Bioinspired Durable Superhydrophobic Surface from a Hierarchically Wrinkled Nanoporous Polymer

Bioinspired Superhydrophobic Fibrous Materials

Natural fibers with robust water repellency play an important role in adapting organisms to various environments, which has inspired the development of artificial superhydrophobic fibrous materials with applications in self-cleaning, antifogging, water harvesting, heat exchanging, catalytic reactions, and microrobots. However, these highly textured surfaces (micro/nanotextured) suffer from frequent liquid penetration in high humidity and abrasion-induced destruction of the local environment. Herein, bioinspired superhydrophobic fibrous materials are reviewed from the perspective of the dimension scale of fibers. First, the fibrous dimension characteristics of several representative natural superhydrophobic fibrous systems are summarized, along with the mechanisms involved. Then, artificial superhydrophobic fibers are summarized, along with their various applications. Nanometer-scale fibers enable superhydrophobicity by minimizing the liquid-solid contact area. Micrometer-scale fibers are advantageous for enhancing the mechanical stability of superhydrophobicity. Micrometer-scale conical fibrous structures endow a Laplace force with a particular magnitude for self-removing condensed tiny dewdrops in highly humid air and stably trapping large air pockets underwater. Furthermore, several representative surface modification strategies for constructing superhydrophobic fibers are presented. In addition, several conventional applications of superhydrophobic systems are presented. It is anticipated that the review will inspire the design and fabrication of superhydrophobic fibrous systems.

Fabrication, Characterization, and In Vitro Cytotoxicity Assessment of Tri-Layered Multifunctional Scaffold for Effective Chronic Wound Healing

Chronic wounds have been a global health risk that demands intensive exploration. A tri-layered biomaterial scaffold has been developed for skin wounds. The top layer of the scaffold is superhydrophobic, and the bottom layer is hydrophilic, both of which were electrospun using recycled expanded polystyrene (EPS) and monofilament fishing line (MFL), respectively. The intermediate layer of the scaffold comprised hydrogel by cross-linking chitosan (CS) with polyethylene glycol. The surface morphology, surface chemistry, thermal degradation, and wettability characteristics of each layer of the scaffold were examined. Also, the antibacterial activity and in vitro cytotoxicity study on the combined tri-layered scaffold were assessed against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Data revealed exceptional water repellency of the heat-treated electrospun top superhydrophobic layer (TSL) with a high-water contact angle (WCA) of 172.44°. A TSL with 15 wt% of micro-/nano-inclusions had the best thermal stability above 400 °C. The bottom hydrophilic layer (BHL) displayed a WCA of 9.91°. Therapeutically, the synergistic effect of the combined tri-layered scaffold significantly inhibited bacteria growth by 70.5% for E. coli and 68.6% for S. aureus. Furthermore, cell viability is enhanced when PEG is included as part of the intermediate CS hydrogel layer (ICHL) composition.

Simple Method for Creating Hydrophobic Ultraflexible Photovoltaics

Optoelectronic devices, such as photodetectors and photovoltaics, are susceptible to surface contamination or water damage that can lead to reductions in performance or stability. Applying superhydrophobic coatings to these devices can introduce self-cleaning behavior and water resistance to extend their lifetime and improve their efficiency. However, existing methods for inducing superhydrophobicity have not been compatible with ultraflexible devices because of their thickness and complexity requirements. In this work, we introduce a procedure for inducing superhydrophobic and self-cleaning behavior on ultraflexible components using a combination of shrinkage-induced wrinkles and a low-surface-energy coating. We apply these techniques to an ultraflexible organic photovoltaics and demonstrate excellent hydrophobicity and self-cleaning behavior.

Superhydrophobic SLA 3D printed materials modified with nanoparticles biomimicking the hierarchical structure of a rice leaf

The rice leaf, combining the surface properties of lotus leaves and shark skin, presents outstanding superhydrophobic properties motivating its biomimesis. We created a novel biomimetic rice-leaf superhydrophobic surface by a three-level hierarchical structure, using for a first time stereolithographic (SLA) 3D printed channels (100µm width) with an intrinsic roughness from the printing filaments (10µm), and coated with TiO₂ nanoparticles (22 and 100nm). This structure presents a maximum advancing contact angle of 165° characterized by lower both anisotropy and hysteresis contact angles than other 3D printed surfaces, due to the presence of air pockets at the surface/water interface (Cassie-Baxter state). Dynamic water-drop tests show that the biomimetic surface presents self-cleaning, which is reduced under UV-A irradiation. The biomimetic surface further renders an increased floatability to 3D printed objects meaning a drag-reduction due to reduced water/solid contact area. Numerical simulations of a channel with a biomimetic wall confirm that the presence of air is essential to understand our results since it increases the average velocity and decreases the friction factor due to the presence of a wall-slip velocity. Our findings show that SLA 3D printing is an appropriate approach to develop biomimetic superhydrophobic surfaces for future applications in anti-fouling and drag-reduction devices.

