Usually they are implants, a control unit with battery embedded under the skin in your shoulder and then a wire up to the vagus nerve in the neck. They are used

Usually they are implants, a control unit with battery embedded under the skin in your shoulder and then a wire up to the vagus nerve in the neck. They are used to control epilepsy among other things. I think there may be external versions too but they are newer and I'm not sure how effective they are.