MIT optical tweezers grip cells over larger distances

09 Oct 2024

Design based on integrated silicon photonics expands utility of optical tweezers for biological research.

A project at MIT Research Laboratory of Electronics (RLE) has developed a new design of optical tweezers that could help the manipulation technology be utilized in new areas of research.

The principle behind optical tweezers exploits the intensity gradients that can be created using focused lasers, gradients capable of trapping small volumes of matter at the focus points.

Challenges to putting this principle to use in clinics have remained, however. Fragile biological samples can suffer thermal damage while held by the lasers, and the instrumental platforms themselves have required bulky microscope set-ups.

Advances in silicon photonics can now allow the integration of different optical components onto a single chip, something that could help reduce the complexity of optical tweezer devices. But to date, demonstrations of this integrated concept have been limited to small micron-scale stand-off distances.

As described in Nature Communications the new MIT breakthrough involves demonstrating optical trapping and tweezing using integrated optical phased arrays (OPAs), capable of emitting and controlling arbitrary free-space radiation patterns from compact silicon photonics chips.

Such arrays "provide a promising approach to extending the standoff operating distance by orders of magnitude, resolving the fundamental limitation on operating distance imposed by using evanescent-feld trapping modalities," commented the MIT project in its published paper.

Controlled deformation of cancer cells

OPA technology involves a series of microscale antennas fabricated on a chip using semiconductor manufacturing processes. By electronically controlling the optical signal emitted by each antenna, researchers can shape and control the beam of light being emitted by the chip.

To date most integrated OPAs have been designed for long-range applications like lidar, noted MIT, and not intended to generate the tightly focused beams needed for optical tweezing. But the project team discovered that more intensely focused emissions could in fact be engineered, by creating specific phase patterns for each antenna in the array.

The ability to steer the position of the emitted focal spot, a vital part of optical tweezing, was created by varying the input wavelength, which in turn changed the effective period of the radiating antennas and their angle of emission. A focused beam can be steered over a range larger than a millimeter and with microscale accuracy in this way, according to MIT.

"With silicon photonics, we can take this large, typically lab-scale system and integrate it onto a chip," said Jelena Notaros, a member of MIT RLE. "This presents a great solution for biologists, since it provides them with optical trapping and tweezing functionality without the overhead of a complicated bulk-optical setup."

In trials, the new OPA approach was used to induce controlled deformation of mouse lymphoblast cells, the first time this had been demonstrated using single-beam integrated optical tweezers. MIT's next steps include refining the system to enable an adjustable focal height for the beam of light, along with applying it to different biological systems and using multiple trap sites at the same time.

"This work opens up new possibilities for chip-based optical tweezers," commented Notaros. "It's exciting to think about the different applications that could be enabled by this technology."