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01 Aug 2005
An optoelectronic tweezer reported in Nature traps and manipulates 15000 particles simultaneously.
Scientists in the US have developed an optoelectronic tweezer (OET) that can manipulate up to 15,000 particles in parallel. Based around an LED and a digital micromirror display (DMD), the team believes its OET could be used for cell sorting or screening as well as assembling colloidal particles. (Nature 436 370)
"The unique advantage of the OET is that each particle in a large array can be individually manipulated and controlled," explained research leader Ming Wu from the University of California at Berkeley. "This is also the first time that an LED has been used to trap particles."
In Wu's tweezer, the liquid containing the particles is sandwiched between an upper conductive layer of ITO-coated glass and a lower photoconductive surface.
The particles are manipulated by images generated from a DMD illuminated with a red (625 nm) LED. The pattern from the DMD is then projected on to the lower photoconductive surface using a 10x objective lens.
"When the projected light illuminates the photoconductive layer, it turns on virtual electrodes, creating non-uniform electric fields and enabling particle manipulation via dielectrophoresis (DEP) forces," explained Wu. "The particles are trapped in darker areas by the induced negative DEP forces, which push the particles into the non-illuminated regions where the electric field is weaker."
Wu estimates that the minimum optical intensity required to turn on a virtual electrode is 3 nW/µm2, which is around 100,000 times lower than that used in optical tweezers. The current set-up also boasts a manipulation area of 1.3 x 1.0 mm.
To date the team has succeeded in sorting dielectric particles such as polystyrene, glass and silica as well as biological cells such as white blood cells. Wu says the OET has manipulated particles with sizes from 4.5 microns to greater than 45 microns.
Using this direct imaging approach, sophisticated virtual electrodes can be created and constantly updated on the DMD. For example, Wu's team has created an optical conveyor belt that can carry particles and sort them by diameter. The team has also sorted living and dead cells by exploiting dielectric differences between the cells.
According to Wu, there is the potential to increase the number of particles that can be manipulated to improve the throughput of the system. "Our current DMD has a resolution of 1024x768 pixels," he said. "In principle that can support around 193,800 traps if we use a single dark pixel surrounded by one layer of bright pixels for a trap."
Author
Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.
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