Low cost optical particle detection for lab on chip systems based on DVD technology

Abstract

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.