Cell adhesion on nanotextured slippery superhydrophobic substrates

Boosted Spontaneous Formation of High-Aspect Ratio Nanopeaks on Ultrafast Laser-Irradiated Ni Surface

The capacity to synthesize and design highly intricated nanoscale objects of different sizes, surfaces, and shapes dramatically conditions the development of multifunctional nanomaterials. Ultrafast laser technology holds great promise as a contactless process able to rationally and rapidly manufacture complex nanostructures bringing innovative surface functions. The most critical challenge in controlling the growth of laser-induced structures below the light diffraction limit is the absence of external order associated to the inherent local interaction due to the self-organizing nature of the phenomenon. Here high aspect-ratio nanopatterns driven by near-field surface coupling and architectured by timely-controlled polarization pulse shaping are reported. Electromagnetic coupled with hydrodynamic simulations reveal why this unique optical manipulation allows peaks generation by inhomogeneous local absorption sustained by nanoscale convection. The obtained high aspect-ratio surface nanotopography is expected to prevent bacterial proliferation, and have great potential for catalysis, vacuum to deep UV photonics and sensing.

Novel Omniphobic Platform for Multicellular Spheroid Generation, Drug Screening, and On-Plate Analysis

Multicellular spheroids are superior to other culture geometries in reproducing critical physiological conditions of tumors, such as the diffusion of oxygen, nutrients, waste, and drugs, leading to a more precise model of in vivo drug sensitivity and resistance. Previously reported spheroid culture platforms are often difficult to use, expensive, single-use, or mechanically unstable. Here, we report a facile, mechanically stable, high-throughput spheroid culture platform based on hierarchically textured omniphobic surfaces. The developed omniphobic surfaces display very high contact angles with a range of different liquids, including the cell-laden culture media, thereby minimizing the cell surface contact area. Additionally, these surfaces maintain these high contact angles for extended periods of time to ensure cell aggregation. Using this novel platform, we demonstrate the generation and maintenance of robust multicellular spheroids, as well as heterogeneous, multicell-type spheroids. The platform is extremely robust, resistant to mechanical shock, allows for on-plate imaging, and is also the first-ever spheroid generation platform that can be reused repeatedly. Finally, the platform is suitable for on-plate drug screening and enables the first-ever, on-plate immunofluorescence staining and imaging of spheroids.

Antimicrobial Copper-Based Materials and Coatings: Potential Multifaceted Biomedical Applications

Surface contamination by microbes leads to several detrimental consequences like hospital- and device-associated infections. One measure to inhibit surface contamination is to confer the surfaces with antimicrobial properties. Copper's antimicrobial properties have been known since ancient times, and the recent resurgence in exploiting copper for application as antimicrobial materials or coatings is motivated by the growing concern about antibiotic resistance and the pressure to reduce antibiotic use. Copper, unlike silver, demonstrates rapid and high microbicidal efficacy against pathogens that are in close contact under ambient indoor conditions, which enhances its range of applicability. This review highlights the mechanisms behind copper's potent antimicrobial property, the design and fabrication of different copper-based antimicrobial materials and coatings comprising metallic copper/copper alloys, copper nanoparticles or ions, and their potential for practical applications. Finally, as the antimicrobial coatings market is expected to grow, we offer our perspectives on the implications of increased copper release into the environment and the potential ecotoxicity effects and possibility of development of resistant genes in pathogens.

Targeting Pathogenic Biofilms: Newly Developed Superhydrophobic Coating Favors a Host-Compatible Microbial Profile on the Titanium Surface

Polymicrobial infections are one of the most common reasons for inflammation of surrounding tissues and failure of implanted biomaterials. Because microorganism adhesion is the first step for biofilm formation, physical-chemical modifications of biomaterials have been proposed to reduce the initial microbial attachment. Thus, the use of superhydrophobic coatings has emerged because of their anti-biofilm properties. However, these coatings on the titanium (Ti) surface have been developed mainly by dual-step surface modification techniques and have not been tested using polymicrobial biofilms. Therefore, we developed a one-step superhydrophobic coating on the Ti surface by using a low-pressure plasma technology to create a biocompatible coating that reduces polymicrobial biofilm adhesion and formation. The superhydrophobic coating on Ti was created by the glow discharge plasma using Ar, O_2, and hexamethyldisiloxane gases, and after full physical, chemical, and biological characterizations, we evaluated its properties regarding oral biofilm inhibition. The newly developed coating presented an increased surface roughness and, consequently, superhydrophobicity (contact angle over 150°) and enhanced corrosion resistance (p < 0.05) of the Ti surface. Furthermore, proteomic analysis showed a unique pattern of protein adsorption on the superhydrophobic coating without drastically changing the biologic processes mediated by proteins. Additionally, superhydrophobic treatment did not present a cytotoxic effect on fibroblasts or reduction of proliferation; however, it significantly reduced (≈8-fold change) polymicrobial adhesion (bacterial and fungal) and biofilm formation in vitro. Interestingly, superhydrophobic coating shifted the microbiological profile of biofilms formed in situ in the oral cavity, reducing by up to ≈7 fold pathogens associated with the peri-implant disease. Thus, this new superhydrophobic coating developed by a one-step glow discharge plasma technique is a promising biocompatible strategy to drastically reduce microbial adhesion and biofilm formation on Ti-based biomedical implants.

