performed contact angle studies for water on the silk surface. With an advancing contact angle of 150.0 ± 0.1° and an average hysteresis of 114°, the silk exhibits a property known as the rose petal effect, where water beads up on a hydrophobic surface but adheres to the surface due to nanoscale hierarchical roughness. We believe the results from our research into the structural and physical properties of this previously unstudied silk material will inspire the design of new lightweight, strong, and hydrophobic materials.

Reversible, suction based adhesion employed by many marine organisms may provide unique, adaptable technologies for biologically inspired grasping devices that function in difficult submerged environments. Remora, lumpsuckers, clingfishes, and octopi are notable organisms capable of forming temporary robust seals against their host using only their soft tissues to create a region of sub-ambient pressure that facilitates attachment. In the past, many studies have focused only on the adhesive strength and neglected other measures of attachment performance such as the seal leakage rate which is critical for understanding prolonged attachment. Here a theoretical framework based on measurable material and topological properties is developed to better understand this critical parameter, and the utility of the approach is demonstrated on an experimental apparatus designed to mimic the flow conditions experienced by a suction-based grasper. Furthermore, the sealing effectiveness of a remora fish on sharkskin is investigated as an example of the model’s application. The model’s success in simulating the flow conditions within the artificial grasper demonstrates its appropriateness as a design tool for translating biological observations to biologically inspired devices.

Biological organisms naturally synthesize complex, highly ordered, multi-functional materials that combine structures with drastically different mechanical properties. For instance, the teeth found in the radular belt of the giant chiton, Cryptochiton stelleri, are composed of the world’s hardest biologically produced mineral, yet they are connected to the soft body tissue in the mouth of the chiton by alpha-chitin fibers. The interface between the tooth and the stylus sharply defines each region across only a few microns, and yet this structure is able to repeatedly transfer load to jagged, rocky substrates that harbor chiton sustenance without failure or de-bonding of the two materials. The chiton’s ultra-hard, tricuspid tooth is composed of magnetite rods that are highly aligned parallel to the tooth surface. These rods are templated by alpha-chitin fibers that are also found in the stylus of the tooth. These fibers not only add to the fracture toughness of the tooth, but also increase the interfacial strength between the tooth and stylus by spanning the two regions, effectively stitching them together. By controlling chitin functionalization, the animal can create regions in the base of the stylus that prohibit nucleation of mineral, but can also encourage nucleation in other regions that experience higher stresses and require reinforcement during rasping. Toughening mechanisms at the junction zone will be revealed through the use of microscopic and spectroscopic techniques, and their mechanical properties will be tested with nano-indentation and verified with finite element modeling.

Woodpeckers peck at trees up to 20 times per second with speeds of 6-7 m/s all while avoiding brain injury despite undergoing decelerations up to 1200g's. Amongst the adaptations allowing this is a highly functionalized impact-absorption system consisting of the head, beak, tongue and hyoid bone. There have been a few attempts to characterize the effect of the shape and mechanical properties of skull on its anti-shock capability, and even these have focused mainly on finite element analysis and microstructural characterizations. This study aims to examine the anatomical structure, mechanical properties, and compositional constituents of the skull to determine its role in energy absorption and stress dissipation. Four different woodpeckers (Ivory-bellied Woodpecker, Golden-fronted Woodpecker, Whited-headed Woodpecker, and Acorn Woodpecker) were assessed through μ-CT to obtain a 3D model along with a control made up of chicken and turkey skulls. Scanning electron microscopy with energy dispersive X-ray spectroscopy was used to identify the structural and chemical components and nanoindentation was carried out to obtain mechanical properties. Results showed the skull bone from four different woodpeckers are very similar with a smooth and elliptical shape, while chickens and turkeys provided a significantly different structure. This structural difference is proposed to have been evolved to dissipate the stress waves created by pecking. Bioinspired applications will be suggested to prevent traumatic brain injury.
This work is supported by a Multi-University Research Initiative through the Air Force Office of Scientific Research (AFOSR-FA9550-15-1-0009).

Cephalopods are well known for their remarkable camouflage abilities; they can modify their
coloration, texture, pattern, and reflectivity to blend into the surrounding environment. Such
dazzling camouflage abilities are enabled by specialized intracellular structures comprised of
unique structural proteins known as reflectins. However, the nano- and micro-scale organization
of these proteins within such structures remains poorly understood. We have fabricated reflectin-
based films and characterized them via environmental scanning electron microscopy (ESEM),
small-angle x-ray scattering (SAXS), grazing incidence small-angle x-ray scattering (GISAXS),
and grazing incidence wide-angle x-ray scattering (GIWAXS). These experiments allowed us to
formulate a model for the hierarchical organization of self-assembled structures from reflectins.
Our findings hold implications for gaining an improved fundamental understanding of the
mechanisms that cephalopods employ to dynamically control their coloration, as well as for the
development of improved cephalopod-inspired materials.

Feathers are lightweight, flexible, strong, and spring-like, facilitating a bird’s ability to fly. Flight feathers possess a tiered hierarchical structure consisting of the rachis (main shaft), barbs (beams that branch from the rachis) and barbules (beams that branch from barbs). Neighboring barbs adhere to each other via the Velcro-like barbule connections to form a “zipped” feather vane. This “zipped” vane traps air to allow for lift, but “unzips” and lets air through when forces become dangerously large. This mechanism allows for failure prevention in the feather. Results from previous experiments show that the adherence of barbs to one another via barbules enables a damage resistant structure due to minimized barb rotation in bending. We examine the mechanism of this adhesion and propose a feather inspired design. The unique structure of the feather has potential for the design and synthesis of new bio-inspired materials and devices. This research is funded by AFOSR MURI (AFOSR-FA9550-15-1-0009).

Stomatopods are a group of aggressive marine crustaceans that have evolved highly specialized raptorial appendages used for feeding and hunting prey. In the smashing variety, such as the species, Odontodactylus scyllarus, the terminal segment (dactyl) takes the form of a bulbous hammer-like club, which is used to smash through the tough exoskeletal structures of mollusks, crustaceans, and other shelled marine organisms with incredible force and speed. The dactyl club can reach speeds up to 23 m/s from rest and impact with up to 1500 N of force, generating tremendous stresses including cavitation at the club surface. The success of the dactyl club’s mechanical response, namely its resistance to catastrophic failure from repeated high-energy impacts, lies in its multi-regional and hierarchical composite architecture. This natural material features a compliant inner layer featuring helicoidal and sheet-like arrangements of alpha-chitin fibrils mineralized by amorphous forms of calcium carbonate and calcium phosphate. An enamel-like surface region that allows for momentum transfer to prey caps this soft core. Here we investigate ultrastructure-mechanical property relationships of this outer “impact” region. Using high-resolution electron microscopy, we identify unique fiber architecture in conjunction with a highly textured apatitic mineral phase. In-situ nano-mechanical testing in combination with finite element modeling reveals a number of fracture-mitigation strategies that yield toughness to an already stiff and hard biomaterial.

