M.L. González-Martín

Ultrasmall Liquid Droplets on Solid Surfaces: Production, Imaging, and Relevance for Current Wetting Research

The investigation of micro- and nanoscale droplets on solid surfaces offers a wide range of research opportunities both at a fundamental and an applied level. On the fundamental side, advances in the techniques for production and imaging of such ultrasmall droplets will allow wetting theories to be tested down to the nanometer scale, where they predict the significant influence of phenomena such as the contact line tension or evaporation, which can be neglected in the case of macroscopic droplets. On the applied side, these advances will pave the way for characterizing a diverse set of industrially important materials such as textile or biomedical micro- and nanofibers, powdered solids, and topographically or chemically nanopatterned surfaces, as well as micro-and nanoscale devices, with relevance in diverse industries from biomedical to petroleum engineering. Here, the basic principles of wetting at the micro- and nanoscales are presented, and the essential characteristics of the main experimental techniques available for producing and imaging these droplets are described. In addition, the main fundamental and applied results are reviewed. The most problematic aspects of studying such ultrasmall droplets, and the developments that are in progress that are thought to circumvent them in the coming years, are highlighted.

Bactericidal behaviour of Ti6Al4V surfaces after exposure to UV-C light.

TiO(2)-coated biomaterials that have been excited with UV irradiation have demonstrated biocidal properties in environmental applications, including drinking water decontamination. However, this procedure has not been successfully applied towards the killing of pathogens on medical titanium-based implants, mainly because of practical concerns related to irradiating the inserted biomaterial in situ. Previous researchers assumed that the photocatalysis on the TiO(2) surface during UV application causes the bactericidal effects. However, we show that a residual post-irradiation bactericidal effect exists on the surface of Ti6Al4V, not related with photocatalysis. Using a combination of staining, serial dilutions, and a biofilm assay, we show a significant and time-dependent loss in viability of different bacterial strains of Staphylococcus epidermidis and Staphylococcus aureus on the post-irradiated surface. Although the duration of this antimicrobial effect depends on the strains selected, our experiments suggest that the effect lasts at least 60 min after surface irradiation. The origin of such phenomena is discussed in terms of the physical properties of the irradiated surfaces, which include the emission of energy and changes in surfaces charge occurring during electron-hole recombination processes. The method here proposed for the preparation of antimicrobial titanium surfaces could become especially useful in total implant surgery for which the antimicrobial challenge is mainly during or shortly after surgery.

Wettability and surface free energy of zirconia ceramics and their constituents

Measurements of contact angle for water, glycerol, formamide and diiodomethane on surfaces of zirconia ceramic stabilized with 4% magnesia, zirconia ceramic stabilized with 3% yttria, zirconia ceramic stabilized with 5% yttria, tetragonal-Y-zirconia ceramic, cubic-Y-zirconia ceramic, zirconium (IV) oxide, aluminum oxide, magnesium oxide and yttrium oxide were conducted. Using the values of contact angle, the total surface free energy of these solids, and its components resulting from different kinds of intermolecular interactions were calculated. It was found that the Lifshitz-van der Waals component of the surface free energy of zirconia ceramics only slightly depends on the amount and kind of metal oxides present in the ceramics. The acid-base component of all the studied solids is lower than 13 mJ/m2, showing a small dependence on the kind of metal oxide. For all samples studied the electron-donor parameter of the acid-base components of the surface free energy is many times higher that electron-acceptor one. It was stated that the acid-base component probably depends on the density of OH groups on the surface of the solids studied.

On the role of RhoA/ROCK signaling in contact guidance of bone-forming cells on anisotropic Ti6Al4V surfaces.

Patterned surfaces direct cell spatial dynamics, yielding cells oriented along the surface geometry, in a process known as contact guidance. The Rho family of GTPases controls the assembly of focal adhesions and cytoskeleton dynamics, but its role in modulating bone-cell alignment on patterned surfaces remains unknown. This article describes the interactions of two human cell types involved in osseointegration, specifically mesenchymal stem cells and osteoblasts, with submicron- or nano-scale Ti6Al4V grooved surfaces generated by mechanical abrasion. The surface chemistry of the alloy was not affected by grinding, ensuring that the differences found in cellular responses were exclusively due to changes in topography. Patterned surfaces supported cell growth and stimulated mesenchymal stem cell viability. Anisotropic surfaces promoted cell orientation and elongation along the grates. Both cell types oriented on nanometric surfaces with grooves of 150 nm depth and 2 μm width. The number of aligned cells increased by approximately 30% on submicrometric grooves with sizes of about 1 μm depth and 10 μm width. Cells were treated with drugs that attenuate the activities of the GTPase RhoA and one of its downstream effectors, Rho-associated kinase (ROCK), and contact guidance of treated cells on the grooved surfaces was investigated. The data indicate that the RhoA/ROCK pathway is a key modulator of both mesenchymal stem cell and osteoblast orientation on nanometric surface features. RhoA and its effector participate in the alignment of mesenchymal stem cells on submicrometric grooves, but not of osteoblasts. These findings show that RhoA/ROCK signaling is involved in contact guidance of bone-related cells on metallic substrates, although to a varying extent depending on the specific cell type and the dimensions of the pattern.

