The crux of the technology lies in a sequence of magnetic elements that store information based on their magnetic alignment. Using rectangular or elliptical elements naturally defines two stable states, parallel (and anti-parallel) to the long axis. By assigning each magnetisation direction to be either state 1 or 0, a magnetic barcode is created, and the number of possible combinations doubles with every additional bit, e.g. 10-bit tags can code for up to 1024 different biological compounds .
The group is working on two different tag designs that will be capable of offering a medium number of codes (32 - 8192) in the short-term, and a very large number for gene (re)sequencing applications in the long-term.
This tag design features magnetic elements encapsulated in a bio-compatible polymer backbone of size 100μm x 30μm and less than 2μm in thickness. The elliptical magnetic elements are spaced along the length of the backbone and typically only 20nm high, which makes these tags free-floating in aqueous solutions. The tags will be flown through microfluidic channels with integrated sensors where they will be read and sorted in a high-throughput system (see section 3).
Figure 1 a) schematic of a 7-bit equi-element magnetic tag, b) schematic of a 5-bit `multi-coercivity' tag and an F-MOKE image of a real tag.
These structures were grown in a sandwich process, where the SU8 backbones are spin-coated to a thickness of 1μm and shaped using standard photolithographic techniques. A second photolithography step is used to grow and align a PDMS mask on these backbones before vacuum deposition (e.g. evaporation, MBE, sputtering) of the magnetic elements. Finally another 1μm of SU8 is patterned on top, which now protects the magnetic bits from oxditation and the environment.
Figure 2 the main steps in the fabrication of free floating planar tags, (the detailed photolithography has been omitted).
A simplified fabrication procedure uses ion etching and reduces the number of complicated alignment steps, however the pay-off is reduced chemical functionality (see section 4). For details on these tags and their new fabrication procedure, please read the following paper: "Design and Fabrication of SU8 Encapsulated Digital Magnetic Carriers for High Throughput Biological Assays." B. Hong et al., J. App. Phys. 105 034701
These tags are aimed at creating an extremely large number of codes (106 to 109), so must be easily scalable for mass production and must be highly cost-effective to manufacture. The coding density is dramatically increased in this design by using a pure metal stack, featuring magnetic elements interleaved with non-magnetic spacers (figure 2). The top is capped with gold to allow a route for chemical modification (see section 4).
Figure 3 (left) schematic of a 10-bit pillar tag, (right) SEM image of a 15μm diameter pillar tag with 8 magnetic layers (some Cu layers are highlighted to aid the eye) and a gold cap, (inset) SEM demonstrating the homogenity of the electrodeposition process.
A single photolithography process is used to define elliptical holes in a non-conducting photo-resist, which is spin-coated onto a conductive substrate. This is usually a 4" silicon wafer with an evaporated novel aluminium/copper bi-layer, which is important for releasing the tags at the end of the process. The substrate is used as the cathode of an electrochemical cell, and when a voltage is applied the metallic ions in the electrolyte are deposited onto the substrate
Figure 4 The two-step fabrication process: (left) a template is photolithographically defined on a conducting substrate, (right) the substrate is used as the cathode in a basic 3-electrode electrochemical cell.
It is possible to deposit multilayers from a single bath containing a mixture of nickel, cobalt, iron and copper ions, by varying the applied potential, different ions deposit preferentially. This makes this process highly automatable, and material waste is minimised making this procedure ideal for mass production. Further details of this design and fabrication procedure can be found in the following paper: "Digital Biomagnetism: Electrodeposited Multilayer Magnetic Barcodes." J.J. Palfreyman et al., JMMM 321 (10) 1662-1666