Holographically encoded microparticles for bead-based assays

We demonstrate a re-writable, high capacity holographic encoding technique for multiplexed bead-based suspension assays. The microparticles are made from SU8 doped with a photochromic diarylethene dye and manufactured using multilayer photolithography and dry etching. Each particle is encoded with a unique hologram, whose diffraction pattern consists of bright and dark regions, representing a binary number that identifies the particle. Theoretically up to 10(24) unique codes are available on a 100 mu m particle using this method, when the code is read with a standard 2/3" CMOS camera. Encoding capacities of 512 unique codes have been demonstrated on a 500 mu m SU8 particle. The code is thermally stable for 3 days at 25 degrees C, and once written, the code can be erased and re-written once whilst still remaining readable. The code can be written into the particle during an assay experiment (no pre-encoding is required) and requires simple optics for reading.