Bioinspired shape-memory graphene film with tunable wettability

Nature-inspired adhesive systems

Many organisms in nature thrive in intricate habitats through their unique bio-adhesive surfaces, facilitating tasks such as capturing prey and reproduction. It's important to note that the remarkable adhesion properties found in these natural biological surfaces primarily arise from their distinct micro- and nanostructures and/or chemical compositions. To create artificial surfaces with superior adhesion capabilities, researchers delve deeper into the underlying mechanisms of these captivating adhesion phenomena to draw inspiration. This article provides a systematic overview of various biological surfaces with different adhesion mechanisms, focusing on surface micro- and nanostructures and/or chemistry, offering design principles for their artificial counterparts. Here, the basic interactions and adhesion models of natural biological surfaces are introduced first. This will be followed by an exploration of research advancements in natural and artificial adhesive surfaces including both dry adhesive surfaces and wet/underwater adhesive surfaces, along with relevant adhesion characterization techniques. Special attention is paid to stimulus-responsive smart artificial adhesive surfaces with tunable adhesive properties. The goal is to spotlight recent advancements, identify common themes, and explore fundamental distinctions to pinpoint the present challenges and prospects in this field.

A model for SARS-CoV-2 virus transmission on the upper deck of a passenger ship bound for a short trip

Public transportation plays a critical role in meeting transportation demands, particularly in densely populated areas. The COVID-19 pandemic has highlighted the importance of public health measures, including the need to prevent the spread of the virus through public transport. The spreading of the virus on a passenger ship is studied using the Computational Fluid Dynamic (CFD) model and Monte Carlo simulation. A particular focus was the context of Bangladesh, a populous maritime nation in South Asia, where a significant proportion of the population utilizes passenger ships to meet transportation demands. In this regard, a turbulence model is used, which simulates the airflow pattern and determines the contamination zone. Parameters under investigation are voyage duration, number of passengers on board, social distance, the effect of surgical masks, and others. This study shows that the transmission rate of SARS-CoV-2 infection on public transport, such as passenger ships, is not necessarily directly proportional to voyage duration or the number of passengers onboard. This model has the potential to be applied in various other modes of transportation, including public buses and airplanes. Implementing this model may help to monitor and address potential health risks effectively in the public transport networks.

Mechanically Strengthened Aerogels through Multiscale, Multicompositional, and Multidimensional Approaches: A Review

In recent decades, aerogels have attracted tremendous attention in academia and industry as a class of lightweight and porous multifunctional nanomaterial. Despite their wide application range, the low mechanical durability hinders their processing and handling, particularly in applications requiring complex physical structures. "Mechanically strengthened aerogels" have emerged as a potential solution to address this drawback. Since the first report on aerogels in 1931, various modified synthesis processes have been introduced in the last few decades to enhance the aerogel mechanical strength, further advancing their multifunctional scope. This review summarizes the state-of-the-art developments of mechanically strengthened aerogels through multicompositional and multidimensional approaches. Furthermore, new trends and future directions for as prevailed commercialization of aerogels as plastic materials are discussed.

Droplet Manipulation on Bioinspired Slippery Surfaces: From Design Principle to Biomedical Applications

Droplet manipulation with high efficiency, high flexibility, and programmability, is essential for various applications in biomedical sciences and engineering. Bioinspired liquid-infused slippery surfaces (LIS), with exceptional interfacial properties, have led to expanding research for droplet manipulation. In this review, an overview of actuation principles is presented to illustrate how materials or systems can be designed for droplet manipulation on LIS. Recent progress on new manipulation methods on LIS is also summarized and their prospective applications in anti-biofouling and pathogen control, biosensing, and the development of digital microfluidics are presented. Finally, an outlook is made on the key challenges and opportunities for droplet manipulation on LIS.

Two-Photon Polymerization of Butterfly Wing Scale Inspired Surfaces with Anisotropic Wettability

Wings of Morph aega butterflies are natural surfaces that exhibit anisotropic liquid wettability. The direction-dependent arrangement of the wing scales creates orientation-turnable microstructures with two distinct contact modes for liquid droplets. Enabled by recent developments in additive manufacturing, such natural surface designs coupled with hydrophobicity play a crucial role in applications such as self-cleaning, anti-icing, and fluidic manipulation. However, the interplay among resolution, architecture, and performance of bioinspired structures is barely achieved. Herein, inspired by the wing scales of the Morpho aega butterfly, full-scale synthetic surfaces with anisotropic wettability fabricated by two-photon polymerization are reported. The quality of the artificial butterfly scale is improved by optimizing the laser scanning strategy and the objective lens movement path. The corresponding contact angles of water on the fabricated architecture with various design parameters are measured, and the anisotropic fluidic wettability is investigated. Results demonstrate that tuning the geometrical parameters and spatial arrangement of the artificial wing scales enables anisotropic behaviors of the droplet's motion. The measured results also indicate a reverse phenomenon of the fabricated surfaces in contrast to their natural counterparts, possibly attributed to the significant difference in equilibrium wettability between the fabricated microstructures and the natural Morpho aega surface. These findings are utilized to design next-generation fluid-controllable interfaces for manipulating liquid mobility on synthetic surfaces.

