Ultrascalable Multifunctional Nanoengineered Copper and Aluminum for Antiadhesion and Bactericidal Applications

Reusable mechano-bactericidal surface with echinoid-shaped hierarchical micro/nano-structure

Biofilms formed owing to the attachment of bacteria to surfaces have caused various problems in industries such as marine transportation/logistics and medicine. In response, many studies have been conducted on bactericidal surfaces, and nanostructured surfaces mimicking cicada and dragonfly wings are emerging as candidates for mechano-bactericidal surfaces. In specific circumstances involving mechano-bactericidal activity, certain nanostructured surfaces could exhibit their bactericidal effects by directly deforming the membranes of bacteria that adhere to these nanostructures. Additionally, in most cases, debris of bacterial cells may accumulate on these nanostructured surfaces. Such accumulation poses a significant challenge: it diminishes the mechano-bactericidal effectiveness of the surface, as it hinders the direct interaction between the nanostructures and any new bacteria that attach subsequently. In specific circumstances involving mechano-bactericidal activity, certain nanostructured surfaces could exhibit their bactericidal effects by directly deforming the membranes of bacteria that adhere to these nanostructures. Additionally, in most cases, debris of bacterial cells may accumulate on these nanostructured surfaces. Such accumulation poses a significant challenge: it diminishes the mechano-bactericidal effectiveness of the surface, as it hinders the direct interaction between the nanostructures and any new bacteria that attach subsequently.In other words, there is a need for strategies to remove the accumulated bacterial debris in order to sustain the mechano-bactericidal effect of the nanostructured surface. In this study, hierarchical micro/nano-structured surface (echinoid-shaped nanotextures were formed on Al micro-particle's surfaces) was fabricated using a simple pressure-less sintering method, and effective bactericidal efficiency was shown against E. coli (97 ± 3.81%) and S. aureus (80 ± 9.34%). In addition, thermal cleaning at 500 °C effectively eliminated accumulated dead bacterial debris while maintaining the intact Al2O3 nanostructure, resulting in significant mechano-bactericidal activity (E. coli: 89 ± 6.86%, S. aureus: 75 ± 8.31%). As a result, thermal cleaning maintains the intact nanostructure and allows the continuance of the mechano-bactericidal effect. This effect was consistently maintained even after five repetitive use (E. coli: 80 ± 16.26%, S. aureus: 76 ± 12.67%).

In vitro comparison of methods for sampling copper-based antimicrobial surfaces

IMPORTANCE

Self-sanitizing surfaces such as copper (Cu) are increasingly used on high-touch surfaces to prevent the spread of harmful viruses and bacteria. Being able to monitor the antimicrobial properties of Cu is fundamental in measuring its antimicrobial efficacy. Thorough investigations into reliable methods to enumerate bacteria from self-sanitizing surfaces are lacking in the literature. This study demonstrates that direct use of Petrifilm on Cu surfaces most likely revives stressed and dying bacteria, which induces increased bacterial counts. This phenomenon was not observed with indirect collection methods. Studies assessing time-kill kinetics or long-term efficacy of Cu should consider the impact of the collection method chosen.

Automated Prediction of Bacterial Exclusion Areas on SEM Images of Graphene-Polymer Composites

To counter the rising threat of bacterial infections in the post-antibiotic age, intensive efforts are invested in engineering new materials with antibacterial properties. The key bottleneck in this initiative is the speed of evaluation of the antibacterial potential of new materials. To overcome this, we developed an automated pipeline for the prediction of antibacterial potential based on scanning electron microscopy images of engineered surfaces. We developed polymer composites containing graphite-oriented nanoplatelets (GNPs). The key property that the algorithm needs to consider is the density of sharp exposed edges of GNPs that kill bacteria on contact. The surface area of these sharp exposed edges of GNPs, accessible to bacteria, needs to be inferior to the diameter of a typical bacterial cell. To test this assumption, we prepared several composites with variable distribution of exposed edges of GNP. For each of them, the percentage of bacterial exclusion area was predicted by our algorithm and validated experimentally by measuring the loss of viability of the opportunistic pathogen Staphylococcus epidermidis. We observed a remarkable linear correlation between predicted bacterial exclusion area and measured loss of viability (R² = 0.95). The algorithm parameters we used are not generally applicable to any antibacterial surface. For each surface, key mechanistic parameters must be defined for successful prediction.

