Optical Tweezers

What are optical tweezers?

Optical tweezers are able to hold and move microscopic particles just employing laser light. Therefore, particles can be manipulated in a precise and contact-free way, only using the momentum generated by laser light. This makes laser light tweezers a versatile and powerful tool for various research areas.

How were optical tweezers first discovered and how do they work?

Arthur Ashkin has been awarded with the Nobel Prize for physics in 2018 for “optical tweezers and their application to biological systems”. The optical tweezers principle is based on the discovery that light has momentum. Once a laser beam is encountering an object such as a glass sphere or a cell, the light will be refracted. In the center of the beam, the light will be brighter and more light is refracted from here than from the outside of the beam. As momentum needs to be constant, the object will be trapped in the center of the beam where the forces generated by refracted light is cancelling out. For optical trapping, laser based optical tweezers are applied which have the optimal properties to generate the optical trap.

In this video, the optical trapping theory as well as optical tweezers are explained and the video also visualizes how laser traps are generated and how laser tweezers work:

What are optical tweezers being used for?

Optical tweezers are applied in a plethora of different research areas such as in cell biology, biomedicine, drug discovery, biochemistry, biophysics as well as in physics or material sciences. Therefore, the optical tweezers uses are highly diverse. This list just mentions a small selection:

– Cell-based studies: Cell interactions and intracellular manipulations
– Measurement of Binding Forces: Bacterial and viral adhesion studies
– Molecular Motor Studies: Actin & myosin, kinesin & dynein motors
– DNA or RNA binding: Interaction forces, helicase and translocase activities
– Laser Raman Tweezers: Isolation of cells and microorganisms
– Lab-on-a-Chip Device: Biosensor assays

Can optical tweezers be used for single cells?

Yes, optical tweezers are ideally suited to study single cells. Using optical traps, the target cells can be manipulated in a precise and contact-free way thereby avoiding any risks of contaminations. In addition, using infra-red lasers, the cells viability is not compromised during the experiment. Thus, the laser tweezer technology is also suitable for the work with living cells, such as for drug discovery projects.Some optical tweezer systems, such as the MMI CellManipulator, are equipped with a second tweezer level to independently hold two cells or particles at the very same time and to precisely measure their interaction forces by force spectroscopy.

Are there any limitations of optical tweezers that I need to be aware of?

Optical tweezers are able to optically trap particles that are transparent for infra-red light and have an optical refraction index n of the object higher than surrounding medium nO > nM. In addition, the particles needs to be between 200 nm – 20 µm in size and should have a relatively smooth surface.

What optical tweezer solutions does MMI offer?

With the MMI CellManipulator, MMI offers a highly versatile multi-beam laser tweezer system to manipulate cells or particles as well as to do force spectroscopy.

The MMI CellManipulator optical tweezers system is based on the mechanical forces arising from a strongly focused laser beam. It enables comfortable, ultra-precise and contact-free manipulation of microscopic particles, single or living cells, or subcellular organisms and the measurement of intracellular activities. Thus, it can hold, move, rotate, join, separate, stretch or otherwise manipulate up to 2 x10 microscopic objects simultaneously or separately in three dimensions by laser trapping. The wavelength of the laser does not interfere with the integrity of living specimens.

Cell sorting and cell positioning can also be accomplished together with the quadrant detector enabling the measurement of binding forces or viscosities at sub-cellular level. Due to multiple ports and dual-level laser integration, the seamless use of different modules and imaging technologies is possible. Automated quadrant detector calibrations routines allow force-distance measurements, so called Force Spectroscopy. An additional feedback module is available for isometric force detections and force clamping.The MMI CellManipulator is highly modular and can be mounted on numerous microscope brands from entry level, mid-range to high-end instruments. In addition, the MMI CellManipulator can be combined with the MMI CellCut laser microdissection system or the MMI CellEctor for microcapillary-based single cell picking. These unique combinations make the MMI CellManipulator a highly versatile optical tweezer system for a plethora of different applications in many research areas from biology and medicine to physics and material sciences.