Pathogen-Repellent Plastic Warp with Built-In Hierarchical Structuring Prevents the Contamination of Surfaces with Coronaviruses

Amidst the COVID-19 pandemic, it is evident that viral spread is mediated through several different transmission pathways. Reduction of these transmission pathways is urgently needed to control the spread of viruses between infected and susceptible individuals. Herein, we report the use of pathogen-repellent plastic wraps (RepelWrap) with engineered surface structures at multiple length scales (nanoscale to microscale) as a means of reducing the indirect contact transmission of viruses through fomites. To quantify viral repellency, we developed a touch-based viral quantification assay to mimic the interaction of a contaminated human touch with a surface through the modification of traditional viral quantification methods (viral plaque and TCID50 assays). These studies demonstrate that RepelWrap reduced contamination with an enveloped DNA virus as well as the human coronavirus 229E (HuCoV-229E) by more than 4 log 10 (>99.99%) compared to a standard commercially available polyethylene plastic wrap. In addition, RepelWrap maintained its repellent properties after repeated 300 touches and did not show an accumulation in viral titer after multiple contacts with contaminated surfaces, while increases were seen on other commonly used surfaces. These findings show the potential use of repellent surfaces in reducing viral contamination on surfaces, which could, in turn, reduce the surface-based spread and transmission.

Ti3C2Tx MXene-Based Flexible Piezoresistive Physical Sensors

MXenes have received increasing attention due to their two-dimensional layered structure, high conductivity, hydrophilicity, and large specific surface area. Because of these distinctive advantages, MXenes are considered as very competitive pressure-sensitive materials in applications of flexible piezoresistive sensors. This work reviews the preparation methods, basic properties, and assembly methods of MXenes and their recent developments in piezoresistive sensor applications. The recent developments of MXene-based flexible piezoresistive sensors can be categorized into one-dimensional fibrous, two-dimensional planar, and three-dimensional sensors according to their various structures. The trends of multifunctional integration of MXene-based pressure sensors are also summarized. Finally, we end this review by describing the opportunities and challenges for MXene-based pressure sensors and the great prospects of MXenes in the field of pressure sensor applications.

Single-layer graphene prevents Cassie-wetting failure of structured hydrophobic surface for efficient condensation

HYPOTHESIS

Structured hydrophobic surfaces often suffer from Cassie-wetting failure due to trapped water in structure gaps for a long-term operation. Sustainable Cassie-wetting on such surface could be achieved by coating an atom-thick and moisture-impermeable graphene on it.

EXPERIMENTS

Water contact angles were measured to clarify the effect of graphene on wetting, and water impermeability was verified by moisture deposition and evaporation. Sliding angle measurements and vapor condensation were carried out to demonstrate the stable Cassie-state wetting and application.

FINDINGS

Interestingly we found the graphene does not significantly disrupt the wetting behavior of the structured hydrophobic surface, showing a wettability transparency. Moreover, the impermeability of graphene keeps moisture away from the structure gaps. Owning to the combination of these two properties, droplets on the graphene-coated structured surface exhibit a stable Cassie-state hydrophobic wetting, even under the situation of moisture deposition and evaporation. Using the modified surface, we also found a 40-100% increase in condensation efficiency for a 5-hour vapor condensation at a subcooling of 40 °C. These results suggest an effective strategy to prevent Cassie-wetting failure of structured hydrophobic surface and are expected to promote its further application in more complex conditions.

Flexible selection of the functional-group ratio on a polytetrafluoroethylene (PTFE) surface using a single-gas plasma treatment

During plasma treatment of polymers, etching occurs and functional groups are introduced on their surface.

