(Invited) Atomic Layer Deposition Coatings for Medical Devices
Authors: Blomberg Tom , Ritasalo Riina , Matvejeff Mikko , Ylivaara Oili , Kärkkäinen Anu , Kivioja Jani 
Published: February 2020

Implantable medical devices must withstand the corrosive environment inside the human body for prolonged periods of time. Hermetic sealing of the device to protect it from the corrosive environment of human body and vice versa is a key step to enable long life time for the smart medical devices.

Atomic layer deposition (ALD) is an advanced deposition technique belonging to the family of chemical vapor deposition (CVD) techniques. In ALD the substrate to be coated is exposed sequentially to precursor chemicals that react with the reactive surface sites on the substrate and form a chemically bonded adsorption layer. This growth mode of the film enables extremely conformal and uniform films to be coated on three-dimensional objects having any shape. Even the smallest, nanometer sized features on the objects can be coated with ease, making the technique extremely useful when hermetic sealing of complex shaped objects are required. The advantages of ALD over other deposition techniques makes it powerful method for applications where sensitive substrate materials combine with extreme demands on coating quality and temperature/chemical resistance, such as those often seen in the medical applications.

In order to make a robust protective sealing for the medical device, the film properties must fulfill certain key criteria. First and foremost, the film material needs to be non-toxic for human cells in order to be biocompatible. Secondly, the film needs to function as an highly efficient diffusion barrier against the ions and molecules present in body fluids. In addition, the adhesion of the film with the device surface needs to be strong enough to prevent the delamination of the film.

In our recent work [1] we have shown that SiO2-HfO2 coated sub-millimeter-sized microelectronic chiplets for wireless body implants can last without degradation and electronic failure in accelerated aging tests in saline at T = 87 °C for more than 180 days. This corresponds an equivalent lifetime of more than 10 years at human body equivalent temperature of 37 °C.

Latest results on ALD films (deposited with Picosun® R-200 Advanced ALD reactor) for medical applications will also be discussed. Biocompatibility tests for Al2O3, TiO2, HfO2, SiO2, Ta2O5 films using the ISO standard Cytotoxicity of Medical Device (ISO 10993-5) procedure will be presented. In addition, we will show results of corrosion tests in PBS solution at elevated temperature (87 °C) for a number of ALD oxides and their nanolaminates, highlighting a stress control method that can improve the long-term delamination resistance of the coatings. Furthermore, we will show some results on the diffusion barrier properties of Al2O3, TiO2, HfO2, SiO2, Ta2O5 and their nanolaminates on different electrode materials (Al, Au, Pt, TiN) when exposed to PBS solution.

[1] Conformal Hermetic Sealing of Wireless Microelectronic Implantable Chiplets by Multilayered Atomic Layer Deposition (ALD) by J. Jeong, F. Laiwalla, J. Lee, R. Ritasalo, M. Pudas, L. Larson, V. Leung, and A. Nurmikko, https://doi.org/10.1002/adfm.201806440.

Acknowledgements: This work is part of the ULIMPIA project funded by PENTA under grant number PENTA-2017-Call2-16101-ULIMPIA