Polymer derived ceramics process in biomedical applications: pacemaker electrode
Current biomedical material research involves the use of a large variety of materials depending on the specific functions and characteristics required. The high mechanical performances and the relatively easy forming techniques allowed metals to be the most widely used material for implantations. Nevertheless, the development of new high performances ceramics is an established trend in this field because their superior biocompatibility (e.g. alumina and zirconia) and corrosion resistance. Despite these improvements the replacement of metal with ceramics is limited, due to the complex shapes or the specific properties required, e.g. electrical conductivity. An alternative is represented from polymer-derived-ceramics (PDCs) technology. Starting from a liquid organosilicon polymer, ceramics with free shapes can be produced using polymer and micro-electrical-mechanical-systems (MEMS) fabrication techniques such as micromoulding and photolithography. Depending from the selected application, material properties can be tuned using different starting polymer precursors, varying the process parameters or adding suitable fillers.
This dissertation is part of the Swiss National Science Foundation (SNSF) research project CERAMED developed at Empa High Performance Ceramics Laboratory in collaboration with EPFL Microsystems Laboratory and Bern Inselspital Clinic for Cardiovascular Surgery. The project aims to combine recent findings in the field of conductive ceramic materials with advanced micromoulding methods to study novel 3D implantable electrodes for low-power, long-term pacemaker applications. The scientific work presented here involves the development of a suitable material that possess all the main requirement, from process to physical and biocompatibility properties, to replace metals in pacemaker electrodes.
In the first chapter an overview is given of the pacing device, its different components and biocompatibility of metal electrodes compared to state-of-the-art ceramic ones. Current advancement in commercial pacemakers and an outlook on research challenges are also presented. Fundamentals of PDCs technology such as precursors, fillers and processing are introduced in chapter one. Specific topics regarding electric conductivity, filler suspensions and carbon nanotube (CNTs) composites are presented in more details at the beginning of each following chapter.
The second chapter contains a detailed description of the technical approach to obtain a crackfree and electrically conductive ceramic from commercial polycarbosilane SMP10. The proposed fabrication method is based on the addition of divinylbenzene as liquid carbon precursor. When an optimum divinylbenzene amount is added, hydrosilylation is promoted and ceramic crack-free disc samples up to 10 mm in diameter can be produced. The advantages of the developed method are the use of a liquid-fabrication route, where the precursor can replicate any mould shape without limitations, and a pressureless curing step. The process is also demonstrated suitable also for production of microsized samples. A further advantage in the addition of divinylbenzene to polycarbosilane SMP10 is the development of a carbon percolative network after pyrolysis that raise the electrical conductivity up to 1 S/cm. Due to these favorable characteristics the material is used as a matrix for the development of novel ceramic composites described in the following chapters.
Chapter three presents a novel and efficient method for CNTs dispersion. The polycarbosilane SMP10 and the polysilazane Ceraset are shown to be able to cover the nanotube wall if hydrosilylation is induced with a Pt(0) catalyst. The result is the stable suspension of CNTs in several organic solvents. Process parameters such as catalyst amount and sonication time are investigated and optimized. This work shows that the developed method is comparable with traditional ones based on surfactants and it also has several advantages: it is simple, can be completed in few minutes, and an up to 0.50 mg/ml of CNTs are suspended, which is a sufficient concentration for many applications.
Chapters four analyze the preparation of a CNTs-ceramic composite. Despite the filler presence, the process maintains all the peculiarity of the original matrix with a pressureless fabrication method suitable for the production of micro and macro samples. However the effects of nanotubes introduction do not significantly improves the electrical properties of the ceramic due to the already high carbon content of the matrix. In addition the more complex process leads to a higher cracking probability during pyrolysis. All the results are critically discussed and compared with existing methods in PDC technology.
Chapter five analyzes the cytotoxicity of the ceramic developed in chapter two. The test, conducted in comparison with standard biocompatible materials such as alumina and biogradestainless- steel, demonstrates the non-cytotoxicity of the developed material and its potential use in medical applications.