Holographic optical tweezers combined with a microfluidic device for exposing cells to fast environmental changes

Abstract

Optical manipulation techniques have become an important research tool for single cell experiments in microbiology. Using optical tweezers, single cells can be trapped and held during long experiments without risk of cross contamination or compromising viability. However, it is often desirable to not only control the position of a cell, but also to control its environment. We have developed a method that combines optical tweezers with a microfluidic device. The microfluidic system is fabricated by soft lithography in which a constant flow is established by a syringe pump. In the microfluidic system multiple laminar flows of different media are combined into a single channel, where the fluid streams couple viscously. Adjacent media will mix only by diffusion, and consequently two different environments will be separated by a mixing region a few tens of micrometers wide. Thus, by moving optically trapped cells from one medium to another we are able to change the local environment of the cells in a fraction of a second. The time needed to establish a change in environment depends on several factors such as the strength of the optical traps and the steepness of the concentration gradient in the mixing region. By introducing dynamic holographic optical tweezers several cells can be trapped and analyzed simultaneously, thus shortening data acquisition time. The power of this system is demonstrated on yeast (<i>Saccharomyces cerevisiae</i>) subjected to osmotic stress, where the volume of the yeast cell and the spatial localization of green fluorescent proteins (GFP) are monitored using fluorescence microscopy.