E Radeva

Study and application of glow discharge polymer layers as humidity sensors

Abstract Some parameters of glow discharge polymer layers of acetonitrile, o -phthalic acid dinitrile, malonic acid dinitrile and hexamethyldisiloxane have been studied with the aim of preparing a sensor for measuring the relative humidity of ambient air. The sensor is based on the piezoresonance effect and is obtained from a quartz crystal with an AT-cut. The resonance frequency changes are different depending on the absorbed gases, and could be used as an indicator of the kind of gas, the percentage of water molecules in ambient air, etc. Glow discharge poly(hexamethyldisiloxane) has proved to be the most suitable polymer for the preparation of humidity sensors.

Thin plasma-polymerized layers of hexamethyldisiloxane for acoustoelectronic humidity sensors

Plasma-polymerized layers are obtained from hexamethyldisiloxane (HMDSO). Using scanning electron microscopy (SEM), the effect of glow discharge duration and modification with ammonia on polymer surface and morphology is studied. It was shown that polymer layers were uniform and without defects. The effect of some plasma polymerization parameters on the humidity-sorptive properties of the polymers are investigated by measuring the mass changes as a result of water sorption. For this purpose, a system consisting of a sensor array of quartz crystal microbalances coated with plasma polymer films is used. The resonant frequency of the coated quartz resonators decreases with increasing relative humidity. The results showed that the increase in the glow discharge current density, gas phase pressure, layer thickness and modification with ammonia in plasma, increases the sensitivity of the polymers to humidity. The study shows that plasma-polymerized HMDSO layers may be used as a hygroscopic element for sensing the humidity of ambient air and therefore, for the development of quartz resonator sensors.

High-resolution humidity measurements with surface transverse wave based resonant devices. Applications to wireless remote sensing

Abstract Results from an experimental study on the humidity sorptive properties of plasma polymer-coated surface transverse wave (STW) resonators, for possible applications in high-resolution humidity sensors, are presented. We used 767- and 998-MHz STW resonators covered with hexamethyldisiloxane (HMDSO) polymer films, obtained in a glow discharge process. The STW resonator frequency shift versus the sorbed water vapor mass was measured at controlled relative humidity (RH) and temperature. An average sensitivity of 1.4 ppm/%change in RH was obtained which was found to be 4–10 times higher compared to 14-MHz bulk crystal resonators coated with the same polymer. HMDSO coated and uncoated STW resonators featuring high- Q , low-loss, excellent temperature stability and low aging are implemented in a system for wireless transmission of humidity and temperature data over a 1-GHz radio channel.

Study of Organosilicon Plasma Polymer Used in Composite Layers with Biomedical Application

In this work we study the ability of plasma polymer (PP) films obtained from hexamethyldisiloxane (HMDS) on silica glass (SG) to induce hydroxyapatite (HA)‐based composite layers from a mixture of simulated body fluid (SBF) and clear solution of detonation nanodiamond (DND) by a biomimetic process. The grown composites (PPHMDS/HADND) were studied by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and Rutherford backscattering (RBS) techniques. FTIR spectra of the PPHMDS indicated diminishing of the polymer characteristic bands when the polymer is immersed in DND clear solution. Furthermore, after sample immersion in the SBF‐DND mixture, the FTIR spectra showed the presence of carbonate‐containing HA through the characteristic vibration modes of P‐O in the phosphate group and C‐O in the carbonate group. The formation of HA layers, rich in silica and/or carbon was confirmed by RBS and SEM. The cell viability measured after 7 days on the polymer surface is more then 95% for...

Effect of nanodiamond modification of siloxane surfaces on stem cell behaviour

Mesenchymal stem cells (MSCs) hold a great promise for use in many cell therapies and tissue engineering due to their remarkable potential to replicate indefinitely and differentiate into various cell types. Many efforts have been put to study the factors controlling stem cell differentiation. However, still little knowledge has been gained to what extent biomaterials properties influence stem cell adhesion, growth and differentiation. Research utilizing bone marrow-derived MSCs has concentrated on development of specific materials which can enhance specific differentiation of stem cells e.g. osteogenic and chondrogenic. In the present work we have modified an organosilane, hexamethyldisiloxane (HMDS) with detonation nanodiamond (DND) particles aiming to improve adhesion, growth and osteodifferentiation of rat mesenchymal stem cells. HMDS/DND films were deposited on cover glass using two approaches: premixing of both compounds, followed by plasma polymerization (PP) and PP of HMDS followed by plasma deposition of DND particles. We did not observe however an increase in rMSCs adhesion and growth on DND-modified PPHMDS surfaces compared to unmodified PPHMDS. When we studied alkaline phosphatase (ALP) activity, which is a major sign for early osteodifferentiation, we found the highest ALP activity on the PPHMDS/DND material, prepared by consequent deposition while on the other composite material ALP activity was the lowest. These results suggested that DND-modified materials were able to control osteodifferention in MSCs depending on the deposition approach. Modification of HMDS with DND particles by consequent plasma deposition seems to be a promising approach to produce biomaterials capable to guide stem cell differentiation toward osteoblasts and thus to be used in bone tissue engineering.

