Creating a robust synthetic surface that repels various liquids would have broad technological implications for areas ranging from biomedical devices and fuel transport to architecture but has proved extremely challenging. Inspirations from natural nonwetting structures, particularly the leaves of the lotus, have led to the development of liquid-repellent microtextured surfaces that rely on the formation of a stable air-liquid interface. Despite over a decade of intense research, these surfaces are, however, still plagued with problems that restrict their practical applications: limited oleophobicity with high contact angle hysteresis, failure under pressure and upon physical damage, inability to self-heal and high production cost. To address these challenges, we developed a new material that we call SLIPS (Slippery Liquid-Infused Porous Surfaces) that show exceptional liquid- and solid-repellency, pressure stability, enhanced optical transparency, and unique self-healing and self-cleaning capabilities. Our approach—inspired by Nepenthes pitcher plants—relies on the use of nano/microstructured substrates that lock in place the infused lubricating fluid and form a defect-free, stable liquid overlayer. The application of these surfaces for directional transport of fluids, and as antifouling materials operating in extreme environments will be discussed.

Bio-inspired smart materials should be a “live” material with various functions like organism in Nature, they must have three essential elements as sense, drive and control. The studies on lotus and rice leaves reveal that a super-hydrophobic surface with both a large CA and small sliding angle needs the cooperation of micro- and nanostructures. Considering the arrangement of the micro- and nanostructures, the surface structures of the water-strider&’s legs were studied in detail. Accordingly, super-hydrophobic surfaces of aligned carbon nanotube films, aligned polymer nanofibers and differently patterned aligned carbon nanotube films have been fabricated. Many methods had been applied in making superhydrophobic films with multi-functional properties, such as structural colored, transparent and/or conductive superhydrophobic films. Under certain circumstances, a surface wettability can switch between superhydrophilicity and superhydrophobicity, just like in Chinese ancient Taiji philosophy that “Yin” and “Yang”, the two opposing fundamental properties of nature, are switchable. The cooperation between surface micro- and nanostructures and surface modification of poly (N-isopropylacrylamide) gave reversible switching. By grafting the copolymer of temperature-sensitive and pH-sensitive components, a dual-responsive surface can be controlled by either or both of temperature and pH was fabricated. Besides organic surfaces, a series of inorganic switchers were also made. UV light stimulated transition between superhydrophobic and superhydrophilic by aligned ZnO, TiO2, and SnO2 films are successfully prepared respectively. Most recently, we developed a superoleophobic and controllable adhesive water/solid interface which opens up a new strategy to control self-cleaning properties in water. To expand the “switching” concept of the smart 2D surface, we also did a lot of interesting work in 1D system. For example, we discovered the water collection ability of capture silk of the cribellate spider Uloborus walckenaerius and then prepared artificial spider silk which will have great applications in water collection. In addition, we developed the novel biomimetic ion channel systems with a variety of intelligent properties (pH responsive, temperature responsive, potassium responsive, zinc activated, and dual-responsive single nanochannels), which were controlled by our designed biomolecules or smart polymers responding to the single external stimulus, provided an artificial counterpart of switchable protein-made nanochannels (highlight by Nature, and Nature China). These intelligent nanochannels could be used in energy-conversion system, such as photoelectric conversion system inspired by rhodopsin from retina or bR, and concentration-gradient-driven nanofluidic power source that mimic the function of the electric eels.

We study with experiments and simulations the motion of small solid objects and liquid drops on a surface when they are subjected to an external driving that may constitute a stochastic or a periodic vibration in conjunction with or without an external field. We focus on understanding how the non-linear friction forces rectify the imposed vibration. An interesting case is the random coalescence of condensed drops on a surface that can overcome pinning forces of the type that give rise to contact angle hysteresis. The directed motion of these coalesced drops with the aid of an imposed field (i.e. surface energy gradient or gravity) can be useful in certain technologies.

A microfludic device is fabricated using micro channels covered with olefin based cilia-like anisotropic nanorods obtained with a simple stamping method. The resulting bioinspired device is used to directionally transport water, isopropyl alcohol as well as whole blood, assisted by fluidic drag at a maximum flow rate of 10 ml/min . This passive pump device is able to operate at low-frequencies reaching low Reynolds numbers and laminar flow mimicking the blood circulation in veins. The anisotropic nanorods prevent the system from having a backflow during the circulation process. Unlike other micropumps, having no check valves in the device aims to cause no damage to cells or clogging. The nanoratchet platform based on olefin stamping method demonstrates a directional drag mechanism derived from the deformation of nanoscale asperities. The deformation mechanism functions with a maximum backpressure of 10 psi. The flexible cilia-like nanofibers lay along the microchannels moving back and forth as the liquid flows. Due to anisotropy of the fibers, a net amount fluid is pumped in a given direction. The device is designed to circulate whole blood repeatedly to increase sensing yield for bioapplications. This naturally inspired micropump device holds a great promise to combine fluidic pumps with biosensors for simple, inexpensive, efficient, and hand-operated devices for medical and bioengineering applications.

Understanding fluid flow in nanoconfined geometries is crucial for a broad range of scientific problems relevant to the behavior of porous materials in biology, nanotechnology and the built environment. Due to the dominant importance of surface effects at the nanoscale, long standing assumptions that are valid for macroscopic systems must be revisited when modeling nanoconfined fluids, since boundary conditions and the confined behavior of liquids are challenging to discern from experiments. To address this issue, here we present a novel coarse-grained model that combines parameters calibrated for water with a dissipative particle dynamics thermostat for the purpose of investigating hydrodynamics under confinement at scales exceeding current capabilities with all-atomistic simulations. Conditions pertaining to slip boundary conditions and confinement emerge naturally from particle interactions, with no need for assumptions a priori. The model is used to systematically investigate the imbibition dynamics of water into cylindrical nanopores of different diameters and surface properties. We present a systematic study that explains how contact angle dynamics and imbibition dynamics vary with nano pore radius. We extend our simulations and analyses to complex fluids where guided assembly of particles can be achieved using directional fields and hydrodynamic forces. Our modeling approach lays the foundation for broader investigations on the dynamics of fluids in nanoporous materials in conjunction with experimental efforts.

Evaporation of sessile droplet is a complex, non-equilibrium phenomenon [1-3]. Up to now, significant progress has been made on the investigation of the contact line dynamics on superhydrophobic surfaces during the evaporation process and it is known that the evaporating droplets exhibit distinctive evaporation modes such as constant contact line (CCL), constant contact angle (CCA) or both of them. However, our fundamental understanding of the effects of surface roughness on the wetting transition remains elusive [4-7]. Here we show that the surface roughness has a remarkable influence on the onset time for the CCL-CCA transition at the early stage of evaporation and the critical base size at the Cassie-Wenzel transition at the late stage of evaporation. Two models were developed to quantitatively analyze the whole scenario of evaporation on rough surfaces. In the first model, we theoretically derive the CCL-CCA transition onset time and explain its reliance on the surface roughness, which is consistent with our experimental observation. Also, we present a global interfacial energy argument which allows for the accurate prediction of the critical base size for the Cassie-Wenzel transition at the late stage of evaporation process [8]. In particular, we show that when the droplet size is comparable to the feature size of the surface roughness, the line tension becomes important in the energy calculation. Finally, we show that both CCL evaporation mode and Cassie-Wenzel transition can be effectively inhibited by engineering surface with hierarchical roughness. We believe that this work provides important insights for the fundamental understanding and controlling of the evaporation dynamics, and will be of interest to the researchers in the area of surface science and engineering. References: [1] P. Tsai, R. G. H. Lammertink, M. Wessling and D. Lohse, Phys. Rev. Lett. 104, 116102 (2010). [2] M. Reyssat, J. M. Yeomans and D. Quéré, Europhys. Lett. 81, 26006 (2008). [3] S. Moulnet and D. Bartolo, Eur. Phys. J. E 24, 251 (2007). [4] X. Chen, J. Wu, R. Ma, M. Hua, N. Koratkar, S. Yao and Z. Wang, Adv. Funct. Mater. 21, 4617 (2011). [5] K. -H. Chu, R. Xiao and E. N. Wang, Nat. Mater. 9, 413 (2010). [6] H. Kusumaatmaja, M. L. Blow, A. Dupuis and J. M. Yeomans. Europhys. Lett. 81, 36003 (2008). [7] P. Forsberg, F.Nikolajeff and M. Karlsson, Soft Matter 7, 104 (2011). [8] T. Liu, W. Sun, X. Sun and H. Ai, Langmuir 26, 14835 (2010).

