Following the reaction step with the sample, tags carrying targets that have reacted can be labelled with a fluorophore (or even magnetic beads). The next task is to isolate the positive tags and decode them - an efficient way to do this is to flow the tags through a microfluidic channel where fluorescing (positive) tags can be picked out of the main flow and sent to a decoding area. This ensures that only a fraction of the total tags are read, increasing the throughput of the process.
Figure 1 schematic of a microfluidic device with hydrodynamic focussing used to measure and separate magnetic and polystyrene beads, magnetic beads are labelled with a red dye, and polystyrene are coated with a green dye. Insets: a) a laminar stream of ink (non-magnetic) is directed to the waste channel, b) metal strip-lines can apply a magnetic field across the split channel to direct individual tags, a pair of AMR ring sensors are shown upstream of the branch, c) magnetic and polystyrene beads were coated with probe antibodies and coupled to their fluorescent target before microfluidic sorting with a permanent magnet.
Magnetic particles can be pulled out of the main flow and into a side-channel by applying an external magnetic field. To separate magnetic particles from a non-magnetic mixture, simply positioning a permanent magnet next to the sorting branch is enough (as shown in figure 1 a) & c)). Alternatively, individual magnetic tags can be picked out by applying an electrical current through metal strip-lines located at the sorting gate just as a selected tag approaches (figure 1 b)).
The tags are read using commercial TMR (tunnelling magnetoresistance) sensors, which are embedded in the microfluidic chip. Focussing techniques centre the tag in the channel and as the tag flows a few microns over the sensor, the electronic response is measured and a digital read-out produced.
Figure 2 (left) optical microscope image of a 5-bit tag flowing through a 50μm channel just before reaching the TMR sensor (right) the measured TMR response signal from a tag in the (1,1,1,1,1) configuration - all bits in the same direction is the hardest to decode.