Active Perception in Non-Visual Recognition Environments by Stretchable Tentacle Sensor Arrays.

Smoke fog or other light-interference environments have intrinsic obstruction for visual recognition techniques to explore objects and surroundings. Alternatively, tactile perceptions, rather than visual observations, are commonly used by burrowing or deep-sea animals to communicate with environments. Bio-inspired by this natural wisdom, here, we demonstrate stretchable tentacle sensor arrays, which can recognize surrounding objects located in non-visual conditions such as smoke fog or dark environment. Each tentacle sensor is composed of two functional parts: a retractable tentacle with a magnetic top and an elastomer bottom containing copper coils. Different from traditionally passive tactile sensors, these tentacle sensors can actively stretch under the control of a syringe pump, yielding different electrical signals when in contact with the objects. Analyzing collected sensing signals of those tactile sensor arrays by the feature analysis model, complex morphological information of irregular objects in the smoke fog can be recognized. Our study reveals a fundamental connection between stretchable tactile sensors and feature analysis and demonstrates its practical potential for active perception in a non-visual recognition environment.

Localized Electrons Enhanced Ion Transport for Ultrafast Electrochemical Energy Storage.

The rate-determining process for electrochemical energy storage is largely determined by ion transport occurring in the electrode materials. Apart from decreasing the distance of ion diffusion, the enhancement of ionic mobility is crucial for ion transport. Here, a localized electron enhanced ion transport mechanism to promote ion mobility for ultrafast energy storage is proposed. Theoretical calculations and analysis reveal that highly localized electrons can be induced by intrinsic defects, and the migration barrier of ions can be obviously reduced. Consistently, experiment results reveal that this mechanism leads to an enhancement of Li/Na ion diffusivity by two orders of magnitude. At high mass loading of 10 mg cm-2 and high rate of 10C, a reversible energy storage capacity up to 190 mAh g-1 is achieved, which is ten times greater than achievable by commercial crystals with comparable dimensions.

Intrinsically reversible superglues via shape adaptation inspired by snail epiphragm.

Adhesives are ubiquitous in daily life and industrial applications. They usually fall into one of two classes: strong but irreversible (e.g., superglues) or reversible/reusable but weak (e.g., pressure-sensitive adhesives and biological and biomimetic surfaces). Achieving both superstrong adhesion and reversibility has been challenging. This task is particularly difficult for hydrogels that, because their major constituent is liquid water, typically do not adhere strongly to any material. Here, we report a snail epiphragm-inspired adhesion mechanism where a polymer gel system demonstrates superglue-like adhesion strength (up to 892 N⋅cm-2) that is also reversible. It is applicable to both flat and rough target surfaces. In its hydrated state, the softened gel conformally adapts to the target surface by low-energy deformation, which is locked upon drying as the elastic modulus is raised from hundreds of kilopascals to ∼2.3 GPa, analogous to the action of the epiphragm of snails. We show that in this system adhesion strength is based on the material's intrinsic, especially near-surface, properties and not on any near-surface structure, providing reversibility and ease of scaling up for practical applications.

Ultrastable Underwater Anti-Oil Fouling Coatings from Spray Assemblies of Polyelectrolyte Grafted Silica Nanochains.

Surfaces that have superhydrophilic characteristics are known to exhibit extreme oil repellency under water, which is attractive for applications including anti-fogging, water-oil separations, and self-cleaning. However, superhydrophilic surfaces can also be easily fouled and lose their extreme oil repellency, which limits their usage in practical applications. In this work, we create an anti-oil fouling coating by spray coating poly(acrylic acid) (PAA)-grafted SiO2 nanochains (approximately 45 nm wide and 300 nm long) onto solid surfaces, forming a nanoporous film exhibiting superhydrophilicity (water contact angle in air ≈ 0°) and underwater superoleophobicity (dichloroethane contact angle ≥ 165°). The polymer-grafted nanochain assemblies exhibit extremely low contact angle hysteresis (<1°) and small adhesion hysteresis (-0.05 mN m-1), and thus, oil can readily roll off from the surface when the coating is immersed in water. Compared to other superhydrophilic surfaces, we show that both the unique structure of spray-assembled nanochains and the hygroscopic nature of PAA are essential to enable ultrastable anti-oil fouling. Even after the PAA-grafted nanochain coating is purposely fouled by oil, oil can be readily and completely expelled and lifted-off from the coating within 10 s when placed under water. Further, we show that our coating retains underwater superoleophobicity even after being subjected to shearing under water for more than 168 h. Our approach offers a simple yet versatile method to create an ultrastable superhydrophilic and anti-oil fouling coating via a scalable manufacturing method.

