Transient Electronics for Smart Biodegradable Medical Implants

Bioresorbable implantable medical devices show great potential for applications requiring medical care over well-defined periods of time. Such implants naturally degrade and resorb in the body, which eliminates adverse long-term effects or the need for a secondary surgery to extract the device. Since biodegradable materials are water-soluble, their fabrication requires special care and relies solely on dry processing steps without exposure to aqueous solutions. Another challenge is the in vivo powering of medical implants that are only constituted of biodegradable materials. The objective of this thesis is to develop a fully biodegradable drug delivery implant with multiple reservoirs for on-demand wireless drug delivery. Then to integrate and miniaturize all the components to reduce the volume of material used, whilst limiting the fabrication process complexity. The design, fabrication and characterization of selectively addressable biodegradable magnesium microheaters is presented, together with the development of an innovative microfabrication process. It consists of four steps: physical vapor deposition, photolithography, ion beam etching and resist stripping. The microheaters consist of several spiral resonators. An electrical current is induced in a specific spiral resonator when coupling an external magnetic field at a matching frequency. Adding a meander to the resonator increases the current density locally and creates a hot-spot. Slightly varying the geometry of the devices enables the tuning of their resonance frequency and makes them selectively addressable. The frequency-selective wireless heating of different resonators is demonstrated in air and in liquid, plus enables the melting of the surrounding environment. Additionally, small geometrical variations by design induce large frequency shifts. As a result, several resonators with multiple resonance frequencies can be integrated in one device without much of an increase in diameter. These microheaters are used as power receivers and triggering elements to selectively release drugs from multiple reservoirs in a wirelessly controlled drug delivery capsule. This prototype is made of non-biodegradable materials to demonstrate the feasibility of the concept. It consists of a 3D printed capsule with several reservoirs filled with drugs. Each reservoir is sealed with a polyimide membrane and a resonant gold microheater on top of it. These microheaters are used to thermally trigger the breaking of the membranes and release the drugs from the reservoirs. The powering and triggering mechanisms are combined into one element, minimizing the volume of material and maximizing the drug-to-implant volume ratio. The successful fabrication of leak-free capsules and the wireless release of liquid from the reservoirs is demonstrated. This capsule is compatible with the drug requirements needed in the context of local analgesia after a knee arthroplasty. However, to have a clinical outcome, it should be fabricated from biodegradable materials to prevent a second surgery to extract the device. Finally, to explore the development of a fully biodegradable version of the device, biodegradable microheaters are fabricated on biodegradable membranes by transfer printing. The capsules are then micro-molded from biodegradable elastomers.

Design, characterisation and modelling of a high current DC arc plasma source for silicon and silicon carbide processing at low pressure

