Lead-Free Ferroelectric KNN Films for Biomedical Applications

Abstract

Ferroelectric materials are utilized in a wide range of different electronic devices throughout our modern society, including all kinds of consumer electronics, ultrasound equipment, minimally invasive surgical tools and intersatellite communication devices, just to name a few. The crave for ferroelectric materials is due to their excellent piezoelectric properties. Piezoelectric materials can passively “translate” between the mechanical world of us humans and the electronic world we are increasingly surrounding us with. As such, these materials can be used as sensors, actuators and energy harvesters. A major advantage of these materials over alternative systems is their conservation of functionality with scaling down to the nanometer range. This makes the ferroelectric materials attractive as components in micro-electromechanical systems (MEMS), and especially in MEMS technology for medical implantation due to the strict size restrictions in vivo. One of the main challenges in the field of ferroelectric materials is to develop lead-free alternatives to the state-of-the-art ferroelectric materials, the lead-titanate-zirconates (PZTs). This work was therefore concerned with the development of lead-free piezoelectric films for biomedical applications based on the ferroelectric material potassium sodium niobate (K0.5Na0.5NbO3, KNN). The principle goal of the project was to develop an aqueous wet chemical processing route to KNN films with properties appropriate for medical applications.

The first part of the thesis was focused on the development of an aqueous synthesis platform to phase pure KNN thin films. Two water-based precursor solutions were investigated, prepared from the combination of alkali metal nitrates and either oxalic acid (KNN-Ox) or malic acid (KNN-MA) complexed niobium as cation precursors. The decomposition process of these precursor solutions during film pyrolysis was demonstrated to be crucial for promoting nucleation of KNN and suppressing the formation of secondary phases. By combining thermal analysis, X-ray diffraction and infrared spectroscopy on powders prepared by drying and calcining the precursor solutions, it was shown that the decomposition temperature could be manipulated by the solution chemistry and processing conditions. Based on the analysis of the powder processing, a processing route to phase pure KNN films on single crystal SrTiO3 substrates was developed. Transmission electron microscopy and X-ray diffraction was applied to investigate the microstructure of the KNN films, and the films were shown to have a columnar microstructure with out-of-plane texturing. A correlation between the thermal processing and the observed film texture was discussed.

Compositional engineering of the KNN films was further targeted with the aim of improving the ferroelectric properties. Ca2+ doping and Ca2+-Ti4+ co-doping (CaTiO3 doping) was implemented in the aqueous processing route developed in the first part of the thesis. Platinized silicon (SiPt) substrates were used since Si-based substrates are more relevant for integration in electronic applications. It was demonstrated that the dopants were dissolved into the KNN crystal lattice and the ferroelectric properties of the doped KNN films were significantly improved relative to the undoped KNN films. Doping was observed to promote ferroelectric switching, resulting in films with remnant polarizations of 6.37±0.47 and 7.40±0.09 μC cm-2 for the Ca2+ and CaTiO3 doped films, respectively. A high dielectric permittivity (1800-3200) was observed for all three compositions of the films.

Fabrication of piezoelectric oxide films on flexible supports allows for high-performing microelectromechanical system (MEMS) devices with minimal loss of functionality due to mechanical clamping. Motivated by this, a wet chemical processing route to flexible KNN films on polymer supports was explored. The synthesis involved a lift-off procedure using ZnO as a release layer for the flexible KNN film. X-ray diffraction and infrared spectroscopy were used to investigate the phase composition of the different oxide layers during the processing of the films, and introduction of a BaTiO3 buffer layer was shown to mitigate the formation of secondary phases in the KNN film grown directly on ZnO. Microstructural investigation using scanning electron microscopy revealed that the mechanical integrity of the flexible KNN film was greatly improved by the addition of a platinum bottom electrode between the film and PDMS support. A proof-of-concept for a complete aqueous processing route of a flexible KNN film was demonstrated, and future development possibilities for the preparation of flexible lead-free ferroelectric films were discussed.

In the final part of the thesis, in vitro cytotoxicity assays were performed to evaluate the biocompatibility of the KNN films with living cells. Undoped and CaTiO3 doped KNN films on SiPt substrates were tested using three different cell lines. The cells were grown on the films for up to 10 days, and cell morphology, viability and proliferation were assessed using scanning electron microscopy, immunofluorescence microscopy and spectrophotometry. The results from the in vitro experiments showed that proliferation rates for the cells grown on the KNN thin films were equally high or higher than those on glass control samples, and no cytotoxic effect from either the films or the substrate was observed. The results demonstrated that the KNN films prepared in this thesis are very promising candidates for components in implantable medical devices.

To summarize this work, an aqueous synthesis platform to phase pure ferroelectric KNN films was established, and the processing route was demonstrated to be compatible with compositional alterations and the use of both rigid and flexible substrates. in vitro biocompatibility of the films with cells was also demonstrated. These results highlight the potency of aqueous chemistry as an environmentally friendly and cost-effective tool in oxide film fabrication. New ideas and techniques regarding processing of films on polymeric supports were also introduced and explored, and this part of the project is suggested to hold great potential for further exploitation. Finally, this thesis demonstrates the potential of KNN films as components in implantable medical devices.