With today’s aging population and the prevalence of chronic illnesses, healthcare systems worldwide are struggling to accommodate the increasing number of patients. This projected growth and rising demand underscore the importance of advancing wearable devices. In the realm of diagnostic healthcare, the pursuit of non-invasive and safe techniques is paramount. Until now, body patches have predominantly focused on monitoring surface-level body parameters such as temperature, humidity, pH, oxygen saturation, and electric potentials. The introduction of ultrasound patches extends the realm of possibilities, enabling a deeper exploration of physiological processes within the body. Additionally, ultrasound serves as a non-invasive diagnostic technique. To facilitate medical ultrasound imaging, an acoustic interface is indispensable for the unimpeded transmission of waves through the skin and tissue. This interface must maintain proper hydration and adhere to the skin to ensure a conforming acoustic connection. Initially, this MSc. thesis research aimed to conduct lifetime performance tests on promising ultrasound acoustic interface materials. These tests were conducted by placing wearable ultrasound patches with the acoustic interface materials in place on an ultrasound phantom. With the ultrasound transducer, ultrasound images were made over a fixed period. This experiment was done to see to which extent the image quality would degrade over time for the different interface materials. The available ultrasound phantoms did not meet the requirements of skin-mimicking properties, on which the lifetime of the acoustic materials would be tested. Consequently, this research opted to simulate specific skin conditions: temperature, moisture, and acoustic properties like human tissue. Different iterations were made and evaluated during the development of the final ultrasound phantom model. In this thesis, five different models were evaluated, and eventually, the final model was presented: A three-layer model. The phantom model consists of a gel wax filled with scattering objects, visible with ultrasound, at specific depths inside the filling. To mimic the water loss rate of the skin, a hydrating layer of agar was placed on top of the gel wax filling. A PET foil was deployed with a specific number of holes to let water through from the agar layer to regulate the amount of water evaporation over time. To mimic skin temperature, the phantom model was placed in an oven at T = 34 °C. With this final model, the lifetime experiments were conducted with six potential interface materials: AquaFlex (solid hydrogel), HydroAid (solid hydrogel), Ecoflex (silicone), Axelgaard (ECG solid hydrogel), HH5023 (ECG solid hydrogel), and HH5450 (ECG solid hydrogel). The duration of this experiment was eight days, after which the agar layer started to degrade and shrink. The filling and scattering objects of the phantom model are reusable, while the hydration layer has to be replaced or disposed of after five days. Further, acoustic properties, like materials-specific attenuation coefficient, of potential acoustic interface materials and materials used in the phantom were measured using a though-transmission setup. Also, a validation assessment of skin compatibility of the potential interface materials for a long duration of time was conducted. This was done with the consultation of experts in medical devices, medical professionals and literature. With these three different subjects of this MSc. the attenuation coefficient of six different acoustic interface materials are characterized and validated to be compatible with human skin for longer periods. The phantom model developed satisfies the requirements set and, most importantly, mimics skin temperature and water loss rate. One round lifetime (eight days) performance experiments of acoustic interface materials using the phantom model. It is difficult to conclude to which extend the image quality degraded over time for the different interface materials due to the agar layer dehydration after eight days and that the experiment was only conducted once. For future recommendations, it is suggested that the lifetime experiments be repeated using the phantom model for these six different interface materials. It could also be an option to renew the hydration layer every seven days to prevent the agar layer from dehydrating over the acceptable limit if the experiment requires longer periods.

