Nanoscale topography reduces fibroblast growth, focal adhesion size and migration-related gene expression on platinum surfaces

Controlling cellular responses on biomaterial surfaces is crucial in biomedical applications such as tissue engineering and implantable prosthetics. Since cells encounter various nanoscale topographic features in their natural environment, it has been postulated that surface nanotopography may be an alternative route to fabricate biomaterials with a desirable cellular response. In this framework, we investigated the responses of primary human fibroblasts to platinum substrates with different levels of surface roughness at the nanoscale. The nanorough surfaces were fabricated by using the glancing angle deposition technique (GLAD). We found that levels of cellular responses depended on the surface roughness and the size of the nanoscale features. We showed that in response to nanotopography cells spread less and have an elongated morphology, displaying signs of actin cytoskeleton impairment and reduced formation of focal adhesion complexes. Although cell growth and adhesion were impaired on the nanorough substrates, cell viability was not affected by topography. To a minor extent our results also indicate that cell migration might be reduced on the nanorough surfaces, since a significantly lower gene expression of migration related genes were found on the roughest surfaces as compared to the flat reference. The results presented here demonstrate that surface nanotopography influences fibroblasts responses on platinum, which may be used to reduce cellular adhesion on platinum implant surfaces such as implantable neural electrodes.     

Roughness assessment and wetting behavior of fluorocarbon surfaces

The wetting behavior of fluorocarbon materials has been studied with the aim of assessing the influence of the surface chemical composition and surface roughness on the water advancing and receding contact angles. Diamond like carbon and two fluorocarbon materials with different fluorine content have been prepared by plasma enhanced chemical vapor deposition and characterized by X-ray photoemission, Raman and FT-IR spectroscopies. Very rough surfaces have been obtained by deposition of thin films of these materials on polymer substrates previously subjected to plasma etching to increase their roughness. A direct correlation has been found between roughness and water contact angles while a superhydrophobic behavior (i.e., water contact angles higher than 150° and relatively low adhesion energy) was found for the films with the highest fluorine content deposited on very rough substrates. A critical evaluation of the methods currently used to assess the roughness of these surfaces by atomic force microscopy (AFM) has evidenced that calculated RMS roughness values and actual surface areas are quite dependent on both the scale of observation and image resolution. A critical discussion is carried out about the application of the Wenzel model to account for the wetting behavior of this type of surfaces.     

Wetting characteristics of aqueous rhamnolipids solutions

The wetting properties of surfactants on solid surfaces form the basis of many industrial and biological processes. The preferential adsorption of the surfactants from aqueous solutions onto solid surfaces alter the adhesion tension of the surface and this behavior may cause partial to complete wetting of the surfaces by the aqueous surfactant solutions. However, different types of surfactants show different wetting characteristics. To study the wetting properties of biologically produced rhamnolipids (RL), advancing contact angles of the aqueous solutions of the RL mixture of R1 and R2 in a ratio of R2/R1=1.1 were measured as a function of surfactant concentration. For a comparison of the wetting performance, sodium dodecyl sulfate (SDS) was chosen as the reference surfactant. A hydrophilic glass surface, a hydrophobic polymer, polyethylene terephthalate (PET), and gold surface were used as the solid surfaces to determine the wetting characteristics of rhamnolipids. At low surfactant concentrations (RL concentration <3x10(-5)M, SDS concentration<3x10(-4)M) contact angle (Theta) varied in a certain range depending on the character of the surfactant interactions with the surface. This was followed by a decrease in contact angle. Parallel to this behavior, at low surfactant concentrations the adhesion tension decreased, then remained constant and an increase at higher surfactant concentrations was obtained on hydrophobic surfaces. On hydrophilic surfaces a steady decrease in adhesion tension was observed with both surfactant solutions.     