Designing Effective Antimicrobial Nanostructured Surfaces: Highlighting the Lack of Consensus in the Literature

Research into nanostructured materials, inspired by the topography of certain insect wings, has provided a potential pathway toward drug-free antibacterial surfaces, which may be vital in the ongoing battle against antimicrobial resistance. However, to produce viable antibacterial nanostructured surfaces, we must first understand the bactericidal mechanism of action and how to optimize them to kill the widest range of microorganisms. This review discusses the parameters of nanostructured surfaces that have been shown to influence their bactericidal efficiency and highlights the highly variable nature of many of the findings. A large-scale analysis of the literature is also presented, which further shows a lack of clarity in what is understood about the factors influencing bactericidal efficiency. The potential reasons for the ambiguity, including how the killing effect may be a result of multiple factors and issues with nonstandardized testing of the antibacterial properties of nanostructured surfaces, are then discussed. Finally, a standard method for testing of antimicrobial killing is proposed that will allow comparison between studies and enable a deeper understanding about nanostructured surfaces and how to optimize their bactericidal efficiency.

Hierarchically Patterned Self-Cleaning Polymer Composites for Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) has the potential to reduce energy demand and mitigate global warming. However, surface contamination from dust and bacterial buildup limits practical PDRC applications. Here, we develop a hierarchically patterned nanoporous composite (HPNC) using a facile template-molding fabrication method to integrate PDRC materials with self-cleaning and antibacterial functions. The HPNC design decouples multifunctional control into different characteristic length scales that can be optimized simultaneously. The nanoporous polymer matrix embedded with tunable fillers enables 7.8 and 4.4 °C temperature reduction for outdoor personal and building cooling, respectively, under intense solar irradiance. Meanwhile, a microscale pillar array pattern integrated into the HPNC enables superhydrophobicity with self-cleaning and antisoiling functions to mitigate surface contamination. Moreover, the surface coating of photocatalytic agents can generate photoinduced antibacterial effects. The scalable fabrication and multifunctional capabilities of our HPNC design offer a promising solution for practical PDRC applications with minimal maintenance needs.

Advances in micro and nanoengineered surfaces for enhancing boiling and condensation heat transfer: a review

Liquid-vapor phase change phenomena such as boiling and condensation are processes widely implemented in industrial systems such as power plants, refrigeration and air conditioning systems, desalination plants, water processing installations and thermal management devices due to their enhanced heat transfer capability when compared to single-phase processes. The last decade has seen significant advances in the development and application of micro and nanostructured surfaces to enhance phase change heat transfer. Phase change heat transfer enhancement mechanisms on micro and nanostructures are significantly different from those on conventional surfaces. In this review, we provide a comprehensive summary of the effects of micro and nanostructure morphology and surface chemistry on phase change phenomena. Our review elucidates how various rational designs of micro and nanostructures can be utilized to increase heat flux and heat transfer coefficient in the case of both boiling and condensation at different environmental conditions by manipulating surface wetting and nucleation rate. We also discuss phase change heat transfer performance of liquids having higher surface tension such as water and lower surface tension liquids such as dielectric fluids, hydrocarbons and refrigerants. We discuss the effects of micro/nanostructures on boiling and condensation in both external quiescent and internal flow conditions. The review also outlines limitations of micro/nanostructures and discusses the rational development of structures to mitigate these limitations. We end the review by summarizing recent machine learning approaches for predicting heat transfer performance of micro and nanostructured surfaces in boiling and condensation applications.

