Date Published: 27 January 2009
Near field laser 'tweezers' reveal new vista for medical diagnosis
Suspended in mid-air, a solitary red blood cell is rotated, stretched and folded in half. Then the light goes out. In darkness, the cell resumes its disc-like shape. But with the light back on, the cell is again subjected to forces that change its shape.
The forces that are able to have such a profound effect on these tiny human cells are produced by laser beams. And the manipulation is possible using delicate laser ‘tweezers’.
Researchers at Swinburne’s Centre for Micro-Photonics are exploring the science behind these tweezers to see if they can manipulate the cells with less light but more dexterity. The technology has tremendous potential for medical diagnosis.
The centre’s director Professor Min Gu and Associate Professor Xiaosong Gan are leading a team that recently became the first in the world to demonstrate ‘near field’ laser tweezers. Yet, to understand the significance of this achievement it is important to know how laser tweezers work.
“Light is composed of a lot of photons,” Professor Gu said:
“When a photon hits a target there’s a momentum change, a recoiling force. If the object is very big then this recoiling force won’t have much effect.” But if the object is tiny, say one micrometre, the force is large enough to move the object.
This phenomenon can be particularly well observed when an intense light such as a laser shines through a transparent sphere. When this occurs the light is refracted to one side, exerting a very slight force, which tends to push the sphere in the opposite direction.
Professor Gu said red blood cells, which are fairly transparent, can be manipulated by laser beams almost as if they were tiny glass balls.
A transparent sphere (or red blood cell) illuminated by a narrow laser seeks to remain centred in the laser beam even as the beam moves. The effect can be compared to suspending a table tennis ball in a rising jet of air. Because of the Bernoulli effect, the ball remains centred in the jet, even if the jet moves or changes direction.
When a second laser beam is introduced the sphere hovers between the two beams. The beams then act like tweezers and the sphere (or red blood cell) can be manipulated by steering the beams.
Laser tweezers are of particular interest to medical scientists as they can reveal a great deal about cell mechanics without permanently altering the cell, Professor Gu said.
“Red blood cells are the standard model for understanding cell mechanics. If the shape of a red blood cell changes it may be an indication of disease,” he said.
Laser tweezers are frequently used to study interactions between cells and how these interactions might influence disease development.
“The benefit of the tweezers is they have a temporary effect. As soon as the laser beam is switched off the cell returns to normal. We can squeeze, bend and rotate the blood cell, all without destroying it.”
Although laser tweezers have been used in biological applications since the late 1980s, they still carry the risk of damaging the cell being analysed. The tweezers also require the use of two laser beams for cell manipulation, which can be difficult to synchronise, Associate Professor Gan said:
This is where Professor Gu and Associate Professor Gan’s demonstration of ‘near field’ laser tweezers is significant.
Near field tweezers use an evanescent wave rather than a propagating wave, which means only one laser beam, not two, is needed to trap and manipulate samples. “With one beam we can achieve all the same mechanical actions – rotating, folding and stretching cells – as with two beams,” Associate Professor Gan said.
“Using the evanescent beam you need significantly less light to achieve the same effect. Too much light can cause functional change and damage the cell, so using less light is better as there’s less phototoxicity.”
Although the scientists have demonstrated the ‘near field’ tweezers on red blood cells their actions have opened a whole new vista for medical science. The tweezers could also be used to manipulate biological samples on the nano-scale – single molecules such as proteins.
“With the near field tweezers we can do a big object like a cell, but the evanescent beam is ideal for small objects like DNA – objects that are one-hundredth the size of a red blood cell,” Associate Professor Gan said.
Professor Gu said money was the only factor stopping the team from laser trapping a single molecule.
“There is a worldwide competition to see who will be the first person to achieve this nano-optical tweezer,” he said. “We are pioneers in this field. We have developed the technique, now we have to demonstrate it.”