Influence of Surface Roughness, Nanostructure, and Wetting on Bacterial Adhesion.

Bacterial fouling is a persistent problem causing the deterioration and failure of functional surfaces for industrial equipment/components; numerous human, animal, and plant infections/diseases; and energy waste due to the inefficiencies at internal and external geometries of transport systems. This work gains new insights into the effect of surface roughness on bacterial fouling by systematically studying bacterial adhesion on model hydrophobic (methyl-terminated) surfaces with roughness scales spanning from ∼2 nm to ∼390 nm. Additionally, a surface energy integration framework is developed to elucidate the role of surface roughness on the energetics of bacteria and substrate interactions. For a given bacteria type and surface chemistry; the extent of bacterial fouling was found to demonstrate up to a 75-fold variation with surface roughness. For the cases showing hydrophobic wetting behavior, both increased effective surface area with increasing roughness and decreased activation energy with increased surface roughness was concluded to enhance the extent of bacterial adhesion. For the cases of superhydrophobic surfaces, the combination of factors including (i) the surpassing of Laplace pressure force of interstitial air over bacterial adhesive force, (ii) the reduced effective substrate area for bacteria wall due to air gaps to have direct/solid contact, and (iii) the reduction of attractive van der Waals force that holds adhering bacteria on the substrate were summarized to weaken the bacterial adhesion. Overall, this study is significant in the context of designing antifouling coatings and systems as well as explaining variations in bacterial contamination and biofilm formation processes on functional surfaces.

Recent developments in antimicrobial and antifouling coatings to reduce or prevent contamination and cross-contamination of food contact surfaces by bacteria.

Illness as the result of ingesting bacterially contaminated foodstuffs represents a significant annual loss of human quality of life and economic impact globally. Significant research investment has recently been made in developing new materials that can be used to construct food contacting tools and surfaces that might minimize the risk of cross-contamination of bacteria from one food item to another. This is done to mitigate the spread of bacterial contamination and resultant foodborne illness. Internet-based literature search tools such as Web of Science, Google Scholar, and Scopus were utilized to investigate publishing trends within the last 10 years related to the development of antimicrobial and antifouling surfaces with potential use in food processing applications. Technologies investigated were categorized into four major groups: antimicrobial agent-releasing coatings, contact-based antimicrobial coatings, superhydrophobic antifouling coatings, and repulsion-based antifouling coatings. The advantages for each group and technical challenges remaining before wide-scale implementation were compared. A diverse array of emerging antimicrobial and antifouling technologies were identified, designed to suit a wide range of food contact applications. Although each poses distinct and promising advantages, significant further research investment will likely be required to reliably produce effective materials economically and safely enough to equip large-scale operations such as farms, food processing facilities, and kitchens.

Visualization of Aluminum Ions at the Mica Water Interface Links Hydrolysis State-to-Surface Potential and Particle Adhesion.

When hydrolyzable cations such as aluminum interact with solid-water interfaces, macroscopic interfacial properties (e.g., surface charge and potential) and interfacial phenomena (e.g., particle adhesion) become tightly linked with the microscopic details of ion adsorption and speciation. We use in situ atomic force microscopy to directly image individual aluminum ions at a mica-water interface and show how adsorbate populations change with pH and aluminum activity. Complementary streaming potential measurements then allow us to build a triple layer model (TLM) that links surface potentials to adsorbate populations, via equilibrium binding constants. Our model predicts that hydrolyzed species dominate the mica-water interface, even when unhydrolyzed species dominate the solution. Ab initio molecular dynamics (AIMD) simulations confirm that aluminum hydrolysis is strongly promoted at the interface. The TLM indicates that hydrolyzed adsorbates are responsible for surface-potential inversions, and we find strong correlations between hydrolyzed adsorbates and particle-adhesion forces, suggesting that these species mediate adhesion by chemical bridging.

Distinctive Stress-Stiffening Responses of Regenerated Silk Fibroin Protein Polymers under Nanoscale Gap Geometries: Effect of Shear on Silk Fibroin-Based Materials.

