Nonlinear Frameworks for Reversible and Pluripotent Wetting on Topographic Surfaces

Wetting-enhanced adhesion of photo-polymerized supramolecular adhesives for both smooth and rough surfaces

Efficient interactions between an adhesive and a substrate surface at the molecular level are the basis for the formation of robust adhesion, which substantially relies on interfacial wetting. However, strong adhesives usually improve cohesion but compromise interfacial properties. Herein, we have reported a kind of robust supramolecular adhesive based on the outstanding mobility and interfacial wettability of adhesive precursors. In situ fast photopolymerization endows supramolecular adhesives with more outstanding adhesion for both smooth and rough surfaces in air and underwater in contrast to their counterparts from thermal polymerization. In addition to their low viscosity and high monomer concentration, supramolecular adhesive precursors without any organic solvents possess well-defined hydrogen bonding interactions. These superior properties consistently contribute to the wetting of the substrate and the formation of adhesive polymers with high molecular weights. This work highlights that enhancing interfacial wetting between an adhesive and a substrate is a promising route to achieving robust adhesion.

Chip-less wireless electronic skins by remote epitaxial freestanding compound semiconductors

Recent advances in flexible and stretchable electronics have led to a surge of electronic skin (e-skin)-based health monitoring platforms. Conventional wireless e-skins rely on rigid integrated circuit chips that compromise the overall flexibility and consume considerable power. Chip-less wireless e-skins based on inductor-capacitor resonators are limited to mechanical sensors with low sensitivities. We report a chip-less wireless e-skin based on surface acoustic wave sensors made of freestanding ultrathin single-crystalline piezoelectric gallium nitride membranes. Surface acoustic wave-based e-skin offers highly sensitive, low-power, and long-term sensing of strain, ultraviolet light, and ion concentrations in sweat. We demonstrate weeklong monitoring of pulse. These results present routes to inexpensive and versatile low-power, high-sensitivity platforms for wireless health monitoring devices.

Rotating Cylinder-Assisted Nanoimprint Lithography for Enhanced Chemisorbable Filtration Complemented by Molecularly Imprinted Polymers

Rotating cylindrical stamp-based nanoimprint technique has many advantages, including the continuous fabrication of intriguing micro/nanostructures and rapid pattern transfer on a large scale. Despite these advantages, the previous nanoimprint lithography has rarely been used for producing sophisticated nanoscale patterns on a non-planar substrate that has many extended applications. Here, the simple integration of nanoimprinting process with a help of a transparent stamp wrapped on the cylindrical roll and UV optical source in the core to enable high-throughput pattern transfer, particularly on a fabric substrate is demonstrated. Moreover, as a functional resin material, this innovative strategy involves a synergistic approach on the synthesis of molecularly imprinted polymer, which are spatially organized free-standing perforated nanostructures such as nano/microscale lines, posts, and holes patterns on various woven or nonwoven blank substrates. The proposed materials can serve as a self-encoded filtration medium for selective separation of formaldehyde molecules. It is envisioned that the combinatorial fabrication process and attractive material paves the way for designing next-generation separation systems in use to capture industrial or household toxic substances.

Artificial Leaf for Switchable Droplet Manipulation

Droplet manipulation plays an important role in scientific research, daily life, and practical production such as biological and chemical analysis. Inspired by the structure and function of three typical leaf veins, the bionic texture was replicated by the template method, and the artificial leaf was selectively treated by nanoparticles to obtain a quasi-three-dimensional hybrid superhydrophobic-hydrophilic surface. When the droplet touches the surface of the leaf, it will be attracted to the bottom of the main vein from different directions even in horizontal conditions due to the Laplace pressure gradient and energy gradient. The simulation analysis demonstrates that the reason for directional transportation is the energy gradient of the droplets on the different levels of veins, including the thin veins, lateral veins, and main vein. Meanwhile, the experimental result of water collection also showed an outstanding directional transportation effect and excellent water collection efficiency. In addition, when the sample is tilted upside down, the droplet will flow back to the main vein along the lateral vein and then flow down the main vein, showing a good droplet pumping effect. Therefore, the directional and polydirectional transportation of droplets on the same sample is successfully realized, and the conversion between executing single and multiple tasks simultaneously can be realized only by upright and inverted samples. This work provided a new strategy for directional and polydirectional water manipulation, water collection, directional drainage, and microfluidic devices.

