18 September 2013

Injectable, wireless LEDs help researchers understand brain activity

Miniaturization of optoelectronic components has driven many advances in computers and communications, however it is in bridging those fields to the mature technologies of life itself that the greatest reward will be gained. A paper published in Science, titled “Injectable, Cellular-Scale Optoelectronics with Applications for Wireless Optogenetics” reveals just how far we have come.

One of the corresponding authors on the paper, Michael Bruchas, was looking for better ways to study how various transmitter-receptor systems come into play in disease states. He started a collaboration with John Rogers, who had been working on some novel ways to probe neural tissue. Rogers has been quietly pioneering the new field of dissolvable electronics. Recently he has also developed flexible electronic tattoos to apply sensors and communications directly to the skin. The real market however, is the brain. Working with neurosurgeons Rogers has developed unique, silk-based electrodes which can meld with the folded cortical surface of the brain and read electrical activity.

To delve beneath that cortical surface, at least without digging through it, light is the medium of choice. Videos released just last week show what is possible when you can create completely transparent mammalian brains. Rather than looking at inert neurons, optogenetics also gives the ability to control them, and watch their resulting activity. Some fantastic tools have previously been developed, which combine optodes and electrodes on the same piece of hardware. The new device developed by Rogers and Bruchas, by virtue of its size and complete integration of diverse silicon components, make them pale in comparison.

In delivering light to the brain, as in electronic circuits, the same question emerges time and again — do you pipe in perfectly structured and powerfully-sourced light through bulky fibers, or do you locally generate it from a common electrical currency? Because we now have the ability to put arrays of semiconductor photodiodes, temperature probes, heating elements, microelectrodes, and LED lights of not-inconsiderable power, on a device that can fit through the eye of a needle, the choice is a foregone conclusion.

The title of the paper indicates that the new probe technology is both wireless and injectable. That is true, however some clarifications are in order. The device is not injected into the bloodstream, and does not home to the brain in science fiction fashion. Rather the probe is mounted to a rigid injector with degradable components that can later release the probe. It remains tethered to the wireless transmitter which is not integral to the probe itself (that will be the next great feat of miniaturization). Having all that sensing and control technology on a single probe tip gives a lot of diagnostic power.

When you have closed-loop control where heat, light, and electrical actuators operate astride the appropriate associated sensors, new measurables pop out automatically. Just as when you combine Google with GPS on a cell phone you might find lunch or gas, here you can suddenly measure things like PH, blood oxygenation, glucose levels, and other chemical changes associated with cellular activity. The four GaN microLEDs sit right between the temperature sensor and the light detector on the probe. In principle, a temperature-calibrated light source detector combo should be able to do all kinds of local spectrometry. Detection of action potentials from neurons could be done in diverse and redundant ways.

Several decades ago, researchers at the NIH were able to measure action potentials by the mechanical disturbance that is associated with their propagation. They also could detect the actual heat evolved during the spike. In some cases, they even found that the exchanged heat flowed in a direction opposite to that which would normally be expected. Today we understand that action potentials are more involved than just the flow of ions which we measure electronically. Combining several measurements together will give us a much clearer picture of what is going on inside a crowded neural environment.

The researchers were also able to functionally evaluate their probes inside the brains of mice. The mice were given tasks to perform, such as choosing which path of a Y-maze to take. By recording and stimulating activity in certain parts of the brain, they could influence what the mice were doing, and perhaps, by implication, thinking. While it is not always critical to vet every advance with animal trials, it is an important proof-of-principle to demonstrate the practicality of the this new and exciting tool.

Read more: Piezo-phototronic nanowire LEDs can scan your fingerprint, and then display it back to you.

Source paper: Injectable, Cellular-Scale Optoelectronics with Applications for Wireless Optogenetics, DOI: 10.1126/science.1232437


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