High-Performance Pressure Sensors Based on Shaped Gel Droplet Arrays.

Polymer gel-based pressure sensors offer numerous advantages over traditional sensing technologies, including excellent conformability and integration into wearable devices. However, challenges persist in terms of their performance and manufacturing technology. In this study, a method for fabricating gel pressure sensors using a hydrophobic/hydrophilic patterned surface is introduced. By shaping and fine-tuning the droplets of the polymer gel prepolymerization solution on the patterned surface, remarkable sensitivity improvements compared to unshaped hydrogels have been achieved. This also showcased the potential for tailoring gel pressure sensors to different applications. By optimizing the configuration of the sensor array, an uneven conductive gel array is fabricated, which exhibited a high sensitivity of 0.29 kPa-1 in the pressure range of 0-30 kPa, while maintaining a sensitivity of 0.13 kPa-1 from 30 kPa up to 100 kPa. Furthermore, the feasibility of using these sensors for human motion monitoring is explored and a conductive gel array for 2D force detection is successfully developed. This efficient and scalable fabrication method holds promise for advancing pressure sensor technology and offers exciting prospects for various industries and research fields.

Mosaic Patterned Surfaces toward Generating Hardly-Volatile Capsular Droplet Arrays for High-Precision Droplet-Based Storage and Detection.

Precise detection involving droplets based on functional surfaces is promising for the parallelization and miniaturization of platforms and is significant in epidemic investigation, analyte recognition, environmental simulation, combinatorial chemistry, etc. However, a challenging and considerable task is obtaining mutually independent droplet arrays without cross-contamination and simultaneously avoiding droplet evaporation-caused quick reagent loss, inaccuracy, and failure. Herein, a strategy to generate mutually independent and hardly-volatile capsular droplet arrays using innovative mosaic patterned surfaces is developed. The evaporation suppression of the capsular droplet arrays is 1712 times higher than the naked droplet. The high evaporation suppression of the capsular droplet arrays on the surfaces is attributed to synergistic blocking of the upper oil and bottom mosaic gasproof layer. The scale-up of the capsular droplet arrays, the flexibility in shape, size, component (including aqueous, colloidal, acid, and alkali solutions), liquid volume, and the high-precision hazardous substance testing proves the concept's high compatibility and practicability. The mutually independent capsular droplet arrays with amazingly high evaporation suppression are essential for the new generation of high-performance open-surface microfluidic chips used in COVID-19 diagnosis and investigation, primary screening, in vitro enzyme reactions, environmental monitoring, nanomaterial synthesis, etc.

Surface Modification, Topographic Design and Applications of Superhydrophobic Systems.

Superhydrophobic surfaces with expanded wetting behaviors, like tunable adhesion, hybrid surface hydrophobicity and smart hydrophobic switching have attracted increasing attention due to their broad applications. Herein, the construction methods, mechanisms and advanced applications of special superhydrophobicity are reviewed, and hydro/superhydrophobic modifications are categorized and discussed based on their surface chemistry, and topographic design. The formation and maintenance of special superhydrophobicity in the metastable state are also examined and explored. In addition, particular attention is paid to the use of special wettability in various applications, such as membrane distillation, droplet-based electricity generators and anti-fogging surfaces. Finally, the challenges for practical applications and future research directions are discussed.

Mixed Fluorinated/Hydrogenated Self-Assembled Monolayer-Protected Gold Nanoparticles: In Silico and In Vitro Behavior.

Gold nanoparticles (AuNPs) covered with mixtures of immiscible ligands present potentially anisotropic surfaces that can modulate their interactions at complex nano-bio interfaces. Mixed, self-assembled, monolayer (SAM)-protected AuNPs, prepared with incompatible hydrocarbon and fluorocarbon amphiphilic ligands, are used here to probe the molecular basis of surface phase separation and disclose the role of fluorinated ligands on the interaction with lipid model membranes and cells, by integrating in silico and experimental approaches. These results indicate that the presence of fluorinated amphiphilic ligands enhances the membrane binding ability and cellular uptake of gold nanoparticles with respect to those coated only with hydrogenated amphiphilic ligands. For mixed monolayers, computational results suggest that ligand phase separation occurs on the gold surface, and the resulting anisotropy affects the number of contacts and adhesion energies with a membrane bilayer. This reflects in a diverse membrane interaction for NPs with different surface morphologies, as determined by surface plasmon resonance, as well as differential effects on cells, as observed by flow cytometry and confocal microscopy. Overall, limited changes in monolayer features can significantly affect NP surface interfacial properties, which, in turn, affect the interaction of SAM-AuNPs with cellular membranes and subsequent effects on cells.