Designing of anisotropic gradient surfaces for directional liquid transport: Fundamentals, construction, and applications

Many biological surfaces are capable of transporting liquids in a directional manner without energy consumption. Inspired by nature, constructing asymmetric gradient surfaces to achieve desired droplet transport, such as a liquid diode, brings an incredibly valuable and promising area of research with a wide range of applications. Enabled by advances in nanotechnology and manufacturing techniques, biomimetics has emerged as a promising avenue for engineering various types of anisotropic material system. Over the past few decades, this approach has yielded significant progress in both fundamental understanding and practical applications. Theoretical studies revealed that the heterogeneous composition and topography mainly govern the wetting mechanisms and dynamics behavior of droplets, including the interdisciplinary aspects of materials, chemistry, and physics. In this review, we provide a concise overview of various biological surfaces that exhibit anisotropic droplet transport. We discussed the theoretical foundations and mechanisms of droplet motion on designed surfaces and reviewed recent research advances in droplet directional transport on designed plane surfaces and Janus membranes. Such liquid-diode materials yield diverse promising applications, involving droplet collection, liquid separation and delivery, functional textiles, and biomedical applications. We also discuss the recent challenges and ongoing approaches to enhance the functionality and application performance of anisotropic materials.

Bioinspired Underwater Superoleophilic Two-Dimensional Surface with Asymmetric Oleophobic Barriers for Unidirectional and Long-Distance Oil Transport

Unidirectional and long-distance liquid transport is critically important to a range of practical applications, e.g., water harvesting, microfluidics, and chemical reactions. Great efforts have been made on liquid manipulation; most of which, however, are limited in the air environment. It is still a great challenge to achieve unidirectional and long-distance oil transport in an aqueous environment. Herein, we have successfully fabricated an underwater superoleophilic two-dimensional surface (USTS) with asymmetric oleophobic barriers to arbitrarily manipulate oil in aqueous medium. The behavior of oil on USTS was carefully investigated, of which the unidirectional spreading capability was originated from the anisotropic spreading resistance resulted from the asymmetric oleophobic barriers. Accordingly, an underwater oil/water separation device has been developed, which can achieve continuous and efficient oil/water separation and further prevent the secondary pollution caused by oil volatilization.

A Review of Smart Superwetting Surfaces Based on Shape-Memory Micro/Nanostructures

Bioinspired smart superwetting surfaces with special wettability have aroused great attention from fundamental research to technological applications including self-cleaning, oil-water separation, anti-icing/corrosion/fogging, drag reduction, cell engineering, liquid manipulation, and so on. However, most of the reported smart superwetting surfaces switch their wettability by reversibly changing surface chemistry rather than surface microstructure. Compared with surface chemistry, the regulation of surface microstructure is more difficult and can bring novel functions to the surfaces. As a kind of stimulus-responsive material, shape-memory polymer (SMP) has become an excellent candidate for preparing smart superwetting surfaces owing to its unique shape transformation property. This review systematically summarizes the recent progress of smart superwetting SMP surfaces including fabrication methods, smart superwetting phenomena, and related application fields. The smart superwettabilities, such as superhydrophobicity/superomniphobicity with tunable adhesion, reversible switching between superhydrophobicity and superhydrophilicity, switchable isotropic/anisotropic wetting, slippery surface with tunable wettability, and underwater superaerophobicity/superoleophobicity with tunable adhesion, can be obtained on SMP micro/nanostructures by regulating the surface morphology. Finally, the challenges and future prospects of smart superwetting SMP surfaces are discussed.

Prediction of COVID-19 Infection in Dental Clinic by CFD and Wells-Riley Model, Identifying Safe Area and Proper Ventilation Velocity: Prédiction de l'infection au COVID-19 dans une clinique dentaire par CFD et modèle Wells-Riley, identification de la zone de sécurité et de la vitesse de ventilation appropriée

The COVID-19 virus is recognized worldwide as a significant public health threat. A dental clinic is one of the most dangerous places in the COVID-19 epidemic, and disease transmission is rapid. Planning is essential to create the right conditions in the dental clinic. In this study, the cough of an infected person is examined in a 9 × 6 × 3 m³ area. Computational fluid dynamic (CFD) is applied to simulate the flow field and to determine the dispersion path. The innovation of this research is checking the risk of infection for each person in the designated dental clinic, choosing the suitable velocity for ventilation, and identifying safe areas. In the first step, the effects of different ventilation velocities on the dispersion of virus-infected droplets are investigated, and the most appropriate ventilation flow velocity has been identified. Then, the results of the presence or absence of a dental clinic separator shield on the spread of respiratory droplets have been identified. Finally, the risk of infection (by the Wells-Riley equation) is assessed, and safe areas are identified. The effect of RH on droplet evaporation in this dental clinic is assumed to be 50%. The N_Tn values in an area with a separator shield are less than 1%. When there is a separator shield, the infection risk of people in A3 and A7 (the other side of the separator shield) is reduced from 23% to 4%, and 21% to 2%, respectively.

Slippery Shape Memory Tube for Smart Droplet Transportation

Recently, research about controllable droplet transportation in tubes has aroused increased interest. However, existing strategies mainly depend on the elastic tube's shape variation that needs constant external stimuli. Meanwhile, these reported tubes are only suitable for wetting liquids. To achieve the transportation of diverse liquids, different coatings are needed to modify the tube's inner surface to realize complete wetting of different liquids. Herein, we advance a design principle by combining a shape memory polymer (SMP) tube and Nepenthes pitcher plant-inspired slippery surface, which can solve the above-mentioned problems. The SMP offers a tunable tube shape owing to its shape memory effect (SME); the slippery surface reduces the adhesion and expands the applicable range of liquids. Transportation of both water and oils in a wide range of surface tension values can be smartly controlled. The results show that not only the transportation speed and direction can be adjusted but also diverse modes including round-trip transportation, segmented transportation, and antigravity transportation can be achieved. Moreover, applications of the tube in batch inspection of different droplets and step-by-step control of multiple microreactions are also displayed. This work reports a strategy for droplet transportation control based on the tube's SME, which initiates some fresh ideas for designing new superwetting materials toward smart liquid transportation.

