In the 21st century, the broad fields of medical science and engineering are more closely linked than ever. Advances in bio-medical technology are driven by combining the detailed knowledge of the processes in the human body with the latest technical innovations in order to diagnose and treat patients more efficiently. One key element of this revolutionary progress is the automated treatment of diseases and continuous monitoring of vital parameters. This is mainly achieved with the help of electronic medical implants and wearable sensors. So far, these devices mainly rely on common batteries as power supplies. With decreasing implant sizes the need for power supplies capable of miniaturization arises. In this thesis, implantable glucose fuel cells are investigated as power supply concepts for medical implants and wearable sensors. Abiotic glucose fuel cells are energy harvesters. They convert the human body’s ambient chemical energy stored in glucose and oxygen to usable electrical energy. In contrast to batteries, they are theoretically able to continuously supply energy to medical implants without the threat of exhaustion. Surgical replacements of implants due to depletion of their power supply would become obsolete. However, body-fluid constituent as for instance amino acids poison the anode’s surface and drastically diminish the performance and lifetime of the glucose fuel cell. Furthermore, the power outputs of abiotic glucose fuel cells are low compared to commonly used batteries preventing them from being a competitive power supply technology for medical implants and wearable sensors. In this dissertation, two major strategies are pursued in order to enhance the applicability of abiotic glucose fuel cells: Developing abiotic glucose fuel cells towards medical applications requiring rather short lifespans, and enhancing the fuel cells’ anodes in order to overcome one or more of the current drawbacks (poisoning issues, poor long-term stability, and low current densities). First, abiotic glucose fuel cells are investigated towards their applicability as power supply for electronic contact lenses. These smart lenses are a promising, non-invasive platform for medical sensors measuring and monitoring a variety of vital signs and medical parameters. Electronic contact lenses are focused on a single day usage reducing the required lifetime of the power supply by far compared to medical implants such as e.g. cardiac pacemakers. The performance of abiotic glucose fuel cells is studied in simulated artificial tear fluid. Considering the available space on standard contact lenses, the results show that abiotic glucose fuel cells would be able to generate 2 µW for a period of at least 24 h. The power output provided by the fuel cell is sufficient for some low power applications intended for the integration in contact lenses. It was found that coating the galvanic cells with the commonly used soft contact lens material pHEMA (Poly(2-hydroxyethyl methacrylate)) drastically diminishes the performance. In comparison to abiotic glucose fuel cells, the applicability of Mg/air batteries has been studied as well. In Mg/air batteries a sacrificial magnesium anode and an oxygen reducing cathode provide electrical energy. Even though, Mg/air batteries have a limited lifetime due to the restricted magnesium mass, the high energy density identifies them as a promising candidate for short term implants and wearable sensors. On an electronic contact lens, Mg/air batteries would be able to generate 40 µW for a period of at least 24 h. Even though the performance of Mg/air batteries is negatively affected by the pHEMA coating as well, the presumably generated 7 µW would still be sufficient to power many electronic contact lens applications. Second, abiotic fuel cell anodes are enhanced by two different approaches: Enlarging the specific surface of PtCu anodes by using a three dimensional electrode substrate, and fabricating highly porous PtCu-alloy anodes by introducing an additional noble metal to the fabrication process. In approach one, the catalytic surface area of the standard PtCu anodes is enlarged in order to harvest larger current densities despite the poisoning effect. A novel anode material is fabricated by electro-deposition of highly porous platinum onto a 3-dimensional carbon nanofiber support. This way, the anode’s roughness factor is increased by approx. a 2-fold (up to 16500 ± 2300) compared to standard PtCu anodes fabricated on flat 2-dimensional supports. In horse serum, current densities of (7.2 ± 1.9) µA cm^-2 are continuously generated for at least 30 days. Compared to state-of-the-art anodes under the same experimental conditions, the current density is enhanced by approximately a six-fold. The material’s superior performance is not only related to its high specific surface area but also to an improved catalytic activity and/or poisoning resistance. This result presents a major step forward in the development of implantable glucose fuel cells. In approach two, the commonly used highly porous PtCu anodes are alloyed with different additional metal partners (ruthenium, palladium, and gold) respectively. To this end, an anode fabrication process with repeated co-deposition of platinum, the alloy metal partner, and Cu from an aqueous electrolyte followed by selective copper dealloying is developed resulting in a porous platinum alloy structure. The obtained highly porous PtCu alloy anodes exhibit a roughness factor of 400 to 4000. Au-rich PtCu-Au alloys (~75 at% Au) significantly improve the catalytic activity for glucose oxidation compared to standard PtCu anodes. Since these PtCu alloy anodes are of interest to other biofuel cells as well, they are additionally investigated for the use in methanol and formic acid fuel cells. Compared to PtCu anodes, small additions of Ru (~3 at% Ru) lead to significant enhanced catalytic activities towards methanol and formic acid. Within this dissertation, an improvement of the performance of abiotic glucose fuel cell anodes has been successfully achieved. This is a major step forward in the realization of abiotic glucose fuel cells as power supply for medical implants and wearable sensors. Furthermore, the work confirms that electronic contact lenses are a promising short-term application for abiotic glucose fuel cells. The presented work provides a good platform for further research and development on the miniaturization and integration of abiotic glucose fuel cells in electronic contact lenses. In addition to abiotic glucose fuel cells, it has been successfully shown that Mg/air batteries qualify as energy supply for electronic contact lenses with regard to the achieved power output. Moreover, it has been demonstrated that platinum alloy anodes with small additions of Ru are promising candidates for methanol and formic acid fuel cells. Future research will have to focus on a deeper understanding of the anode’s poisoning behavior and a further improvement of the long-term stability beyond the shown 30 day period. In case of electronic contact lenses, experiments in more realistic tear fluid media, investigating the influence of tear fluid constituents such as amino acids, proteins and lipids will be necessary. Miniaturization and implementation of abiotic glucose fuel cells (as well as Mg/air batteries) in functional contact lenses will be a further step towards application.
Towards abiotic glucose fuel cells as powering concepts for medical implants and sensors
2019
Sonstige
Elektronische Ressource
Englisch
DDC: | 629 |
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