Nanocrystalline carbon coatings and powders for medicine

Int. J. Nanomanufacturing, Vol. x, Nos. x, 200x 29

Copyright © 2008 Inderscience Enterprises Ltd.

Nanocrystalline carbon coatings and powders for medicine

Katarzyna Mitura* Institute of Materials Science and Engineering, Technical University of Lodz, Poland E-mail: [email protected] E-mail: [email protected] *Corresponding author

Anna Karczemska Institute of Turbomachinery, Technical University of Lodz, Poland

Piotr Niedzielski, Jacek Grabarczyk and Witold Kaczorowski Institute of Materials Science and Engineering, Technical University of Lodz, Poland

Petr Louda Institute of Materials Science and Engineering, Technical University of Liberec, Czech Republic

Stanisław Mitura Institute of Materials Science and Engineering, Technical University of Lodz, Poland

Abstract: All the allotropic forms of carbon, i.e., diamond, graphite and carbine, find applications in different areas of medicine, but diamond is specifically preferred. The unique properties of thin diamond layers, due to the highest biocompatibility of carbon resulting from the presence of this element in human body, make them candidates for producing biomaterials. Especially carbon in the form of a nanocrystalline diamond film has found industrial applications in the area of medical implants. Diamond Powder Particles (DPP), as an extended surface NCD, are useful for medical examinations. Different medical implants are covered with Nanocrystalline Diamond Coatings (NCD). NCD forms a diffusion barrier between implant and human environment.

Keywords: carbon films; Nanocrystalline Diamond Coatings; NCD; biocompatibility; Diamond Powder Particles; DPP; medical implants.

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Reference to this paper should be made as follows: Mitura, K., Karczemska, A., Niedzielski, P., Grabarczyk, J., Kaczorowski, W., Louda, P. and Mitura, S. (2008) ‘Nanocrystalline carbon coatings and powders for medicine’, Int. J. Nanomanufacturing, Vol. x, Nos. x, ppxxx–xxx

Biographical notes: Katarzyna Mitura (de domo Bakowicz) received job title Physician in 1998 (Medical University of Lodz). In 1999, she started working in Department of Pneumonology and Allergology, Medical University of Lodz, and in Institute of Materials Science and Engineering, Biomedical Engineering Division. She received PhD (‘Bioactivity of Diamond’) in Materials Science in 2003 (Technical University of Lodz). She has medical practices in hospital.

Anna Karczemska received the MSc in Solid State Physics from Technical University of Lodz (1997) after her five months stay in ECAM Lyon. Then she stayed for three years in Imperial College of Science, Technology and Medicine in London, in High Energy Physics Group. She received her Doctorate Degree from the Faculty of Materials Science, Warsaw University of Technology (2002). She is the author and co-author of more than 20 papers. She is an Assistant Professor in the Institute of Turbomachinery, Technical University of Lodz.

Piotr Niedzielski received his MSc in Materials Science (Technical University of Lodz, 1993), PhD Degree in Mechanics (Technical University of Lodz, TUL, 1998) and DSc in Materials Science (Warsaw University of Technology, 2006). He is the Vice-Supervisor of the EU Center of Excellence NANODIAM. He is the author and co-author of three patents and more than 60 papers.

Jacek Grabarczyk received the MSc in Biomedical Engineering (Technical University of Lodz, 1998) and PhD Degree in Mechanics (Technical University of Lodz, TUL, 2003). He is the author and co-author of two patents and more than 20 papers.

Witold Kaczorowski received the MSc and PhD Degrees in Materials Science (Technical University of Lodz, TUL, 2000, 2005), He joined the Institute of Materials Science and Engineering TUL as an Assistant Professor in 2006. His research interests include the technology of the synthesis of carbon using microwave and radio frequency plasma CVD. He is the author and co-author of more than 15 papers.

Petr Louda received his MSc in Automatic Control Systems for Engineering (Technical University of Liberec, 1984) and PhD Degree in Physical Metallurgy (University of West Bohemia in Pilsen, 1993). He was an Associated Professor of Engineering Technology in TUL (2000). From 2003, he had been the Dean of Faculty of Mechanical Engineering of TU Liberec and from 2005, he has been working as a Professor of Engineering Technology in TUL. He is the author of more than 110 papers.

Stanislaw Mitura received his MSc in Solid-State Physics (University of Lodz, 1974), PhD Degree in Materials Science (Technical University of Lodz, TUL, 1985) and DSc Degree from the Faculty of Materials Science, Warsaw University of Technology (1993). From 1995, he had been a Professor of Materials Science in the TUL, specialising in PA CVD synthesis of materials. During 2002–2005, he was the Vice-Rector of TUL. He is the Supervisor of the EU Center of Excellence NANODIAM. He is the author and co-author of ten patents and more than 180 papers. In 2006, he was awarded Dr h.c. from the Technical University of Liberec.