Using micro-patterned surfaces to inhibit settlement and biofilm formation by Bacillus subtilis.

Biofilm is a biological complex caused by bacteria attachment to the substrates and their subsequent reproduction and secretion. This phenomenon reduces heat transfer efficiency and causes significant losses in treated sewage heat-recovering systems. This paper describes a physical approach to inhibit bacteria settlement and biofilm formation by Bacillus subtilis, which is the dominant species in treated sewage. Here, micro-patterned surfaces with different characteristics (stripe and cube) and dimensions (1-100 µm) were fabricated as surfaces of interest. Model sewage was prepared and a rotating coupon device was used to form the biofilms. Precision balance, scanning electron microscopy, and confocal laser scanning microscopy (CLSM) were employed to investigate the inhibitory effects and the mechanisms of the biofilm-surface interactions. The results have shown that surfaces with small pattern sizes (1 and 2 µm) all reduced biofilm formation significantly. Interestingly, the CLSM images showed that the surfaces do not play a role in "killing" the bacteria. These findings are useful for future development of new process surfaces on which bacteria settlement and biofilm formation can be inhibited or minimized.

Patterned Manipulated Surface Based on Femtosecond Laser with Adjustable Wetting Speed and Directional Fluid Delivery.

Recently, due to the crucial roles of multifunctional liquid manipulation surfaces in biomedical transportation, microfluidics, and chemical engineering, the demand for controllable and functional aspects of directed liquid transportation has increased significantly. However, designing an intelligent manipulation surface that is easy to manufacture and fully functional remains an immense challenge. To address this challenge, a smart surface that can regulate the rate of liquid transport within a patterned channel by temperature is reported. A synergistically controlled approach of poly(N-isopropylacrylamide) and micropillar shape-memory polymers (SMPs) was used to modulate the wetting rate of liquids on surfaces. By femtosecond laser direct writing, temperature-responsive composite surfaces are embedded in the microstructure of shape-memory polymers (SMPs) in a patterned manner, resulting in the preparation of novel programmable liquid manipulation surfaces incorporating boundaries possessing asymmetric wettability. Since the smart surface is based on SMP, the superhydrophobic part in the superhydrophobic/controllable wettability patterning platform is also programmed for droplet directional transport, which takes advantage of the difference in wettability between the rewritable indentation track and the periphery to allow droplets to flow into the temperature-controlled velocity track, enriching the functionality of the surface. In addition, based on its excellent controllability and patterning, the surface has been shown to be used in microfluidic circuit chips with self-cleaning properties, which provides new ideas for circuit timing control. This study provides promising prospects for the effective development of multifunctional liquid steering surfaces, lab-on-a-chip, and microfluidic devices.

Wrinkled Interfaces: Taking Advantage of Anisotropic Wrinkling to Periodically Pattern Polymer Surfaces.

Periodically patterned surfaces can cause special surface properties and are employed as functional building blocks in many devices, yet remaining challenges in fabrication. Advancements in fabricating structured polymer surfaces for obtaining periodic patterns are accomplished by adopting "top-down" strategies based on self-assembly or physico-chemical growth of atoms, molecules, or particles or "bottom-up" strategies ranging from traditional micromolding (embossing) or micro/nanoimprinting to novel laser-induced periodic surface structure, soft lithography, or direct laser interference patterning among others. Thus, technological advances directly promote higher resolution capabilities. Contrasted with the above techniques requiring highly sophisticated tools, surface instabilities taking advantage of the intrinsic properties of polymers induce surface wrinkling in order to fabricate periodically oriented wrinkled patterns. Such abundant and elaborate patterns are obtained as a result of self-organizing processes that are rather difficult if not impossible to fabricate through conventional patterning techniques. Focusing on oriented wrinkles, this review thoroughly describes the formation mechanisms and fabrication approaches for oriented wrinkles, as well as their fine-tuning in the wavelength, amplitude, and orientation control. Finally, the major applications in which oriented wrinkled interfaces are already in use or may be prospective in the near future are overviewed.