Increased elastic modulus of plasma polymer coatings reinforced with detonation nanodiamond particles improves osteogenic differentiation of mesenchymal stem cells

In the present study we demonstrated that composite PPHMDS/DND coatings with elastic moduli close to those of mature bone tissue (0.2-2.8 GPa) stimulated growth and osteogenic differentiation of human adipose-derived mesenchymal stem cells (hADMSCs). Composite coatings were prepared by a method of plasma polymerization (PP) where detonation nanodiamond (DND) particles in different amounts (0.1, 0.5, and 1 mg/mL) were added to hexamethyldisiloxane (HMDS) before plasma deposition. This method allows variation only in the reduced elastic modulus (Er´) with increase in the particle concentration, while the other surface properties, including surface wettability and topography, did not change. The response of hAD-MSCs to the increasing stifness showed an effect on adhesion and osteogenic differentiation but not on cell proliferation. Matrix mineralization and cell spreading were maximized on PPHMDS/DND coatings with the highest elastic modulus (2.826 GPa), while the differences in proliferation rates among the samples were negligible. In general, PPHMDS/DND coatings provide better conditions for growth and osteogenic differentiation of hAD-MSCs in comparison to glass coverslips, confirming their suitability for osteo-integration applications. Additionally, our findings support the hypothesis that biomaterials with elasticity similar to that of the native tissue can improve the differentiation potential of mesenchymal stem cells.

Optical characterization of composite layers prepared by plasma polymerization

Thin composite layers from polymer/nanoparticles (Ag-nanoparticles and detonation nanodiamonds) were prepared by plasma polymerization process on the base of hexamethyldisiloxane. The variation of the layer composition was achieved by changing the type of nanoparticles. The optical measurement techniques used were UV-VIS-NIR ellipsometry (SE), Fourier-transformed infrared spectroscopy (FTIR) and Raman spectroscopy. The values of the refractive index determined are in the range 1.30 to 1.42. All samples are transparent with transmission between 85-95% and very smooth. The change in Raman and FTIR spectra of the composites verify the expected bonding between polymer and diamond nanoparticles due to the penetration of the fillers in the polymer matrix. The comparison of the spectra of the corresponding NH3 plasma treated composites revealed that the composite surface becomes more hydrophilic. The obtained results indicate that preparation of layers with desired compositions is possible at a precise control of the detonation nanodiamond materials.

Plasma deposited composite coatings to control biological response of osteoblast-like MG-63 cells

The successful osseointegration of a bone implant is greatly dependent on its ability to support cellular adhesion and functions. Deposition of thin composite coatings onto the implant surface is a promising approach to improve interactions with cells without compromising implant bulk properties. In this work, we have developed composite coatings, based on hexamethyldisiloxane (HMDS) and detonation nanodiamond (DND) particles and have studied adhesion, growth and function of osteoblast-like MG-63 cells. PPHMDS/DND composites are of interest for orthopedics because they combine superior mechanical properties and good biocompatibility of DND with high adherence of HMDS to different substrata including glass, metals and plastics. We have used two approaches of the implementation of DND particles into a polymer matrix: pre-mixture of both components followed by plasma polymerization and layer-by-layer deposition of HMDS and DND particles and found that the deposition approach affects significantly the surface properties of the resulting layers and cell behaviour. The composite, prepared by subsequent deposition of monomer and DND particles was hydrophilic, with a rougher surface and MG-63 cells demonstrated better spreading, growth and function compared to the other composite which was hydrophobic with a smooth surface similarly to unmodified polymer. Thus, by varying the deposition approach, different PPHMDS/DND composite coatings, enhancing or inhibiting osteoblast adhesion and functions, can be obtained. In addition, the effect of fibronectin pre-adsorption was studied and was found to increase greatly MG-63 cell spreading.