Bead incubation and washing on a droplet actuator

Abstract

Methods are provided for separating magnetically responsive beads from a droplet in a droplet actuator. Droplet operations electrodes and a magnet are arranged in a droplet actuator to manipulate a bead-containing droplet and position it relative to a magnetic field region that attracts the magnetically responsive beads. The droplet operations electrodes are operated to control the droplet shape and transport it away from the magnetic field region to form a concentration of beads in the droplet. The continued transport of the droplet away from the magnetic field causes the concentration of beads to break away from the droplet to yield a small, concentrated bead-containing droplet immobilized by the magnet.

1 . A droplet actuator comprising:
 (i) droplet operations electrodes to conduct 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 to attract magnetically responsive beads in the droplet;   wherein the magnet is positioned such that three or more droplet operations electrodes are within the magnetic field of the magnet.   
     
     
         2 . The method of  claim 1 , wherein the magnet is an electromagnet. 
     
     
         3 . The method of  claim 1 , wherein the magnetically responsive beads are coated with a primary antibody with an affinity for a specific target antigen.