Optical tweezers allow for manipulating micron-sized objects using pN level optical forces. In this work, we use an optical trapping setup to aid in three separate experiments, all related to the physics of the cellular membrane.
In the first experiment, in conjunction with Brian Henslee, we use optical tweezers to allow for precise positioning and control of cells in suspension to evaluate the cell size dependence of electroporation. Theory predicts that all cells porate at a transmembrane potential of roughly 1 V. The Schwann equation predicts that the transmembrane potential depends linearly on the cell radius r, thus predicting that cells should porate at threshold electric fields that go as 1/r. The threshold field required to induce poration is determined by applying a low voltage pulse to the cell and then applying additional pulses of greater and greater magnitude, checking for poration at each step using propidium iodide dye. We find that, contrary to expectations, cells do not porate at a constant value of the transmembrane potential but at a constant value of the electric field which we find to be 692 V/cm for K562 cells.
Delivering precise dosages of nanoparticles into cells is of importance for assessing toxicity of nanoparticles or for genetic research. In the second experiment, we conduct nano-electroporation---a novel method of applying precise doses of transfection agents to cells---by using optical tweezers in conjunction with a confocal microscope to manipulate cells into contact with 100 nm wide nanochannels. This work was done in collaboration with Pouyan Boukany of Dr. Lee's group. The small cross sectional area of these nano channels means that the electric field within them is extremely large, 60 MV/m, which allows them to electrophoretically drive transfection agents into the cell. We find that nano electroporation results in excellent dose control (to within 10% in our experiments) compared to bulk electroporation. We also find that, unlike bulk electroporation, nano-electroporation directly injects nanoparticles, such as quantum dots, to the cell interior, bypassing the cell membrane without the need for endocytosis.
The aging of RBC's can render them rigid, an issue for the survivability of transfusion patients. This rigidity can be assessed by examining the fluctuations in the cell membrane. In the third experiment, we use back focal plane detection---an interferometric detection scheme using an optical tweezers setup---to measure the membrane fluctuations of RBC's and K562 cells. Membrane fluctuations have long been observed in RBC's and a well developed theory exists linking them to the cells internal viscosity, the membrane bending modulus and the surface tension of the membrane. We use back focal plane detection to measure the effect of ascorbic acid treatment on RBC aging and find no improvement in cell flexibility. K562 cells differ from RBC's in that they possess an actin cortex which the membrane attaches to. We demonstrate that K562 cells exhibit as much as an order of magnitude more variation in their fluctuations than RBC's do.