Symposium BM6 : Fabrication, Characterization and Applications of Bioinspired Nanostructured Materials

Detecting target analytes with high specificity and sensitivity in any fluid is of fundamental importance to analytical science and technology. Surface-enhanced Raman scattering (SERS) has proven to be capable of detecting single molecules with high specificity, but achieving single-molecule sensitivity in any highly diluted solutions remains a challenge. Here we demonstrate a universal platform that allows for the enrichment and delivery of analytes into the SERS-sensitive sites in both aqueous and nonaqueous fluids, and its subsequent quantitative detection of Rhodamine 6G (R6G) down to ∼75 fM level (10⁻¹⁵ mol/L). Our platform, termed slippery liquid-infused porous surface-enhanced Raman scattering (SLIPSERS), is based on a slippery, omniphobic substrate that enables the complete concentration of analytes and SERS substrates (e.g., Au nanoparticles) within an evaporating liquid droplet. Combining our SLIPSERS platform with a SERS mapping technique, we have systematically quantified the probability, p(c), of detecting R6G molecules at concentrations c ranging from 750 fM (p > 90%) down to 75 aM (10⁻¹⁸ mol/L) levels (p ≤ 1.4%). The ability to detect analytes down to attomolar level is the lowest limit of detection for any SERS-based detection reported thus far. We have shown that analytes present in liquid, solid, or air phases can be extracted using a suitable liquid solvent and subsequently detected through SLIPSERS. Based on this platform, we have further demonstrated ultrasensitive detection of chemical and biological molecules as well as environmental contaminants within a broad range of common fluids for potential applications related to analytical chemistry, molecular diagnostics, environmental monitoring, and national security.

Keywords: SERS | slippery surfaces | nanoparticles

References
1. S. M. Nie & S. P. Emory, Probing single molecules and single nanoparticles by surface enhanced Raman scattering. Science 275, 1102 – 1106 (1997).
2. T.-S. Wong, S. H. Kang, S. K. Y. Tang, E. J. Smythe, B. D. Hatton, A. Grinthal & J. Aizenberg, Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 477, 443 – 447 (2011).
3. S. Yang, X. Dai, B. B. Stogin, & T.-S. Wong, Ultrasensitive surface-enhanced Raman scattering detection in common fluids. Proc. Natl. Acad. Sci. USA 113, 268 – 273 (2016).

Nature-inspired liquid-repellent surfaces are primarily modeled after two classes of biological surfaces – leaves of lotus¹ and pitcher plant². Lotus leaves rely on air-infused textured surfaces to repel impinging liquid droplets¹; while the leaves of pitcher plant utilize liquid-infused textured surface to maintain a highly slippery interface³. Natural and synthetic surfaces that can switch between these two liquid-repellent states are rare due to the distinctive repellent mechanisms. Here, we demonstrated a magnetically shape-shifting surface that can reversibly transform the liquid-repellent states between the modes of lotus leaves and pitcher plant. The surface property change can be programmed on-demand by external magnetic field. The ability to alter surface interfacial properties dynamically will open up new opportunities for smart liquid-repellent skin, programmable fluid control and transport, adaptive drag reduction, and controlled-release devices.

References
1. Barthlott, W. & Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 202, 1-8, doi:DOI 10.1007/s004250050096 (1997).
2. Bohn, H. F. & Federle, W. Insect aquaplaning: Nepenthes pitcher plants capture prey with the peristome, a fully wettable water-lubricated anisotropic surface. Proc. Natl Acad Sci USA 101, 14138-14143, doi:10.1073/pnas.0405885101 (2004).
3. Wong, T. S. et al. Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 477, 443-447, doi:10.1038/nature10447 (2011).

To a certain degree, it is possible to control the macroscopic wetting properties of a surface by its nano- and microstructure. In particular, super liquid-repellant-surfaces have received interest due to their many potential applications, such as anti-fouling for for example. Super liquid-repellency can be achieved by nano- and microstructuring a low energy surface in a way, that the structure can entrap air underneath
the liquid. The common criteria for super liquid-repellency are a high apparent advancing contact angle and a low contact angle hysteresis.
For a better understanding of how a drop advances and recedes on such a structured surface, we imaged the motion of a water drop on a superhydrophobic array of micropillars by laser scanning confocal microscopy (LSCM). With LSCM, we imaged an advancing water front on a superhydrophobic surface at a resolution of 1 µm. The results give a qualitatively new picture of how water advances on the microscopic
scale. We demonstrate that in contrast to traditional goniometer measurements, the advancing contact angle is close to 180° or even higher.
In contrast, the apparent receding contact angle is determined by the strength of pinning. We propose that the apparent receding contact angle should be used for characterizing super liquid-repellent surfaces [1,2].