In Situ Observation of Micro-Patterned Elastomeric Surfaces: The Formation of the Area of Real Contact and the Influence on Its Friction and Deformation Behaviour.

Structured surfaces, which are the basis of the lotus blossom effect, have great potential to serve/operate as functionalised surfaces, i.e., surfaces with specific and/or adjustable properties. In the present study, the aim is to use micro-structured elastomeric surfaces to specifically influence the friction and deformation behaviours on the basis of the shape and arrangement of the structures. Thiol-acrylate-based photopolymers patterned via nanoimprint lithography were investigated by using an in situ tribological measurement set-up. A clear influence of the different structures on the surface's friction behaviour could be shown, and, furthermore, this could be brought into relation with the real area of contact. This finding provides an important contribution to further development steps, namely, to give the structures switchable properties in order to enable the control of friction properties in a targeted manner.

Simple and Scalable Chemical Surface Patterning via Direct Deposition from Immobilized Plasma Filaments in a Dielectric Barrier Discharge.

In this work, immobilization of the often unwanted filaments in dielectric barrier discharges (DBD) is achieved and used for one-step deposition of patterned coatings. By texturing one of the dielectric surfaces, a discharge containing stationary plasma filaments is ignited in a mix of argon and propargyl methacrylate (PMA) in a reactor operating at atmospheric pressure. From PMA, hydrophobic and hydrophilic chemical and topographical contrasts at sub-millimeter scale are obtained on silicon and glass substrates. Chemical and physical characterizations of the samples are performed by micrometer-scale X-ray photoelectron spectroscopy and infrared imaging and by water contact angle and profilometry, respectively. From the latter and additional information from high-speed imaging of the plasma phase and electrical measurements, it is suggested that filaments, denser in energetic species, lead to higher deposition rate with higher fragmentation of the precursor, while surface discharges igniting outwards the filaments are leading to smoother and slower deposition. This work opens a new route for a one-step large-area chemical and morphological patterning of surfaces at sub-millimeter scales. Moreover, the possibility to separately deposit coatings from filaments and the surrounding plasma phase can be helpful to better understand the processes occurring during plasma polymerization in filamentary DBD.

Patterned Carboxymethyl-Dextran Functionalized Surfaces Using Organic Mixed Monolayers for Biosensing Applications.

The deposition of biomolecules on biosensing surface platforms plays a key role in achieving the required sensitivity and selectivity for biomolecular interactions analysis. Controlling the interaction between the surface and biomolecules is increasingly becoming a crucial design tool to modulate the surface properties needed to improve the performance of the assay and the detection outcome. Carboxymethyl-dextran (CMD) coating can be exploited to promote chemical grafting of proteins, providing a hydrophilic, bioinert, nonfouling surface and a high surface density of immobilized proteins. In the present work, we developed and optimized a technique to produce a cost-effective CMD-based patterned surface for the immobilization of biomolecules to be used on standard protocols optimization. They consist of silicon or glass substrates with patterned bioactive areas able to efficiently confine the sampling solution by simply exploiting hydrophilic/hydrophobic patterning of the surface. The fabrication process involves the use of low-cost instruments and techniques, compatible with large scale production. The devices were validated through a chemiluminescence assay we recently developed for the analysis of binding of DNA nanoassemblies modified with an affinity binder to target proteins immobilized on the bioactive areas. Through this assay we were able to characterize the chemical reactivity of two target proteins toward a dextran matrix on patterned surfaces and to compare it with model CMD-based surface plasmon resonance (SPR) surfaces. We found a high reproducibility and selectivity in molecular recognition, consistent with results obtained on SPR sensor surfaces. The suggested approach is straightforward, cheap, and provides the means to assess patterned functionalized surfaces for bioanalytical platforms.

