Surface engineering using PDMS and functionalized nanoparticles for superhydrophobic coatings: Selective liquid repellence and tackling COVID-19

Recently, synthesis and design of bioinspired nanostructured coated surfaces with exceptional selective liquid repellency (superhydrophobicity) have been a fascinating area of research because of their excellent utility in various applications from our daily life to industry level. In this context, superhydrophobic coatings by the use of polydimethylsiloxane (PDMS) along with functionalized nanoparticles have been reported widely for oil/water separation, antimicrobial ability and antiviral surface coatings to prevent the transmission of contagious coronavirus disease 2019 (COVID-19). PDMS is mechanically stable and highly flexible silicone polymer and can be irreversibly bound to various types of surfaces in order to provide superhydrophobicity. This review highlights the latest innovations in the research area of PDMS based nano-engineered superhydrophobic coatings on various surfaces. Particular attention has been paid toward the application of such superhydrophobic surfaces for the separation of oily contaminants from water as well as antimicrobial and antiviral efficacy in order to reduce the transmission of toxic pathogens including contagious COVID-19. Technical breakthrough and mechanistic concepts behind the success of PDMS based superhydrophobic coatings have been reviewed and discussed in these selected applications. It is expected that this study will be highly useful to lead future research in order to tackle the transmission of viral outbreaks in the coming future similar to the currently ongoing pandemic of COVID-19.

Application of Micro/Nanoporous Fluoropolymers with Reduced Bioadhesion in Digital Microfluidics.

Digital microfluidics (DMF) is a versatile platform for conducting a variety of biological and chemical assays. The most commonly used set-up for the actuation of microliter droplets is electrowetting on dielectric (EWOD), where the liquid is moved by an electrostatic force on a dielectric layer. Superhydrophobic materials are promising materials for dielectric layers, especially since the minimum contact between droplet and surface is key for low adhesion of biomolecules, as it causes droplet pinning and cross contamination. However, superhydrophobic surfaces show limitations, such as full wetting transition between Cassie and Wenzel under applied voltage, expensive and complex fabrication and difficult integration into already existing devices. Here we present Fluoropor, a superhydrophobic fluorinated polymer foam with pores on the micro/nanoscale as a dielectric layer in DMF. Fluoropor shows stable wetting properties with no significant changes in the wetting behavior, or full wetting transition, until potentials of 400 V. Furthermore, Fluoropor shows low attachment of biomolecules to the surface upon droplet movement. Due to its simple fabrication process, its resistance to adhesion of biomolecules and the fact it is capable of being integrated and exchanged as thin films into commercial DMF devices, Fluoropor is a promising material for wide application in DMF.

Mask Waste: A Sustainable Mask-Based Epoxy Resin/SiO2 Composite for Efficient Purification of Water-in-Oil Emulsions

Since the emergence of the COVID-19 pandemic, there has been a tremendous increase in the production of masks worldwide, with more than 1.5 billion masks having been disposed of during this time. The damage caused by mask pollution is a global threat;highlighting the need to dispose of discarded masks correctly. Herein, we report a recycling approach that uses discarded masks to fabricate a superhydrophobic epoxy resin/SiO2 membrane for separating emulsions. The composite has a high flux value (2123 L. m(-2).h(-1)) and high separation efficiency (>98%). The filter maintained its excellent superhydrophobic property (WCA > 150 degrees) after tape-peel cycles, clamping cycles with tweezers, abrasion cycles with 800 grit SiC sandpaper, pressure with fingertips, and kneading cycles. This study proposes a renewable, eco-friendly, and low-cost product, which can be used for oil spill cleanup and water purification. The filter not only removes oil from oily wastewater (such as oil spills) but also solves pollution caused by discarded masks. This study provides insights for resource recovery that may contribute to the purification of oily water emulsions.

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.

Enhancement of Bacterial Anti-Adhesion Properties on Robust PDMS Micro-Structure Using a Simple Flame Treatment Method.

Biofilm-associated infections caused by an accumulation of micro-organisms and pathogens significantly impact the environment, health risks, and the global economy. Currently, a non-biocide-releasing superhydrophobic surface is a potential solution for antibacterial purposes. This research demonstrated a well-designed robust polydimethylsiloxane (PDMS) micro-structure and a flame treatment process with improved hydrophobicity and bacterial anti-adhesion properties. After the flame treatment at 700 ± 20 °C for 15 s, unique flower-petal re-entrant nano-structures were formed on pillars (PIL-F, width: 1.87 ± 0.30 µm, height: 7.76 ± 0.13 µm, aspect ratio (A.R.): 4.14) and circular rings with eight stripe supporters (C-RESS-F, width: 0.50 ± 0.04 µm, height: 3.55 ± 0.11 µm, A.R.: 7.10) PDMS micro-patterns. The water contact angle (WCA) and ethylene glycol contact angle (EGCA) of flame-treated flat-PDMS (FLT-F), PIL-F, and C-RESS-F patterns were (133.9 ± 3.8°, 128.6 ± 5.3°), (156.1 ± 1.5°, 151.5 ± 2.1°), and (146.3 ± 3.5°, 150.7 ± 1.8°), respectively. The Escherichia coli adhesion on the C-RESS-F micro-pattern with hydrophobicity and superoleophobicity was 42.6%, 31.8%, and 2.9% less than FLT-F, PIL-F, and Teflon surfaces. Therefore, the flame-treated C-RESS-F pattern is one of the promising bacterial anti-adhesion micro-structures in practical utilization for various applications.

