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High-pressure chemistry can be used to determine the contents of blood or water samples and to discover new chemistries. However, working with chemistry at pressures of many tens, or even hundreds, of bars often requires expensive and stationary equipment, such as autoclaves or chromatographic systems like high-performance liquid chromatography (HPLC).

Since the introduction of microfluidics in the '90s, researchers have attempted to develop microfluidic chips as microreactors to speed up synthesis with faster mass and heat transfer. Other researchers have made efforts to create microfluidic chip-based HPLC to reduce the cost, increase the separation quality, speed up the analysis, and even enable portable systems for on-site medical or environmental analysis. Still, fully integrated systems have not yet been realized due to a lack of fluidic control components.

This thesis presents novel methods for on-chip regulation and monitoring of pressure, flow, and temperature. Papers I and II specifically suggest a method for regulating backpressure and stabilizing pressure and flows using thermally controlled restrictors. Furthermore, a collaboration was made where a pressure-regulating chip was connected to an on-chip HPLC. The purpose of this was to activate sample plugs and therefore reduce the requirement for expensive surrounding equipment and enable portability, Paper III. Paper IV explores the use of a pressurized capsule to generate high-pressure flows that are coupled to a pressure-regulating chip to stabilize and regulate the pressure. Finally, an approach for integrating pressure sensors into high-pressure tolerant microchannels has been proposed, Paper V.

The work conducted has provided new insights into fluid dynamics. The regulating method employed in Paper I-IV utilizes a restrictor that alters the pressure drop as temperature changes, hence changing the viscosity of the fluid. Although this technology has been known since before, new understandings have emerged regarding how the compressibility of incompressible fluids must be considered at higher pressures. Additionally, the concept of buffer capacitance is presented, which is central when working with high-pressure microfluidics.

Through this thesis, discoveries of high-pressure microfluidics have been accomplished, which enable micro-total-analysis systems that could serve as portable HPLC equipment.

This thesis explores flow control and sensing in microfluidic chips for high-pressure applications. Sub-cm glass chips have been designed and fabricated with the aim of miniaturizing chemical analysis systems. Today, chemical analyses are performed worldwide for medical and environmental purposes. As more tests become available for a wider audience, the demand further increases. The large instruments that are typically used are expensive and have high chemical and power consumption. Miniaturizing components has, on the contrary, the ability to decrease volumes, costs, and environmental impacts. In addition to lower consumption, miniaturization carries several features: quick heat distribution, laminar flow, and higher pressure tolerances, to name a few. In this thesis, microfluidic chips are developed aiming to replace larger-scale instruments. Applications are centered around high-performance chromatography, which is a separation method used to separate and detect compounds in a sample. Different flow phenomena are also investigated, including fluid compressibility and capacitance, which become interesting when working at elevated pressures. Experiments have been made showing how this impacts the regulation of microfluidic flow. Thermal regulation of viscosity has been a centerpiece of this work. Controlling flow rate and pressure in a system by changing the viscosity of a fluid has proven effective for several applications. This was utilized to maintain back pressure at the end of a system as well as to control and stabilize flow at the beginning. It was also used to regulate composition and adjust parallel flows during experiments. Multiple chips were also connected to utilize several features and to get close to fully miniaturized and portable analysis systems. Apart from flow actuation, the microfluidic chips were also equipped with sensors for accurate sensing in close proximity.