The ostraciidae family (i.e., boxfish) is known for their slow, well controlled swimming motion and complex dermal armor that can be traced back 55 million years. Their carapace is made of rigid scutes (plates), consisting of a mineralized plate and compliant collagen base, which connect by interdigitating in suture structures as opposed to overlapping (the more common design in fish species). The complex scheme of this carapace is structurally examined through in-situ mechanical testing (SEM and SAXS/WAXD) as well as computational analysis. Results show that, in shear, the mineralized suture interfaces are capable of interlocking while they simply pull apart in tension. The underlying collagen fibers are oriented in a complex cross-hatched pattern so as to provide resistance against stresses in multiple directions. Application to impact and piercing resistant bioinspired materials are discussed.

This work is supported by funding provided by the Multi-University Research Initiative through the Air Force Office of Scientific Research (AFOSR-FA9550-15-1-0009).

Keratins are among the toughest biological materials, which have been a continuing source of inspiration to develop new functional materials. The pangolins scales and the feather shaft stand out with interesting functions: the scales are strong and resilient to resist sudden and repeated loads from predators, and the feather shaft is light-weight, strong and stiff, yet reasonably flexible. The structure and mechanical properties of scales from Chinese and African pangolins were correlated for the first time. They consist of flattened keratinocytes and show crossed lamellar structure (2~8 µm). Tensile and compressive behaviors under different strain rates, and along different orientations were studied. The feather shaft shows a cross sectional change of cortex and a layered structure with differentially oriented fibers along the shaft length, which work in synergy to provide adjusted bending stiffness and reasonable flexibility optimized for flight. The structure and mechanical functions were investigated and correlated through nanoindentation, tension, compression and flexure tests.

The Sonoran desert has an incredible diversity of cacti, which poses an equally diverse number of morphological and physical characters. Somewhat surprisingly considering the xeric climatic conditions, strong water repellency is one of these physical characteristics. In particular, Opuntia, also known as prickly pear cactus, is among numerous plants that poses superhydrophobic characteristics.¹ This hydrophobic property stems from waxy exterior (cuticle) with hierarchical nanoscale and microscale texture.^2-4 In addition, Opuntia microdasys has complex multi-level, hair-like spines with a gradient of wettability that may efficiently collect water droplets from fog.⁵ Besides efficient fog collection technology,⁶ this mechanism has also inspired a new route to separate oil droplets from water.⁷ These two examples illustrate the potential rich diversity of technological innovations that could be inspired by cacti.
We investigated wetting properties of over 100 species within the genus Opuntia at the Desert Botanical Gardens’ living plant collection and found that only some species are superhydrophobic. In fact, different Opuntia species display a rich variety of wetting properties ranging from superhydrophilic to superhydrophobic. We observed that wetting properties depend not only on the particular species investigated but also on the location and age of the stem tissue analyzed. For example, on some plants the wetting properties can involve a macroscopic gradient, with old cladodes being superhydrophilic while new ones being superhydrophobic. To gain an insight into this rich diversity of wetting behaviors, we correlated observed contact angles to age-dependent surface hierarchical nano-to-microscale topology. We also used genetic information to relate different wetting patterns to evolutionary relationships and biogeographic origins of prickly pear cacti⁸. We show how this information can be used to infer relationships among climate, wetting characteristics, and their functionality for the different species of Opuntia.


References
1. Neinhuis, C.; Barthlott, W., Annals of Botany 1997, 79, 667-677.
2. Koch, K.; Barthlott, W., Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 2009, 367, 1487-1509.
3. Koch, K.; Bohn, H. F.; Barthlott, W., Langmuir 2009, 25, 14116-14120.
4. Salem-Fnayou, A. B.; Zemni, H.; Nefzaoui, A.; Ghorbel, A., Micron 2014, 56, 68-72.
5. Ju, J.; Bai, H.; Zheng, Y.; Zhao, T.; Fang, R.; Jiang, L., Nature communications 2012, 3, 1247.
6. Cao, M.; Ju, J.; Li, K.; Dou, S.; Liu, K.; Jiang, L., Adv. Funct. Mater. 2014, 24, 3235-3240.
7. Li, K.; Ju, J.; Xue, Z.; Ma, J.; Feng, L.; Gao, S.; Jiang, L., Nature communications 2013, 4.
8. Majure, L.C.; Puente, R.; Griffith, M.P., Judd, W.s.; Soltis, P.S., Soltis, D.E., American Journal of Botany,2012,99,847-864.

Spider silks are protein-based biopolymers with extraordinary physical and mechanical properties. Much is known about the secondary molecular structure of spider silk fibers and its relationship to physical and mechanical properties. However, even in the most heavily studied spider silks little is know about organization or secondary structural motifs or further hierarchical organization. It is known that in the major ampullate gland two proteins (MaSp1 & 2) are generated for dragline silk production. The relative amounts of these two proteins within the silk gland and resulting fiber are known and vary between species. This variation is well correlated with the resulting fiber physical and mechanical properties. Our research group has utilized several spectroscopy and imaging modalities to investigate the protein structure at several length scales to gain insight into protein-protein interactions and structure-function relationships in spider silks. In this presentation, we will show our recent investigation of spider silks probed using Nuclear Magnetic Resonance (NMR), Magnetic Resonance Imaging (MRI), x-ray diffraction (XRD) and imaging modalities to better understand the hierarchical proteins structures that make-up spider silk fibers.