Robust and Eco-Friendly Superhydrophobic Starch Nanohybrid Materials with Engineered Lotus Leaf Mimetic Multiscale Hierarchical Structures

The use of superhydrophobic surfaces in a broad range of applications is receiving a great deal of attention due to their numerous functionalities. However, fabricating these surfaces using low-cost raw materials through green and fluorine-free routes has been a bottleneck in their industrial deployment. This work presents a facile and environmentally friendly strategy to prepare mechanically robust superhydrophobic surfaces with engineered lotus leaf mimetic multiscale hierarchical structures via a hybrid route combining soft imprinting and spin-coating. Direct soft-imprinting lithography onto starch/polyhydroxyurethane/cellulose nanocrystal (SPC) films formed micro-scaled features resembling the pillar architecture of lotus leaf. Spin-coating was then used to assemble a thin layer of low-surface-energy poly(dimethylsiloxane) (PDMS) over these microstructures. Silica nanoparticles (SNPs) were grafted with vinyltriethoxysilane (VTES) to form functional silica nanoparticles (V-SNPs) and subsequently used for the fabrication of superhydrophobic coatings. A further modification of PDMS@SPC film with V-SNPs enabled the interlocking of V-SNPs microparticles within the cross-linked PDMS network. The simultaneous introduction of hierarchical microscale surface topography, the low surface tension of the PDMS layer, and the nanoscale roughness induced by V-SNPs contributed to the fabrication of a superhydrophobic interface with a water contact angle (WCA) of ∼150° and a sliding angle (SA) of <10°. The PDMS/V-SNP@SPC films showed an ∼52% reduction in water vapor transmission rate compared to that of uncoated films. These results indicated that the coating served as an excellent moisture barrier and imparted good hydrophobicity to the film substrate. The coated film surfaces were able to withstand extensive knife scratches, finger-rubbing, jet-water impact, a sandpaper-abrasion test for 20 cycles, and a tape-peeling test for ∼10 repetitions without losing superhydrophobicity, suggesting superior mechanical durability. Self-cleaning behavior was also demonstrated when the surfaces were cleared of artificial dust and various food liquids. The green and innovative approach presented in the current study can potentially serve as an attractive new tool for the development of robust superhydrophobic surfaces without adverse environmental consequences.

Recent Progress of Bioinspired Scalephobic Surfaces with Specific Barrier Layers

Bioinspired superwettable surfaces have been widely harnessed in diverse applications such as self-cleaning, oil/water separation, and liquid transport. So far, only a little work is focused on scalephobic capability of those superwettable surfaces. However, the troublesome scale deposition will inevitably be observed in our daily production and life, greatly reducing heat transfer efficiency and inhibiting the liquid transport. To address this annoying problem, as the emerging strategy, specific barrier layers are introduced onto superwettable surfaces to reduce or even avoid the direct contact between scale and the surfaces. In this feature article, we first provide the basic concept of bioinspired scalephobic surfaces with specific barrier layers. Then, we briefly introduce the typical fabrication methods of scalephobic surfaces. Later, we summarize recent progress of bioinspired scalephobic surfaces with specific barrier layers. Furthermore, we point out the guiding theory and criteria for the stability of barrier layers. Finally, we put forward the forecast on the existing problems and future direction in bioinspired scalephobic surfaces.

Robust Hierarchical Porous PTFE Film Fabricated via Femtosecond Laser for Self-Cleaning Passive Cooling

Passive cooling materials that spontaneously cool an object are promising choices for mitigating the global energy crisis. However, these cooling effects are usually weakened or lost when dust contaminates the surface structure, greatly restricting their applications. In this work, a robust hierarchical porous polytetrafluoroethylene (PTFE) film with coral-like micro/nanostructures is generated by a facile and efficient femtosecond laser ablation technique. Owing to its unique micro/nanostructures, the as-prepared surface exhibits an outstanding self-cleaning function for various liquids with ultralow adhesion. This self-cleaning characteristic enhances the durability of its passive cooling effect. It is demonstrated that the titanium (Ti) sheet covered with laser-ablated PTFE film can realize a maximum temperature decrease of 4 and 10 °C compared to the Ti sheet covered with pristine PTFE film and uncovered, respectively. This study reveals that femtosecond laser micromachining is a facile and feasible avenue to produce robust self-cleaning passive cooling devices.

Earthworm-Inspired Ultradurable Superhydrophobic Fabrics from Adaptive Wrinkled Skin

Wrapped by periodically wrinkled skin, soft earthworm shows excellent robustness against sticky soil. Mimicking this deformation adaptability, here, we report an ultradurable superhydrophobic fabric by exploiting the formation of adaptive, soft wrinkled poly(dimethylsiloxane) (PDMS) skins. Uniform wrinkles are created on woven fabric fibers due to the surface instability of PDMS coating with a cross-linking gradient induced by Ar plasma treatment. Both the surface topography of wrinkles and the viscoelasticity of the underlying compliant layer to release stress endow the treated superhydrophobic fabrics with extraordinary durability, withstanding 800 standard laundries or 1000 rubbing cycles under 44.8 kPa. Additionally, superhydrophobic fabrics are self-healable after heating or plasma treatment. This insight of engineering soft skins with periodic submicron surface topography and gradient modulus provides a pathway for the design of ultradurable, multifunctional wearables.

