Cell cytometry with a light touch: Sorting microscopic matter with an optical lattice

Lab-on-a-chip design is a key technology for increasing both the reliability and the functionality of many different preparation and diagnostic techniques in biomedicine. The drive towards ever more integrated lab-on-a-chip designs requires increasingly complex microfluidic systems. In order to build these systems, non-invasive actuators such as pumps, filters and mixers are required (1). We have demonstrated microfluidic sorting based on a 3D interference pattern, formed from multiple coherent laser beams, which has the potential to fulfil all the above criteria (2). By interfering five laser beams from a fibre laser at 1070 nm, we have formed a 3D optical lattice. When particles flow through the optical lattice their trajectories depend upon the force exerted on the particle by the optical lattice, in combination with the drag force exerted by the fluid flow. Hence, with the strength of a particle's interaction with the lattice determining the total force exerted upon it, its trajectory is determined by its physical properties (3). These properties include refractive index, size and shape, giving a range of criteria with which to sort an analyte. We have shown separation at 45° of polymer from silica microspheres (by refractive index), the separation of protein microcapsules and the sorting of erythrocytes from lymphocytes. The interference pattern can be tailored to the particles and if a blockage occurs, the laser can simply be switched off, unlike solid-state micro-sorters (4), so that no jamming occurs. Efficiencies in excess of 95% have been achieved (2).