Vagus Nerve Stimulators Market, By Product Type (Implantable Devices and External Devices), By Application (Epilepsy, Depression & Anxiety, Migraine, and Others), By Material Type (Ceramics, Metallics, and Polymerics) , By End User (Hospitals, Specialty Clinics, Ambulatory Surgical Centers, and Others), By Geography (North America, Latin America, Europe, Asia Pacific, Middle East & Africa)