Photomicrograph of two micro-rotors with one particle

A system designed by scientists at three U.K. universities uses robotically controlled micro-rotors, created with direct laser writing and turned using optical tweezers, to hydrodynamically control and steer tiny particles. The system could protect some samples, such as fragile living biological particles, from the photo-damage sometimes entailed by direct manipulation with optical tweezers. [Image: David Phillips / University of Exeter]

Optical tweezers, which use gradient forces of strong light fields to grab onto and manipulate nanoparticles, cells and even viruses, have brought increasing sophistication to the study of cell division, intracellular forces and a range of other biophysical problems. But the powerful light fields of optical tweezers can damage some delicate, living biological specimens.

To get around that, a team of scientists from three U.K. universities has proposed a different take on optical tweezers (Nat. Commun., doi: 10.1038/s41467-019-08968-7). Instead of using light to trap and manipulate particles directly, the team’s setup uses holographic optical tweezers to nudge into place and optically drive tiny, robotically controlled micro-rotors. These spinning rotors in turn create precise, highly localized fluid-flow fields that can manipulate particles and biological cells in aqueous media without the risk of particle damage by concentrated laser power.

The team believes that the method opens up some interesting new application possibilities for moving biological samples through aqueous environments, such as those in microfluidic chips.

Direct laser writing to fashion tiny rotors

Numerous researchers have explored the prospects for manipulating small particles using microfluidic controls, as an alternative to optical-, magnetic- or electric-field approaches. But conventional hydrodynamic techniques, which often involve external pressure controllers or syringe pumps, have tended to generate relatively large-scale flow fields. These fields can sweep up other particles in the solution or suspension in addition to the particles of interest, risking contamination of the experiment.

To get around that problem, the research team behind the current work—including scientists from the University of Glasgow, the University of Bristol, and the University of Exeter, U.K.—began by using direct laser writing to create exquisitely small flow-actuating elements. Taking advantage of a commercial 3-D laser lithography system from the company Nanoscribe, the researchers fashioned circular three-blade plastic rotors around 20 microns in diameter. Each rotor blade included a tiny, bulbous “handle” that could be grabbed onto by optical tweezers.

Robotic control

The team immersed the micro-rotors into an aqueous suspension that also included target particles, such as 5-micron-radius silica beads. The suspension, in a clean glass sample cell, was then put on the stage of a lab-built holographic-optical-tweezers setup, focused with an inverted microscope objective and controlled with a spatial light modulator (SLM). The SLM imprinted the beam with a rapidly reconfigurable phase pattern that could be used to generate multiple optical traps in the sample field. Two digital cameras tracked each experiment’s progress in real time.

To provide robotic control of the micro-rotors, the researchers tied the tweezer setup to computer software that implemented a hydrodynamic feedback loop. Real-time information from the cameras on target location and speed were fed into the computer routines, which solved a hydrodynamic matrix equation relating the rotation speed of the various rotors to the particle velocity, and calculated the changes in rotor rotation and location needed to change the flow field. Those data were used in feedback loops to refresh the SLM for the next cycle on the tweezers.

Keeping it local

Using this setup, the team found that was able to spin the micro-rotors using the optical-tweezers setup, and use the hydrodynamic currents from those spinning rotors to move around the tiny particles, as well as to hold them in place in specific spots. Because the amounts of fluid displaced by each rotor are on the order of picoliters, the flow fields are precisely confined, affecting nearby target particles but leaving surrounding particles unaffected.

The team was also able to use the system to move and orient individual yeast cells, and to control multiple objects independently at the same time. And the spatial separation between the lasers driving the optical tweezers and the hydrodynamically trapped and manipulated particles, according to the authors, is enough to protect the particles from photo-damage.

The combination of optical control, automated feedback and reconfigurability, and precise hydrodynamic steering could, in the view of the researchers, open up “a variety of new experimental paradigms.” One could be the creation of “a dynamically reconfigurable microfluidic chip,” with light- and computer-controlled micro-rotors that move along with particles and steer them through the chip architecture.