Recent Advances in Metal-Based Antimicrobial Coatings for High-Touch Surfaces.

International interest in metal-based antimicrobial coatings to control the spread of bacteria, fungi, and viruses via high contact human touch surfaces are growing at an exponential rate. This interest recently reached an all-time high with the outbreak of the deadly COVID-19 disease, which has already claimed the lives of more than 5 million people worldwide. This global pandemic has highlighted the major role that antimicrobial coatings can play in controlling the spread of deadly viruses such as SARS-CoV-2 and scientists and engineers are now working harder than ever to develop the next generation of antimicrobial materials. This article begins with a review of three discrete microorganism-killing phenomena of contact-killing surfaces, nanoprotrusions, and superhydrophobic surfaces. The antimicrobial properties of metals such as copper (Cu), silver (Ag), and zinc (Zn) are reviewed along with the effects of combining them with titanium dioxide (TiO2) to create a binary or ternary contact-killing surface coatings. The self-cleaning and bacterial resistance of purely structural superhydrophobic surfaces and the potential of physical surface nanoprotrusions to damage microbial cells are then considered. The article then gives a detailed discussion on recent advances in attempting to combine these individual phenomena to create super-antimicrobial metal-based coatings with binary or ternary killing potential against a broad range of microorganisms, including SARS-CoV-2, for high-touch surface applications such as hand rails, door plates, and water fittings on public transport and in healthcare, care home and leisure settings as well as personal protective equipment commonly used in hospitals and in the current COVID-19 pandemic.

Masks for COVID-19.

Sustainable solutions on fabricating and using a face mask to block the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spread during this coronavirus pandemic of 2019 (COVID-19) are required as society is directed by the World Health Organization (WHO) toward wearing it, resulting in an increasingly huge demand with over 4 000 000 000 masks used per day globally. Herein, various new mask technologies and advanced materials are reviewed to deal with critical shortages, cross-infection, and secondary transmission risk of masks. A number of countries have used cloth masks and 3D-printed masks as substitutes, whose filtration efficiencies can be improved by using nanofibers or mixing other polymers into them. Since 2020, researchers continue to improve the performance of masks by adding various functionalities, for example using metal nanoparticles and herbal extracts to inactivate pathogens, using graphene to make masks photothermal and superhydrophobic, and using triboelectric nanogenerator (TENG) to prolong mask lifetime. The recent advances in material technology have led to the development of antimicrobial coatings, which are introduced in this review. When incorporated into masks, these advanced materials and technologies can aid in the prevention of secondary transmission of the virus.

Can Machine Learning and Nanotechnology Help the Fight Against the Current COVID-19 Crisis?

The entire world is now in a state of caution since the outbreak of the COVID-19 pandemic. The overwhelmingly high spread and mortality rate due to the SARS-CoV-2 virus has not only made the headlines but also raised alarming concerns for the human community. Applications of nano-biotechnology, along with machine learning, have excellent potential in dealing with serious health issues, mainly in medical science. This review article aims to augment the multidimensional use of silver nanoparticles, especially in the fabrication of textiles and face masks, which could represent a new avenue for prevention. Furthermore, the disinfection of COVID-19, along with other pathogens using silver nanoparticles and machine learning could help in the risk assessment.

Coal-Derived Functionalized Nano-Graphene Oxide for Bleach Washable, Durable Antiviral Fabric Coatings

This work demonstrates a coal-derived functionalized nano-graphene oxide coating applied to fabrics that exhibits antiviral properties even after mechanical abrasion or bleach washing. Nano-graphene oxide is chemically exfoliated from low cost coal and functionalized with octadecylamine to render repellency properties. The functionalized nano-graphene oxide is applied to polyethylene terephthalate (PET) fabric after wet etching which roughens the microfiber surface for better coating adhesion and liquid repellency. An additional polydimethylsiloxane (PDMS) layer on top of the functionalized nano-graphene oxide further improves the repellency and durability. The functionalized nano-graphene oxide/PDMS coating robustly repels droplets of water and human saliva. Additionally, we demonstrate antiviral properties with human adenovirus type 5 (HAdVS), herpes simplex virus type 1 (HSV-1), and betacoronavirus (CoV) even after mechanical abrasion and bleach washing. The coating reduces titers of HAdV5 by 1.8 log (98.6%), HSV-1 by 2.2 log (99.4%), and CoV by 2.4 log (99.6%). The coating may have applications in reusable, antiviral personal protective equipment or other large-area, high production coating applications.

