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.