Optoelectronic Tweezers (OET) trap stiffness with HeLa cells

Optoelectronic Tweezers (OET) creates patterned electrical fields by selectively illuminating a photoconductive layer sandwiched between two electrodes. The resulting electrical gradients are used to manipulate microscopic particles, including biological cells, using the dielectrophoresis (DEP) force. Previously it has been shown that up to 15,000 traps can be created with just 1 mW of optical power, and that OET traps are 470 times stiffer than traps created with optical tweezers of the same power. In this paper we explore the use of OET for trapping HeLa cells. First, experiments are performed using glass beads as a model particle, and the results are compared with numerical simulations to confirm our ability to model the electrical field gradients in the OET device. We then track trapped HeLa cells in different sizes of traps, showing maximum cell velocities of 60 μm s using an illumination intensity of just 2.5 W cm. We measure the electrical properties of the cell’s membrane by analyzing the cell’s DEP frequency response and use this information to model the forces on the cell. We find that it is possible to create a trap with a stiffness of 3x10 N m that does not vary with position within the trap.