Nanoscale design of snake skin for reptation locomotions via friction anisotropy

Multi-mode scanning probe microscopy is employed to investigate the nanostructure of dermal samples from three types of snakes. Sophisticated friction modifying nanostructures are described. These include an ordered microfibrillar array that can function to achieve mission adaptable friction characteristics. Significant reduction of adhesive forces in the contact areas caused by the 'double-ridge' nanoscale microfibrillar geometry provides ideal conditions for sliding in forward direction with minimum adhesive forces and friction. Low surface adhesion in these local contact points may reduce local wear and skin contamination by environmental debris. The highly asymmetric, 'pawl-like' profile of the microfibrillar ends with radius of curvature 20-40 nm induces friction anisotropy in forward backward motions and serves as an effective stopper for backward motion preserving low friction for forward motion. The system of continuous micropores penetrating through the snake skin may serve as a delivery system for lubrication/anti-adhesive lipid mixture that provides for boundary lubrication of snake skins.

Robust Chiral Organization of Cellulose Nanocrystals in Capillary Confinement

We showed large area uniformly aligned chiral photonic bioderived films from a liquid crystal phase formed by a cellulose nanocrystal (CNC) suspension placed in a thin capillary. As a result of the spatial confinement of the drying process, the interface between coexisting isotropic and chiral phases aligns perpendicular to the long axis of the capillary. This orientation facilitates a fast homogeneous growth of chiral pseudolayers parallel to the interface. Overall, the formation of organized solids takes hours vs weeks in contrast to the slow and heterogeneous process of drying from the traditional dish-cast approach. The saturation of water vapor in one end of the capillary causes anisotropic drying and promotes unidirectional propagation of the anisotropic phase in large regions that results in chiral CNC solid films with a uniformly oriented layered morphology. Corresponding ordering processes were monitored in situ at a nanoscale, mesoscale, and microscopic scale with complementary scattering and microscopic techniques. The resulting films show high orientation order at a multilength scale over large regions and preserved chiral handedness causing a narrower optical reflectance band and uniform birefringence over macroscopic regions in contrast to traditional dish-cast CNC films and those assembled in a magnetic field and on porous substrates. These thin films with a controllable and well-identified uniform morphology, structural colors, and handedness open up interesting possibilities for broad applications in bioderived photonic nanomaterials.

Adaptive nanomechanical response of stratified polymer brush structures

We have fabricated a stratified polymer surface film with tunable thickness (within 17-34 nm) through facile, room-temperature, UV-initiated polymerization with a temperature-sensitive pNIPAAM layer confined beneath a hydrophobic layer. AFM morphology and ellipsometric measurements were measured at each grafting step, along with XPS measurements of the overall layer to verify layer growth. The strong characteristic LCST behavior of pNIPAAM was observed in water, with a 100% change in thickness above and below this transition. The AFM nanomechanical results demonstrate vertical gradients of the elastic response tunable to a desired state by the external temperature. These temperature-sensitive, adaptive polymer structures with the pNIPAAM layer "hidden" beneath the rubbery, hydrophobic PBA topmost layer represent an interesting example of nanoengineering surfaces with properties such as adhesion, elastic modulus, and multi-level structural reorganization responsive to fluidic and temperature variations that can be important for biological purposes such as implant coatings, cell-surface mimicry, and drug delivery vehicles.

Biological thermal detection: micromechanical and microthermal properties of biological infrared receptors

Bioinspired design of biomimetic sensors relies upon the complete understanding of properties and functioning of biological analogues in conjunction with an understanding of their microstructural organization at various length scales. In the spirit of this approach, the microscopic properties of infrared (IR) receptors of snakes with "infrared vision" were studied with scanning thermal microscopy and micromechanical analysis. Low surface thermal conductivity of 0.11 W/(m K) was measured for the IR receptor surfaces as compared to the nonspecific skin areas. This difference in surface thermal conductivity should result in a significant local temperature gradient around the receptor areas. Micromechanical analysis showed that pit organs were more compliant than surrounding skin areas with an elastic modulus close to 40 MPa. In addition, the maximum elastic modulus was detected for the outermost layer with gradually reduced elastic resistance for the interior. The porous microstructure of the underlying tissue combined with the highly branched microfibrillar network (Biomacromolecules 2001, 2, 757) is thought to be responsible for such a combination of biomaterial properties. Considering these biomaterials features, we postulated a possible design of an artificial photothermal detector inspired by the microstructure of natural receptors. This bioinspired design would include a microfabricated cavity filled with an ordered lattice of microspheres with a gradient periodicity from the surface to the interior. Such a "photonic cavity" could provide an opportunity for multiple scattering at wavelength tuned to 8-12 microm as a range of highest sensitivity.

