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A cell generates green laser light in this microscope image. The pattern of the laser spot is due to the internal structure of the cell. (Malte Gather/Wellman Center for Photomedicine)

A living, human cell has been used as the key component in the first ever biological laser.

"What was most surprising to me is that the cells remain alive," said Malte Gather, one of the two researchers who created the laser, in an interview Monday.

He said the discovery could lead to the development of "self-healing" lasers that can repair themselves and biocompatible lasers that can be put inside the human body for use in photodynamic therapy — a treatment for diseases such as cancer that targets certain cells with a light sensitive drug, and then hits them with a light source such as a laser.

How the laser works

The researchers used a cell line based on human embryonic kidney cells because they are easy to keep alive and well. The cells were genetically modified to contain a green fluorescent protein that emits green light when it is charged up or "excited" by light of higher energy.  A single, modified cell is placed in a cavity filled with a nourishing liquid, between two mirrors.

A pulse of blue light is used to charge up the fluorescent protein molecules inside the cell. When one of them emits a photon of green light, the photon is reflected back and forth by the mirrors on either side of the cell. When the photon hits another charged up protein, it causes that molecule to emit another green photon that has all the same properties as the original – that is, it is travelling in the same direction with the same phase and can work cooperatively with the first. The light gets amplified as an increasing number of green photons bounce back and forth.

Once the amount of laser light  reaches a certain threshold, some can exit through one of the mirrors, which is transparent enough to let out half a per cent of the light.

The results were published online Sunday in Nature Photonics.

A laser amplifies waves of light in such a way that they all cooperate with each other and travel in the same direction, making  them far more powerful and precise than regular light sources.

A key component of a laser is an "optical gain material" — the material used to boost the light. Up until now, laser light has been amplified using inorganic materials such as semiconductors or carbon dioxide.

But a human embryonic kidney cell genetically modified to produce a green, fluorescent protein (GFP) — a dye that originally came from jellyfish — can also be used as an optical gain material, Gather discovered, along with Seok Hyun Yun, both physicists at the Wellman Center for Photomedicine at the Massachusetts General Hospital.

To make a laser, the cell is placed in a laser cavity — the space between two mirrors.

The cell is hit with a blue laser, which "charges" the fluorescent protein, getting it ready to emit green light. Light bounces back and forth between the two mirrors, getting amplified every time it passes between the mirrors. Once the light is amplified to a certain threshold, a small amount can pass out one of the mirrors as a laser pulse.

Gather said he created the laser because he was curious whether there was a fundamental reason why laser light doesn't occur in nature or if there was a way to create a laser system using biological substances or living things.

Initially, he was afraid the cell itself would scatter too much light. It turned out the spherical cell actually reduced scattering.

Cell focuses light

"The sphere acts as a lens and it kind of focuses the light," Gather said.

Usually dyes such as GFP stop fluorescing after they've been exposed to light for some time, making them unsuitable for lasers. But Gather said living cells have the advantage of being able to make more of the protein to replace the molecules that have been deactivated.

They are also biocompatible, which means they may be used in the future to build lasers that can be placed inside the human body for use in photodynamic therapy. That isn't feasible now because the mirrors are not biocompatible, Gather acknowledged. But he is currently working on making the mirrors smaller so they will fit inside the cell itself.

"The bigger challenge is probably to get FDA approval," he added.