Bioinspired Flexible Volatile Organic Compounds Sensor Based on Dynamic Surface Wrinkling with Dual‐Signal Response

Living systems can respond to external stimuli by dynamic interface changes. Moreover, natural wrinkle structures allow the surface to switch dynamically and reversibly from flat to rough in response to specific stimuli. Artificial wrinkle structures have been developed for applications such as optical devices, mechanical sensors, and microfluidic devices. However, chemical molecule‐triggered flexible sensors based on dynamic surface wrinkling have not been demonstrated. Inspired by human skin wrinkling, herein, a volatile organic compound (VOC)‐responsive flexible sensor with a switchable dual‐signal response (transparency and resistance) is achieved based on a multilayered Ag nanowire (AgNW)/SiO_x/polydimethylsiloxane (PDMS) film. Wrinkle structures can form dynamically in response to VOC vapors (such as ethanol, toluene, acetone, formaldehyde, and methanol) due to the instability of the multilayer induced by their different swelling capabilities. By controlling the modulus of PDMS and the thickness of the SiO_x layer, tunable sensitivities in resistance and transparency of the device are achieved. Additionally, the proximity mechanism of the solubility parameter is proposed, which explains the high selectivity of the device toward ethanol vapor compared with that of other VOCs well. This stimuli‐responsive sensor exhibits the dynamic visual feedback and the quantitative electrical signal, which provide a novel approach for developing smart flexible electronics.     

Epitaxially Grown Ferroelectric PVDF‐TrFE Film on Shape‐Tailored Semiconducting Rubrene Single Crystal

Epitaxial crystallization of thin poly(vinylidene fluoride‐co‐trifluoroethylene) (PVDF‐TrFE) films is important for the full utilization of their ferroelectric properties. Epitaxy can offer a route for maximizing the degree of crystallinity with the effective orientation of the crystals with respect to the electric field. Despite various approaches for the epitaxial control of the crystalline structure of PVDF‐TrFE, its epitaxy on a semiconductor is yet to be accomplished. Herein, the epitaxial growth of PVDF‐TrFE crystals on a single‐crystalline organic semiconductor rubrene grown via physical vapor deposition is presented. The epitaxy results in polymer crystals globally ordered with specific crystal orientations dictated by the epitaxial relation between the polymer and rubrene crystal. The lattice matching between the c‐axis of PVDF‐TrFE crystals and the (210) plane of orthorhombic rubrene crystals develops two degenerate crystal orientations of the PVDF‐TrFE crystalline lamellae aligned nearly perpendicular to each other. Thin PVDF‐TrFE films with epitaxially grown crystals are incorporated into metal/ferroelectric polymer/metal and metal/ferroelectric polymer/semiconductor/metal capacitors, which exhibit excellent nonvolatile polarization and capacitance behavior, respectively. Furthermore, combined with a printing technique for micropatterning rubrene single crystals, the epitaxy of a PVDF‐TrFE film is formed selectively on the patterned rubrene with characteristic epitaxial crystal orientation over a large area.     

Finite element formulation for anisotropic coupled piezoelectro‐hygro‐thermo‐viscoelasto‐dynamic problems

A finite element algorithm has been developed for the efficient analysis of smart composite structures with piezoelectric polymer sensors or/and actuators based on piezoelectro‐hygro‐thermo‐viscoelasticity. Variational principles for anisotropic coupled piezoelectro‐hygro‐thermo‐viscoelasto‐dynamic problems have also been proposed in this study. As illustrative studies, dynamic responses in laminated composite beams and plates with PVDF sensors and actuators are obtained as functions of time using the present finite element procedures. The voltage feedback control scheme is utilized. The proposed numerical method can be used for analysing problems in the design of smart structures as well as smart sensors and actuators. Copyright © 1999 John Wiley & Sons, Ltd.     

Vapor sensing performances of PVDF nanocomposites containing titanium dioxide nanotubes decorated multi-walled carbon nanotubes

