Volume 3, Issue 4 (April 2006)
Parameters in Ultrasonic Cleaning for Implants and Other Critical Devices
Ultrasonic cleaning is widely accepted as an invaluable tool for minimizing contamination of critical devices, particularly where complex geometries such as blind holes are present. In order to use this powerful technique effectively, it is necessary to identify multiple parameters and to optimize conditions. Optimizing ultrasonic effectiveness involves removal of undesirable contaminants. Because ultrasonic cleaning generates significant force, effectiveness also involves minimizing the potential for substrate damage. Cavitation quality, and cavitation effects, are dependent on a number of factors including the frequency, amplitude, chemistry (aqueous or solvent), time, and pressure. The negative impact of ultrasonic erosion is often assumed to be minimal, particularly where more sophisticated ultrasonic systems are employed. Where the surface itself is more complex and contains what might be termed microstructure (or nanostructure), the potential for damage during ultrasonic cleaning must not be ignored. Several examples are presented to illustrate the desirability for independent, documented studies of the impact of ultrasonic cleaning or of ultrasonic extraction on critical products. As one simple model, studies were conducted on various thicknesses of aluminum foil. Efficacy of cavitation and potential for surface modification effects was indicated by foil erosion. In a second example, loss of metal substrate from critical components (or subassemblies) used in inertial navigation systems was observed after one hour of ultrasonic extraction. Both examples involve softer metals than are typically used in biomedical devices. However, depending on the size and configuration of the device, the issue of erosion and/or stress must be considered. The potential for ultrasonic damage in miniature devices and indications of such damage is discussed. Many newer cleaning chemistries, particularly biobased chemistries, are ineffective at ambient temperature. Discussion indicating the impact of elevated temperature are included. In some cases, it appears that ultrasonic cleaning may be superfluous in certain biobased chemistries at elevated temperature. It is typically assumed that ultrasonic cleaning will be conducted at atmospheric pressure. However, many newer cleaning systems operate at reduced pressure that reduce the boiling point. Therefore, studies of cavitation effects at reduced pressure or at high temperature are discussed. Typically, extensive studies of appropriate conditions for ultrasonics are not conducted. Instead, conditions are selected based on previous pragmatic observations or on vendor recommendations. In recent years, the number of available options in ultrasonic equipment and chemistries has increased markedly. At the same time, there has been a trend toward miniaturization and increased complexity of configuration in biomedical devices; this trend means increased surface, increased potential for contamination, and increased potential for unintended surface modification. With increasing use of ultrasonics for such critical applications as medical implants, the need for additional independent studies of ultrasonic efficacy is proposed.