Adhesion hysteresis from interdependent capillary and electrostatic forces

Adhesion hysteresis commonly occurs at the nanoscale in humid atmospheres, yet mechanisms are not entirely understood. Here, the adhesion forces between silicon (111) oxide surfaces and tungsten oxide probes have been examined using interfacial force microscopy. The results show that the adhesion forces during surface approach and separation differ not only in magnitude but also in mechanism, arising mainly from capillary and electrostatic forces, respectively. Surface contact leads to a transient intersurface potential on dewetting. This mechanism of adhesion hysteresis differs in not relying singly on hysteretic wetting. Furthermore, by biasing the surfaces, nonadditivity is demonstrated between the capillary and electrostatic forces at the onset of condensation. These results hold important implications on the interpretation of force in nanoprobe geometries in humid atmospheres.     

Wettability conversion from superoleophobic to superhydrophilic on titania/single-walled carbon nanotube composite coatings

Superoleophobic surfaces were demonstrated on perfluorosilane-rendered titania (TiO(2))/single-walled carbon nanotube (SWNT) composite coatings. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations revealed that SWNTs play a key role in the formation of overhanging structures and the nanoscale roughness on the coating surface, which compose the two critical morphologic factors for a superoleophobic surface. The wettability conversion from superoleophobic to superhydrophilic of the composite coatings was realized by the gradual decomposition of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) on the coating surface using UV irradiation. Contact angle measurement on both smooth TiO(2) surface and rough composite coating surface under different UV irradiation time revealed that the wetting behavior of the liquids on the composite coating surface passes from the Cassie to the Wenzel and finally to the inversed-Cassie regime. Different liquids show different irradiation time for the wetting state change. By controlling the UV irradiation dose, liquids with surface tension difference smaller than 5 mN/m can exist in completely converse wetting states on the same coating surface, that is, superphobic for one liquid while superphilic for another with lower surface tension. Mixed organic liquids with different surface tension can be completely separated through a coated grid using this wettability tuning technique.     

Controlling the wettability of hierarchically structured thermoplastics

Surfaces play an important role in defining the properties of materials, controlling wetting, adsorption, or desorption of biomolecules, and sealing/bonding of different materials. We have combined microscale features with plasma-etched nanoscale roughness and chemical modification to tailor the wettability of the substrates. Cyclic olefin polymers and copolymers (COPs/COCs) were processed to make a range of surfaces with controlled superhydrophobic or -hydrophilic properties. The hydrophobic properties of the polymers were increased by the introduction of microstructures of varying geometry and spacing through hot embossing. The COC/COP substrates were functionalized by plasma activation in O(2), CF(4), and a mixture of both gases. The plasma etching introduces nanoscale roughness and also chemically modifies the surface, creating either highly hydrophilic or highly hydrophobic (contact angle >150°) surfaces depending on the gas mixture. The influence of geometry and chemistries was characterized by atomic force microscopy, contact angle measurements, and X-ray photoelectron spectroscopy. Measurements of the contact angle and contact angle hysteresis demonstrated long-term stability of the superhydrophobic/superhydrophilic characteristics (>6 months).     

Nanostructures increase water droplet adhesion on hierarchically rough superhydrophobic surfaces

Hierarchical roughness is known to effectively reduce the liquid-solid contact area and water droplet adhesion on superhydrophobic surfaces, which can be seen for example in the combination of submicrometer and micrometer scale structures on the lotus leaf. The submicrometer scale fine structures, which are often referred to as nanostructures in the literature, have an important role in the phenomenon of superhydrophobicity and low water droplet adhesion. Although the fine structures are generally termed as nanostructures, their actual dimensions are often at the submicrometer scale of hundreds of nanometers. Here we demonstrate that small nanometric structures can have very different effect on surface wetting compared to the large submicrometer scale structures. Hierarchically rough superhydrophobic TiO(2) nanoparticle surfaces generated by the liquid flame spray (LFS) on board and paper substrates revealed that the nanoscale surface structures have the opposite effect on the droplet adhesion compared to the larger submicrometer and micrometer scale structures. Variation in the hierarchical structure of the nanoparticle surfaces contributed to varying droplet adhesion between the high- and low-adhesive superhydrophobic states. Nanoscale structures did not contribute to superhydrophobicity, and there was no evidence of the formation of the liquid-solid-air composite interface around the nanostructures. Therefore, larger submicrometer and micrometer scale structures were needed to decrease the liquid-solid contact area and to cause the superhydrophobicity. Our study suggests that a drastic wetting transition occurs on superhydrophobic surfaces at the nanometre scale; i.e., the transition between the Cassie-Baxter and Wenzel wetting states will occur as the liquid-solid-air composite interface collapses around nanoscale structures. Consequently, water adheres tightly to the surface by penetrating into the nanostructure. The droplet adhesion mechanism presented in this paper gives valuable insight into a phenomenon of simultaneous superhydrophobicity and high water droplet adhesion and contributes to a more detailed comprehension of superhydrophobicity overall.     

