Elsevier

Thin Solid Films

Volumes 398–399, November 2001, Pages 602-606
Thin Solid Films

Deposition of anti-bacterial silver coatings on polymeric substrates

https://doi.org/10.1016/S0040-6090(01)01326-8Get rights and content

Abstract

A low temperature (70°C) magnetron deposition process has been developed for the deposition of silver coatings on thermally sensitive polymeric substrates. This low temperature deposition was achieved by combining magnetron sputtering with a neutral atom beam (Saddle Field) plasma source. A range of polymer sheet and tube substrates was coated with silver thicknesses in the range 5–50 nm. The bacterial adhesion and bactericidal effects of the coated polymers was assessed using Staphylococcus epidermidis and the cytotoxicity using fibroblast cells. Up to a 3 log reduction in bacterial adhesion was achieved for silver coatings on polyurethane, which did not exhibit cytotoxicity.

Introduction

Silver exhibits good anti-bacterial properties and in recent years has been used on medical devices ranging from wound dressings to urinary catheters [1], [2], [3], [4]. In addition to silver's anti-bacterial properties it also seems to possess anti-inflammatory properties and improved healing rates as shown by Bishop et al. in the treatment on non-healing venous stasis ulcers [5]. The anti-bacterial activity of silver is dependent on the silver cation (Ag+), which binds strongly to electron donor groups on biological molecules containing sulfur, oxygen or nitrogen. The silver ions act by displacing other essential metal ions such as Ca2+ or Zn+[6].

Ion beam-assisted treatments, which operate at pressures of 10−3 Pa and lower, have been widely applied for the deposition of silver coatings [7], [8]. At higher pressures (0.01–0.1 Pa) magnetron sputtering can also be used. However, in the case of the latter technique in order to obtain good adhesion between the metal and the polymer substrate, it is necessary to apply an rf bias on the substrate holder. The combination of the magnetron and rf plasma sources can result in substrate temperatures of 200–300°C. These relatively high temperatures cause considerable difficulty when coating medical devices, many of which contain thermally sensitive polymer parts. In this study the problem of silver coating adhesion on thermally sensitive polymers was addressed by combining a magnetron sputtering source with a Saddle Field plasma source. This latter source [9], [10] produces a neutral atom beam which was used both for activating the polymer prior to coating deposition and for substrate bombardment during the sputtering of silver from the magnetron source. The atom beam source consists of two water cooled steel rods surrounded by a rectangular cathode made of carbon (graphite). The source has a beam aperture of 7×15 cm. Ions formed in the source are neutralized both by colliding with residual gas molecules within the source and electrons particularly in the electrostatic field close to the source aperture [11]. The degree of plasma neutralization is also reported to be dependent on distance from the source [12]. The advantages of the combination of magnetron sputtering with the atom beam source is the deposition of silver at higher pressures than conventional ion beam treatments in addition to the ability to operate at low substrate coating temperatures.

Section snippets

Experimental

The initial study involved the deposition of silver coatings on a polyurethane sheet by magnetron sputtering. A 20×13 cm rectangular silver target was used in an Ar plasma (deposition current of 1.0 A at 0.4 Pa). The flat polymer sheet samples were mounted on a substrate holder approximately 10 cm from the target. It was observed that in the absence of a rf bias (13.56 MHz) on the substrate on which the polymer was held, coating adhesion was poor. The silver coating was easily removed using an

Coating thickness

The thickness and deposition rates of the silver deposited using the combined magnetron and saddle field source were measured on glass slides by glancing angle X-ray diffraction (XRD) [13] and ellipsometry methods which both corresponded closely with each other. It was not possible to use either of these techniques to measure the thickness of the coatings on the polymer sheets as a very flat surface is required for these measurements. The XRD results are presented in Fig. 2 and show that the

Conclusions

Silver coatings were also deposited using a combination of magnetron sputtering with an rf biased substrate holder. Some thermal decomposition of the polymer was observed with this arrangement. The combination of magnetron sputtering with the atom beam source has been successfully demonstrated for the deposition of silver coatings on thermally sensitive polymer substrates. Silver coatings exhibiting good adhesion have been deposited at 70°C through the combination of these sources. Based on

Acknowledgements

This work was part funded by the EU under the programme BRITE-Euram, contract BRPR-CT97-0415.

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