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Plasma-Enhanced Chemical Vapor Deposition (PE-CVD) yields better Hydrolytical Stability of Biocompatible SiOx Thin Films on Implant Alumina Ceramics compared to Rapid Thermal Evaporation Physical Vapor Deposition (PVD) Frederik Böke,*,† Ignacio Giner,*,‡ Adrian Keller,‡ Guido Grundmeier,‡ and Horst Fischer† †
Department of Dental Materials and Biomaterials Research, RWTH Aachen University Hospital, Pauwelsstrasse 30, 52074 Aachen, Germany ‡ Technical and Macromolecular Chemistry, University of Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany S Supporting Information *
ABSTRACT: Densely sintered aluminum oxide (α-Al2O3) is chemically and biologically inert. To improve the interaction with biomolecules and cells, its surface has to be modified prior to use in biomedical applications. In this study, we compared two deposition techniques for adhesion promoting SiOx films to facilitate the coupling of stable organosilane monolayers on monolithic α-alumina; physical vapor deposition (PVD) by thermal evaporation and plasma enhanced chemical vapor deposition (PE-CVD). We also investigated the influence of etching on the formation of silanol surface groups using hydrogen peroxide and sulfuric acid solutions. The film characteristics, that is, surface morphology and surface chemistry, as well as the film stability and its adhesion properties under accelerated aging conditions were characterized by means of X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), inductively coupled plasma−optical emission spectroscopy (ICP-OES), and tensile strength tests. Differences in surface functionalization were investigated via two model organosilanes as well as the cell-cytotoxicity and viability on murine fibroblasts and human mesenchymal stromal cells (hMSC). We found that both SiOx interfaces did not affect the cell viability of both cell types. No significant differences between both films with regard to their interfacial tensile strength were detected, although failure mode analyses revealed a higher interfacial stability of the PE-CVD films compared to the PVD films. Twenty-eight day exposure to simulated body fluid (SBF) at 37 °C revealed a partial delamination of the thermally deposited PVD films whereas the PE-CVD films stayed largely intact. SiOx layers deposited by both PVD and PE-CVD may thus serve as viable adhesion-promoters for subsequent organosilane coupling agent binding to α-alumina. However, PE-CVD appears to be favorable for long-term direct film exposure to aqueous solutions. KEYWORDS: PVD, PE-CVD, oxide ceramics, SiOx thin film, silanization
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resistance.6 However, this class of inert ceramic materials is not suitable for implants with direct contact to bone due to their insufficient osseointegration behavior. When implanted into biological tissue, their chemical and biological inertness effectively hinders direct cell-attachment and causes the formation of fibrous tissue-encapsulation.7 This results from the fact that those oxide ceramics do not possess comparable reactive interfaces. Consequently, also chemical and biological surface modification strategies are difficult to develop and can often only be realized via highly aggressive approaches which affect the materials properties.8,9 To overcome this hindrance, different surface strategies, which involved the use of inorganic coatings,
INTRODUCTION It is well-known that biological responses toward implanted materials are mediated through the composition of the interfacial layer between material and tissue. As such, tweaking the chemical and physical composition of these layers on biomedical surfaces is a promising approach to stimulate or suppress specific responses from its surroundings. Over the past, numerous approaches have been developed to improve the cell-material interaction by biologically activating surfaces of the most prominent implant materials such as titanium1,2 and magnesium alloys.3,4 These materials are particularly accessible for surface modifications due to their tendency to form chemically reactive interfacial oxide layers.5 High-strength ceramics such as alumina and zirconia are a versatile and reliable alternative especially for articulating components of artificial joints due to their high strength and wear © 2016 American Chemical Society
Received: April 13, 2016 Accepted: June 14, 2016 Published: June 14, 2016 17805
DOI: 10.1021/acsami.6b04421 ACS Appl. Mater. Interfaces 2016, 8, 17805−17816
ACS Applied Materials & Interfaces