[1] F. Schellenberger et al., Phys. Rev. Lett. 116, 096101 (2016)
[2] P. Ball, Nature Materials 15, 376 (2016)

Multiscale hierarchical structures show engineered interfacial properties that are important for controlled wetting, structural color, and selective filtration. In particular, bioinspired three-dimensional (3D) substrates have achieved such properties with superior mechanical stability over large areas (>cm² ). The fabrication of 3D patterns with length scales spanning several orders of magnitude (e.g., nm to μm), however, is usually done with complex top-down processes such as multistep photolithography or imprinting. Moreover, these tools cannot easily manipulate order/disorder of multiscale features over large areas. Here we found that memory-based, sequential wrinkling process can transform flat polystyrene (PS) sheets into bioinspired, three-dimensional hierarchical textures. Multiple cycles of plasma-mediated skin growth followed by directional strain relief of the substrate resulted in hierarchical architectures with characteristic generational (G) features. Independent control over wrinkle wavelength and wrinkle orientation for each G was achieved by tuning plasma treatment time and strain-relief direction for each cycle. As a practical application, we demonstrated stretchable superhydrophobicity on elastomeric hierarchical wrinkles monolithically formed by high fidelity pattern trasfer of PS templates designed by the sequential wrinkling. The poly(dimethysiloxane) (PDMS) wrinkles consisting of three different length scales showed wetting properties characteristic of static superhydrophobicity with water contact angles (>160°) and very low contact angle hysteresis (<5°). To examine how superhydrophobicity was maintained as the substrate was stretched, we investigated the dynamic wetting behavior of bouncing and splashing upon droplet impact with the surface. The substrate remained superhydrophobic up to 100% stretching with no structural defects after 1000 cycles of stretching and releasing. Stretchable superhydrophobicity was possible because of the monolithic nature of the hierarchical wrinkles as well as partial preservation of nanoscale structures under stretching.

The ability to control the repellent properties of bio-inspired immobilized liquid layers is of interest for a wide range of applications. Liquid layers created using infused polydimethyl siloxane (PDMS) polymers offer a potentially simple way of accomplishing this goal through the adjustment of nanoscale parameters such as cross-linker ratio and infused oil viscosity. In this work, we examine how tuning these parameters affects the material properties of the infused polymer, the stability of the liquid overlayer, and finally the overall performance of this system against bacterial adhesion and biofilm formation. We find that cross-linker density appears to have the greatest impact on the system, with a lower cross-linker:base ratio resulting in both an increased liquid overlayer stability and improved performance against bacteria. We further demonstrate how this finding may be exploited to produce patterns of slippery/sticky areas on the surface of the infused polymers for controlling the spatial arrangement of proteins and bacteria. These results demonstrate a new degree of control over immobilized liquid layers and may help facilitate their use in new applications.

Tree and torrent frogs are able to adhere and move about their wet or even flooded environments without falling. The secret of their outstanding adhesive performance is the complex hierarchical structure of their attachment pads, including microchannels of different length scales, anisotropically fiber-reinforced micropillars and different constitutional materials. Understanding the design principles behind this original surface design opens the door to novel adhesion strategies for reversible attachment in artificial systems. Over the last years, strategies to prepare frog-like microstructured surfaces of different soft materials have been reported [1-4]. However, hybrid structured surfaces with oriented fibers embedded in soft microstructures represent a fabrication challenge. I will present new fabrication strategies to obtain hierarchical, microstructured surfaces containing aligned nanofibers. The role of the anisotropic morphology in mechanical stabilisation, adhesion/friction properties and detachment will be described. Our results will clarify the role that oriented keratin fibers might have for directional and reversible attachment of frogs in wet environments.

Torrent-frog inspired adhesives: attachment to flooded surfaces. J. Iturri, L. Xue, M. Kappl, L. García-Fernández, W.J.P. Barnes, H.J. Butt, A. del Campo. Adv. Funct. Mater. 2015, 25(10), 1499-1505
Morphological studies of the toe pads of the rock frog, staurois parvus (family: Ranidae) and their relevance to the development of new biomimetically inspired reversible adhesives. D.M. Drotlef, E. Appel, H. Peisker, K. Dening, A. del Campo, S.N. Gorb, W.J.P. Barnes, Interface Focus 2015, 5(1), 1-11
Bioinspired orientation dependent friction. L. Xue, J. Iturri, M. Kappl, H.J. Butt, A. del Campo*. Langmuir 2014, 30(37), 11175–11182
Insights into the adhesive mechanisms of tree-frogs using artificial mimics. D.-M. Drotlef, L. Stepien, M. Kappl, W. J. P. Barnes, H.-J. Butt, A. del Campo. Adv. Funct. Mater. 2013, 23(9), 1137-1146

