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We will demonstrate how optical tweezers can be combined with a microfluidic system to create a versatile microlaboratory. Cells are moved between reservoirs filled with different media by means of optical tweezers. We show that the cells, on a timescale of a few seconds, can be moved from one reservoir to another without the media being dragged along with them. The system is demonstrated with an experiment where we expose E. coli bacteria to different fluorescent markers. We will also discuss how the system can be used as an advanced cell sorter. It can favorably be used to sort out a small fraction of cells from a large population, in particular when advanced microscopic techniques are required to distinguish various cells. Patterns of channels and reservoirs were generated in a computer and transferred to a mask using either a sophisticated electron beam technique or a standard laser printer. Lithographic methods were applied to create microchannels in rubber silicon (PDMS). Media were transported in the channels using electroosmotic flow. The optical system consisted of a combined confocal and epi-fluorescence microscope, dual optical tweezers and a laser scalpel.

We introduce a novel setup combining a micro-Raman spectrometer with external optical tweezers, suitable for resonance Raman studies of single functional trapped cells. The system differs from earlier setups in that two separate laser beams used for trapping and Raman excitation are combined in a double-microscope configuration. This has the advantage that the wavelength and power of the trapping and probe beam can be adjusted individually to optimize the functionality of the setup and to enable the recording of resonance Raman profiles from a single trapped cell. Trapping is achieved by tightly focusing infrared (IR) diode laser radiation (830 nm) through an inverted oil-immersion objective, and resonance Raman scattering is excited by the lines of an argon:krypton ion laser. The functionality of the system is demonstrated by measurements of trapped single functional erythrocytes using different excitation lines (488.0, 514.5, and 568.2 nm) in resonance with the heme moiety and by studying spectral evolution during illumination. We found that great care has to be taken in order to avoid photodamage caused by the visible Raman excitation, whereas the IR trapping irradiation does not seem to harm the cells or alter the hemoglobin Raman spectra. Stronger photodamage is induced by Raman excitation using 488.0- and 514.5-nm irradiation, compared with excitation with the 568.2-nm line., ((c) 2004 Society of Photo-Optical Instrumentation Engineers.)