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such as glass10,11 and hydroxyapatite,12−14 were followed. Also direct organic bonding to the immediate ceramic surface has been investigated using organosilane coupling agents on ceramic colloids.15−19 However, direct organosilane coupling without prior surface modification toward adherence promotion was found to be inferior in comparison to surfaces previously modified by adhesion interlayers.20 Furthermore, the huge difference in effective interface surface area between colloids and dense monolithic structures, ranging from several 100 m2 down to below 1 mm2, hinders a technological transfer of the individual findings. Recently, the deposition of an interfacial layer of silicon suboxide (SiOx) on alumina ceramics21 via physical vapor deposition (PVD) by rapid thermal evaporation, has been proposed as an alternative approach to apply an adhesion promoting interlayer for organosilane coupling agents and subsequent protein coupling.22 Such silicon oxide thin films have been investigated extensively for numerous applications such as optical or hydrophobic coatings23,24 and also in combination with subsequent organosilane coupling as adhesion and bonding agents between various inorganic and organic substrates.25,26 However, despite these findings, modifications on biomaterial interfaces have to meet a set of different, highly distinct requirements in terms of hydrolytical stability and elicited response from the surrounding host matter. In the present study, we propose plasma enhanced chemical vapor deposition (PE-CVD) as a superior technique for the deposition of SiOx. The SiOx interlayer serves as an interfacial adhesion promoter for organosilane coupling agents on densely sintered α-alumina for the use in biomaterial applications. In direct comparison with PVD by thermal evaporation, film deposition by PE-CVD possesses several advantages, for example, regarding the accurate control of the thickness, homogeneity over large areas, and chemical composition of the SiOx films.27,28 PE-CVD uses volatile organic−silicon precursors which are excited and partially decomposed in a plasma reaction chamber. By careful control of plasma conditions and reagent supply, the resulting chemical composition of the created layer can be arbitrarily adjusted.29 In addition, PE-CVD allows for the coating of more complex structures featuring overhangs and cavities in comparison to PVD, which requires a direct line of sight between precursor and substrate.30 Also, tests under physiological conditions showed that the PE-CVD films were robust and adhered well to different surfaces.31 On the basis of these findings, the deposition techniques compared within this study, PVD and PE-CVD, were chosen to determine SiOx thin film composition dependent influences, originating for example from different stoichiometry due to deposition parameters on, for example, silanization potential, hydrolytical stability, and biocompatibility between both techniques when applied on implant alumina ceramics. We compare key film properties generated by both deposition techniques and furthermore investigate the deposited films after treatment with piranha solution as a means to remove organic residues and to increase the surface hydroxyl groups and subsequent organosilane coupling agent density. The deposited films on densely sintered medical grade α-alumina substrates are compared in terms of their atomic composition, interfacial binding strength, hydroxyapatite formation, layer integrity upon prolonged exposure to simulated body fluid, silanization potential as well as cytotoxicity and influence on cell viability of both mouse fibroblasts and human mesenchymal stromal cells.
Research Article
MATERIALS AND METHODS
Sample Preparation. Alumina disks were prepared from uniaxial pressed (100 MPa) Al2O3-powder (purity ≥99.7%, CT 3000 SDP, Almatis, Germany) with subsequent sintering at 1600 °C for 1 h. The surface was polished (Saphir 5202, ATM, Germany) until arithmetic average of surface amplitudes Ra < 0.2 μm to yield a surface area of 314 mm2 per disk. The specimen were cleaned for 15 min in an ultrasonic cleaner (Ultrasonic Cleaner 200, Branson Danbury, USA) successively in acetone, 70% ethanol, and deionized water, dried at 120 °C for 20 min and subsequently burned at 450 °C for 15 min to remove any remaining organic residues from the surface. SiOx Film Deposition and Functionalization. Silicon suboxide (SiOx, 0 < x < 2) was deposited on the polished and cleaned alumina disks through physical vapor deposition (PVD) and plasma-enhanced chemical vapor deposition (PE-CVD). PVD was carried out in vacuum (
