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Optical trapping moves towards microfluidics

20 Mar 2008

Optical trapping is a rapidly advancing and versatile field. Kishan Dholakia speaks to Marie Freebody about the breakthroughs so far and his expectations for the future.

Kishan Dholakia heads the Optical Trapping Group at the University of St Andrews in the UK. His pioneering research into optical landscapes has enabled complex patterns of light to sort, separate or transform many molecules or cells at once. Applications that benefit from this research range from single-cell analysis to fundamental studies of light.

Can you summarize how optical trapping works?
If we focus a laser beam to a small spot of the order of 1/1000th of a millimetre, we can form a trap. A transparent object or bead will refract light causing a change in the light's momentum. This attracts the bead to the centre of the laser point where an equal amount of light is refracted in all directions. Some light is reflected, which means that the final equilibrium position of the sphere is close to, but not exactly at, the beam focus.

Why is it important to pursue the development of this technology?
Optical tweezers have made the trans-ition to become an exceptionally powerful interdisciplinary tool. Since its conception in the mid-1980s, it has revolutionized our understanding of molecular motors in single molecule biophysics as well as making a huge impact in colloidal physics and fundamental studies of light. It is important to sustain the momentum of the field as it is difficult to think of another technology so versatile and widely applicable. With improvement, new and exciting applications are emerging, including those in microfluidics and single-cell analysis.

What are the main applications and when do you expect them to occur?
There are studies appearing that can measure angstrom-level motion and femtonewton or smaller forces. Major advances are likely so that technology can do this in a more regular and simplified manner, and hopefully in the next few years will become a mainstay in biology laboratories.

On a larger scale level, I expect applications for organizing and arranging particles to advance. If we can organize cells into certain arrays, then we can look at cell differentiation and even tissue growth. This is likely to grow in the next year with multiple traps giving insight into colloidal systems such as complex solvent/particle mixtures, as well as glass systems. We might even see tweezers making a bigger impact in self-assembled structures.

Multiple traps can create light patterns similar to a large array of egg boxes and these are commonly known as "optical landscapes". This technology is relatively new and will allow for new forms of sorting. If used in combination with other sorting and separation techniques in the microscopic scale, some exciting new ways to separate, probe and select cells in microfluidic environments could be found. This will help develop new types of microchips.

Until recently, the area of metal nanoparticles and nanowires had been relatively unexplored. I believe that this area will see a rich advance in the next few years where we will understand and organize nanoparticles into arrays and gain new insight into light-metal interactions.

Traps are also becoming useful in moving small droplets and microcapsules. This is likely to lead to new forms of microreactors and greater understanding of reaction dynamics – perhaps the world's smallest "test tube"? Traps will also play a more prominent role in aerosol studies.

What would you say is the most important recent advance?
This is a hard one – can I pick two? The Block group at Stanford University in the US is using optical tweezers to measure angstrom-level displacements by reducing the noise and vibration in the system. This is amazing.

Another important advance is the emergence of multiple traps and the new applications that this is creating such as multicell analysis, cell sorting and cell organization.

What are the key challenges left to overcome in this field?
Calibrating each trap within an array of multiple traps and combining this with technologies such as confocal spectroscopy, multiphoton microscopy, fluorescence and Raman. Understanding how a wider array of objects such as nanoparticles may be trapped and integrating optical systems with microfluidics for real biomedical science on a chip.

What do you expect the next big breakthroughs to be?
Optical trapping has been progressing steadily. In the short term there will be a greater uptake of multiple calibrated traps as well as the development of a bio-workstation where trapping is readily incorporated with other modalities such as imaging and spectroscopy. Advanced particle tracking and microscopy methods will add value and we may gain new insight into Brownian dynamics.

On a longer time frame I would expect to see a deeper understanding of the light-matter interaction leading to the creation of larger, truly self-assembled structures using tweezers.

A variety of schemes will come forward that will integrate optics and microfluidics, and I expect to see major advances. This will lead to a compact optical microfluidic device that will look at sourcing analysis and will prove to be a very strong and powerful device in years to come.

• This article originally appeared in the March 2008 issue of Optics & Laser Europe magazine.

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