Nanocrystalline carbon coatings and powders for medicine 31

1 Introduction

All the allotropic forms of carbon, i.e., diamond, graphite and carbine, find applications in different areas of medicine, but diamond is specifically preferred (Guseva et al., 1995; Thomson et al., 1991; Lettington and Smith, 1992; McLaughlin et al., 1996; Mitura et al., 1995, 1996, 2000; Mitura, 1997). Carbon is useful in medicine, particularly in a form of thin films, coatings and plates.

Research and development studies on carbon (diamond) films are carried out in the Institute for Materials Science and Engineering, Technical University of Lodz, in the framework of Center of Excellence NANODIAM. The scope of research activities, carried out in the centre, comprises development and tests of new materials with intentionally modified surfaces. Crystalline carbon, synthesised with the help of Radio Frequency Plasma-Activated Chemical Vapour Deposition (RF PA CVD) and Microwave/Radio Frequency Plasma-Activated Chemical Vapour Deposition (MW/RF PA CVD) methods, exhibits diamond structure with crystallite dimensions of several nanometres. This is why it is called nanocrystalline. In addition, polycrystalline diamond is produced using the CVD technique.

A synthesis of crystalline carbon is performed on surfaces of several materials, broadly used in medical practice, such as medical grade steel, titanium or polymers. The scope of present and future applications of our achievements is becoming broader, and it is closely connected with orthopaedics, brain and spinal cord surgery, dentistry, implantation of artificial organs and other biomedical uses, such as novel surgery tools or diagnostic devices.

Diamond is a form of carbon, which is also present in our body, and that is why a patient does not reject an implant as a foreign entity. A diamond coating not only does not alter mechanical properties of implants, but, in addition, it improves their durability and corrosion resistance and protects against allergic effect of metals, of which they are made. Recent results of our investigations present still another significant property of nanodiamonds – their bioactivity. Our technology is also utilised in a quick diagnostics device, designed for a general practitioner’s use (a utilisation of nanocrystalline diamond on a surface of a biosensor).

Apart from the applications listed above, diamond layers are also studied with respect to their application to medical tools. An appropriately selected, with regards to the substrate, layer substantially improves properties of the bulk material, thus improving working parameters of the element coated.

2 Experimental

2.1 Radio Frequency Plasma-Activated Chemical Vapour Deposition (RF PA CVD)

One of the methods used to produce carbon layers for medical applications is the RF PA CVD technique. Its description has already been given in the literature (Holland, 1976; Mitura et al., 2000; Mitura, 1997). The technique utilises radio frequency plasma initiation with an application of a 13.56 MHz power generator. Deposition of films is carried out in a two-electrode system, where one electrode is an RF-powered and negatively biased working electrode, while the other is comprised of a grounded reactor

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chamber (Figure 1). Films are deposited in the pressure range of 10–80 Pa. The working atmosphere consists of methane, hydrogen and argon. This technique enables deposition of films on all metallic materials, used in medicine. The properties of the carbon layers are controlled by a mutual selection of such parameters as the pressure in the working chamber, the composition of gas mixture and the negative self-bias potential of the working electrode. An appropriate selection of the process parameters enables deposition of carbon films of very different properties, ranging from amorphous Diamond-Like Carbon (DLC) coatings to NCD layers.

Figure 1 A schematic representation of the equipment for carbon film deposition using the RF PA CVD technique

Apart from typical heteroepitaxial deposition of films on metallic substrates, the method allows one to also seed carbon crystals in a homoepitaxial way. Crystallisation seeds are formed and undergo growth in the gas phase, which leads to the production of carbon powders. Similarly to the film deposition, the phase composition and structure of the powders strictly depend on process parameters. Most often, they are synthesised in a form of conglomerates, with the diamond phase content of 40–95%.

2.2 Dual-frequency plasma technology

The dual-frequency plasma technology makes use of the system, presented in Figure 2. In this system, in order to initiate plasma, two frequencies have been applied:

microwave frequency and radio frequency. The device allows one either to apply both frequencies simultaneously or to use MW or RF plasma separately. The system is additionally equipped with vacuum pumping, gas dosing and control modules. Microwave energy is transferred from the generator to the chamber by means of a circulator, stub tuner and quartz tubing. The RF energy is supplied to the system through an isolated electrode, just as it is in the RF PA CVD system. The MW power used in the studies comprises a 100–1000 W range, while the RF power range amounts to 60–1200 W. In each case, matching of both RF and MW circuitries is carried out in order to achieve a minimum of reflected power.

