A biosensor-based digital microfluidic system for neuroanalysis in the intensive care unit

The primary focus of this thesis is the development of a digital microfluidic based system with integrated microelectrode biosensors, designed for the analysis of human brain microdialysate in the intensive care unit. The main aim is to measure the neurochemical effects of spreading depolarisations (SD waves), which have been shown to be detrimental to the injured human brain. A combined electrode was developed containing working electrodes in the range of 25 to 125 μM, a reference electrode and an auxiliary electrode, within a needle of outer diameter 300 to 500 μm. Glucose, lactate and ATP biosensors were developed with detection limits of 2 to 10 μM and response times of under 10 seconds. A digital microfluidic system was designed to segment the dialysate at a microdialysis probe outlet, thereby eliminating Taylor dispersion and reducing the time lag between the sample leaving the brain and analysis. Different designs are discussed for the manipulation of droplets for optimal analysis, thus creating a microfluidic toolkit. The analysis chamber was analysed mathematically, the optimal placement of electrodes found and the sensor performance assessed on-chip. The on-chip glucose biosensor was used in vivo in a translational pilot study. The biosensor performance was validated against rapid sampling microdialysis with excellent results. The glucose biosensors successfully monitored concentration changes, in response to stimulations, in the range of 10 to 400 μM. The data shows that during a SD wave, there is a time delay between the increase in potassium and the decrease in glucose, due to the uncoupling of blood flow and metabolism. For the first time, the microfluidic system was used in the intensive care unit, monitoring brain injury patients at the bedside