Low-Cost Laser Micromachining Super Hydrophilic-Super Hydrophobic Microgrooves for Robotic Capillary Micromanipulation of Microfibers.

Capillary self-alignment technique can achieve highly accurate and fast alignment of micro components. Capillary self-alignment technique relies on the confinement of liquid droplets at receptor sites where hydrophobic-hydrophilic patterns are widely used. This paper reports a low-cost microsecond pulse laser micromachining method for fabrication of super hydrophilic-super hydrophobic grooves as receptor sites for capillary self-alignment of microfibers. We investigated the influence of major manufacturing parameters on groove sizes and wetting properties. The effects of the width (20 µm-100 µm) and depth (8 µm-36 µm) of the groove on the volume of water droplet contained inside the groove were also investigated. We show that by altering scanning speed, using a de-focused laser beam, we can modify the wetting properties of the microgrooves from 10° to 120° in terms of the contact angle. We demonstrated that different types of microfibers including natural and artificial microfibers can self-align to the size matching super hydrophilic-super hydrophobic microgrooves. The results show that super hydrophilic-super hydrophobic microgrooves have great potential in microfiber micromanipulation applications such as natural microfiber categorization, fiber-based microsensor construction, and fiber-enforced material development.

Fabrication of Quasi-2D Shape-Tailored Microparticles using Wettability Contrast-Based Platforms.

The ability to fabricate materials with ultrathin architectures enables the breakthrough of low-dimensional structures with high surface area that showcase distinctive properties from their bulk counterparts. They are exploited in a wide range of fields, including energy harvesting, catalysis, and biomedicine. Despite such versatility, the fine tuning of the lateral dimensions and geometry of these structures remains challenging. Prepatterned platforms gain significant attention as enabling technologies to process materials with highly controlled shapes and dimensions. Herein, different nanometer-thick particles of various lateral sizes and geometries (e.g., squares, circles, triangles, hexagons) are processed with high precision and definition, taking advantage of the wettability contrast of oleophilic-oleophobic patterned surfaces. Quasi-2D polymeric microparticles with high shape- and size-fidelity can be retrieved as freestanding objects in a single step. These structures show cell-mediated pliability, and their integration in gravity-enforced human adipose-derived stem cell spheroids leads to an enhanced metabolic activity and a modulated secretion of proangiogenic factors.

Harnessing Multiple Surface Deformation Modes for Switchable Conductivity Surfaces.

Surface deformation modes, such as wrinkling, creasing, and cracking, enable a plethora of surface morphologies under mechanical loading, which have been widely exploited to provide flexibility and stretchability to electronic devices. As each phenomenon offers a distinct set of potential advantages, controlling the types and spatial locations of deformation modes is key for their successful application. In this study, we demonstrate a method to simultaneously harness multiple surface deformation modes-wrinkles, creases, and cracks-in patterned multilayer films. The wrinkling of metal-coated stiff patterned films provides flexibility and stretchability, while the reversible formation of creases in the intervening regions of the bare elastomer is used to template the formation of patterned cracks in the metal. While conventional cracks can be difficult to precisely control, the patterned cracks demonstrated here remain straight over long distances and show tunable lateral spacings from hundreds of micrometers to centimeters. Finally, the reversible opening and closing of these cracks under mechanical loading provides mechanically gated electrical switches with small and tunable critical switching strains of 0.05-0.18 and high on/off ratios of >107, enabling the preparation of mechanical NAND and NOR logic gates each composed of multiple patterned switches on a single elastomer surface.

The role of adhesion in contact mechanics.

