Integrated optical microflow cytometer for bead-based immunoassays

Abstract

Flow cytometry is an important tool for medicine and biology with applications from clinical diagnosis to investigations of fundamental cell biology. However, traditional flow cytometers are expensive, bulky and complex to operate. Miniaturised flow cytometers or microflow cytometers offer advantages over traditional devices, being compact, cheap and mass producible and would offer the user ease of operation and low sample consumption. A key challenge for developing a microflow cytometer is the integration of the optical components with the fluidics. Integrated optical waveguides offer an ideal method of light control in microflow cytometers as the waveguides are intrinsically aligned to the analysis region during fabrication. In this thesis the design, fabrication and demonstration of a silica-based microflow cytometer for bead-based immunoassays is presented. The device consists of a rugged monolithic glass chip with integrated waveguides which deliver excitation light to an etched microfluidic channel and collect light transmitted across the channel. The fluidics are designed to employ inertial focusing to reduce signal variation by bringing the flowing beads onto the same plane as the excitation beam. A fabrication process was developed using techniques common to the microelectronics industry which are scalable for mass production. This process allowed the realisation of devices with waveguides of a range of widths from 2.0 µm and upwards allowing both single mode and multimode operation at the immunoassay analysis wavelengths of 532 nm and 637 nm. Microfluidic channels with rectangular cross sections suitable for inertial focussing with depths of 30 µm were also realised. Inertial focussing was demonstrated to confine the flowing beads in two dimensions in the microfluidic channel which effectively reduced the fluorescence signal variation in the device to a CV of 29%. The application of the device was demonstrated by the detection of fluorescence from immunoassay beads incubated with the cytokine tumour necrosis factor alpha at 154 pg/ml. Additional functionality of the device was demonstrated with transmission based detection of flowing beads.