Roughness Evolution and Charging in Plasma-Based Surface Engineering of Polymeric Substrates: The Effects of Ion Reflection and Secondary Electron Emission

The interaction of plasma with polymeric substrates generates both roughness and charging on the surface of the substrates. This work, toward the comprehension and, finally, the control of plasma-induced surface roughness, delves into the intertwined effects of surface charging, ion reflection, and secondary electron-electron emission (SEEE) on roughness evolution during plasma etching of polymeric substrates. For this purpose, a modeling framework consisting of a surface charging module, a surface etching model, and a profile evolution module is utilized. The case study is etching of a poly(methyl methacrylate) (PMMA) substrate by argon plasma. Starting from an initial surface profile with microscale roughness, the results show that the surface charging contributes to a faster elimination of the roughness compared to the case without charging, especially when ion reflection is taken into account. Ion reflection sustains roughness; without ion reflection, roughness is eliminated. Either with or without ion reflection, the effect of SEEE on the evolution of the rms roughness over etching time is marginal. The mutual interaction of the roughness and the charging potential is revealed through the correlation of the charging potential with a parameter combining rms roughness and skewness of the surface profile. A practical implication of the current study is that the elimination or the reduction of surface charging will result in greater surface roughness of polymeric, and generally dielectric, substrates.

Fibrinogen Motif Discriminates Platelet and Cell Capture in Peptide-Modified Gold Micropore Arrays

Human blood platelets and SK-N-AS neuroblastoma cancer-cell capture at spontaneously adsorbed monolayers of fibrinogen-binding motifs, GRGDS (generic integrin adhesion), HHLGGAKQAGDV (exclusive to platelet integrin α_IIbβ₃), or octanethiol (adhesion inhibitor) at planar gold and ordered 1.6 μm diameter spherical cap gold cavity arrays were compared. In all cases, arginine/glycine/aspartic acid (RGD) promoted capture, whereas alkanethiol monolayers inhibited adhesion. Conversely only platelets adhered to alanine/glycine/aspartic acid (AGD)-modified surfaces, indicating that the AGD motif is recognized preferentially by the platelet-specific integrin, α_IIbβ₃. Microstructuring of the surface effectively eliminated nonspecific platelet/cell adsorption and dramatically enhanced capture compared to RGD/AGD-modified planar surfaces. In all cases, adhesion was reversible. Platelets and cells underwent morphological change on capture, the extent of which depended on the topography of the underlying substrate. This work demonstrates that both the nature of the modified interface and its underlying topography influence the capture of cancer cells and platelets. These insights may be useful in developing cell-based cancer diagnostics as well as in identifying strategies for the disruption of platelet cloaks around circulating tumor cells.

Enhancing the Bactericidal Efficacy of Nanostructured Multifunctional Surface Using an Ultrathin Metal Coating