Exploiting available resources, nature has developed formidable damage tolerant composites through centuries of evolution. Exemplifying these traits, beetles possess resilient hardened protective forewings known as elytra. One example, the exoskeleton from the diabolical ironclad beetle, Phloeodes diabolicus, exhibits an outstanding adaptation to resist external loads that would prove fatal to most insects. Composed of layered fibrillar chitinous bundles embedded in an organic protein matrix, the beetle’s exoskeleton represents a biological composite with hierarchical construction. We identified two distinct regions in the structure that contribute to the overall mechanical strength. The elytra consists of an organized layers of uni-directional a-chitin fibers in a helicoidal arrangement. The ventral cuticle possesses a similar fiber arrangement and acts in tandem with the elytra to prevent lateral expansion as force is applied to the architecture. Cross sectional analysis reveals a support mechanism that joins the elytra to the ventral cuticle and engages upon compression. Our analysis provides insight into structural and mechanical properties of the elytra through compression, puncture and nanoindentation. Revealing the organism’s evolutionary design, current optical and mechanical analysis will exposes the structure property relationship of the ironclad beetle’s exoskeleton and can lead to the development of tough light weight composite materials for industrial applications.

Biogenic magnesia calcites (Ca_1-xMg_xCO₃) with x > 0.11-0.13 have greater ion activity products than pure calcites (x = 0, CaCO₃), which implies that natural materials with high x are more soluble in acidified waters. Literature values for the stone part of the grinding tips in sea urchin teeth have x > 0.4, the highest value found among Ca_1-xMg_xCO₃ bearing species. Naturally low pH water in the California Current system off the coast of Southern California occurs largely due to natural oceanographic processes such as upwelling. Expanding regions of high CO₂ and low O₂ from respiration beneath productive upwelled waters in oxygen minimum zones (OMZs) necessitates further study of broad-area persisting species, such as pink sea urchins (Strongylocentrotus fragilis), that inhabit a depth range of 100-1200 meters in habitats subject to acidification. Pink sea urchin teeth, collected from 100, 300, 700 and 1100 meters are analyzed mechanically to assess differences between ocean depths. Polished cross-sections of the stone part of the tooth grinding tip are nanoindented to compare hardness and stiffness values. The natural process for maintenance of a sharp tooth grinding tip is also examined. Individual teeth mounted to a microindenter tip holder are scratch tested on a hard substrate to analyze the self-sharpening mechanism at the serrated edge, where calcite plates are removed from the grinding tip due to shear stress. The tooth grinding tip is observed with nano-computed tomography to provide further insights about the microstructure. This work is supported by Multi-University Research Initiative through the Air Force Office of Scientific Research of the United States (AFOSR-FA9550-15-1-0009).

The changes associated with bone structure and properties can lead to fracture of bone. One of the clinical tests for detecting the quality of bone uses ultrasound waves propagating through the bone to measure the bone mineral density. The ultrasound wave interaction with the medium can lead to information about its microstructure and material properties. At the micrometer scale, bone is made up of fibers, which are indeed composed of mineralized collagen fibrils. The fibers are aligned in different orientations, which make up the osteons and Haversian canals within the bone. The orientation of fibers and defects in fibers could affect the wave propagation. To gain a fundamental understanding of the ultrasound wave interaction with fibers within the bone, we use a multiscale approach, where the elastic constants of the mineralized collagen fibrils are predicted using atomistic simulations, which serves as an input for the continuum model of the fibers. Ultrasound waves are modeled to propagate through bone lamellae using the finite element method along with the Newmark’s constant acceleration method.

In the bone lamellae model, two energy flux waves are produced in plane. The faster quasi-longitudinal (QL) and the slower quasi-shear (QT), both of them deviate from the normal direction depending on the fiber orientations. It has been found out that, wave interaction with defects in lamellae will split the ultrasound wave into two distinct waves and the distance between the peaks of the split waves correspond with the size of the defect found in the bone lamellae. The studies outlined here, would lead to gaining a fundamental understanding of how ultrasound waves can predict fracture properties of bone.

The purpose of this research was to extract arabinoxylans (AX) from maize wastewater generated under different maize nixtamalization conditions and to investigate the polysaccharide gelling capability, as well as the rheological and microstructural characteristics of the gels formed. The nixtamalization conditions were 1.5 hours of cooking and 24 hours of alkaline hydrolysis (AX1) or 30 minutes cooking and 4 hours of alkaline hydrolysis (AX2). AX1 and AX2 presented yield values of 0.9% and 0.5% (w/v), respectively. Both AX samples presented similar molecular identity (Fourier Transform Infra-Red) and molecular weight distribution but different ferulic acid (FA) content. AX1 and AX2 presented gelling capability under laccase exposure. The kinetics of gelation of both AX samples was rheologically monitored by small amplitude oscillatory shear. The gelation profiles followed a characteristic kinetics with an initial increase in the storage modulus and loss modulus followed by a plateau region for both gels. AX1 presented higher storage modulus than AX2. In scanning electron microscopy (SEM) images, both gels present an irregular honeycomb microstructure. The lower FA content in AX2 form gels presenting minor elasticity values and a more fragmented microstructure. These results indicate that nixtamalization process conditions can modify the characteristics of AX gels. Environmental concerns have triggered research on alternative nixtamalization processes rendering residues with less environment impact. AX recovering from this kind of less pollutant maize wastewater could be an interesting research subject in order to explore the structural and functional properties of this hydrocolloid.

Spider silk from N. clavipes have a range of useful properties in addition to remarkable mechanical properties. These silks are uniquely suited for biomedical applications as they are generally biocompatible and biodegradable. Piriform silk is an adhesive protein used by the spider to adhere its web to a variety of substrates. Sequencing of mRNA extracted from the piriform silk gland of N. clavipes has revealed two unique amino acid motifs. The structural role of these motifs in the resulting piriform silk attachment disk is unknown. The specific aim of this project is to produce recombinant piriform spider silk in Escherichia coli. Three biosynthetic proteins will be produced, two based on individual amino acid motifs and one based on the overall piriform sequence. The resulting piriform-analogue proteins will be used to produce fibers, films and gels for mechanical testing. The structural role of piriform’s unique motifs will be characterized using X-Ray Diffraction and Circular Dichroism on solid samples. Elucidating the mechanical and structural roles of these motifs will add to the existing repertoire of characterized motifs for the production of tunable, chimeric spider silk materials.

