Bead incubation and washing on a droplet actuator
The present 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 of resuspending magnetically responsive beads between wash cycles, the method comprising:
(a) providing a droplet actuator comprising:
(i) 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 suspended therein at a location at least partially overlapping the first region of the magnetic field, wherein at least a portion of the magnetically responsive beads become unsuspended;
(c) transporting the droplet through activation of selected droplet operations electrodes away from the first region of the magnetic field without removing the magnet;
(d) transporting the droplet through activation of selected droplet operations electrodes towards the first region of the magnetic field; and
(e) repeating steps (b) and (c) to cause sufficient resuspension of beads such that unbound material may be effectively removed in subsequent wash cycles.
2. The method of claim 1 , wherein step (b) is conducted by positioning the droplet at a location partially overlapping a droplet operations electrode located within the first region of the magnetic field.
3. The method of claim 1 , further comprising conducting a wash cycle after resuspension of the beads to remove unbound material.
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.