Insects and plants exhibit bactericidal behavior through nanostructures, which leads to physical contact killing that does not require antibiotics or chemicals. Also, certain metallic ions (e.g., Ag⁺ and Cu²⁺) are well-known to kill bacteria by disrupting their cellular functionalities. The aim of this study is to explore the improvement in bactericidal activity by combining extreme physical structure with surface chemistry. We have fabricated tall (8-9 μm high) nanostructures on silicon surfaces (NSS) having sharp tips (35-110 nm) using a single-step, maskless deep reactive ion etching technique inspired by dragonfly wing. Bactericidal efficacy of the nanostructured surfaces coated with a thin layer of silver (NSS_Ag) or copper (NSS_Cu) was measured quantitatively using standard viability plate-count method and flow cytometry. NSS_Cu surfaces kill bacteria very efficiently (killing 97% within 30 min) when compared to the uncoated NSS. This can be attributed to the addition of a surface chemistry to the nanostructures. The antibacterial activity of NSS_Cu is further indicated by the morphological differences of the dying/dead bacteria observed in the SEM images. The nanostructured surfaces demonstrate excellent superhydrophobic behavior, even with an ultrathin layer of metal (Ag/Cu) coating. The nanostructured surfaces exhibit static contact angle greater than 150° and contact hysteresis less than 10°. Moreover, reflectance is found to be <1% (for NSS_Cu < 0.5%) for all the nanostructured surfaces in the wavelength range 250-800 nm. The results obtained suggest that the fabricated nanostructured surfaces are multifunctional and can be used in various practical applications.

Drops on a Superhydrophobic Hole Hanging On under Evaporation

Drops with larger volumes placed over a superhydrophobic (SH) surface with a hole do not fall through unless they are evaporated to a size that is small enough. This feature offers the ability to preconcentrate samples for biochemical analysis. In this work, the influence of pinning on the behavior of drops placed on a 0.1 mm thick SH substrate with a 2 mm diameter hole as they evaporated was investigated. With 16 μL of water dispensed, the sessile drop component volume was initially higher than that of the overhanging drop component and maintained this until the later stages where almost identical shapes were attained and full evaporation was achieved without falling off the hole. With 15 μL of water dispensed, the volume of the sessile drop was initially higher than that of the overhanging drop component but the liquid body was able to squeeze through the hole after 180 s due to the contact line not having sufficient pinning strength when it encountered the edge of the hole. This resulted in the liquid body either falling through the hole or remaining pinned with an oval-like shape. When it did not fall-off, the liquid body had volume and contact angle characteristics for the sessile drop and overhanging drop components that were reversed. In the later stages, however, nearly identical shapes were again attained and full evaporation was achieved without falling off the hole. The effects of pinning, despite the substrate being SH, offer another path toward achieving practical outcomes with liquid bodies without the need for chemical surface functionalization. Similarities and differences could be seen in the behavior of a sessile drop on a SH plate that was inclined at 30° to the horizontal and evaporated.

Peptide-Mediated Platelet Capture at Gold Micropore Arrays

Ordered spherical cap gold cavity arrays with 5.4, 1.6, and 0.98 μm diameter apertures were explored as capture surfaces for human blood platelets to investigate the impact of surface geometry and chemical modification on platelet capture efficiency and their potential as platforms for surface enhanced Raman spectroscopy of single platelets. The substrates were chemically modified with single-constituent self-assembled monolayers (SAM) or mixed SAMs comprised of thiol-functionalized arginine-glycine-aspartic acid (RGD, a platelet integrin target) with or without 1-octanethiol (adhesion inhibitor). As expected, platelet adhesion was promoted and inhibited at RGD and alkanethiol modified surfaces, respectively. Platelet adhesion was reversible, and binding efficiency at the peptide modified substrates correlated inversely with pore diameter. Captured platelets underwent morphological change on capture, the extent of which depended on the topology of the underlying substrate. Regioselective capture of the platelets enabled study for the first time of the surface enhanced Raman spectroscopy of single blood platelets, yielding high quality Raman spectroscopy of individual platelets at 1.6 μm diameter pore arrays. Given the medical importance of blood platelets across a range of diseases from cancer to psychiatric illness, such approaches to platelet capture may provide a useful route to Raman spectroscopy for platelet related diagnostics.

Superhydrophobic materials for biomedical applications

Superhydrophobic surfaces are actively studied across a wide range of applications and industries, and are now finding increased use in the biomedical arena as substrates to control protein adsorption, cellular interaction, and bacterial growth, as well as platforms for drug delivery devices and for diagnostic tools. The commonality in the design of these materials is to create a stable or metastable air layer at the material surface, which lends itself to a number of unique properties. These activities are catalyzing the development of new materials, applications, and fabrication techniques, as well as collaborations across material science, chemistry, engineering, and medicine given the interdisciplinary nature of this work. The review begins with a discussion of superhydrophobicity, and then explores biomedical applications that are utilizing superhydrophobicity in depth including material selection characteristics, in vitro performance, and in vivo performance. General trends are offered for each application in addition to discussion of conflicting data in the literature, and the review concludes with the authors' future perspectives on the utility of superhydrophobic biomaterials for medical applications.