From water to oil, droplets of liquids slide on omniphobic surfaces at small angles: a defining characteristic of such surfaces that makes it difficult to manipulate droplets on them. Here, we demonstrate a responsive omniphobic surface that consists of a porous surface infiltrated by a ferrofluid, a responsive liquid made of magnetic nanoparticles. We show that the application of a magnetic field changes the local topography of such surfaces and make the surface capable of pinning droplets of water and oil at 90° angle. Through the movement of the pinning site, these surfaces can perform tasks such as moving droplet uphill, delivering cargo to desired locations to carry out chemical reactions, and pumping along a tube. We envision that these magnetically responsive surfaces will have potential applications in areas such as fluid transport, microfluidics, and biomedical devices.

Hydroxyapatite and type I Collagen are two major components of bone tissues. While extensive efforts have been made in preparing pure apatite or pure collagen coatings, there have only been a few reports on the preparation of collagen-apatite composite coatings. In the present study, a simple UV treatment was applied to the substrate prior to the coating process to induce the coating formation. XPS and wettability test proved that UV treatment decreased the carbon contamination as well as produce a hydrophilic surface condition. As a result, apatite nucleation was facilitated and the coating formation process was accelerated. A collagen-apatite composite coating has been successfully deposited onto treated Ti6Al4V surfaces in a collagen-containing modified simulated body fluid (m-SBF) within 24h. The coating exhibits a homogeneous porous structure and its morphology can be tailored by adjusting carefully the collagen concentration in the m-SBF solution. XRD and FTIR proved that the coating is made of carbonated bonelike apatite and collagen, which is similar to the composition of natural bone. Especially, the bonding strength of the composite coating on UV-treated substrate was higher than that without the UV treatment. Thus, UV irradiation can be potentially applied to Ti6Al4V implant/device surfaces as a simple technique to enhance the biomimetic coating formation.

Controlling or guiding the formation of wrinkles of a thin stiff film on a soft material such as poly(dimethylsiloxane) (PDMS) has been a focus of intensive research for various applications. Recently, surface folding, a morphological instability driven by further compression of a wrinkled surface, has gained increasing attention and some effort has been devoted to understanding the wrinkle-to-fold transition and exploring the potential applications of folded structures. In fact, we can find such wrinkled/folded structures in our biological systems (e.g. brains, white blood cells, and internal elastic lamina of arteries) and those structures are believed to play significant roles in their functions. In this study, inspired by those structures, we focused on controlling the formation and evolution of folds of oxygen plasma-induced stiff layers on flat or tubular PDMS substrates under large compressive strain for the development of cellular scaffolds and controlled drug release systems. PDMS substrates prepared by a mixture of silicone elastomer base with a curing agent were stretched uniaxially and subsequently treated with oxygen plasma. The surface topographies and the cross-sections of the specimens were observed in detail with an atomic force microscope (AFM) and a scanning electron microscope (SEM), respectively. Our results demonstrate that we can control the formation as well as the size of the folded structures simply by controlling the prestrain applied to the substrates and/or the duration of oxygen plasma treatment. To confirm the feasibility of the aforementioned biomedical applications (i.e. cellular scaffolds and controlled drug release systems), further in vitro experiments using those specimens are ongoing and detailed results will be provided in the presentation.

Preventing bacterial adhesion to surfaces is one of the key steps in limiting biofouling and biofilm formation. Many bacterial pathogens readily attach to surfaces and form robust biofilms which limit the efficacy of antimicrobial and antibiotic treatments. This can be prevented by treating surfaces with bacteriostatic agents, or by tuning the surface chemistry to reduce bacterial attachment via cellular adhesion factors. Recent studies have shown that ordered, microscale topographic patterns can limit bacterial attachment to surfaces and significantly reduce biofilm formation. In this study, repeating linear and block patterns were fabricated with length scales ranging from 500 nanometers to 2 microns. Bacterial surface adhesion and biofilm formation was assessed under laminar fluid flow and in static conditions. Cell attachment was highly dependent upon the size, spacing, and geometry of the presented topography. In addition to the two dimensional aspect of the topography (eg. linear vs. block patterns), the vertical aspect of the features was also varied. For each topographical pattern both “raised” and “recessed” features were presented. Raised pillar-like structures of all sizes yielded lower surface attachment than linear patterns and arrays of holes. This was also seen for previously reported patterns (used as reference controls in this study) in which fewer cells attached to raised features than recessed features. Beyond initial cell attachment, biofilm formation was also assessed, showing that Initial attachment patterns were predictive of subsequent biofilm formation and coverage. This suggests a direct role for surface topography in bacterial biofouling. Beyond these studies, we have demonstrated a computational approach towards designing antifouling topographical patterns. Current approaches to designing such patterns follow an iterative process of pattern modification, fabrication and testing. To enhance this process we have developed methods for in-silico pattern characterization. These techniques have enabled the use of an evolutionary algorithm to generate non-intuitive surface designs.

Hydroxyapatite (HA) reinforced polymers composites have received much attention due their wide clinical applications, specifically, composites composed of chitosan and PMMA polymers matrix due to their bioactivity and biocompatibility with the biological cells [1-2]. In this work we prepared a poly(methyl methacrylate)-chitosan/hydroxyapatite (further referred as PMMA-CTS/HA) composite coating. This study was focused to evaluate the PMMA-CTS/HA composite as coating on UHMWPE substrates by its wear and friction behavior in dry conditions. The hardness characteristic of PMMA-CTS/HA coating has been also studied. PMMA-CTS/HA composite was prepared by a solution of chitosan-hydroxyapatite (CTS-HA) and the in situ polymerization of poly(methyl methacrylate) (PMMA) obtained from its monomer methyl methacrylate (MMA) with benzoyl peroxide (PBO) as the initiator. FTIR technique was used to study the hybrid composite coating. The coating morphology was investigated by SEM; finding out a well-connected pored structure surface provided by the incorporation of HA and chitosan with an average surface roughness (Ra ) of 1 µm. A ball-on-disk test was run with a 6 mm diameter WC ball in dry conditions applying 2, 4, 6, 8 and 10 N loads. The lowest mean kinetic friction coefficient was µk= 0.04 at 10 N, whereas the lowest wear rate was k = 5.1 x 10^-5 mm³(Nm)^-1 at 4 N. The coating hardness was evaluated with a Vickers hardness test employing a load of 50 mN, 10 indentations were performed obtaining a mean value of 0.21 Gpa. The experimental results opened the possibility to use the PMMA-CTS/HA coating on UHMWPE bearing material in biomedical applications taking into account the potential properties of these systems with the human tissue. [1] M.P. Casaletto, et al. Surface and Interface Analysis, 2002; 34: 45-49. [2] G.Soccol, et al, Materials Science and Engineering B, 2010; 169: 159-168.

Morphologically complex cellulosic substance (e.g., commercial filter paper) was employed as solid substrates for immobilization of different guest species, which endowed the natural cellulose substance with engineered properties, leading to the development of a series of novel surface-functionalized cellulosic materials. A uniform ultrathin zirconia gel film was first deposited on each cellulose nanofiber in bulk filter paper by a facile sol-gel process. Relying on the large surface area of filter paper and the strong affinity of zirconia for the phosphate group, terminal-phosphate probe DNA was abundantly immobilized on the zirconia-modified filter paper so as to convert the composite to a biofunctional material for the sensitive and repetitive recognition of the corresponding complementary target DNA on the nanomolar level. By contrast, the amount of captured probe DNA or recognized target DNA on a zirconia-coated flat substrate (e.g., quartz plate) was much less than that captured or recognized on filter paper, resulting in a relatively insensitive recognition event. Moreover, immobilization of ruthenium dye-Hg²⁺ complex (N719-Hg²⁺ complex) monolayer on ultrathin zirconia gel film coated cellulose nanofibers of filter paper yielded a solid-phase sensing device with high sensitivity, selectivity and reversibility for colorimetric detection of cysteine in aqueous media. This assay relies on the specific mercury dispossession by cysteine from the immobilized N719-Hg²⁺ complex. And the dissociation of N719 and Hg²⁺ caused by mercury snatch behavior of cysteine engenders obvious orange-to-purple color change of the zirconia gel layer and N719-Hg²⁺ complex modified filter paper, hence realizing the colorimetric detection of cysteine. The detection limit of this assay is 20 mu;M by the naked eye and the selectivity for cysteine against interference of the other 19 natural amino acids and their mixture is extremely high.

