Ilya Reviakine

Hearing what you cannot see and visualizing what you hear: interpreting quartz crystal microbalance data from solvated interfaces.

Over the last 2 decades, the quartz crystal microbalance (QCM or QCM-D) has emerged as a versatile tool for investigating soft and solvated interfaces between solid surfaces and bulk liquids because it can provide a wealth of information about key structural and functional parameters of these interfaces. In this Feature, we offer QCM users a set of guidelines for interpretation and quantitative analysis of QCM data based on a synthesis of well-established concepts rooted in rheological research of the last century and of new results obtained in the last several years.

Model-Independent Analysis of QCM Data on Colloidal Particle Adsorption

Quartz crystal microbalance (QCM) is widely used for studying soft interfaces in liquid environment. Many of these interfaces are heterogeneous in nature, in the sense that they are composed of discrete, isolated entities adsorbed at a surface. When characterizing such interfaces, one is interested in determining parameters such as surface coverage and size of the surface-adsorbed entities. The current strategy is to obtain this information by fitting QCM data--shifts in resonance frequency, DeltaF, and bandwidth, DeltaGamma--with the model derived for smooth, homogeneous films using the film acoustic thickness and shear elastic moduli as fitting parameters. Investigating adsorption of liposomes and icosahedral virus particles on inorganic surfaces of titania and gold, we demonstrate that the predictions of this model are at variance with the experimental observations. In particular, while the model predicts that the ratio between the bandwidth and frequency shifts, DeltaGamma/DeltaF (the Df ratio), should increase with both surface coverage and particle size, we observe that this ratio increases with increasing particle size but decreases with increasing surface coverage, demonstrating that QCM response in heterogeneous films, such as those composed of adsorbed colloidal particles, does not conform with the predictions of the homogeneous film model. Employing finite element method (FEM) calculations, we show that hydrodynamic effects are the cause of this discrepancy. Finally, we find that the size of the adsorbed colloidal particles can be recovered from a model-independent analysis of the plot of the DeltaGamma/DeltaF ratio versus the frequency shift on many overtones.

Dissipation in Films of Adsorbed Nanospheres Studied by Quartz Crystal Microbalance (QCM)

The quartz crystal microbalance (QCM) has become a popular method to study the formation of surface-confined films that consist of discrete biomolecular objects--such as proteins, phospholipid vesicles, virus particles--in liquids. The quantitative interpretation of QCM data--frequency and bandwidth (or, equivalently, dissipation) shifts--obtained with such films is limited by the lack of understanding of the energy dissipation mechanisms that operate in these films as they are sheared at megahertz frequencies during the QCM experiment. Here, we investigate dissipation mechanisms in such films experimentally and by finite-element method (FEM) calculations. Experimentally, we study the adsorption of globular proteins and virus particles to surfaces with various attachment geometries: direct adsorption to the surface, attachment via multiple anchors, or attachment via a single anchor. We find that the extent of dissipation caused by the film and the evolution of dissipation as a function of surface coverage is not dependent on the internal properties of these particles but rather on the geometry of their attachment to the surface. FEM calculations reproduce the experimentally observed behavior of the dissipation. In particular, a transient maximum in dissipation that is observed experimentally is reproduced by the FEM calculations, provided that the contact zone between the sphere and the surface is narrow and sufficiently soft. Both a small-angle rotation of the sphere in the flow field of the background fluid (rocking) and a small-amplitude slippage (sliding) contribute to the dissipation. At high coverage, lateral hydrodynamic interactions between neighboring spheres counteract these modes of dissipation, which results in a maximum in dissipation at intermediate adsorption times. These results highlight that, in many scenarios of biomolecular adsorption, the dissipation is not primarily determined by the adsorbate itself, but rather by the link by which it is bound to the substrate.

Hemocompatibility study of a bacterial cellulose/polyvinyl alcohol nanocomposite

Bacterial cellulose (BC) has been suggested to be a suitable biomaterial for the development of cardiovascular grafts. The combination of BC with polyvinyl alcohol (PVA) results in nanocomposites with improved properties. Surprisingly, there are very few studies on the BC-blood interaction. This is the focus of this paper. We present the first thorough assessment of the hemocompatibility of the BC/PVA nanocomposite. Whole blood clotting time, plasma recalcification, Factor XII activation, platelet adhesion and activation, hemolytic index and complement activation are all determined. The platelet activation profiles on BC and BC/PVA surfaces are comprehensively characterized. BC and BC/PVA outperformed ePTFE--used as a point of comparison--thus evidencing their suitability for cardiovascular applications.

Time-dependent release of growth factors from implant surfaces treated with plasma rich in growth factors†‡

Plasma rich in growth factors (PRGFs) technology is an autologous platelet-rich plasma approach that provides a pool of growth factors and cytokines that have been shown to increase tissue regeneration and accelerate dental implant osseointegration. In this framework, the spatiotemporal release of growth factors and the establishment of a provisional fibrin matrix are likely to be key aspects governing the stimulation of the early phases of tissue regeneration around implants. We investigated the kinetics of growth factor release at implant surfaces functionalized either with PRGFs or platelet-poor plasma and correlated the results obtained with the morphology of the resulting interfaces. Our main finding is that activation and clot formation favors longer residence times of the growth factors at the interfaces studied, probably due to their retention in the adsorbed fibrin matrix. The concentration of the platelet-derived growth factors above the interfaces becomes negligible after 2-4 days and is significantly higher in the case of activated interfaces than in the case of nonactivated ones, whereas that of the plasmatic hepatocyte growth factor is independent of platelet concentration and activation, and remains significant for up to 9 days. Platelet-rich plasma preparations should be activated to permit growth factor release and thereby facilitate implant surface osseointegration.

