Nano and Microscale Topographies for the Prevention of Bacterial Surface Fouling

Bioinspired Photocatalytic Shark-Skin Surfaces with Antibacterial and Antifouling Activity via Nanoimprint Lithography

By combining antifouling shark-skin patterns with antibacterial titanium dioxide (TiO₂) nanoparticles (NPs), we present a simple route toward producing durable multifunctional surfaces that decrease microbial attachment and inactivate attached microorganisms. Norland Optical Adhesive, a UV-crosslinkable adhesive material, was loaded with 0, 10, or 50 wt % TiO₂ NPs from which shark-skin microstructures were imprinted using solvent-assisted soft nanoimprint lithography on a poly(ethylene terephthalate) (PET) substrate. To obtain coatings with an exceptional durability and an even higher concentration of TiO₂ NPs, a solution containing 90 wt % TiO₂ NPs and 10 wt % tetraethyl orthosilicate was prepared. These ceramic shark-skin-patterned surfaces were fabricated on a PET substrate and were quickly cured, requiring only 10 s of near infrared (NIR) irradiation. The water contact angle and the mechanical, antibacterial, and antifouling characteristics of the shark-skin-patterned surfaces were investigated as a function of TiO₂ composition. Introducing TiO₂ NPs increased the contact angle hysteresis from 30 to 100° on shark-skin surfaces. The hardness and modulus of the films were dramatically increased from 0.28 and 4.8 to 0.49 and 16 GPa, respectively, by creating ceramic shark-skin surfaces with 90 wt % TiO₂ NPs. The photocatalytic shark-skin-patterned surfaces reduced the attachment of Escherichia coli by ∼70% compared with smooth films with the same chemical composition. By incorporating as low as 10 wt % TiO₂ NPs into the chemical matrix, over 95% E. coli and up to 80% Staphylococcus aureus were inactivated within 1 h UV light exposure because of the photocatalytic properties of TiO₂. The photocatalytic shark-skin-patterned surfaces presented here were fabricated using a solution-processable and roll-to-roll compatible technique, enabling the production of large-area high-performance coatings that repel and inactivate bacteria.

Controlled Attachment of Pseudomonas aeruginosa with Binary Colloidal Crystal-Based Topographies

Micro- and nanotopographies can interfere with bacteria attachment, however, the interplay existing between surface chemistry and topography remains unclear. Here, self-assembled spherical micrometer- silica and nanometer poly(methyl methacrylate) (PMMA)-sized particles are used to make binary colloidal crystal (BCC) topographical patterns to study bacterial attachment. A uniform surface chemistry of allylamine plasma polymer (AAMpp) is coated on the top of the BCCs to study only the topography effects. The uncoated and coated BCCs are exposed to Pseudomonas aeruginosa, and the surfaces and bacteria are characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and fluorescence microscopy. It is found that bacteria attachment to the uncoated BCCs is delayed and individual cells are attracted to the small particle regions of the patterns. Surprisingly, this phenomenon is also observed for the AAMpp-coated BCCs, with bacteria attaching to the small particle regions of the pattern, in stark contrast to uniform flat films of AAMpp that are highly adhesive toward P. aeruginosa. Also, the overall levels of bacterial attachment are significantly reduced by the BCC patterns compared to controls. Thus, there is a trade-off that exists between chemistry and topography that can be exploited to delay the onset of P. aeruginosa biofilm formation on surfaces.

Nanostructured surface topographies have an effect on bactericidal activity

Background

Due to the increased emergence of antimicrobial resistance, alternatives to minimize the usage of antibiotics become attractive solutions. Biophysical manipulation of material surface topography to prevent bacterial adhesion is one promising approach. To this end, it is essential to understand the relationship between surface topographical features and bactericidal properties in order to develop antibacterial surfaces.

