Development of Implantable Electrodes Based on Polymer Derived Ceramics

The invention of heart pacemakers in the 50's aims to help patients to overcome heart strokes, which represent the first cause of death worldwide. In Europe and in the USA, more than 3000 pacemakers are implanted every day. Despite the constant evolution of the pacemaker system, the lifetime of the implants is limited to several years. State of the art pacemaker electrodes suffer from pacing signal inhibition related to fibrotic tissue growth. This thesis describes the development of novel electrically conductive ceramic based materials, which promise to be more bio-inert than the currently used metals electrodes. Several Polymer Derived Ceramic (PDC) materials (Ceraset, SMP10 and SU8) have been investigated. In order to induce electrical conductance our project partner at EMPA doped the ceramics by an increase in carbon contents. However, for SU8 this step was not necessary. Photolithography and moulding were the two main microfabrication methods used for shaping the PDC materials. Moulding showed to be the most suitable method for shaping Ceraset and SMP10 materials. Thanks to the optimization work of the mould coatings, the main advantage of moulding consists in the possibility to release the PDC from the mould prior the sintering at high temperature. The release can be further facilitated when using elastomeric mould. SU8 was shaped by means of photolithography on top of a water soluble Dextran release layer and released before sintering. The materials were characterized at each fabrication step. The material composition, surface analysis and hardness of the densified ceramic samples have been measured. Furthermore an electrochemical characterization was performed in PBS and 1M KCl solution, by means of cyclic voltammetry and electrochemical impedance spectrometry. The measurements of the new materials show comparable values and behaviours to standard Pt metal electrodes. As the electrodes are meant to be implanted, it was necessary to investigate their cytotoxicity and in vitro biocompatibility for non-conductive and conductive compositions. Low cytotoxicity levels comparable to the state of the art positive reference implant materials have been assessed. To elucidate the fibrosis inhibition, long-term implantation tests (1 week to 6 months) have been carried out without the presence of electric signals. The fibrosis level turned out to be low for the new materials and comparable the reference materials. The pacing functionality of the new electrodes has been experimentally assessed as follows: (a) In vitro pacing of isolated cardiomyocytes by applying an electric potential of 20-40 V between two electrodes. The cell contraction was quantified by correlating the pacing signal with the hemodynamic parameters by means of video recording and image processing; (b) In vivo and ex vivo tests on a rat model were conducted by implanting SU8 micro-electrodes into peripheral muscles and in an isolated and perfused heart, respectively. The analysis confirmed that both muscle tissues were successfully actuated by using typical signal parameters. In conclusion, this thesis presents the successful development, optimisation and fabrication of pacemaker electrodes with new materials, which possess properties that are comparable to state of the art platinum electrode. This work provides a selection of materials for further characterization of long term implanted electrodes, studying the influence of an electric pacing signal on the fibrosis.

Inkjet printing and high aspect ratio structuring for polymer-based micro and nano systems

Vahid Fakhfouri

Inkjet printing of functional materials, photolithography and micromolding are 3 different techniques largely used in the field of micro-electromechanical systems (MEMS) with increasing interest for the fabrication of new devices and the development of new applications. This Ph.D. thesis provides new insights into each of these fields and, by combining them, aims to enable the access to new application fields. Complementary to photolithography, inkjet printing is increasingly considered a cost-effective and flexible microfabrication method to structure functional materials. The ease of mass fabrication and the inherent flexibility of inkjet technology make it a suitable method for the manufacturing of microsystems and components. The results presented in this dissertation summarize recent achievements in this relatively new technology for the development of miniaturized devices. First the piezo-acoustic inkjet printing technology is described and a theory to explain the drop generation process is developed and evaluated. The experimental work was focused on the drop-on-demand inkjet printing of viscous polymers, particularly SU-8, a negative tone photo-resist epoxy. It validates the theoretical predictions and shows the shortcomings of the existing theories. The camera-assisted high accuracy alignment (< 3 µm misalignment) technique developed in the frame of this work enables the precise positioning of microdrops on pre-structured substrates and accurate filling of high aspect ratio micromolds. Additionally, polymeric microlenses were also Inkjet-printed and characterized. The highly reproducible optical properties of such microlenses, i.e. the focal length in the range of 40-150 µm and the numerical aperture in the range of 0.25-0.75, can be adjusted for different applications in micro-optical systems. An innovative technique, based on micromolding, for the microfabrication of high aspect ratio ceramic structures using polymeric precursors is also presented. Polyureasilazane, a precursor for silicon-carbide-based ceramics was investigated. Pipettes and inkjet printers were used to fill high-aspect ratio SU-8 molds. The micromolded structures were released using a sacrificial layer. After a pyrolysis step which enables the polymer-to-ceramic transformation in a simple tube furnace, the final structures showed that this process is very reliable as the mold shapes were perfectly replicated and a smooth surface was obtained. This technique is an interesting candidate for the fabrication of ceramic devices for harsh environments and high temperatures. Incorporation of luminescent nanocrystals (NCs) in SU-8 is another contribution of this thesis. For this purpose, the selection of an appropriate solvent for both NCs and the epoxy-resist formulation is fundamental in directing the nanocomposite resist preparation. The NC-modified SU-8 was successfully patterned and high aspect ratio structures with micrometer resolution were fabricated, showing that the overall UV-structuring capability of the modified resist was not affected by the NC incorporation. The obtained microstructures were shown to retain the NC emission properties. Finally the possibility to combine inkjet printing, micromolding and photopolymerization for the fabrication of Hybrid AFM probes by a single-plane surface-micromachining technique was demonstrated. Hybrid AFM probes, with a tip made of a hybrid organic/inorganic polymer and a cantilever and a holder made of SU-8 were micro-structured, characterized and tested for AFM imaging. The results were compared with a commercial silicon probe and a SU-8 AFM cantilever. This technology can be used to convey new properties to scanning microscopy probes for new sensing applications or to improve the quality of the existing devices. Functional materials can be inkjet-printed to form the tips with a minimum material waste. This thesis is intended to contribute to the advancement of micromachining technologies in the area of micro-devices with enhanced properties. In particular, it shall contribute to the cost effective fabrication of hybrid components made of different functionalized materials.