Inorganic nanocarbon hybrid materials are good alternatives for superior electrochemical performance and specific capacitance to their traditional counterparts. Nanocarbons act as a good template for the growth of metal nanoparticles on it and their hybrid combinations enhance the charge transport and rate capability of electrochemical materials without sacrificing the specific capacity. In this study, titanium dioxide nanotubes (TNT) are synthesized hydrothermally in the presence of multi-walled carbon nanotubes (MWCNT) where the latter acts as base template material for the metal oxide nanotube growth. The MWCNT–TNT hybrid material possesses very high dielectric strength and this is used to enhance the dielectric property of the polymer polyvinyledene fluoride (PVDF). Solution mixing was used to prepare the PVDF/MWCNT–TNT nanocomposites by varying the filler concentrations from 0.5 to 2.5 wt%. Excellent vapor sensing was noticed for the PVDF nanocomposites with different rate of response towards commonly used laboratory solvents. The composites and the fillers were characterized for its morphology and structural properties using scanning and transmission electron microscopy, X-ray diffraction studies and infrared spectroscopy. Vapor sensing was measured as relative resistance variations against the solvent vapors, and the dielectric properties of the composites were measured at room temperature during the frequency 10²–10⁷ Hz. Experimental results revealed the influence of filler synergy on the properties of PVDF and the enhancement in the solvent vapor detectability and dielectric properties reflects the ability of these composite films in flexible vapor sensors and in energy storage.     

Self‐Suppression of Lithium Dendrite in All‐Solid‐State Lithium Metal Batteries with Poly(vinylidene difluoride)‐Based Solid Electrolytes

Polymer‐based electrolytes have attracted ever‐increasing attention for all‐solid‐state lithium (Li) metal batteries due to their ionic conductivity, flexibility, and easy assembling into batteries, and are expected to overcome safety issues by replacing flammable liquid electrolytes. However, it is still a critical challenge to effectively block Li dendrite growth and improve the long‐term cycling stability of all‐solid‐state batteries with polymer electrolytes. Here, the interface between novel poly(vinylidene difluoride) (PVDF)‐based solid electrolytes and the Li anode is explored via systematical experiments in combination with first‐principles calculations, and it is found that an in situ formed nanoscale interface layer with a stable and uniform mosaic structure can suppress Li dendrite growth. Unlike the typical short‐circuiting that often occurs in most studied poly(ethylene oxide) systems, this interface layer in the PVDF‐based system causes an open‐circuiting feature at high current density and thus avoids the risk of over‐current. The effective self‐suppression of the Li dendrite observed in the PVDF–LiN(SO₂F)₂ (LiFSI) system enables over 2000 h cycling of repeated Li plating–stripping at 0.1 mA cm^?2 and excellent cycling performance in an all‐solid‐state LiCoO₂||Li cell with almost no capacity fade after 200 cycles at 0.15 mA cm^?2 at 25 °C. These findings will promote the development of safe all‐solid‐state Li metal batteries.     

Indium Tin Oxide Thin-Film Sensor for Detection of Volatile Organic Compounds (VOCs)

Indium tin oxide (ITO) (In₂O₃ + 17% SnO₂) thin films were grown on glass substrate by direct evaporation method. Two thick gold pads were deposited to take out contacts. The response of these films at different operating temperatures, when exposed to various volatile organic compounds (VOCs) such as methanol, ethanol, butanol, and acetone in the concentration range 200-2500 ppm was evaluated. Additionally, the effect of film thickness on the response charateristics of methanol and acetone was studied. The linearity and sensitivity of the sensors were measured. The ITO thin-film sensors showed a sensitivity of 0.256 ohms/ppm to acetone vapors, which was almost linear in the range 200-2500 ppm. In order to improve sensitivity and selectivity, a thin layer of various metal and metal oxides such as Cu and PbO was deposited on the sensor surface to work as catalytic layer and the effect on the performance of the sensor was studied. The response and recovery times of the sensor were determined for acetone vapors and were found to be 155 sec and 110 sec, respectively.     

A micro-system based on glass-nanoporous silicon for optical sensing of organic solvent vapor

We present a recent experimental study on the application of nanoporous silicon (np-Si) to an optical vapor sensor. We fabricated the micro-system based on a glass-nanoporous silicon layer on a p(+)-type silicon wafer. To check the selectivity and sensitivity of the np-Si layer to organic vapors, we prepared three types of np-Si layer samples--a single layer, distributed Bragg reflector (DBR) layer, and microcavity layer--and investigated its reflectance spectra upon exposure to different concentrations of various organic vapors. When the np-Si layer samples were exposed to the organic vapors, a red-shift occurred in the reflectance spectrum, and we determined that this red-shift can be attributed to the changes in the refractive index induced by the capillary condensation of the organic vapor within the pores of the np-Si layer. The np-Si layer samples showed excellent sensing ability to different types and concentrations of organic vapors. After removing the organic vapors, the reflectance spectrum immediately returned to its original state.     