A Review of Crystallization Fouling in Heat Exchangers

A vast majority of heat exchangers suffer from unwanted deposition of material on the surface, which severely inhibits their performance and thus marks one of the biggest challenges in heat transfer. Despite numerous scientific investigations, prediction and prevention of fouling remain unresolved issues in process engineering and are responsible for large economic losses and environmental damage. This review article focuses specifically on crystallization fouling, providing a comprehensive overview of the state-of-the-art of fouling in heat exchangers. The fundamentals of the topic are discussed, as the term fouling resistance is introduced along with distinct fouling behaviour, observed in laboratory and industrial environments. Insight into subsequent phases of the fouling process is provided, along with the accompanying microscale events. Furthermore, the effects of fluid composition, temperature, flow velocity, surface condition, nucleate boiling and composite fouling are comprehensively discussed. Fouling modelling is systematically reviewed, from the early work of Kern and Seaton to recently used artificial neural networks and computational fluid dynamics. Finally, the most common fouling mitigation approaches are presented, including design considerations and various on-line strategies, as well as off-line cleaning. According to our review, several topics require further study, such as the initial stage of crystal formation, the effects of ageing, the interplay of two or more fouling mechanisms and the underlying phenomena of several mitigation strategies.

Multifunctional superhydrophobic surfaces

Surface wetting has a significant influence on the performance and applications of the materials. The superhydrophobic surfaces have water repellency due to low surface energy chemistry and micro/nanostructure roughness. The amazing applications of superhydrophobic surfaces (SHSs) lead to increase attention to superhydrophobicity in recent decades. The SHSs have been fabricated through chemical and physical methods. The further properties of SHSs as functions such as self-healing, anti-bacterial, anti-fouling, and stimuli-responsiveness are considered as the functions of the SHSs. The Multifunctional SHSs (MSHSs) that contained superhydrophobicity and at least two other properties as the next generation of the SHSs are swiftly developed in recent years. The multiple applications of the MSHSs are originated from specific morphology and functional groups of the MSHSs. The functions (properties) of the MSHSs are categorized into three groups including self-cleaning properties, restrictive properties, and smart properties. Designing and keeping surface structure plays a significant role in fabricating durable MSHSs. However, there is a big challenge to design and also scale up mechanochemical durable MSHSs. Based on state-of-the-art investigations, establishing a self-healing function can improve the durability of SHSs. The durable self-healing MSHSs can enhance the performance of the other functions and lifespan of the surface. In this review, all surface structures and superhydrophobic agents in MSHSs are investigated. The perspective of the MSHSs determined the next generation of the MSHSs have several significant parameters including durability, stability, more functions, more responsiveness, and environmentally friendly features for fabricating the large-scale MSHSs and enhancing their applications.

Implication of Surface Properties, Bacterial Motility, and Hydrodynamic Conditions on Bacterial Surface Sensing and Their Initial Adhesion

Biofilms are structured microbial communities attached to surfaces, which play a significant role in the persistence of biofoulings in both medical and industrial settings. Bacteria in biofilms are mostly embedded in a complex matrix comprised of extracellular polymeric substances that provide mechanical stability and protection against environmental adversities. Once the biofilm is matured, it becomes extremely difficult to kill bacteria or mechanically remove biofilms from solid surfaces. Therefore, interrupting the bacterial surface sensing mechanism and subsequent initial binding process of bacteria to surfaces is essential to effectively prevent biofilm-associated problems. Noting that the process of bacterial adhesion is influenced by many factors, including material surface properties, this review summarizes recent works dedicated to understanding the influences of surface charge, surface wettability, roughness, topography, stiffness, and combination of properties on bacterial adhesion. This review also highlights other factors that are often neglected in bacterial adhesion studies such as bacterial motility and the effect of hydrodynamic flow. Lastly, the present review features recent innovations in nanotechnology-based antifouling systems to engineer new concepts of antibiofilm surfaces.

