We report a versatile, and automatic method for sorting cells and particles in a three dimensional
polydimethylsiloxane (PDMS) structure consisting of two crossmicrochannels. As microspheres or yeast cells
are fed continuously into a lower channel, a line shaped focused laser beam is applied (perpendicular to the
direction of flow) at the crossing junction of the two channels. The scattering force of the laser beam was
employed to push microparticles matching specific criteria upwards from one channel to another. The force
depends on the intrinsic properties of the particles such as their refractive index and size, as well as the laser
power and the fluid flow speed. The combination of these parameters gives a tunable selection criterion for
the effective and efficient sorting of the particles. The introduction of the cylindrical lens into the optical train
allows for simultaneous manipulation of multiple particles which has significantly increased the efficiency
and throughput of the sorting. A high aspect ratio microchannel (A.R. = 1.6) was found to enhance the
sorting performance of the device. By careful control of the microparticle flow rate, near 100% sorting
efficiency was achieved.
Proton beam writing (PBW) is a lithographic technique that utilizes MeV protons in a direct write mode to
fabricate micro/nano features in suitable resist material (E.g PMMA, SU-8, silicon, Foturan). These micro/nano
structures may be used in an electroplating step to yield robust metallic stamps/molds for the replication of
the original and lends itself to the fabrication of micro/nano fluidic channels that are important components in
devices such as biophotonic chips. Another feature of proton bombardment is its ability to induce an increase
in refractive index along the ions path, in particular at the end of its range where there is substantial nuclear
scattering. This allows PBW to directly write buried waveguides that can be accurately aligned with fluidic
Polydimethylsiloxane (PDMS) is an optically clear, biocompatible polymer that can be readily used with a
mold (such as that created with PBW) and easily sealed so as to produce biophotonic chips containing micro/nano
fluidic channels. This has lead us to favour PDMS as the base material for our work on the development of these
biophotonic chips. The present work is concerned with the production of integrating channel waveguides in
PDMS chips, so as to have a working device that may be used to detect fluorescently tagged biological samples.
For this we have adopted two approaches, namely(1) directly embedding optical fibres in the polymer and (2)
using PBW to directly write buried waveguides in the polymer.
Proton beam writing is a lithographic technique that can be used to fabricate microstructures in a variety of materials including PMMA, SU-8 and FoturanTM. The technique utilizes a highly focused mega-electron volt beam of protons to direct write latent images into a material which are subsequently developed to form
structures. Furthermore, the energetic protons can also be used to modify the refractive index of the material at a precise depth by using the end of range damage. In this paper we apply the proton beam writing technique to the fabrication of a lab-on-a-chip device that integrates buried waveguides with microfluidic channels. We have chosen to use FoturanTM photostructurable glass for the device because both direct patterning and refractive index modification is possible with MeV protons.