Making tweezers out of holograms and using them to manipulate living tissue sounds like something from a yet-to-be-made Star Trek episode.
The technology, however, has been around since the year 2000, when Matthew Dearing and Gabriel Spaulding of Illinois Wesleyan University in the US demonstrated the idea at a conference.
Dearing and Spaulding’s “holographic optical tweezers”, in theory at least, used a laser to trap and move individual nanoparticles, manipulating them by exploiting the angular divergence of the laser beam to gently push them and combine them into structures.
Since then, the development of the tweezers (known as HOT) has continued apace, achieving ever greater precision and manipulating ever smaller particles.
In 2003 a Chinese team reported deploying a HOT so delicate that it was able to tie a trefoil in double-stranded DNA. Not to be outdone, earlier this year another team, led by Mervyn Miles of the University of Bristol, UK, demonstrated “the process of tying increasingly complex knots” in a segment of DNA 50 micrometres long.
HOT works through using a tightly focused laser beam to create optical traps, which permit the holding or manipulation of particles on a nano or micro scale. They make use of a phenomenon known as wavefront shaping – where light is pushed through a scattering medium, creating multiple traps and allowing operators to move captured particles through three dimensions.
Holographic tweezers are the tool kit of choice for scientists who build and operate molecular motors, and have permitted unprecedented insight into the workings of small structures within individual cells.
However, because of the scale on which they operate, they are of little use more than just a few micrometres beneath any given surface, and function poorly in turbid conditions – inside actual living cells functioning as part of a larger organism, for instance.
Now, however, a team led by Ivo Lette from the University of Dundee in Scotland has demonstrated a new approach which overcomes both obstacles.
In a paper published in the journal Nature Photonics, Lette and his colleagues describe a HOT system that creates multiple optical traps by using an optical fibre with a “high numerical aperture” – meaning it enables a large number of angles through which light can be transmitted along it.
The team’s approach overcomes one of the principle problems associated with using this type of optical tool – known as multimode fibres – which is that light transmission through them becomes a chaotic process, producing images with significant interference patterns.
Combining multimodal fibres with wavefront shaping, the team reports, allows the “the engineering of various elaborate beam shapes including laser foci precisely positioned”.
To achieve this, Lette and his colleagues created a rigid multimodal fibre with a diameter of just 35 micrometres, incorporating very high contrast gradients between its core and cladding, thus producing a high numerical aperture.
The new design – coupled with an equally new and powerful algorithm – allowed the team to successfully trap and manipulate nanoscale objects using fibres up to 150 millimetres long. In so doing, they were able to reach normally inaccessible cavities in turbid liquid.
The research, the scientists conclude, opens the door to the development and use of “single-core-fibre endoscopes deep inside living tissues and other complex environments”.
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