Lab on chip (LOC) systems often require the controlled movement of individual biological cells. Automated operation
of these systems usually requires detectors to track individual cells. Electrical methods involving measurement of the
conductivity or permittivity of regions between two electrodes are capable of providing this information. However, these
detection systems can interfere with other dielectrophoretic LOC cell handling systems. Conversely optical systems are
immune to electrical interference. Many LOC devices are fabricated with only the top surface of the device being
transparent to light, precluding the use of transmitted optical detection. This is often due to the use of silicon, a favoured
substrate. Here we present a low cost optical system suitable for detecting biological cells in microfluidic channels.
A flow cell with a fluid microlayer approximately 105±10μm deep was fabricated having a 100±10μm thick glass
window, and a reflective base layer. The reflective base was formed by thermal evaporation of gold onto a substrate.
Particles within a microfluidic layer were epi-illuminated by a standard (red) laser DVD pickup unit. The flow cell
permitted the laser beam to be focussed onto the gold reflector, and back through a beamsplitter onto a photodiode. This
system was tested using polystyrene beads that were representative of biological cells. The position of the focal point significantly affected the base line reflected signal, but this micron scale position sensitivity could be overcome using the magnetic focussing coil of the DVD pickup. In this system, polystyrene beads down to 3μm in diameter were successfully detected.