Wood is an ideal material having outstanding mechanical properties particularly in tension. We report the preparation of wood-mimetic materials from microfibrillated cellulose (MFC) by unidirectional freezing and subsequent alkaline treatment. The directional freezing is a simple and eco-friendly technique to produce a membrane with through-thickness porosity [1]. The MFC, an eco-friendly polymer for composites, was obtained by acid hydrolysis of bacterial cellulose produced by Gluconacetobacteria xylinus strain in Kombucha. Thick MFC suspension was directionally frozen, and then a freeze-drying subsequently followed. The freeze dried membrane dipped in a concentrated alkali solution at 50 degree Celsius for 24 hrs to obtain a hydrogel [2]. The obtained hydrogel was characterized by swelling ratio, and the crystal forms and morphology of dried gel were confirmed by XRD and FE-SEM, respectively. The XRD results showed the crystal form of natural cellulose [cellulose I] converts to cellulose II [2]. The FE-SEM of dry film confirmed through thickness aligned pores of ice crystals. The results from FE-SEM indicated that freeze-drying induced aligned and through-thickness pores. The alkali treatment causes gelation through coalescence and the axial shrinkage of the cellulose fibers. This study makes a simple and versatile technique to prepare natural wood mimetic structures of well-ordered pores available for polysaccharides, particularly for cellulose materials which have suffered from difficult processing. 1. Min Kyung Lee, Nae-Oh Chung, Jonghwi Lee, Membranes with through-thickness porosity prepared by unidirectional freezing. Polymer 51 (2010) 6258-6267. 2. Kentaro Abe, Hiroyuki Yano; Formation of hydrogels from cellulose nano#64257;bers. Carbohydrate Polymers 85 (2011) 733-737.

Inspiration from natural designs offers opportunities to develop new functional materials having unique properties. One of the active areas of research in this field is the construction of anisotropic nanostructured surfaces, which exhibit direction dependent wetting behavior. Here, we demonstrated the effects of surface chemistry and morphology on the directional wetting phenomenon. The nanofilms having directionality were fabricated at varying tilt angles (β) via oblique angle deposition (OAD) technique. The chemical modifications of engineered nanofilms were then carried out by using thiol molecules having different end groups (i.e., -CF3, -CH3, and -phenyl). We found that surface morphology and chemistry are extremely important parameters for the manipulation of a water droplet. Such a control would have a great impact for several technological applications involving catalysis, tissue engineering, and biosensors.

Bulk metallic glasses (BMGs) are recognized as one of promising candidate materials for implant due to their unique physical, chemical and mechanical properties such as high strength, low Young&’s modulus, high elastic limit and high corrosion resistance. Recently, Ni- and Cu-free nontoxic Zr-based BMG were developed [1]. However, the BMG could not be directly joined to human bone because of bio-inertness and too high chemical resistance. Thus, it is required to give biocompatibility such as bone inducing ability to this BMG by fabricating ceramic coating on its surface. In this study, the hydrothermal(H), electrochemical(E) and Hydrothermal-Electrochemical (H-E) treatments were investigated to fabricate bioactive layer on Zr-based BMG. The BMG sheets (3.0 x 1.0 cm in area) were degreased in acetone under ultrasonic cleaning, dried and eventually rinsed with distilled and deionized water (Millipore Milli-Q). BMG substrate and a platinum plate were suspended in NaOH solution as working and counter electrodes, respectively. The concentration of NaOH was ranged from 0.2 to 5.0 M. The current of 50 mA/cm2 were applied between these electrodes keeping temperature at 90 - 150°C. After preparation, all specimens were washed with distilled water and dried in incubator at 600°C. The surface morphology was observed by scanning electron microscopy (SEM). From SEM images, flake-shaped products were obtained by only H-E treatment, while other treatments did not change the surface conditions. This result indicates that the H-E treatment has the highest potential to modify highly corrosive surface of Zr-based BMG. XPS data and Raman spectra showed that amount of Co were increased for flake-shaped surface and suggested that the surface layer formed by the H-E treatment was amorphous sodium cobaltate. The immersion test in the simulated body fluid (SBF) for 12 days revealed that this flake-shaped structure on BMG was useful to enhance the growth of bone like hydroxyapatite crystallites. [1] T. Zhang et.al. Mater. Trans., JIM 43, 267 (2002).

Conventional surface texturing methods such as cutting, boring, drilling, and shaping may have limited regional controllability of surface roughness. Moreover, textured surfaces are typically evaluated by standard roughness parameters. They describe the height profiles well, but provide no information of the surface periodicity which is important in the tactile feel of the texture. The roughness parameters also offer only an average value over a measured surface, since they do not offer information of surface spatial distribution. Therefore, a method producing systematically controlled textures and a characterization approach perceiving both height and spatial profiles are needed. We have fabricated systematically controlled ordered and randomized textures by micropillars onto polypropylene surfaces. Our texturing method is based on injection moulding with the mould inserts fabricated by a micro-working robot technique. The advantage of the method is the casting of rigorously controlled three-dimensional structures where directional structures can easily be produced. Surface textures have been analyzed by power spectral density (PSD) curves combined with filtered root-mean-square (RMS) roughness values providing periodicities and roughness at various spatial frequencies. Hence, textured surfaces can be evaluated both in horizontal and vertical directions. The developed characterization method is valid for directional surfaces since it enables the structure characterization in arbitrary angles. The analysis tool is also useful for mimicking and designing textures. The frequency and roughness values of a mimicked surface can be reproduced in the cast polymer surfaces. The tool can also be used in the design stage of the texture since the input data of the micro-working data can be analyzed in a similar fashion.

Most of semi-aquatic insects like water strider, fisher spider have micro/nano hair on their legs and body that can help them repel liquid to specific direction. Inspired by micro/nano structure, researchers take an effort to apply the structures to microfluidic devices. Nowadays, Controlling fluidic characteristics like spreading velocity and direction in microchannel is of significant interest for broad range of microfluidics, including DNA microarrays, lab-on-a-chip, inkjet printing. With advancements in micro/nano fabrication, patterned surface have enabled control microfluidic characteristics. However, most of the surfaces have symmetric spreading property. Herein, we demonstrate that we can control asymmetric liquid spreading property by regulate asymmetry of structure. For the formation of asymmetric micro pillar structure, fill the UV-cured polymer in microfluidic channel bonded with microhole patterned Cr glass substrate. Then, backside UV exposure after contacting backside of Cr glass with Lucius prism enables to from an asymmetric micro structure. As previously reported, the Lucius prism can allocate light to specific direction by oblique metal deposition technique. Hence, micro structure is formed following in the UV light pathway. The fabricated asymmetric micro structure has uni-directional wetting property. It is because of the critical angle effect on edge side. It is expected to apply to the flowing control in a microfluidic device.

Polymer substrates with a built-in capability for alignment of nanometer-sized objects are of interest for the development, performance, and large-scale production of robust, flexible devices. We have used atomic force microscopy (AFM) in friction mode to investigate the effects of uniaxial stretching on the chain orientation and nanoscale adhesion and friction of glassy polymer substrates. Examples will be shown of the different friction responses of a commercial low-density polyethylene along and across the stretching direction, and how this friction response is altered as the strength of adhesion between the polymer and the AFM tip is deliberately changed.

The Belousov-Zhabotinsky (BZ) reaction, an oscillating reaction accompanying a rhythmical change in the redox state, is a subject of both chemical and biological interest. We have attempted to induce the BZ reaction within the gel that undergoes a volume phase transition in response to changes in the surrounding conditions such as temperature. Here, we develop preparation and characterization of novel block copolymer that can induce dynamic structural transition of micelle in constant condition. Target block copolymers (PEO-b-P(Ru(bpy)3-r-NIPAAm)) were successfully prepared by using RAFT random copolymerization of N-isopropylacrylamide (NIPAAm) and vinyl monomer having ruthenium side chain (Ru(bpy)3 monomer), respectively as monomers for 2nd segment from poly(ethylene oxide) (PEO)-based macro chain transfer agent (CTA) as 1st block. After polymerization, CTA residue at the terminal of the block copolymer was removed by AIBN treatment. The number average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of the block copolymer were 12.2 kDa and 1.27, respectively from GPC measurements. Composition of Ru(bpy)3 monomer in 2nd block was estimated to be 1 mol%. Dynamic light scattering (DLS) measurements confirmed that the thermo sensitivity of the block copolymer in equilibrium. The diblock copolymer showed low temperature unimer/high temperature micelle (lower critical micellization temperature (LCMT)-type) phase behavior in aqueous solution owing to a thermo sensitivity of 2nd P(NIPAAm-r-Ru(bpy)3) segment. The diblock copolymer self-assembles into micelle irrespective of redox state of Ru(bpy)3, however; aggregation temperature of oxidized state is higher than that of reduced one. When Ru(bpy)3 is oxidized, thermo sensitive segment becomes more hydrophilic compared to that in reduced state, thus higher aggregation temperature is observed. The hydrodynamic radius (Rh) of micelles both in oxidized and in reduced form was over 100 nm. Since volume fraction of insoluble block was over 0.5, it might form bilayer vesicles at higher temperature, although there are reports on block copolymers that do not exhibit the full morphological sequence even with widely changing volume fraction of insoluble block. We finally demonstrated the rhythmical scattering intensity oscillation of micelles synchronized with the BZ reaction at temperature between oxidized- and reduced-form. We further confirmed that the size of micelle oscillated under constant condition without providing any external stimuli.

