The production of base chemicals by electrochemical conversion of captured CO₂ has the potential to close the carbon cycle, thereby contributing to a future energy transition. With the feasibility of low-temperature electrochemical CO₂ conversion demonstrated at lab scale, research is shifting toward optimizing electrolyser design and operation for industrial applications, with target values based on techno-economic analysis. However, current techno-economic analyses often neglect experimentally reported interdependencies of key performance variables such as the current density, the faradaic efficiency, and the conversion. Aiming to understand the impact of these interdependencies on the economic outlook, we develop a model capturing mass transfer effects over the channel length for an alkaline, membrane electrolyser. Coupling the channel scale with the higher level process scale and embedding this multiscale model in an economic framework allows us to analyze the economic trade-off between the performance variables. Our analysis shows that the derived target values for the performance variables strongly depend on the interdependencies described in the channel scale model. Our analysis also suggests that economically optimal current densities can be as low as half of the previously reported benchmarks. More generally, our work highlights the need to move toward multiscale models, especially in the field of CO₂ electrolysis, to effectively elucidate current bottlenecks in the quest toward economically compelling system designs.

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Two decades of research on droplet formation in microchannels have led to the widely accepted view that droplets form through the squeezing mechanism when interfacial forces dominate over viscous forces. The initially surprising finding that the volume of the droplets is insensitive to the relative importance of these two forces is nowadays well understood from the constrained deformation of the droplet interface during formation. In this work, we show a lower limit of the squeezing mechanism for droplets produced in microfluidic cross-junctions. Below this limit, in the leaking regime, which was recently discovered for droplets produced in T-junctions, the volume of the produced droplets strongly depends on the relative importance of interfacial and viscous forces, as captured by the capillary number. We reveal a fundamental difference in the mechanisms at play in the leaking regime between T- and cross-junctions. In cross-junctions, the droplet neck elongates substantially, and unlike the case of the T-junction, the magnitude of this elongation depends strongly on the value of the capillary number. This elongation significantly affects the final droplet volume in a low capillary number regime. Generalizing the classical squeezing law by lifting the original assumptions and incorporating both identified mechanisms of leaking through gutters and neck elongation, we derive a model for droplet formation and show that it agrees with our experiments.

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It is common practice in the development of bioprocesses to genetically modify a microorganism and study a large number of resulting mutants in order to select the ones that perform best for use at the industrial scale. At industrial scale, strict nutrient-controlled growth conditions are imposed to control the metabolic activity and growth rate of the microorganism, thereby enhancing the expression of the product of interest. Although it is known that microorganisms that perform best under these strictly controlled conditions are not the same as the ones that perform best under uncontrolled batch conditions, screening, and selection is predominantly performed under batch conditions. Tools that afford high throughput on the one hand and dynamic control over cultivation conditions on the other hand are not yet available. Microbioreactors offer the potential to address this problem, resolving the gap between bioprocess development and industrial scale use. In this review, we highlight the current state-of-the-art of microbioreactors that offer the potential to screen microorganisms under dynamically controlled conditions. We classify them into: (i) microtiter plate-based platforms, (ii) microfluidic chamber-based platforms, and (iii) microfluidic droplet-based platforms. We conclude this review by discussing the opportunities of nutrient-fed microbioreactors in the field of biotechnology.

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In recent years, biotechnological processes have gained increased interest due to their potential for high-value compound production and waste recycling. This shift towards biotechnology is driven by global challenges such as food security, climate change, and the transition to renewable resources. To address the limitations of large-scale fermentations, scale-down approaches have been recommended to minimize microbial performance losses during scale-up procedures. Computational fluid dynamics (CFD) coupled with omics-based technologies offer valuable insights into the environmental and intracellular
lifelines of cells. However, current laboratory-scale setups have certain limitations, emphasizing the need for dynamic microfluidic single-cell cultivation (dMSCC) devices. These devices enable the analysis of single-cell behavior in dynamic environments with high temporal resolution.

This thesis focuses on improving the amplitude control while maintaining temporal resolution in dMSCC devices. A new dMSCC device design was analyzed using a 2D model, which was experimentally validated. The results demonstrated that the design mechanism effectively generated concentration profiles resembling discrete and smooth lifelines, albeit with a relatively high response time (30 seconds). A mesh independence study indicated minimal deviations (2 %) in results for different mesh refinements, while complex geometric structures introduced greater variations.