Colloidal inks from bumpy colloidal nanoparticles for the assembly of ultrasmooth and uniform structural colors.

Angle-independent structural colors obtained from colloidal nanoparticles (NPs) are of interest for potential applications in displays, color printing, 3D printing, and direct writing. However, it remains challenging to prepare uniform structural colors that can be directly written from colloidal inks that not only have no coffee-ring effect, but also have ultrasmooth film thickness, which will be important for layer-by-layer stacking. Herein, we synthesize polypyrrole (PPy) black coated silica NPs that have a low coverage (∼10.7 wt%) of bumpy PPy nanogranules (10-30 nm in diameter). When the PPy@silica NPs are drop-cast on a substrate, the surface roughness of the PPy@silica NPs effectively suppresses the coffee-ring effect and center aggregation during the drying of the colloidal ink, leading to ultrasmooth surfaces with sub-micron thickness and uniform structural colors with wide viewing angles. The color can be fine-tuned by the size of silica NPs, and the presence of PPy black significantly enhances the color contrast by suppressing incoherent and multiple light scattering. Moreover, we show that the bumpy colloidal ink is very versatile: the ink can be formulated from both low and high surface tension liquids as solvents and applied to a hydrophilic or hydrophobic substrate. We demonstrate direct writing of uniformly colored lines and three different color drops stacked on top of each other.

Cuts Guided Deterministic Buckling in Arrays of Soft Parallel Plates for Multifunctionality.

Harnessing buckling instability in soft materials offers an effective strategy to achieve multifunctionality. Despite great efforts in controlling the wrinkling behaviors of film-based systems and buckling of periodic structures, the benefits of classical plate buckling in soft materials remain largely unexplored. The challenge lies in the intrinsic indeterminate characteristics of buckling, leading to geometric frustration and random orientations. Here, we report the controllable global order in constrained buckling of arrays of parallel plates made of hydrogels and elastomers on rigid substrates. By introducing patterned cuts on the plates, the randomly phase-shifted buckling in the array of parallel plates transits to a prescribed and ordered buckling with controllable phases. The design principle for cut-directed deterministic buckling in plates is validated by both mechanics model and finite element simulation. By controlling the contacts and interactions between the buckled parallel plates, we demonstrate on-demand reconfigurable electrical and optical pathways, and the potential application in design of mechanical logic gates. By varying the local stimulus within the plates, we demonstrate that microscopic pathways can be written, visualized, erased, and rewritten macroscopically into a completely new one for potential applications such as soft reconfigurable circuits and logic devices.

Varying and unchanging whiteness on the wings of dusk-active and shade-inhabiting Carystoides escalantei butterflies.

Whiteness, although frequently apparent on the wings, legs, antennae, or bodies of many species of moths and butterflies, along with other colors and shades, has often escaped our attention. Here, we investigate the nanostructure and microstructure of white spots on the wings of Carystoides escalantei, a dusk-active and shade-inhabiting Costa Rican rain forest butterfly (Hesperiidae). On both males and females, two types of whiteness occur: angle dependent (dull or bright) and angle independent, which differ in the microstructure, orientation, and associated properties of their scales. Some spots on the male wings are absent from the female wings. Whether the angle-dependent whiteness is bright or dull depends on the observation directions. The angle-dependent scales also show enhanced retro-reflection. We speculate that the biological functions and evolution of Carystoides spot patterns, scale structures, and their varying whiteness are adaptations to butterfly's low light habitat and to airflow experienced on the wing base vs. wing tip.

Confined Assemblies of Colloidal Particles with Soft Repulsive Interactions.