Instantaneous,Simple, and Reversible Revealing of Invisible Patterns Encrypted in Robust Hollow Sphere Colloidal Photonic Crystals

The colors of photonic crystals are based on their periodic crystalline structure. They show clear advantages over conventional chromophores for many applications, mainly due to their anti‐photobleaching and responsiveness to stimuli. More specifically, combining colloidal photonic crystals and invisible patterns is important in steganography and watermarking for anticounterfeiting applications. Here a convenient way to imprint robust invisible patterns in colloidal crystals of hollow silica spheres is presented. While these patterns remain invisible under static environmental humidity, even up to near 100% relative humidity, they are unveiled immediately (≈100 ms) and fully reversibly by dynamic humid flow, e.g., human breath. They reveal themselves due to the extreme wettability of the patterned (etched) regions, as confirmed by contact angle measurements. The liquid surface tension threshold to induce wetting (revealing the imprinted invisible images) is evaluated by thermodynamic predictions and subsequently verified by exposure to various vapors with different surface tension. The color of the patterned regions is furthermore independently tuned by vapors with different refractive indices. Such a system can play a key role in applications such as anticounterfeiting, identification, and vapor sensing.     

Characterization of a cross-reactive electronic nose with vapoluminescent array elements

A three-channel cross-reactive sensor array based on vapoluminescent platinum(II) double salt materials has been characterized. Two arrays were studied, one consisting of [Pt(CN-cyclododecyl)4][Pt(CN)4] (1), [(phen)Pt(CN-cyclohexyl)2][Pt(CN)4] (2), and [Pt(CN-n-tetradecyl)4][Pt-(CN)4] (3) materials, where phen = 1,10-phenanthroline, and a second array that has compound 3 replaced by the mixed double salt material [(phen)Pt(CN-cyclododecyl)Cl)]2[(phen)Pt(CN-cyclododecyl)2]2[Pt(CN)4]3 (4). Compounds 2, 3 and 4 are characterized here for the first time. Inclusion of solvent vapors into these materials often leads to dramatic shifts in their solid-state absorption and luminescence spectra. In these studies the arrays were exposed to a set of 10 test solvent vapors to determine the ability of each cross-reactive array to give reproducible vapoluminescent spectra characteristic of each solvent vapor. It was discovered that temperature programming between solvent vapor exposures greatly improved the reproducibility of the luminescence spectra obtained. A statistical analysis of three-dimensional resolution factors between pairs of solvent clusters in principal component space supported this assertion. The success of the temperature programming protocol was limited by the thermal stability and the sensitivity to low background water vapor levels of some platinum(II) double salt materials. The ability of the cross-reactive sensor array to differentiate between two different solvent vapors over a large concentration range was also investigated. Acetone and methanol were found to occupy two distinct regions of the three-dimensional principal component space. Detection limits for acetone and methanol were estimated from the principal component analysis as 75 and 6 g/m3, respectively.     

Dielectric Elastomers in Actuator Technology

Electroactive polymers (EAPs) are characterized by their ability to respond to external electric stimulation by displaying a significant shape or size displacement. Actuators, based on dielectric elastomers exhibiting low elastic stiffness and high dielectric constants, can produce high strain levels from 10 to 380 %. Typically, acrylic and silicone materials are used as dielectric layer in such actuators. Their potential to mimic the movement of animals, insects and even human body parts are increasingly of interest for researchers in the field of biomimetics, as well as more classical application fields like robotics. The control of the most important material properties, elastic moduli and dielectric constants of the dielectric elastomers and electrode materials, together with the control of fabrication parameters i.e. film thickness, electrode manufacturing as well as design of the actuator configuration allow the fabrication of tailor‐made actuators, which match the necessary requirements for a given application. Theoretical models contribute to a deeper understanding of EAP actuators and improve design and optimisation.     

Switchable Helical Structures Formed by the Hierarchical Self‐Assembly of Laterally Tethered Nanorods

The formation of helical scrolls formed by self‐assembly of tethered nanorod amphiphiles and their molecular analogs are investigated. A model bilayer sheet assembled by laterally tethered nanorods is simulated and shown that it can fold into distinct helical morphologies under different solvent conditions. The helices can reversibly transform from one morphology to another by dynamically changing the solvent condition. This model serves both to inspire the fabrication of laterally tethered nanorods for assembling helices at nanometer scales and as a proof‐of‐concept for engineering switchable nanomaterials via hierarchical self‐assembly.     