Nanocrystalline carbon coatings and powders for medicine 33

Figure 2 A schematic diagram of the MW/RF PA CVD system

The working chamber pressure covers a range of several Pa to several dozens Pa. For the synthesis of carbon films, either pure methane or its mixture with such gases

as argon, nitrogen or hydrogen is used. The total flow rate of the gas mixture supplied to the system is comprised in the range of 10–100 sccm. Methane, as a source of carbon, is supplied through a shower device in a vicinity of the sample. Other plasma-forming gases are supplied through the quartz tubing. Before the deposition, the chamber is evacuated to the pressure of a few Pa. During the process, the following parameters are being controlled: total pressure, gas flow rates and MW and RF power supplied and reflected.

The deposition parameters have been optimised, in order to obtain homogeneous carbon coatings on such different substrates as AISI 316L steel, silicon or synthetic polymers. Prior to the deposition, the substrates are pre-cleaned, first in an acetone bath and then in argon plasma, directly in the working chamber.

The advantages of the presented technology comprise a capability to produce carbon coatings at low temperatures and an ease of process control.

3 Results and discussion

3.1 Nanocrystalline diamond coatings as an antiallergic diffusion barrier on medical implants

Allergic contact dermatitis is one of the most frequent skin diseases, whose clinic symptoms is erythemic itching with small blisters and papules. Dermal lesions usually occur in those spots where the sensitiser substance interacts with skin. The contact with an exogenous chemical leads to inflammation and is generally classified as immunologic and T-cell-mediated, delayed type of reaction. Hypersensitivity lasts many years, and it is a cause of recurrent eczema. Therefore, people suffering from allergic contact dermatitis suffer from impediments in daily life and job’s problems.

The sensitising substances comprise usually simple chemical compounds (500 Da) exhibiting enhanced chemical activity. The most common allergens are metals (nickel, cobalt and chromium), fragrances, preservatives, rubber chemicals, epoxy resins and acrolates (Kiec-Świerczynska et al., 2003, 2005).

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A frequency of contact allergy to nickel in European countries amounts to 18% in general population and is far more frequent in women (Kiec-Świerczynska et al., 2005). Nickel is a component in many metal alloys, of which jewellery, metal parts of machines, tools or coins are made. Orthopaedic implants and orthodontic braces also include nickel. An original cause of allergy to nickel is ear piercing and wearing cheap jewellery. People sensitive to nickel can obtain adverse reaction after orthopaedic operations, in which metal–bones–implants are used. In those cases, an inflammation of tissues around the implants is observed and eczema occurs on the skin.

Because nickel sensitivity constitutes a serious social problem, European Union enacted the Nickel Directive (94/27/EEC) determining the concentration of nickel in various objects, referring to their purpose. This law has been obligating since June 2001, and products that do not fulfil its requirements cannot be produced and sold in EU.

Contact allergy induced by metal implants is a very popular subject of biomedical engineering. Different medical implants are covered by nanocrystalline diamond coatings. NCD form a diffusion barrier between an implant and the human environment. The research on NCD proved that diamond layers are biocompatible with the living organism.

We examined mechanical and biological properties of medical screw coated with NCD after two years exposition to the human body environment (compare Figure 3).

Figure 3 Micrograph of medical screw coated with Nanocrystalline Diamond Coatings (NCD) after two years exposition to the human body environment

The first results indicate that NCD coating makes a diffusion barrier for metal ions. We have not observed corrosion. Biological examinations indicate that the tissues around the implant exhibit no congestion, inflammatory process and proliferation of fibrous connective tissue.

3.2 Bioactivity of diamond powder particles in contact with allergic diseases

DPP (Mitura, 1987) constitute a new antioxidant and anti-inflammatory factor in the living organism. DPP activity is probably based on the reaction between the nanoparticle diamond surface and the living organism molecules, responsible for toxic processes (Bakowicz, 2003).

Nanocrystalline carbon coatings and powders for medicine 35

We examined patients with contact allergic diseases, for example: with contact allergy for nickel, chromium, euxyl. We used the method of patch tests (Figure 5).

A modification of this method, in a presence of diamond powder particles manufactured by RF/MW PACVD method (Figure 4), was also tested. The positive control was the Elocom (mometasone furoate) – antiallergic drug.

Figure 4 SEM micrograph of diamond powder particles manufactured by MW/RF PA CVD method

Figure 5 Human skin after 96 h test: examination of diamond powder particles manufactured by MW/RF PA CVD and Elocom

After 96 h, the results indicate that diamond powder particles have the same antiallergic properties in the patients with contact dermatitis as antiallergic drug.

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3.3 Applications of carbon coatings to medical tools

A separate issue concerning a production and application of carbon coatings comprises their use in order to improve medical tools. The main purpose of their modification is to increase wear resistance of the tools, primarily in order to protect the patient against the products of the tool wear. In the case of metallic materials, not to exclude titanium alloys, these products are hazardous (carcinogenic, allergic and irritating). An additional aspect of the modification of medical tools is an improvement of their biocompatibility and a protection of their surface against colonisation by bacteria.