Robust superhydrophobicity: mechanisms and strategies

Superhydrophobic surfaces hold great prospects for extremely diverse applications owing to their water repellence property. The essential feature of superhydrophobicity is micro-/nano-scopic roughness to reserve a large portion of air under a liquid drop. However, the vulnerability of the delicate surface textures significantly impedes the practical applications of superhydrophobic surfaces. Robust superhydrophobicity is a must to meet the rigorous industrial requirements and standards for commercial products. In recent years, major advancements have been made in elucidating the mechanisms of wetting transitions, design strategies and fabrication techniques of superhydrophobicity. This review will first introduce the mechanisms of wetting transitions, including the thermodynamic stability of the Cassie state and its breakdown conditions. Then we highlight the development, current status and future prospects of robust superhydrophobicity, including characterization, design strategies and fabrication techniques. In particular, design strategies, which are classified into passive resistance and active regeneration for the first time, are proposed and discussed extensively.

The effects of bio-inspired micro/nano scale structures on anti-icing properties

Ice formation and accumulation have detrimental effects on commercial surfaces and people's lives. The ice adhesion strength decreases with increasing surface hydrophobicity, and the superhydrophobicity of a surface can be constructed by a combination of low surface free energy and high surface roughness. Conversely, the characteristics of biological surfaces have aroused wide attention as a result of the superhydrophobicity of plants and animals, deriving from the synergistic effects of chemical compositions and multi-scale hierarchical structures. Therefore, inspired by bio-mimetic studies on biological surfaces, a lot of artificial bio-inspired superhydrophobic surfaces have been broadly designed and constructed. Herein, we aim to summarize the fundamental theories of surface wettability and recent progress in the fabrication of bio-inspired surfaces. The bio-inspired surfaces prepared by different facile methods not only have superhydrophobicity, but also have anti-icing/icephobic properties. In the end, some challenges and problems in the future study and advancement of bio-inspired superhydrophobic surfaces are proposed.

Asymmetric Sc-PLA Membrane with Multi-scale Microstructures: Wettability, Antifouling, and Oil-Water Separation

In this work, an eco-friendly superhydrophobic stereo-complex polylactic acid (Sc-PLA) membrane was fabricated by a facile non-solvent-induced phase separation (NIPS) method, followed by peeling off its skin layer. By adjusting the thickness and roughness, membranes with various multi-scale microstructures could be obtained due to the formation of stereo-complex crystals during the process of phase separation. The Sc-PLA membranes display a hydrophobic wetting property. Interestingly, when the skin layer of the membrane with a 600 μm thickness was peeled off, the water contact angle on the surface of the membrane significantly improved from 142 to 152°, and the membrane displayed superhydrophobic wetting properties, which may be owing to the improvement of roughness for the surface by enlarging the exposure opportunity of finger holes and microstructures. In addition, the Sc-PLA membrane with superhydrophobicity shows excellent antifouling performance and large oil absorption capacity. Predictably, the Sc-PLA membranes may have potential applications in antifouling and oil-water separation.

Symmetrical Wrinkles in Single-Component Elastomers with Fingerprint-Inspired Robust Isotropic Dry Adhesive Capabilities

Robust and inexpensive dry adhesives have a great potential in multitudinous industrial applications. However, to date, the fabrication of dry adhesives, prepared using high aspect ratio structures in general, requires specific equipment and time-consuming processes, which limit their practicable utilization. Inspired from human fingerprints, in this study, we created durable single-component elastomer surfaces with symmetric and multiple concentric-shaped wrinkled patterns that exhibit isotropic dry adhesion capabilities. The dynamic interfacial release-induced surface wrinkling property of a rigid degradable polymeric capping layer [i.e., poly(l-lactide) (PLLA)] was exploited on a soft elastomer substrate [i.e., polydimethylsiloxane (PDMS)] to spontaneously form wrinkled PLLA/PDMS bilayer composites. After conducting a two-step thermal curing process on the composite and hydrolysis of the PLLA capping layer, a single-component microwrinkled PDMS surface with a large area and symmetric patterns could be generated. The patterns show flexible, durable, and isotropic dry adhesion capabilities that could be controlled by tuning their geometrical parameters (wrinkle wavelengths and amplitudes) and elastic modulus. In particular, the formation of symmetrically wrinkled patterns without using expensive lithography for patterning and costly material precursors is an advantage and could be extended to other industrial applications, such as damage-free transportation, biomimetic climbing robots, and biocompatible medical patches.