Thermoelectric generators (TEGs) are critical in environments where alternative energy harvesting methods are necessary, such as space exploration. This thesis investigates using a two-dimensional hole gas (2DHG) as a p-type material within a Gallium Nitride (GaN) based TEG. This study focuses on determining the thermoelectric performance of the 2DHG by measuring the Seebeck coefficient, electrical conductivity, and power factor for extremely cold environments.

This work introduces a novel simulation method used to evaluate different structures to find the optimal hole densities for maximizing the power factor (PF) of the 2DHG. A structure was realized, and its thermoelectric properties were experimentally assessed. Key results, including a Seebeck coefficient of 100 − 200 μV /∆K, a sheet resistance of 70 − 120 kΩ , and a resulting Power Factor (PF) of 60 − 100 μW m−1K−2, are measured over an extremely low-temperature range of 50 - 300 Kelvin and discussed for future applications.

These results demonstrate that GaN-based TEGs can significantly broaden the temperature range and improve the performance of TEGs in extreme conditions. These findings pave the way for more efficient thermoelectric systems, with potential applications in aerospace, deep-space missions, and other challenging environments.

This Bachelor of Science thesis presents the development of a portable readout system for a graphenebased gas sensor array, aiming to bring advanced gas sensing technology from a controlled laboratory environment to practical field applications, such as greenhouses and vineyards. The project focuses on integrating various hardware components into a single printed circuit board (PCB) equipped with a microcontroller, ensuring accurate resistance measurement of graphene strips, generating programmable waveforms for micro hotplate control, and facilitating data communication via USB and Modbus interfaces. The design includes high-resolution analog-to-digital converters (ADCs) for precise current and voltage measurements, a digital-to-analog converter (DAC) for waveform generation, and a robust power management system supporting multiple power inputs. The system supports dual operation modes: interfacing with a desktop for laboratory use and functioning autonomously as an integrated sensor unit. Key challenges addressed include ensuring measurement accuracy, reliable sensor array connectivity, and user-friendly interfacing for both lab and field scenarios. Comprehensive test plans are outlined to verify the system’s performance, focusing on critical requirements such as resistance measurement accuracy, waveform generation, and communication interfaces. The successful implementation of a large part of this readout system demonstrates its potential for enhancing gas detection and monitoring in agricultural applications, providing a scalable solution for future deployment and development.

Tissue vitality monitoring is a crucial process to preserve patients’ health during post-surgical recovery or to ensure full adaptation and healing of transplanted organs. Among the many local factors for vitality assessment, tissue oxygenation gives an insight into entire tissue recovery and is directly linked to cell metabolism. To be able to assess oxygenation at the cellular level, NADH fluorescence sensing is used due to its contribution to the cellular respiratory cycle and high sensitivity to oxygen concentration. The current devices for NADH fluorescence sensing are limited to external measurements, where they are suitable for hospital use. This raises the need for a device that is implantable and bioresorbable so that it can stay in the body after the surgery and does not require secondary surgery for removal that puts the patient at risk. The goal of this master’s thesis is to introduce the design of a bioresorbable optical filter and photodetector for the measurements of oxygen through the detection of NADH fluorescence. The design consists of an absorption layer, a Fabry-Perot filter, and a wavelength-specific photodetector; where the overall response is designed to have high sensitivity to the emission wavelength of NADH (470nm) and low sensitivity to the excitation of NADH (350nm). ZnO nanoparticles are chosen to be the absorption layer due to their high absorption properties to 350nm and biodegradability, where the optical response is then tested through spectroscopy measurements. For the Fabry-Perot filter, a design with SiO2 and SiNx layers has been created with simulation and tested with optical measurements. The complete design consists of 15 layers and has a total thickness of approximately 1μm. Lastly; for the photodetector, Spectra simulations have been conducted for the determination of the optimal design properties. The choices are then adapted to a mask design for the fabrication, in which the back side etching of the silicon wafer is required for biodegradability and optical performance. The fabrication of the photodetector has not been completed due to the time frame of the project. The measurements on the combination of ZnO and Fabry-Perot filter show that the transmission of 470nm is 2.5-4 times larger than the transmission of 350nm, which is expected to increase to at least an order of magnitude when combined with the photodetector. For future experiments, it is recommended to conduct more tests on the filters and ZnO to ensure repeatability and consistency.