Spider silks are incredible natural materials that possess combinations of strength and elasticity that exceed man-made materials. Traditionally harsh chemicals and long processing times have been required to form materials from these proteins. However, with the development of a new aqueous solubilization method for recombinant spider silk proteins, new possibilities for these proteins have been unlocked. This solvation method is easy, cheap, fast, and has even allowed for novel materials and proteins be investigated and characterized. Most notable among these materials developed from this method are fibers, films, hydrogels, lyogels, and sponges. These materials have been investigated using several different protein types, both native-like and chimeric, which have been purified from different expression systems. Finally, the structure, orientation, and consequently the mechanical properties of these various materials were the primary focus of these studies. Using various techniques and processing methods these mechanical properties can be increased or tuned to a range of desired applications.

The spider aggregate gland from orb weaving spiders produces a glue from aggregate protein that helps the web keep insects immobile long enough for the spider to capture it. Very little is known about the glue other than a partial genetic sequence and that the glue is sticky when wet. This opens a variety possible applications for a synthetically produced aggregate protein rangeing from underwater adhesives to surgical glue. We are cloning the aggregate cDNA to confirm and extend existing knowledge about the aggregate gene(s). mRNA was extracted from aggregate glands using TRIzol reagent, and then reverse transcribed using MMLV-RT to create single stranded DNA. The second strand was synthesized employing DNA Polymerase I in conjunction with RNaseH to form cDNA. The cDNA was ligated into pBluescript II SK (+) and transformed into electrocompetent DH10B bacterial cells. Bacteria were plated on ampicillin containing media and plates used had a colony density of approximately 1000 colonies per plate. Library screening has commenced with a probe designed using data from two previously reported sequences for the aggregate gene. Preliminary screening of the library using a fluorescent labeled probe was unsuccessful, but screening with a positive control sequence using a biotinylated probe has yielded promising results. We expect to obtain 2.5-3Kb of clear aggregate sequence from this library in order to confirm previous sequences and to allow us to express this gene synthetically.

Eggshells can be easily broken by striking them at a local point. However, breaking it by squeezing the egg with a distriuted load, such as using one's hand to do so, is nearly impossible. Although the shell is a very thin porous calcium carbonate structure, its compressive resistance is an interesting feature. In this study, a new insight on these mechanical properties is provided through a series of experiments of compression of various kinds of bird eggs along their long-axis. Failure force scales with the thickness of the shell reaching loads upwards of five-thousand Newtons for ostrich eggs. Failure occurs by axial splitting parallel to the loading direction, spaced by a characteristic length. The spacing is the result of radial tensile stresses that develop from the applied compressive traction. These observations were then compared with calculations executed on a finite element model of an ovoide volume under compression using a maximum tensile stress criterion.

The alligator gar (A. spatula) is covered with bony scales and an enamel-like surface layer. The scales form a tridimensional pattern in which neighboring scales overlap in such a manner that the thickness conforms and the sum of the overlaps is constant. The mechanical properties and structure are correlated and the tridimensional pattern revealed by computerized tomography is transferred by additive manufacturing to a magnified array of idealized, identical tiles. It is demonstrated that flexibility is maintained while protection is retained. This design is proposed as a model for bioinspired flexible ceramic composite personal armor. Research funded by AFOSR MURI.

Coelacanths (Latimeria chalumnae) are known as living fossils because their morphology has little change in the last 300 million years. This lob-finned fish plays an important role in evolution because it bridges an evolutionary gap between fish and tetrapods. They have highly modified scales which are known as cosmoid scales which are typically found on extinct species of fish. The surface of the fish body with overlapped scales is rough and hard, which help protect the fish from predators. The scales are overlapped to each other and the degree of imbrication is about 0.3. The exposed part bears spines of denticles and annular ridges centerd on the apex of the scale are apparent on the surface of the overlapped part. The intervals of the ridges are about 30 μm. The out layer of the scales shows higher mineralization than the inner layers and provide protection.The major component of the overlapped region is isopedine, which is composed of layers of parallel collagen bundles. Each bundle is composed of collagen fibrils with typical 67nm d spacing. These bundles form layers with a double twisted system in which the directions of bundles in neighboring layers are almost perpendicular to each other, but successive bilayers show right-handed twisting arrangement. This double twisted arrangement can be clearly illustrated in 3D models and such unique structure results in an isotropic mechanical behavior. Such hierarchical structure does not provide much different tensile strength in longitudinal and transverse directions as the collagen bundles contribute equally in both orientations.

Dragline spider silk is one of the strongest biomaterials known and is primarily composed of major ampullate spidroin 1 (MaSp1) proteins. This long repetitive protein forms fibrils with alternating crystalline and non-crystalline regions, but many aspects of its structure and dynamics remain unclear. We have performed temperature replica exchange simulations of MaSp1 minifibrils consisting of one non-crystalline region surrounded by crystalline regions. The simulations show significant residual structure in the non-crystalline regions, consisting of 3₁₀ helical structures. In addition, persistent clusters of tyrosines were found. The occurrence of these clusters may help explain the anomalously large barrier of Tyr ring flips observed in solid state NMR measurements.

Architectured materials are characterized by specific structural features which are larger than what is typically considered microstructure (i.e. grains) but smaller than the size of the component. This class of materials includes lattice materials and foams, but also dense materials composed of building blocks of well-defined size and shape. While the deformations of the blocks typically remain small and within elastic limits, their interfaces can channel cracks and undergo large deformations. These features lead to building blocks which can slide, rotate, separate or interlock collectively, provide a wealth of tunable mechanisms. Well-designed architectured materials can therefore generate new and attractive combinations of properties which are unattainable in monolithic materials. Interestingly, there are many analogies between emerging architectured materials and hard biological materials such as bone, teeth or mollusk shells. I will particularly discuss nacre and fish scales, two examples or architectured natural materials which demonstrate how the interplay between the properties, shape, size and arrangement of the building blocks and nonlinear behaviors at the interfaces generates high properties. Duplicating these structures and mechanisms into engineering materials is very attractive, but the assembly of building blocks from the bottom-up and into highly ordered structures still presents major challenges. The top-down approach is a new strategy whereby weak interfaces are carved within the bulk of hard materials such as glass. This architectured / bio-inspired approach leads to materials with highly unusual combinations of properties which usually conflict: High stiffness and high toughness in a nacre-like glass, hardness and flexural compliance in fish scale-inspired coatings.