Adhesive (e.g. van der Waals) forces were not generally taken into account in contact mechanics until 1971, when Johnson, Kendall and Roberts (JKR) generalized Hertz' solution for an elastic sphere using an energetic argument which we now recognize to be analogous to that used in linear elastic fracture mechanics. A significant result is that the load-displacement relation exhibits instabilities in which approaching bodies 'jump in' to contact, whereas separated bodies 'jump out' at a tensile 'pull-off force'. The JKR approach has since been widely used in other geometries, but at small length scales or for stiffer materials it is found to be less accurate. In conformal contact problems, other instabilities can occur, characterized by the development of regular patterns of regions of large and small traction. All these instabilities result in differences between loading and unloading curves and consequent hysteretic energy losses. Adhesive contact mechanics has become increasingly important in recent years with the focus on soft materials (which generally permit larger areas of the interacting surfaces to come within the range of adhesive forces), nano-devices and the analysis of bio-systems. Applications are found in nature, such as insect attachment forces, in nano-manufacturing, and more generally in industrial systems involving rubber or polymer contacts. In this paper, we review the strengths and limitations of various methods for analysing contact problems involving adhesive tractions, with particular reference to the effect of the inevitable roughness of the contacting surfaces.

Heat-treated carbon coatings on poly (l-lactide) foils for tissue engineering.

Carbon-based materials have emerged as promising candidates for a wide variety of biomedical applications, including tissue engineering. We have developed a simple but unique technique for patterning carbon-based substrates in order to control cell adhesion, growth and phenotypic maturation. Carbon films were deposited on PLLA foils from distances of 3 to 7 cm. Subsequent heat-treatment (60 °C, 1 h) created lamellar structures with dimensions decreasing from micro- to nanoscale with increasing deposition distance. All carbon films improved the spreading and proliferation of human osteoblast-like MG 63 cells, and promoted the alignment of these cells along the lamellar structures. Similar alignment was observed in human osteoblast-like Saos-2 cells and in human dermal fibroblasts. Type I collagen fibers produced by Saos-2 cells and fibroblasts were also oriented along the lamellar structures. These structures increased the activity of alkaline phosphatase in Saos-2 cells. Carbon coatings also supported adhesion and growth of vascular endothelial and smooth muscle cells, particularly flatter non-heated carbon films. On these films, the continuity of the endothelial cell layer was better than on heat-treated lamellar surfaces. Heat-treated carbon-coated PLLA is therefore more suitable for bone and skin tissue engineering, while carbon-coated PLLA without heating is more appropriate for vascular tissue engineering.

Perfection in Nucleation and Growth of Blue-Phase Single Crystals: Small Free-Energy Required to Self-Assemble at Specific Lattice Orientation.

Chemically patterned surfaces can be used to selectively stabilize blue phases as macroscopic single crystals with a prescribed lattice orientation. By tailoring the interfacial free energy through the pattern characteristics, it is possible to set, with nanoscale precision, the optimal conditions to induce spontaneously blue-phase crystal nucleation on the patterned substrate where a uniform, defect-free, blue-phase single crystal is finally formed in a matter of seconds. The chemical patterns taken into consideration in this work are made up of alternated stripelike regions of homeotropic and planar anchoring. By varying the stripe pattern dimension, including the period and ratio of the planar/homeotropic anchoring width, it is possible to generate blue-phase I single crystals with (110) lattice orientation and blue-phase II single crystals with either the (100), (110), or (111) lattice orientation. Continuum mean-field calculations of the studied systems serve to explain, in terms of the free energy of the systems, how the pattern dimensions favor certain crystallographic orientations while penalizing the others. We found that a small free-energy difference is sufficient to drive the nucleation and growth of blue phases into a certain lattice orientation. Therefore, a processing window for obtaining arbitrary large blue-phase single crystals with predesigned lattice orientation, highly aligned reflective peaks, and significantly short forming time is provided here, which is essential for manufacturing and modulating optical devices and photonics.

Capillary Transport of Miniature Soft Ribbons.

Manipulation of soft miniature devices is important in the construction of soft robots, wearable devices, and biomedical devices. However, transport of soft miniature devices is still a challenging task, and few studies has been conducted on the subject. This paper reports a droplet-based micromanipulation method for transporting miniature soft ribbons. We show that soft ribbons can be successfully picked up and released to the target location using water droplets. We analyze the forces involved during the process numerically and investigate the influence of the width of the ribbon on the deformation. We verify that the deformation of a soft ribbon caused by elasto-capillary phenomena can be calculated using a well-known equation for calculating the deflection of a cantilever beam. The experimental and theoretical results show that the deformability of a soft miniature device during manipulation depends on its width.