Interfacial dynamics, assembly processes, and changes in nanostructures and mechanical properties of Bombyx mori silk fibroin (SF) proteins under varying degrees of nanoconfinement without and with lateral shear are investigated. When only compressive confinement forces were applied, SF proteins adsorbed on the surfaces experienced conformational changes following the Alexander-de Gennes theory of polymer brushes. By contrast, when SF proteins were exposed to a simultaneous nanoconfinement and shear, remarkable changes in interaction forces were observed, displaying the second order phase transitions, which are attributed to the formation of SF micelles and globular superstructural aggregates via hierarchical assembly processes. The resultant nanostructured SF aggregates show several folds greater elastic moduli than those of SF films prepared by drop-casting and compression-only and even degummed SF fibers. Such a striking improvement in mechanical strength is ascribed to a directional organization of ß-sheet nanocrystals, effectively driven by nanoconfinement and shear stress-induced stiffing and ordering mechanisms.

Thermo-responsive gels based on supramolecular assembly of an amidoamine and citric acid.

In this work, we report the formation of a novel, aqueous-based thermo-responsive, supramolecular gelling system prepared by a convenient and efficient self-assembly of a long-chain amino-amide and citric acid. To determine the viscosity behavior and to gain insights into the gelation mechanism, a complementary combination of techniques, including Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), dynamic light scattering (DLS), and sinusoidal oscillatory tests, were used. The supramolecular gelator exhibited remarkably reversible sol-gel transitions induced by temperature at 76 °C. At a concentration of 5 wt%, the zero-frequency viscosity of the supramolecular system increased by about four orders of magnitude (from 4.2 to 12 563 Pa s) by changing the temperature from 23 °C to 76 °C. The viscous nature of the supramolecular gel could be preserved up to 90 °C. The synergistic combination of the hydrogen bonding between amino and carboxylic acid groups and the electrostatic interactions arising from the protonation of the amino-group and the deprotonation of carboxylic acid groups enhanced at higher temperatures is presumably responsible for the thermo-responsive behavior. We anticipate that these supramolecular gelators can be beneficial in various applications such as hydrogel scaffolds for regenerative medicine, personal care products and cosmetics, and enhanced oil recovery as viscosity modifiers.

The effect of nanoconfinement on the glass transition temperature of ionic liquids.

This work is concerned with investigating the glass transition behavior of ionic liquids as a function of nanoconfinement. The glass transition temperature was found to increase with a decrease in confinement length, below a critical confinement of 40-50 nm and 80-90 nm for 1-butyl-3-methylimidazolium tetrafluoro-borate and 1-methyl-3-octylimidazolium tetrafluoro-borate between alumina surfaces, respectively.

Nanoporous aerogel as a bacteria repelling hygienic material for healthcare environment.

Healthcare-associated infections (HAIs) caused by pathogenic bacteria are a worldwide problem and responsible for numerous cases of morbidity and mortality. Exogenous cross-contamination is one of the main mechanisms contributing to such infections. This work investigates the potential of hydrophobically modified nanoporous silica aerogel as an antiadhesive hygienic material that can inhibit exogenous bacterial contamination. Nanoporous silica aerogels were synthesized via sol-gel polymerization of tetraethyl orthosilicate and hydrophobized using trimethylsilyl chloride. Bacterial adhesion characteristics were evaluated via dip-inoculation in suspensions of Gram-negative Escherichia coli O157:H7 and Gram-positive Staphylococcus aureus. The attachment of E. coli O157:H7 and S. aureus to hydrophobic nanoporous silica aerogel (HNSA) was found to be significantly lower than that to hydrophilic and hydrophobic nonporous silica materials: 99.91% (E. coli O157:H7) and 99.93% (S. aureus) reduction in comparison to hydrophilic nonporous silica, and 82.95% (E. coli O157:H7) and 84.90% (S. aureus) reduction in comparison to hydrophobic nonporous silica. These results suggest that the use of HNSA as surfaces that come into contact with bacterial pathogens in the healthcare environment can improve bacterial hygiene, and therefore may reduce the rate of HAIs.

Adsorption mechanism of myelin basic protein on model substrates and its bridging interaction between the two surfaces.