Marine organisms have developed a wide variety of protective strategies to thrive in their native environments. These biological materials, although formed from simple biopolymer and biomineral constituents, take on many intricate and effective designs. The specific environmental conditions that shape all marine organisms have helped modify these materials into their current forms: complete hydration, and variation in hydrostatic pressure, temperature, salinity, as well as motion from currents and swells. These conditions vary throughout the ocean, being more consistent in the pelagic and deep benthic zones while experiencing more variability in the nearshore and shallows (e.g., intertidal zones, shallow bays and lagoons, salt marshes and mangrove forests). Of note, many marine organisms are capable of migrating between these zones. In this, the basic building blocks of these structural biological materials and a variety of protective strategies in marine organisms are discussed with a focus on their structure and mechanical properties. These protective strategies can be organized based upon their primary defensive and structural function into four distinct categories: crushing, flexure, piercing and impact resistant structures. Finally, the bioinspired potential of these biological materials is discussed.

This work is supported by funding provided by the Multi-University Research Initiative through the Air Force Office of Scientific Research (AFOSR-FA9550-15-1-0009).

Diatoms are single-cell algae that form a hard cell wall made of a silica/organic composite. One fascinating aspect of such silica glass shells is their intricate, varied and detailed architecture. It is well accepted that the diatom frustules mainly serve as protection, among other evolutionary functions. The question remains that how do the geometry and the microstructure of diatom frustules each contribute to their amplified structural resilience.
We conducted three-point bending on extracted lammelae from the Coscinodiscus sp. frustule with approximate square cross-section with dimensions ~3-4 µm. We observe failure by brittle fracture at an average stress of 1.1 GPa. Analysis of crack propagation and shell morphology reveals a differentiation in the function of the frustule layers with the basal layer pores seen deflecting crack propagation. We calculate the relative density of the frustule beam samples to be ~30% and show that the frustule has the highest strength to density ratios among reported biologic materials, at 1702 kN-m/kg. We also performed nanoindentation on both the single basal layer of the frustule and the girdle band and show that these components display similar mechanical properties that agree well with bending tests. TEM analysis reveals that the frustule is composed almost entirely of amorphous silica with a 275 nm-thick proximal layer displaying nanocrystalline microstructure. No flaws are observed down to 2 nm. FEM simulations of the three-point bending experiments reveal the stress distribution with a frustule beam sample and show that the load is carried primarily by the basal layer, while stresses within the cribrum and areolae layer are an order of magnitude lower.
We further fabricated silica architectures that fully replicate the diatom geometry using two-photon lithography. These biomimetic diatoms have a completely amorphous microstructure, and its elastic modulus matches that of the natural diatom biosilica. Three-point bending experiments of these synthetic diatoms reveal that cracks initiated and propagated in the same fashion as in Coscinodiscus sp. samples. Analysis of specific strength show that geometry does not solely account for the exceptional strength of natural diatom frustules. Comparison of failure strength also reveal potential mechanisms for the microstructure and silicification process of diatom frustules and provide insights to their evolutionary design.

Tubular structure are one of the most impact resistant and energy absorbent structural designs and although studied for many years, remains an attractive research interest. Among the biological materials that have tubular structures, horns and hooves are excellent examples of impact resistant structures that have evolved for fighting or galloping of bovidae and equidae, respectively. Microstructures and mechanical behavior of the bighorn sheep (Ovis Canadensis) horn and African zebra (Equus burchelli) hoof were studied to examine mechanisms of impact resistant. Hooves and horns are composed of α–keratin crystalline intermediate filaments embedded in an amorphous matrix. Both have a tubular structure and a surrounding laminated structure. Optical and electronic microscope images showed the shape of tubule cross section in both horn and hoof are ellipses, ∼80μm (major axis) and ∼40μm (minor axis). However, the porosity in the horn is ∼7%, while in the hoof is ∼3.5%. The intertubular lamellae area is much larger in the hoof than that in the horn. Since anisotropy is an important factor in material properties, mechanical properties of horns and hooves were evaluated in three orthogonal directions (longitudinal, transverse and radial). Quasi-static compression test results showed strength and Young’s modulus were much higher in longitudinal and transverse direction than that in radial direction, while the radial direction was more energy absorbent because of the tubule collapse. Comparing horn and hoof structures, although the porosity in horn is two times that of the hoof, the compressive yield strength and Young’s modulus are higher in the horns in all the three directions, which demonstrated that horn structure was more energy absorbent in a quasi-static load. To determine how porosity affects the mechanical properties of tubular structures, 3D printed models of two different tubular densities (7% and 3.5%) were fabricated. At the same time, quasi-static and dynamic simulation of these two kinds of tubular models were carried out using finite element analysis, which showed a more uniformly distributed stress in transverse direction, leading to a higher stiffness. Kolsky bar dynamic test results as well as further finite element analysis on the tubular models will be presented. This work is supported by funding provided by a Multi-University Research Initiative through the Air Force Office of Scientific Research (AFOSR-FA9550-15-1-0009).

Hard biomaterials such as horn, teeth and jaws are essential in food digestion, defense, or prey hunting for survival. In particular, grasshoppers need strong mandibles (the organ functioning as teeth) to eat tough leaves. Herein, we propose a chemical mechanism demonstrating the mechanically strong mandible formation in grasshopper. The hardness of mandibles was approximately 0.4 Gpa, which is stronger than that of insect cuticle (approximately 0.25 Gpa). The mechanical strength of mandibles was exclusively achieved by the organic sources (i.e. carbon, nitrogen and oxygen), which is dissimilar from the teeth of other animals containing abundant inorganic elements. We found that catecholamine (i.e. dopamine) is the key molecule in hardening mandibles and presents in the protein chains as a form of dopamine-histidine. It has been reported that the acetylated-dopamine, N-acetyl-dopamine or N-β-alanyl-dopamine, plays a role in cuticular sclerotizaiton and tanning. The use of chemically unprotected dopamine for hardening tissues is the first report, suggesting that the air-sensitive dopamine might be an alternative molecule for constructing hard surfaces via prompt oxidation.

Eggshells serve as multifunctional shields for successful embryogenesis, such as protection, moisture control and thermal regulation. Unlike calcareous avian eggshells which are brittle and hard, reptilians have leathery eggshells that are tough and flexible. Reptilian eggshells can withstand collision damages when laid in holes and dropped onto each other, and reduce abrasion caused by buried sand. In this study, we investigate structure and mechanical properties of eggshells of Taiwan cobra snake (Naja atra) which composed mainly of keratin. Mechanical tests showed that snake eggshells are highly extensible and reversible. The exceptional deformability (110-230% tensile strain) and toughness of snake eggshells are contributed by the wavy and random arrangement of keratin fibers as well as collagen layers. Multi-scale toughening mechanisms of snake eggshells were observed and elucidated, including crack deflection and twisting, fibers reorientation, sliding and bridging, inter-laminar shear effect, as well as the a-b phase transition of keratin. Inspirations from the structural and mechanical designs of snake eggshells may lead to the synthesis of tough, extensible, lightweight composites which could be further applied in the flexible devices, packaging and bio-medical fields.

