05 Dec 2003
A grid of light beams creates a powerful way for separating cells and other microscopic particles.
An optical sorter that can separate different types of microscopic particles suspended in a fluid by simply “deflecting” them with beams of light has been demonstrated by scientists in Scotland and the US (Nature 426 421).
Since the technique is non-invasive, sterile and highly accurate it could be ideal for medical applications, say its developers, from the Universities of St Andrews and Illinois Wesleyan. They have already demonstrated that it can separate red and white blood cells as well as protein microcapsules that are used to deliver drugs.
The optical sorter relies on the same phenomenon that is used to create optical traps and tweezers. However, instead of using a focused laser beam to hold or trap a particle, the researchers create a 3D optical interference pattern known as an “optical lattice”.
The fluid containing the particles is pumped through the lattice to perform the sorting. By optimizing the flow rate and lattice parameters, it is possible to use the interaction between the particles and the lattice to strongly deflect and separate one of the particle types. The exact deflection angle depends on the size, shape and optical properties of the particle type.
“So far we’ve only played around with particles on the cellular scale, that’s 1 to 10 microns, but in theory it could be possible to sort particles as large as 100 microns,” Mike MacDonald from St. Andrews told optics.org. “It [the sorter] should be able to operate within the same range as optical tweezers.”
Experiments with water containing 2 micron diameter silica and polymer spheres (protein microcapsules) have demonstrated that a deflection angle of 45° and a sorting efficiency of at least 96% is achievable. Preliminary experiments have also shown that it is possible to separate white and red blood cells.
The lattice is made by passing a 1070 nm laser beam with a power of a few hundred milliwatts through a diffractive beam splitter and recombining the output.
As for the speed of the sorting, the team says that the throughput of its prototype system was 25 particles per second. It believes that this could be scaled up by increasing the volume of its sample call and moving to a parallel process. “There’s a trade-off between deflection angle and flow velocity,” commented MacDonald.
The St Andrews team is not the only group building such optical sorters, a group in the US headed by David Grier is also developing a similar system.
Given time, the technology may evolve into a commercial lab-on-a-chip solution. “We believe that we have only touched the surface of this technology,” said MacDonald. “We now want to extend the technique and see if we can distinguish between cancerous and non-cancerous cells.”
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.
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