The effect of thickness of titanium nitride coatings on bacterial adhesion.
Abstract: We report an interesting observation of minimization of bacterial attachment on titanium nitride coatings of increased thickness. DC magnetron sputtering method was used to prepare titanium nitride coatings of different thickness and the attachment of an oral bacteria Porphyromonas gingivalis was studied. With increase in thickness of the coatings the particle size increased and the order of bacterial attachment decreased while no variation was observed in the chemical composition.
Subject: Coatings industry
Nitrides
Biomedical engineering
Coatings
Authors: Jeyachandran, Y.L.
Narayandass, Sa.K.
Pub Date: 08/01/2010
Publication: Name: Trends in Biomaterials and Artificial Organs Publisher: Society for Biomaterials and Artificial Organs Audience: Academic Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2010 Society for Biomaterials and Artificial Organs ISSN: 0971-1198
Issue: Date: August, 2010 Source Volume: 24 Source Issue: 2
Product: Product Code: 2850000 Paints & Allied Products; 2852330 Wire & Insulating Coatings NAICS Code: 32551 Paint and Coating Manufacturing SIC Code: 2851 Paints and allied products
Accession Number: 308129451
Full Text: Introduction

The excellent intrinsic biocompatibility of titanium nitride (TiN) is well documented in various biomedical applications [1-3]. Dion et al. [4] demonstrated the use of TiN in left ventricular assisted devices. Mitamura et al. [5] showed that the blood compatibility of TiN films was as good as that of low-temperature isotropic pyrolytic carbon and the durability of titanium heart valve cages coated with TiN increased seven-fold. Coll and Jacquet [6] observed great reduction of wear rate in implants with TiN surface coating. Chung et al. [7] found that the TiN based coatings significantly improve electrochemical and biocompatibility properties of the base material. Huang et al. [8] reported good corrosion resistance and cell adhesion behavior of TiN coated implants. However, the antibacterial property of TiN coatings is not well determined. Scarano et. al. [9] and Koerner et. Al [10] showed reduced bacterial adhesion on TiN surfaces. On the other hand, Yoshineri et. al [11, 12] and Jeyachandran et.al. [13] reported no influence of TiN coatings on bacterial adhesion. In this communication, we report an interesting observation of reduced bacterial adhesion on TiN surfaces with increase in thickness of the coatings.

Materials and Methods

A detailed description of the coating method and bacterial experimental conditions is presented in our previous paper [13]. Briefly, TiN coatings were prepared on silicon substrates by d.c. magnetron sputtering at a typical base pressure of 4 x [10.sup.-4] Pa, sputtering pressure of 1.1 Pa, cathode power of 100 W and substrate-target distance of 0.1 cm. At 27 % and 37 % of nitrogen reactive gas concentrations, TiN coatings of thicknesses 145 and 230 nm and 160, 220 and 270 nm respectively were prepared. The bacterial strain used for adhesion studies was an oral bacterium Porphyromonas gingivalis (Pg, ATCC 33277).

The thicknesses of the coatings were obtained using a VASE spectroscopic ellipsometer (SE, J A Woollam Inc.). The surface composition of the coatings was measured using Perkin Elmer PHI 5400 ESCA X-ray photoelectron spectrometer (XPS). A scanning electron microscopes (SEM) Philips XL20 system was used to measure the microstructure of the coatings and a KYKY 2800 SEM system was used to measure bacterial adhesion results.

[FIGURE 1 OMITTED]

Results and Discussion

In our previous paper [13], we reported the bacterial adhesion property of TiN coatings of thickness 100 nm. Therein tested TiN coatings of different surface chemical composition (Ti/N = 1.1 to 3.0) or microstructure (roughness = 1.3 to 3.5) showed higher bacterial adhesion and also biofilm formation.

In the present work, we observed a decrease in bacterial adhesion with increase in thickness of the TiN coatings. The SEM pictures of TiN coatings (thickness > 100 nm) obtained after bacterial culture are shown in Fig. 1. For reference the bacterial adhesion results of 100 nm TiN coatings are also shown. The coatings deposited at both 27 and 34 % nitrogen concentrations showed a significant decrease in bacterial adhesion with increase in thickness. For quantitative information, the bacterial counts on TiN coatings obtained by image count method are presented in Fig. 2. The counts are given for the coatings those showed relatively low bacterial adhesion.

[FIGURE 2 OMITTED]

The surface of the coatings was found to be smooth and uniform as observed from SEM images (Fig. 3). The particles size in the coatings prepared at 27 % and 34 % nitrogen concentrations varied from 32 to 85 nm and 25 to 60 nm respectively, with increase in thickness. The surface chemical composition of coatings, as measured by XPS, showed the presence of titanium oxy nitride stoichiometry. Moreover, no significant variation of the surface chemical composition was observed with increase in thickness of the coatings. The Ti/N and Ti/O ratio of the coatings were found to be around 1.5 and 0.4, respectively.

[FIGURE 3 OMITTED]

Bacteria-surface interactions are explained on the basis of microstructure or surface charge mechanism [14-17]. In case of TiN, a difference of opinion is reported in literature on surface microstructure or chemical composition effect of bacterial adhesion [9, 11, 12, 18]. The surface chemical composition of the test TiN coatings, in the present study, is nearly similar and the microstructure may not have significant effect. The microstructural features of the coatings are in a range of 25 to 85 nm that is least significant comparing the dimension of a bacterium (~3 im) for possible mechanical retention effect. A more reasonable explanation for the observed decrease of bacterial adhesion with increase in coating thickness could be made on the basis of surface free charge. The surface free charge carriers are responsible for the electrostatic interaction of bacteria with surfaces. The surface charge carriers' presence is more on the coatings of lower thicknesses or particle size. With increase in coating thickness or particle size, the charge carriers on surface decrease due to the diffusion mechanism towards the bulk of the coatings. The decrease of surface free charge carriers with increase in thickness (or particle size) of TiN coatings may be the reason of reduced bacterial adhesion.

Conclusion

TiN coatings of different thickness were prepared at 27 % and 34 % nitrogen concentrations. Bacterial adhesion was found to reduce with increase of the coating thickness. The surface composition of the coatings was similar while the microstructural features varied in the range of 25 nm to 85 nm with increase in thickness. The decrease of bacterial adhesion with increase of TiN coatings thickness is attributed to the decrease of surface free charge carriers due to inward diffusion into bulk of the coatings. Preliminary results look promising and are worth further analysis.

Acknowledgement

The authors are grateful to Dr. C.Y. Bao, Sichuan University, China and Dr. D. Mangalaraj, Bharathiar University, India, for their kind help.

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Y.L. Jeyachandran *, Sa.K. Narayandass

Department of Physics, Bharathiar University, Coimbatore, Tamilnadu 641046, India.

Corresponding author: yljchandran@gmail.com (Dr. Y.L. Jeyachandran)

Received 7 January 2010, Accepted 23 January 2010, Published online 27 January 2010.
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