Remarkable Vapochromic Behavior of Pure Organic Octahedron Embedded in Porous Frameworks

Vapochromic behavior is employed to selectively monitor the vapor changes in surrounding environment, particularly for toxic gas leaking and floating detection. Thus, sensitive trapping and accurate response to different toxic vapors are critical factors in vapochromic sensing. In this work, a self‐assembled hybrid that consists of fluorescent organic octahedron encapsulated by metal–organic polyhedron (MOP) is reported. The fluorescent octahedron is used as a responsive sensor to probe various solvent vapors, while the MOP is employed as a protector to prevent the corrosion of solvents to the organic octahedron. The hybrid exhibits remarkable vapochromic behavior to different solvents, and shows the highest selectivity and sensitivity specifically to acetone. In addition, acetone vapor under different conditions is utilized for further studying the response mechanism of the hybrid. This work presents a promising vapochromic sensor with good stability, selectivity, and sensitivity. The study is expected to open up the applicability of MOP‐based hybrids for specific molecular capture, interim storage, controlled release, and advanced sensing.     

Cholesteric liquid crystals doped with dodecylamine for detecting aldehyde vapors

In this paper, we report a study of using cholesteric liquid crystals (CLCs) doped with dodecylamine for detecting aldehyde vapors. CLCs doped with dodecylamine show color change from red to yellow-green upon exposure to 300 ppmv pentyl aldehyde within 60 s. In contrast, no colorimetric response is observed when pure CLCs are used. Characterization by using FT-IR shows that the possible mechanism responsible for the colorimetric response is the formation of an imine bond between dodecylamine and pentyl aldehyde. A new C=N peak at 1670 cm(-1) appears in the spectrum after the exposure to aldehyde vapors. The CLCs doped with dodecylamine show good selectivity for pentyl aldehyde; they do not respond to 200 ppmv pentyl alcohol, pentylamine, acetone, ethanol, and water vapor. This study demonstrates the potential applications of doped CLCs as low-cost and portable gas sensors.     

ATP-independent contractile proteins from plants

Emerging technologies are creating increasing interest in smart materials that may serve as actuators in micro- and nanodevices. Mechanically active polymers currently studied include a variety of materials. ATP-driven motor proteins, the actuators of living cells, possess promising characteristics, but their dependence on strictly defined chemical environments can be disadvantagous. Natural proteins that deform reversibly by entropic mechanisms might serve as models for artificial contractile polypeptides with useful functionality, but they are rare. Protein bodies from sieve elements of higher plants provide a novel example. sieve elements form microfluidics systems for pressure-driven transport of photo-assimilates throughout the plant. Unique protein bodies in the sieve elements of legumes act as cellular stopcocks, by undergoing a Ca2+-dependent conformational switch in which they plug the sieve element. In living cells, this reaction is probably controlled by Ca2+-transporters in the cell membrane. Here we report the rapid, reversible, anisotropic and ATP-independent contractility in these protein bodies in vitro. Considering the unique biological function of the legume 'crystalloid' protein bodies and their contractile properties, we suggest to give them the distinctive name forisome ('gate-body'; from the Latin foris, the wing of a gate).     

Differentiation of chemical components in a binary solvent vapor mixture using carbon/polymer composite-based chemiresistors

We demonstrate a "universal solvent sensor" constructed from a small array of carbon/polymer composite chemiresistors that respond to solvents spanning a wide range of Hildebrand solubility parameters. Conductive carbon particles provide electrical continuity in these composite films. When the polymer matrix absorbs solvent vapors, the composite film swells, the average separation between carbon particles increases, and an increase in film resistance results, as some of the conduction pathways are broken. The adverse effects of contact resistance at high solvent concentrations are reported. Solvent vapors including isooctane, ethanol, diisopropylmethylphosphonate (DIMP), and water are correctly identified ("classified") using three chemiresistors, their composite coatings chosen to span the full range of solubility parameters. With the same three sensors, binary mixtures of solvent vapor and water vapor are correctly classified; following classification, two sensors suffice to determine the concentrations of both vapor components. Poly(ethylenevinyl acetate) and poly(vinyl alcohol) (PVA) are two such polymers that are used to classify binary mixtures of DIMP with water vapor; the PVA/carbon particle composite films are sensitive to less than 0.25% relative humidity. The Sandia-developed visual-empirical region of influence (VERI) technique is used as a method of pattern recognition to classify the solvents and mixtures and to distinguish them from water vapor. In many cases, the response of a given composite sensing film to a binary mixture deviates significantly from the sum of the responses to the isolated vapor components at the same concentrations. While these nonlinearities pose significant difficulty for (primarily) linear methods such as principal component analysis, VERI handles both linear and nonlinear data with equal ease. In the present study, the maximum speciation accuracy is achieved by an array containing three or four sensor elements, with the addition of more sensors resulting in a measurable accuracy decrease.     