Many natural surfaces such as butterfly wings, beetles’ backs, and rice leaves exhibit directional liquid adhesion or transport; this is of fundamental interest as well as for applications including self-cleaning surfaces, microfluidic devices, and phase change energy conversion. For example, the intricate scales on the wings of the Morpho aega give rise to hydrophobicity and anisotropic droplet roll-off behavior. Previous studies have explained anisotropic roll-off, for example, via the directionality of a rigid rachet surface or the re-arrangement of nanoscale tips. Inspired by the butterfly wing, we demonstrate the fabrication of flexible synthetic scale surfaces from arrays of thin carbon nanotube (CNT) microstructures. Uniform centimeter-scale arrays of CNT scales are synthesized by a strain-engineered chemical vapor deposition (CVD) technique, using an offset-patterned catalyst layer that imparts a spatial gradient in the CNT growth rate, causing the scales to curve during growth. The scale height and curvature is controlled via the CNT growth parameters. After growth, the scales are conformally coated by a thin ceramic layer (i.e., Al₂O₃, by atomic layer deposition) followed by a hydrophobic polymer (divinylbenzene, by CVD) to tune their compliance and surface wettability. We demonstrate that the CNT scales exhibit anisotropic droplet roll-off, and via high-resolution optical imaging we observe how the droplet pinning and motion are influenced by the scale geometry and flexibility. The electrical conductivity and mechanical robustness of the CNTs, and the ability to fabricate complex multi-directional patterns, suggest further opportunities to create engineered scale surfaces.

The term biofouling describes the agglomeration of microorganisms on surfaces mainly in contact with liquid [1]. Free-floating cells freely swim and approach surfaces until they undergo irreversible attachment. At this point, thanks to the quorum sensing effect a bacterial colony will start to grow and disseminate along the surface. These bacterial layers can be found on pipelines, hulls of boats or food packaging, leading to corrosion, increase on fuel consumption due to friction and food poisoning [2]. Furthermore, when they form in medical devices nosocomial infections and failure due to clogging arise. Accounting to the economical losses and mortality related to biofilm formation [3], new approaches battling this field have been proposed in the past years. However, the increased resistance (up to a factor of 1000) of enclosed bacteria compared to free-floating cells as well as the possibility to restore films within hours inspired me to focus on hindering or delaying the first adhesion events.
On this behalf, we focused on super-liquid repellent surfaces as a platform to prevent biofilm formation. Candle-soot based superamphiphobic coatings were proved to prevent wetting by both water and low surface tension liquids thanks to the existence of a mobile air layer (Cassie state) between solid features and liquid [4,5]. By means of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) we confirmed an adsorption of proteins (bovine serum albumin and human serum plasma) below the instrument detection limit of 2 ng/cm², provided by the synergy between topography in the nano-scale and chemistry [6]. Furthermore the stability of the air layer and ability to hinder bacterial adhesion was visualized by the study through laser scanning confocal microscopy (LSCM) of E. coli (GFP expressed) biofilm formation.
[1] Costerton, J. W.; Stewart, P. S.; Greenberg, E. P. Science 284, 1318-1322, 1999.
[2] Klevens, R. M. et al.; Public Health Rep 122, 160-166, 2007.
[3] Davies D. Nat. Rev. Drug Discov. 2, 114, 2003.
[4] Deng X., Mammen L., Butt H.-J., Vollmer D., Science 335, 67-70, 2012.
[5] Paven M., Papadopoulos P., Schöttler S., Deng X., Mailänder V., Vollmer D., Butt H.-J., Nature Communication 4, 2013.
[6] Schmüser L., Encinas N., Paven M., Graham D., Castner D.G., Vollmer D., Butt H.-J., Weidner T. (submitted ).

Semiaquatic water bugs and plants efficiently move and breathe while submerged underwater due to an air film retained on their superhydrophobic hair-covered surfaces. Air entrapped between the hairs forms a shear-free air-water interface, resulting in a non-zero velocity and reduced drag at such surfaces. Artificial polymeric nanofur covered with dense layer of nano- and microhairs is fabricated using a hot pulling technique in which softened polymer is locally elongated during separation from a heated sandblasted steel plate [1]. Similarly to natural surfaces, artificial bioinspired superhydrophobic polymeric nanofur film submerged underwater traps air between its hairs, forming a fixed air film on the surface. The trapped air significantly reduces the pressure drop across the microchannels lined with bioinspired polycarbonate nanofur compared to unstructured flat polymer, indicating reduction in fluid drag. Additionally, high stability of the retained air film under external stimuli is required for underwater applications. The robustness of the underwater retained air film on the bioinspired nanofur against pressure was estimated by analyzing the air-water-interface at different applied pressures. Furthermore, by perforating the nanofur and applying additional pressure to support the air-water interface, we demonstrate a fourfold increase of the air layer stability against pressure. The response of the air-water-interface to varying pressure difference between the hydrostatic pressure and the pressure of the retained air layer was analyzed in order to estimate the stability of the nanofur under pressure fluctuations. Moreover, we observed a significant increase in lifetime of the air-water interface retained by the perforated nanofur under different hydrostatic pressures.
[1] Kavalenka et al., ACS Appl. Mater. & Interfaces 7, 1065 (2015)

Antimicrobial surfaces have been critical for many areas including medical and industry. There are two main strategies for antimicrobial surfaces; to reduce bacterial adhesion or kill them by using antibacterial agents. Bioinspired soft polydimethylsiloxane (PDMS) shark skin structures show reduced bacterial attachment due to highly rough microstructured surface design. However, they wear off by the time and are not good enough to prevent bacterial adhesion in the long term. Herein, we combine antibacterial and antifouling properties by incorporating antibacterial titanium dioxide (TiO2) nanocomposite material with shark skin structure. We demonstrated that shark skin patterns prevented bacterial attachment and also induced 80-95% bacteria death in an hour. Improved mechanical properties help them to be durable in the long term. Our method is solution processable, robust and roll to roll compatible method.