Results

In this work a systematic study of topographical effects on bactericidal activity of nanostructured surfaces is presented. Nanostructured Ormostamp polymer surfaces are fabricated by nano-replication technology using nanoporous templates resulting in 80-nm diameter nanopillars. Six Ormostamp surfaces with nanopillar arrays of various nanopillar densities and heights are obtained by modifying the nanoporous template. The surface roughness ranges from 3.1 to 39.1 nm for the different pillar area parameters. A Gram-positive bacterium, Staphylococcus aureus, is used as the model bacterial strain. An average pillar density at ~ 40 pillars μm⁻² with surface roughness of 39.1 nm possesses the highest bactericidal efficiency being close to 100% compared with 20% of the flat control samples. High density structures at ~ 70 pillars μm⁻² and low density structures at < 20 pillars μm⁻² with surface roughness smaller than 20 nm reduce the bactericidal efficiency to almost the level of the control samples.

Conclusion

The results obtained here suggests that the topographical effects including pillar density and pillar height inhomogeneity may have significant impacts on adhering pattern and stretching degree of bacterial cell membrane. A biophysical model is prepared to interpret the morphological changes of bacteria on these nanostructures.

Electronic supplementary material

The online version of this article (10.1186/s12951-018-0347-0) contains supplementary material, which is available to authorized users.

Antibacterial effects of the artificial surface of nanoimprinted moth-eye film

The antibacterial effect of a nanostructured film, known as “moth-eye film,” was investigated. The moth-eye film has artificially formed nano-pillars, consisting of hydrophilic resin with urethane acrylate and polyethylene glycol (PEG) derivatives, all over its surface that replicates a moth’s eye. Experiments were performed to compare the moth-eye film with a flat-surfaced film produced from the same materials. The JIS Z2801 film-covering method revealed that the two films produced a decrease in Staphylococcus aureus and Esherichia coli titers of over 5 and 3 logs, respectively. There was no marked difference in the antibacterial effects of the two surfaces. However, the antibacterial effects were reduced by immersion of the films in water. These results indicated that a soluble component(s) of the resin possessed the antibacterial activity, and this component was identified as PEG derivatives by time-of-flight secondary ion mass spectrometry (TOF-SIMS) and Fourier transform infrared spectroscopy (FT-IR). When a small volume of bacterial suspension was dropped on the films as an airborne droplet model, both films showed antibacterial effects, but that of the moth-eye film was more potent. It was considered that the moth-eye structure allowed the bacteria-loaded droplet to spread and allow greater contact between the bacteria and the film surface, resulting in strong adherence of the bacteria to the film and synergistically enhanced bactericidal activity with chemical components. The antibacterial effect of the moth-eye film has been thus confirmed under a bacterial droplet model, and it appears attractive due to its antibacterial ability, which is considered to result not only from its chemical make-up but also from physical adherence.

High-Throughput Fabrication Method for Producing a Silver-Nanoparticles-Doped Nanoclay Polymer Composite with Novel Synergistic Antibacterial Effects at the Material Interface

In this study, we report a high-throughput fabrication method at industrial pilot scale to produce a silver-nanoparticles-doped nanoclay-polylactic acid composite with a novel synergistic antibacterial effect. The obtained nanocomposite has a significantly lower affinity for bacterial adhesion, allowing the loading amount of silver nanoparticles to be tremendously reduced while maintaining satisfactory antibacterial efficacy at the material interface. This is a great advantage for many antibacterial applications in which cost is a consideration. Furthermore, unlike previously reported methods that require additional chemical reduction processes to produce the silver-nanoparticles-doped nanoclay, an in situ preparation method was developed in which silver nanoparticles were created simultaneously during the composite fabrication process by thermal reduction. This is the first report to show that altered material surface submicron structures created with the loading of nanoclay enables the creation of a nanocomposite with significantly lower affinity for bacterial adhesion. This study provides a promising scalable approach to produce antibacterial polymeric products with minimal changes to industry standard equipment, fabrication processes, or raw material input cost.

Analyzing the influence of microstructured surfaces on the lactic acid production of Lactobacillus delbrueckii lactis in a flow-through cell system

Microorganisms growing in biofilms might be possible biocatalysts for future biotechnological production processes. Attached to a surface and embedded in an extracellular polymeric matrix, they create their preferred environment and form robust cultures for continuous systems. With the objective of implementing highly efficient processes, productive biofilms need to be understood comprehensively. In this study, the influence of microstructured metallic surfaces on biofilm productivity was researched. To conduct this study, titanium and stainless steel sheets were polished, micromilled, as well as coated with particles. Subsequently, the metal sheets were exposed to the lactic acid producing Lactobacillus delbrueckii subsp. lactis under laminar and homogeneous flow conditions in a custom-built flow cell. A proof-of-concept showed that biofilm formation in the system only occurred on the designated substratum. Following a 24-h batch cultivation for primary biofilm development, the culture was continuously provided with glucose containing medium. As different experimental series have indicated, the process resulted to be stable for up to eleven days. Primary metabolite productivity averaged around 6-7 g/(L h). Interestingly, the productivity was shown to be affected neither by the type of metal, nor by the applied microstructures. Nevertheless, a higher dry biomass weight determined on micro-milled substratum indicates a complementary differentiation of biofilm components in future experiments.

Mechanosensing of shear by Pseudomonas aeruginosa leads to increased levels of the cyclic-di-GMP signal initiating biofilm development

Biofilms are communities of sessile microbes that are phenotypically distinct from their genetically identical, free-swimming counterparts. Biofilms initiate when bacteria attach to a solid surface. Attachment triggers intracellular signaling to change gene expression from the planktonic to the biofilm phenotype. For Pseudomonas aeruginosa, it has long been known that intracellular levels of the signal cyclic-di-GMP increase upon surface adhesion and that this is required to begin biofilm development. However, what cue is sensed to notify bacteria that they are attached to the surface has not been known. Here, we show that mechanical shear acts as a cue for surface adhesion and activates cyclic-di-GMP signaling. The magnitude of the shear force, and thereby the corresponding activation of cyclic-di-GMP signaling, can be adjusted both by varying the strength of the adhesion that binds bacteria to the surface and by varying the rate of fluid flow over surface-bound bacteria. We show that the envelope protein PilY1 and functional type IV pili are required mechanosensory elements. An analytic model that accounts for the feedback between mechanosensors, cyclic-di-GMP signaling, and production of adhesive polysaccharides describes our data well.

Temperature-Controlled Three-Stage Switching of Wetting, Morphology, and Protein Adsorption

The novel polymeric coatings of oligoperoxide-graft-poly(4-vinylpyridine-co-oligo(ethylene glycol)ethyl ether methacrylate246) [oligoperoxide-graft-P(4VP-co-OEGMA246)] attached to glass were successfully fabricated. The composition, thickness, morphology, and wettability of resulting coatings were analyzed using X-ray photoelectron spectroscopy, ellipsometry, atomic force microscopy, and contact angle measurements, respectively. In addition, adsorption of the bovine serum albumin was examined with fluorescence microscopy. The thermal response of wettability and morphology of the coatings followed by that of protein adsorption revealed two distinct transitions at 10 and 23 °C. For the first time, three stage switching was observed not only for surface wetting but also for morphology and protein adsorption. Moreover, the influence of the pH on thermo-sensitivity of modified surfaces was shown.

Hybrid Hairy Janus Particles as Building Blocks for Antibiofouling Surfaces

Herein, we report a new strategy for the design of antifouling surfaces by using hybrid hairy Janus particles. The amphiphilic Janus particles possess either a spherical or a plateletlike shape and have core-shell structures with an inorganic core and hydrophilic/hydrophobic polymeric shells. Subsequently, these bifunctional Janus particles enable the fabrication of surfaces with modularity in chemical composition and final surface topography, which possess antifouling properties. The antifouling and fouling-release capability of the composite Janus particle-based surfaces is investigated using the marine biofilm-forming bacteria Cobetia marina. The Janus particle-based coatings are robust and significantly reduce bacterial retention under both static and dynamic conditions independent of the particle geometry. The plateletlike (kaolinite-based) Janus particles represent a scalable system for the rational design of antifouling coatings as well as their large-scale production and application in the future.

Pattern Formation by Staphylococcus epidermidis via Droplet Evaporation on Micropillars Arrays at a Surface

We evaluate the effect of epoxy surface structuring on the evaporation of water droplets containing Staphylococcus epidermidis (S. epidermidis). During evaporation, droplets with S. epidermidis cells yield to complex wetting patterns such as the zipping-wetting1-3 and the coffee-stain effects. Depending on the height of the microstructure, the wetting fronts propagate circularly or in a stepwise manner, leading to the formation of octagonal or square-shaped deposition patterns.4,5 We observed that the shape of the dried droplets has considerable influence on the local spatial distribution of S. epidermidis deposited between micropillars. These changes are attributed to an unexplored interplay between the zipping-wetting1 and the coffee-stain6 effects in polygonally shaped droplets containing S. epidermidis. Induced capillary flows during evaporation of S. epidermidis are modeled with polystyrene particles. Bacterial viability measurements for S. epidermidis show high viability of planktonic cells, but low biomass deposition on the microstructured surfaces. Our findings provide insights into design criteria for the development of microstructured surfaces on which bacterial propagation could be controlled, limiting the use of biocides.

How Escherichia coli lands and forms cell clusters on a surface: a new role of surface topography

Bacterial response to surface topography during biofilm formation was studied using 5 μm tall line patterns of poly (dimethylsiloxane) (PDMS). Escherichia coli cells attached on top of protruding line patterns were found to align more perpendicularly to the orientation of line patterns when the pattern narrowed. Consistently, cell cluster formation per unit area on 5 μm wide line patterns was reduced by 14-fold compared to flat PDMS. Contrasting the reduced colony formation, cells attached on narrow patterns were longer and had higher transcriptional activities, suggesting that such unfavorable topography may present a stress to attached cells. Results of mutant studies indicate that flagellar motility is involved in the observed preference in cell orientation on narrow patterns, which was corroborated by the changes in cell rotation pattern before settling on different surface topographies. These findings led to a set of new design principles for creating antifouling topographies, which was validated using 10 μm tall hexagonal patterns.

The impact of structure dimensions on initial bacterial adhesion

Substrate topography can have profound effects on initial bacterial adhesion during biofilm formation. We applied Staphylococcus epidermidis and Escherichia coli cells onto periodically structured substrates with different structure dimensions, structure types and wetting properties. We found a strong dependence of cell retention on the structure dimensions of the applied substrates. Periodicities in the range of the cell size increased, whereas smaller periodicities decreased cell retention, independent of contact time (minutes to hours) and hydrophobicity. These novel insights on the role of surface topography on bacterial retention might facilitate the development of non-fouling surfaces in the future.

Human Mesenchymal Stem Cell Morphology and Migration on Microtextured Titanium

The implant used in spinal fusion procedures is an essential component to achieving successful arthrodesis. At the cellular level, the implant impacts healing and fusion through a series of steps: first, mesenchymal stem cells (MSCs) need to adhere and proliferate to cover the implant; second, the MSCs must differentiate into osteoblasts; third, the osteoid matrix produced by the osteoblasts needs to generate new bone tissue, thoroughly integrating the implant with the vertebrate above and below. Previous research has demonstrated that microtextured titanium is advantageous over smooth titanium and PEEK implants for both promoting osteogenic differentiation and integrating with host bone tissue; however, no investigation to date has examined the early morphology and migration of MSCs on these surfaces. This study details cell spreading and morphology changes over 24 h, rate and directionality of migration 6–18 h post-seeding, differentiation markers at 10 days, and the long-term morphology of MSCs at 7 days, on microtextured, acid-etched titanium (endoskeleton), smooth titanium, and smooth PEEK surfaces. The results demonstrate that in all metrics, the two titanium surfaces outperformed the PEEK surface. Furthermore, the rough acid-etched titanium surface presented the most favorable overall results, demonstrating the random migration needed to efficiently cover a surface in addition to morphologies consistent with osteoblasts and preosteoblasts.

