Analysis of Microscale Transport for BioMEMS

Department of Mechanical Engineering
University of California, Santa Barbara

Abstract-
During recent years there has been significant development in the field of microfluidics and its application to BioMEMS devices. The scalability and sensitivity of BioMEMS make them well suited for manipulating and analyzing macromolecules. Microfluidics plays a key role in the transport processes inside these devices, which include advection, Brownian motion, electrokinetic phenomena, and surface-dominated forces.
Recent developments at UCSB of a fully-integrated tunable laser cavity sensor for optical immunoassays will be presented. This device incorporates a pair of Distributed Bragg Reflector (DBR) lasers to sense specific antigen/antibody binding events that occur in the evanescent field of the laser cavity. The binding event modifies the modal index of the laser through coupling of the evanescent field. The modal index can be detected theoretically to within a resolution of n ~ 10^-7. Dielectrophoresis (DEP) is proposed as a method for manipulating the antigen concentration fields, thereby enhancing the sensitivity of the device.
The length scales of microfluidic devices typically range between 100 - 102 microns. In order to make full use of the physical phenomena at this scale and to understand how these devices function, accurate non-intrusive diagnostic techniques are required. To this end, a micron-resolution Particle Image Velocimetry (micro-PIV) system has been developed to measure velocity-vector fields with order one-micron spatial resolution. The resolution of the PIV system is demonstrated by measuring the flow field in a 30 x 300 micron channel. By overlapping the interrogation spots by 50%, a velocity-vector spacing of 450 nm is achieved. Surprisingly, the velocity measurements indicate that the well-accepted no-slip boundary condition may not be valid for hydrophobic/hydrophilic boundaries at the microscale. These results represent the first direct experimental measurement of this phenomenon.

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