The unique mechanical properties observed in biological materials distinguish themselves from common engineering materials. Some naturally occurring high-performance ceramics, like the external veneer of the Chiton tooth show a common feature, a rod-like microstructure. Nanoindentation tests not only provide some key mechanical properties of the surface of the tooth but are also an effective test to induce and analyze damage mechanisms at the nano- and micron-scale level. Despite these advantages, our ability to make direct observations about the potential abrasion resistance mechanisms acting at these small scales during these tests remains a challenging task. In this presentation, we will present a 3D finite element-based micromechanical model for the simulation of nanoindentation cracking in a fully mineralized tooth. This proposed model is capable of capturing damage, fracture and fragmentation of the mineral rods, and energy dissipation at the organic interfaces. A correlation between the rod-like ultrastructure and the abrasion tolerant properties will be discussed. A comparison of the model with indentation test in a 3D printing prototypes with similar features confirms the key role of the interactions between organic and mineral phases in the mechanical properties of biomaterials.

Spider silk is the toughest known biomaterial, and its energy absorption capacity is known to be further enhanced if the silk is arranged into an orb-web structure. The webs of non-orbweaving spider species, which are often more disorganized, have been less studied. But these alternatives offer potentially intriguing designs that are just as superbly adapted to their respective evolutionary niches. The recluse genus of spiders (Loxosceles) is one such non-orbweaver that spins an especially curious silk: instead of a cylindrical strand spun by most other spiders, it produces a flat ribbon 6–8 µm wide and only 40–80 nm thick. We show that Loxosceles spins these flat ribbons into a web structure that enhances toughness. Modeling of ideal elastic and strain-hardening plastic fibers arranged into this structure confirmed that the advantage is generally applicable, and key design parameters were identified. In addition to enhancing toughness, the Loxosceles web design was also found to contribute to the unique prey capture capacity of a ribbon-like silk. These advantages make the recluse silk-web system an ideal candidate for biomimicry in future synthetic materials.

The hierarchical organization of components within bone has been credited with its remarkable combination of strength and toughness that outperforms those of its two main constituents, hydroxyapatite and collagen. Investigations of the mineral phase in other biogenic minerals revealed their amorphous microstructure in the mineralization stage. It has been proposed that in human bone, an amorphous mineral serves as a precursor to the formation of crystalline hydroxyapatite, i.e. bone growth, yet the mechanism for amorphous-to-crystalline transformation remains unknown. We use Transmission Electron Microscopy (TEM) to provide direct evidence of 100-300 nm-sized amorphous calcium phosphate (ACP) regions present in the disordered phase within the bone, which is located in the nodes of the trabecular network. We deprotinate the disordered phase and observe crystallization within the ACP. We extracted site-specific, cylindrical samples with diameters between 250 nm and 3000 nm from each phase, ordered and disordered, and performed dry, nanomechanical compression experiments on them. These experiments revealed a transition from plastic deformation to brittle failure and at least a factor of 2 higher strength in the nano-sized samples compared with micro-sized ones. We postulate that this ductile to brittle transition is caused by a suppression of interfibrillar shearing, and the emergent size effect of “smaller is stronger” is attributed to the scaling of the distribution of flaws with sample size. These findings have significant implications in our understanding of the multi-scale nature of bone, establishing a link between the microstructural detail and macroscopic properties of bone, as well as provide insights into its biomineralization process.

The nano-mechanical properties of tortoise shell will be elucidated in this paper. This material experiences, and has been designed to endure, very different loading conditions in their environment and during their function. This work will explore the deformation and failure mechanisms of tortoise shell with interfaces between relatively "hard" and "soft" layers. Nano-indentation experiments will be used to study the hardness and deformation of the different layers. The work will also develop mechanics-based models for the design of robust materials. Insights from this work will be explored for the design of energy-absorbing materials such as helmets for motorcyclists and polo players which protect them from injury due to falls or hammer impact

Learning from nature and based on lotus leaves and fish scale, we developed super-wettability system: superhydrophobic, superoleophobic, superhydrophilic, superoleophilic surfaces in air and superoleophobic, superareophobic, superoleophilic, superareophilic surfaces under water [1]. Further, we fabricated artificial materials with smart switchable super-wettability [2], i.e., nature-inspired binary cooperative complementary nanomaterials (BCCNMs) that consisting of two components with entirely opposite physiochemical properties at the nanoscale, are presented as a novel concept for the building of promising materials [3-4].
The smart super-wettability system has great applications in various fields, such as self-cleaning glasses, water/oil separation, anti-biofouling interfaces, and water collection system [5].
The concept of BCCNMs was further extended into 1D system. Energy conversion systems that based on artificial ion channels have been fabricated [6]. Also, we discovered the spider silk’s and cactus's amazing water collection and transportation capability [7], and based on these nature systems, artificial water collection fibers and oil/water separation system have been designed successfully [8].
Learning from nature, the constructed smart multiscale interfacial materials system not only has new applications, but also presents new knowledge: Super wettability based chemistry including basic chemical reactions, crystallization, nanofabrication arrays such as small molecule, polymer, nanoparticles, and so on [9].
Reference:
1. Adv. Mater. 2006, 18 (23), 3063-3078.
2. Adv. Mater. 2008, 20 (15), 2842-2858.
3. Pure Appl. Chem. 2000, 72 (1-2), 73-81.
4. Small 2015, 11, 1071-1096.
5. Adv. Mater. 2011, 23 (6), 719-734.
6. (a)Chem. Soc. Rev. 2011, 40 (5), 2385-2401; (b) Acc. Chem. Res. 2013, 46 (12), 2834-2846; (c) Adv. Mater. 2010, 22 (9), 1021-1024. (d) ACS Nano 2009, 3 (11), 3339-3342; (e) Angew. Chem. Int. Ed. 2012, 51 (22), 5296-5307;
7. (a) Nature 2010, 463 (7281), 640-643; (b) Nat Commun 2012, 3, 1247.
8. (a) Nat Commun 2013, 4, 2276; (b) Adv. Mater. 2010, 22 (48), 5521-5525.
9. (a) Chem. Soc. Rev. 2012, 41 (23), 7832-7856; (b) Adv. Funct. Mater. 2011, 21 (17), 3297-3307; (c) Adv. Mater. 2012, 24 (4), 559-564; (d) Nano Research 2011, 4 (3), 266-273; (e) Soft Matter 2011, 7 (11), 5144-5149; (f) Soft Matter 2012, 8 (3), 631-635; (g) Adv. Mater. 2012, 24 (20), 2780-2785; (h) Adv. Mater. 2013, 25 (29), 3968-3972; (i) J. Mater. Chem. A 2013, 1 (30), 8581-8586; (j) Adv. Mater. 2013, 25 (45), 6526-6533; (k) Adv. Funct. Mater. 2012, 22 (21), 4569-4576