We investigate the microconfinement of charged silica nanoparticles dispersed in refractive index matching monomers in poly(dimethylsiloxane) (PDMS) porous membrane. Here, the silica colloidal particles interact with each other and the pore wall via electrostatic double layer forces. Different from the hard sphere systems where the assembled morphologies are prescribed by the diameter ratio between the cylindrical confinement and the nanoparticles, here we observe a much richer variety of assemblies that are highly sensitive to both bulk and local nanoparticle concentration with fixed particle size and channel size. The experimentally observed assembly morphologies are consistent with theoretical predictions from the literature, based on Yukawa potential in the low packing density regime. Also, most of the configurations found in the experiment are well described by computer simulations using pairwise additive long-range repulsive interactions, demonstrating the ability to control the system to obtain a desired structure.

Carrier Step-by-Step Transport Initiated by Precise Defect Distribution Engineering for Efficient Photocatalytic Hydrogen Generation.

Semiconductor photocatalysts have been widely used for solar-to-hydrogen conversion; however, efficient photocatalytic hydrogen generation still remains a challenge. To improve the photocatalytic activity, the critical step is the transport of photogenerated carriers from bulk to surface. Here, we report the carrier step-by-step transport (CST) for semiconductor photocatalysts through precise defect engineering. In CST, carriers can fast transport from bulk to shallow traps in the defective subsurface first, and then transfer to the surface active acceptors. The key challenge of initiating CST lies in fine controlling defect distribution in semiconductor photocatalysts to introduce the special band matching between the crystalline bulk and defect-controllable surface, moderate bridgelike shallow traps induced by subsurface defects, and abundant surface active sites induced by surface defects. In our proof-of-concept demonstration, the CST was introduced into typical semiconductor TiO2 assisted by the fluorine-assisted kinetic hydrolysis method, and the designed TiO2 can exhibit the state-of-the-art photocatalytic hydrogen generation rate among anatase TiO2 up to 13.21 mmol h-1 g-1, which is 120 times enhanced compared with crystalline anatase TiO2 under sunlight. The CST initiated by precise defect distribution engineering provides a new sight on greatly improving photocatalytic hydrogen generation performance of semiconductor catalysts.

Production of Structural Colors with High Contrast and Wide Viewing Angles from Assemblies of Polypyrrole Black Coated Polystyrene Nanoparticles.

Structural color with wide viewing angles has enormous potential applications in pigment, ink formulation, displays, and sensors. However, colors obtained from colloidal assemblies with low refractive index contrast or without black additives typically appear pale. Here, we prepare polypyrrole (PPy) black coated polystyrene (PS) nanoparticles and demonstrate well-defined colors with high color contrast and wide viewing angles under ambient light. Depending on the loading of pyrrole during polymerization, PPy nanogranules of different sizes and coverages are grafted to the surface of PS nanoparticles. The bumpy particles can self-assemble into quasi-amorphous arrays, resulting in low angle dependent structure colors under ambient light. The color can be tuned by the size of the PS nanoparticles, and the presence of the PPy black on PS nanoparticles enhances the color contrast by suppressing incoherent and multiple scattering.

A high-efficiency superhydrophobic plasma separator.

To meet stringent limit-of-detection specifications for low abundance target molecules, a relatively large volume of plasma is needed for many blood-based clinical diagnostics. Conventional centrifugation methods for plasma separation are not suitable for on-site testing or bedside diagnostics. Here, we report a simple, yet high-efficiency, clamshell-style, superhydrophobic plasma separator that is capable of separating a relatively large volume of plasma from several hundred microliters of whole blood (finger-prick blood volume). The plasma separator consists of a superhydrophobic top cover with a separation membrane and a superhydrophobic bottom substrate. Unlike previously reported membrane-based plasma separators, the separation membrane in our device is positioned at the top of the sandwiched whole blood film to increase the membrane separation capacity and plasma yield. In addition, the device's superhydrophobic characteristics (i) facilitates the formation of well-defined, contracted, thin blood film with a high contact angle; (ii) minimizes biomolecular adhesion to surfaces; (iii) increases blood clotting time; and (iv) reduces blood cell hemolysis. The device demonstrated a "blood in-plasma out" capability, consistently extracting 65 ± 21.5 µL of plasma from 200 µL of whole blood in less than 10 min without electrical power. The device was used to separate plasma from Schistosoma mansoni genomic DNA-spiked whole blood with a recovery efficiency of >84.5 ± 25.8%. The S. mansoni genomic DNA in the separated plasma was successfully tested on our custom-made microfluidic chip by using loop mediated isothermal amplification (LAMP) method.