Microgels based on 0D-3D carbon materials: Synthetic techniques, properties, applications, and challenges

Microgels are three-dimensional (3D) colloidal hydrogel particles with outstanding features such as biocompatibility, good mechanical properties, tunable sizes from submicrometer to tens of nanometers, and large surface areas. Because of these unique qualities, microgels have been widely used in various applications. Carbon-based materials (CMs) with various dimensions (0-3D) have recently been investigated as promising candidates for the design and fabrication of microgels because of their large surface area, excellent conductivity, unique chemical stability, and low cost. Here, we provide a critical review of the specific characteristics of CMs that are being incorporated into microgels, as well as the state-of-the art applications of CM-microgels in pollutant adsorption and photodegradation, H₂ evoluation, CO₂ capture, soil conditioners, water retention, drug delivery, cell encapsulation, and tissue engineering. Advanced preparation techniques for CM-microgel systems are also summarized and discussed. Finally, challenges related to the low colloidal stability of CM-microgels and development strategies are examined. This review shows that CM-microgels have the potential to be widely used in various practical applications.

Structural Color Medical Patch with Surface Dual-Properties of Wet Bioadhesion and Slipperiness

Developing a self-reporting bioadhesive patch that has strong adhesion to the wet tissues and meanwhile can avoid adhering to the adjacent tissues is a current research difficulty and challenge. In this paper, inspired by the wet adhesion of spider web, slippery surface of Nepenthes, and structural color phenomena of chameleons, a novel structural color medical patch with surface dual-properties of wet bioadhesion and slipperiness for internal tissue repair based on inverse opal scaffold is presented. The adhesive surface made by poly(acrylic acid)-polyethylene glycol-N-hydroxysuccinimide ester and gelatin hydrogel can attain tough adhesion to internal wet tissues by absorbing tissue interfacial water and the covalent cross-linking between the hydrogel and tissue. Besides, the slippery surface made by liquid paraffin infused inverse opal scaffold can avoid adhesion to the adjacent tissues. It is demonstrated that the designed patch can adhere tightly to the defect tissue and improve the tissue repair without adjacent adhesion when applied in a rat model with full-thickness perforation of the stomach wall. In addition, the responsive structural color can supply a color-sensing monitoring to evaluate the adhesive and repair process. These features impart the bioinspired patch with great scientific significance and broad clinical application prospects.

Self-Transportation of Superparamagnetic Droplets on a Magnetic Gradient Slippery Surface with On/Off Sliding Controllability

Recently, research about droplet self-transportation on slippery surfaces has become a hotspot. However, to achieve on/off sliding control during the self-transportation process is still difficult. Herein, we report a magnetic slippery surface, and demonstrate on/off sliding control during the self-transportation of superparamagnetic droplets. The surface is prepared through integrating a substrate that has a gradient magnetic region with a layer of paraffin infused hydrophobic SiO₂ nanoparticles. On the surface, a superparamagnetic droplet is pinned at room temperature (about 25 °C), while it can self-transport directionally as the temperature is increased to about 70 °C. When the temperature is cooled down again, the droplet would return to the pinned state, indicating that on/off sliding control during the self-transportation process can be achieved. Furthermore, based on the excellent controllability, controllable coalescence of two droplets from opposite direction is displayed, demonstrating its potential application in numerous areas.

On-Demand Maneuvering of Diverse Prodrug Liquids on a Light-Responsive Candle-Soot-Hybridized Lubricant-Infused Slippery Surface for Highly Effective Toxicity Screening

At present, microscale high-throughput screening (HTS) for drug toxicity has drawn increased attention. Reported methods are often constrained by the inability to execute rapid fusion over diverse droplets or the inflexibility of relying on rigid customized templates. Herein, a light-responsive candle-soot-hybridized lubricant-infused slippery surface (CS-LISS) was reported by one-step femtosecond laser cross-scanning to realize highly effective and flexible drug HTS. Due to its low-hysteresis merits, the CS-LISS can readily steer diverse droplets toward arbitrary directions at a velocity over 1.0 mm/s with the help of tracing lateral near-infrared irradiation; additionally, it has the capability of self-cleaning and self-deicing. Significantly, by integrating the CS-LISS with a GFP HeLa cell chip, high-efficiency drug toxicity screening can be successfully achieved with the aid of fluorescence imaging. This work provides insights into the design of microscale high-throughput drug screening.

Controllable droplet sliding on smart shape memory slippery surface

Recently, slippery surfaces with controllable droplet sliding have aroused much attention in both fundamental research and realistic application. However, for almost all existing surfaces, constant stimuli such as thermal, light, magnetic fields, etc., are indispensable. Herein, by constructing pit structures on shape memory polymer and further infusing oil with low surface tension, we report a shape memory slippery surface that can overcome the above imperfection. Based on the shape memory performance, the surface can memorize diverse pit size as the surface is stretched or recovered. With the variation of pit structure, the sliding performances for both water and organic liquid droplets can be reversibly adjusted between the rolling and pinning states. This work, based on the shape memory effect, reports smart droplet sliding control through regulating surface microstructure, which not only provides a strategy for droplet sliding control, but also offers some fresh ideas for designing novel intelligent slippery surface.

Green syntheses of graphene and its applications in internet of things (IoT)-a status review

Internet of Things (IoT) is a trending technological field that converts any physical object into a communicable smarter one by converging the physical world with the digital world. This innovative technology connects the device to the internet and provides a platform to collect real-time data, cloud storage, and analyze the collected data to trigger smart actions from a remote location via remote notifications, etc. Because of its wide-ranging applications, this technology can be integrated into almost all the industries. Another trending field with tremendous opportunities is Nanotechnology, which provides many benefits in several areas of life, and helps to improve many technological and industrial sectors. So, integration of IoT and Nanotechnology can bring about the very important field of Internet of Nanothings (IoNT), which can re-shape the communication industry. For that, data (collected from trillions of nanosensors, connected to billions of devices) would be the 'ultimate truth', which could be generated from highly efficient nanosensors, fabricated from various novel nanomaterials, one of which is graphene, the so-called 'wonder material' of the 21st century. Therefore, graphene-assisted IoT/IoNT platforms may revolutionize the communication technologies around the globe. In this article, a status review of the smart applications of graphene in the IoT sector is presented. Firstly, various green synthesis of graphene for sustainable development is elucidated, followed by its applications in various nanosensors, detectors, actuators, memory, and nano-communication devices. Also, the future market prospects are discussed to converge various emerging concepts like machine learning, fog/edge computing, artificial intelligence, big data, and blockchain, with the graphene-assisted IoT field to bring about the concept of 'all-round connectivity in every sphere possible'.

