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.

A green laser being tested at the Naval Research Lab ionizes the water when it strikes, generating acoustic waves. Credit: Navy Research Lab

Researchers at the United States Naval Research Laboratory are developing laser acoustic technology that works underwater. It could make for better underseas acoustic imaging, and be used for remote communications between aircraft and underwater hardware and vessels because the signals can travel from the air into the water without degrading.

According to a press release from the Navy, the laser light--generated using a commercial laser--ionizes a small area of water, which superheats, creating an explosion of steam that generates pulses of sound waves at about 220 decibels.

Laser light was focused on the region of this mouse cell indicated by the red dot, activating a hybrid version of a protein called Rac and causing the cell to change shape and move. Credit: Yi Wu, UNC-Chapel Hill

Researchers at the University of North Carolina in Chapel Hill and the Max Planck Institute in Heidelberg, Germany have genetically engineered animal cells to make proteins that can be turned on and off using visible light.

The researchers spliced a gene for a light-activated protein with the gene for a protein called Rac, which is known to be involved in regulating healthy cell movements as well as the movement of cancer cells. Researchers then focused laser light to locally activate the proteins, causing protrusions that led to cell movement. The work was described this week in the journal Nature.

The location of a protein within the cell plays a role in determining the cell's behaviors, but this has been difficult to study. The researchers hope using light activation will be a good method.

Here's how it works: the light-activated portion of the protein blocks the binding site on Rac. When it's illuminated, the block is removed and Rac can function. A second pulse of light at a different wavelength causes the block to move back into position, deactivating the protein.

The technique should be compatible with other proteins in addition to Rac. In the past, researchers have made proteins that are activated by ultraviolet radiation, which is toxic to cells. And these previous proteins couldn't be turned off again; the new ones can.