Mother nature provides the ultimate inspiration for various topologically ordered patterns, structures, and flexible motion from one-dimensional (1D) linear structures such as actin filaments and muscle fibers, two-dimensional (2D) arrayed compound eyes of insects, Morpho butterfly wings composed of three-dimensional (3D) hierarchical complex structures, etc. With self-assembly and self-organization, which are the driving principles in the formation of these natural structures, a number of biologically inspired artificial materials have been prepared.
Surface wrinkling is an inventive and unconventional technique that is also fast and inexpensive for various types of surface patterning involving sinusoids (ripples), herringbones, labyrinthine designs, etc. It is especially suited for large-area surfaces of poly(dimethylsiloxane) (PDMS) elastomers based on mechanical (buckling) instability. This self-organization buckling phenomenon is widely observed in natural systems such as humanskin, brain cortex, fruits, and plants. Owing to the periodic structure and dynamically tunable wrinkles, it has been used in many applications.
Previously, we have succeeded in fabricating ultrasmall attoliter-sized (10⁻¹⁸ L) 1D metallic nanocup arrays embedded in PDMS films by colloidal soft-lithography and wrinkle processing (H. Endo et al., Langmuir 2013, 29, 15058). Moreover, we described the fabrication of various topological 1D colloidal arrays, including single, helical, zigzag, triple-line, and random arrays integrated in sinusoidal wrinkle grooves, through simple spin-coating (H. Endo et al., Coll. Surf. A 2014, 443, 576). The particles in these arrays can be connected using plasma etching, forming beaded, robust, and long (>100 mm) colloidal chains.
In this study, we succeeded in the fabrication of a large-area ultra-water-repellent film on which water drops can be flexibly controlled by utilizing original 3D-streching method. It found that surfaces with different properties—an ultra-water-repellent and high-adsorption area and an ultra-water-repellent area—can be generated on the basis of two different pattern structures by applying water-repellent coating to the wrinkle film. Moreover, we succeeded fabrication of film with highly adhesive superhydrophobic surface and SERS activity. The results of this study will not only contribute to resolving issues of conventional top-down lithography techniques but will also be applicable to environmental, water-saving, medical and many other fields. In addition, we propose fabrication of 3D microobjects using controlled folding/bending of wrinkled-thin films besed on elastocapillary force toward new type of 3D-imprinting technology.

Combining high optical transmission with self-cleaning and water-repellency is of great interest for optical systems, especially for those operating in outdoor conditions such as solar cells. Natural surfaces of water plants Salvinia Cucullata and Pistia Stratiotes combine these functionalities in a transparent layer of dense microhairs. This layer renders their surface superhydrophobic without affecting the absorption of sunlight necessary for photosynthesis. Inspired by these natural surfaces, we introduce superhydrophobic flexible thin nanofur films made from optical grade polycarbonate, which can be used as a transparent coating on optoelectronic devices. Thin nanofur films are fabricated using a highly scalable cost-effective hot pulling technique, in which heated sandblasted steel plates are used to locally elongate softened polymer resulting in a surface covered in microcavities surrounded by randomly distributed high aspect ratio micro- and nanohairs. The superhydrophobic nanofur exhibits high water contact angles (166±6°), low sliding angles (< 6°) and is self-cleaning against various contaminants. Additionally, subjecting the nanofur to argon plasma reverses the film wettability to underwater superoleophobic, enabling its use as an underwater oil-repelling coating.

The thin nanofur exhibits transmission values above 85% with high forward scattering when used as a translucent self-standing film and reflection values of less than 4% for the visible spectrum when used as a coating on a polymer substrate. Those properties make it suitable for light extraction in organic light emitting diodes (OLEDs). We demonstrate a 10% relative increase of luminous efficacy for a nanofur coated OLED with respect to a bare device. Lastly, thin nanofur coatings can be used as light-collecting and -diffusing elements for enhancing absorption in solar cells. We report on the optical coupling of the thin nanofur film to a multi-crystalline silicon solar cell, which results in a relative gain of 5.8% in photogenerated current compared to a bare photovoltaic device.