Scalable Corrosion-Resistant Coatings for Thermal Applications

Corrosion of metallic substrates is a problem for a variety of applications. Corrosion can be mitigated with the use of an electrically insulating coating protecting the substrate. Thick millimetric coatings, such as paints, are generally more corrosion-resistant when compared to nanoscale coatings. However, for thermal systems, thick coatings are undesirable due to the resulting decrease in the overall heat transfer stemming from the added coating thermal resistance. Hence, the development of ultrathin (<10 μm) coatings is of great interest. Ultrathin inorganic silicon dioxide (SiO₂) coatings applied by sol-gel chemistries or chemical vapor deposition, as well as organic coatings such as Parylene C, have great anticorrosion performance due to their high dielectric breakdown and low moisture permeability. However, their application to arbitrarily shaped metals is difficult or expensive. Here, we develop a sol-gel solution capable of facile and controllable dip coating on arbitrary metals, resulting in a very smooth (<5 nm roughness), thin (∼3 μm), and conformal coating of dense SiO₂. To benchmark our material, we compared the corrosion performance with in-house synthesized superhydrophobic aluminum and copper samples, Parylene C-coated substrates, and smooth hydrophobic surfaces functionalized with a hydrophobic self-assembled monolayer. For comparison with state-of-the-art commercial coatings, copper substrates were coated with an organo-ceramic SiO₂ layer created by an elevated temperature and atmospheric pressure metal organic chemical vapor deposition process. To characterize corrosion performance, we electrochemically investigated the corrosion resistance of all samples through potentiodynamic polarization studies and electrochemical impedance spectroscopy. To benchmark the coating durability and to demonstrate scalability, we tested internally coated copper tubes in a custom-built corrosion flow loop to simulate realistic working conditions with shear and particulate saltwater flow. The sol-gel and Parylene C coatings demonstrated a 95% decrease in corrosion rate during electrochemical tests. Copper tube weight loss was reduced by 75% for the sol-gel SiO₂-coated tubes when seawater was used as the corrosive fluid in the test loop. This work not only demonstrates scalable coating methodologies for applying ultrathin anticorrosion coatings but also develops mechanistic understanding of corrosion mechanisms on a variety of functional surfaces and substrates.

Interactions Between 2D Materials and Living Matter: A Review on Graphene and Hexagonal Boron Nitride Coatings

Two-dimensional material (2DM) coatings exhibit complex and controversial interactions with biological matter, having shown in different contexts to induce bacterial cell death and contribute to mammalian cell growth and proliferation in vitro and tissue differentiation in vivo. Although several reports indicate that the morphologic and electronic properties of the coating, as well as its surface features (e.g., crystallinity, wettability, and chemistry), play a key role in the biological interaction, these kinds of interactions have not been fully understood yet. In this review, we report and classify the cellular interaction mechanisms observed in graphene and hexagonal boron nitride (hBN) coatings. Graphene and hBN were chosen as study materials to gauge the effect of two atomic-thick coatings with analogous lattice structure yet dissimilar electrical properties upon contact with living matter, allowing to discern among the observed effects and link them to specific material properties. In our analysis, we also considered the influence of crystallinity and surface roughness, detailing the mechanisms of interaction that make specific coatings of these 2DMs either hostile toward bacterial cells or innocuous for mammalian cells. In doing this, we discriminate among the material and surface properties, which are often strictly connected to the 2DM production technique, coating deposition and post-processing method. Building on this knowledge, the selection of 2DM coatings based on their specific characteristics will allow to engineer desired functionalities and devices. Antibacterial coatings to prevent biofouling, biocompatible platforms suitable for biomedical applications (e.g., wound healing, tissue repairing and regeneration, and novel biosensing devices) could be realized in the next future. Overall, a clear understanding on how the 2DM coating's properties may modulate a specific bacterial or cellular response is crucial for any future innovation in the field.

