Case Study CS-LDTC05 Rev. A

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Once sorted, based on either fluorescence method, particles

exhibiting the desired parameters were then removed from

the microwells using an infrared diode laser (emitting at

980 nm, fiber-coupled). The location of the desired cells

was stored during the fluorescence measurements, and the

laser beam was then steered to these locations to remove

the particles.

Removal of the particles was accomplished via a method

similar to optical tweezing.

Figure 2 shows the removal

sequence. The microchip structure was designed to allow

transmission of infrared light. The optical scattering force,

once focused on a particle, then levitated the particle

vertically out of the well, allowing the fluid flow to remove

it from the microgrid array for storage. Once the beam was

aligned to a chosen spot, it took between 3-10 seconds to

remove the particle from the well.

Figure 2. Cell levitation and removal sequence.

Adapted with permission from [1]. Copyright 2007

American Chemical Society.

Infrared light was chosen for two main factors. First, infrared

light has much higher damage thresholds for cells, allowing

for enough optical power to reach the cell to levitate it out of

the array without damaging it. Second, water is not strongly

absorbing at the wavelength chosen, producing low cell

temperature changes. This again allows for minimal cell

damage during the process.

LASERS IN MICROFLUIDICS

Researchers from Georgia Institute of Technology utilized

the laser for the opposite approach. Instead of choosing

a wavelength where water absorption does not contribute

meaningfully to the temperature of the medium, they

utilized a water absorption peak to vary the temperature of

the microchip [2].

In this way, the temperature range required for polymerase

chain reaction (PCR) could be achieved. This PCR required

the cell's medium to vary in temperature between 72°C and

93°C (shown in

Figure 3) in 50 second cycles.

Figure 3. Representation of two thermal cycles used

for the microchip PCR. For the 500 base-pair PCR

reported, 30 cycles were used. Data from [2].

Again, the microchip was designed to allow transmission

of light (here a 1450 nm infrared diode laser) through the

base of the microchip, with negligible heating of the chip

itself. 1450 nm was chosen due to its alignment with an

absorption peak of water, enhancing the temperature

change of the water (the cell's surrounding medium) upon

radiation.

The heating and cooling cycles required for PCR are well

known. In this application, a calibration between the laser

setpoint voltage (i.e. output current) and the temperature

of the water needed to be applied to ensure the cycling

was easily and precisely repeatable. This was achieved

empirically, by varying the driving voltage of the laser and

recording the corresponding temperature of the water.

To collect the data for the calibration, the voltage of the laser

was changed, and radiation allowed to occur for 2 minutes

to reach a steady-state temperature of the fluid. To reach

the desired temperatures, the entire operation range of the

laser was used.

Once the calibration was finalized, it was found that the

heating and cooling rates were on the order of 3°C/s, with

cooling having a slightly higher rate than heating.