Shear Stress-Triggered Deformation of Microparticles in a Tapered Microchannel

We demonstrate that it is possible to produce microparticles with high deformability while maintaining a high effective volume. For significant particle deformation, a particle must have a void region. The void fraction of the particle allows its deformation under shear stress. Owing to the importance of the void fraction in particle deformation, we defined an effective volume index (V*) that indicates the ratio of the particle’s total volume to the volumes of the void and material structures. We chose polyethylene glycol diacrylate (Mn ~ 700) for the fabrication of the microparticles and focused on the design of the particles rather than the intrinsic softness of the material (E). We fabricated microparticles with four distinct shapes: discotic, ring, horseshoe, and spiral, with various effective volume indexes. The microparticles were subjected to shear stress as they were pushed through a tapered microfluidic channel to measure their deformability. The deformation ratio R was introduced as R = 1−W_deformed/D_original to compare the deformability of the microparticles. We measured the deformation ratio by increasing the applied pressure. The spiral-shaped microparticles showed a higher deformation ratio (0.901) than those of the other microparticles at the same effective volume index.

Fabricating 3D Metastructures by Simultaneous Modulation of Flexible Resist Stencils and Basal Molds

Metamaterials have gained much attention thanks to their extraordinary and intriguing optical properties beyond natural materials. However, universal high-resolution fabrications of 3D micro/nanometastructures with high-resolution remain a challenge. Here, a novel approach to fabricate sophisticated 3D micro/nanostructures with excellent robustness and precise controllability is demonstrated by simultaneously modulating of flexible resist stencils and basal molds. This method allows arbitrary manipulations of morphology, size, and orientation, as well as contact angles of the objects. Combined with a new alignment strategy of high-resolution, previously inaccessible architectures are fabricated with ultrahigh precision, leading to an excellent spectra response from the fabricated metastructures. This method provides a new possibility to realize true 3D metamaterial fabrications featuring high-resolution and direct-compatibility with broad planar lithography platforms.

Artificial Perspiration Membrane by Programmed Deformation of Thermoresponsive Hydrogels

Thermal management is essential for living organisms and electronic devices to survive and maintain their own functions. However, developing flexible cooling devices for flexible electronics or biological systems is challenging because conventional coolers are bulky and require rigid batteries. In nature, skins help to maintain a constant body temperature by dissipating heat through perspiration. Inspired by nature, an artificial perspiration membrane that automatically regulates evaporation depending on temperature using the programmed deformation of thermoresponsive hydrogels is presented. The thermoresponsive hydrogel is patterned into pinwheel shapes and supported by a polymeric rigid frame with stable adhesion using copolymerization. Both shape of the valve and mechanical constraint of the frame allow six times larger evaporation area in the open state compared to the closed state, and the transition occurs at a fast rate (≈1 s). A stretchable membrane is selectively coated to prevent unintended evaporation through the hydrogel while allowing swelling or shrinking of the hydrogel by securing path of water. Consequently, a 30% reduction in evaporation is observed at lower temperature, resulting in regulation of the skin temperature at the thermal model of human skins. This simple, small, and flexible cooler will be useful for maintaining temperature of flexible devices.