Cephalopods (squid, octopuses, and cuttlefish) are known as the chameleons of the sea – these animals can alter their skin’s coloration, patterning, and texture to blend into the surrounding environment. These remarkable capabilities are enabled by unique proteins and self-assembled nanostructures found within cephalopod skin. I will discuss our work on new types of photonic and protonic devices fabricated from cephalopod-inspired materials. Our findings hold implications for the next generation of infrared stealth, renewable energy, and bioelectronics technologies.

Understanding the structure-function relationships of pigment based nanostructures can provide insight into the molecular mechanisms behind biological signaling, camouflage, or communication experienced in many species. In squid Doryteuthis pealeii, combinations of phenoxazone-based pigments are identified as the source of visible color within the nanostructured granules that populate dermal chromatophore organs. In the absence of the pigments, granules experience a reduction in diameter with the loss of visible color, suggesting important structural and functional features. Energy gaps are estimated from electronic absorption spectra, revealing HOMO-LUMO energies that are dependent on the varying carboxylated states of the pigment. These results implicate a hierarchical mechanism for the bulk coloration in cephalopods originating from the molecular components confined within in the nanostructured granules of chromatophore organs.

In nature, some marine organisms, such as Vogtia and Cephalopods, have evolved to possess camouflage traits by dynamically and reversibly altering their transparency, fluorescence, and coloration via muscle controlled surface structures and morphologies. To mimic this display tactics, we designed similar deformation controlled surface engineering via strain-dependent cracks and folds to realize four types of novel mechanochromic devices: (1) transparency change mechanochromism (TCM), (2) luminescent mechanochromism (LM), (3) color alteration mechanochromism (CAM), and (4) encryption mechanochromism (EM), based on a simple bilayer system containing a rigid thin film and a soft substrate. These devices exhibit a wide scope of mechanochromic response with excellent sensitivity and reversibility. The TCM device can reversibly and instantly switch between transparent and opaque state upon stretching and releasing. The LM can emit intensive fluorescence as stretched with an ultrahigh strain sensitivity in comparison to strain sensors based on electrical resistance change. The CAM can turn fluorescent color from green to yellow to orange as stretched within 20% strain. The EM device can reversibly reveal and conceal any desirable patterns. These novel devices are promising for applications in smart windows, dynamic optical switches, strain sensors, encryption, etc.

Vivid, saturated structural colors such as many violet, blue and green hues provide a conspicuous and important aspect of the appearance of many animals^1,2,3. In birds, butterflies, beetles, bees and arachnids, both iridescent and isotropic colors are produced by constructive interference from a staggering diversity of biophotonic nanostructures. The nanostructures in arthropods, in particular, span the phase space of morphologies commonly seen in block copolymer melts, lipid-water or surfactant systems but at harder to achieve optical length scales³. These nanostructures are architecturally sculpted within the arthropod scale cells by the in-folding of membranes, biologically “back-filled” with chitin, followed by cell death leaving behind chitin nanostructures in air, reminiscent of an engineering process^1,3. Whereas the amorphous or quasi-ordered biophotonic nanostructures in bird feather barbs appear to be self-assembled by a visco-elastic phase separation process followed by a dynamic self-arrest. I will summarize the structure, optical function, development, and biomimetic potential of these meso-scale biophotonic nanostructures (with a special focus on the single gyroid I4₁32), when defect-free, long-range synthetic fabrication of photonic morphologies remains challenging.
[1] Saranathan et al., Nano Letters (2015), 15, 3735–3742
[2] Saranathan et al., J. Roy. Soc. Interface (2012), 9, 2563–2580
[3] Saranathan et al., PNAS (2010), 107, 11676–11681

There have been significant efforts on developing novel ways to efficiently transform sunlight to energy in photovoltaics and photobioreactors by using for example high refractive index materials and nanostructures. However, it remains a major challenge in harvesting solar energy to limit photodamage in low surface area devices. In a previous study, we show that Tridacnid giant clams are highly efficient “solar transformers”. The clams have evolved a layer of forward-scattering cells, or iridocytes, overlying vertically arranged algae pillars. The iridocytes redirect photosynthetically efficient wavelengths of downwelling sunlight from the top surface of the clam tissue evenly to the algae located in the narrow pillar arrays underneath. This redistribution of solar flux in forward direction seems to allow algae to achieve maximally efficient photosynthesis within their unique pillar geometries. Here, we have designed and fabricated synthetic iridocytes via the self-assembly of silica nanoparticles (120 – 300 nm in diameter) into micron-sized beads by way of Pickering emulsion. Our synthetic iridocytes show very similar phase functions (scattered light intensity per scattering angle) as the wild iridocytes. Compared to natural clam iridocytes, the synthetic ones show a similarly wide angular distribution of light in the forward-scattering direction. The ratio between the forward (0 – 50 °) and backward (120 – 165 °) scattered light intensities and the peaks of back-scattered wavelengths from the synthetic iridocytes can be tuned by varying the size of nanoparticles and microbeads. Thus, we can engineer the synthetic iridocytes that favor forward-scattering of photosynthetically efficient light while rejecting less-efficient wavelengths. We further investigate the redistribution of downwelling solar irradiance from the synthetic iridocytes in a wavelength-tunable manner for potential applications as photobioreactors.

