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We describe a microfluidic cytometer that performs simultaneous optical and electrical characterisation of particles. The microfluidic chip measures side scattered light, signal extinction and fluorescence using integrated optical fibres coupled to photomultiplier tubes. The channel is 80 mm high and 200 mm wide, and made from SU-8 patterned and sandwiched between glass substrates. Particles were focused into the analysis region using 1-D hydrodynamic focusing and typical particle velocities were 0.1 ms�1. Excitation light is coupled into the detection channel with an optical fibre and focused into the channel using an integrated compound air lens. The electrical impedance of particles is measured at 1 MHz using micro-electrodes fabricated on the channel top and bottom. This data is used to accurately size the particles. The system is characterised using a range of different sized polystyrene beads (fluorescent and non-fluorescent). Single and mixed populations of beads were measured and the data compared with a conventional flow cytometer.

Knowledge of the distribution and concentration of phytoplankton (microscopic algae that live in the oceans) is important, as this reflects and may modify environmentally important events such as pollution, climate change and carbon dioxide sequestration. Optical based flow cytometry has proved to be strategically important for the study of marine environments in general and of phytoplankton in particular because it enables rapid, simultaneous quantitative analysis of multiple optical properties of particles. Unfortunately, cost, complexity and size limit the widespread use of this technique for the study and monitoring of phytoplankton. The objective of this research was to solve some of the above mentioned problems by developing a small device for deployment on remote underwater vehicles (RUV) for in situ flow cytometric analysis of marine organisms. The core of the system was a lab-on-a-chip (LOC) device with a microfluidic channel with integrated optics and electrodes for the detection of the optical and electrical characteristics of phytoplankton. The work describes the design, fabrication, characterisation and testing of various LOC devices, together with data and analysis of results. The devices were able to clearly distinguish different populations of phytoplankton and other micro-particles on the basis of their optical and electrical properties. The systems had performance comparable with commercial instruments. Research also included bio-physical measurements (electro rotation and dielectrophoresis) of marine organisms to allow further modelling of the information obtained from the LOC cytometer.