Wearable and implantable biosensors are emerging as promising devices for the continuous monitoring of individuals in the context of sports and health. The successful development and deployment of these devices could yield tremendous health benefits to the general population. Biosensors have to perform in complex biological environments that contain a wide variety of biomolecules that can interact with the biosensor in an undesirable way, affecting the biosensor performance and lifetime. Among these interactions, the blockage of bioreceptors by biomolecules (biofouling) and the non-specific adsorption of biomolecules to the sensor are two major processes that limit the successful deployment of biosensors.
This thesis presents a novel cytomorphic double network hydrogel (CDNH) structure to address biofouling and the non-specific adsorption of biomolecules. The CDNH mimics the functioning of a cell, the best biosensor ever created in nature. Cells have a membrane that separates the interior and outer world. The cell membrane selectively allows specific molecules to pass to the interior, where the biorecognition and metabolic processes happen. The CDHN comprises a structural hydrogel hemisphere to mimic the inner cell cytoskeleton and an outer network to mimic the cell membrane. The inner network is a poly (ethylene glycol) diacrylate (PEGDA) hydrogel with a large pore size. With the open structure of the inner network, the aim is to improve the diffusion of the target molecules to the bioreceptors (aptamers). Furthermore, the aim is to reduce steric hindrance for the binding of the target molecules to the bioreceptors. The outer network is a polyacrylamide hydrogel with a very dense mesh (nanopores). The outer network aims to function as a size exclusion filter, selectively allowing only small biomarkers to access the sensing site. By separating the dense outer network from the open structured inner network, the CDNH aims to reduce biofouling and ANSM while still ensuring adequate diffusion of the target biomarkers to the receptors and binding of the target biomarkers to the receptors.
Integration of the CDNH with a relatively simple 2-electrode sensor was achieved which enabled the electrical characterization (EIS) of the CDNH, providing crucial insights into the electrical behaviour of the CDNH in an electrochemical solution.
Overall, this work presents the foundation for a novel structure that is expected to improve biosensing drastically, contributing to the realization of continuous sports and health monitoring.

Defect centers in diamond has outstanding properties, including long coherence times and the presence of a nuclear spin bath, which make them highly promising for quantum applications. However, the technology for large-scale integration of defect centers is still in its early stages. There are various methods for the integration of diamond centers for quantum applications. Here we focus on the hybrid integration of diamond nanophotonic structures in three aspects: 1) adiabatic coupler design, 2) pick-and-place integration of diamond chiplets with silicon nitride (SiN) photonic platform, and 3) preparation of a diamond membrane for nanofabrication. An adiabatic coupler has been designed for an existing diamond chiplet. The optimal performance has been obtained at 12 μm taper length and 120 nm tip width with an insertion loss of 0.16dB, and coupling efficiency of 78.2%. It has also been demonstrated that much better performance could be obtained by reducing the diamond waveguide thickness from 485 nm to 200 nm, giving an insertion loss of 0.094dB and a coupling efficiency of 97.2%. The optimal taper lengths in this case have been determined as 12 μm for the SiN waveguide and 8 μm for the diamond waveguide. The effect of surface treatment on the pick-and-place transfer has been investigated as well. The time dependence of the contact angle of various surface treatments applied on the SiN surface has been characterized. It has been found that the optimal pick-and-place condition can be obtained with HMDS treatment at a contact angle range of 20∘ −30∘ measured by water, corresponding to a time between 1 and 2 hours after the treatment. One issue found in pick-and-place experiments is the roughness of the diamond nanophotonic chiplet bottom surface. Etching of diamond on insulator (DOI) substrates has been performed for two kinds of diamond samples. It is found that the minimum surface roughness achievable by Ar/Cl2 + Ar/O2 etching is limited by the initial surface quality. The average surface roughness was lowered from 5.71 nm to 1.19 nm using a higher-quality diamond.

Early-stage post-surgical infection monitoring is crucial for preserving patients' health and reducing public healthcare costs. Among various parameters and symptoms, local pH shifts can provide valuable insight into the development of infections. An implantable pH sensor can be employed to monitor localised pH changes. The Ion-Sensitive Field-Effect Transistor (ISFET) is a precise pH sensing device that is compact in size and can be integrated with standard CMOS technology. Due to its small size, the ISFET is promising as an implantable sensor for in vivo pH sensing. To avoid a second surgery for sensor removal after its functional lifespan, biodegradable sensors offer an alternative, as they can degrade and be absorbed by the human body after completing their function.

The aim of this project was to design and fabricate the first fully biodegradable ISFET pH sensor, addressing the challenges of accurate real-time monitoring for post-surgical infection, specifically in hip implantation surgery. This project successfully led to the design and fabrication of an 8-mask ISFET/MOSFET device using fully biodegradable materials and incorporating two rounds of ion implantation. Chip thinning via TMAH wet etching was also explored. Several alternatives were studied to overcome challenges with Molybdenum and Mo-Si contact in having suitable interconnections and Ohmic contact between Molybdenum and Silicon.

The current drug development process is a costly effort both in terms of budget and time. Organ-on-Chip (OoC) technologies are rapidly developing and are used for various applications and studies that aim to reduce the costs of this process. They represent the convergence between tissue engineering and microfluidic technology to provide researchers with an in vitro tool that can better recreate the environmental conditions in which cells would normally develop in vivo. One of the applications of this tool is the implementation of such platforms to study heart and muscle tissue. Electrical stimulation of this tissue is of great importance for these studies. There exist different ways in which these cells can be electrically stimulated such as microelectrode arrays (MEAs) or external electrodes. These methods pose several challenges that need to be addressed.

The scope of this project is to implement 3D microelectrodes into an already existing device from the company Bi/ond to electrically stimulate the cells developing inside. This approach aims to solve the existing challenges in current methods for the electrical stimulation of tissue. This project succeeded in developing such 3D microelectrodes although the functionality of the device could not be proven. However, several approaches were also studied and propositions were made on how to continue with this project.