Nanoscale analyses of modified polypropylene microtubes internal surface: an approach covering topographical and force spectroscopic parameters

A nanoscale characterization of modified and unmodified polypropylene (PP) microtubes internal surface was performed to investigate their structural, chemical, and physical properties. Nanoroughness, stiffness, elasticity, attraction behavior, adhesion forces, and chemical environment were investigated to test some manufacturer statements regarding Axygen MAXYMum Recovery® products. They announced that this class of material presented special features, originated from a modification to the original PP resin and by using a diamond polished mould, providing lower retention and minor interference on laboratorial tests, such as low roughness and little interaction tendency. Then, in this study, modified and control internal surfaces of PP microtubes were compared by atomic force microscopy, scanning electron microscopy, and Fourier transform infrared spectroscopy. Nanoroughness and force spectroscopy parameters assessed by atomic force microscopy showed out as a sensible and high‐resolution technique, crucial to discriminate differences between the surfaces. This type of investigation can be considered as a promising approach that can be applied to other polymeric systems, considering nanoscale properties, physical/chemical modifications, and as an alternative route for quality control checking concerning polymeric surfaces. Copyright © 2013 John Wiley & Sons, Ltd.     

Fabrication of polymeric replicas of cell surfaces with nanoscale resolution

We report an approach for fabricating biomimetic surface replicas of cells with nanoscale resolution. Fixed cells serve as a template for a two-stage replica molding process. Cast from the template, the impression replica contains a reproduction of cellular topographical features indented into its surface, and cast from the impression replica, the relief replica contains a copy of these features protruding from its surface. Various polymers and cells can be utilized, and scanning electron microscopy, atomic force microscopy, and white light interferometry analyses confirm the replication of nanoscale features. These replicas are useful for investigating cellular function and for biomimetic tissue engineering.     

Correction of random surface roughness on colloidal probes in measuring adhesion

Atomic force microscopes (AFM) are commonly used to measure adhesion at nanoscale between two surfaces. To avoid uncertainties in the contact areas between the tip and the surface, colloidal probes have been used for adhesion measurements. We measured adhesion between glass spheres and silicon (100) surface using colloidal probes of different radii under controlled conditions (relative humidity of < 3%, temperature of 25 +/- 1 degrees C). Results showed that the adhesion forces did not correlate with the radii of the spheres as suggested by elastic contact mechanics theories. Surface roughness and random surface features were found on the surfaces of the colloidal probes. We evaluated various roughness parameters, Rumpf and Rabinovich models, and a load-bearing area correction model in an attempt to correct for the roughness effects on adhesion, but the results were unsatisfactory. We developed a new multiscale contact model taking into account elastic as well as plastic deformation in a successive contacting mode. The new model was able to correct for most of the surface roughness features except for surface ridges with sharp angular features, limited by the spherical asperity assumption made in the model.     

Adhesion between coating layers based on epoxy and silicone

The adhesion between a silicon tie-coat and epoxy primers, used in marine coating systems, has been studied in this work. Six epoxy coatings (with varying chain lengths of the epoxy resins), some of which have shown problems with adhesion to the tie-coat during service life, have been considered. The experimental investigation includes measurements of the surface tension of the tie-coat and the critical surface tensions of the epoxies, topographic investigation of the surfaces of cured epoxy coatings via atomic force microscopy (AFM), and pull-off tests for investigating the strength of adhesion to the silicon/epoxy systems. Calculations for determining the roughness factor of the six epoxy coatings (based on the AFM topographies) and the theoretical work of adhesion have been carried out. The coating surfaces are also characterized based on the van Oss-Good theory. Previous studies on the modulus of elasticity of the polymers involved have also been considered. It was found that adhesion problems might be due to inadequate wetting, the significantly different topographies, and differences in the mechanical strengths of the epoxies. Acid-base interactions calculated from the van Oss-Good theory were found useful in explaining the enhanced adhesion for some epoxy/silicon surfaces.     

