OPTICAL TWEEZERS: Optical trapping empowers biophysics
Newly available to broad audiences, optical-tweezers technology is becoming easier to implement and operate, and is poised to open new vistas—especially for single-molecule studies.
KISHAN DHOLAKIA AND MIKE ELLIOT
The miniscule forces that light exerts on micron-size particles have empowered scientists, particularly those in biomedicine, enabling them to perform important studies on single molecules, cells, and colloids, without inflicting damage (see Fig. 1).
FIGURE 1. Optical trapping makes it possible to obtain a Raman spectrum of a single cell. Each color denotes the spectra from a different cell type. Using statistical analysis one can differentiate between abnormal and normal cells.
The harnessing of these light forces, known as optical tweezing has for many years been dabbled in by only a brave few, primarily because the technique is difficult and multifaceted, involving lasers, microscopes, imaging systems, specialized software, and complex optomechanical design. Indeed many students have earned Ph.D.s by designing and building an optical tweezers and then used their post-doc time to undertake meaningful experimentation. Design, integration, and testing to produce a useful calibrated system typically requires one to two years—a threshold too high for many researchers.
To make the technology accessible for a larger audience, including those not well versed in photonics, we assembled a team of engineers at Elliot Scientific (Harpenden, England) and researchers from the Optical Trapping Group at the University of St. Andrews (Scotland) to design a simple, single-beam trap system. The inherent flexibility of the system's design, paired with our experience, allows us to address individual needs—and we can manufacture the system in a multitude of configurations tailored to the needs of specific researchers. In fact, many of the first systems were for Master's-level course experiments, for which we built and installed custom systems. A milestone was the commercial release of a prealigned, self-contained, fully interlocked portable workstation for single beam trapping that can be set up in minutes and carried from room to room to demonstrate technique.
Beyond the basics
The research community wanted more, however, including the ability to add more exotic beam shapes, incorporate optical tweezers into existing microscope systems, and make high-quality precision measurements. The technique was ripe to grow beyond physics into biology applications (see Fig. 2).
FIGURE 2. Using optical tweezers in a fluid flow environment it is possible to separate untagged erythrocytes and lymphocytes. Cells are passed though an optical gradient that acts as a sieve, allowing separation by size, shape, or refractive index.
One of the main challenges for those setting up the systems is the beam path and its layout on an optical table. To address this issue, we designed a small-footprint optical assembly to bolt directly onto almost any commercial microscope. This development enables fast, reliable set-up, does away with the "optics strewn all over the table" scenario, and made it possible to add optical tweezing just as one might add an epi-fluorescence or total-internal-reflection fluorescence (TIRF) attachment to an existing microscope.
To enable manipulation of multiple traps in a field of view rather than one at a time, we developed software that allows the operator to select one or more traps or arrays of traps and manipulate each group individually.
The software works with acousto-optic deflectors to enable creation of multiple traps, each held using time-division multiplexing of the single infrared-laser source beam.
Because many researchers want to measure trapping force, we developed particle imaging based on a quadrant photodetector (QPD). The setup allows the operator to monitor the position of a trapped bead to nanometer accuracy. The associated measurement software allows the user to calibrate the motion of a single trapped object and thus makes it possible to infer forces exerted (see Fig. 3). This makes optical tweezing particularly attractive for the growing number of scientists probing single-molecule systems.
FIGURE 3: Optically trapped microspheres can be held in a pattern.
Applying the power
Now that optical tweezers can be installed as other standard laboratory equipment, we forecast a significant surge in the techniques and applications that this technology enables. As we look to the future, we see major interest in the following areas:
"Fingerprinting" with Raman spectroscopy. Combining tweezers with other photonics methods is an exciting way forward. Why not perform spectroscopic examination of a trapped particle to gather additional information? Adding Raman analysis to optical trapping would enable, for instance, identification of the onset of abnormalities in one cell among a given cell population (see Fig. 4).
FIGURE 4. Two Chinese hamster ovary cells (green), among nontransfected cells, the nuclei of which are visible because of co-staining with the blue nuclear dye DAPI. The ovary cells were transfected with green fluorescent protein using optical trapping and photoporation.
Probing with pulsed lasers. In another combination of tweezers with other photonics methods, one could trap a cell and use ultrashort laser pulses to create small holes in the membrane, which are able to heal quickly with no long-term damage. This method may be used for delivering therapeutic agents to cells that otherwise would not normally be taken up, and could enable testing of new drug compounds and treatments at the single-cell level.
Cell and particle sorting. Using different trap configurations and positioning it becomes possible to sort cells or particles in a microfluidic flow. Because this approach is based on the physical attributes of the cells and particles, it does not rely on biological markers (fluorophores) as do other method. Thus, it allows separation of populations without chemical interference, opening up an array of new possibilities.
Characterizing cell and molecule interaction. Measurement of interaction forces between cells and the dynamics of single-molecule processes is increasingly important and optical tweezing enables such exploration. For instance, with optical tweezers one can probe the physical properties of DNA by twisting and stretching the molecule, watch the process of transcription occurring in real time, and study other macromolecules such as the actin-myosin system or kinesin motion on microtubules. Similarly, optical tweezers may enable study of a wide range of biological macromolecules and other polymers.
Now accessible to a wider audience, optical tweezers have come of age. The trapping method expands the photonics toolbox, enabling exciting new applications and promising equally exciting discoveries.
KISHAN DHOLAKIA is head of the Optical Trapping Group, University of St. Andrews, Scotland, st-andrews.ac.uk/~atomtrap/group.html, and MIKE ELLIOT is managing director of Elliot Scientific, Harpenden, England; elliotscientific.com; e-mail: firstname.lastname@example.org;