During the past decade, Implantable biomedical devices have emerged as an efficient treatment for neurological disorders. Among them, low-intensity focused ultrasound (LIFU) has recently gained interest due to its focusing ability, depth of penetration and reversible effects. Although the exact biophysical mechanism that leads to the interaction between ultrasound and neurons is not clear yet, many successful applications have been reported both in vitro and in vivo in animals, raising prospects for clinical applications. Nevertheless, treatments were mainly delivered transcranially due to the lack of implantable technologies, constraining the frequency range to below 1 MHz. Thanks to the development of biocompatible capacitive micromachined ultrasound transducers (CMUTs), ultrasound has become implantable, opening the way for high frequency ultrasound neuromodulation. In addition, implantable biomedical devices are becoming smaller and smaller, requiring their power supply to follow this trend. Batteries can be a limiting factor for their miniaturisation and other powering methods have been investigated. Among them, ultrasound as a means to wirelessly transfer power represents the best trade-off between the amount of available power, implanted receiver size, and depth of penetration. Also in this case, biocompatible CMUTs could be used as transducers to power an implanted device. Yet they require a DC bias source in the order of tens of volts which is not practical for an implanted device. In this thesis the application of ultrasound for implantable biomedical devices is investigated. First, the modulation of the excitability of in vitro mice hippocampal brain slices (normal and epileptic) using high frequency sonication protocols is explored. Secondly, wireless power transfer using CMUT devices with charges trapped in the dielectric which do not require a DC bias voltage is investigated. The results indicate that high frequency ultrasound clearly affects the excitability of neurons, having an excitatory effect when delivered using a 5 MHz continuous wave sonication protocol, while it inhibits the neurons when they are exposed to pulsed wave ultrasound at 13 MHz. The intensities used were in line or much lower compared to values used by other groups of researchers. In addition, ultrasound power transfer with an efficiency higher than 48% was achieved using CMUT devices with charges permanently trapped in their Al2O3 layer. This efficiency was in line with values obtained by other groups using piezoelectric transducers. Based on these results, it is clear that ultrasound, specifically using implantable CMUT devices, has a great potential to be used in bioelectronic medicine applications for both the treatment of neurological disorders as well as a means of power transfer.

Particle sorting is an important step in many biological and biomedical techniques. One sorting technique that has received a lot of attention is based on acoustophoresis. Manipulating cells with acoustic forces allows for a sorting method based on physical properties in a non-invasive, label-free, and biocompatible manner. To date, acoustic sorting devices have been developed using either an interdigitated transducer (IDT) or a lead zirconate titanate (PZT) as an acoustic source. IDTs generate surface acoustic waves (SAW) that can sort particles with high precision, however only low throughput is possible. PZT, on the other hand, generates bulk acoustic waves (BAW) that allows for a simple device architecture with high throughput. Nevertheless, the current research done on BAW sorting devices also shows the disadvantage of frequency restrictions. An attractive alternative to PZT in acoustic sorting devices that has yet to be researched, are the capacitive Micromachined Ultrasonic Transducers (CMUT). CMUTs are well-known for their broad bandwidth, batch production and the possibility for complex designs and integration with electronic circuits. In this work, we present proof-of-concept devices to examine that CMUTs can be used as an acoustic source for particle sorting. Two different acoustic-microfluidic device interfaces were considered, namely the sidewall and the top area of the CMUT. Additionally, the presence of an acoustic pressure within the silicon substrate of the CMUT's die was investigated. Experiments show that CMUTs generate an acoustic pressure within the silicon substrate of the CMUT's die. This acoustic pressure increased 6.4 times when a phase alignment approach was used compared to when no phase alignment approach was used. Following these findings, acoustic-based sorting devices were assembled to perform particle alignment experiments. It was shown that continuous particle sorting could be demonstrated efficiently for both interfaces by using a pressure field based on standing waves. Additionally, structures integrated into the microfluidic devices, such as a vacuum horn, have the potential to amplify the acoustic signal. These findings indicate that CMUT-based microfluidic devices could provide a promising method for sheathless particle sorting.