Myelin basic protein (MBP) is an intrinsically disordered (unstructured) protein known to play an important role in the stability of myelin's multilamellar membrane structure in the central nervous system. The adsorption of MBP and its capacity to interact with and bridge solid substrates has been studied using a surface forces apparatus (SFA) and a quartz crystal microbalance with dissipation (QCM-D). Adsorption experiments show that MBP molecules adsorb to the surfaces in a swollen state before undergoing a conformational change into a more compact structure with a thickness of ∼3 nm. Moreover, this compact structure is able to interact with nearby mica surfaces to form adhesive bridges. The measured adhesion force (energy) between two bridged surfaces is 1.0 ± 0.1 mN/m, (Ead = 0.21 ± 0.02 mJ/m(2)), which is slightly smaller than our previously reported adhesion force of 1.7 mN/m (Ead = 0.36 mJ/m(2)) for MBP adsorbed on two supported lipid bilayers (Lee et al., Proc. Natl. Acad. Sci. U.S.A. 2014, 111, E768-E775). The saturated surface concentration of compact MBP on a single SiO2 surface reaches a stable value of 310 ± 10 ng/cm(2) regardless of the bulk MBP concentration. A kinetic three-step adsorption model was developed that accurately fits the adsorption data. The developed model is a general model, not limited to intrinsically disordered proteins, that can be extended to the adsorption of various chemical compounds that undergo chemical reactions and/or conformational changes upon adsorbing to surfaces. Taken together with our previously published data (Lee et al., Proc. Natl. Acad. Sci. U.S.A. 2014, 111, E768-E775), the present results confirm that conformational changes of MBP upon adsorption are a key for strong adhesion, and that such conformational changes are strongly dependent on the nature of the surfaces.

Polymer multilayer tattooing for enhanced DNA vaccination.

DNA vaccines have many potential benefits but have failed to generate robust immune responses in humans. Recently, methods such as in vivo electroporation have demonstrated improved performance, but an optimal strategy for safe, reproducible, and pain-free DNA vaccination remains elusive. Here we report an approach for rapid implantation of vaccine-loaded polymer films carrying DNA, immune-stimulatory RNA, and biodegradable polycations into the immune-cell-rich epidermis, using microneedles coated with releasable polyelectrolyte multilayers. Films transferred into the skin following brief microneedle application promoted local transfection and controlled the persistence of DNA and adjuvants in the skin from days to weeks, with kinetics determined by the film composition. These 'multilayer tattoo' DNA vaccines induced immune responses against a model HIV antigen comparable to electroporation in mice, enhanced memory T-cell generation, and elicited 140-fold higher gene expression in non-human primate skin than intradermal DNA injection, indicating the potential of this strategy for enhancing DNA vaccination.

Relating domain size distribution to line tension and molecular dipole density in model cytoplasmic myelin lipid monolayers.

We fit the size distribution of liquid-ordered (L(o)) domains measured from fluorescence images of model cytoplasmic myelin monolayers with an equilibrium thermodynamic expression that includes the competing effects of line tension, λ, dipole density difference, Δm, and the mixing entropy. From these fits, we extract the line tension, λ, and dipole density difference, Δm, between the L(o) and liquid-disordered (L(d)) phases. Both λ and Δm decrease with increasing surface pressure, , although λ/Δm(2) remains roughly constant as the monolayer approaches the miscibility surface pressure. As a result, the mean domain size changed little with surface pressure, although the polydispersity increased significantly. The most probable domain radius was significantly smaller than that predicted by the energy alone, showing that the mixing entropy promotes a greater number of smaller domains. Our results also explain why domain shapes are stable; at equilibrium, only a small fraction of the domains are large enough to undergo theoretically predicted shape fluctuations. Monolayers based on the composition of myelin from animals with experimental allergic encephalomyelitis had slightly lower values of λ and Δm, and a higher area fraction of domains, than control monolayers at all . While it is premature to generalize these results to myelin bilayers, our results show that the domain distribution in myelin may be an equilibrium effect and that subtle changes in surface pressure and composition can alter the distribution of material in the monolayer, which will likely also alter the interactions between monolayers important to the adhesion of the myelin sheath.

