Unwanted gas bubbles are a challenge for microfluidic-based systems, as adherence to channel networks can disrupt fluid delivery. This is especially true for devices with biological applications, as the presence of a single bubble creates thin fluid films with extremely high shear stresses, which can damage biological samples. Current strategies to remove bubbles require complicated fabrication or off-chip components. This thesis describes an on-chip microfluidic strategy utilizing permeation for in-plane trapping and removal of occasional gas bubbles. The trap was demonstrated with nitrogen bubbles, which were consistently removed at a rate of 0.14 µL/min for a single trap, and shown to have long-term operation capability by removing approximately 4,000 bubbles during one day without failure. The trap was integrated with a microfluidic system for the study of small blood vessels. Experiments were complemented with analytical and numerical models to characterize the bubble removal process.

Unwanted gas bubbles are a challenge for microfluidic-based systems, as adherence to channel networks can disrupt fluid delivery. This is especially true for devices with biological applications, as the presence of a single bubble creates thin fluid films with extremely high shear stresses, which can damage biological samples. Current strategies to remove bubbles require complicated fabrication or off-chip components. This thesis describes an on-chip microfluidic strategy utilizing permeation for in-plane trapping and removal of occasional gas bubbles. The trap was demonstrated with nitrogen bubbles, which were consistently removed at a rate of 0.14 µL/min for a single trap, and shown to have long-term operation capability by removing approximately 4,000 bubbles during one day without failure. The trap was integrated with a microfluidic system for the study of small blood vessels. Experiments were complemented with analytical and numerical models to characterize the bubble removal process.

The field of organ-on-chip is a relatively new area of research and builds upon the principle of engineering microfluidic systems to mimic the body’s internal environment as precisely as possible. Eventually these models could hopefully simulate whole organ-systems and enable the examination of the cell’s or organ’s reaction to foreign substances like new pharmaceuticals in a better way than current models. Previously this has been done with in vitro models such as petri dishes that only offer static culturing conditions. These are not very realistic environments compared to the human body where the cells are exposed to both variations in pressure and flows among other things. The purpose of this bachelor’s thesis project has been to evaluate and improve the design of an organ-on-chip system developed by the EMBLA-group at Ångströmslaboratoriet, Uppsala university. This has been done by evaluating the manufacturing process to find areas of improvements of the current chip design, as well as conducting a literature study to understand key components of similar organ-on-chip systems and see if it is possible to implement relevant parts to the organ-on-chip of this project. One of these important parts is a so-called bubble trap. A bubble trap is a construction that enables the capturing and elimination of bubbles in the system since the bubbles can harm the chips components, kill the cells, and compromise measurements.  A first prototype of the bubble trap was developed in Polydimethylsioxane (PDMS) and integrated on the EMBLA-group’s chip design. The principle behind the bubble trap was to use the natural buoyancy of the bubbles to trap them. This was done by introducing an upwards going slope before the inlets to the chip. In this manner the bubbles would float up to the top of the slope and accumulate at the roof as the liquid moved on into the chip without bubbles. To make the bubbles leave the chip a low-pressure chamber was added on top of the bubble trap to help the process of the bubble’s diffusion through the roof and out of the chip. The development of an improved chip design turned out to be a time-consuming endeavor and the time left for evaluation the functionality of the chip became too short. One test was performed which showed that the bubbles did accumulate at the top of the slope as expected, but it rapidly became full and thus started to let bubbles through to the microfluidic chip. The bubbles did not diffuse as efficiently as required and the removal of the bubbles became inefficient. To understand and correct the problem areas of this bubble trap design further tests and experiments will have to be conducted. / Organ-på-chip (Organ-on-chip eller OoC) är ett relativt nytt forskningsområde som bygger på att mikrofluidiksystem utvecklas till att efterlikna människokroppen i så stor utsträckning som möjligt. Detta då det är attraktivt att kunna undersöka cellers/organs beteende vid tillförsel av vissa substanser, till exempel nya läkemedel. I tidigare in vitro modeller har det endast observerats och utförts tester på celler odlade i statiska förhållanden vilket inte är likt den omgivning cellerna har i människokroppen där de tex utsätts för olika vätskeflöden och tryckförändringar.    Syftet med detta examensarbete har varit att utvärdera och förbättra designen på ett OoC system utvecklat av EMBLA-gruppen på Ångströmlaboratoriet vid Uppsala universitet. Detta har gjorts genom att studera den nuvarande tillverkningsprocessen för att hitta relevanta förbättringsområden samt att genom en litteraturstudie undersöka viktiga delar som bör ingå i dessa typer av system. En av dessa delar är en bubbelfälla (bubble trap eller BT) vilket innebär att det i chippet bör finnas ett sätt att eliminera/fånga upp bubblor. Detta eftersom bubblorna kan orsaka stor skada på både chipet, cellerna och mätningarna som skall utföras. En första prototyp av en BT design i Polydimetylsiloxan (PDMS) utvecklades och integrerades på EMBLA-gruppens OoC design. Principen bakom BT-designen var att utnyttja bubblornas flytkraft vilket gjordes genom att introducera en uppåtgående backe innan ingångskanalen. Bubblorna kan därmed flyta upp till toppen av lutningen och vätskan kan fortsätta in i mikrochipset utan bubblor. För att bubblorna ska ta sig ut ur chippet integrerades en tryckkammare ovanpå BT-designen för att få bubblorna att diffundera ut genom taket i den uppåtgående kammaren och ut ur chippet. Utvecklingen av den förbättrade chip-designen visade sig var tidskrävande och tiden för att utvärdera designens funktionalitet blev för kort. Ett test gjordes på den nya chip-designen vilket visade att den utvecklade BT som väntat fångade upp bubblor men att den snabbt blev full i och med att bubblorna inte diffunderade ut genom taket i den takt som behövdes. Vidare undersökningar och experiment behövs för att evaluera vad som orsakade detta och rätta till eventuella felkällor i design och experimentuppställning.