Analysis of the adsorption isotherms of a non-ionic surfactant from aqueous solution onto activated carbons

Abstract The adsorption isotherms at 20°C onto four activated carbons of the non-ionic surfactant Triton X-100 from aqueous solution have been studied over a wide concentration range. The adsorption was explained using one or a combination of two Langmuir equations, depending on the equilibrium concentration range studied. A description of the adsorbed layer was constructed on the basis of the information obtained from the isotherms and assuming the model of van Oss et al. for the interfacial interaction between adsorbent and adsorbate. The results indicate that there are at least two kinds of interactions, the first related to a direct interaction between the activated carbon surface and adsorbate molecules, and the second mainly due to the interaction between surfactant molecules at the adsorbent–solution interface leading to the formation of interfacial aggregates.

Nitrogen adsorption isotherms on carbonaceous materials. Comparison of BET and Langmuir surface areas

Abstract This study compares the Brunauer–Emmett–Teller (BET) and Langmuir equations when applied to N2 adsorption isotherms at 77 K on various carbon blacks and activated carbons. Adsorbent samples varied greatly in the degree of development of their surface area and porosity. The activated carbons were more microporous and less mesoporous than the carbon blacks. The equations were applied up to p/p0=0.20, 0.30, and 0.40. The values of the lineal correlation coefficient as a rule were above 0.990. From the derived BET (SBET) and Langmuir (SL) surface areas, the conversion factor (α) of SL to SBET was calculated. The α mean value (ᾱ) and the standard deviation coefficient (σ) were also obtained. The N2 isotherms fit better to the Langmuir equation for the activated carbons and to the BET equation for the carbon blacks. The factor α was markedly higher than the unit for all carbons. It increased significantly with increasing p/p0 range. The α values were closer for each series of carbons, and larger for the carbon blacks than for the activated carbons; in spite of the fact that the carbon blacks were more mesoporous. As shown by the ᾱ increments, the dependence of ᾱ on the p/p0 range was less strong for the activated carbons for the two narrowest p/p0 ranges. The σ values indicated that the influence of the type of carbon on ᾱ was greater for the carbon blacks when fitting up to p/p0=0.30 or 0.40.

The contribution of double layers to the free energy of interactions in the cassiterite-SDS solution system

Abstract From the values of contact angles for water, glycerol and diiodomethane taken from the literature. the Lifshitzvan der Waals, electron-acceptor and electron-donor parameters corresponding to the polar head of the surface free energy of sodium dodecyl sulphate (SDS[pwere determined and compared with those found in the literature. From these values, the Lifshitz-van der Waals and acid-base components of the free energy of interaction between SDS molecules through water were evaluated. The electrostatic component of the free energy was estimated from the ψ value for SDS found in the literature. These calculations allow the estimation of the total free energy of interaction between the SDS apolar and polar moieties through water on the basis of two models for interactions: plane-plane and cylinder cylinder. The results were compared with data from the literature, showing good agreement (for electrolyte of 0.1 M ionic strength) for both models, and with the value obtained from CMC data. However, for other ionic strengths some differences are observed. The free energy of interaction between SDS and cassiterite through water was evaluated from the components of the surface free energy for both the apolar and the polar moieties of SDS and its ψ value, and from the Lifshitz-van der Waals and acid-base components of the surface free energy of cassiterite with different surface treatments, taken from the literature. On the basis of these calculations, it was possible to infer the main influence of pH on the SDS-cassiterite interactions: they are predominantly electrostatic forces at low pH, and acid base forces at a pH corresponding to the ZPC. Also, at high pH, when negative charges are present at the cassiterite-water interface, SDS adsorption onto cassiterite is possible only by acid-base interactions between the apolar chain of SDS and the cassiterite surface.

