Implementation of a Chemiluminescent Immunoassay for S. Aureus Protein A in a Cd Microfluidic Device

Microfluidics devices allows performing analyses using very small amounts of sample and reagents (micro-nanoliter range). The techniques for fabrication of microfluidic devices are relatively inexpensive and allows to produce integrated devices able to perform the different functions required for the analytical procedure in the same chip.

Microfluidic devices can work with different types of fluids and analytes (aqueous solutions and buffers, antibody solutions, biological fluids, bacterial or cell suspensions) and microfluidic-based devices have been already designed and used to measure molecular diffusion coefficients, fluid viscosity, pH, binding constants and enzyme reaction kinetics. They can also perform complex analytical procedures such as analyte separation based on capillary electrophoresis, isoelectric focusing, immunoassays, flow cytometry analysis, cell separation and patterning. Most of these microfluidic devices can be used for clinical diagnostics, indeed an important field of application of these devices is the development of portable diagnostic devices for home and bedside use, which could allow eliminating expensive and time-consuming laboratory procedures.

The main feature of microfluidic device is the scale of fluidic channels, whose dimensions are less than 1 mm. The flow of a fluid through a microfluidic channel is described by the Reynolds number:
Re = LVavgP/μ

where L is the length scale of the channel (which is equal to 4A/P, where A is the cross section and P is the wetted perimeter of the channel), μ is the viscosity of the fluid and Vavg is its average velocity. In microfluidic systems, Re is usually much lower than 100 ad often lower than 1.0, thus the flow is always laminar (turbulent flow only occurs if the Reynolds number is higher than 2000). The laminar flow implies peculiar aspects of the diffusion of molecules within the device, which must be taken into account in the design of the device itself.

Another particular aspect of microfluidic devices is the pumping system. In microfluidic devices, two main common pumping systems are used. In pressure driven microfluidic systems, the fluid is pumped through the device via positive displacement pumps, such a syringe pumps. The main feature of the laminar pressure driven flow is the velocity profile of the fluid: the profile is parabolic and the velocity of the fluid in the correspondence of the walls of the channel is zero (fig.1.11). Pressure driven flow is quite inexpensive and can be easily implemented in microfluidic devices (Yager Research Laboratory).