Multibioinspired slippery surfaces with wettable bump arrays for droplets pumping

Droplet manipulation is playing an important role in various fields, including scientific research, industrial production, and daily life. Here, inspired by the microstructures and functions of Namib desert beetles, Nepenthes pitcher plants, and emergent aquatic plants, we present a multibioinspired slippery surface for droplet manipulation by employing combined strategies of bottom-up colloidal self-assembly, top-down photolithography, and microstructured mold replication. The resultant multilayered hierarchical wettability surface consists of hollow hydrogel bump arrays and a lubricant-infused inverse opal film as the substrate. Based on capillary force, together with slippery properties of the substrate and wettability of the bump arrays, water droplets from all directions can be attracted to the bumps and be collected through hollow channels to a reservoir. Independent of extra energy input, droplet condensation, or coalescence, these surfaces have shown ideal droplet pumping and water collection efficiency. In particular, these slippery surfaces also exhibit remarkable features including versatility, generalization, and recyclability in practical use such as small droplet collection, which make them promising candidates for a wide range of applications.

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.

Hydrogel-Templated Transfer-Printing of Conductive Nanonetworks for Wearable Sensors on Topographic Flexible Substrates

Transfer-printing enables the assembly of functional nanomaterials on unconventional substrates with a desired layout in a controllable manner. However, transfer-printing to substrates with complex surfaces remains a challenge. Herein, we show that hydrogels serve as effective template material platforms for the assembly and transfer-printing of conductive nanonetwork patterns for flexible sensors on various topographic surfaces in a very simple yet versatile manner. The non-adherence, nanoporous structure, and molding capability of the hydrophilic hydrogel enable the assembly of conductive nanonetwork patterns on the hydrogel surface and transfer of the nanonetworks onto various flexible and topographic substrates. Flexible strain sensors and pressure sensors that monitor finger motions and arterial pulses are successfully demonstrated using the hydrogel-templated approach. The rich chemistry of polymeric networks, facile molding capability, and biocompatibility of hydrogels could be further combined with additive technology for hydrogels and electronic materials for emerging four-dimensional functional materials and soft bioelectronics.

Fabrication and design of mechanically stable and free-standing polymeric membrane with two-level apertures

Herein, we report the fabrication process and the investigation of mechanically stable, flexible and free-standing polymeric membranes with two-level apertures. By using overlapped oxygen inhibition layers (OILs) with variation in diameters of the micro-sized supporting layer, we successfully fabricated the mechanically stable and free-standing polymeric membrane with micro/nano two-level apertures. The nano aperture membrane was stably sustained on the micro aperture membrane with a diameter of 50 μm and 100 μm, but was torn off in the case of 300 μm and 500 μm sized supporting layers. To analyze the results, we propose a simple model to set the criteria of the geometrical features which are mechanically stable during the demolding process. It is worth noting that an appropriate material modulus, length, and thickness of the membrane are required for designing and achieving the robust free-standing hierarchical polymeric membrane.

Utilization of Resist Stencil Lithography for Multidimensional Fabrication on a Curved Surface

The limited ability to fabricate nanostructures on nonplanar rugged surfaces has severely hampered the applicability of many emerging technologies. Here we report a resist stencil lithography based approach for in situ fabrication of multidimensional nanostructures on both planar and uneven substrates. By using the resist film as a flexible stencil to form a suspending membrane with predesigned patterns, a variety of nanostructures have been fabricated on curved or uneven substrates of diverse morphologies on demand. The ability to realize 4 in. wafer scale fabrication of nanostructures as well as line width resolution of sub-20 nm is also demonstrated. Its extraordinary capacity is highlighted by the fabrication of three-dimensional wavy nanostructures with diversified cell morphologies on substrates of different curvatures. A robust general scheme is also developed to construct various complex 3D nanostructures. The use of conventional resists and processing ensures the versatility of the method. Such an in situ lithography technique has offered exciting possibilities to construct nanostructures with high dimensionalities that can otherwise not be achieved with existing nanofabrication methods.