Geocolloidal interactions and relaxation dynamics under nanoconfinement: Effects of salinity and particle concentration.

HYPOTHESIS: Energy-related contaminants are frequently associated with geocolloids that translocate in underground fissures with dimensions comparable with geocolloids. To assess the transport and impact of energy-related contaminants in geological systems, fundamental understandings of interfacial behaviors of nanoparticles under confinement is imperative. We hypothesize that the dynamic properties of geocolloids, as well as their dependence on aqueous medium conditions would deviate from bulk behaviors under nanoconfinement. EXPERIMENTS: Force profiles and rheological properties of 50 nm silica nanoparticles in aqueous media confined between mica surfaces as a function of surface separation, particle concentrations, and salinity were measured utilizing the surface forces apparatus. FINDINGS: Force profiles revealed the critical surface separation for nonlinear rheological behaviors coincides with the onset of exponential repulsion between mica surfaces. When salts were absent, the normal forces and viscosity values of colloidal suspensions resembled pure water. In contrast, with salts, the force profiles and corresponding critical length scales were found to be highly sensitive to the particle concentration and the degree of confinement. A Newtonian to shear-thinning transition was captured with increasing degrees of confinement. Our results show that the interplay among confinement, particle, and ionic concentrations can alter the interparticle forces and rheological responses of true nanosized-colloidal suspensions and thus their transport behaviors under nanoconfinement for the first time.

Bioinspired Superhydrophobic Nanocoating Based on Polydopamine and Nanodiamonds to Mitigate Bacterial Attachment to Polyvinyl Chloride Surfaces in Food Industry Environments.

Polyvinyl chloride (PVC) is commonly utilized as a food-contact surface by the food industry for processing and storage purposes due to its durability, ease of fabrication, and cost-effectiveness. Herein, we report a composite coating for the superhydrophobization of PVC without the use of polyfluoroalkyl chemistry. This coating rendered the PVC superhydrophobic, exhibiting a static water contact angle of 151.9 ± 0.7° and a contact angle hysteresis of only 3.1 ± 1.0°. The structure of this composite coating, consisting of polydopamine, nanodiamonds, and an alkyl silane, was investigated by utilizing both scanning electron microscopy and atomic force microscopy. Surface chemistry was probed using attenuated total reflectance-Fourier transform infrared, and the surface wetting behavior was thoroughly characterized using both static and dynamic water contact angle measurements. It was demonstrated that the superhydrophobic PVC was cleanable using a food-grade surfactant, becoming wet in contact with high concentration surfactant solutions, but regaining its nonwetting property upon rinsing with water. It was demonstrated that the coating produced a 2.1 ± 0.1 log10 reduction (99.2%) in the number of Escherichia coli O157:H7 cells and a 2.2 ± 0.1 log10 reduction (99.3%) in the number of Salmonella enterica Typhimurium cells that were able to adsorb onto PVC surfaces over a 24 h period. The use of this fluorine-free superhydrophobic coating on PVC equipment, such as conveyor belts within food production facilities, may help to mitigate bacterial cross-contamination and curb the spread of foodborne illnesses.

Edible nano-encapsulated cinnamon essential oil hybrid wax coatings for enhancing apple safety against food borne pathogens.

Post-harvest losses of fruits due to decay and concerns regarding microbial food safety are significant within the produce processing industry. Additionally, maintaining the quality of exported commodities to distant countries continues to pose a challenge. To address these issues, the application of bioactive compounds, such as essential oils, has gained recognition as a means to extend shelf life by acting as antimicrobials. Herein, we have undertaken an innovative approach by nano-encapsulating cinnamon-bark essential oil using whey protein concentrate and imbibing nano-encapsulates into food-grade wax commonly applied on produce surfaces. We have comprehensively examined the physical, chemical, and antimicrobial properties of this hybrid wax to evaluate its efficacy in combatting the various foodborne pathogens that frequently trouble producers and handlers in the post-harvest processing industry. The coatings as applied demonstrated a static contact angle of 85 ± 1.6°, and advancing and receding contact angles of 90 ± 1.1° and 53.0 ± 1.6°, respectively, resembling the wetting properties of natural waxes on apples. Nanoencapsulation significantly delayed the release of essential oil, increasing the half-life by 61 h compared to its unencapsulated counterparts. This delay correlated with statistically significant reductions (p = 0.05) in bacterial populations providing both immediate and delayed (up to 72 h) antibacterial effects as well as expanded fungal growth inhibition zones compared to existing wax technologies, demonstrating promising applicability for high-quality fruit storage and export. The utilization of this advanced produce wax coating technology offers considerable potential for bolstering food safety and providing enhanced protection against bacteria and fungi for produce commodities.