Injectable Dual-Dynamic-Bond Cross-Linked Hydrogel for Highly Efficient Infected Diabetic Wound Healing

Diabetic wound is a significant challenge for clinical treatment with high morbidity and mortality. Plenty of hydrogels with good biocompatibility have been widely used in diabetic wound healing. However, most of them cannot be directly absorbed and utilized by the wounds, which prolongs the regeneration time. Here a new type of healing hydrogel is developed that is based on histidine, a natural dietary essential amino acid that is significant for tissue formation. The amino acid is cross-linked with zinc ions (Zn²⁺ ) and sodium alginate (SA) via dynamic coordinate and hydrogen bonds, respectively, forming a histidine-SA-Zn²⁺ (HSZH) hydrogel with good injectable, adhesive, biocompatible, and antibacterial properties. Application of this dual-dynamic-bond cross-linked HSZH hydrogel accelerates the migration and angiogenesis of skin-related cells in vitro. Furthermore, it significantly promotes the healing of infected diabetic wounds in vivo and uniquely allows a full repair of wounds within ≈13 days, while ≈27 days are required for the healing process of the control group. This work provides a new strategy for designing wound dressing materials, that weakly cross-linked material based on tissue-friendly micromolecules can heal the wounds more efficiently than highly cross-linked materials based on long-chain polymers.

Precise Controlling of Friction and Adhesion on Reprogrammable Shape Memory Micropillars

Microstructured surfaces with stimuli-responsive performances have aroused great attention in recent years, but it still remains a significant challenge to endow surfaces with precisely controlled morphological changes in microstructures, so as to get the precise control of regional properties (e.g., friction, adhesion). Herein, a kind of carbonyl iron particle-doped shape memory polyurethane micropillar with precisely controllable morphological changes is realized, upon remote near-infrared light (NIR) irradiation. Owing to the reversible transition of micropillars between bent and upright states, the micro-structured surface exhibits precisely controllable low-to-high friction transitions, together with the changes of friction coefficient ranging from ∼0.8 to ∼1.2. Hence, the changes of the surface friction even within an extremely small area can be precisely targeted, under local NIR laser irradiation. Moreover, the water droplet adhesion force of the surface can be reversibly switched between ∼160 and ∼760 μN, demonstrating the application potential in precisely controllable wettability. These features indicate that the smart stimuli-responsive micropillar arrays would be amenable to a variety of applications that require remote, selective, and on-demand responses, such as a refreshable Braille display system, micro-particle motion control, lab-on-a-chip, and microfluidics.

Sunlight Recovering the Superhydrophobicity of a Femtosecond Laser-Structured Shape-Memory Polymer

Superhydrophobic surfaces have aroused increasing attentions in the fields of self-cleaning, anti-fouling, heat transfer, etc. However, one of the major problems of the artificial superhydrophobic surface in practical applications is the poor durability. Inspired by the self-healing property of nature organism, we developed a sunlight-driven recoverable superhydrophobic surface by femtosecond laser constructing micropillar array on the surface of the photo-responsive shape-memory polymer (SMP). The photo-responsive SMP composite was prepared by adding reduced graphene oxide (RGO) into thermal-responsive SMP matrix. Due to the excellent sunlight-to-heat transformation property of RGO, the temperature of the as-fabricated RGO-SMP composite could be rapidly increased above the shape transformation temperature of the RGO-SMP under one sunlight irradiation. Once the micropillar array of the RGO-SMP composite was deformed by pressing or stretching treatments, the surface would lose superhydrophobicity. Upon sunlight irradiation, the surface morphology and the wettability of the RGO-SMP micropillars could completely recover to the original states. Meanwhile, this reversible morphology and wettability transformation process could be repeated multiple times. We envision that such a sunlight-recoverable superhydrophobic surface will have great applications in the future.

Durably Self-Sustained Droplet on a Fully Miscible Liquid Film

Droplets impacting onto a solid or liquid surface inducing wetting, floatation, splash, coalescence, etc. is ubiquitous in nature and industrial processes. Here, we report that liquid droplets exhibit spherical caps upon contact with a fully miscible liquid film of lower surface tension, despite the spontaneous mixing of the two liquids. Such a spherical cap on a continuous liquid surface sustains a long lifespan up to minutes before ultimately merging into the film. Benefiting from large viscous forces in a thin film as a result of spatial confinement, the surface flow is substantially suppressed. Therefore, the surface tension gradient responsible for this phenomenon is maintained because the normal diffusion of film liquid into the droplet can timely dilute film liquid supplied by uphill Marangoni flow at the droplet surface. The present finding removes the conventional cognition that droplet coalescence is prompt on fully miscible continuous liquid surfaces, thus benefiting design of new types of microfluidic devices.

Sneezing Aerosol Transport in an Indoor Farmers’ Market

The transportation and surface deposition of aerosols from sneezing in a small indoor farmers’ market are studied numerically. The effects of numbers and locations of the entrances and exits of the market are discussed under the condition of natural convection. The results indicate that aerosols leave the indoor environment more quickly when two doors are designed face to face on the walls perpendicular to the natural wind direction compared to other natural ventilation strategies. The concentrations of aerosols accumulated on the surfaces of the stalls and human bodies inside the market are also lower. In this case, the risk of contacting the virus is relatively low among susceptible individuals in the indoor farmers’ markets. Moreover, opening more doors on the walls parallel to the natural wind direction is not beneficial for the fast exhaust of aerosols.

