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 manipulating a droplet having magnetically responsive beads therein, 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 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 at a location on the droplet operations surface within the region of the magnetic field to form a concentration of beads in the droplet; and
(c) removing the droplet from the region of the magnetic field by transporting the droplet through activation of selected droplet operations electrodes away from the region of the magnetic field without removing the magnet, thereby resuspending the magnetically responsive beads in the droplet.
2. The method of claim 1 , wherein at least one of steps (c) and (d) is conducted with the magnetically responsive beads positioned at a locus of the droplet operations surface sufficiently near the magnet such that they are actively attracted to the first region of the magnetic field.
3. The method of claim 1 , wherein at least one of steps (c) and (d) is conducted with the magnetically responsive beads positioned at a locus of the droplet operations surface at a sufficient distance from the magnetic field such that they are not substantially aggregated by the first region of the magnetic field.
4. The method of claim 1 , wherein the droplet transported has a footprint approximately one of one, two, or three times the area of a single droplet operations electrode.
5. 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.
6. The method of claim 5 , wherein the target substance is at least one of cells, protein, nucleic acid, and an antigen.
7. The method of claim 1 , wherein the transporting, splitting, and merging operations are repeated a plurality of times.
8. The method of claim 1 , wherein in step (b) the concentration of magnetically responsive beads occurs at an edge of the droplet adjacent the magnet.
9. The method of claim 1 , wherein in step (c), because of the activation of selected droplet operation electrodes, the droplet becomes elongated as a result of the transporting.
10. The method of claim 1 , wherein in step (e) the merging occurs within a region where the magnet is located.
11. The method of claim 1 , wherein the droplet operations are electrowetting mediated.
12. The method of claim 1 , wherein the droplet operations are dielectrophoresis mediated.