Blue LED implant wirelessly controls a mouse – the sort with whiskers
The mouse’s own body transmits energy to the implantable device which delivers light to stimulate the mouse’s leg nerves and activate neurons of the brain, spinal cord or limbs.
A miniature device that combines optogenetics – using light to control the activity of the brain – with a technique for wirelessly powering implanted devices is the first fully internal method of delivering optogenetics.
The device expands the scope of research that can be carried out through optogenetics to include experiments involving mice in enclosed spaces or interacting freely with other animals. The work is published in the publication Nature Methods.
"This is a new way of delivering wireless power for optogenetics," said Ada Poon, an assistant professor of electrical engineering at Stanford. "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," said Poon.
Traditionally, optogenetics has required a fiber optic cable attached to a mouse’s head to deliver light and control nerves. With the restrictive headgear, mice can
move in an open cage but cannot 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.
The 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.
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 said that developing the tiny device to deliver light was the easy part. She and her colleagues developed that and had it working a few months after the workshop.
What was hard was figuring out how to power it over a large area without compromising power efficiency.
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.
Poon had the inspiration 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 did not know how to build a chamber to amplify and store radio frequency energy. Poon 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 key is that there is 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.
The 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.
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