Bactericidal mechanism of nanopatterned surfaces

The quest to design and fabricate new antibacterial surfaces is an important task to meet the urgent demands of biomedical applications. Recently, a mechanical mechanism for killing adherent bacteria was discovered on nanopatterned surfaces, but there is a lack of understanding of the bactericidal mechanism. Here we present a quantitative thermodynamic model to study the bactericidal mechanism of nanopatterned surfaces through analyzing the total free energy change of bacterial cells. By comparing the bacterial cells on a flat surface and nanopatterned surface, our theoretical results reveal that cicada wing-like nanopatterned surfaces have more effective bactericidal properties than flat surfaces because a patterned surface leads to a drastic increase of the contact adhesion area. Our model also reveals some details of the influence mechanism, and gives some important information about how to improve the bactericidal properties through designing the morphology of the patterned surface.

Liquid body formation from a semispherical superhydrophobic well on a small incline

In this work, drop formation on a slightly inclined superhydrophobic substrate with liquid at various flow rates delivered through a semispherical well was investigated. Due to the initial dry well condition in the first drop produced, the inertial force from liquid filling allowed the well's edge hysteresis to be more readily breached, in which flow rates of 16 mL/min and above could create a jet that appeared to be able to "pierce" through the top of the semispherical drop without disrupting its form and growth very much. For subsequent drops, the well's edge hysteresis at flow rates of 14 mL/min and above helped to support an "egg" like form. In contrast, this form could not be developed on a similarly inclined superhydrophobic substrate without a well. The findings here assist to establish the flow rate ranges for consistent discrete volume delivery in biochemical analysis and serves as a means to conduct investigations to better reconcile the tendency of liquids to assume drops or develop jets.

Adsorption of "soft" spherical particles onto sinusoidally-corrugated substrates

We utilize a Monte Carlo simulation scheme based on the bond fluctuation model to simulate settlement of "soft" adhesive particles onto sinusoidally-corrugated substrates. Particles are composed of a hard inner core with a "soft" adhesive shell made of surface-grafted polymer chains. These chains adhere to surface lattice sites via pair wise non-specific interactions acting between the substrate and the last two segments of the polymer grafts on the particle. This simulation scheme is aimed at comprehending single particle adsorption behavior to find the highest adhesion energy locations for given test surfaces and elucidate test surfaces that reduce adhesion energy. Parameters in this study are set by the particle, the substrate and an interaction parameter between the two. Particle parameters include core diameter (D), grafting density of polymer (σ) and length of grafted polymer (N). Substrate parameters include wavelength (λ) and amplitude (A). Our results show that the wavelength of substrate features plays a significant role in the settlement of single particle systems. At λ = D/2 we observe a minimum in the adhesion energy and at λ = D we observe a uniform settlement location of the particles. Increasing N leads to a reduction in the effectiveness of substrate topography to direct the settlement of individual particles into specific sites on the substrate.

Bactericidal activity of black silicon

Black silicon is a synthetic nanomaterial that contains high aspect ratio nanoprotrusions on its surface, produced through a simple reactive-ion etching technique for use in photovoltaic applications. Surfaces with high aspect-ratio nanofeatures are also common in the natural world, for example, the wings of the dragonfly Diplacodes bipunctata. Here we show that the nanoprotrusions on the surfaces of both black silicon and D. bipunctata wings form hierarchical structures through the formation of clusters of adjacent nanoprotrusions. These structures generate a mechanical bactericidal effect, independent of chemical composition. Both surfaces are highly bactericidal against all tested Gram-negative and Gram-positive bacteria, and endospores, and exhibit estimated average killing rates of up to ~450,000 cells min⁻¹ cm⁻². This represents the first reported physical bactericidal activity of black silicon or indeed for any hydrophilic surface. This biomimetic analogue represents an excellent prospect for the development of a new generation of mechano-responsive, antibacterial nanomaterials.

The topographical features of insect wings result in some interesting surface properties, including hydrophobicity and antibacterial activity. Here the authors identify the surface of black silicon as a mimic of dragonfly wings and show that it too possesses antibacterial activity.