The fabrication of micro-/nano-structures with defined surface chemistry on surfaces has been explored and enabled the production of advanced biomaterials and biodevices, and the understanding of fundamentals in biology. In the present study, we report the use of a negative photoresist that is comprised of the photodefinable poly(4-benzoyl-p-xylylene-co-p-xylylene). This polymer belongs to the parylene family and is compatible with biological applications and can be prepared via chemical vapor deposition (CVD) polymerization process on various substrates. Upon UV irradiation, its unique solvent stability in acetone was identified in the study and was characterized by using infrared reflection adsorption spectroscopy (IRRAS), scanning electron microscope (SEM) and imaging ellipsometry. The discovery has made the polymer a powerful tool to work as a negative photoresist for surface microstructuring and biointerface engineering purposes. Plus, the coating process provides an access to non-conventional and complex substrates compared to spin-coated resists that are limited to flat assembly. The demonstration of using this photoresist technology has seamlessly incorporated with other functional parylenes including aldehyde-functionalized, acetylene-functionalized, and amine-functionalized parylenes to create surface microstructures that are chemically and topographically defined. The photopatterning and immobilization protocols have provided an approach that avoids harmful substances to contact with sensitive biomolecules. Finally, the harmonic presentations of multiple biomolecules on planar substrates and on non-conventional substrates, e.g. stents, are also highlighted.

Experimental and theoretical approaches for the design, synthesis, and characterization of new bioinspired surfaces demonstrating unidirectional surface properties will be summarized. The experimental approaches focus on bottom-up and top-down synthesis methods of unidirectional micro- and nanoscale films to explore and characterize their anomalous features. The theoretical component focuses on computational tools to predict the physicochemical properties of unidirectional surfaces.

Leidenfrost droplets are known to accelerate on ratchet-shaped surfaces, driven by the drag of directional vapor flow underneath the droplets (Phys. Rev. Lett. 96, 154502 (2006)). Here, we report a related phenomenon by which Leidenfrost droplets can "jump" up a step, sometimes leaving the surface and jumping as high as their own size. I will illustrate the phenomenon, discuss the circumstances when it is observed, and propose a physical mechanism.

Recent experiments have shown that gecko adhesion exhibits seemingly contradicting or incompatible multifunctionalities including self-cleaning, strong attachment and easy detachment. Currently, there exists no theoretical framework in contact mechanics that is capable of integrating all these features in one unified model. Here we propose an accordion model of adhesion that accomplishes this integration with an adhesive pad consisting of a compliant backbone covered by a foldable hard skin containing hidden adhesives which are initially folded into the structure to avoid adhesion in the stress-free state. At the default, stress-free state, the pad is non-adhesive and self-cleaning. Under a tensile prestress large enough to expose the hidden adhesives, the pad becomes highly stiffened and strongly adhesive. We investigate the adhesion behavior of such a structure and show that strong attachment can be established spontaneously when the pad is dragged on an external surface at a low angle, with prestress trapped in the attached portion of the pad. We then investigate the orientation-dependent peel-off force of an attached accordion pad and show that it can be spontaneously detached above a critical pulling angle, leading to strongly reversible adhesion. Our present work reinforces an earlier conjecture that prestress is an important principle in achieving orientation-dependent adhesion strength of a surface containing elastic fibrillar structures on substrate. The prestress can significantly increase the peel-off force at small peeling angles while decreasing it at large peeling angles, leading to strongly reversible adhesion. More interestingly, there exists a critical value of prestress beyond which the peel-off force plunges to zero at a force independent critical peeling angle. The prestress required for such force-independent detachment at a critical angle can be induced by simply dragging a spatula pad along a substrate at sufficiently low angles. Our present work generalizes the prestress model to an initially wrinkled elastic film on a substrate, with the feature of a tension-induced transition from a self-cleaning, non-adhesive state to a prestressed, strongly adhesive state, while at the same time undergoes spontaneous detachment at a critical angle. Our study suggests possible mechanisms by which nature evolves soft adhesive systems with self-cleaning, strong attachment and easy detachment capabilities.

Surfaces with controllable liquid mobility have attracted great attention for their potential applications in areas such as anti-bioadhesion, liquid transportation, and dropwise manipulation in microfluidics. By combining subtle surface chemistry with micro/nano-hierarchical structures, superhydrophobic surfaces were shown to finely tune water mobility. However, low-surface-tension hydrocarbon oils generally infiltrate and stain these surfaces, thus reducing the liquid repellency and resulting in the loss of function in areas such as anti-oil-adhesion of pipe-transportation, oil repellence of marine exploration instrumentation and oil-spill cleanup. Recently, the discovery of slippery surface on pitcher plant inspired us to develop a new way to fabricate antifouling surfaces with a broad range of liquid repellency. This novel strategy takes advantage of the lubricant infiltration into porous materials to create ultra-slippery, defect-free, liquid-like interfaces with the ability to repel crude oil, among other complex fluids. Here, we report on the design of a novel lubricant-infused porous membrane that adds to the slippery surfaces a dynamic, switchable character. Oil-drop mobility can be now dynamically controlled from moving to pinning by applying external stimuli, which change the affinity between the target liquid and the underlying substrate. Such a surface design can be widely extended to various material systems, and it opens a new way to design adaptive liquid manipulation systems.

Controlling the movement of water droplets is necessary for the development of microfluidics devices. Superhydrophobic surfaces enable facile droplet movement, however the direction can be difficult to control as the droplet can slip freely in all directions. In addition, this high mobility can be problematic when maintaining a droplet in a specific location is required. Some naturally occurring surfaces, such as the wing of the Morpho Aega butterfly1, as well as the leaves of rice plants2 offer an approach to limit droplet motion. Water droplets on these surfaces exhibit ratchet-like droplet motion due to partial wetting of anisotropic surface features. Such partial wetting, however, can be unstable as droplets may transition to a fully wetted Wenzel state if subjected to sufficient vibrations or pressures. Based on insights from these natural surfaces, we have developed superhydrophobic surfaces that exhibit ratchet-like slip angle anisotropy3 as well as reversible wetting along the features in the vertical direction. These superhydrophobic surfaces are composed of arrays of polydimethylsiloxane (PDMS) circular conical posts printed on a porous web substrate. A robotic printer dispenses the PDMS in a controlled manner such that the posts are formed with slopes ranging from 0 to 50o relative to the plane of the surface. When the tilt direction is aligned with the slope of the posts, droplets rolled more easily than when the substrate was tilted in the opposite direction. Slip-angle anisotropy values as large as 32o were observed as a function of droplet size and post slope. This anisotropy results from partial wetting of the surface features and changes in the triple contact line (TCL) length as a function of tilt. Vertical fluid motion could be achieved by controlling the air pressure below the porous web substrate. As the height of a water column supported on the surface increased, the TCL moved down along the conical features, thereby increasing the wetted fraction of the posts. Partial wetting occurs gradually, with no abrupt Wenzel transition, because the TCL lengthens as water pressure increases. By increasing the pressure in the plenum, the water pressure can be compensated and partial wetting can be reversed thereby restoring the liquid to a non-wetted Cassie-Baxter state. Thus the vertical position of the TCL can be controlled by simple adjustment of the pressure. Because the PDMS features are large (0.5mm diameter, 1mm tall), the vertical translation of the TCL can be easily observed. Videos of the reversible movement will be shown and an analysis of the wetting phenomena presented. 1 Zheng, Y.; Gao, X.; Jiang, L. Soft Matter. 2007, 3 (178). 2 Feng, L.; et. al. Advanced Materials 2002, 14(24) 1857. 3. Barahman, M.; Lyons, A. M. Langmuir, 2011, 27 9902.