Platelet activation profiles on TiO2: effect of Ca2+ binding to the surface.

Surface ion equilibrium is hypothesized to play an important role in defining the interactions between foreign materials and biological systems. In this study, we compare two surfaces with respect to their ability to activate adhering platelets. One is a commonly used implant material TiO2, which binds Ca2+, and the other one is glass, which does not. We show, that in the presence of Ca2+, TiO2 acts as an agonist, activating adhering platelets and causing the expression on their surface of two well-known activation markers, CD62P (P-selectin) and CD63. On the contrary, in the absence of Ca2+, platelets adhering on TiO2 express only one of the two markers, CD63. Platelets adhering on glass, as well as platelets challenged with soluble agonists in solution, express both markers independently of whether Ca2+ is present or not. The expression of CD62P and CD63 is indicative of the exocytosis of the so-called α- and dense granules, respectively. It is a normal response of platelets to activation. Differences in the expression profiles of these two markers point to differential regulation of the exocytosis of the two kinds of granules, confirming the recent notion that platelets can tune their microenvironment in a trigger-specific fashion.

Quartz crystal microbalance based on torsional piezoelectric resonators

A quartz crystal microbalance (QCM) is described, which is based on a torsional resonator, rather than a conventional thickness-shear resonator. Typical applications are measurements of film thickness in the coating industry and monitoring of biofouling. The torsional QCM is about a factor of 100 less sensitive than the conventional QCM. On the other hand, it can probe film thicknesses in the range of hundreds of microns, which is impossible with the conventional QCM due to viscoelastic artifacts. Data acquisition and data analysis proceed in analogy to the conventional QCM. An indicator of the materials softness can be extracted from the bandwidth of the resonance. Within the small-load approximation, the frequency shift is independent of whether the sample is applied to the face or to the side of the cylinder. Details of the geometry matter if the viscoelastic properties of the sample are of interest.

Platelet Activation Profiles on TiO 2 : Effect of Ca 2+ Binding to the Surface

Surface ion equilibrium is hypothesized to play an important role in defining the interactions between foreign materials and biological systems. In this study, we compare two surfaces with respect to their ability to activate adhering platelets. One is a commonly used implant material TiO2, which binds Ca2+, and the other one is glass, which does not. We show, that in the presence of Ca2+, TiO2 acts as an agonist, activating adhering platelets and causing the expression on their surface of two well-known activation markers, CD62P (P-selectin) and CD63. On the contrary, in the absence of Ca2+, platelets adhering on TiO2 express only one of the two markers, CD63. Platelets adhering on glass, as well as platelets challenged with soluble agonists in solution, express both markers independently of whether Ca2+ is present or not. The expression of CD62P and CD63 is indicative of the exocytosis of the so-called α- and dense granules, respectively. It is a normal response of platelets to activation. Differences in the expression profiles of these two markers point to differential regulation of the exocytosis of the two kinds of granules, confirming the recent notion that platelets can tune their microenvironment in a trigger-specific fashion.

Platelet Activation Profiles on TiO2

Surface ion equilibrium is hypothesized to play an important role in defining the interactions between foreign materials and biological systems. In this study, we compare two surfaces with respect to their ability to activate adhering platelets. One is a commonly used implant material TiO2, which binds Ca2+, and the other one is glass, which does not. We show, that in the presence of Ca2+, TiO2 acts as an agonist, activating adhering platelets and causing the expression on their surface of two well-known activation markers, CD62P (P-selectin) and CD63. On the contrary, in the absence of Ca2+, platelets adhering on TiO2 express only one of the two markers, CD63. Platelets adhering on glass, as well as platelets challenged with soluble agonists in solution, express both markers independently of whether Ca2+ is present or not. The expression of CD62P and CD63 is indicative of the exocytosis of the so-called α- and dense granules, respectively. It is a normal response of platelets to activation. Differences in the expression profiles of these two markers point to differential regulation of the exocytosis of the two kinds of granules, confirming the recent notion that platelets can tune their microenvironment in a trigger-specific fashion.

Use of Torsional Acoustic Resonators for Microweighing, Polymer Rheology, and Monitoring of Biofilm Formation

The work describes the use of readily available and economical quartz crystal torsional resonators for microweighing. The resonance behavior was probed by impedance analysis. Torsional resonators are straightforwardly adapted to the detection electronics usually employed for driving a quartz crystal microbalance. The analysis proceeds as with quartz crystal resonators. The measurement of the viscoelastic parameters of a silicone oil gave results consistent with the corresponding low-frequency data. Thickness determinations on a film formed from a polymer dispersion have confirmed that torsional resonators indeed function as microbalances when the sample is thin enough. Biofilm formation is easily monitored, as well.