Adsorption of multiple spherical particles onto sinusoidally corrugated substrates

We utilize a Monte Carlo simulation scheme based on the bond fluctuation model to simulate settlement of adhesive particles onto sinusoidally corrugated substrates. The particles are composed of a hard inner core with either an effective potential shell or a "soft" adhesive shell made of flexible arms attached to the particle surface. These chains adhere via either the effective potential shell or the sticky chain ends to the surface via pairwise nonspecific interactions. This simulation model allows for multiple particles to settle onto each tested substrate to elucidate the behavior of the collective adhesive layer featuring multiparticle assembly. Particles move within a 3D lattice space and settle on the substrate due to attractive particle/substrate interactions. Once a single particle adheres to the substrate, a new particle is introduced into the lattice to begin a new settlement. Through this multiparticle settlement mode, we explore the interplay among the characteristics of the particles (i.e., size, interaction shell) and the substrates (i.e., wavelength and periodicity) as well as interparticle interactions. We report that the adhesion of particles with an effective interaction shell to the substrates is reduced dramatically when the particle size is smaller than the feature width of the periodic substrate. The settlement of particles with flexible hair on the sinusoidally corrugated substrates is more complex. Specifically, the presence of flexible polymeric hairs makes the particle settlement more likely to occur on nearly all substrates studied irrespective of the characteristics of the substrate.

Challenges in the characterization of plasma-processed three-dimensional polymeric scaffolds for biomedical applications

Low-temperature plasmas offer a versatile method for delivering tailored functionality to a range of materials. Despite the vast array of choices offered by plasma processing techniques, there remain a significant number of hurdles that must be overcome to allow this methodology to realize its full potential in the area of biocompatible materials. Challenges include issues associated with analytical characterization, material structure, plasma processing, and uniform composition following treatment. Specific examples and solutions are presented utilizing results from analyses of three-dimensional (3D) poly(ε-caprolactone) scaffolds treated with different plasma surface modification strategies that illustrate these challenges well. Notably, many of these strategies result in 3D scaffolds that are extremely hydrophilic and that enhance human Saos-2 osteoblast cell growth and proliferation, which are promising results for applications including tissue engineering and advanced biomedical devices.

Bioinspired patterning with extreme wettability contrast on TiO2 nanotube array surface: a versatile platform for biomedical applications

Binary wettability patterned surfaces with extremely high wetting contrasts can be found in nature on living creatures. They offer a versatile platform for microfluidic management. In this work, a facile approach to fabricating erasable and rewritable surface patterns with extreme wettability contrasts (superhydrophilic/superhydrophobic) on a TiO2 nanotube array (TNA) surface through self-assembly and photocatalytic lithography is reported. The multifunctional micropatterned superhydrophobic TNA surface can act as a 2D scaffold for site-selective cell immobilization and reversible protein absorption. Most importantly, such a high-contrast wettability template can be used to construct various well-defined 3D functional patterns, such as calcium phosphate, silver nanoparticles, drugs, and biomolecules in a highly selective manner. The 3D functional patterns would be a versatile platform in a wide range of applications, especially for biomedical devices (e.g., high-throughput molecular sensing, targeted antibacterials, and drug delivery). In a proof-of-concept study, the surface-enhanced Raman scattering and antibacterial performance of the fabricated 3D AgNP@TNA pattern, and the targeted drug delivery for site-specific and high-sensitivity cancer cell assays was investigated.

A simple way to achieve pattern-dependent tunable adhesion in superhydrophobic surfaces by a femtosecond laser

In this paper, we present a new approach to the tunable adhesive superhydrophobic surfaces consisting of periodic hydrophobic patterns and superhydrophobic structures by femtosecond (fs) laser irradiation on silicon. The surfaces are composed of periodic hydrophobic patterns (triangle, circle, and rhombus) and superhydrophobic structures (dual-scale spikes induced by a fs laser). Our results reveal that the adhesive forces of as-prepared surfaces can be tuned by varying the area ratio (AR(s-h)) of superhydrophobic domain to hydrophobic domain, thus resulting in tunable static and dynamic wettabilities. By increasing AR(s-h), (i) the static wetting property, which is characterized by the minimum water droplet volume that enables a droplet to land on the surface, can be tailored from 1 μL to 9 μL; (ii) the sliding angle can be flexibly adjusted, ranging from >90° (a droplet cannot slide off when the sample is positioned upside down) to 5°; and (iii) the droplet rebound behaviors can be modulated from partial rebound to triple rebound. In addition, the Cassie-Baxter model and the sliding angle model are used to speculate the contact angles and sliding angles to provide potentially theoretical models to design slippery-to-sticky superhydrophobic surfaces. The tunable adhesive superhydrophobic surfaces achieved by fs laser microfabrication may be potentially used in microfluidic systems to modulate the mobility of liquid droplets.