Manipulation of liquid spreading is important for a broad range of microfluidic, biological, and thermal management applications. Recent advances in surface engineering have enabled opportunites to control liquid film thickness, as well as to create various elongated droplet shapes. However, typically, the wetting behavior on engineered surfaces is symmetric. In addition, dynamic control of spreading behavior is difficult to achieve. In this work, we demonstrate asymmetric nanostructured surfaces that actively control the directionality of liquid spreading with applied electric potentials. In addition, we show that we can manipulate droplet spreading in a single direction on-demand, which suggests new opportunities to tailor advanced nanostructures for active control of complex flow patterns. Asymmetric nanostructures were fabricated with pillar diameters of 500 nm, heights of 9 µm, spacings of 3.5 µm, and deflection angles ranging from 2.4° to 25°. The silicon-based nanopillars statically deflect due to residual stresses generated by a thin gold film deposited on one side of the nanopillars. While a water droplet deposited on typical vertical nanopillars spreads symmetrically, the same droplet on the deflected nanopillar array spreads only in the direction which the pillar is deflected to. This uni-directional spreading is distinct from previous studies because the asymmetric nanostructures introduce local energy barriers via geometry to prevent spreading in all but one direction. In addition, the spreading behavior can be dynamically controlled by electrowetting, which utilizes an electric potential, V, across the droplet and nanostructured surface to change the surface energy. With this approach, different droplet spreading directionalities can be obtained based on the magnitude of the electric potential. By applying V= 1.5 V to an initially static symmetric droplet, the liquid pins in one direction and shows uni-directional spreading. In the case of an applied V= 2.1 V, the liquid starts spreading bi-directionally. The spreading, however, is asymmetric: the rate is three times faster in one direction than the other. Moreover, with increasing applied V, the asymmetry decreases. In the case of an applied V> 2.5V, the liquid spreading is nearly symmetric, i.e., the rates in all directions are approximately equal. A simple two-dimensional geometric model was developed to explain the liquid propagation behavior observed in the experiments. Based on the model, a regime map is attained to predict the spreading behavior and shows excellent agreement with our experimental observations. Our study provides design guidelines to tune the droplet behavior from uni-directional to bi-directional asymmetric or symmetric spreading using both nanostructure design and applied electric fields for a variety of microfluidic applications.

The adhesion and locomotion mechanisms employed by tree frogs under flooded conditions offer the ultimate solution for the need of strong, reversible, reusable, tunable, and water tolerant adhesives. The presence of structured toe-pads (hexagonal epithelial cells separated by channels) has been proposed to facilitate drainage of fluid and reduce hydrodynamic repulsion during approach. These channels may remain closed during retraction for hydrodynamic adhesion and open for pull-off, both of which require active/passive control of toe-pad deformation. We present results from our investigation of the role of draining channels on normal hydrodynamic interactions in the absence of elastohydrodynamics (rigid surfaces). The surface force apparatus is employed to measure hydrodynamic forces between a smooth silver film and a structured SU-8 surface driven towards or away from each other at a constant drive velocity. The structured SU-8 comprises of an array of cylindrical posts. The hydrodynamic forces are analyzed within the framework of Reynolds&’ continuum approach in the lubrication limit for smooth surfaces in cross-cylinder geometry. We observe a reduction in repulsion upon approach with respect to that predicted by Reynolds&’ theory. Our results are analyzed using a scaling argument based on the geometric parameters associated with the structures (diameter of posts, channel width and depth, and surface coverage). Viscosity-dominated drainage of fluid in the presence of a network of channels intrinsically results in competition between drainage from in contact region and drainage through the network of channels. The scaling argument can, then, be used to predict regimes of preferential drainage of fluid through network of channels and drainage of fluid from the contact region for a single post or the cross-cylinders as a whole as the surface separation changes.

A texture ratchet is a surface region whose microstructured, asymmetric features propel drops when vibrated. Drop motion can persist over long distances, along circular paths, up- or down-hill, and even upside-down. Specific drop volumes resonate at specific frequencies, providing a means to separately control the motion of different-sized drops. We analyze the physical mechanism responsible for drop transport and reveal the relationship between design parameters such as microscale geometry and surface wettability, operation parameters such as drop volume and viscosity, vibration frequency and amplitude, and performance parameters such as drop velocity. Experiments demonstrate water drop transport on silicon and elastomer substrates. Texture ratchets can move liquid samples without electric fields, pressure gradients or gravity, making them an attractive low-cost platform for lab-on-chip applications. Texture ratchets are one instance of a broader class of "wetting barrier ratchets" that include transparent and optically flat microfluidic devices.

To climb on smooth surfaces many insects possess adhesive footpads, which broadly divide into two morphologies, namely 'hairy' (fibrillar) and 'smooth'. Both types of pad are efficient directional adhesives, which increase contact area and adhesion when pulled towards the body but detach easily when pushed away from it, thereby allowing rapid changes between attachment and detachment for locomotion. Nevertheless, climbing and jumping insects routinely use their feet for pushing, even on smooth substrates. They achieve this by using different tarsal attachment pads with opposite directionality, engaging pads on the proximal tarsus for pushing and pads at the distal end for pulling. In smooth adhesive pads of insects, the detailed mechanisms of directional adhesion are still unclear. While pads of some insects such as ants can be unfolded and retracted to increase or reduce contact area, even pads lacking such motility show directional adhesion. The cuticle of smooth pads is characterized by numerous branched, internal fibrils that are oriented almost perpendicularly to the surface, terminating in a smooth film of epicuticle. Assuming that this fluid-filled fibrous cuticle is a constant-volume hydrostat, a pull toward the body could laterally expand the pad's contact area via a reduction of the fibril angle. In vivo fluorescence microscopy of smooth pad adhesive cuticle in stick insects demonstrated that pulls toward the body indeed reduce the fibril angle. However, the observed fibril angle variation was insufficient to explain the increase in pad contact area. Direct strain measurements in the contact zone showed that pulls not only expand the cuticle laterally, but also add new contact area at the pad&’s outer edge. Force measurements in ants revealed an abrupt loss of adhesion for pull-off angles greater than ca. 35°, analogous to the situation in geckos. This biologically important effect is neither explained by standard peeling models nor by the shear-induced variation of contact area. Instead, the steep increase of force for smaller pull-off angles may be based on pre-tension along the pad cuticle or on shear-induced changes in the pad's bending stiffness.

We discuss how textures on hot solids can generate special properties for liquids levitating on them, such as enhanced friction and self-propulsion. We discuss the different mechanisms involved in these dynamics, and show that they result from an interplay between inertia and viscosity.

The limbless locomotion of snakes relies fundamentally upon friction. Snakes can adjust their frictional properties temporally and spatially in order to generate high friction anisotropy. Their ventral scales which resemble overlapping shingles play a major role in this regard. In this combined experimental and theoretical study we measure the mechanical and frictional properties of snakeskin. We numerically model a flexible snake scale interacting with an elastic cantilever beam using elastica theory. We find the combination of the scale's geometry and angle of attack leads to the high frictional anisotropy. Fabricating engineering surfaces such as artificial snakeskin with optimized scale geometry and orientation of scales will improve the efficiency of snake-like robots.

Remoras (echeneid fish) reversibly attach and detach to marine hosts, almost instantaneously, to “hitchhike” and feed. The adhesion mechanisms that they use are remarkably insensitive to substrate topology and quite different from the latching and suction cup-based systems associated with other species at similar length scales. Remora adhesion is also anisotropic; drag forces induced by the swimming host increase adhesive strength, while rapid detachment occurs when the remora reverses this shear load. In this work, an investigation of the adhesive system&’s functional morphology and tissue properties was carried out initially through dissection and x-ray microtomographic analyses. Resulting finite element models of these components have provided new insights into the adaptive, hierarchical nature of the mechanisms and a path toward a wide range of engineering applications.

