A device the size of a peppercorn can activate neurons of the brain, spinal cord, or limbs in mice and is powered wirelessly using the mouse's own body to transfer energy. The miniature device combines optogenetics - using light to control the activity of the brain - with a newly developed technique for wirelessly powering implanted devices, and is the first fully internal method of delivering optogenetics. The device, developed at Stanford University, dramatically 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 researchers used the mouse's own body to transfer radio frequency energy that was just the right wavelength to resonate in a mouse. The energy is transmitted to an implantable device that delivers light to stimulate leg nerves. 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.