凝胶浴温度对PVDF膜的对称结构及晶型的调控

以聚偏氟乙烯(PVDF)为膜材料,通过浸没沉淀法制备得到开放性网络状对称结构的PVDF膜。分别考察不同凝固浴温度对膜结构和晶型的影响。结果表明,当温度较高时,溶剂与非溶剂的扩散传质速度也较快,有利于形成α晶型的PVDF晶体。温度较低时,成膜较慢,PVDF大分子在极性的凝胶浴中有足够的时间由非极性的α晶型扭转形成极性的β晶型。而β晶型的PVDF由于H、F均匀分布在碳链的两侧,分子呈极性,更有利于蛋白质的吸附。当磷酸三乙酯(TEP)凝胶浴温度为20℃,纯水凝胶浴温度为8℃时,所制备的膜以β晶型为主,蛋白吸附量高达160μg/cm2。     

Soft Ultrathin Electronics Innervated Adaptive Fully Soft Robots

Soft robots outperform the conventional hard robots on significantly enhanced safety, adaptability, and complex motions. The development of fully soft robots, especially fully from smart soft materials to mimic soft animals, is still nascent. In addition, to date, existing soft robots cannot adapt themselves to the surrounding environment, i.e., sensing and adaptive motion or response, like animals. Here, compliant ultrathin sensing and actuating electronics innervated fully soft robots that can sense the environment and perform soft bodied crawling adaptively, mimicking an inchworm, are reported. The soft robots are constructed with actuators of open‐mesh shaped ultrathin deformable heaters, sensors of single‐crystal Si optoelectronic photodetectors, and thermally responsive artificial muscle of carbon‐black‐doped liquid‐crystal elastomer (LCE‐CB) nanocomposite. The results demonstrate that adaptive crawling locomotion can be realized through the conjugation of sensing and actuation, where the sensors sense the environment and actuators respond correspondingly to control the locomotion autonomously through regulating the deformation of LCE‐CB bimorphs and the locomotion of the robots. The strategy of innervating soft sensing and actuating electronics with artificial muscles paves the way for the development of smart autonomous soft robots.     

Highly tunable self-assembled nanostructures from a poly(2-vinylpyridine-b-dimethylsiloxane) block copolymer

An extraordinarily large degree of tunability in geometry and dimension is demonstrated in films of a self-assembled block copolymer. A poly(2-vinylpyridine-b-dimethylsiloxane) block copolymer with highly incompatible blocks was spun-cast on patterned substrates and treated with various solvent vapors. The degree of selective swelling in the poly(2-vinylpyridine) matrix block could be controlled over an extensive range, leading to the formation of various microdomain morphologies such as spheres, cylinders, hexagonally perforated lamellae, and lamellae from the same block copolymer. The systematic control of swelling ratio and the choice of solvent vapors offer the unusual ability to control the width of very well-ordered linear features within a range between 6 and 31 nm. This methodology is particularly useful for nanolithography based on directed self-assembly in that a single block copolymer film can form microdomains with a broad range of geometries and sizes without the need to change molecular weight or volume fraction.     

Bio‐Inspired,Smart, Multiscale Interfacial Materials

In this review a strategy for the design of bioinspired, smart, multiscale interfacial (BSMI) materials is presented and put into context with recent progress in the field of BSMI materials spanning natural to artificial to reversibly stimuli‐sensitive interfaces. BSMI materials that respond to single/dual/multiple external stimuli, e.g., light, pH, electrical fields, and so on, can switch reversibly between two entirely opposite properties. This article utilizes hydrophobicity and hydrophilicity as an example to demonstrate the feasibility of the design strategy, which may also be extended to other properties, for example, conductor/insulator, p‐type/n‐type semiconductor, or ferromagnetism/anti‐ferromagnetism, for the design of other BSMI materials in the future.     

Arbitrarily 3D Configurable Hygroscopic Robots with a Covalent–Noncovalent Interpenetrating Network and Self‐Healing Ability

Soft materials that can reversibly transform shape in response to moisture have applications in diverse areas such as soft robotics and biomedicine. However, the design of a structurally transformable or mechanically self‐healing version of such a humidity‐responsive material, which can arbitrarily change shape and reconfigure its 3D structures remains challenging. Here, by drawing inspiration from a covalent–noncovalent network, an elaborately designed biopolyester is developed that features a simple hygroscopic actuation mechanism, straightforward manufacturability at low ambient temperature (≤35 °C), fast and stable response, robust mechanical properties, and excellent self‐healing ability. Diverse functions derived from various 3D shapes that can grasp, swing, close–open, lift, or transport an object are further demonstrated. This strategy of easy‐to‐process 3D structured self‐healing actuators is expected to combine with other actuation mechanisms to extend new possibilities in diverse practical applications.