Papers by Author: J. Schoonman

Abstract: In situ measurements of electrospray droplet sizes and velocities were performed by Phase Doppler Anemometry during Electrostatic Spray Deposition (ESD) of calcium phosphate (CaP) coatings. Numerous processing parameters were varied (nozzle-to-substrate distance, deposition temperature, nozzle geometry, and composition of the precursor solution), whereafter the morphological characteristics of these ESD-derived CaP coatings were correlated with measured droplet characteristics. Equal droplet sizes and velocities were measured for nozzle-to-substrate distances up to 40 mm and deposition temperatures up to 400 °C, indicating that electrospray droplets did not shrink at all during droplet flight using an involatile solvent butyl carbitol with a high boiling point (Tb = 231 °C). Nevertheless, coatings with considerably different surface morphologies were obtained under these conditions, varying from microporous structures with coalesced pore walls to morphologies revealing isolated rings on top of dense or grainy underlayers. The chemical composition of the precursor solutions and the mixing characteristics of the calcium and phosphate precursor components strongly influenced the initial droplet sizes, precipitation kinetics of the CaP solute, and subsequent coating morphology. Unique, reticular coating morphologies were deposited at a deposition rate of 3.2 µm/hour, which have a graded structure consisting of a dense underlayer, a submicron-porous intermediate layer, and a roughened toplayer revealing droplet-derived features such as isolated rings or coalesced, hollow surface pits.

Abstract: The dissolution and precipitation behavior of various porous, ESD-derived calcium phosphate coatings was investigated a) in vitro after soaking in Simulated Body Fluid (SBF) for several time periods (2, 4, 8, and 12 weeks), and b) in vivo after subcutaneous implantation in the back of goats for identical time periods. At the end of these studies, the physicochemical properties of the coated substrates were characterized by means of Scanning Electron Microscopy (SEM), XRay Diffraction (XRD), Fourier-Transform InfraRed spectroscopy (FTIR) and Energy Dispersive Spectroscopy (EDS). Moreover, part of the implants was prepared for light microscopical evaluation of the tissue response. In vitro, a highly bioactive behavior was observed for all ESD-coatings, characterized by the deposition of a thick and homogeneous carbonate hydroxyapatite precipitation layer on top of the porous coatings. Regarding the in vivo study, no adverse tissue reactions (toxic effects/inflammatory cells) were observed using light microscopy, and all coatings became surrounded by a thin, dense fibrous tissue capsule after implantation. The ESD-coatings degraded gradually at a dissolution rate depending on the specific chemical phase, thereby enabling synthesis of CaP coatings with a tailored degradation rate.

Abstract: The recently developed Electrostatic Spray Deposition (ESD) technique was used in order to deposit calcium phosphate (CaP) coatings onto various substrates, since this technique enables deposition of inorganic thin films with a variety of morphological and chemical properties. In the present study, the relationship between various deposition parameters and the physicochemical properties of deposited coatings was investigated in order to be able to deposit CaP coatings with defined chemical and morphological properties using ESD. The results showed that the chemical characteristics of the coatings were determined by both the composition of the precursor solutions (solution acidity, absolute and relative precursor concentrations) and apparatus-related parameters, such as the liquid flow rate and the nozzle-tosubstrate distance. By varying these parameters, several crystal phases and phase mixtures were obtained (carbonate apatite, carbonated hydroxyapatite, a/b-tricalciumphosphate, monetite, b/g-pyrophosphate, metaphosphate, calcite/calcium oxide). Coating morphology was also shown to be strongly dependent on several deposition parameters. A wide range of coating morphologies was obtained, varying from relatively dense to highly porous, reticular coating surface architectures.