Amorphous carbon — Biomaterial for implant coatings

Diamond and Related Materials, 3 (1994) 899 901

899

Amorphous c a r b o n - biomaterial for implant coatings A. O l b o r s k a a n d M. S w i d e r Department qf Maxillo-Facial Surgery, Military Medical Academy, ul. Zeromskiego 113, 90-540 Lodz (Poland)

R. Wolowiec, P. Niedzielski, A. R y l s k i a n d S. M i t u r a Institute of Materials Science and Engineering, Lodz Technical University ul. Ste['anowskiego l, 90-924 Lodz (Poland)

Abstract Thin, hard amorphous carbon (a-C) layers were the object of investigations, in particular the microstructure of the layer in the carbon layeF steel substrate system with the aim of optimization of the synthesis and applicability of this new system for medical implants. The results of Auger electron spectroscopy studies show that the composition of the layer is complex. It consists of carbon and a transitional layer containing both carbides and carbon regions. The structure of the coatings ensures a high mechanical strength and very good adhesion to steel. The studies were aimed at investigations of the biological resistance of implants in the oral cavity. Human saliva is an aqueous solution of salts (organic compounds) and contains small amounts of organic substances and gases. The results of the investigations have shown very good resistance of a-C to elements of the oral cavity environment.

1. Introduction

Dental implants have been accepted world-wide as a therapeutic procedure, and in many centers the implants are chosen purposely. The historic background of implantology is much older than generally realized. The first attempts at implantation date back to 6000 BC when in Central America the Mayans put specially selected and adapted shells into tooth gaps [ 1]. Modern implantology developed in the 1950s. This development

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isconnectedw.th.nlio ormigg n whod s.gnedan implant in the form of a spiral screw [2]. An important stage in the field of dental implants was the concept of osseointegration [3]. Osseointegration is a direct structural and functional connection between ordered living bone and the surface of a load-carrying implant [3]. The requirements of optimal biointegration of the implant [3, 4] require specific conditions to be satisfied by the implant material. It was found that they are met by titanium coated using a plasma method [5]. After plasma coating, the surface of titanium implants has roundedoutside a the plasma,f°rminandcontactiS verYwithPOrOUSair,anbUtoxideC°hesiVe'and nitride form spontaneously on the clean Ti surface. This oxide and nitride are highly chemically resistant materials and protect the metal against chemical attack, including that of aggressive body fluids. Carbon reveals similar properties. Additionally, owing to the high degree of biocompatibility of carbon resulting from the presence of this element in the human body, it appears to be a new potential biomaterial. The possibility

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© 1994

Elsevier Sequoia. All rights reserved SSDI 0925-.9635(93)05126-W

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of using carbon as an implant material appeared when a method for low-pressure synthesis of diamond and diamond-like carbon layers was developed. The first reports on the subject were presented by Mitura et al. [6] and Lettington and Smith [7].

layers used in general surgery [9, 10]. The specificity of the oral cavity requires the application of special materials. Additional analysis of properties of the coatings has involved, among others, histopathological investigations using laboratory animals [9], tests of corrosion resistance [103, measurements of mechanical properties [9, 11], and breakdown tests in Tyrod solution [6].

2. Experimental details The purpose of the present study was to investigate

Differences between hard a-C layers produced by the r.f. dense plasma method and typical diamond-like carbon layers have been found. First the layers show very good

the applicability of amorphous carbon (a-C), as a coating for implants in maxillofacial surgery. The implants were coated with carbon deposited by the method of dense

adhesion to AISI 316L steel which is explained by us on the basis of investigations carried out by Auger electron spectroscopy (AES) using a 918-2000 Varian

plasma produced from methane in an r.f. electric field, The idea of this method, described in detail elsewhere, is to excite a plasma in methane or other hydrocarbons in an r.f. electric field at a relatively high gas pressure of about 100-300 Pa [8].

spectroscope. Initial studies on the biotolerance of the layers in laboratory animals and in the human oral cavity have also been carried out. The results of AES studies are presented graphically in Figs. 1-3. They show analyses of a differential spectrum of Auger electrons on the surface (Fig. 1) and at different depths of the coating (Figs. 2 and 3). The distribution of elements inside the coating was analyzed after argon ion etching with a 2000 eV Ar ÷ ion beam.

3. Results The results of studies carried out so far confirm the very good applicability of implants coated with a-C

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Fig. 2. AES spectra from the carbon layer after Ar + ion etching, showing the distribution of elements inside the coatings at different depths; the characteristic peaks on the soft energy side of the C KLL line are conected with metal carbides.

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scope after 1 week, 2 weeks and 1 month of use were compared. No changes were observed.

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4. Conclusions : .........................................

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The results of AES studies show that the surface layer several thousand fingstrGms thick consists of carbon. The carbon film passes smoothly into a thick layer (micrometers) consisting of metal carbides of metals which included in the steel. The composition of the transitional layer containing both carbides and carbon regions changes. This structure of the coatings ensures a higher mechanical strength and very good adhesion to AISI 316L steel. Therefore, the coatings are also good protection for metal implants against corrosion, and protect the subject from a serious illness, metalosis [ 10]. Preliminary histopathological investigations confirm the biocompatibility of a-C and its biointegration in the oral cavity. Additionally, investigations of mechanical, anticorrosive and structural properties of a new type of coating produced by a dense methane plasma excited in an r.f. electric field show that this can be a very good material for use in maxillofacial surgery.

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Fig. 3. AES spectrum from the inside of AISI 316L steel (59% Fe, 18% Cr, 16% Ni, 3% Mo, 2% Mg, 1% Si, 0.5% Ca, 0.03% C).

The surface layer which is several thousand 5,ngstrGms thick consists of carbon. The carbon films are contaminated with a small amount of oxygen and nitrogen (Fig. 1). The carbon film passes smoothly into a thick layer (micrometers) consisting of metal carbides of metals included in AISI 316L steel ( i . e . Cr, Ni, Mo, Fe). This is confirmed by characteristic peaks occurring at the lefthand side of the C KLL peak at 276 eV (Fig. 2). The composition of the transitional layer containing both carbides and carbon regions changes. The AISI 316L steel does not contain more than 0.03% carbon. This is illustrated by a very small peak C KLL, 276 eV (Fig. 3). Thus a s t r u c t u r e of the coatings e n s u r e s a higher mechanical strength. Therefore, the coatings are also good protection for metal implants against corrosion, and can protect the subject from a serious illness, metalosis [10].

The results of preliminary histopathological investigations of laboratory animals were so promising (for example, there were no changes in the animals after several months) that attempts were made at implanting dental prostheses in humans. Images of carbon-coated dental prostheses obtained by a scanning electron micro-

The authors are indebted to Jan Swiderski from OPTIMUS who contributed to the completion of this work. The work was partly financed by Grant no. 3.3602.91.02. from the Polish State Committee for Scientific Research.

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