Piezoelectric Materials create electrical charge when mechanically stressed. Natural materials with this property are quartz, human skin, and human bone. In medical field this material is used as medical sensors and low power medical devices. The material is specifically formulated with high coupling values and high dielectric properties and is well suited for thin and small parts. Applications include sensors for catheters, blood pressure, auditory sounds, vascular flow, and medical imaging. Depending on the design of a sensor, different 'modes' to load the piezoelectric element can be used: longitudinal, transversal and shear. Detection of pressure variations in the form of sound is the most common sensor application. Piezoelectric microphones (sound waves bend the piezoelectric material, creating a changing voltage) and piezoelectric pickups for electrically amplified guitars. A piezo sensor attached to the body of an instrument is known as a contact microphone.Piezoelectric sensors especially are used with high frequency sound in ultrasonic transducers for medical imaging and also for industrial nondestructive testing.