Micro- and nano-patterned conductive graphene-PEG hybrid scaffolds for cardiac tissue engineering

A lack of electrical conductivity and structural organization in currently available biomaterial scaffolds limits their utility for generating physiologically representative models of functional cardiac tissue. Here we report on the development of scalable, graphene-functionalized topographies with anisotropic electrical conductivity for engineering the structural and functional phenotypes of macroscopic cardiac tissue constructs. Guided by anisotropic electroconductive and topographic cues, the tissue constructs displayed structural property enhancement in myofibrils and sarcomeres, and exhibited significant increases in the expression of cell-cell coupling and calcium handling proteins, as well as in action potential duration and peak calcium release.

Ultrasensitive and Highly Stable Resistive Pressure Sensors with Biomaterial-Incorporated Interfacial Layers for Wearable Health-Monitoring and Human-Machine Interfaces

Flexible piezoresistive sensors have huge potential for health monitoring, human-machine interfaces, prosthetic limbs, and intelligent robotics. A variety of nanomaterials and structural schemes have been proposed for realizing ultrasensitive flexible piezoresistive sensors. However, despite the success of recent efforts, high sensitivity within narrower pressure ranges and/or the challenging adhesion and stability issues still potentially limit their broad applications. Herein, we introduce a biomaterial-based scheme for the development of flexible pressure sensors that are ultrasensitive (resistance change by 5 orders) over a broad pressure range of 0.1-100 kPa, promptly responsive (20 ms), and yet highly stable. We show that employing biomaterial-incorporated conductive networks of single-walled carbon nanotubes as interfacial layers of contact-based resistive pressure sensors significantly enhances piezoresistive response via effective modulation of the interlayer resistance and provides stable interfaces for the pressure sensors. The developed flexible sensor is capable of real-time monitoring of wrist pulse waves under external medium pressure levels and providing pressure profiles applied by a thumb and a forefinger during object manipulation at a low voltage (1 V) and power consumption (<12 μW). This work provides a new insight into the material candidates and approaches for the development of wearable health-monitoring and human-machine interfaces.

Shaping micro-clusters via inverse jamming and topographic close-packing of microbombs

Designing topographic clusters is of significant interest, yet it remains challenging as they often lack mobility or deformability. Here we exploit the huge volumetric expansion (up to 3000%) of a new type of building block, thermally expandable microbombs. They consist of a viscoelastic polymeric shell and a volatile gas core, which, within structural confinement, create micro-clusters via inverse jamming and topographical close-packing. Upon heating, microbombs anchored in rigid confinement underwent balloon-like blowing up, allowing for dense clusters via soft interplay between viscoelastic shells. Importantly, the confinement is unyielding against the internal pressure of the microbombs, thereby enabling self-assembled clusters, which can be coupled with topographic inscription to introduce structural hierarchy on the clusters. Our strategy provides densely packed yet ultralight clusters with a variety of complex shapes, cleavages, curvatures, and hierarchy. In turn, these clusters will enrich our ability to explore the assemblies of the ever-increasing range of microparticle systems.

Self-assembled systems are normally composed of incompressible building blocks, which constrain their space filling efficiency. Yu et al. show programmable, densely packed clusters using thermally expandable soft microparticles, whereby the self-assembling process is realized via a jamming transition.

Multidirectional Wrinkle Patterns Programmed by Sequential Uniaxial Strain with Conformal yet Nontraceable Masks

Surface wrinkling is a promising route to control the mechanical, electrical, and optical properties of materials in a wide range of applications. However, previous artificial wrinkles are restricted to single or random orientation and lacks selectivity. To address this challenge, this study presents multidirectional wrinkle patterns with high selectivity and orientation through sequential uniaxial strain with conformal polymeric shadow masks. The conformal but nontraceable polymeric stencil with microapertures are adhered to a flat substrate prior to oxidation, which forms discrete and parallel wrinkles in confined domains without any contamination. By fully investigating the process, this study displays compound topography of wrinkles consisting of wrinkle islands and surrounding secondary wrinkles on the same surface. With this topography, various diffusion properties are presented: from semi-transparent yet diffusive films to multidirectional diffusive films, which will be available for new types of optical diffuser applications.