Durable superhydrophobic coatings for stainless-steel: An effective defense against Escherichia coli and Listeria fouling in the post-harvest environment.

Increasing concerns revolve around bacterial cross-contamination of leafy green vegetables via food-contact surfaces. Given that stainless-steel is among the commonly used food-contact surfaces, this study reports a coating strategy enhancing its hygiene and microbiological safety through an antifouling approach via superhydrophobicity. The developed method involves growing a nickel-nanodiamond nanocomposite film on 304 stainless-steel via electroplating and sequential functionalization of the outer surface layer with nonpolar organosilane molecules via polydopamine moieties. The resultant superhydrophobic stainless-steel surfaces had a static water contact angle of 156.3 ± 1.9° with only 2.3 ± 0.5° contact angle hysteresis. Application of the coating to stainless-steel was demonstrated to yield 2.3 ± 0.6 log10 and 2.0 ± 0.9 log10 reductions in the number of adherent gram-negative Escherichia coli O157:H7 and gram-positive Listeria innocua cells, respectively. These population reductions were shown to be statistically significant (α = 0.05). Coated stainless-steel also resisted fouling when contacted with contaminated romaine lettuce leaves and maintained significant non-wetting character when abraded with sand or contacted with high concentration surfactant solutions. The incorporation of superhydrophobic stainless-steel surfaces into food processing equipment used for washing and packaging leafy green vegetables has the potential to mitigate the transmission of pathogenic bacteria within food production facilities.

Lipid-protein interactions alter line tensions and domain size distributions in lung surfactant monolayers.

The size distribution of domains in phase-separated lung surfactant monolayers influences monolayer viscoelasticity and compressibility which, in turn, influence monolayer collapse and set the compression at which the minimum surface tension is reached. The surfactant-specific protein SP-B decreases the mean domain size and polydispersity as shown by fluorescence microscopy. From the images, the line tension and dipole density difference are determined by comparing the measured size distributions with a theory derived by minimizing the free energy associated with the domain energy and mixing entropy. We find that SP-B increases the line tension, dipole density difference, and the compressibility modulus at surface pressures up to the squeeze-out pressure. The increase in line tension due to SP-B indicates the protein avoids domain boundaries due to its solubility in the more fluid regions of the film.

Interaction forces and adhesion of supported myelin lipid bilayers modulated by myelin basic protein.

Force-distance measurements between supported lipid bilayers mimicking the cytoplasmic surface of myelin at various surface coverages of myelin basic protein (MBP) indicate that maximum adhesion and minimum cytoplasmic spacing occur when each negative lipid in the membrane can bind to a positive arginine or lysine group on MBP. At the optimal lipid/protein ratio, additional attractive forces are provided by hydrophobic, van der Waals, and weak dipolar interactions between zwitterionic groups on the lipids and MBP. When MBP is depleted, the adhesion decreases and the cytoplasmic space swells; when MBP is in excess, the bilayers swell even more. Excess MBP forms a weak gel between the surfaces, which collapses on compression. The organization and proper functioning of myelin can be understood in terms of physical noncovalent forces that are optimized at a particular combination of both the amounts of and ratio between the charged lipids and MBP. Thus loss of adhesion, possibly contributing to demyelination, can be brought about by either an excess or deficit of MBP or anionic lipids.

Bacterial Antifouling Characteristics of Helicene-Graphene Films.

