Reducing the dissolution rate of silicon nitride coatings for spinal implants using Fe and C as alloying elements

• INTRODUCTION: Wear and corrosion may lead to a release of particles and ions from spinal implants, which is a concern because of their potentially detrimental effect on the life span of the implant [1]. Silicon nitride-based coatings have been suggested as an option to reduce the release of metal ions from an implant. In addition, any particles produced will slowly dissolve, releasing only biocompatible ions [2]. It is of high interest to reduce the dissolution rate of the coating to ensure an adequate lifetime [3]. The present study aimed to assess the effect of Fe and C additions to silicon nitride coatings in terms of dissolution rate as well as the impact of the released ions on the in vitro neural cell response.METHODS: Using a combinatorial approach, SiFeCN coatings were deposited on CoCr disc substrates by reactive sputtering in an in-house built equipment. The coatings were characterized in 9 points using x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The dissolution behaviour was evaluated by exposing the coated samples to cell media for 14 days. The obtained extracts were used to measure ion release with inductively coupled plasma - optical emission spectrometry (ICP-OES) and to assess cell viability of microglia (C8-B4 cell line) using the MTT assay. RESULTS: The XPS results showed compositional gradients of Si ranging from 35.0 to 47.3 at.%, Fe from 1.4 to 9.3 at.% and C from 4.5 to 13.9 at.%. SEM of focused ion beam (FIB) cross-sections revealed coating thicknesses between 427-534 nm. SEM of the coating after exposure showed substantial signs of dissolution with visibly increased porosity for the SiN coating, while the SiFeCN coatings appeared less affected. SiFeCN coatings appeared more affected by dissolution for increasing Si contents. The estimated dissolution rate of the SiN coating was 8.3 nm/day, while the rate of SiFeCN coatings was 5.2-6.8 nm/day. The ICP results showed a reduction in Co ions from the substrate in the coated samples compared to uncoated CoCr. Moreover, the levels of detected Si ions were lower for the SiFeCN compared to SiN reference. Indirect biocompatibility tests suggested that microglia cell viability was comparable for the SiFeCN coatings, the uncoated CoCr and the SiN coating.DISCUSSION & CONCLUSIONS: The compositional gradients influenced the thickness of the coating, giving a slight thickness increase in the coatings with the increment of Si content. In addition, the ICP results showed the capability of the coating to act as a barrier to the release of ions from the substrate. Furthermore, the presence of Fe and C in the coating causes a decrease in the ion release from the coating, indicative of a lower dissolution rate, which was supported by the thickness measurements. The findings from this study indicate that using Fe and C as alloying elements can lower the dissolution rate of the silicon nitride-based coating while showing positive indications of biocompatibility on neural cells. Therefore, SiFeCN coatings merit further investigation as a future option for spinal implants.REFERENCES: 1Y. Shimamura et al (2008) Spine. 33:351–355. 2M.  Pettersson et al (2016) ACS Biomater. Sci. Eng. 2:998–1004. 3C. Skjöldebrand et al (2022) Biomater Sci, 10:3757–3769.ACKNOWLEDGEMENTS: This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 812765 and from the European Union’s Seventh Framework Programme (FP7/2007-2013), grant agreement GA-310477(LifeLongJoints).

Ämnesord

MEDICIN OCH HÄLSOVETENSKAP  -- Medicinsk bioteknologi -- Biomaterialvetenskap (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Medical Biotechnology -- Biomaterials Science (hsv//eng)

Nyckelord

Engineering Science with specialization in Biomedical Engineering

Teknisk fysik med inriktning mot medicinsk teknik

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