Purpose
The main purpose of this report is to find out whether the OpenBCI "Ultracortex Mark IV" Electroencephalogram (EEG) headset is capable of differentiating EEG-signals of motor execution from neutral state with recorded data and to find out whether it can differ motor executions between left and right hand. Next to that, it is to be determined whether the OpenBCI headset was the optimal one for this purpose.

Method
First, the specifications of different headsets were compared. Afterwards, a montage of the electrodes was designed to detect motor execution and motor imagery, mainly centered around the locations C3, Cz and C4, on the top of the scalp. The software "Openvibe" was used to extract data from the headset during experiments and to record it in a csv file. A subject was asked to follow a video with a sound cue followed by a visual cue instructing to move either its left hand or right hand.


Result
Merging the left and right hand trial data together, the result is that the headset shows in the alpha band (7-12 Hz) mostly a decrease (ERD) in magnitude around the visual cue, sometimes followed by a bigger increase in magnitude (ERS). Looking at the extremes after the cue, it is seen that mostly the difference in magnitude is around a factor 1.5 compared to the average magnitude of before the visual cue. Splitting the trial data between left and right hand, similar results can be seen, but one hand produces slightly more ERD or ERS than the other hand depending on the position of the electrode on the left or right hemisphere of the brain.

Conclusion
The OpenBCI headset can in fact detect a difference between movement of the hands and the neutral state. Differentiating between the movements of left and right hands seems possible from the results, but the difference in the signal of left and right hand is minimal. It is recommended to repeat the experiment with more trials and different subjects to get a more solid conclusion.

This thesis details the design of a selection operator used in a Genetic Algorithm. The Genetic Algorithm is used for loudspeaker filter design of three way loudspeakers for which tournament selection was chosen as selection operator. A methodology is proposed and used to tune the parameters of tournament selection, which is based on diversity and fitness of the population. Besides basic tournament selection, two new adaptive selection operators based on tournament selection are proposed to improve its functionality. The first adaptive selection operator uses noise proportional to the fitness variance of the population to improve the efficiency of the genetic algorithm. The second adaptive selection operator uses a convergence stage to speed up the convergence towards the optimal filter. After the presented tuning process in this thesis, the latter adaptive selection operator was found to perform better. The optimal selection operator and parameters found in this thesis will not translate to every application, because they heavily depend on the design and the application of the genetic algorithm. However, the presented comparison of selection operators, the provided performance metrics and design methodology can still be used to guide the choice and the tuning process of a selection operator used in any genetic algorithm.

Vagus nerve stimulation serves as an approach to manage drug-resistant epilepsy. Focused ultrasound stands out as a unique tool, offering a non-invasive modality for VNS. With its excellent depth of penetration and sub-millimeter focusing capabilities in soft tissue, ultrasound allows for precise targeting of the vagus nerve without the need for surgical intervention. This technology relies on piezoelectric transducers that convert electrical energy into ultrasonic waves. However, the high costs and technical difficulties associated with the production of these arrays have led to limited commercialization of piezoelectric transducer arrays.

This thesis investigates the design, fabrication, and optimization of a small, low-cost phased-array piezoelectric ultrasonic transducer. The transducer is specially designed for non-invasive UVNS applications in the form of a small patch for the neck. The research is dedicated to the fabrication of the transducer array, designing a transducer capable of both transmitter (TX) and receiver (RX) functions, which in the future can be used for simultaneous image-guided neural regulation. Two prototypes were fabricated: one is a 256-element 2MHz rectangular transducer array integrated into an 8-layer stacked rigid printed circuit board (PCB), and the other is a dual-frequency array integrated into a 4-layer flexible PCB substrate, explicitly designed for simultaneous imaging and neural modulation.

The study utilized two key materials, the well-known PZT and the lesser-known but promising <001> oriented PMN-0.3PT piezoelectric single crystal. Through extensive optimization, air-filled PZT transducer arrays were realized, and the cutting parameters for PMN-PT were significantly improved. A unique aspect of the fabrication was the use of flip-chip technology to directly integrate the air-filled 2D array onto the PCB, a method seldom detailed in existing research.

While exhaustive testing on phase-delayed beam-forming and imaging capabilities could not be conducted due to equipment limitations, preliminary evaluations of the capacity to generate plane waves were made. The properties of the bulk PZT and PMN-PT transducers were characterized. It was observed that the PMN-PT transducer generated significantly lower ultrasonic pressure compared to the PZT transducer, which aligns with expectations due to the fragile nature of PMN-PT. The first prototype of the PZT array transducer was successfully fabricated and assessed, achieving a pressure of 176 kPa at its 10 mm natural focal point. Subsequently, the internal array second of the prototype was fabricated and evaluated, registering a pressure of 1.14 MPa at the 1.2 mm natural focus point.

The results demonstrate the feasibility of building complex piezoelectric arrays using optimized fabrication methods. This lays the foundation for future prototypes to test beam-forming capabilities and for fabrication of the complete equipment afterwards.