A new type of optical tweezer consisting of a Fresnel zone plate microfabricated on a glass slide can trap particles without using high-performance objective lenses. It has the potential to make biological and microfluidic force measurements inside microfluidic chips and other integrated systems. The device was created by researchers at the Harvard School of Engineering and Applied Sciences (SEAS): postdoctoral fellow Ethan Schonbrun and undergraduate researcher Charles Rinzler under the direction of electrical engineering assistant professor Ken Crozier. Optical tweezers, which trap and move objects through the forces supplied by tightly focused laser beams, have become important tools for biological research over the last 20 years, such as in cell sorting. Most of the time, optical tweezers are built by making extensive modifications to a standard optical microscope. (a) Photograph of microfabricated Fresnel zone plate optical tweezer, consisting of concentric gold rings (50-nm thick) on a microscope slide. The zone plate outer diameter is 100 µm, and the focal length 8 µm. (b) CCD camera image of fluorescent bead (2-µm diameter) trapped in zone plate focus. (Image courtesy Ken Crozier, Harvard School of Engineering and Applied Sciences) "The microfabricated nature of the new optical tweezer offers an important advantage over conventional optical tweezers based on microscope objective lenses," said Crozier. "High-performance objective lenses usually have very short working distances -- the trap is often about 200 mm or less from the front surface of the lens. This prevents their use in many microfluidic chips since these frequently have glass walls that are thicker than this." The researchers said that the Fresnel zone plate optical tweezers could be fabricated on the inner walls of microfluidic channels or even inside cylindrical or spherical chambers and could perform calibrated force measurements in a footprint of only 100 x 100 µm. Traditional tweezers, by contrast, would suffer from crippling aberrations in such locations. Also, in experimental trials, the optical tweezers exhibited trapping performance comparable to conventional optical tweezers when the diffraction efficiency was taken into account, the researchers said. They envision using their new tweezer inside microfluidic chips to carry out fluid velocity, refractive index, and local viscosity measurements. Additional applications include biological force measurements and sorting particles based on their size and refractive index. Particle-sorting chips based on large arrays of tweezers could be used to extract the components of interest of a biological sample in a high-throughput way. The work was supported by the Microsystems Technology Office of DARPA and the Harvard Nanoscale Science and Engineering Center of the National Science Foundation. The team's results were published in the Feb. 18 edition of Applied Physics Letters and the researchers have filed a provisional patent on the device. For more information, visit: www.seas.harvard.edu