Bioinspired adhesive surfaces have received much attention in scientific research over the last decade. Observation of adhesion organs in animals such as insects, spiders and geckos has led to important patterning concepts to enhance adhesion (the so-called contact splitting concept). Several laboratories have successfully fabricated artificial gecko structures, with 3D features in the range between sub-micron and hundreds of microns, on a laboratory scale and thus demonstrated that the principles from nature can be emulated with different polymeric materials. What is missing, however, is the link from small-scale laboratory samples to prototype products that demonstrate the feasibility of artificial gecko surfaces for targeted applications and the step to economic large-scale fabrication of such surfaces. At INM, we have recently taken the step to realistic engineering of such structures and thus created prototype demonstrators of our newly created Gecomer^® technology. Advanced Gecomer^® surfaces were implemented in pick-and-place procedures, including commercial robotic systems. Several release mechanisms were integrated to enable the handling of materials ranging from wafers to glass and paper. Realistic field and endurance tests were carried out under conditions of e.g. vacuum or higher temperatures. The talk will describe some of the critical issues whose solution may make Gecomer^® technology one of the first successful examples of commercialized bioinspiration.

Controlling dropwise condensation is fundamental to water harvesting systems, desalination, thermal power generation, air conditioning, distillation towers, and numerous other applications. For any of these, it is essential to design surfaces that enable droplets to grow rapidly and be shed as quickly as possible. However, state-of-the-art approaches based on micro/nano or molecular scale textures suffer from intrinsic trade-offs that make it difficult to optimize both growth and transport at once. Here we present a conceptually different design approach based on principles derived from Namib desert beetles, cacti, and pitcher plants that synergistically couples both aspects of condensation and significantly outperforms other synthetic surfaces. Inspired by an unconventional interpretation of the role of the beetle’s bumpy surface geometry in promoting condensation, and based on theoretical modeling, we show how to maximize vapor diffusion flux at the apex of convex millimetric bumps by optimizing radius of curvature and cross-sectional shape. Integrating this apex geometry with a widening slope analogous to cactus spines directly couples facilitated droplet growth with fast directional transport, by creating a free energy profile that drives the droplet down the slope before its growth rate can decrease. This coupling is further enhanced by a slippery, pitcher plant-inspired nanocoating that facilitates feedback between coalescence-driven growth and capillary-driven motion on the way down. Bumps that are rationally designed to integrate these mechanisms are able to grow and transport large droplets even against gravity. We further observe an unprecedented six-fold higher exponent in growth rate, much faster shedding time and an order of magnitude greater volume of water collected compared to other surfaces. We envision that our fundamental understanding and rational design strategy can be applied to a wide range of water harvesting and phase change applications.

Geckos have the extraordinary ability to keep their sticky feet from fouling while running on dusty walls and ceilings. Understanding gecko adhesion and self-cleaning mechanisms is essential for elucidating animal behaviors and rationally designing gecko-inspired devices. We report a unique self-cleaning mechanism possessed by the nano-pads of gecko spatulae in both dry and wet conditions. This study has provided direct evidence that the unique shape of nanoscale spatula pads plays a crucial role in generating robust and stable adhesion while permitting efficient self-cleaning capabilities in dynamic regimes. Inspired by this natural design, we have fabricated micro/nano-pad-terminated artificial spatulae and micromanipulators that show similar effects, and that provide a new way to manipulate microparticles in dry and aqueous environments. By simply tuning the pull-off velocity, our gecko-inspired micromanipulators, made of synthetic microfibers with graphene-decorated micro-pads, can easily pick up, transport, and drop off microparticles for precise assembling. This work should open the door to the development of novel highly-efficient biomimetic self-cleaning adhesives, smart surfaces, MEMS, tunable micro/nano-manipulators, biomedical devices, and more.

Polyvinylidene difluoride (PVDF) microsphere aggregate films with exceptionally high specific surface area have been synthesized using the Nonsolvent-Induced Phase Separation (NIPS) method. These structures are similar to the highly hydrophobic leaves of the plant Colocasia esculenta (Taro) in both geometry and physical properties. The individual spheres mimic the rough micro-nodules of the leaf, while clustering of the microspheres provides an additional layer of hierarchy, emulating the water-shedding capabilities of Colocasia esculenta.

The microsphere aggregates have been created by spinodal decomposition of a ternary solution of PVDF, dimethylformamide (DMF), and water, initiated by submerging spin-cast films in a water bath. The sphere diameter has been varied from 250 nm to 4 µm by modulating the initial composition of the system, while the film thickness is determined by the spinning speed. Additionally, the sphere surface roughness has been controlled by changing the phase separation temperature, and consequently, the relative diffusion rates of DMF and water.

These microsphere clusters are among the most superhydrophobic pure polymer structures produced to date, with contact angles in excess of 171°, contact angle hysteresis less than 12°, and a slide angle of 3°. Furthermore, the synthesis technique is reproducible and easily scalable, and the coating can be sprayed on to any surface to impart anti-microbial, self-cleaning, chemical-resistant, and water-repellent properties.

