First 'living' laser made from kidney cell

It's not quite Cyclops, the sci-fi superhero from the X-Men franchise whose eyes produce destructive blasts of light, but for the first time a laser has been created using a biological cell.

The human kidney cell that was used to make the laser survived the experience. In future such "living lasers" might be created inside live animals, which could potentially allow internal tissues to be imaged in unprecedented detail.

 A human cell, alive and lasing (Image: Malte Gather)

It's not the first unconventional laser. Other attempts include lasers made of Jell-O and powered by nuclear reactors (see box below). But how do you go about giving a living cell this bizarre ability?

Typically, a laser consists of two mirrors on either side of a gain medium – a material whose structural properties allow it to amplify light. A source of energy such as a flash tube or electrical discharge excites the atoms in the gain medium, releasing photons. Normally, these would shoot out in random directions, as in the broad beam of a flashlight, but a laser uses mirrors on either end of the gain medium to create a directed beam.

As photons bounce back and forth between the mirrors, repeatedly passing through the gain medium, they stimulate other atoms to release photons of exactly the same wavelength, phase and direction. Eventually, a concentrated single-frequency beam of light erupts through one of the mirrors as laser light.

Alive and well

Hundreds of different gain media have been used, including various dyes and gases. But no one has used living tissue. Mostly out of curiosity, Malte Gather and Seok-Hyun Yun of Harvard University decided to investigate with a single mammalian cell.

They injected a human kidney cell with a loop of DNA that codes for an enhanced form of green fluorescent protein. Originally isolated from jellyfish, GFP glows green when exposed to blue light and has been invaluable as a biological beacon, tracking the path of molecules inside cells and lighting up when certain genes are expressed.

After placing the cell between two mirrors, the researchers bombarded it with pulses of blue light until it began to glow. As the green light bounced between the mirrors, certain wavelengths were preferentially amplified until they burst through the semi-transparent mirrors as laser light. Even after a few minutes of lasing, the cell was still alive and well.

Christopher Fang-Yen of the University of Pennsylvania, who has worked on single-atom lasers but was not involved in the recent study, says he finds the new research fascinating. "GFP is similar to dyes used to make commercial dye lasers, so it's not surprising that if you put it in a little bag like a cell and pump it optically you should be able to get a laser," he says. "But the fact that they show it really works is very cool."

Internal imaging?

Yun's main aim was simply to test whether a biological laser was even possible, but he has also been mulling over a few possible applications. "We would like to have a laser inside the body of the animal, to generate laser light directly within the animal's tissue," he says.

In a technique called laser optical tomography, laser beams are fired from outside the body at living tissues. The way the light is transmitted and scattered can reveal the tissues' size, volume and depth, and produce an image. Being able to image from within the body might give much more detailed images. Another technique, called fluorescence microscopy, relies on the glow from living cells doped with GFP to produce images. Yun's biological laser could improve its resolution with brighter laser light.

To turn cells inside a living animal into lasers, they would have to be engineered to express GFP so that they were able to glow. The mirrors in Yun's laser would have to be replaced with nanoscale-sized bits of metal that act as antennas to collect the light.

"Previously the laser was considered an engineering material, and now we are showing the concept of the laser can be integrated into biological systems," says Yun.

You might also like to check out this gallery charting the evolution of the laser.

Journal reference: Nature Photonics, DOI: 10.1038/nphoton.2011.99

Source New Scientist