Effects of Stiff Film Pattern Geometry on Surface Buckling Instabilities of Elastic Bilayers.

Buckling instabilities-such as wrinkling and creasing-of micropatterned elastic surfaces play important roles in applications, including flexible electronics and microfluidics. In many cases, the spatial dimensions associated with the imposed pattern can compete with the natural length scale of the surface instabilities (e.g., the wrinkle wavelength), leading to a rich array of surface buckling behaviors. In this paper, we consider elastic bilayers consisting of a spatially patterned stiff film supported on a continuous and planar soft substrate. Through a combination of experimental and computational analyses, we find that three surface instability modes-wrinkling, Euler buckling, and rigid rotation-are observed for the stiff material patterns, depending on the in-plane dimensions of the film compared to the natural wrinkle wavelength, while the intervening soft regions undergo a creasing instability. The interplay between these instabilities leads to a variety of surface structures as a function of the pattern geometry and applied compressive strain, in many cases yielding contact between neighboring stiff material elements because of the formation of creases in the gaps between them.

Design of Nanofiber Coatings for Mitigation of Microbial Adhesion: Modeling and Application to Medical Catheters.

Surface-associated microbial communities, known as biofilms, pose significant challenges in clinical and industrial settings. Micro-/nanoscale substratum surface features have been shown to disrupt firm adhesion of planktonic microbes to surfaces, thereby interfering with the earliest stage of biofilm formation. However, the role of geometry and size of surface features in microbial retention is not completely understood. In this study, we developed a biophysical model that describes the changes in the total free energy (adhesion energy and stretching energy) of an adherent Candida albicans cell on nanofiber-coated surfaces as a function of the geometry (i.e., diameter) and configuration (i.e., interfiber spacing) of the surface features (i.e., nanofibers). We then introduced a new nondimensional parameter, Π, to represent the ratio of cell rigidity to cell-substratum interfacial energy. We show that the total free energy is a strong function of topographical feature size at higher Π and lower spacing values. To confirm our biophysical model predictions, we performed 24 h dynamic retention assays and quantified cell attachment number density on surfaces coated with highly ordered polystyrene nanofibers. We show that the total free energy of a single adherent cell on a patterned surface is a key determinant of microbial retention on that surface. The cell attachment density trend closely correlates with the predictions based on the adherent single-cell total energy. The nanofiber coating design (1.2 µm diameter, 2 µm spacing) that maximized the total energy of the adherent cell resulted in the lowest microbial retention. We further demonstrate the utility of our biophysical model by showing close correlation between the computed single-cell total free energy and biofilm nucleation on fiber-coated urinary and central venous catheters of different materials. This biophysical model could offer a powerful new paradigm in ab initio design of patterned surfaces for controlled biofilm growth for medical applications and beyond.

A Maugis-Dugdale cohesive solution for adhesion of a surface with a dimple.

We study the adhesion of a surface with a 'dimple' which shows a mechanism for a bi-stable adhesive system in surfaces with spaced patterns of depressions, leading to adhesion enhancement, high dissipation and hysteresis. Recent studies were limited mainly to the very short range of adhesion (the so-called JKR regime), while we generalize the study to a Maugis cohesive model. A 'generalized Tabor parameter', given by the ratio of theoretical strength to elastic modulus, multiplied by the ratio of dimple width to depth has been found. It is shown that bistability disappears for generalized Tabor parameter less than about 2. Introduction of the theoretical strength is needed to have significant results when the system has gone in full contact, unless one postulates alternative limits to full contact, such as air entrapment, contaminants or fine scale roughness. Simple equations are obtained for the pull-off and for the full contact pressure in the entire set of the two governing dimensionless parameters. A qualitative comparison with results of recent experiments with nanopatterned bioinspired dry adhesives is attempted in light of the present model.