Tiny, glowing probes packed with LEDs and sensors are scientists' newest tool for measuring and manipulating the brain and other living tissues. They're flexible, they can operate wirelessly, and yes, they're small enough to fit through the eye of a needle.

This kind of device could potentially improve researchers' ability to influence neural activity in live animals and measure a variety of physiological and biochemical processes, says applied physicist and neuroscientist Mark Schnitzer of Stanford University, who was not involved in the work. Such bio-compatible electronics also offer new possibilities for manipulating living tissue based on rapid feedback from sensors embedded in the tissue.

One obvious application in brain research is for optogenetics experiments, which involve genetically modifying neurons to make them fire in response to light. In recent years neuroscientists have used these methods to examine the neural circuits involved in everything from drug addiction, to depression, to Parkinson's disease. But getting light to areas deep inside the brain is tricky.

An LED probe lights up a mouse brain. Image: John Rogers, University of Illinois/Beckman Institute

Schnitzer and others have developed optical fibers to do this, but the new devices have several advantages, says materials scientist John Rogers of the University of Illinois at Urbana-Champaign, a co-leader of the team that developed them. "Their dimensions are much smaller than those of an optical fiber, and they are much more mechanically compliant," Rogers said. Those features help minimize tissue damage. "Also, they are powered and controlled wirelessly, in a way that allows free motion of the animals, social interactions and other natural behaviors."

Rogers and colleagues describe the probes today in Science, along with several demonstrations of their potential.

In one experiment, the researchers implanted a probe into the brain of a mouse. Then they used pulses of light to stimulate neurons in a part of the brain's reward pathway. Mice who received pulses in a particular arm of a Y-shaped maze soon learned to spend more time there, just as they would if they'd been rewarded with food.

Some researchers have speculated that optogenetics could eventually improve on deep brain stimulation, a therapy in which surgeons implant electrodes in a patient's brain to treat movement disorders, drug-resistant depression, and other conditions. The metal electrodes they use now can't target a specific type of neuron, but optogenetics can. Conceivably that could allow doctors to target just the neurons that are malfunctioning and cut down on side effects by leaving normally-functioning neurons alone.

The new probes are minimally invasive and biocompatible, but Rogers cautions that clinical applications of optogenetics are probably still quite a ways off. "The molecular biology and the need for gene therapy represent the major hurdles in that sense," he said.

In any case, tweaking neural activity is just a small part of what the probes can do. In addition to LED arrays, they also contain photodetectors, electrodes for stimulating and recording electrical activity, and temperature sensors that double as microheaters. "We view the technology as providing a generically useful way for introducing electronics and optoelectronics directly into the 3D depths of tissues," Rogers said.