Active implantable devices are conventionally powered with batteries. The miniaturisation of implants necessitates the need to find an efficient way to transfer power wirelessly. Ultrasound energy transfer with its short wavelength, small receiving transducer size, beamforming capability makes deep tissue penetration possible making it a good alternative for powering deep implants. There is also the advantage of using the reflected energy (ultrasound echo) from the implants to perform passive uplink data telemetry. The ultrasound power transfer efficiency depends on how well the energy is focused on the implant. So, the implant needs to be located first and then an initial contact needs to be established that can then be used to fine-tune beam forming attributes such as focus depth and steering angle. In this work, a simple low power uplink data telemetry circuit to establish the 'first contact' that sends information on the implant's energy status to the transmitter is designed. The telemetry uses load impedance modulation to passively transmit back information. This principle is tested using an experimental setup with a 4 MHz burst frequency and different resistive loads varied from 0 Ω to 2000 Ω. Next, an algorithm was developed that automatically finds the optimal focus and steering angle settings so that the maximum amount of power is transferred to the ultrasound energy scavenger. This algorithm works based on the information sent from the uplink data telemetry circuit. The results show that load impedance modulation can effectively be used for backscatter communication. A reflected energy change of up to 40% is observed between matched and mismatched load conditions. This load modulation was implemented in the uplink data telemetry circuit. A simple low power circuit that consumes less than 0.5 μW was designed for this. The uplink telemetry protocol uses pulse width modulation to send the storage capacitor voltage which powers the implant, and an almost linear correlation was found between the pulse width and the supply voltage. The algorithm produces a heat plot showing the maximum power transferred at the location where CMUT was placed and gives the optimal value of focus and steering angle.

Implantable medical devices (IMDs) such as cardiac pacemakers, cochlear implants, and neurostimulators improve millions of people’s lives every day. Over the years, IMDs have significantly improved in function, increased in lifetime, and decreased in size. However, further miniaturisation is limited by the batteries used to power these devices, and therefore methods to power them wirelessly are gaining in popularity. The technique most used to wirelessly power IMDs is electromagnetic energy transfer. However, at low frequencies, this method is limited to shallow implants due to its low directivity, while at high frequencies it is limited by attenuation in the human body. Therefore, ultrasound energy may be the future of wirelessly powering implants. Its high directivity and low attenuation in the human body opens up the possibility to wirelessly power deep implants without any harmful side effects. An ultrasound energy transfer set-up was established in this work to demonstrate that Philips’ capacitive micromachined ultrasound transducers (CMUTs) can be used as ultrasound energy harvesters. Normally, CMUTs need a bias voltage to function as an energy harvester. However, the CMUTs in this work are chargeable and therefore eliminate the need for this bias voltage. The results show that charged CMUTs can indeed be used as efficient energy harvesters of ultrasound and that information can be sent back to the ultrasound probe using echo modulation. The charged CMUTs showed to be able to harvest sufficient energy to stimulate a nerve and power a Bluetooth module, even when ultrasound sent through 90mm of biological (pork) tissue. The maximum power harvested during the ultrasound energy transfer experiments was 250mW over a transferring distance of 75mm.

The application of implantable medical devices (IMDs) is increasing rapidly due to the many health benefits they provide in diagnosing and treating diseases. However, these devices have to be able to survive harsh body conditions to ensure their reliability and functionality. Bodily fluids initiate chemical degradation and corrosion in the devices especially in the metal interconnects. Therefore, the devices are encapsulated by various forms of hermetic and non-hermetic packaging. The current standard hermetic packaging is not suitable for miniature microelectronics. As a result, conformal encapsulation is currently being developed. Multiple inorganic and organic layers of materials or a combination of different layers are used to achieve corrosion protection. Polyimide (PI) is a type of polymer that is used as encapsulation material in bioelectronic devices. Recently, there has been an increasing interest in using thin inorganic layers such as SiC and SiO₂ between the polyimide layers in order to improve the PI to PI adhesion. In this thesis, first, the corrosion phenomenon in bioelectronics and the available packaging methods are explained. Next, test structures that contain polyimide encapsulation with and without the SiC and SiO₂ ceramic layers are microfabricated. In order to provide accelerated aging conditions to the samples, a lifetime measurement set-up is modeled and built. Finally, the test structures are tested in the lifetime set-up to evaluate their reliability performances in accelerated aging conditions. The leakage currents of the test structures are measured as a function of the soaking time. Samples with SiC and SiO₂ thin layers exhibit a high leakage current value and fail relatively fast. In addition, samples are analyzed by electrochemical impedance spectroscopy (EIS) measurements and the samples without the ceramic layers demonstrate a capacitive behavior in lower frequencies.