Mechanisms of pulsed laser microbeam release of SU-8 polymer "micropallets" for the collection and separation of adherent cells

Abstract

The release of individual polymer micropallets from glass substrates using highly focused laser pulses has been demonstrated for the efficient separation, collection, and expansion of single, adherent cells from a heterogeneous cell population. Here, we use fast-frame photography to examine the mechanism and dynamics of micropallet release produced by pulsed laser microbeam irradiation at lambda = 532 nm using pulse durations ranging between 240 ps and 6 ns. The time-resolved images show the laser microbeam irradiation to result in plasma formation at the interface between the glass coverslip and the polymer micropallet. The plasma formation results in the emission of a shock wave and the ablation of material within the focal volume. Ablation products are generated at high pressure due to the confinement offered by the polymer adhesion to the glass substrate. The ablation products expand underneath the micropallet on a time scale of several hundred nanoseconds. This expansion disrupts the polymer-glass interface and accomplishes the release of the pallet from its glass substrate on the microsecond time scale (approximately 1.5 micros). Our experimental investigation demonstrates that the threshold energy for pallet release is constant (approximately 2 microJ) over a 25-fold range of pulse duration spanning the picosecond to nanosecond domain. Taken together, these results implicate that pallet release accomplished via pulsed laser microbeam irradiation is an energy-driven plasma-mediated ablation process.

Figure 1

(a) Array of SU-8 polymer pallets. The pallets are cubes with a dimension of 50 μm. (b) Side view of the irradiation geometry.

Figure 2

Schematic of laser–microscope setup for pallet release and time-resolved imaging.

Figure 3

Probability of pallet release as a function of pulse energy for different pulse durations.

Figure 4

Series of time-resolved photographs showing the pallet release dynamics using a single 540-ps laser pulse. Pallets are 50 μm in size. The formation of a plasma, followed by the emission of a shock wave, emergence of vapor from under the pallet, and pallet release are all visible on the nanosecond to microsecond time scale.

Figure 5

Series of time-resolved photographs showing the pallet release dynamics using a single 6-ns laser pulse. Pallets are 50 μm in size. The formation of a plasma, followed by the emission of a shock wave, emergence of vapor from under the pallet, and pallet release are all visible on the nanosecond to microsecond time scale.