In this paper, we report the preliminary development of a fiber coupled microfluidic flow cytometer with its potential
application of sorting the very small embryonic like (VSEL) stem cells out of a mixture of platelets and VSEL stem
cells. The identification of a VSEL stem cell from a platelet is based on the large difference of their abilities to scatter
light. A simple cytometer prototype was built by cutting the fluidic and other channels into a polymer sheet and bonding
it with epoxy between two standard glass slides. Standard photolithography was used to expose an observation window
over the upper coated glass to reduce background scattered light. Liquid sample containing micro-particles (such as
cells) is injected into the microfluidic channel. Light from a 532-nm CW diode laser is coupled into the optical fiber that
delivers the light to the detection region in the channel to interrogate the flowing-by micro-particles. The scattering light
from the interrogated micro-particle is collected by a photodiode placed over the observation window. The device sorts
the micro-particle into the sort or waste outlet depending on the level of the photodiode signal. We used fluorescent latex
beads to test the detection and sorting functionalities of the device. It was found that the system could only detect about
half of the beads but could sort almost all the beads it detected.
The characterization of single biological cells in a microfluidic flow by using a 2D light scattering microfluidic
cytometric technique is described. Laser light is coupled into a microfluidic cytometer via an optical fiber to illuminate a
single scatterer in a fluidic flow. The 2D light scattering patterns are obtained by using a charge-coupled device (CCD)
detector. The system is tested by using standard polystyrene beads of 4 μm and 9.6 μm in diameter, and the bead
experimental results agree well with 1D Mie theory simulation results. Experiments on yeast cells are performed using
the microfluidic cytometer. Cell results are studied by finite-difference time-domain (FDTD) method, which can
simulate light scattering from non-homogeneous cells. For example, a complex biological cell model with inner
mitochondrial distribution is studied by FDTD in this paper. Considering the yeast cell size variations, the yeast cell 2D
scatter patterns agree well with the FDTD 2D simulation patterns. The system is capable of obtaining 2D side scatter
patterns from a single biological cell which may contain rich information on the biological cell inner structures. The
integration of light scattering, microfluidics and fiber optics described here may ultimately allow the development of a
lab-on-chip cytometer for label-free detection of diseases at a single cell level.
Femtosecond laser ablation is an important process in micromachining and nanomachining of microelectronic,
optoelectronic, biophotonic and MEMS components. It is also important in the damage of optical components and
materials. A thorough understanding of all aspects of femtosecond matter interaction processes in the near-threshold
regime is required if one wants to have complete control of these processes. Two aspects of the interaction process for
metals and semiconductors are examined in detail in the present paper, namely the effect of a more complete model for
the temperature dependent electron thermal conductivity in metals and the avalanche ionization process in
semiconductors. These are included in two temperature and molecular dynamics modeling calculations respectively.
The proper inclusion of these processes allows the model calculations to better reproduce published experimental
measurements for copper and silicon.
Femtosecond laser ablation is an important process in the micromachining and nanomachining of microelectronic, optoelectronic, biophotonic and MEMS components. The process of laser ablation of silicon is being studied on an atomic level using molecular dynamics
(MD) simulations. We investigate ablation thresholds for Gaussian
laser pulses of 800 nm wavelength, in the range of a few hundred
femtoseconds in duration. Absorption occurs into a hot electron bath
which then transfers energy into the crystal lattice. The simulation
box is a narrow column approximately 6 nm x 6 nm x 80 nm with periodic
boundaries in the x and y transverse directions and a 1-D heat flow
model at the bottom coupled to a heat bath to simulate an infinite
bulk medium corresponding to the solid bulk material. A modified
Stillinger-Weber potential is used to model the silicon atoms. The
calculated thresholds are compared to various reported experimental
values for the ablation threshold of silicon. We provide an overview
of the code and discuss the simulation techniques used.
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