Machine Learning for Shape Memory Graphene Nanoribbons and Applications in Biomedical Engineering

Shape memory materials have been playing an important role in a wide range of bioengineering applications. At the same time, recent developments of graphene-based nanostructures, such as nanoribbons, have demonstrated that, due to the unique properties of graphene, they can manifest superior electronic, thermal, mechanical, and optical characteristics ideally suited for their potential usage for the next generation of diagnostic devices, drug delivery systems, and other biomedical applications. One of the most intriguing parts of these new developments lies in the fact that certain types of such graphene nanoribbons can exhibit shape memory effects. In this paper, we apply machine learning tools to build an interatomic potential from DFT calculations for highly ordered graphene oxide nanoribbons, a material that had demonstrated shape memory effects with a recovery strain up to 14.5% for 2D layers. The graphene oxide layer can shrink to a metastable phase with lower constant lattice through the application of an electric field, and returns to the initial phase through an external mechanical force. The deformation leads to an electronic rearrangement and induces magnetization around the oxygen atoms. DFT calculations show no magnetization for sufficiently narrow nanoribbons, while the machine learning model can predict the suppression of the metastable phase for the same narrower nanoribbons. We can improve the prediction accuracy by analyzing only the evolution of the metastable phase, where no magnetization is found according to DFT calculations. The model developed here allows also us to study the evolution of the phases for wider nanoribbons, that would be computationally inaccessible through a pure DFT approach. Moreover, we extend our analysis to realistic systems that include vacancies and boron or nitrogen impurities at the oxygen atomic positions. Finally, we provide a brief overview of the current and potential applications of the materials exhibiting shape memory effects in bioengineering and biomedical fields, focusing on data-driven approaches with machine learning interatomic potentials.

Computational fluid dynamics-based disease transmission modeling of SARS-CoV-2 intensive care unit

Covid-19 has become one of the most severe diseases causing acute respiratory problems and has killed millions of people worldwide. It was declared as the ongoing pandemic by the World Health Organization. It is an infectious virus which can be transmitted by sneezing, coughing and exhalation of air by any infected person. There are certain places having high chances of becoming contaminated like hospital rooms. In this context, we studied the transmission of Covid-19 particles in an ICU room. We have considered the combined effect of both of air-conditioning (AC) and ceiling fan in the room. The infected person can transmit the disease when under influence of fan and AC. The work highlights the flow of aerosol particles considering the combined effect as well as the individual effects of fan and AC. The results also emphasized that the aerosol particle flow have a promising application in sanitizing the room.

COVID-19 spread in a classroom equipped with partition - A CFD approach

In this study, the motion and distribution of droplets containing coronaviruses emitted by coughing of an infected person in front of a classroom (e.g., a teacher) were investigated using CFD. A 3D turbulence model was used to simulate the airflow in the classroom, and a Lagrangian particle trajectory analysis method was used to track the droplets. The numerical model was validated and was used to study the effects of ventilation airflow speeds of 3, 5, and 7 m/s on the dispersion of droplets of different sizes. In particular, the effect of installing transparent barriers in front of the seats on reducing the average droplet concentration was examined. The results showed that using the seat partitions for individuals can prevent the infection to a certain extent. An increase in the ventilation air velocity increased the droplets' velocities in the airflow direction, simultaneously reducing the trapping time of the droplets by solid barriers. As expected, in the absence of partitions, the closest seats to the infected person had the highest average droplet concentration (3.80 × 10-8 kg/m3 for the case of 3 m/s).

Crosslinking Trends in Multicomponent Hydrogels for Biomedical Applications

Multicomponent-based hydrogels are well established candidates for biomedical applications. However, certain aspects of multicomponent systems, e.g., crosslinking, structural binding, network formation, proteins/drug incorporation, etc., are challenging aspects to modern biomedical research. The types of crosslinking and network formation are crucial for the effective combination of multiple component systems. The creation of a complex system in the overall structure and the crosslinking efficiency of different polymeric chains in an organized fashion are crucially important, especially when the materials are for biomedical applications. Therefore, the engineering of hydrogel has to be, succinctly understood, carefully formulated, and expertly designed. The different crosslinking methods in use, hydrogen bonding, electrostatic interaction, coordination bonding, and self-assembly. The formations of double, triple, and multiple networks, are well established. A systematic study of the crosslinking mechanisms in multicomponent systems, in terms of the crosslinking types, network formation, intramolecular bonds between different structural units, and their potentials for biomedical applications, is lacking and therefore, these aspects require investigations. To this end, the present review, focuses on the recent advances in areas of the physical, chemical, and enzymatic crosslinking methods that are often, employed for the designing of multicomponent hydrogels.

Tailoring Materials with Specific Wettability in Biomedical Engineering

As a fundamental feature of solid surfaces, wettability is playing an increasingly important role in our daily life. Benefitting from the inspiration of biological paradigms and the development in manufacturing technology, numerous wettability materials with elaborately designed surface topology and chemical compositions have been fabricated. Based on these advances, wettability materials have found broad technological implications in various fields ranging from academy, industry, agriculture to biomedical engineering. Among them, the practical applications of wettability materials in biomedical-related fields are receiving remarkable researches during the past decades because of the increasing attention to healthcare. In this review, the research progress of materials with specific wettability is discussed. After briefly introducing the underlying mechanisms, the fabrication strategies of artificial materials with specific wettability are described. The emphasis is put on the application progress of wettability biomaterials in biomedical engineering. The prospects for the future trend of wettability materials are also presented.