EPFL2008

Préparation et caractérisation de nanoparticules bimétalliques (Pt-Au) sur un substrat en diamant dopé au bore

Bahaa El Roustom

Using petrol as a main source of energy, during the 20th century, has caused considerable amounts of damage among which are atmospheric pollution and global warming. At the same time, many geologists and economists expect that the discovery rate of new petrol sources will not follow the market's demand causing a serious shortage of petrol in the near future. These alarming perspectives have motivated scientists around the world to develop new clean and renewable energy sources. It is in that objective that electrochemists intensified their research in order to develop the fuel cell technology. This latter consists in directly converting chemical energy into electricity. The main advantage of this technology over traditional energy production is that the fuel cell energy efficiency is Carnot cycle independent. The theoretical efficiency of such an energy source is nearly 90%. Direct Alcohol Fuel Cell (DAFC), fed by ethanol on one hand and by air (oxygen) in the other hand, is a very interesting option for such kind of energy production. Ethanol is a renewable energy source of vegetal origin whose CO2 production is consumed by plants during the cycle. Even though Pt has long been known to be a perfect catalyst for DAFC, it has some principal limitations, the most important ones being poisoning by intermediate products (OH on the cathodic side and CO on the anodic side) and a relatively high cost compared to other materials. These problems have pushed the experts in the field to focalize their research on the development of more suitable and better performing materials. Gold nanoparticles have shown a surprising catalytic activity, very different from the bulk, making them a preferential candidate to replace Pt. A system based on using two or more metals as catalysts (example Au-Pt) dispersed on an appropriate substrate seems to be also an interesting candidate to enhance the cell's efficiency. The substrate should be chemically inert, non-oxydable and should not present any interference with the electrochemical and electrocatalytic behaviour of the nanoparticles. In the first part, the employed substrate, which is boron doped diamond (BDD) is characterized. The substrate's main characteristics are its chemical inertness, its weak residual current and its resistance to corrosion. These characteristics contribute to making the substrate an ideal support for nanoparticles. In this work, the substrate is a diamond film, but for fuel cell applications diamond should be used as powder or dispersed particles. In chapters III and IV, we studied the effect of boron incorporation on the morphology and the behaviour of the diamond film. For this purpose, samples with different ratios of boron/carbon (B/C) in the vapour phase during preparation were characterized (B/C = 300, 1000, 2500 and 6000 ppm). Results showed that the grain size decreases with increasing boron content in the film. This is accompanied by a transition towards a quasi-metallic character. It was also found that graphitic impurities in the diamond film increase when the B/C ratio increases. We also established some graphitic impurities increase in the diamond film when the ratio of B/C increases. The identity of these impurities was established in chapter III as being surface redox couples (semi-quinone/semi-hydroquinone). Using cyclic voltametry we were able to observe a total elimination (B/C=300 ppm) or partial elimination (B/C=6000 ppm) of these surface redox couples. We also found in chapter IV that these surface redox couples are responsible of the electrochemical activity of the BDD electrode. BDD electrodes with different doping level B/C=300, 1000 and 6000 ppm) in the vapour phase are modified with IrO2 nanoparticles using the method of thermal decomposition. Our results showed that, at low potentials, the behaviour of the composite electrode IrO2/BDD with respect to the reaction of oxygen evolution is independent of the doping level. On the other hand, at high potentials, the weakly doped electrode (B/C=300 ppm) presents a supplementary overpotential due to the interface BDD-IrO2. An approach based on the double junction is developed in this work describing two distinct behaviours depending on the boron concentration in the diamond film. A new method for Au nanoparticles deposition on highly doped BDD electrodes (B/C= 2500 ppm) is presented in chapter VI. This method involves two steps: the first step consists on sputtering the equivalent of few nanolayers of Au on the diamond surface; the second step is a heat treatment in air at high temperatures. Several parameters that could influence the dispersion, the stability as well as the diameter of the Au nanoparticles are studied. In chapter VII, the behaviour of the composite electrode Au/BDD is studied in a solution containing redox couples in acid media. This electrode was simulated as an arrangement of active Au microelectrodes dispersed on a less active BDD substrate. The overall behaviour of the composite electrode (Au/BDD) has been related to the electrochemical rate constants on each material. In chapter VIII, another new synthesis method for a bimetallic system formed by Au and Pt nanoparticles dispersed on a BDD substrate is developed. This method consists in electrodepositing a small amount of Pt on a composite electrode Au/BDD. Our results showed that Pt was preferentially deposited on Au covering its whole surface. In fact, Au nanoparticles serve as germ of nucleation for Pt. In the following step, the bimetallic PtbAu/BDD is heated to 400°C in air to enhance the interaction between Au and Pt. The experiments showed that Au reappeared on the surface. The electrochemical behaviour of the bi-metallic electrode is characterized by a negative shift in the peak potential of reduction of Pt oxides. This is not the case of heterogeneous alloys obtained by the fusion of two metals at high temperatures. The composite electrode Au/BDD as well as the bi-metallic electrode Pt-Au/BDD are used for electrocatalytic applications. The studied reaction in chapter IX is the oxygen reduction reaction (ORR) in an acid medium. We found that the number of exchanged electrons increases with increasing the amount of Pt at the surface. Oxygen is reduced into H2O2 on the Au/BDD electrode and into H2O on the Pt-Au/BDD electrode. In chapter X we studied the reaction of oxidation of oxalic acid. The behaviour of both Au and Pt electrodes seemed very similar for this reaction. The bi-metallic electrode containing both Pt and Au on its surface has shown an intermediate behaviour between the behaviour of the two metals.

