Bead incubation and washing on a droplet actuator
The invention relates to bead incubating and washing on a droplet actuator. Methods for incubating magnetically responsive beads that are labeled with primary antibody, a sample (i.e., analyte), and secondary reporter antibodies on a magnet, on and off a magnet, and completely off a magnet are provided. Also provided are methods for washing magnetically responsive beads using shape-assisted merging of droplets. Also provided are methods for shape-mediated splitting, transporting, and dispensing of a sample droplet that contains magnetically responsive beads. The apparatuses and methods of the invention provide for rapid time to result and optimum detection of an analyte in an immunoassay.
1. A method comprising:
(a) providing a droplet actuator comprising:
droplet operations electrodes arranged for conducting droplet operations on a droplet operations surface; and
(ii) a magnet positioned relative to the droplet operations surface such that a droplet controlled by one or more of the droplet operations electrodes may be positioned within or away from a first region of the magnet's magnetic field capable of substantially attracting magnetically responsive beads in the droplet;
(b) positioning a droplet having magnetically responsive beads therein within the first region of the magnetic field of the magnet to cause the beads to be attracted to the magnet, and activating the droplet operations surface in a manner to cause the droplet to take an elongate shape;
(c) operating the droplet operations surface to activate one electrode at a time, to cause the droplet to move away from the magnet, and thereby cause the geometry of the droplet to be distorted; and
(d) continuing to operate the droplet operations surface to transport the droplet further away from the magnet and inactivating an electrode intermediate to the droplet to cause the droplet to split into a supernatant droplet and a smaller droplet that has the magnetically responsive beads therein.
2. The method of claim 1 , wherein step (b) is conducted by positioning the droplet at a droplet operations electrode within the first region of the magnetic field of the magnet, and wherein an adjacent droplet operations electrode is activated to cause the droplet to take an elongate shape, and wherein step (c) is conducted by operating one electrode at a time.
3. The method of claim 1 , wherein the sample droplet is of a size wherein its footprint is three times the area of a single droplet operations electrode.
4. The method of claim 1 , wherein the droplet comprises a sample comprising a target substance, and the magnetically responsive beads comprise beads having an affinity for the target substance.
5. The method of claim 4 , wherein the target substance is at least one of cells, protein, nucleic acid, and an antigen.