Superhydrophobicity on two-tier rough surfaces fabricated by controlled growth of aligned carbon nanotube arrays coated with fluorocarbon

Considerable effort has been expended on theoretical studies of superhydrophobic surfaces with two-tier (micro and nano) roughness, but experimental studies are few due to the difficulties in fabricating such surfaces in a controllable way. The objective of this work is to experimentally study the wetting and hydrophobicity of water droplets on two-tier rough surfaces for comparison with theoretical analyses. To compare wetting on micropatterned silicon surfaces with wetting on nanoscale roughness surfaces, two model systems are fabricated: carbon nanotube arrays on silicon wafers and carbon nanotube arrays on carbon nanotube films. All surfaces are coated with 20 nm thick fluorocarbon films to obtain low surface energies. The results show that the microstructural characteristics must be optimized to achieve stable superhydrophobicity on microscale rough surfaces. However, the presence of nanoscale roughness allows a much broader range of surface design criteria, decreases the contact angle hysteresis to less than 1 degrees , and establishes stable and robust superhydrophobicity, although nanoscale roughness could not increase the apparent contact angle significantly if the microscale roughness dominates.     

The wetting of gold and silicon nanoscale arrays

The relative hydrophobicity of surfaces containing highly regular, nanoscale (<100 nm) topological features (50 nm diameter, 5 nm height, 100 nm spacing) was measured and compared with their flat counterparts. The results are described in the context of both the Wenzel and Cassie models for wetting rough surfaces. Wenzel-type wetting is observed for these high areal density nanostructures (approximately 10(10)/cm2) whose aspect ratios range from 0.2 to 0.7.     

超声刻蚀法构建分级结构的超疏水表面

在湿法刻蚀和超声空化的基础上, 采用超声刻蚀法制备了具有微纳米分级结构的超疏水表面. 以等体积比的硝酸/乙醇(体积分数为4%)和双氧水(质量分数为30%)的混合溶液作为刻蚀剂, 在室温下对60Si2Mn钢、 60^#钢、 T10钢、 Cr06钢、 65Mn钢和硅钢表面超声刻蚀2~10 min, 构建出多种形貌的微纳米分级结构. 上述表面经氟硅烷修饰后具有超疏水性, 水的表观接触角高达157.0°, 155.8°, 157.4°, 154.9°, 157.6°和156.8°, 滚动角分别为6.5°, 19.2°, 6.1°, 7.8°, 6.7°和7.2°. 与常规刻蚀方法相比, 超声刻蚀的化学刻蚀作用因与空化作用耦合而得到强化和改变, 从而在钢表面构建出分级结构. 由于材料表面微结构形貌和固/液界面接触状态不同, 制得的超疏水表面表现出的润湿行为也不同. 超声刻蚀法简单易行, 成本低廉, 适用于其它金属表面构建微纳米分级结构和超疏水表面.     

Influence of surface topography on osteoblast response to fibronectin coated calcium phosphate thin films