EPFL2006

Advances in the characterization of nanowire photovoltaic devices

Dmitry Mikulik

III-V nanowires (NWs) have a great potential for solar energy applications due to their diameter-dependent optical properties, which may enhance absorption of light. In addition, core-shell radial p-i-n structures, in which the direction of light absorption is orthogonal to the carrier collection, can provide efficient carrier collection. The main goal of this thesis is the experimental study of the challenges of NW-based solar cells, related to materials and device fabrication. In the first part of the thesis, we present an analysis of where the electrical losses can be originated. By applying an equivalent circuit analysis approach, we classified them into three main groups: (i) the non-uniformity of NWs which may result in a reduction of the parallel resistance, (i) potential barriers originated at the different materials interfaces in the solar cell structure may result in an increase of the series resistance or addition of a second diode and (iii) surface recombination resulting in the reduction of the open-circuit voltage. In this thesis, we propose separate strategies to characterize and tackle these factors. The electric scheme of a NW-based solar cell consists of an ensemble of p-n junctions connected in parallel. We show how conductive-probe atomic force microscopy, C-AFM, is an essential tool for the characterization and optimization of these parallel-connected NW devices. We demonstrate topography and current mapping of the NW arrays, combined with current-voltage (IV) measurements of the individual NW junctions from the ensemble. Our results provide discussion elements on some of the factors limiting the performance of a NW-based solar cell, such as uniformity and photosensitivity of the individual NW p-n junctions within the array, and thereby a path for their improvement. Besides parallel losses due to uniformity issues, barriers in the carrier collection through the various heterointerfaces composing the device is discussed. To analyze it, we illuminate GaAs NW-based solar cells at different levels of light intensity and extract IV characteristics. This analysis helps to separately study the NW p-n junction response and the series resistance. The high series resistance of the NW-ensemble device can be attributed to the following interfaces: 1) GaAs-ITO, forming a photoactive Schottky diode, which suppresses the p-n junction at high concentrations of light, and 2) Si-GaAs heterojunction, disturbing the flow of majority carriers. Finally, the characterization of surface passivation in high-aspect-ratio nano/micro structures is addressed by electrochemical impedance spectroscopy (EIS). The method is applied to Si micropillars, as a proof-of-concept prior to the application to III-V nanowires. We tested structures passivated by a dielectric layer. The effect of different surface treatments on the interface state density were quantified by the analysis of the capacitance-voltage and conductance-voltage characteristics. This method allows the electrical measurements on rough vertical surfaces, which would otherwise suffer from high gate leakage currents if tested using solid-state metal-insulator-semiconductor scheme. The results and characterization methods, demonstrated in this work, contribute to the overall efforts of the scientific community on how to reveal the main engineering challenges in NW-based solar cells. It thus paves the way to approach the fundamental conversion efficiencies predicted by theory.

EPFL2018