The experimental validation of the 2D COMSOL Multiphysics model highlighted discrepancies between the experimental data and model predictions, both at the outlets of the microfluidic concentration gradient generator (μCGG) and inside the chamber (RMSE=0.1-0.75; >10% of experimental data). However,
the observed trends of the concentration profiles inside the chamber were well-captured. Optimization studies were conducted based on these findings, leading to valuable conclusions. Narrowing the chamber width increased the chip’s response time. Moreover, increasing the space between μCGG outlets
as well as increasing fluid velocity inside the μCGG (while keeping the maximum velocity constant) improved gradient width. The latter approach is preferred to maintain temporal resolution. A comparison between COMSOL Multiphysics (RMSE=0.14) and Ansys Fluent (RMSE=0.15) models revealed that Ansys Fluent better captures experimental trends but has lower prediction accuracy. Further investigations involved a Design of Experiments (DoE), which indicated that the current μCGG design is suitable for fluid velocities preferably lower than 1 · 10−5 m/s and tracers with high diffusion coefficients. These conclusions provide insights into optimizing dMSCC devices and contribute to the broader understanding of mimicking microbial lifelines.

166Ho is a promising radionuclide for targeted radionuclide therapy, because of its beneficial decay characteristics and the possibility to track its dose delivery in a patient. However, a problem for the use of 166Ho in these treatments is its limited maximally produced specific activity in neutron irradiation of a pure 165Ho target, since the produced 166Ho is chemically inseparable from the 165Ho target. The Szilard-Chalmers method is known to be able to increase the achieved specific activity. It does so by irradiating the target isotope while it is incorporated in a compound. As the target isotope undergoes (n, γ) neutron capture it forms the product isotope and escapes the compound due to recoil from prompt gamma emissions. Then by separating the product isotopes from the compound, which still contains the target isotopes, a high specific activity can be achieved. However, batch irradiations using the Szilard-Chalmers method for production of high specific activity gives low production yields. In order to increase these production yields, a continuous flow Szilard-Chalmers system is proposed. The objective of this research is to investigate the main processes at play in a continuous flow system using the Szilard-Chalmers method on 165HoDOTAfor the production of high specific activity 166Ho. Investigations on the labelling of Ho3+ to DOTA4- have shown that 96±3 % labelling efficiency is achievable. Experiments on relabelling and isotope-exchange have shown that less than 8 % of 166Ho3+ undergoes relabelling with DOTA4- or isotope-exchange with 165HoDOTA-. Also, a cation exchange separation method for separation of Ho3+ from DOTA4- and HoDOTA- was developed. Moreover, investigations on Szilard-Chalmers separation of a dried 165HoDOTAsample have shown that with a 4-minute irradiation it was possible to achieve a specific activity of 2.8±0.2 MBq mg-1 . This corresponds to an enrichment factor of 1.62±0.03 compared to an irradiation without Szilard-Chalmers separation. Decreased radiolysis by shielding and slowed neutrons, and increased initial labelling are expected to be able to increase the achieved enrichment factor even more. The continuous flow Szilard-Chalmers system could in potential prove very useful in high specific activity radionuclide production, but further improvements on its subprocesses are necessary before practical implication is worthwhile.

A key bottleneck in bioprocess development is that state-of-the-art tools used for screening of cells and optimization of cultivation conditions do not represent the conditions enforced at industrial scale. At industrial scale, cell growth is strictly controlled (“fed-batch”) to optimize the metabolites produced by the cells. In contrast, cell growth is uncontrolled (“batch”) in microwells commonly used for bioprocess development due to the difficulty to continuously supply minute amounts of nutrients to the cells in these wells over the course of the cultivation experiment. This work addresses this bottleneck through the development of a droplet-based fed-batch nanobioreactor. A key challenge addressed in this work is the implementation of the required non-steady droplet operations on chip to establish a semi-continuous nutrient supply, while keeping the chip and its operation as simple as possible. The ability to study micro-organisms under nutrient-controlled fed-batch conditions is demonstrated using the yeast Cyberlindnera (Pichia) jadinii, with the cell growth rate controlled through the glucose concentration. Given the relative ease of operation and the potential to extend its features, the presented nanobioreactor provides a solid platform technology for further development and use in the field of bioprocess development and beyond.

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Urine is commonly used for clinical diagnosis and biomedical research. The discovery of extracellular vesicles (EV) in urine opened a new fast-growing scientific field. In the last decade urinary extracellular vesicles (uEVs) were shown to mirror molecular processes as well as physiological and pathological conditions in kidney, urothelial and prostate tissue. Therefore, several methods to isolate and characterize uEVs have been developed. However, methodological aspects of EV separation and analysis, including normalization of results, need further optimization and standardization to foster scientific advances in uEV research and a subsequent successful translation into clinical practice. This position paper is written by the Urine Task Force of the Rigor and Standardization Subcommittee of ISEV consisting of nephrologists, urologists, cardiologists and biologists with active experience in uEV research. Our aim is to present the state of the art and identify challenges and gaps in current uEV-based analyses for clinical applications. Finally, recommendations for improved rigor, reproducibility and interoperability in uEV research are provided in order to facilitate advances in the field.