Microfluidics for Drug Development: From Synthesis to Evaluation

Drug development is a long process whose main content includes drug synthesis, drug delivery, and drug evaluation. Compared with conventional drug development procedures, microfluidics has emerged as a revolutionary technology in that it offers a miniaturized and highly controllable environment for bio(chemical) reactions to take place. It is also compatible with analytical strategies to implement integrated and high-throughput screening and evaluations. In this review, we provide a comprehensive summary of the entire microfluidics-based drug development system, from drug synthesis to drug evaluation. The challenges in the current status and the prospects for future development are also discussed. We believe that this review will promote communications throughout diversified scientific and engineering communities that will continue contributing to this burgeoning field.

Rapid generation of hybrid biochemical/mechanical cues in heterogeneous droplets for high-throughput screening of cellular responses

Cells in their native microenvironment are subjected to varying combinations of biochemical cues and mechanical cues in a wide range. Although many signaling pathways have been found to be responsive for extracellular cues, little is known about how biochemical cues crosstalk with mechanical cues in a complex microenvironment. Here, we introduced heterogeneous droplets on a microchip, which were rapidly assembled by combining wettability-patterned microchip and programmed droplet manipulations, for a high-throughput cell screening of the varying combinations of biochemical cues and mechanical cues. This platform constructed a heterogeneous droplet/microgel array with orthogonal gradual chemicals and materials, which was further applied to analyze the cellular Wnt/β-catenin signaling in response to varying combinations of Wnt ligands and substrate stiffness. Thus, this device provides a powerful multiplexed bioassay platform for drug development, tissue engineering, and stem cell screening.

Stimuli-responsive surfaces for switchable wettability and adhesion

Diverse unique surfaces exist in nature, e.g. lotus leaf, rose petal and rice leaf. They show similar contact angles but different adhesion properties. According to the different wettability and adhesion characteristics, this review reclassifies different contact states of droplets on surfaces. Inspired by the biological surfaces, smart artificial surfaces have been developed which respond to external stimuli and consequently switch between different states. Responsive surfaces driven by various stimuli, e.g. stretching, magnetic, photo, electric, temperature, humidity and pH, are discussed. Studies reporting on either atmospheric or underwater environments are discussed. The application of tailoring surface wettability and adhesion includes microfluidics/droplet manipulation, liquid transport and harvesting, water energy harvesting and flexible smart devices. Particular attention is placed on the horizontal comparison of smart surfaces with the same stimuli. Finally, the current challenges and future prospects in this field are also identified.

Superhydrophobicity preventing surface contamination as a novel strategy against COVID-19

Surface contact with virus is ubiquitous in the transmission pathways of respiratory diseases such as Coronavirus Disease 2019 (COVID-19), by which contaminated surfaces are infectious fomites intensifying the transmission of the disease. To date, the influence of surface wettability on fomite formation remains elusive. Here, we report that superhydrophobicity prevents the attachment of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on surfaces by repelling virus-laden droplets. Compared to bare surfaces, superhydrophobic (SHPB) surfaces exhibit a significant reduction in SARS-CoV-2 attachment of up to 99.99995%. We identify the vital importance of solid-liquid adhesion in dominating viral attachment, where the viral activity (N) is proportional to the cube of solid-liquid adhesion (A), N ∝ A³. Our results predict that a surface would be practically free of SARS-CoV-2 deposition when solid-liquid adhesion is ≤1 mN. Engineering surfaces with superhydrophobicity would open an avenue for developing a general approach to preventing fomite formation against the COVID-19 pandemic and future ones.

Ultra-Conformable Ionic Skin with Multi-Modal Sensing, Broad-Spectrum Antimicrobial and Regenerative Capabilities for Smart and Expedited Wound Care

While rapid wound healing is essential yet challenging, there is also an unmet need for functional restoration of sensation. Inspired by natural skin, an ultra-conformable, adhesive multi-functional ionic skin (MiS) with multi-modal sensing capability is devised for smart and expedited wound care. The base of MiS is a unique skin-like, conductive and self-adaptive adhesive polyacrylamide/starch double-network hydrogel (PSH) and self-powered, flexible, triboelectric sensor(s) is integrated on top of PSH for multi-tactile sensing. MiS could enhance wound contraction, collagen deposition, angiogenesis, and epidermis formation in a full-thickness skin defect wound model in vivo, while significantly inhibiting the biofilm formation of a wide range of microorganisms. MiS also exhibits multi-modal sensing capability for smart and instant therapeutics and diagnostics, including skin displacement or joint motion, temperature, pressure and tissue exudate changes of wound bed, and locally releasing drugs in a pH-responsive manner. More importantly, MiS could restore the skin-mimicking tactile sensing function of both touch location and intensity, and thus could be used as a human-machine interface for accurate external robotic control. MiS demonstrates a new comprehensive paradigm of combining wound diagnosis and healing, broad-spectrum anti-microbial capability and restoration of multi-tactile sensing for the reparation of severe wound.

A Review of Smart Lubricant-Infused Surfaces for Droplet Manipulation

The nepenthes-inspired lubricant-infused surface (LIS) is emerging as a novel repellent surface with self-healing, self-cleaning, pressure stability and ultra-slippery properties. Recently, stimuli-responsive materials to construct a smart LIS have broadened the application of LIS for droplet manipulation, showing great promise in microfluidics. This review mainly focuses on the recent developments towards the droplet manipulation on LIS with different mechanisms induced by various external stimuli, including thermo, light, electric, magnetism, and mechanical force. First, the droplet condition on LIS, determined by the properties of the droplet, the lubricant and substrate, is illustrated. Droplet manipulation via altering the droplet regime realized by different mechanisms, such as varying slipperiness, electrostatic force and wettability, is discussed. Moreover, some applications on droplet manipulation employed in various filed, including microreactors, microfluidics, etc., are also presented. Finally, a summary of this work and possible future research directions for the transport of droplets on smart LIS are outlined to promote the development of this field.