In order to fulfil increasingly demanding requirements, put on medical tools, such surface-processing technologies as glow discharge processing (Czarnowska et al., 2000), oxidation (Aladjem, 1973) or ion implantation (Krupa et al., 1999) are applied to the tool materials. The presented RF PA CVD method constitutes one of such surface-processing techniques (Mitura et al., 1995, 1996, 2000; Mitura, 1997). Carbon coating is most often used to modify tools in orthopaedics, as well as in brain and spinal cord surgery (See Figure 6).

Figure 6 Surgical tools after their modification with: (a) carbon coating and (b) during the surgery

(a) (b)

An investigation of carbon coating-modified tools shows a uniformity of the coating on the entire surface of the tool and very good adhesion to the surface. An open-loop (OCP) corrosion potential Ecorr measurements of the tools made of titanium alloy Ti6Al14V, carried out in Tyrode solution, show a 600 mV shift of corrosion potential towards higher values with respect to the same material deprived of the NCD coating. This is a strong indication that the coating presents a tight diffusion barrier for metal ions. The character of corrosion pits, resulting from a damage of the coating, also speaks to the advantage of the samples coated with the NDC layer. The corrosion pits formed on the surfaces

Nanocrystalline carbon coatings and powders for medicine 37

of uncoated samples are very large and several 100 µm deep, in contrast to shallow and point-like pits developed in the NCD-coated samples (Scholl et al., 2004, 2006).

A carbon coating on a surface of a tool also limits a likelihood of a colonisation of its surface by bacteria. Studies have revealed nearly 80-fold lower number of bacteria on the medical steel surface modified with carbon layer and eight-fold lower number of bacteria on modified titanium surface (Jakubowski et al., 2004). Numerous chemical cleaning and sterilisation cycles at 140°C of the modified tools in hospital environment have neither caused delamination nor introduced any other surface changes.

3.4 Diamond for microfluidic devices

Microfluidic devices are made of wafers with micropatterns in them (microchannels, having microchambers, etc.), having at least one dimension of the channels of an order of micrometres. Such devices have many possible applications, such that they can be used for DNA and protein’s separation. It is worth to underline that miniaturisation of flow devices enables manipulation of fluid samples within the channels to the order of nanolitres.

Many materials are used for manufacturing of microfluidic devices, among them are silicon, glass and different polymers.

Choosing an optimum material and technique for the microfluidic device fabrication is of great importance (usually techniques known from microelectronics are adapted for microfluidics).

Diamond’s extreme properties make it a new generation material for this application. As monocrystalline diamond has the highest-known thermal conductivity at room temperature, heat dissipation problems can be easily solved; diamond is chemically inert, so it can be used even in contact with highly reactive chemicals (can be easily cleaned and, also, application for reaction microchambers are worthy to think about). Diamond also has good bio-adhesion properties and is biocompatible, which is important in contact with biomolecules; it has high electrical resistivity, is extremely hard and possesses very good optical properties (is transparent to UV). All this gives new possibilities of diamond microfluidic devices, allowing them to go further, to the end of their possibilities.

Good-quality CVD polycrystalline diamond (Karczemska and Sokołowska, 2002; Karczemska, 2002) was used for fabrication of a diamond microfluidic device. The channel was obtained by ablation with a KrF excimer laser. The channel is 11 mm long and 150 µm deep (compare Figure 7).

An application of diamond for microfluidics gives advantages in comparison to different materials. However, diamond is a very hard material and difficult to shape; for now, technologies for micropatterning diamond are not easy to handle and are still expensive.

Often good surface properties of microfluidic device are advantageous, so applications of carbon coatings, especially of nanocrystalline diamond obtained by RF PACVD method, to change the surface properties are worth to think about. A preliminary research has been done, and it gave interesting results. Such a coating can be deposited on glass or silicon microfluidic device, improving its surface properties.

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Figure 7 Microfluidic device (Karczemska and Sokołowska, 2002; Karczemska, 2002) made of CVD polycrystalline diamond: (a) top view and (b) microchannel cross section

(a)

(b)

4 Conclusions

Carbon in the form of a nanocrystalline diamond film has found industrial applications in the area of medical implants. DPP, as an extended surface NCD, are useful for medical examinations.

Different medical implants are covered with NCD. NCD forms a diffusion barrier between implant and human environment.

Acknowledgements

The works were financed from the Multi-Year Programme PW-004 “Development of innovativeness systems of manufacturing and maintenance 2004–2008” and grant MŠM4674788501 (Czech Republic).

Nanocrystalline carbon coatings and powders for medicine 39

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