Integrated lab-on-a-chip devices, also known as micro total analysis systems (p-TAS), are expected to play a leading role in biological research and medicine in the 21st century, and on chip sample processing is a key function of such devices. A new class of ultrasonic microfluidic sample processing devices is presented, based on a single common fundamental unit - a capacitive micromachined ultrasonic transducer and fabricated using a single common process. Arrays of the transducers are integrated with fluidic microchannels, allowing devices with different functions to be realized simply by altering the physical arrangement and electrical drive signals of the array elements. The efficient, in-plane manipulation of particle-laden liquids is achieved by the use of phased, co-planar transducers, allowing the generation of in-plane, cavity mode standing waves in the microchannels, and permitting the efficient manipulation of suspended particles such as cells by acoustic radiation forces. Fabricated prototype devices include several types of ultrasonic particle filters, flow-through particle fractionators, particle collimators for cell alignment, devices for the ultrasonic lysing of cells, ultrasonic pumps and ultrasonic mixers.
As part of the development effort, an investigation of the thin film silicon material known as "permeable polysilicon" was performed, resulting in the discovery that the material's liquid permeability properties are caused by nanoscale pores that form spontaneously within an unusual morphological growth regime. A new, one-step porous polysilicon process is presented that allows the quick and easy fabrication of porous polysilicon films for a wide range of applications. The process is used to fabricate the ultrasonic immersion transducers used in the device arrays, and allows the convenient fabrication of a wide variety of microstructures that would be difficult or impossible to fabricate by other means.
In addition, a new simulation code is developed that permits the ultrasonic device designer to determine the trajectories of suspended particles with arbitrary properties as they pass through cavities containing resonant ultrasonic fields. This code, called PSIM, accepts output files generated by ANSYS finite element acoustic field simulations, and utilizes a numerical implementation of a general three-dimensional acoustic potential theory to plot simulated particle trajectories in a variety of user-defined formats.
The new devices, common fabrication process, and simulation capability support the development of integrated ultrasonic on-chip sample processing to meet the requirements of lab-on-a-chip systems. Additional optimization steps are identified to further improve the devices and process for future applications.
November 30, 2002
Dougherty, G. M. (2002). Ultrasonic Microdevices for Integrated On-chip Biological Sample Processing. United States: Materials Science and Mineral Engineering)--University of California, Berkeley.