Bioinspired Metal-Intermetallic Laminated Composites for the Fabrication of Superhydrophobic Surfaces with Responsive Wettability

Hundreds of copper and titanium foils were applied to prepare biomimetic metal-intermetallic laminated composites by diffusion bonding. The cross sections of the obtained diffusion bonded bulks were etched selectively with FeCl₃ solution to get regular microarray structures. This kind of microstructure was controlled accurately and promptly by simple parameter adjustment. The etched surfaces were modified with 1-dodecanethiol, and the water contact angles (WCAs) were measured. The relationship between the microstructure and wettability of the achieved material was discussed, and the reason for the anisotropic wettability was also analyzed. Then etched surfaces were anodized in different electrolyte solutions to obtain different nanostructures. The morphology and chemical compositions of the surfaces were analyzed. The surfaces with CuO nanostructures by modification show superhydrophobicity with self-cleaning, on which the WCA and water sliding angle are 160.9° and 0.8°, respectively. The surfaces with TiO₂ nanostructures without modification show ultraviolet light-responsive wettability. After modification with 11-mercaptoundecanoic acid and 1-decanethiol, the surfaces also exhibit pH-responsive wettability. The superhydrophobic surfaces with responsive wettability have potential applications in biotechnology and microfluidics.

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

Superwetting Shape Memory Microstructure: Smart Wetting Control and Practical Application

Smart control of wettability on superwetting surfaces has aroused much attention in the past few years. Compared with traditional strategies such as adjusting the surface chemistry, regulating the surface microstructure is more difficult, though it can bring lots of new functions. Recently, it was found that, based on the shape memory effect of a shape memory polymer, the surface microstructure can be controlled more easily and precisely. Here, recent developments in the smart control of wettability on superwetting shape memory microstructures and corresponding applications are summarized. The primary concern is the superhydrophobic surfaces that have demonstrated numerous attractive functions, including controllable droplet storage, transportation, bouncing, capture/release, and reprogrammable gradient wetting, under variation of the surface microstructure. Finally, some achievements in wetting control on other superwetting surfaces (such as superomniphobic surfaces and superslippery surfaces) and perspectives on future research directions are also discussed.

An air distribution optimization of hospital wards for minimizing cross-infection

Currently, the "2019-CoV-2" has been raging across the world for months, causing massive death, huge panic, chaos, and immeasurable economic loss. Such emerging epidemic viruses come again and again over years, leading to similar destructive consequences. Air-borne transmission among humans is the main reason for the rapid spreading of the virus. Blocking the air-borne transmission should be a significant measure to suppress the spreading of the pandemic. Considering the hospital is the most probable place to occur massive cross-infection among patients as emerging virus usually comes in a disguised way, an air distribution optimization of a general three-bed hospital ward in China is carried out in this paper. Using the Eulerian-Lagrangian method, sneeze process from patients who are assumed to be the virus carrier, which is responsible for a common event to trigger cross-infection, is simulated. The trajectory of the released toxic particle and the probability of approaching others in the same ward are calculated. Two evaluation parameter, total maximum time (TMT) and overall particle concentration (OPC) to reflect the particle mobility and probability to cause cross-infection respectively, are developed to evaluate the proposed ten air distributions in this paper. A relatively optimized air distribution proposal with the lowest TMT and OPC is distinguished through a three-stage analysis. Results show that a bottom-in and top-out air distribution proposal is recommended to minimize cross-infections.

Humidity effect on peeling of monolayer graphene and hexagonal boron nitride

Ambient humidity introduces water adsorption and intercalation at the surfaces and interfaces of low-dimensional materials. Our extensive molecular dynamics (MD) simulations reveal the completely opposite contributions of interfacial water to the peeling of monolayer graphene and hexagonal boron nitride (h-BN) sheets from graphite and BN substrates. For graphene, interfacial water decreases the peeling force, due to lower adhesion at the graphene/water interface. The peeling force of h-BN increases with an increase in the thickness of interfacial water, owing to stronger adhesion at the h-BN/water interface and the detachment of the water layer from the substrates. In this work, a theoretical model considering graphene/water and water/substrate interfacial adhesion energies is established, to predict the peeling forces of graphene and h-BN, which coincides well with the peeling forces predicted by the MD simulations. Our results should provide a deeper insight into the effect of interfacial water, induced by ambient humidity, on mechanical exfoliation and the transfer of two-dimensional van der Waals crystals.

Magnetically Controllable Isotropic/Anisotropic Slippery Surface for Flexible Droplet Manipulation

Controllable wetting surfaces play a significant role in numerous applications such as smart liquid manipulation, lab-on-a-chip, drug delivery, liquid robot, and so on. A novel type of magnetically controllable isotropic/anisotropic slippery surface was prepared by femtosecond laser ablation. The slippery liquid-infused porous surface (SLIPS) can be switched between an isotropic smooth state and an anisotropic groove state by the magnetic field. The relationship between the sliding property of the SLIPS and the magnetic flux density, water droplet volume, microgroove width, and microgroove height are systematically studied. Passively flexible movement on the isotropic SLIPS and actively directional movement on the anisotropic SLIPS of water droplets were realized. This work provides a fresh understanding of the controllable isotropic/anisotropic SLIPS and reveals great potential in versatile applications which are related to magnetically controllable smart liquid manipulation.

