Metamaterial improves resolution in light microscopes

Electrical engineers at the University of California San Diego have developed a technology that improves the resolution of an ordinary light microscope so that it can be used to directly observe finer structures and details in living cells. Their work, which was published in Nature Communications, overcomes a big limitation of conventional light microscopes: low resolution.

Light microscopes are useful for imaging live cells, but they cannot be used to see anything smaller. Conventional light microscopes have a resolution limit of 200 nm, meaning that any objects closer than this distance will not be observed as separate objects. And while there are more powerful tools out there such as electron microscopes, which have the resolution to see subcellular structures, they cannot be used to image living cells because the samples need to be placed inside a vacuum chamber.

“The major challenge is finding one technology that has very high resolution and is also safe for live cells,” said UC San Diego Professor Zhaowei Liu. He and his team developed technology that combines both features, allowing a conventional light microscope to image live subcellular structures with a resolution of up to 40 nm — effectively turning it into a super-resolution microscope.

The technology consists of a microscope slide that’s coated with a type of light-shrinking material called a hyperbolic metamaterial. It is made up of nanometres-thin alternating layers of silver and silica glass. As light passes through, its wavelengths shorten and scatter to generate a series of random high-resolution speckled patterns. When a sample is mounted on the slide, it gets illuminated in different ways by this series of speckled light patterns. This creates a series of low-resolution images, which are all captured and then pieced together by a reconstruction algorithm to produce a high-resolution image.

“This material converts low-resolution light to high-resolution light,” Prof Liu said.

“It’s very simple and easy to use. Just place a sample on the material, then put the whole thing under a normal microscope — no fancy modification needed.”

The researchers tested their technology with a commercial inverted microscope. They were able to image fine features, such as actin filaments, in fluorescently labelled Cos-7 cells — features that are not clearly discernible using just the microscope itself. The technology also enabled the researchers to clearly distinguish tiny fluorescent beads and quantum dots that were spaced 40 to 80 nm apart.

The super-resolution technology has great potential for high-speed operation, the researchers said. Their goal is to incorporate high speed, super resolution and low phototoxicity in one system for live cell imaging.

The team are now expanding the technology to do high-resolution imaging in a three-dimensional space. The latest paper shows that the technology can produce high-resolution images in a two-dimensional plane, but the team previously published a paper showing that the technology is also capable of imaging with ultrahigh axial resolution (about 2 nm). They are now working on combining the two together.

Image caption: Artistic rendering of the new super-resolution microscopy technology. Animal cells (red) are mounted on a slide coated with the multilayer hyperbolic metamaterial. Nanoscale structured light (blue) is generated by the metamaterial and then illuminates the animal cells. Image credit: Yeon Ui Lee.

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