Living laser: This live cell, which makes a large amount of green fluorescent protein, is the core of a new laser design. Credit: Malte Gather.

A laser based on living cells has been created by researchers at Harvard Medical School and the Massachusetts General Hospital in Boston. They were motivated to overcome one of the fundamental limitations on biological imaging: it’s very difficult to get visible and infrared light in and out of the body.

Living lasers have a few basic parts that are drawn from the same list as any laser. First, the researchers genetically modified human liver cells so that they produce large amounts of green fluorescent proteins that are scattered throughout the cell. A cell carrying these proteins acts as the “gain medium”—the part of the laser that amplifies light energy. ‘

Like any laser, the cell laser needs an energy source to “pump” it and increase the power of the light it can emit. The researchers pumped the living lasers by pulsing the cells with light through a microscope. As light bounces around inside the cell and is re-emitted by the fluorescent proteins, it’s amplified, increasing in power before being emitted in a coherent beam. To keep the light bouncing around as long as possible, to gain as much power as possible, the Boston group placed these cells inside a biocompatible optical cavity—essentially a tiny, highly reflective, cell-shaped hole.

In a paper in Nature Photonics, the Boston researchers suggest that living lasers would help get light-encoded information into and out of the body. These living lasers are fundamentally different from cells that simply make fluorescent proteins: by definition, a laser emits a strong, coherent beam of light. Laser light is great for carrying information over distances, whether that’s from country-to-country in the optical fibers that make up the backbone of the internet.

Optical imaging labels can report on the molecular workings of tissues and cells in the body. Fluorescent protein tags that emit visible or infrared light are now common tools for studying cell biology in test tubes. But getting such light in and out of the body is difficult because light diffuses as it passes through biological tissues. Living lasers, if they’re made into practical systems, have the potential to change that. One can imagine having a hybrid living-nonliving medical implant under the skin that would beam out a stream of information about biomarkers in the blood, for example.

The main challenge with any new kind of laser is figuring out how to pump it in a practical way. Using a microscope to pump the living lasers is a good way to prove that they work but it’s not that practical for applications. Lasers can either be pumped with electricity or light, but how would that be accomplished inside the body?

Perhaps this work can dovetail with other projects directed at developing implantable electronics. Other groups have already developed implantable light sources and electrical diodes that might pump a living laser. A group at the University of Illinois and Tufts University, for example, have made biocompatible and high quality LEDs, transistors, electrodes, and other electronics, and have shown they work when implanted in living animals.