Nanometer-Thick Superhydrophobic Coating Renders Cloth Mask Potentially Effective against Aerosol-Driven Infections.

The advent of COVID-19 pandemic has made it necessary to wear masks across populations. While the N95 mask offers great performance against airborne infections, its multilayered sealed design makes it difficult to breathe for a longer duration of use. The option of using highly breathable cloth or silk masks especially for a large populace is fraught with the danger of infection. As a normal cloth or silk mask absorbs airborne liquid, it can be a source of plausible infection. We demonstrate the chemical modification of one such mask, Eri silk, to make it hydrophobic (contact angle of water is 143.7°), which reduces the liquid absorption capacity without reducing the breathability of the mask significantly. The breathability reduces only 22% for hydrophobic Eri silk compared to the pristine Eri silk, whereas N95 shows a 59% reduction of breathability. The modified hydrophobic silk can repel the incoming aqueous liquid droplets without wetting the surface. The results indicate that a multilayered modified silk mask to make it hydrophobic can be an affordable and breathable alternative to the N95 mask.

Superhydrophobic, photo-sterilize, and reusable mask based on graphene nanosheet-embedded carbon (GNEC) film.

The 2019 coronavirus disease (COVID-19) has affected more than 200 countries. Wearing masks can effectively cut off the virus spreading route since the coronavirus is mainly spreading by respiratory droplets. However, the common surgical masks cannot be reused, resulting in the increasing economic and resource consumption around the world. Herein, we report a superhydrophobic, photo-sterilize, and reusable mask based on graphene nanosheet-embedded carbon (GNEC) film, with high-density edges of standing structured graphene nanosheets. The GNEC mask exhibits an excellent hydrophobic ability (water contact angle: 157.9°) and an outstanding filtration efficiency with 100% bacterial filtration efficiency (BFE). In addition, the GNEC mask shows the prominent photo-sterilize performance, heating up to 110 °C quickly under the solar illumination. These high performances may facilitate the combat against the COVID-19 outbreaks, while the reusable masks help reducing the economic and resource consumption. Electronic Supplementary Material: Supplementary material (further details of electron cyclotron resonance (ECR) sputtering system, deposition of GNEC film, fabrication of GNEC mask, and characterization of the GNEC mask) is available in the online version of this article at 10.1007/s12274-020-3158-1.

Plasmonic and Superhydrophobic Self-Decontaminating N95 Respirators.

The COVID-19 pandemic is endangering the world due to the spread of respiration droplets with viruses. Medical workers and frontline staff need to wear respirators to protect themselves from breathing in the virus-containing respiration droplets. The most frequently used state-of-the-art respirators are of N95 standard; however, they lack self-decontamination capabilities. In addition, the viruses and bacteria can accumulate on the respirator surfaces, possessing high risks to the wearers over long-term usage. Photothermal decontamination is a contactless, fast, low-cost, and widely available method, capable of decontaminating the respirators. Herein, we report a plasmonic photothermal and superhydrophobic coating on N95 respirators, possessing significantly better protection than existing personal protection equipment. The plasmonic heating can raise the surface temperature to over 80 °C for this type of respirator within 1 min of sunlight illumination. The superhydrophobic features prohibit respiration droplets from accumulating on the respirator surfaces. The presence of the silver nanoparticles can provide additional protection via the silver ion's disinfection toward microbes. These synergistic features of the composite coatings provide the N95 respirator with better protection and can inspire experts from interdisciplinary fields to develop better personal protection equipment to fight the COVID-19 pandemic.

Reusable and Recyclable Graphene Masks with Outstanding Superhydrophobic and Photothermal Performances.

The 2019 coronavirus outbreak (COVID-19) is affecting over 210 countries and territories, and it is spreading mainly by respiratory droplets. The use of disposable surgical masks is common for patients, doctors, and even the general public in highly risky areas. However, the current surgical masks cannot self-sterilize in order to reuse or be recycled for other applications. The resulting high economic and environmental costs are further damaging societies worldwide. Herein, we reported a unique method for functionalizing commercially available surgical masks with outstanding self-cleaning and photothermal properties. A dual-mode laser-induced forward transfer method was developed for depositing few-layer graphene onto low-melting temperature nonwoven masks. Superhydrophobic states were observed on the treated masks' surfaces, which can cause the incoming aqueous droplets to bounce off. Under sunlight illumination, the surface temperature of the functional mask can quickly increase to over 80 °C, making the masks reusable after sunlight sterilization. In addition, this graphene-coated mask can be recycled directly for use in solar-driven desalination with outstanding salt-rejection performance for long-term use. These roll-to-roll production-line-compatible masks can provide us with better protection against this severe virus. The environment can also benefit from the direct recycling of these masks, which can be used for desalinating seawater.