Tiny wireless device inside mice sends light to nerves

Traditionally, optogenetics has required a fiber optic cable attached to a mouse's head to deliver light and control nerves. These restrictions limit what can be learned through optogenetics, which is being investigated as a treatment for chronic pain and to relieve tremors in Parkinson's disease. (Credit: iStockphoto)

Scientists have implanted a wireless device the size of a peppercorn inside mice to stimulate nerves.

The technique combines optogenetics—using light to control the activity of the brain—with a newly developed method for wirelessly powering implanted devices. It’s described in a paper published in Nature Methods.

“This is a new way of delivering wireless power for optogenetics,” says Ada Poon, an assistant professor of electrical engineering at Stanford University. “It’s much smaller and the mouse can move around during an experiment.”

The device can be assembled and reconfigured for different uses in a lab, and the design of the power source is publicly available. “I think other labs will be able to adapt this for their work,” Poon says.

Less stress for mice

Traditionally, optogenetics has required a fiber optic cable attached to a mouse’s head to deliver light and control nerves. With this somewhat restrictive headgear, mice can move in an open cage but can’t navigate an enclosed space or burrow into a pile of sleeping cage-mates the way an unencumbered mouse could. Also, before an experiment a scientist has to handle the mouse to attach the cable, stressing the mouse and possibly altering the outcome of the experiment.

These restrictions limit what can be learned through optogenetics.

People have successfully investigated a range of scientific questions, including how to relieve tremors in Parkinson’s disease, the function of neurons that convey pain, and possible treatments for stroke. However, addressing issues with a social component like depression or anxiety or that involve mazes and other types of complex movement is more challenging when the mouse is tethered.

How optogenetics works

Before a new wave of tinfoil hat designs takes over the internet, it is important to clarify one point: Optogenetics only works on nerves that have been carefully prepared to contain the proteins that respond to light.

In the lab, scientists either breed mice to contain those proteins in select groups of nerves or they carefully and painstakingly inject viruses carrying the protein DNA into nerves the size of dental floss. Shining a light—whether through a fiber optic cable or a wireless device—on neurons that haven’t been prepared has no effect.

Poon says developing the tiny device to deliver light was the easy part. She and her colleagues developed it and had it working in a few months. Figuring out how to power it over a large area without compromising power efficiency was the hard part.

How to power the implant

In behavioral experiments, the mouse would be moving all around, and the researchers needed a way of tracking that movement to provide localized power. Poon knew other labs were tackling the same problem using bulky devices that affix to the skull and complex arrays of coils paired with sensors to locate the mouse and deliver localized power.

“We were lazy,” Poon says. “That sounded like a lot of work.”

So instead she got what she called a crazy idea to use the mouse’s own body to transfer radio frequency energy that was just the right wavelength to resonate in a mouse. Crazy maybe, but it worked, and she published the results in Physical Review Applied with co-first authors John Ho, a graduate student who is now an assistant professor at the National University of Singapore, and Yuji Tanabe, a research associate in her lab.

Poon had the idea but initially didn’t know how to build a chamber to amplify and store radio frequency energy. She and Tanabe consulted with Tanabe’s father, who had worked at Stanford’s SLAC research center and knew a thing or two about machining such a cavity, and then traveled to Japan to do the initial assembly and testing.

Tanabe’s dad referred to their final chamber as a “kindergarten project,” but it worked. However, in its native state the open chamber would radiate energy in all directions. Instead, a grid was overlaid on top of the chamber with holes that were smaller than the wavelength of the energy contained within. That essentially trapped the energy inside the chamber.

The mouse’s body transfers energy

The key is that there’s a bit of wiggle room at the grid. So if something like, say, a mouse paw were present, it would come in contact with the boundary of all that stored energy. And remember how the wavelength is the exact wavelength that resonates in mice? The mouse essentially becomes a conduit, releasing the energy from the chamber into its body, where it is captured by a 2 mm coil in the device.

Wherever the mouse moves, its body comes in contact with the energy, drawing it in and powering the device. Elsewhere, the energy stays tidily contained. In this way, the mouse becomes its own localizing device for power delivery.

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This novel way of delivering power is what allowed the team to create such a small device. And in this case, size is critical.

The device is the first attempt at wireless optogenetics that is small enough to be implanted under the skin and may even be able to trigger a signal in muscles or some organs, which were previously not accessible to optogenetics.

The team says the device and the novel powering mechanism open the door to a range of new experiments to better understand and treat mental health disorders, movement disorders, and diseases of the internal organs. They have a Stanford Bio-X grant to explore and possibly develop new treatments for chronic pain.

Source: Stanford University