The ability to engineer biomaterial surfaces that are capable of a dynamic interaction with cells and tissues is central to the development of medical implants with improved functionality. An important consideration in this regard is the role played by the extracellular proteins that bind to an implant surface in vivo. Deliberate use of an ad-layer of such proteins on an implant surface has been observed to guide and direct cell response. However, the role that changes in surface topography might play in determining the nature of this cell–protein–surface interaction has not been investigated in detail. In this study, calcium phosphate (CaP) thin films have been deposited onto substrates with varying topography such that this is reflected in the (conformal) CaP surface features. A fibronectin (FN) ad-layer was then deposited from solution onto each surface and the response of MG63 osteoblast-like cells investigated. The results revealed that in all cases, the presence of the adsorbed FN layer on the CaP thin films improved MG63 cell adhesion, proliferation and promoted early onset differentiation. Moreover, the nature and scale of the response were shown to be influenced by the underlying CaP surface topography. Specifically, MG63 cell on FN-coated CaP thin films with regular topographical features in the nanometer range showed statistically significant differences in focal adhesion assembly, osteocalcin expression and alkaline phosphase activity compared to CaP thin films that lacked these topographical features. As such, these data indicate that surface topography can be used to further influence cell adhesion and downstream differentiation by enhancing the effects of a surface adsorbed FN layer.     

Influence of nanoscale particle roughness on the stability of pickering emulsions

The wetting behavior of solid surfaces can be altered dramatically by introducing surface roughness on the nanometer scale. Some of nature's most fascinating wetting phenomena are associated with surface roughness; they have inspired both fundamental research and the adoption of surface roughness as a design parameter for man-made functional coatings. So far the attention has focused primarily on macroscopic surfaces, but one should expect the wetting properties of colloidal particles to be strongly affected by roughness, too. Particle wettability, in turn, is a key parameter for the adsorption of particles at liquid interfaces and for the industrially important use of particles as emulsion stabilizers; yet, the consequence of particle roughness for emulsion stability remains poorly understood. In order to investigate the matter systematically, we have developed a surface treatment, applicable to micrometer-sized particles and macroscopic surfaces alike, that produces surface coatings with finely tunable nanoscale roughness and identical surface chemistry. Coatings with different degrees of roughness were characterized with regard to their morphology, charging, and wetting properties, and the results were correlated with the stability of emulsions prepared with coated particles of different roughness. We find that the maximum capillary pressure, a metric of the emulsions' resistance to droplet coalescence, varies significantly and in a nonmonotonic fashion with particle roughness. Surface topography and contact angle hysteresis suggest that particle roughness benefits the stability of our emulsions as long as wetting occurs homogeneously (Wenzel regime), whereas the transition toward heterogeneous wetting (Cassie-Baxter regime) is associated with a loss of stability.     

Visualizing cell extracellular matrix (ECM) deposited by cells cultured on aligned bacteriophage M13 thin films

Topographical features ranging from micro- to nanometers can affect cell orientation and migratory pathways, which are important factors in tissue engineering and tumor migration. In our previous study, a convective assembly of bacteriophage M13 resulted in thin films which could be used to control the alignment of cells. However, several questions regarding its underlying reasons to dictate cell alignment remained unanswered. Here, we further study the nanometer topographical features generated by the bacteriophage M13 crystalline film, which results in the alignment of the cells and extracellular matrix (ECM) proteins. Sequential imaging analyses at micro- and nanoscale levels of aligned cells and fibrillar matrix proteins were documented using scanning electron microscopy and immunofluorescence microscopy. As a result, we observed baby hamster kidney cells with higher degree of alignment on the ordered M13 substrates than NIH-3T3 fibroblasts, a difference which could be attributed to the intrinsic nature of the cells' production of ECM proteins. The results from this study provide a crucial insight into the topographical features of a biological thin film, which can be utilized to control the orientation of cells and surrounding ECM proteins.     

Patterned nonadhesive surfaces: superhydrophobicity and wetting regime transitions

Nonadhesive and water-repellent surfaces are required for many tribological applications. We study mechanisms of wetting of patterned superhydrophobic Si surfaces, including the transition between various wetting regimes during microdroplet evaporation in environmental scanning electron microscopy (ESEM) and for contact angle and contact angle hysteresis measurements. Wetting involves interactions at different scale levels: macroscale (water droplet size), microscale (surface texture size), and nanoscale (molecular size). We propose a generalized formulation of the Wenzel and Cassie equations that is consistent with the broad range of experimental data. We show that the contact angle hysteresis involves two different mechanisms and how the transition from the metastable partially wetted (Cassie) state to the homogeneously wetted (Wenzel) state depends upon droplet size and surface pattern parameters.