Despite the limitation to a restricted range of organic materials, evolution has optimized the color response of many organisms to an amazing extent that often appears to surpass the physical limits of the employed organic materials. One such example are the common pierid butterflies which show bright colors ranging from white to red caused by various pterin pigments concentrated in scattering spheroidal beads in the wing scales. The final coloration arises from the interplay of absorption and scattering of light by these pigment-loaded granules. Given the sparsity of the beads in the wing scales, the high color brightness suggests a scattering strength of the beads that significantly surpasses that of chitin, from which the beads are composed of. To elucidate this apparent contradiction, we have analyzed the optical signature of the pierids’ highly saturated pigmentary colors by using Jamin-Lebedeff interference microscopy combined with Kramers-Kronig theory and light scattering modeling. Our study shows that both the shape of the beads and the unusually high complex refractive index of these pigmented granules are optimized to give rise to one of the brightest biological materials. Our results present yet another trick of evolution for optimized light scattering that might be useful for bio-inspired applications.

The history of microscopy is a history of human progress fueled by the ability to image, imagine, and then create at ever-smaller length scale. As man-made materials become more similar to the biological structures that inspire them, they increasingly combine nano-sized hard and soft, synthetic and biological components. With its unique spatial resolution and chemical sensitivity, UV laser-pulsed atom probe tomography (APT) is poised to revolutionize our understanding of such complex composites. Specifically, APT has provided unprecedented insights into the nanostructure and phase composition of biological materials such as bone, dentin, and enamel, but also ferritin protein nanocages and even cells.
For any given organism, the hardest materials are typically used to protect the surface of teeth. Optimized to withstand the forces of mastication, they are hierarchically structured, organic/inorganic nanocomposite materials. For example, the radula teeth of the chiton are capped with a composite made from magnetite (Fe₃O₄) and a nanofibrous chitin scaffold.^[1] The excellent hardness and wear resistance of this layer allows the chiton to abrade rocks during feeding. Human tooth enamel, on the other hand, is composed of hydroxylapatite nanowires, thousands of which are bundled into rods that are organized in a three-dimensional weave; this provides great fracture resistance and a much enhanced fatigue life.^[2] It has long been known that the susceptibility of enamel to caries, i.e. acid corrosion, is greatly dependent on the presence of magnesium, carbonate, and fluoride ions. I will discuss our recent insights into the chemistry of organic/inorganic interfaces, and the role of magnesium, fluoride, and iron at grain boundaries, and in amorphous intergranular phases that are integral to the mechanical properties of teeth and their resistance to corrosion. ^[3]
[1] D. Joester, L. R. Brooker, in Iron Oxides: From Nature to Applications (Ed.: D. Faivre), Wiley-VCH Verlag GmbH & Co. KGaA, 2016, pp. 177-205.
[2] A. Nanci, Ten Cate's Oral Histology: Development, Structure, and Function, 8 ed., C.V. Mosby Co., 2012.
[3] a) L. M. Gordon, D. Joester, Nature 2011, 469, 194-197. "Nanoscale chemical tomography of buried organic-inorganic interfaces in the chiton tooth"; b) L. M. Gordon, L. Tran, D. Joester, ACS nano 2012, 6, 10667-10675. "Atom probe tomography of apatites and bone-type mineralized tissues"; c) L. M. Gordon, M. J. Cohen, K. W. MacRenaris, J. D. Pasteris, T. Seda, D. Joester, Science 2015, 347, 746-750. "Amorphous intergranular phases control the properties of rodent tooth enamel."; d) L. M. Gordon, D. Joester, Frontiers in Physiology 2015, 6. "Mapping residual organics and carbonate at grain boundaries and in the amorphous interphase in mouse incisor enamel".

