Medicine: Ultrasonic applications

The inverse piezo effect, i.e. the conversion of electrical energy into mechanical vibrations, is used for various applications in the life sciences. In addition to the familiar systems of medical diagnostics, dental scale removal, scalpels in eye surgery and also the production of aerosols, systems based on ultrasound are gaining in importance in the detection of air bubbles.
 

Aerosol production

Ultrasound makes it possible to atomize liquids without increasing the pressure or the temperature, a fact which is of crucial importance particularly for sensitive substances such as drugs.
Two processes are used at present. As it is the case with high-frequency ultrasonic cleaning (MHz range), a piezoceramic disk is fixed to the bottom of a liquid container and is vibrating at resonance. It generates high-intensity ultrasonic waves in the medium to be nebulized. The capillary waves which form on the surface of the liquid make it possible for minute droplets to “break off”.
With direct nebulization, the piezo element vibrating at high frequency (1 to 3 MHz) is in direct contact with the liquid. Special surface finishes are used for aggressive substances. The ultrasonic waves propagating in the medium reach their maximum intensity at a specific height of the liquid, at which the liquid droplets break off. In both processes, the diameter of the aerosol droplets is determined by the frequency of the ultrasonic waves. The higher the frequency, the smaller the droplets.
 

Dental scale removal

The previously, often painful removal of mineral coatings from teeth is nowadays carried out throughly using ultrasonic tools. Piezoceramic composite systems comprising ring disks clamped together are used. As is the case with ultrasonic machining systems, vibration amplitudes in the µm range at working frequencies of around 40 kHz are transmitted by means of a sonotrode in the form of a dental tool.

Ultrasound therapy

This term is used for a therapeutic method in which ultrasonic waves are used to irradiate the tissue in a type of micromassage. During the application of ultrasound, the deflection of the vibrating particles propagates in a longitudinal wave in liquids and soft tissue. Mechanical longitudinal waves generate mechanical vibrations in the tissue, while at the same time part of the ultrasonic energy is converted into heat. The core of this system of therapy is an ultrasonic transmitter comprising a piezo ceramic disk connected to a metallic sound transmission diaphragm.