Piezoelectric MEMS for Energy Conversion in Biomedical Devices
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Next-generation implantable medical devices are increasingly dependent on the development of sustainable and safe power sources that eliminate the need for bulky battery packs, as well as efficient and miniaturized sensing mechanisms capable of monitoring multiple physical vital signs with low power consumption. In this context, micro-electromechanical systems (MEMS) has emerged as a promising technology, attracting significant attention for their potential in various industries and applications under stringent operational requirements. This thesis explores MEMS technology, focusing on innovative methods for energy harvesting and vital signs monitoring using miniaturized, micromachined piezoelectric transducers. Specifically, the study designs transducers targeting two primary energy sources: internal mechanical movements of body organs and a safe, reliable ultrasound power source.
The first half of this work introduces a frequency comb-vibrational energy harvester, designed and characterized to function under a broadband and random vibration source. The frequency comb phenomenon is traditionally studied in the optical domain for applications in timing and spectroscopy. In this thesis, an application to vibration energy harvesting utilizing a MEMS transducer is studied leveraging its effective signal mixing and mode coupling capabilities when driven off-resonance within a specific range. This thesis marks the first instance of operating a frequency comb harvester with a broadband vibrational signal, mimicking real-world application conditions, and resulting in significant improvements in energy output and performance. Testing across multiple conditions, the study analyzes the dependency of the harvester's performance on various driving conditions, including frequency and signal amplitude. Notably, when operated in frequency comb mode with a single-frequency signal, the device achieved a total output power of 14.74 µW under 0.07 g acceleration, a marked enhancement from the 4.03 µW generated in non-comb mode at the same acceleration level.
IV The latter half of the thesis examines piezoelectric micromachined ultrasonic transducers (PMUTs) to tackle the challenges of energy generation and sensing in biomedical devices via ultrasound sources. An equivalent circuit method with defined lumped parameters was developed to understand the performance metrics of PMUTs. Leveraging insights from these figures of merit, two multi-channel PMUTs prototypes were firstly designed and fabricated in both circular and square configurations, capable of operating across multiple resonant modes simultaneously. An innovative dual-frequency channel concept was tested between two PMUT arrays, assigning the low-frequency mode to wireless power transfer and the high-frequency mode to data telemetry within a single device framework. Experimental tests on a silicone biomimicking phantom demonstrated that the circular PMUT prototype achieved a power output of 411.3 nW and a data transmission rate of 50 kbps with a bit error rate (BER) of 1.7 × 10 −2 . In contrast, the square PMUT prototype delivered a power output of 202.4 nW and a data rate of 10 kbps with a BER of 8.1 × 10 −4 . Prompted by the limitations observed in these initial prototypes, subsequent developments focused on numerically analyzing the mechanical responses of various mode combinations, which led to targeted optimizations in the electrode configuration and an increase in operating frequencies aimed at boosting overall device performance. The optimized multi-channel PMUTs featured customized electrode designs and increased operating frequencies, achieving a marked improvement in system capabilities. Notably, these enhancements led to an increase in the data transfer rate to 945 kbps and enhanced the power output to 504.3 nW, achieving 22.7% boost in power output compared to the initial prototype.
Lastly, aiming to exploit the multi-frequency capabilities with a different approach, a high-density PMUTs array with varied on-chip transducer dimensions and frequencies is designed to improve the transmitting and receiving sensitivity across an expanded bandwidth. This multi-frequency PMUTs array is tested for its ability to sense the dynamic motion of a blood vessel model with a wall thickness of 0.75 mm and temperature of blood-mimicking fluid, achieving a sensitivity of 9.7° phase shift per degree Celsius within a temperature range of 35°C to 45°C. Further testing of its broadened bandwidth and improved sensitivity includes efficiently capturing the emitted ultrasound signals from photoacoustic imaging applications. Tests on imaging phantoms with target dimensions ranging from 0.3 mm to 0.7 mm allow the PMUTs to characterize the corresponding frequency response and reconstruct images with comparable resolution and quality by integrating multiple transducer designs on the same chip.