In recent years there has been a growing interest in the use of optical manipulation techniques, such as optical
tweezers, in biological research as the full potential of such applications are being realized. Biological research is
developing towards the study of single entities to reveal new behaviors that cannot be discovered with more
traditional ensemble techniques. To be able to study single cells we have developed a new method where a
combination of micro-fluidics and optical tweezers was used. Micro-fluidic channels were fabricated using soft
lithography. The channels consisted of a Y-shaped junction were two channels merged into one. By flowing
different media in the two channels in laminar flow we were able to create a sharp concentration gradient at the
junction. Single cells were trapped by the tweezers and the micro-fluidic system allowed fast environmental
changes to be made for the cell in a reversible manner. The time required to change the surroundings of the cell
was limited to how sharp mixing region the system could create, thus how far the cells had to be moved using
the optical tweezers. With this new technique cellular response in single cells upon fast environmental changes
could be investigated in real time. The cellular response was detected by monitoring variations in the cell by
following the localization of fluorescently tagged proteins within the cell.
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