Cellulose is the most abundant natural polymer produced in the biosphere. Renewable, nontoxic, and inexpensive cellulose is expected to be a highly relevant source for the development of sustainable functional materials due to its intrinsic biocompatibility and biodegradability. In nature, cellulose is found in fibers of microfibrils made by individual chains of β-(poly-1,4-D-glucose). Each microfibril consists of amorphous and crystalline domains, and a complex network of hydrogen bonds is formed. Acidic treatment of bulk cellulose selectively hydrolyzes amorphous regions of the biopolymer, thereby leaving rod-shaped cellulose nanocrystals (CNCs) that are hundreds of nanometers in length.
One of the most prominent features of CNCs is their ability to self-assemble into a chiral nematic structure where the unidirectional CNC layers are organized in a long-range helical structure with periodic rotation. This chiral nematic structure, which can be seen throughout nature in plant cell walls and exocuticles of many beetles, is frequently referred to as one-dimensional photonic crystals, selectively reflecting wavelengths of light corresponding with the helical pitch. Moreover, the light reflected from the structure is circularly polarized with a handedness that is determined by the helical orientation. As for the CNC films, the wavelength of light reflected can be tuned over the entire visible region with brilliant coloration, but the handedness of reflected light has been uncontrollable thus far. CNC films are fixed in a left-handed helical orientation which leads to the reflection of left-handed circularly polarized light (CPL). However, there are some exceptions in nature which demonstrate the reflection of both left and right-handed CPL despite having similar components to CNC films. For example, the exocuticles of Plusiotis resplendens mainly composed of chitin is known to reflect both handed CPL due to the coexistence of half-wave retarder sandwiched between two left-handed helical layers. Another renowned example is found in the cell walls of Pollia condensate, based mainly on cellulose; the mechanism has not been clarified to date.
With inspiration from such unique examples found in nature, we thought that it may be possible to convert CNC films composed of cellulose into right-handed CPL reflective materials. We have explored the possibility for controlling the handedness of reflected CPL through the fabrication of many CNC films under varying conditions. These experiments are characterized based on UV-Vis transmission measurements with CPL filters and scanning electron microscopy measurements. In conclusion, it was found that a specific arrangement of CNCs can force the film to reflect right-handed CPL.

Aliphatic amine microcapsules are desirable components for bio-inspired smart materials such as self-healing and self-reporting systems. However, the highly reactive nature of amines excludes traditional microencapsulation methods. In this work, we report an innovative approach to prepare mechanically robust and thermally stable microcapsules containing high loading of aliphatic amines. With designed chemical modification, the reactivity of the targeting amines is masked. The protected amines are thereafter encapsulated via a developed emulsification condensation polymerization method. In order to achieve on-demand activity of the sequestered amines, we incorporated nanoparticles into the microcapsules, which facilitates a stimuli-triggered in capsule deprotection to restore the aliphatic amine functionality. We integrate these microcapsules in polymer coatings and bulk polymer matrices and demonstrate that the recovered amines are fully functional to implement programmed self-healing and self-reporting abilities. With the ready availability of diverse protecting chemistries, this novel microencapsulation strategy establishes a versatile platform to encapsulate a wide range of highly reactive chemicals.

Electrospinning is a favorable technique to prepare artificial architecture mimicking extracellular matrix (ECM) structure. Cell cultivation on the electrospun meshes is often restricted to 2-D cultivation because cells cannot easily penetrate into nanoporous structures of the meshes. Instead of solid ground for depositing nanofiber, liquid ground and ball shape ground were introduced and showed low density of sponge nanofiber mesh. Hydrophilic polymer, however, is limited to use independently that the nanofiber rapidly dissolved out in water without further crosslinking process. Herein, we prepared hydrophilic alginate nanofibrous mesh by co-axial electrospinning with etching polymeric shells, which enables electrospinning of inner alginate and maintains fibrous structure of the alginate core during calcium-dependent crosslinking. Electrospun alginate nanofiber covered with poly (e-caprolactone) (PCL) (Mw 43-50K) shell using co-axial nozzle. Alginate and PCL were injected through inner and outer nozzle with 0.5 and 2ml/h, respectively. The nanofiber was subsequently immersed in various concentration of CaCl₂ solution for alginate crosslinking. After peeling off the PCL shell in chloroform, calcium cross-linked alginate gel nanofiber suspension was finally harvested. Alginate nanofiber was characterized for morphology, water swelling ratio, elastic modulus of single nanofiber, thermal property, and crystallization property. NIH3T3 cells were seeded on the top of the mesh and evaluated cell proliferation rate and cell migration according to calcium crosslinking density of the alginate nanofiber. The morphology of alginate nanofiber with/without PCL shell was confirmed by TEM and SEM that the diameter was changed from around 700nm to 100nm. According to different calcium-crosslinking density from the range, 2wt% (AL NF_high) and 0.02wt% (AL NF_low) showed different pore distribution in the mesh and water swollen ratio. Completely removed PCL was confirmed by disappearance of PCL melting peak at 60°C by DSC. NIH3T3 were seeded on the alginate nanofiber and proliferated for 28 days. At day 3, cells were stained with MTT reagent and AL NF_low showed higher cell proliferation rate than AL NF_high. Phalloidin and nucleus were fluorescently stained and observed by Z-stacking image of confocal laser scanning microscope. As lower calcium concentration, cells migrated into the mesh more deeply. In conclusion, 3-D alginate mesh by shell etching of core-shell electrospun nanofibers was developed for 3D culture scaffold. In future study, these results developed to tissue engineering to fill the defect site based on its mechanical strength.