Nano- and microscale patterning of surfaces enables manipulation of cell function and behavior by controlling adhesion, shape and cytoskeletal structure. This patterning can take the form of topographical, chemical and mechanical modifications to the surface, typically in an attempt to mimic some structural or functional aspect of the native extracellular matrix (ECM). However, to date it has been difficult to combine topographical and chemical patterning on the same surface. Here we report a method to nanopattern ECM proteins onto a surface independent of the underlying microtopography. To do this, we use a protein Patterning on Topography (PoT) printing technique that is able to directly transfer patterned ECM proteins from a smooth release surface onto a topographically complex surface while substantially maintaining pattern fidelity. The process is based on the surface-initiated assembly (SIA) of ECM proteins into nanometer thick layers of polymerized matrix. ECM proteins (e.g., fibronectin, laminin, collagen type IV) are patterned onto poly(N-isopropylacrylamide) (PIPAAm) using microcontact printing and then the PIPAAm layer is thermally-triggered to dissolve and push the ECM patterns onto an adjacent surface. Results show the ability to pattern nanometer thick micropatterns of ECM proteins onto a variety of surfaces including A4 paper, 150 and 220 grit sandpaper and micro-ridges of various size and aspect ratio. We demonstrate that the ECM micropatterns can be applied independently of the underlying micro-ridge topography, either in parallel, orthogonal or any orientation in-between. Cell response to these surfaces is dependent on both the micropatterned ECM and the microtopography, with cardiomyocytes demonstrating preferential alignment to the ECM proteins in most conditions. In summary, we have developed a new technique to pattern ECM proteins onto topographically complex surfaces and evaluate cell response. Looking forward, we hope to use these surfaces to study the interaction between surface chemistry and surface topography on cell behavior during myogenesis.

Micropillar arrays offer precise control of the compliance of the underlying substrates for cellular studies, where the multidirectional mechanical forces exerted by a cell can be isolated to each individual pillar and measured independently and collectively. Alternatively, nanogrooves have been shown to guide cell spreading and alignment. Exploiting these properties, we have fabricated well-defined hierarchical pillar arrays, composed of elastomeric micropillars atop mechanically-tunable wrinkled substrates to study cell spreading, alignment, and adhesion in a dynamic environment. A PDMS substrate patterned with a square array of micropillars (diameter 1 mu;m, spacing 1 mu;m, height 1 mu;m) was stretched uniaxially (strain=0.3), followed by oxygen plasma treatment. Upon release of the strain, wrinkles (wavelength=2mu;m) form spontaneously perpendicular to the stretch direction. Changing only the direction of the stretch, a wide variety of mechanically-induced hierarchical wrinkles with pillars was fabricated. Using these substrates to mimic the extracellular matrix (ECM), we studied adhesion and spreading of vascular smooth muscle cells in response to environmental changes. These substrates are also exploited for applications, including switchable transparency, adhesion, and wetting.

Nature has revealed three critical principles that are needed for efficient climbing on vertical surfaces: conformability, so that van der Waals forces can produce sufficient adhesion to remain attached; directional adhesion, so that one can control the amount of adhesive force by controlling the applied shear; and even load distribution so that local stress concentrations do not cause catastrophic peeling failures. Among these principles, directional adhesion is perhaps the most important for fast, smooth and efficient surface climbing. This talk will show how directional adhesives can produce a convex limit surface in a three-dimensional space of applied contact forces, with properties that are particularly useful for controlling adhesion and for attaching and detaching with little wasted energy. Directional adhesion also works hand-in-hand with conformability and distributed force control to increase the robustness and scalability of climbing solutions to areas of 100 square centimeters and greater. These principles are demonstrated with robots ranging from 0.1-4.0 kg that scale vertical surfaces smoothly and efficiently using directional dry adhesives.

In this study, we present gecko-inspired directional (angled) elastomer micro-fibers with flat or round tip endings as compliant pick-and-place macro- or micro-manipulators. The pillars are 35 µm in diameter, 90 µm tall, and angled at an inclination of about 20 degrees. By gently pressing the tip of a pillar to a part, the fiber adheres to it through intermolecular forces. Next, by retracting quickly, the part is picked from a given donor substrate. During transferring, the adhesion between the pillar and the part is high enough to withstand disturbances due to external forces or the weight of the part. During release of the part onto a receiver substrate, the contact area of the pillar to the part is drastically reduced by controlled vertical or shear displacement, which results in reduced adhesive forces due to the angled fiber structure. The maximum repeatable ratio of pick-to-release adhesive forces was measured as 39 to 1. We find that a flat tip shape and shear displacement control provide a higher pick-to-release adhesion ratio than a round tip and vertical displacement control, respectively. We present a model of forces to serve as a framework for the operation of this macro- or micro-manipulator. Finally, demonstrations of pick-and-place manipulation of µm-scale silicon micro-platelets and a centimeter-scale glass cover slip serve as proofs of concept. The compliant polymer micro-fibers are safe for use with fragile parts, and, due to exploiting intermolecular forces, could be effective on most materials and in air, vacuum, and liquid environments.

The efforts to learn and take inspiration from nature have proven successful in a wide range of areas such as epidermal electronics, dry adhesion, directional wetting, and biomimetic materials. Among many useful functions, reversible binding or interlocking is an attractive feature that nature can provide, which is enabled by a number of different intermolecular, capillary, electric, and mechanical forces. In addition, nature has created unique structural devices with specially designed physical structures such as interlocking between “hooks” and “loops” in burdock&’s seeds (fabric Velcro) and insect locking systems. We report that the wing locking device of beetles can be exploited to form a reversible interlocker based on van der Waals force-assisted binding between high aspect-ratio polymer hairs made of UV-curable polyurethane-based materials. Also, controlling fluidic features like spreading velocity and flow direction in a microchannel is of significant interest for microfluidic applications, including DNA microarrays, lab-on-a-chip, and inkjet printing. We demonstrate that one can take advantage of asymmetric surface structure via oblique metal deposition and polymer etching to generate unidirectional liquid spreading. Specifically, the direction of liquid wetting is determined by a lower critical angle, which can be reversibly switched by suitable surface modifications.

We show how uni-directional, micro-channel based, structuring of a surface can provide highly selective interfacial properties: strongly enhanced work of adhesion between two matched patterns and strongly attenuated adhesion between most others. Relative misalignment is accommodated by screw dislocations that run in a direction orthogonal to the channels. Dislocation energy governs the size of its core; misorientation controls dislocation spacing through the Moire pattern of channels on the two sides of the interface. During separation, interfacial cracks grow primarily by nucleation in a direction normal to the channel and growth along the channels. Energy dissipation is enhanced by a combination of frictional losses and crack trapping, and attenuated by energy release from dislocations.

A large number of engineering applications involve sliding of elastomers over a solid wall. In this situation, friction plays an important role in the overall system performance. Here, we use computer simulations to study the tribological behavior of elastomers in relative motion against surfaces with engineered nanoscale topography. We use the bond-bending lattice spring method to model permanently cross-linked polymer networks representing elastomers. We use a momentum conservative thermostat to account for thermal fluctuations and introduce internal dissipation that depends on the relative velocity of the network filaments. We consider substrates with triangular, forward and backward sawooth asperities. Our simulations reveal that the friction experienced on a weakly repulsive substrate is the lowest for a surface with forward sawtooth roughness, while the surface with backward roughness leads to the highest friction. For adhesive surfaces, friction force is independent of the surface geometry when sliding velocity is sufficiently slow. We demonstrate that this behavior is related to the asymmetric distribution of shear stress near asperities, which is controlled by the roughness geometry. Our findings give insights into the micromechanics of elastomer friction and could be useful for developing new methods for regulating friction and reducing wear using directional surfaces.

Gecko toe pads show strong adhesion on various surfaces yet remain remarkably clean around everyday contaminants. An understanding of how geckos clean their toe pads while being in motion is essential for the elucidation of animal behaviors as well as the design of biomimetic devices with optimal performance. Here, we test the self-cleaning of geckos during animal locomotion. We provide the first evidence that geckos clean their feet through a unique dynamic self-cleaning mechanism via digital hyperextension. When walking naturally with hyperextension, geckos shed dirt from their toes twice as fast as they would if walking without hyperextension, returning their feet to nearly 80% of their original stickiness in only 4 steps. Our dynamic model predicts that when setae suddenly release from the attached substrate, they generate enough inertial force to dislodge dirt particles from the attached spatulae. The predicted cleaning force on dirt particles significantly increases when the dynamic effect is included. The extraordinary design of gecko toe pads perfectly combines dynamic self-cleaning with repeatable attachment/detachment, making gecko feet sticky yet clean. This work thus provides a new mechanism to be considered for biomimetic design of highly re-useable and reliable dry adhesives and devices.