Surface behavior of amphiphilic heteroarm star-block copolymers with asymmetric architecture

We study the surface behavior of the asymmetric amphiphilic heteroarm poly(ethylene oxide) (PEO)/polystyrene (PS) star polymer on solid substrate. These star polymers differ in both architecture (four- and three-arm molecules, PEO-b-PS(3) and PEO-b-PS(2)) and in the length of PS chains (molecular weight from about 10 000 up to 24 000). We observed that, for a given chemical composition with a predominant content of hydrophobic blocks, the compression behavior of the PS domain structure controls the surface behavior and the final morphology of the monolayers. New features of the surface behavior of star-block copolymers are high stretching of the PS arms from the interface and enhanced stability of the circular PS domain structure, even at high compression. We suggest that for asymmetric star-block copolymers both architecture and chemical composition heavily favor the formation of highly curved interfaces and, thus, more stable circular domain structure with stretched PS arms.

Microthermal Probing of Ultrathin Polymer Films

We present the results of microthermal studies of ultrathin polymeric films deposited on a silicon substrate. We propose the procedure for microthermal analysis of ultrathin polymer films, elucidate limits of applicability of this technique to study their microthermal properties, and observe the thermomechanical response during local heating for films with thicknesses above 10 nm. The glass transition temperature of ultrathin polystyrene (PS) films deduced from experiments decreases for film thicknesses below 200 nm. For the thinnest PS film with clearly detectable response(25nm), the glass transition temperature is 20 °C below its bulk value. Our estimation of heat dissipation in the tip–surface contact area supports the conclusion that the observed micro-thermal-analysis response for polymer films is associated mainly with temperature-induced elastic variation of the contact area.

Ultramicrostructure and microthermomechanics of biological IR detectors: materials properties from a biomimetic perspective

Microstructural organization of the biological infrared (IR) receptors was studied to elucidate their materials properties useful for prospective biomimetic design of artificial IR sensors from organic/polymeric materials. The IR receptors in Melanophila acuminata beetles were studied with ultrahigh-resolution scanning probe microscopy (SPM) in a range of temperatures. By application of micromechanical mapping and thermal stage, we made attempts to reveal the micromechanical and thermomechanical properties of the cuticular apparatus of the IR sensillum. The main component of the cuticular apparatus is an internal endocuticular sphere with a diameter of about 15-20 microm. Highly ordered multilayered organization of the lamellated peripheral mantle of the sphere was confirmed and characterized. We observed that the interlayer spacing of this microstructure varied along the circumference and decreased to 300 nm in the vertex of the sphere. We demonstrated that the microlayered structure is composed of nanolayers with very different micromechanical properties and thermal behaviors. Thermal expansion of the outer mantle was observed, and the local thermal expansion coefficient under given preparation conditions was estimated to be below 1.5 x 10(-4) grad(-1).

Biological thermal detection in infrared imaging snakes. 1. Ultramicrostructure of pit receptor organs

The receptor organs of snakes with "thermal vision" were studied with ultra-high-resolution scanning probe microscopy (SPM) at close to in vivo conditions to elucidate their surface morphology and materials properties critical for prospective biomimetic design of "soft matter"-based infrared (IR) sensors. The surfaces of living tissues were scanned under wet ambient conditions in physiological solution, and the resulting parameters were compared with SPM data obtained for chemically treated (formaldehyde-fixed) tissue in ambient air and TEM studies in high vacuum. We found that the microstructural parameters for the living tissue are similar to ones observed for the formaldehyde-fixed snake tissues. However, previous data obtained from TEM analysis in high vacuum underestimated actual dimensions of surface microstructures. The average spacing of the nanopit array observed within receptor surface areas, which was suggested to play a critical role in selective IR adsorption, was determined to be 520 nm. This value is close to the grating spacing required for efficient reflection of electromagnetic radiation characteristic for sunlight without affecting IR adsorbance.

Remote Giant Multispectral Plasmonic Shifts of Labile Hinged Nanorod Array via Magnetic Field

We report a remotely mediated and fast responsive plasmonic-magnetic nanorod array with extremely large variability in optical appearance (up to 100 nm shifts in scattering maxima) and concurrently for multiple wavelengths in a broad range from UV-vis to near-infrared (at 450, 550, and 670 nm) with an external magnetic field with variable direction. The observed phenomenon demonstrates a rapid, wide-range response controlled via a noninvasive remote stimulus. The remotely controlled system suggested here is a magnetic field-directed assembly of an ordered monolayer array of unipolar oriented magnetic-plasmonic nickel-gold nanorods flexibly hinged to a sticky substrate. The unique geometry of the mobile nanorod array allows for the instant alteration of the surface plasmon polariton modes in the gold segment of the controllably tilting nanorods. This design demonstrates the utility of hybrid bimetallic nanoparticles and gives a novel approach to the design of fast-acting, remotely controlled color-changing nanomaterials for sensing and interfacial transport.