Four-Dimensional (Bio-)printing: A Review on Stimuli-Responsive Mechanisms and Their Biomedical Suitability

The applications of tissue engineered constructs have witnessed great advances in the last few years, as advanced fabrication techniques have enabled promising approaches to develop structures and devices for biomedical uses. (Bio-)printing, including both plain material and cell/material printing, offers remarkable advantages and versatility to produce multilateral and cell-laden tissue constructs; however, it has often revealed to be insufficient to fulfill clinical needs. Indeed, three-dimensional (3D) (bio-)printing does not provide one critical element, fundamental to mimic native live tissues, i.e., the ability to change shape/properties with time to respond to microenvironmental stimuli in a personalized manner. This capability is in charge of the so-called “smart materials”; thus, 3D (bio-)printing these biomaterials is a possible way to reach four-dimensional (4D) (bio-)printing. We present a comprehensive review on stimuli-responsive materials to produce scaffolds and constructs via additive manufacturing techniques, aiming to obtain constructs that closely mimic the dynamics of native tissues. Our work deploys the advantages and drawbacks of the mechanisms used to produce stimuli-responsive constructs, using a classification based on the target stimulus: humidity, temperature, electricity, magnetism, light, pH, among others. A deep understanding of biomaterial properties, the scaffolding technologies, and the implant site microenvironment would help the design of innovative devices suitable and valuable for many biomedical applications.

Progress in Bioinspired Dry and Wet Gradient Materials from Design Principles to Engineering Applications

Nature does nothing in vain. Through millions of years of revolution, living organisms have evolved hierarchical and anisotropic structures to maximize their survival in complex and dynamic environments. Many of these structures are intrinsically heterogeneous and often with functional gradient distributions. Understanding the convergent and divergent gradient designs in the natural material systems may lead to a new paradigm shift in the development of next-generation high-performance bio-/nano-materials and devices that are critically needed in energy, environmental remediation, and biomedical fields. Herein, we review the basic design principles and highlight some of the prominent examples of gradient biological materials/structures discovered over the past few decades. Interestingly, despite the anisotropic features in one direction (i.e., in terms of gradient compositions and properties), these natural structures retain certain levels of symmetry, including point symmetry, axial symmetry, mirror symmetry, and 3D symmetry. We further demonstrate the state-of-the-art fabrication techniques and procedures in making the biomimetic counterparts. Some prototypes showcase optimized properties surpassing those seen in the biological model systems. Finally, we summarize the latest applications of these synthetic functional gradient materials and structures in robotics, biomedical, energy, and environmental fields, along with their future perspectives. This review may stimulate scientists, engineers, and inventors to explore this emerging and disruptive research methodology and endeavors.

Superlyophilic Shape Memory Porous Sponge for Smart Liquid Permeation

Recently, smart liquid permeation has aroused much attention. However, existing strategies to achieve such a goal are often based on reversibly controlling hydrophobicity/hydrophilicity on static porous structures, which are unsuitable for oils with low surface tension, and meanwhile they cannot realize tunable permeation flux since the pore sizes are constant. Herein, we report a superlyophilic shape memory porous sponge (SSMS) that can demonstrate tunable pore size from about 28 nm to 900 μm as the material's shape is changed. Based on the controllability in pore size, not only ON/OFF penetration but also precisely tunable permeation flux can be obtained for both water and oil. Furthermore, by using the SSMS, an application in accurate release of small-molecule rhodamine B was also demonstrated. This work reports a material with tunable pore size for controllable liquid permeation, which provides some ideas for designing smart superwetting permeation materials.

Switchable Wettability and Adhesion of Micro/Nanostructured Elastomer Surface via Electric Field for Dynamic Liquid Droplet Manipulation

Dynamic control of liquid wetting behavior on smart surfaces has attracted considerable concern owing to their important applications in directional motion, confined wetting and selective separation. Despite much progress in this regard, there still remains challenges in dynamic liquid droplet manipulation with fast response, no loss and anti-contamination. Herein, a strategy to achieve dynamic droplet manipulation and transportation on the electric field adaptive superhydrophobic elastomer surface is demonstrated. The superhydrophobic elastomer surface is fabricated by combining the micro/nanostructured clusters of hydrophobic TiO2 nanoparticles with the elastomer film, on which the micro/nanostructure can be dynamically and reversibly tuned by electric field due to the electric field adaptive deformation of elastomer film. Accordingly, fast and reversible transition of wetting state between Cassie state and Wenzel state and tunable adhesion on the surface via electric field induced morphology transformation can be obtained. Moreover, the motion states of the surface droplets can be controlled dynamically and precisely, such as jumping and pinning, catching and releasing, and controllable liquid transfer without loss and contamination. Thus this work would open the avenue for dynamic liquid manipulation and transportation, and gear up the broad application prospects in liquid transfer, selective separation, anti-fog, anti-ice, microfluidics devices, etc.

Field-Induced Wettability Gradients for No-Loss Transport of Oil Droplets on Slippery Surfaces

Transporting oil droplets is crucial for a wide range of industrial and biomedical applications but remains highly challenging due to the large contact angle hysteresis on most solid surfaces. A liquid-infused slippery surface has a low hysteresis contact angle and is a highly promising platform if sufficient wettability gradient can be created. Current strategies used to create wettability gradient typically rely on the engineering of the chemical composition or geometrical structure. However, these strategies are inefficient on a slippery surface because the infused liquid tends to conceal the gradient in the chemical composition and small-scale geometrical structure. Magnifying the structure, on the other hand, will significantly distort the surface topography, which is unwanted in practice. In this study, we address this challenge by introducing a field-induced wettability gradient on a flat slippery surface. By printing radial electrodes array, we can pattern the electric field, which induces gradient contact angles. Theoretical analysis and experimental results reveal that the droplet transport behavior can be captured by a nondimensional electric Bond number. Our surface enables no-loss transport of various types of droplets, which we expect to find important applications such as heat transfer, anticontamination, microfluidics, and biochemical analysis.