Many species such as geckos are capable of generating impressively large adhesion forces between their feet and a wide variety of substrates. Equally impressive is their ability to rapidly break the adhesive bond to detach from these substrates in a controllable way. Although the fibrillar structures of gecko feet have garnered the most attention among directional adhesion studies, the attachment mechanisms used by insects may provide attractive alternatives. Aphids are able to cling to smooth surfaces using soft, retractable attachment pads. These pads may then be peeled away from the surface using a combination of the mechanical force of their claws and the active retraction of the pad material. This method of controllably reversible adhesion may be mimicked and extended to macroscale applications through the use of shape memory polymer (SMP) and microscopic surface features designed to facilitate on-demand detachment without significantly affecting adhesive ability. Through the novel use of microstructured large-area SMP surfaces, macroscale objects of substantial mass are lifted, stably held for long periods, and then released when desired. The surfaces may be manufactured with areas of less than one square millimeter, and scaled up to several square centimeters using common microfabrication and molding/demolding techniques, allowing for a wide range of applications. Maximum adhesion to glass is shown to be in excess of 500 kPa, while an optimized surface pattern of microstructures provides the ability to reduce adhesion to <1 kPa when release is desired. The switching from adhesion-on to adhesion-off states is thermally activated. Thermal activation requires only small localized changes in temperature to shift the SMP across its glass transition temperature.

Controlled imbibition is a way of realizing capillarity driven microfluidics on open surfaces. Two basic approaches exists for controlling the imbibition: surface topography (micro and nano-textures) and surface chemistry (patterned areas). With these methods, we can control the imbibition direction as well as the final shape of the droplet. Potential applications for controlled imbibition are presented in the field of analytical chemistry We have demonstrated fluidic diodes by unidirectional surface structures [1]. The channels are packed with isosceles triangular microposts (apex angle 10 degrees, size and height in 10-20 micron range). These surface structures lead to directional imbibition properties, so that the liquid can penetrate to the direction of the bases of the pillars, but the imbibition is pinned in a metastable state to the direction of the tips .The same phenomenon can be realized in 2D: octagonal microposts with two sharp angles induce spreading in one dimension on a 2D plane. Black silicon, aka silicon nanograss, is a quasiregular array of silicon nanopillars that is easily fabricated over large areas in a maskless plasma process utilizing SF6 [2]. Contact angle contrasts of 170 degrees can be created on top of these surfaces by utilizing lithographical patterning of a fluoropolymer and oxygen plasma surface treatments. These surfaces can be used to engineer the shapes of liquid droplets. Combined to evaporation and the "coffee ring" effect, we have demonstrated directional transport of solutes from a central droplet to eight peripheral droplets for use with surface assisted laser desorption ionization mass spectrometry [3]. The key parameter for anisotropic imbibition is the advancing contact angle, which needs to be smaller to the directions of preferred imbibition. The actual liquid/surface chemistry pair is not as important, and anisotropic behavior can also be achieved with oil droplets [4]. This allows solute depositions also from an organic phase. In addition, we have demonstrated multiphase droplets consisting of both aqueous and organic domains, that can be utilized for liquid-liquid-liquid extraction [5]. [1] V. Jokinen, M. Leinikka, S. Franssila: Microstructured Surfaces for Directional Wetting, Advanced Materials 21, (2009), pp. 4835-4838 [2] V. Jokinen, L. Sainiemi, S. Franssila: Complex droplets on chemically modified silicon nanograss, Advanced Materials 20, (2008), pp. 3453-3456 [3] V. Jokinen, S. Franssila, M.Baumann: Engineered droplets for dried droplet solute deposition by mass spectrometric imaging, Microfluids Nanofluids 11 (2011) pp. 145-156 [4] V. Jokinen, L. Sainiemi, S. Franssila: Controlled Lateral Spreading and Pinning of Oil Droplets Based on Topography and Chemical Patterning, Langmuir 27 (2011) pp. 7314-7320 [5] V. Jokinen, T. Sikanen, R. Kostiainen and S. Franssila: Miniaturized liquid-liquid extraction system based on controlled aqueous and organic droplets, Proc. MicroTAS 2011 p.897

We have been studying polymer gels with an autonomous self-oscillating function, since firstly reported in 1996. We succeeded in developing novel self-oscillating polymers and gels by utilizing the oscillating reaction, called the Belousov-Zhabotinsky reaction. The polymer gel undergoes spontaneous cyclic swelling-deswelling changes or soluble-insoluble changes (in the case of uncrosslinked polymer) without any on-off switching of external stimuli. Potential applications of the self-oscillating polymers and gels include several kinds of functional material systems, such as biomimetic actuators, mass transport surface and functional fluid. For example, it was demonstrated that an object was autonomously transported in the tubular self-oscillating gel by the peristaltic pumping motion similar to an intestine. Further, it is possible to create a new dynamic interface by immobilizing the self-oscillating polymer. We prepared a self-oscillating polymer brush surface on the inside surface of a glass capillary and evaluated its dynamic behavior. In this presentation, such recent progress on the gel will be introduced.

Influenza is a membrane-enveloped virus, which infects a host cell through the endocytotic pathway. An essential viral coat protein, hemagglutinin, is responsible for both the attachment of the virus to the host cell (binding) and the fusion of the viral membrane with the host membrane. The virus binds to a glycolipid containing sialic acid groups present on the surface of many host cells, which initiates its uptake into an endosome. Once inside the endosome, the viral membrane must fuse the endosomal membrane in order to delivery its genetic material to the cytosol for replication. In nature, viral fusion to the endosomal membrane is initiated by a conformational change in hemagglutinin, triggered by acidification of the endosome. Different strains and serotypes of virus can have markedly different binding and fusion characteristics and characterizing this behavior is important for basic biological studies of virus entry, identifying new anti-viral drugs that target viral entry processes, and creating new diagnostic tools for differentiating virus types. The development of such platforms requires an appropriate mimic for the host cell surface that presents the receptors for viral interaction and a strategy to acidify the system to mimic the drop in pH inside the endosomal compartment to initiate membrane fusion. We developed an in vitro method for assaying binding and fusion of a single virion particle using an individual virion imaging technique and stochastic analysis of data. In this work, we mimic the host membrane chemistry in a supported bilayer coating the walls of a microfluidic device. The physico-chemical properties of the bilayer can be controlled to present different receptors and surface features to modify virus interaction. In this work, we monitor residence times of viruses bound to the membrane using total internal reflection microscopy, hemifusion (the merging of the two outermost lipid leaflets) using fluorescence dequenching, and the formation of fusion pores by release times of fluorescently-labeled viral contents. The increased sensitivity gained by monitoring individual events can facilitate comparison of binding and fusion kinetics between different influenza strains to better characterize and diagnose mutants and identify host membrane properties that enhance viral entry processes and infection.

Genetically engineered collagen-like triple helix peptides were co-assembled into freestanding films with quantum dots (QDs) via biomolecular recognition. These peptide-based hybrid films show excellent mechanical properties with Young&’s modulus of ~20 GPa, much larger than most of polymer films and other reported nanoparticle freestanding sheets (<10 GPa), and it is even close to that reported for the bone tissue in nature. In addition, nano-indenting tests suggest that these films exhibit little permanent deformation under small indentation; however, the mechanical hysteresis becomes remarkable when the load approaches near and beyond the rupture point, which is also characteristic of the bone tissue.

We used three dimensional computer simulations to examine diffusion of nanoparticles in microchannels with walls decorated with periodic arrays of spiral structures. The channel was filled with a viscous fluid driven by a pressure gradient and tracer particles were used to model diffusive nanoparticles suspended in the fluid. The spiral structures induce mixing in the fluid, which enhances the effective diffusion and deposition of particles on the channel walls. To model the system, we employed the lattice Boltzmann model coupled with the Brownian dynamics model. We systematically varied the pitch, radius, and spacing of the spiral structures and established the optimal conditions leading to enhanced mixing and deposition of nanoparticles. Our findings could be useful for designing structured surfaces that can actively regulate transport and deposition of nanoparticles in microscale sensors and detectors.