Herein, we describe interfacially-assembled [7]helicene films that were deposited on graphene monolayer using the Langmuir-Schaefer deposition by utilizing the interactions of nonplanar (helicene) and planar (graphene) π-π interactions as functional antifouling coatings. Bacterial adhesion of Staphylococcus aureus on helicene-graphene films was noticeably lower than that on bare graphene, up to 96.8% reductions in bacterial adhesion. The promising bacterial antifouling characteristics of helicene films was attributed to the unique molecular geometry of helicene, i.e., nano-helix, which can hinder the nanoscale bacterial docking processes on a surface. We envision that helicene-graphene films may eventually be used as protective coatings against bacterial antifouling on the electronic components of clinical and biomedical devices.

Studies of bilayers and vesicle adsorption to solid substrates: development of a miniature streaming potential apparatus (SPA).

A new miniature streaming potential apparatus (SPA) was developed to determine the streaming potentials (Psi(str)) and zeta potentials (zeta) of substrates under different ionic conditions while simultaneously visualizing the state of the surfaces, such as the adsorption of surfactants or polymers, using fluorescence microscopy and/or fluorescence recovery after photobleaching (FRAP). Experimental results with different surfaces show that the new SPA provides streaming potential values (hence zeta potentials) that agree with results obtained using traditional electrokinetic analyzers. Using the new SPA, the formation of supported lipid bilayers (SLBs) on glass from fluorescently labeled, unilamellar (approximately 100 nm diameter), charge neutral dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles was studied in aqueous electrolyte solutions at different lipid concentrations. Simultaneous zeta potential measurements and fluorescence imaging for measuring diffusion coefficients by confocal microscopy enabled us to precisely monitor the changes in the surface charge as well as in the surface morphology during SLB formation from vesicles. For a fixed incubation time of 5 min, both results revealed that the adsorption of intact vesicles and/or discrete bilayer patches were observed below a threshold concentration, above which the formation of continuous SLBs occurred leading to an estimate for the zeta-potential and for the diffusion coefficient of -9.1 +/- 1.6 mV and (1.1 +/- 0.02) x 10(-12) m(2)/s, respectively.

The role of interparticle and external forces in nanoparticle assembly.

The past 20 years have witnessed simultaneous multidisciplinary explosions in experimental techniques for synthesizing new materials, measuring and manipulating nanoscale structures, understanding biological processes at the nanoscale, and carrying out large-scale computations of many-atom and complex macromolecular systems. These advances have led to the new disciplines of nanoscience and nanoengineering. For reasons that are discussed here, most nanoparticles do not 'self-assemble' into their thermodynamically lowest energy state, and require an input of energy or external forces to 'direct' them into particular structures or assemblies. We discuss why and how a combination of self- and directed-assembly processes, involving interparticle and externally applied forces, can be applied to produce desired nanostructured materials.

Dual-functional core-shell electrospun mats with precisely controlled release of anti-inflammatory and anti-bacterial agents.

Acute wounds are worldwide problems affecting millions of people and causing heavy economic burden to national healthcare systems. Herein, we describe novel wound dressing materials relying on core/shell electrospun mats incorporated with flurbiprofen and vancomycin for achieving programmable release of anti-inflammatory and anti-bacterial agents. The shell matrix of nanofibers consisted of polyethylene oxide while the core matrix was made from a blend of silk and collagen. Several optimal mat architectures were engineered with distinct configurations, of which release profiles displayed an exponential trend, which indicates a first-order process following Fickian diffusion behavior. The flurbiprofen release lasted from 2 to 6 days, which was much faster compared to the one of vancomycin prolonged up to about 20 days. Mechanical data indicated tensile modulus, tensile strength, elongation before break of core/shell electrospun mats became enhanced or comparable to those for human skin after methanol vapor treatment. Desirable release kinetics and mechanical characteristics achieved by novel core/shell electrospun mats were attributable to induced enrichment of ß-sheet phase in silk via methanol vapor treatment as well as water annealing process with time and judicious selections for matrix materials and mat configurations. The design principles considered in this study successfully addressed a range of inflammation and infection requirements in wound healing, potentially guiding construction of other biomedical coatings and devices.