The hierarchical structure of wood constitutes an ideal scaffold for creating functionalized renewable biomaterials. Here the in situ synthesis of ferri- and ferromagnetic mineral phases in the intrinsically anisotropic cell anatomy yields hybrid wood materials with highly directional magnetic properties, which have been characterized by hysteresis curves parallel and perpendicular to the cell axis and by magnetic susceptibility tensors. ¹ Through a detailed characterization of the embedded magnetic phases using light and scanning electron microscopy, Raman mapping, and X-ray diffraction, it is possible to relate the nanocrystal composition, shape and size to the macroscopic magnetization of the modified wood samples. A variation of the chemical synthesis allows for tailoring the nano-crystallinity and aggregation state of the magnetic particles, as well as for their deeper insertion into the cell walls, resulting in higher degrees of magnetic anisotropy compared to previous studies. Based on the results, we envision the application of these magnetic hybrids as light-weight magnetic actuators.

1. Merk, V.; Chanana, M.; Gierlinger, N.; Hirt, A. M.; Burgert, I., Hybrid Wood Materials with Magnetic Anisotropy Dictated by the Hierarchical Cell Structure. ACS Appl. Mater. Interfaces 2014, 6 (12), 9760-9767.

Micro-scale optical components play a crucial role in imaging and display technology, biosensing, beam shaping, optical switching, wavefront-analysis, and device miniaturization. Bi-phase emulsion droplets synthesized from immiscible hydro- and fluorocarbons form liquid compound micro-lenses with a tunable droplet-internal optical interface. Adjustments in interfacial tensions between the constituent liquids and the surrounding medium are used to control the curvature of the internal interface of the droplet based microlenses. This allows the microlenses to be reconfigured to focus or scatter light, form real or virtual images, and allow for a wide tuning range of their focal length. Finite Difference Time Domain and Raytracing simulations were used to predict the lensing and scattering behavior of the droplets. Experiments using the droplets as lenses show their imaging capabilities, and selective tuning of the drops show that the scattering behavior can be observed macroscopically. A wide variety of stimuli can be used to manipulate the shape and orientation of the droplets including altering the chemical environment, light, and heat.

Radula, a rasping organ for feeding, is a characteristic feature for mollusks. Depending on species, the morphology and chemical composition of radula teeth vary largely. Much work has been focused on radula teeth of chitons, marine mollusks known to scrape rocks for food, as a model for biomineralization process. Meanwhile, little is known about the radula teeth of terrestrial mollusks. Banana slugs are terrestrial gastropods in the genus Ariolimax. It is native to North America and some individuals are bright yellow like a ripe banana. Here we use the banana slug radula as an example, and report the morphology and organic-inorganic hierarchical interface of the micro-teeth for the first time. The slug radula is a conveyor-belt like structure. It is comprised of transverse rows of teeth with files pointing in longitudinal directions. Each tooth is a unique natural nanocomposite that is mainly composed of an organic chitin matrix with trace amount of minerals. Unlike the chiton, with only one pair of teeth on each row, there are dozens of teeth on each row for banana slug radula. Interestingly, depending on their specific position, teeth on the same row are in three forms – medial, lateral and marginal. We found that such shape variation results in a slight change in chemical compositions, enabling the distinctive feeding function of each tooth. In addition, during the tooth maturation process, along with the morphological change, an increase in mineral composition is observed which leads to an increase in stiffness. Our findings serve as a first-of-its-kind detailed characterization of terrestrial mollusk radula teeth, and provide insights on correlations from teeth morphology, chemical composition and structural hierarchy, to mechanical behavior and feeding functions.

The structural integrity of bioinspired particles such as red blood cells and bacteria under flow conditions is dependent upon their ability to adapt their shape. Our goal is to examine the role of the composition of bioinspired vesicles on their shape during their flow through in a channel. Via the Dissipative Particle Dynamics simulation technique, we apply laminar flow in a cylindrical channel and investigate the shape transition of a bio-inspired four-component lipid vesicle encompassing DPPC, DMPC, glycolipids and cholesterol, and a hairy vesicle composed of phospholipids and pegylated lipids. We vary the channel dimensions, flow rate and composition to study their impact on the shape of the cell-mimetic and hairy vesicles. We will also characterize the critical flow rate at which the vesicles are ruptured. Our results could be potentially used to accelerate the design of smart particles with the ability to adaptive their shape under diverse flow conditions.

Achieving strong bonding between hydrogels and non-porous rigid surfaces and understanding the interface between the materials is crucial for the development of stretchable electronics, and soft microfluidics and biomedical devices. Here, we present a method to bond hydrogels to rigid substrates with tunable interfacial toughness values ranging from 10 J/m² to 1800 J/m² by controlled treatment of such substrates with functional silanes. To study the source of the large variations (three orders or magnitude) in interfacial toughness, we analyzed the peeled samples with surface characterization techniques such as scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) to identify interfacial chemical compositions and morphology. We discovered that hydrogel polymers were present on the substrate surface of strongly-bonded systems due to the presence of nitrogen and carbon signals. Moreover, the amount of polymer residue increased with increasing interfacial toughness, indicating a relationship between interfacial toughness and microscopic residues at the interface of the hydrogel-substrate bonded structure.

Colorimetric assay represents a class of simple and convenient technique for in-vit