During the last decades biomimetics has attracted increasing attention as well from basic and applied research as from various fields of industry, architecture and especially from building construction. Biomimetics has a high innovation potential and offers the possibility for the development of sustainable technical products and production chains. The huge number of organisms with the specific structures and functions they have developed during evolution in adaptation to differing environments represents the basis for all biomimetic R&D-projects. Novel sophisticated methods for quantitatively analysing and simulating the form-structure-function-relationship on various hierarchical levels allow new fascination insights in multi-scale mechanics and other functions of biological materials and surfaces. On the other hand, new production methods enable for the first time the transfer of many outstanding properties of the biological role models into innovative biomimetic products for reasonable costs. Within the framework of the new Collaborative Research Centre CRC 141 “Biological Design and Integrative Structures” an interdisciplinary team aims to explore the potential of biomimetics for a new smart kind of bioinspired architecture.
After a short introduction into the topic, the interdisciplinary approach and the different process sequences for the development of biomimetic materials for building construction are presented, using examples from CRC 141 and PBG Freiburg. Main focus is laid on bioinspired light-weight and damping materials and structures as well as on self-x-materials. Examples include branched and un-branched fiber-reinforced light-weight composite materials and jackets for concrete pillars, structural materials with a high energy dissipation capacity as fiber-reinforced graded foams and thin-layer compound materials, bioinspired high-load bearing adhesive as well as anti-adhesive materials and surfaces; and various bioinspired materials and structures with self-x-properties as self-repairing structural materials and the biomimetic façade-shading systems flectofin^® and flectofold inspired by the bird of paradise flower and the waterwheel plant, respectively.

The recent interest in biocompatible and bioabsorbable bone implants over permanent devices has driven a multitude of research into bioinspired designs. Key to these designs is mimicking the complex hierarchical structure of cortical bone, which displays porosity on multiple length scales including larger osteons and smaller lacuna spaces. It is this porosity that promotes healthy bone growth. We present a novel method for the fabrication of bioinspired, hydroxyapatite-based materials that mimic this complex and hierarchical porosity. In this, smaller porosity is created by the freeze casting technique, where a ceramic scaffold is templated by the growth of ice crystals, while larger porosity is templated through the use of 3D printed structures. This results in final scaffolds with interpenetration between porosity at two length scales. The 3D printing process allows for a high level of control over the final porosity size and shape. Results show that cell viability is good and scaffolds have not shown toxicity. Cell proliferation and differentiation will be assessed. Applications as biomedical implants and structural materials will be discussed.

This work is supported by funding provided by the Multi-University Research Initiative through the Air Force Office of Scientific Research (AFOSR-FA9550-15-1-0009).

Load-bearing reinforcing elements in continuous matrices allow for improved mechanical properties and can reduce the weight of structural composites. As the mechanical performance of the system is heavily affected by interfacial properties, tailoring the interactions between polymer matrices and reinforcing elements is a crucial problem. Recently, several studies using bio-inspired model systems suggested that interfacial mechanical interlocking can lead to an efficient energy dissipation mechanism in platelet-reinforced composites. While cheap and effective solutions are available at the macroscale, modifying the surface topography of micron-sized reinforcing elements still represents a challenging task. In this study, we report a simple method to create nanoasperities with tailored sizes and densities on the surface of alumina reinforcing platelets and investigate their effect on the energy dissipation mechanisms of bioinspired nacre-like materials. Epoxy-based composites reinforced with surface-roughened platelets exhibited enhanced energy dissipation during fracture, leading to values of work of fracture and resistance against crack propagation that are 110% and 30% higher than its non-roughened counterpart, respectively. Understanding the role of surface nanoasperities in mechanically-interlocked interfaces provides valuable design guidelines for the development of advanced and lightweight materials with a potential impact on the minimization of energy demand and carbon footprint in the transportation and construction sectors.

Nature has many examples of protein-based composites that are optimized for mechanical strength and durability. Silkworm silk fibers are protein composites made up of sericin and silk fibroin where the two proteins are spun together in a coaxial arrangement. The arthropod endocuticle is a tough, flexible component of the exoskeleton and is made up of chitin and proteinaceus layers forming a protein/polysaccharide composite. Due to their complexity and diversity, proteins and carbohydrates hold great promise in the creation of novel biomaterials. This is largely related to their chemical versatility which can result in programmable and complex functions. The knowledge gained in understanding how biological materials are constructed will enable bio-inspired design and fabrication of functional materials with tailored properties. In this talk I will cover research undertaken by our group in exploring the structure-property relationships of silk and other biopolymers, and their use in creating multifunctional materials. I will also discuss our results on the use of 3D direct write processes for protein-based materials. The utilization of proteinaceous materials in direct write processes is particularly attractive as these polymers offer aqueous processability, robust mechanical properties, chemical functionality, biomolecular recognition motifs, and self-assembly capabilities not found in synthetic inks.

Micro- or nanostructures with thin-film-like tips on the feet of climbing creatures contribute to their remarkable adhesion capabilities. Micro- or nanostructures with a protruding thin-film tip can exhibit superior adhesion properties in comparison to chemical-based ones because the adhesion force mainly originates from intermolecular interactions owing to the attractive van der Waals forces between the tips and the contact surface.

Motivated by such fascinating structures, many research groups have developed fabrication methods based on various top-down and bottom-up approaches for designing similar artificial structures. We have demonstrated beetle-inspired microscale structures with mushroom-shaped tips as a biomimetic dry adhesive, which can produce 2−30 times higher pull-off force than pillars with simple flat or hemispherical heads because of the enhanced contact area, pollution tolerance, homogeneous stress distribution, and prevention of crack propagation. Furthermore, these microscale structures are relatively easy to produce in a scalable manner and are mechanically more durable as compared with the slanted hierarchical hairy nanostructures. Therefore, these adhesive pads show a strong potential to replace existing gripping techniques such as vacuum suction or electrostatic chucks since dry adhesives enable precise transportiation of various substrates without any surface contamination in a reversible, repeatable, and durable manner.

To fabricate mushroom-like microstructures, poly-dimethylsiloxane (PDMS) is generally utilized because its favorable mechanical properties (low elastic modulus and high elongation at break) allow for simple prototyping and enhanced normal and shear adhesion strength. Nonetheless, it shows inherent limitations in maintaining its form and adhesion strength at high temperatures (>200°C), which restricts high-temperature applications. To address this problem, we fabricated mushroom-like microstructures using carbon nanotube (CNT)/PDMS composites, which enables enhanced thermal stability (>300°C) while maintaining proper adhesive forces. Furthermore, this composite microstructure exhibits good electrical conductivity (10 S/m). As a result, this bio-inspired composite adhesive pad has a strong potential to be used for a variety of applications from functional adhesives for high-t