Nature employs various mechanisms to modify the wetting properties of free surfaces. It was shown that the tilted fibers on poly(p-choloro-xylylene) (PPX) nanofilm resulted in unidirectional wetting and droplet motion. The nanotextured film provides a directional surface, which is hydrophobic due to its nanoscale texture. Droplet transport or manipulation on a surface by a ratcheting mechanism requires a textured pattern for movement of droplets at low amplitudes of vibration. The tilted PPX nanorods of the nanofilm produce unidirectional wetting, thereby enabling droplet motion in a predetermined direction. Smooth nanotextured film surface, fabricated by oblique angle vapor deposition of PPX, carries microliter droplets along with minimal droplet shape deformations. This approach eliminates the need for complex fabrication procedures and controls for applications such as droplet manipulation and movement. Here, we demonstrated the transport of microscale hydrogels (microgels) encapsulated in water droplets using a unidirectional nanotextured surface, which moves droplets with low vibration amplitudes by a ratcheting mechanism. Time-lapse frames of droplet motion with and without microgels were quantified as a function of vibration frequency. Vibration of the surface at 94 Hz allowed the microgel to move within the 5 µl droplet along the ratchet. Maximum droplet translation speed on the nanofilm was determined to be 3.5 mm/sec, which offer a pathway towards high throughput microgel assembly applications to build complex constructs. Directional transport of droplets on nanotextured surfaces is a promising technology for moving fragile cargo such as microgels and cells. We demonstrated that droplets with and without soft cargo, move unidirectionally as a function of vibration frequency on a nanocoated surface. Efficient assembly of cell encapsulating microgels can potentially overcome the limitations to obtain control over cell-cell proximity with microscale resolution.

Nature assembles anisotropic building blocks in a fashion that maximizes the mechanical performance of natural composites according to the stresses they are subjected to. For instance, the outer and the inner layers of the abalone shell exhibit anisotropic calcium carbonate particles that are aligned perpendicular and parallel to its surface, respectively. (1) The outer layer maximizes the shell hardness, providing substantial resistance against crack initiation on the surface. The inner layer accounts for the shell&’s outstanding resistance against crack propagation. In contrast, the deliberate control of anisotropic particles to locally tailor the material&’s mechanical properties without changing its chemical composition remains largely unexplored in manmade composites. In this study, we produced platelet-reinforced epoxy composites with deliberate reinforcing architectures, including the bilayers found in seashells and teeth. To obtain such composites, micron-sized alumina platelets exhibiting ultra-high magnetic response were produced by electrostatic adsorption of magnetic nanoparticles on the alumina surface. The modified platelets were then added to an epoxy matrix and oriented in a deliberate way by applying magnetic fields in the range of 1 - 10 milliTesla. Magnetic control of anisotropic alumina particles within the epoxy matrix allowed for 120% and 28% increase in flexural modulus and Vickers hardness as compared to conventional composites. Bilayer composites were then produced by laminating bars exhibiting different platelet orientation. Composites mimicking the bilayer structure of the abalone shell showed a 1.4- and 2.1-fold increase in flexural modulus and Vickers hardness, respectively, compared to the pure epoxy matrix. (2) These values are about 50% higher than the bilayer composites containing the platelets aligned horizontally in the outer layer and vertically in the inner layer. The improvement in properties achieved by just controlling the orientation of reinforcing particles is a promising approach to locally tailor the mechanics of the surface and interfaces of man-made composites. References: 1. Qi, H.J.; Bruet, B.J.F.; Palmer, J.S.; Ortiz, C.; Boyce, M.C. Mechanics of Biological Tissues, Springer, 2005. 2. Erb, R.M.; Libanori, R.; Rothfuchs, N.; Studart, A.R. Science 2012, 335, 199.

We discuss the use of free energy lattice Boltzmann algorithms as tools to investigate fluid flow and spreading at surfaces and in microchannels patterned with micron-scale posts. Putting together arguments based on the Gibbs&’ criterion for interface pinning, and the numerical results, we show how capillary filling, imbibition and evaporation depend on the arrangement, shape and tilt of the posts.

Using computational modeling, we show that self-oscillating Belousov-Zhabotinsky (BZ) gels can both emit and sense a chemical signal and thus, drive neighboring gel pieces to spontaneously self-aggregate, so that the system exhibits auto-chemotaxis. To date, this is the closest system to the ultimate self-healing material, which can be divided into separated parts and the parts move autonomously to assemble into a structure resembling the original, uncut sample. We also show that the gels&’ coordinated motion can be controlled by light, allowing us to achieve selective self-aggregation and control over the shape of the gel aggregates. By exposing the BZ gels to specific patterns of light and dark, we design a BZ gel “train” that leads the movement of its “cargo”. Our findings pave the way for creating reconfigurable materials from self-propelled elements, which autonomously communicate with neighboring units and thereby actively participate in constructing the final structure.

We study the impact and rebound dynamics of falling spheres off of a coated surface. The rebound follows a diagonal trajectory determined by the angle of the nanorods. Dependence on the parameters is studied

Volume metallic nanostructured mesostructures resembling such natural objects as plants, mushrooms, shells are grown and presented in their modest elegance. The images of metallic mesostructures are made by using a scanning electron microscope SUPRA -50 VP. These structures occur on porous membranes by means of pulsed current electrodeposition if the electroplating is continued after the nanowires appear on the membrane surface. The membrane geometry, pulsed current electroplating parameters, and alternating electrodeposition from two baths with variety of electrolytes were used to fabricate the diversity of complex shape nanostructured mesostructures of various metals and alloys. The possibility of forms&’ regulation for models was demonstrated. Depending on the form of the membrane and the regime of the pulsed current electroplating either one type of convex-concave structures looking like “shells” or various structures can be grown. It is shown that the obtained complex structures are formed by layers of metallic nanoclusters and nanowires as a result of their self-assembly while growing during the pulsed current electroplating process. It is shown that the electrodeposition with pulsed current on templates is a biomimetic method of the metal nano-and mesostructures&’ fabrication. The amazing resemblance of the obtained structures with natural objects such as plants, mushrooms and shells suggest common laws of morphogenesis. The nanostructured mesostructures of the magnetic (Ni, Pd-Ni, Pd-Co), superconducting (Bi, Pb, Pb-In) and normal (Ag) metals were obtained. Such objects can be of interest for fundamental research as well as for applications in catalysis, electronics, optics, medicine, and for fabrication of superhydrophobic surfaces.

On surfaces with asymmetric or “saw tooth” features, liquid slugs or drops tend to move preferentially in one direction. In these systems, capillarity manifests itself as a retention force that holds back the movement of liquids against externally applied forces, such as gravity or pressure. In this theoretical study, the imbalance of capillary forces that leads to directionally-biased wetting is examined. Capillary tubes with symmetric and asymmetric saw tooth features were used to estimate the ratios of retention forces in opposing directions. Our analysis suggests that the difference between the retention force in one direction versus the other can be maximized by increasing feature asymmetry and minimizing inherent hysteresis (liquid-solid adhesion) of the materials of construction. This work has implications for small channels or surfaces of fluid handling components found in microfluidic devices and fuel cells.

Directional wetting in nature and technology typically results from textured or patterned surfaces. The roughness elements of geometrically textured surfaces typically have edges that pin the contact lines of droplets. Similarly, chemical patterns can also pin and contort the contact line at the boundaries of wetting and non-wetting regions on the substrate. We present the results of two numerical investigations. The first relates the contact angle hysteresis and adhesive force to the geometrical and wetting properties of the substrate. A number of generic surfaces are considered, including arrays of tilted posts and surfaces consisting of lattice chemical patterns. The calculated wetting properties can predict common observable quantities such as the critical tilt angle for drop motion. The second numerical investigation explores how chemical patterns on the substrate can shape microdroplets, including inducing pinch-off at critical fluid volumes.

Axonal growth and the formation of synaptic connections are key steps in the development of the nervous system. Despite notable advances achieved over the past decade, to date the mechanisms of axonal navigation to their target region and their specific interactions with guidance factors such as chemical gradients and mechanical cues are still largely unknown. Here we present experiments on axonal growth and interconnectivity that are performed in simplified, well-controlled growth environments, in order to elucidate some of the basic rules that neuronal cells use to form functional connections with one another. We show that engineered unidirectional surfaces consisting of nanofilms of poly(p-xylylene) (parylene or PPX) nanorods covered with poly-D-lysine are extremely effective in controlling and guiding axonal extensions. We perform a systematic investigation of the adhesion and real time growth of neuronal processes on these surfaces and quantify the role that different types of biomechanical cues play in neuronal growth. We demonstrate that the axonal growth dynamics is strongly influenced by the asymmetric mechanical guidance cues and discuss the implications of our results for directing axonal growth in neuro-regeneration studies, and for engineering neuroprosthetic devices and nerve-material interfaces. 1. E.M. Spedden, R. Beighley, K. Sekeroglu, M. C. Demirel, C. Staii, in press (2012). 2. N. A. Malvadkar, M.J. Hancock, K. Sekeroglu, W. J. Dressick, M. C. Demirel, Nature Materials, 9, 1023 (2010). Funding: NSF-CBET 1067093, NIH R21HL112114-01.