Directing the deformation paths of soft metamaterials with prescribed asymmetric units.

By prescribing asymmetric ligaments with different arrangements in elastomeric porous membranes of pre-twisted kagome lattices, the buckling instability is avoided, allowing for smooth and homogenous structural reconfiguration in a deterministic fashion. The stress-strain behaviors and negative Poisson's ratios can be tuned by the pre-twisting angles.

Buckling, symmetry breaking, and cavitation in periodically micro-structured hydrogel membranes.

We investigated swelling induced instabilities in porous membranes with a square array of micron-sized circular holes prepared from a pH and temperature dual-responsive hydrogel, poly(2-hydroxyethyl methacrylate-co-N-isopropylacrylamide-co-acrylic acid) (PHEMA-co-PNIPAAm-co-PAA). At room temperature (25 °C), the hydrogel swelled to ∼1.5 to 8 times of its dried volume when pH was increased from 2 to 7. Within this regime, we observed four distinctive morphologies of the hydrogel membrane, including a "breathing" mode of the membrane having circular pore arrays, a buckled pore array of alternating mutually orthogonal ellipses, twisted snap-shut pores forming "S" shaped slits, and cusps formed in local regions that perturbed the 2D periodicity of the hydrogel membrane. Using a 3D confocal imaging technique, we followed the post-buckling behaviors of the porous membranes and investigated the pattern evolution process as a function of pH. Amplification of buckling and symmetry breaking were observed when we increased the pH of the buffer solutions from pH 4.0 to 5.0, leading to the transition from an achiral buckled state (pH 4.0) to a chiral twisted state (pH 5.0) driven by the compaction of the hydrogel domains within the space to completely close the pores. When the pH of the aqueous environment was further increased to 7, star-shaped patterns appeared randomly in the film, where the hydrogel domains were compressed by the adjacent neighbors, thus resulting in out-of-plane deformation. Finally, we demonstrated the temperature-dependent reversible switching of the hydrogel membrane among the chiral twisted state, buckled state, and circular state via changing the temperature between 20 °C and 45 °C.

Spray coating of superhydrophobic and angle-independent coloured films.

Angle-independent coloured films with superhydrophobicity were fabricated from quasi-amorphous arrays of monodispersed fluorinated silica nanoparticles via one-step spray coating. The film exhibited a high contact angle (>150°) and a low roll-off angle (~2°) and the colour could be tuned to blue, green and moccasin by varying the size of the nanoparticles.

In situ plasmonic counter for polymerization of chains of gold nanorods in solution.

Self-assembly of gold nanorods (NRs) in linear, polymer-like chains offers the ability to test and validate theoretical models of molecular polymerization. Practically, NR chains show multiple promising applications in sensing of chemical and biological species. Both areas of research can strongly benefit from the development of a quantitative tool for characterization of the structure of NR chains in the course of self-assembly, based on the change in ensemble-averaged optical properties of plasmonic polymers; however, quantitative correlation between the extinction spectra and the structural characteristics of NR chains has not been reported. Here, we report such a tool by a quantitatively correlating the red shift of the longitudinal surface plasmon band of gold NRs and the average aggregation number of NR chains. The generality of the method is demonstrated for NRs with different aspect ratios, for varying inter-rod distances in the chains, and for varying initial concentrations of NRs in solution. We modeled the extinction spectra of the NR chains by combining the theory of step-growth polymerization with finite-difference time-domain simulations and a resistor-inductor-capacitor model, and obtained agreement between the theoretical and experimental results. In addition to capturing quantitatively the ensemble physics of the polymerization, the proposed 'plasmonic counter' approach provides a real-time cost- and labor-efficient method for the characterization of self-assembly of plasmonic polymers.