x-posted at Erica Pratt’s Blog
Anion:a negatively charged ion.asophil, Lymphocyte, Neutrophil: Different types of white blood cells.
Biofouling: Fouling of surface with biological material.
Cation: A positively charged ion.
Conductivity: Measure of a substance’s ability to conduct electricity.
Cytoplasm: Inner contents of the cell, which holds everything outside the nucleus.
CTC: Circulating tumor cell. Read about what they are and why they’re important here.
Erythrocyte: Red blood cell.
PBMCs: Peripheral mononuclear blood cells, aka blood cells that have a nucleus (e.g. white blood cells). Cartoon of the different kinds of white blood cells here.
MDA-MB-***: Human breast carcinoma immortalized cell lines.
Phenotype: Observable characteristics of a cell.
This is the last major sorting technique in this series (for now), and I will be using the review paper I co-wrote with Charlie Huang as the framework for my description of this technique1. Charlie works extensively on electrokinetic manipulation of cells, and his half of the review paper lends itself well to explaining how electrokinetics can be used to sort CTCs.
Why use electrokinetic separation?
Most CTC sorting devices target some observed cancer cell phenotype that was determined from studying tumor tissue directly, or from using immortalized cancer cell lines. This means that active sorting techniques, like size-based selection and immunocapture, require some level of a priori knowledge about CTCs before you can engineer a device to capture them. Microscopic characterization is one CTC identification method that circumvents this problem, fixing (killing) the cells, and then using imaging in combination with rapid scanning to look at almost everything present in the blood sample. Electrokinetic separation of cancer cells is another, but enables live cell isolation without knowing its physical or biochemical properties beforehand.
What types of electrokinetic techniques are used?
There are two commonly used types of electrokinetic manipulation for mammalian cells, electrophoresis (EP) and dielectrophoresis (DEP). Electrophoresis involves applying a uniform electric field across a charged particle, causing it to polarize (i.e. free charge aligns with the electric field), inducing a net particle migration. However, if a uniform electric field is applied to an electrically neutral particle, the charges will polarize to form a dipole, but there is no actuation because the force on each anion is cancelled out by the force on its respective cation, and vice versa. To induce actuation, a non-uniform electric field must be applied (Dielectrophoresis), causing the charge on one side of the particle to feel the electrical force more strongly than the other, resulting in particle migration—as shown in the example below (thanks to the Kirby Lab DEP subgroup for the great schematic!).
Electrophoresis is good for moving charged particles around; however, the net charge from cell type-to-cell type is often not distinct enough to sort cells with high resolution. In contrast, dielectrophoresis is excellent for sorting cells because motion is dependent not on net charge, but on cell membrane and cytoplasm electrical properties as well as cell size, as dictated by this equation: