Microfluidic devices, biostructures, and biochips

The ability of p-beam writing to create smooth three-dimensional channels in polymers allows the direct writing of microfluidic devices. Microchannels down to 100 nm wide in PMMA have been fabricated and a surface smoothness of 2.5 nm for the sidewalls of the channels has been measured using atomic force microscopy (AFM). Characterization of the channels’ electrokinetic behavior has been undertaken [1], and a theoretical model developed to predict the bulk electro-osmotic flow of phosphate buffer solution in the channels has shown good agreement with the measured electro-osmotic mobilities. Encapsulation of the microchannels to avoid fluid evaporation is an important factor in fabricating a working system, and thermal bonding techniques to produce enclosed proton-written nanochannels have proved successful [2].
In work involving the behavior of cells (fibroblasts) in a threedimensional micro-environment, p-beam writing has been used to fabricate a cell ‘corral’ consisting of a circular wall with four ‘outlet channels’ of differing widths enclosing a flat surface [3]. Cells seeded onto the circular flat surface migrate outward to the inside wall of the corral, where further migration is retarded. Cells pass through the outlet channels one by one, at a migration speed dependent on the width of the channels. The ability of p-beam writing to write deep channels with widths below 100 nm demonstrates the potential of fabricating biochips that can sort cells, DNA, and large proteins. One example being investigated by the Singapore group is that of a test chip created using p-beam writing which operates through the process of entropic trapping [4]. In the p-beam written chip, DNA is trapped in a series of reservoirs that are interconnected by 100 nm channels. When an electric field is applied along the channels, the DNA molecules uncoil and pass through the channels at a rate that depends on size and unfolding parameters.

1. Ansari, K., et al., J. Micromech. Microeng. (2006) 16, 1170
2. Shao, P. E., et al., Appl. Phys. Lett. (2006) 88, 093515
3. Sun, F., et al., Tissue Eng. (2004) 10, 267
4. Han, J., et al., Phys. Rev. Lett. (1999) 83, 1688