One-Step and Low-Cost Designing of Two-Layered Active-Layer Superhydrophobic Silicalite-1/PDMS Membrane for Simultaneously Achieving Superior Bioethanol Pervaporation and Fouling/Biofouling Resistance

Recently, the coupling of biofuel fermentation broths and pervaporation has been receiving increasing attention. Some challenges, such as the destructive effects of constituents of the real fermentation broth on the membrane performances, the lethal effects of the membrane surface chemical modifiers on the microorganisms, and being expensive, are against this concept. For the first time, a continuous study on the one-step and low-cost preparation of superhydrophobic membranes for bioethanol separation is made to address these challenges. In our previous work, spraying as a fast, scalable, and low-cost procedure was applied to fabricate the one-layered active-layer hydrophobic (OALH) silicalite-1/polydimethylsiloxane (PDMS) membrane on the low-cost mullite support. In this work, the spraying method was adopted to fabricate a two-layered active-layer superhydrophobic (TALS) silicalite-1/PDMS membrane, where the novel active layer consisted of two layers with different hydrophobicities and densities. Contact-angle measurements, surface charge determination, scanning electron microscopy, atomic force microscopy, and pervaporation separation using a 5 wt % ethanol solution were used to statically evaluate the fouling/biofouling resistance and pervaporation performances of OALH and TALS membranes in this study. The TALS membrane presented a better resistance and performance. For dynamic experiments, the Box-Behnken design was used to identify the effects of substrates, microorganisms, and nutrient contents as the leading indicators of fermentation broth on the TALS membrane performances for the long-term utilization. The maximum performances of 1.88 kg/m²·h, 32.34, and 59.04 kg/m²·h concerning the permeation flux, separation factor, and pervaporation separation index were obtained, respectively. The dynamic fouling/biofouling resistance of the TALS membrane was also characterized using energy-dispersive X-ray spectroscopy of all the tested membranes. The TALS membrane demonstrated the synergistic resistance of membrane fouling and biofouling. Eventually, the novel TALS membrane was found to have potential for biofuel recovery, especially bioethanol.

Mechano-bactericidal actions of nanostructured surfaces

Antibiotic resistance is a global human health threat, causing routine treatments of bacterial infections to become increasingly difficult. The problem is exacerbated by biofilm formation by bacterial pathogens on the surfaces of indwelling medical and dental devices that facilitate high levels of tolerance to antibiotics. The development of new antibacterial nanostructured surfaces shows excellent prospects for application in medicine as next-generation biomaterials. The physico-mechanical interactions between these nanostructured surfaces and bacteria lead to bacterial killing or prevention of bacterial attachment and subsequent biofilm formation, and thus are promising in circumventing bacterial infections. This Review explores the impact of surface roughness on the nanoscale in preventing bacterial colonization of synthetic materials and categorizes the different mechanisms by which various surface nanopatterns exert the necessary physico-mechanical forces on the bacterial cell membrane that will ultimately result in cell death.

Resolving Bio-Nano Interactions of E. coli Bacteria-Dragonfly Wing Interface with Helium Ion and 3D-Structured Illumination Microscopy to Understand Bacterial Death on Nanotopography

Obtaining a comprehensive understanding of the bactericidal mechanisms of natural nanotextured surfaces is crucial for the development of fabricated nanotextured surfaces with efficient bactericidal activity. However, the scale, nature, and speed of bacteria-nanotextured surface interactions make the characterization of the interaction a challenging task. There are currently several different opinions regarding the possible mechanisms by which bacterial membrane damage occurs upon interacting with nanotextured surfaces. Advanced imaging methods could clarify this by enabling visualization of the interaction. Charged particle microscopes can achieve the required nanoscale resolution but are limited to dry samples. In contrast, light-based methods enable the characterization of living (hydrated) samples but are limited by the resolution achievable. Here we utilized both helium ion microscopy (HIM) and 3D structured illumination microscopy (3D-SIM) techniques to understand the interaction of Gram-negative bacterial membranes with nanopillars such as those found on dragonfly wings. Helium ion microscopy enables cutting and imaging at nanoscale resolution, while 3D-SIM is a super-resolution optical microscopy technique that allows visualization of live, unfixed bacteria at ∼100 nm resolution. Upon bacteria-nanopillar interaction, the energy stored due to the bending of natural nanopillars was estimated and compared with fabricated vertically aligned carbon nanotubes. With the same deflection, shorter dragonfly wing nanopillars store slightly higher energy compared to carbon nanotubes. This indicates that fabricated surfaces may achieve similar bactericidal efficiency as dragonfly wings. This study reports in situ characterization of bacteria-nanopillar interactions in real-time close to its natural state. These microscopic approaches will help further understanding of bacterial membrane interactions with nanotextured surfaces and the bactericidal mechanisms of nanotopographies so that more efficient bactericidal nanotextured surfaces can be designed and fabricated, and their bacteria-nanotopography interactions can be assessed in situ.