Antibacterial Au nanostructured surfaces

We present here a technological platform for engineering Au nanotopographies by templated electrodeposition on antibacterial surfaces. Three different types of nanostructures were fabricated: nanopillars, nanorings and nanonuggets. The nanopillars are the basic structures and are 50 nm in diameter and 100 nm in height. Particular arrangement of the nanopillars in various geometries formed nanorings and nanonuggets. Flat surfaces, rough substrate surfaces, and various nanostructured surfaces were compared for their abilities to attach and kill bacterial cells. Methicillin-resistant Staphylococcus aureus, a Gram-positive bacterial strain responsible for many infections in health care system, was used as the model bacterial strain. It was found that all the Au nanostructures, regardless their shapes, exhibited similar excellent antibacterial properties. A comparison of live cells attached to nanotopographic surfaces showed that the number of live S. aureus cells was <1% of that from flat and rough reference surfaces. Our micro/nanofabrication process is a scalable approach based on cost-efficient self-organization and provides potential for further developing functional surfaces to study the behavior of microbes on nanoscale topographies.

A bactericidal microfluidic device constructed using nano-textured black silicon

Nano-structured black silicon (bSi) was used as a substratum for the construction of a microfluidic device of the highly efficient bactericidal action of this nano-textured surface againstPseudomonas aeruginosabacteria.

Tunable surface topography in fluoropolymers using photo-embossing

We describe and characterise a novel method of producing tuneable surface topography in fluorinated elastomers using a modified photo-embossing process.

Black-CuO: surface-enhanced Raman scattering and infrared properties

Large surface area samples of nanotextured black CuO were prepared by chemical etching of Cu for use in surface-enhanced Raman scattering (SERS). The SERS intensity of a self-assembled monolayer (SAM) of thiophenol was proportional to the thickness of a nanoscale-conformal Au film deposited by magnetron sputtering over the black CuO. A very high SERS yield of ∼10(4) counts per s per mW was observed for the thiophenol SAM on the thickest Au films studied here. Synchrotron X-ray photoelectron spectroscopy was used to confirm that the surface of the chemically etched Cu was covered by high purity CuO. IR spectral characterization of the black CuO showed a close to linear increase in reflectivity from 25 to 100% over the range of 4000-500 cm(-1) wavenumbers (or 2.5-20 μm in wavelength). Sensing applications and thermal effects in SERS are discussed.

Characterization and anti-settlement aspects of surface micro-structures from Cancer pagurus

Tuning surface and material properties to inhibit or prevent settlement and attachment of microorganisms is of interest for applications such as antifouling technologies. Here, optimization of nano- and microscale structures on immersed surfaces can be utilized to improve cell removal while reducing adhesion strength and the likelihood of initial cellular attachment. Engineered surfaces capable of controlling cellular behaviour under natural conditions are challenging to design due to the diversity of attaching cell types in environments such as marine waters, where many variations in cell shape, size and adhesion strategy exist. Nevertheless, understanding interactions between a cell and a potential substrate for adhesion, including topographically driven settlement cues, offers a route to designing surfaces capable of controlling cell settlement. Biomimetic design of artificial surfaces, based upon microscale features from natural surfaces, can be utilized as model surfaces to understand cell-surface interactions. The microscale surface features of the carapace from the crustacean Cancer pagurus has been previously found to influence the rate of attachment of particular organisms when compared to smooth controls. However, the nature of microscale topographic features from C. pagurus have not been examined in sufficient detail to allow design of biomimetic surfaces. In this work, the spatial distribution, chemical composition, size and shape descriptors of microscale surface features from C. pagurus are characterized in detail for the first time. Additionally, the influence of topography from C. pagurus on the settlement of marine diatoms is examined under field conditions.