Experimental analysis of the influence of surface topography on the adhesion force as measured by an AFM

Force curves have been acquired using an atomic force microscope (AFM) on homogeneous microspheres of three different materials (latex, glass and yttria), in order to study the possible influence of the surface topography/geometry on the adhesion force as measured by an AFM. Forces were measured in regions at the top of the spheres ( ≈ 90°), at half-heights ( ≈ 0°) and in an intermediate region between these two ( ≈ 45°), where the angle is measured from the equatorial plane of the sphere to its polar axis. A very irregular and non-reproducible behaviour was found at ≈ 0°, so only the other two regions were quantitatively analysed. For all the three materials, a much smaller adhesion force was obtained in the region corresponding to ≈ 45° as compared to ≈ 90°. Moreover, a quite similar adhesion decrease ratio of about 1.60 ± 0.5 was obtained for all the three materials, which may suggest that the observed behavior might be due to geometrical factors. This observed influence could, in part, explain the observed heterogeneity in adhesion maps of microbial cells reported in the literature. The influence of the surface roughness is also discussed and it seems to result in a poor reproducibility of force curves.

AFM probing in aqueous environment of Staphylococcus epidermidis cells naturally immobilised on glass: Physico-chemistry behind the successful immobilisation

AFM probing of microbial cells in liquid environments usually requires them to be physically or chemically attached to a solid surface. The fixation mechanisms may influence the nanomechanical characterization done by force curve mapping using an AFM. To study the response of a microbial cell surface to this kind of local measurement this study attempts to overcome the problem associated to the uncertainties introduced by the different fixation treatments by analysing the surface of Staphylococcus epidermidis cells naturally (non-artificially mediated) immobilised on a glass support surface. The particularities of this natural bacterial fixation process for AFM surface analysis are discussed in terms of theoretical predictions of the XDLVO model applied to the systems bacteria/support substratum and bacteria/AFM tip immersed in water. In this sense, in the first part of this study the conditions for adequate natural fixation of three S. epidermidis strains have been analyzed by taking into account the geometries of the bacterium, substrate and tip. In the second part, bacteria are probed without the risk of any possible artefacts due to the mechanical or chemical fixation procedures. Forces measured over the successfully adhered cells have (directly) shown that the untreated bacterial surface suffers from a combination of both reversible and non-reversible deformations during acquisition of force curves all taken under the same operational conditions. This is revealed directly through high-resolution tapping-mode imaging of the bacterial surface immediately following force curve mapping. The results agree with the two different types of force curves that were repeatedly obtained. Interestingly, one type of these force curves suggests that the AFM tip is breaking (rather than pushing) the cell surface during acquisition of the force curve. In this case, adhesive peaks were always observed, suggesting a mechanical origin of the measured pull-off forces. The other type of force curves shows no adhesive peaks and exhibits juxtaposing of approaching and retraction curves, reflecting elastic deformations.

Flavonoid-Modified Surfaces: Multifunctional Bioactive Biomaterials with Osteopromotive, Anti-Inflammatory, and Anti-Fibrotic Potential

Flavonoids are small polyphenolic molecules of natural origin with antioxidant, anti-inflammatory, and antibacterial properties. Here, a bioactive surface based on the covalent immobilization of flavonoids taxifolin and quercitrin on titanium substrates is presented, using (3-aminopropyl)triethoxysilane (APTES) as coupling agent. FTIR and XPS measurements confirm the grafting of the flavonoids to the surfaces. Using 2-aminoethyl diphenylborinate (DPBA, a flavonoid-specific dye), the modified surfaces are imaged by fluorescence microscopy. The bioactivity of the flavonoid-modified surfaces is evaluated in vitro with human umbilical cord derived mesenchymal stem cells (hUC-MSCs) and human gingival fibroblasts (HGFs) and compared to that of simple flavonoid coatings prepared by drop casting. Flavonoid-modified surfaces show anti-inflammatory and anti-fibrotic potential on HGF. In addition, Ti surfaces covalently functionalized with flavonoids promote the differentiation of hUC-MSCs to osteoblasts--enhancing the expression of osteogenic markers, increasing alkaline phosphatase activity and calcium deposition; while drop-casted surfaces do not. These findings could have a high impact in the development of advanced implantable medical devices like bone implants. Given the broad range of bioactivities of flavonoid compounds, these surfaces are ready to be explored for other biomedical applications, e.g., as stent surface or tumor-targeted functionalized nanoparticles for cardiovascular or cancer therapies.