Anti-Biofouling Properties of Femtosecond Laser-Induced Submicron Topographies on Elastomeric Surfaces

Antibacterial coatings are often employed to elastomer surfaces to inhibit bacterial attachment. However, such approaches could lead to increased antibiotic resistance. Surface micro-/nanotexturing is gaining significant attention recently, as it is a passive approach to reduce bacterial adhesion to surfaces. To this end, this work aims to assess the anti-biofouling functionality of femtosecond laser-induced submicron topographies on biomedical elastomer surfaces. Femtosecond laser processing was employed to produce two types of topographies on stainless-steel substrates. The first one was highly regular and single scale submicron laser-induced periodic surface structures (LIPSS) while the second one was multiscale structures (MSs) containing both submicron- and micron-scale features. Subsequently, these topographies were replicated on polydimethylsiloxane (PDMS) and polyurethane (PU) elastomers to evaluate their bacterial retention characteristics. The submicron textured PDMS and PU surfaces exhibited long-term hydrophobic durability up to 100 h under immersed conditions. Both LIPSS and MS topographies on PDMS and PU elastomeric surfaces were shown to substantially reduce (>89%) the adhesion of Gram-negative Escherichia coli bacteria. At the same time, the anti-biofouling performance of LIPSS and MS topographies was found to be comparable with that of lubricant-impregnated surfaces. The influence of physical defects on textured surfaces on the adhesion behavior of bacteria was also elucidated. The results presented here are significant because the polymeric biomedical components that can be produced by replication cost effectively, while their biocompatibility can be improved through femtosecond surface modification of the respective replication masters.

Extreme Antiscaling Performance of Slippery Omniphobic Covalently Attached Liquids

Scale formation presents an enormous cost to the global economy. Classical nucleation theory dictates that to reduce the heterogeneous nucleation of scale, the surface should have low surface energy and be as smooth as possible. Past approaches have focused on lowering surface energy via the use of hydrophobic coatings and have created atomically smooth interfaces to eliminate nucleation sites, or both, via the infusion of low-surface-energy lubricants into rough superhydrophobic substrates. Although lubricant-based surfaces are promising candidates for antiscaling, lubricant drainage inhibits their utilization. Here, we develop methodologies to deposit slippery omniphobic covalently attached liquids (SOCAL) on arbitrary substrates. Similar to lubricant-based surfaces, SOCAL has ultralow roughness and surface energy, enabling low nucleation rates and eliminating the need to replenish the lubricant. To enable SOCAL coating on metals, we investigated the surface chemistry required to ensure high-quality functionalization as measured by ultralow contact angle hysteresis (<3°). Using a multilayer deposition approach, we first electrophoretically deposit (EPD) silicon dioxide (SiO₂) as an intermediate layer between the metallic substrate and SOCAL. The necessity of EPD SiO₂ is to smooth (<10 nm roughness) as well as to enable the proper surface chemistry for SOCAL bonding. To characterize antiscaling performance, we utilized calcium sulfate (CaSO₄) scale tests, showing a 20× reduction in scale deposition rate than untreated metallic substrates. Descaling tests revealed that SOCAL dramatically decreases scale adhesion, resulting in rapid removal of scale buildup. Our work not only demonstrates a robust methodology for depositing antiscaling SOCAL coatings on metals but also develops design guidelines for the creation of antifouling coatings for alternate applications such as biofouling and high-temperature coking.