Lukas Derendinger

In the frame of this thesis, two similar high current DC arc (HCDCA) plasma sources were investigated in a low gas pressure regime (10-3-10-2 mbar). One of them was initially designed for the epitaxial growth of silicon and silicon germanium (LEP), the other for the industrial deposition of diamond (BAI). The LEP source was analysed using pure argon plasmas. Measurements of the ion saturation current were performed with a custom-built multi-Langmuir probe to analyse plasma density homogeneity. Plasma instabilities at 50 Hz were observed and studied by different means. One source of the instability is the AC current used for the filament heating, whereas lower frequency instabilities are due to the use of a ring shaped anode. A sensitive Hall sensor was used to measure the magnetic field induced by the discharge current. It was found that depending on the plasma parameters gas pressure and external magnetic field the current tends to attach at different points on the anode, leading to a complete loss of reactor symmetry. The installation of an additional cusp field around the reactor chamber lead to an increase of the overall homogeneity of the plasma density, but it could not resolve the problems of current attachment. In the following the ring anode was replaced by a point anode and the maximum external magnetic field strength was increased by a factor of ten. With a novel multi-Hall probe the current density of the now columnar shaped plasma was measured, showing a strongly peaked current density profile, when an external magnetic field is applied. Together with ion saturation current measurements made with a Langmuir probe, the electron temperature inside the plasma column was estimated to be about 4 eV. In earlier works made on the BAI reactor the high dissociation efficiency of the HCDCA plasma source has already been shown. Optical emission spectra were compared between RF plasmas, low pressure (1.5mbar) HCDCA plasmas and our very low pressure (10-3-10-2 mbar) HCDCA plasmas. The dominating species found in RF plasma spectra are molecular, the spectra of low pressure HCDCA plasmas are dominated by atomic species and the spectra of very low pressure HCDCA plasmas are dominated by ions, emphasising the high dissociation efficiency of this system. Silane and Methane was used as precursor gases in the BAI reactor for the deposition of silicon carbide films. Promising high deposition rates up to 9 nm/s were found, but FTIR spectroscopy showed high hydrogen and oxygen concentrations in the porous films, making the not optimised deposited material useless for the application of wear-resistant coatings. Microcrystalline hydrogenated silicon (µc-Si:H) is viewed as a cost- and energy-effective alternative to crystalline silicon for the production of solar cells. A detailed analysis of the deposition rate and the Raman crystallinity of µc-Si:H films deposited in the BAI reactor showed deposition rates up to 6.5 nm/s and a wide range of crystallinity from 0-80%. Film thickness inhomogeneity in silicon solar cells has to be less than 5%. To meet this standard a first substantial improvement was achieved with the installation of a linear gas injection along the plasma column. Tests on large surface glasses (47 × 37 cm2) revealed strong diffusive effects, which could be reproduced with a simple gas diffusion model. The model showed the necessity to reduce the dead volume around the plasma and to set the substrates as close as possible to the plasma column in order to minimise film thickness inhomogeneity due to diffusion. Deposition rate measurements made in these conditions assured the results of the model. The development of a method to estimate the dissociation efficiency of the plasma by simple pressure measurements showed also an important increase of the silane dissociation from 75-92% when the plasma is confined in a smaller volume. Therefore, compared to the reactor with a large dead volume, only a third of the initial silane is lost to the pumps.

EPFL2008

Biodegradable Frequency‐Selective Magnesium Radio‐Frequency Microresonators for Transient Biomedical Implants

Remo Blum, Giovanni Boero, Jürgen Brugger, Matthieu Jean Michel Rüegg

Bioresorbable implantable medical devices show a great potential for applications requiring medical care over well‐defined periods of time. Once their function is fulfilled, such implants naturally degrade and resorb in the body, which eliminates adverse long‐term effects or the need for a secondary surgery to extract the implanted device. Since biodegradable materials are water‐soluble, the fabrication of such transient electronic circuits and devices requires special care and needs to rely solely on dry processing steps without exposure to aqueous solutions. A further challenge is the in vivo powering of medical implants that are only constituted of biodegradable materials. This paper describes the design, fabrication, and testing of radio‐frequency biodegradable magnesium microresonators. To this end, an innovative microfabrication process with minimal exposure to aqueous media is developed to fabricate magnesium‐based, water‐soluble electronic components. It consists of a novel sequence of only three steps: one physical vapor deposition, one photolithography, and one ion beam etching step. The frequency‐selective wireless heating of different resonators is demonstrated. This represents a significant step toward their use as power receivers and microheaters in biodegradable implantable medical devices, for applications such as triggered drug release.

2019

Biodegradable Wireless Microheaters for Transient Biomedical Implants

Remo Blum, Giovanni Boero, Jürgen Brugger, Matthieu Jean Michel Rüegg

Transient electronics show a great potential in the field of bioresorbable medical implants, for applications such as localized drug delivery for wound healing or following a surgical procedure. Once their function is fulfilled, such implants naturally degrade and resorb in the body, which eliminates adverse long-term effects or the need for a secondary surgery to extract the implanted device. Since biodegradable materials are water-soluble, the fabrication of such transient electronic circuits and devices requires special care and needs to rely solely on dry processing steps without exposure to aqueous solutions. A further challenge is the in-vivo powering of medical implants that are only constituted of biodegradable materials. This work describes the design, fabrication and testing of radiofrequency biodegradable magnesium (Mg) microresonators. To this end, an innovative microfabrication process with minimal exposure to aqueous media is developed to fabricate magnesium-based, water soluble electronic components, without the need to fabricate and use fragile stencils. It consists of a novel sequence of only three steps: one physical vapor deposition, one photolithography, and one ion beam etching step. The frequency-selective wireless heating of different resonators is demonstrated.

2019

Design, characterisation and modelling of a high current DC arc plasma source for silicon and silicon carbide processing at low pressure

Lukas Derendinger

EPFL2008