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Electrochemical reduction of CO2 using renewable energy sources is one of the promising avenues to pursue towards mitigating the emissions of the notorious CO2. However, the CO2 electrolysis in aqueous systems, due to the low solubility of CO2, are severely limited by mass transfer. State of the art review shows that a significant amount of the research is done to improve mass transfer, where a variety of electrolyser designs were studied. Despite the effort, the challenge to enhance mass transfer remains and is the focus of the present work. To improve mass transfer, an innovative concept is proposed - Taylor flow in an electrochemical flow cell. Taylor flow has been extensively studied in the literature, especially in micro channels and monolith reactors. Therefore the characteristics of the Taylor flow are known to a certain extent. But they were never tested in an electrochemical system. For that reason, a numerical investigation is carried out to assess the performance of Taylor flow on the flow cell. A simplified 2D model was formulated using a unit cell strategy and is verified based on experimental data and theoretical concepts. The effect of dissolving bubbles is also modelled using a quasi-steady-state analysis The results of the 2D model show a significant improvement in the performance, i.e. in the current densities of the electrochemical cell compared to a typical flow cell. The maximum calculated current densities increase by an order of magnitude under certain flow conditions. The dissolution studies showed that the current densities deteriorate with time. Nevertheless, the overall performance is still higher than the typical flow cell. Finally, based on the insights from the 2D model, a 1D model is suggested to estimate the current densities and dissolution rates. The present study showed promising results for using Taylor flow in an electrochemical cell. The proposed 2D model can help in aiding future modelling studies while the 1D model can give simple estimates for the experimental work.

Underground Hydrogen storage (UHS) is an attractive technology for large-scale energy storage. The UHS safety and efficiency depends highly on accurate characterization of H2 interactions with reservoir fluids, specially wettability analyses for H2/brine/rock systems. This thesis reports experimental measurements of advancing and receding contact angles of H2/water, N2/water and CO2/water systems at P = 10 bar and T = 20 °C using a microfluidic chip (channel widths: 50 - 130 μm). The results indicate strong water-wet conditions with H2/water advancing and receding contact angles of respectively 13 - 39°, and 6 - 23°. It was found that the contact angles decrease with increasing channel widths. Little hysteresis was measured, and consequently, the results are not in line with Morrow’s curve. The receding contact angle measured in the smallest channel agrees well with the literature coreflood tests. The N2/water and CO2/water systems showed similar behaviours as the H2/water system.

Extracellular vesicles (EVs) are a family of small membrane vesicles that carry information about cells by which they are secreted. Growing interest in the role of EVs in intercellular communication, but also in using their diagnostic, prognostic and therapeutic potential in (bio) medical applications, demands for accurate assessment of their biochemical and physical properties. In this review, we provide an overview of available technologies for EV analysis by describing their working principles, assessing their utility in EV research and summarising their potential and limitations. To emphasise the innovations in EV analysis, we also highlight the unique possibilities of emerging technologies with high potential for further development.

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The cellular environment is characterized by confinement and macro-molecular crowding: both concepts that have been studied separately. To understand kinetics of enzymatic reactions, there is a need to understand how the diffusional encounters of enzyme and substrate proceed in an environment
that is confined and crowded simultaneously. The project carried out in this thesis is the first step towards achieving the ultimate goal of studying biochemical reactions in native cellular environment - Understanding diffusion in confinement. Despite multiple investigations of diffusion of analytes in confinement, there exists a research gap. There is inconsistency in the interpretation of the results and in the dependence of diffusion properties of analytes of different sizes in channels of different dimensions. Hence, to bridge the research gap, the main goal of this thesis project was to find the diffusion coefficient of 100 nm polystyrene beads in microchannel (200 µm wide and 4.5 µm high) and nanochannel (5 or 10 µm wide and 300 nm high). The Brownian motion of particles was observed using Confocal Laser Scanning Microscope. A preliminary study first confirmed the reliability and optimization of the particle tracking method of finding diffusion coefficient. Diffusion coefficient of the particles determined experimentally in microchannel (bulk system) was in agreement with the theoretical estimate and statistically significant. Experiments in the nanochannel revealed a reduction in the particle diffusion coefficient of about ∼52% compared to bulk, due to interactions with the confining depth
(300 nm) of nanochannel. An interesting behaviour was also exhibited by particles diffusing close to the side wall along the width of nanochannels, which was not confining (5 or 10 µm). The diffusion coefficient in such a case reduced by around ∼90% relative to bulk. The reduction in both cases can be mainly attributed to hydrodynamic interactions. The experimental investigations of diffusion coefficient carried out in this study were in agreement with long standing theoretical predictions. However, the research gap could not be fully expelled.