Ultrasonic cleaning offers several advantages
by Edward Mant, Ultrawave Ltd.
Cleanrooms are, by very definition, environments that must remain clean at all times and the level of cleanliness required depends on the process within. For example, a Class 100 (ISO Class 5) cleanroom used in hard-disk-drive manufacture can contain no more than 100 particles larger than 0.5 micron in a given cubic foot. The most common methods of cleaning items en route to a cleanroom are those of washer disinfectors or, surprisingly, a simple hand-wash procedure. There is, however, an alternative method that has the ability to clean contaminants to the micron level and has been proven to be more effective than hand-washing alone. Here we explore the process of ultrasonic cleaning and its application in the cleanroom environment.
Ultrasonic cleaning has been around for over forty years but its use within a cleanroom environment is still relatively sparse, which seems somewhat strange when you consider the cleaning efficacy it achieves. This may be because of lack of knowledge of ultrasonics or maybe it is simply overlooked as a viable cleaning option.
The process of ultrasonic cleaning involves the use of high frequency sound waves, which are not audible to human ears, in order to create a phenomenon called cavitation within a fluid. The sound waves are produced through the use of piezoceramic transducers. These resonant structures are made up of a number of parts: a front mass (the area that is bonded to the tank), two piezoceramic rings (PZT composite), beryllium-copper electrodes, a back mass and a tensional bolt. As the transducers are subjected to an alternating voltage, the piezoelectric effect creates a displacement in the crystals (i.e., they become agitated and vibrate), creating high-frequency sound waves proportional to the driving signal. This process changes the electrical energy into mechanical energy, which is then transferred into the liquid cleaning medium.
Figure 1. Fully validated benchtop ultrasonic cleaner. Photo courtesy of Ultrawave, Ltd.
An ultrasonic tank will have a number of these transducers bonded onto the outside of the base of the tank. The mechanical energy that is created is then transferred onto the base of the tank, causing it to vibrate. These vibrations are then radiated through the solution contained within the tank, causing millions of microscopic bubbles to form within the solution. As the sound waves pass through the solution, they cause these bubbles to expand, creating a vacuum within them. They continue to grow until a size is reached at which they cannot support their own density, causing them to implode: cavitation. During this implosion, extreme temperatures and forces are achieved. The implosion process for each bubble will only last nanoseconds, but at any one time there will be millions of these microscopic implosions occurring within even the smallest ultrasonic tank (see Fig. 1). As a bubble implodes, the surrounding fluid rushes in to fill the gap left by the bubble, which creates a cleaning action similar in effect to having millions of microscopic scrubbing brushes cleaning the surface of the item that is immersed. Although the forces achieved appear as though they would be highly damaging to surfaces they come into contact with, this is not the case. Because the implosions are microscopic in scale, the action is very gentle and easily lifts contamination off the surface area of the item immersed in the liquid.
Although ultrasonic cleaning is highly effective on its own, the process can be amplified by using a suitable detergent in the water. There are a whole host of application-specific detergents available for use in ultrasonics that have been created to maximize the efficiency of the process. These detergents act in much the same way as household detergents. It’s often possible to clean items with just water and a good scrub, but adding a detergent to aid the loosening process will always achieve faster and better results. It also lowers tensile values in the water, making it easier to cavitate it at lower pressure amplitudes. This also applies to increasing the temperature of the water used, as it will help soften the debris on the item. It should be noted, however, that this is only beneficial up to a temperature of around 60-70°C. Above this temperature, the level of cavitation reduces. These same principles apply to ultrasonic cleaning; however, it is important to note that for surgical instrument decontamination in the U.K., the solution should not exceed 35°C (just below the temperature of the human body: 36.8°C) as this will potentially lead to proteins becoming baked onto the item and not removed.
The efficiency achieved by ultrasonic cleaners means only a relatively short cleaning cycle is required in order to remove even the most tenacious of substances. In most cases, a cycle time of less than ten minutes is more than sufficient to clean the item, although variants such as temperature and the detergent used will affect the cleaning process. As was previously stated, using a high-frequency ultrasonic cleaner will also speed up the cleaning time, which is significantly less than other methods currently used, meaning a higher throughput of items can be achieved within the same timescale.
The ultrasonic cleaning process is so flexible in its application that it can be incorporated into almost any manufacturing, healthcare or pharmaceutical environment. Its uses within each sector can also be diverse and widespread.
There are two main ways in which ultrasonic cleaning can be incorporated into the cleanroom process, depending on the type of cleanroom. The first is to have the ultrasonic cleaner as part of the cleaning process prior to items entering the cleanroom environment. All items within a cleanroom need to be free from contamination in order not to compromise the cleanroom operation. By using ultrasonic cleaning products, items entering the said environment will be free from unacceptable debris, thus minimizing the risk of contaminants entering the cleanroom.
Second, an ultrasonic cleaner can be present inside the cleanroom itself. Although the products within a cleanroom should, by definition, already be contamination-free, in certain cases there may be a requirement for items to be cleaned within their operational environment. This could include, for example, scientific instrumentation used within the cleanroom. Although items such as these are often removed for cleaning, it would be far more logical to clean them within the cleanroom. This would eliminate the possibility of re-exposure to contaminants after the cleaning process and prior to re-entry into the operating environment. By installing an ultrasonic cleaner in the cleanroom, the items can be cleaned under cleanroom conditions, thus remaining free from potentially hazardous particles.
Because the U.K. has suffered from a vCJD (Creutzfeldt-Jakob disease) scare, the government has introduced a set of guidelines called HTM2030 to cover every aspect of instrument reprocessing, from procedures to equipment specifications and validation. This has lead to the creation of central sterilization supply departments, or CSSDs, in many key hospitals and it is here that ultrasonic cleaners are increasing in numbers. This is because, in order to ensure that surgical instruments are completely decontaminated, the use of an ultrasonic cleaner prior to normal disinfection and sterilization procedures will produce better debris removal results than other methods available such as washer disinfectors. Throughout the world, more and more hospitals are having CSSDs installed in order to reprocess their surgical instruments. These CSSDs have a high throughput of instruments and need to ensure that each time a batch enters into the cleaning process it is cleaned to the required level-the first time, every time. It is because of this that ultrasonic cleaning is increasingly being incorporated into the cleaning operations of such environments. Hospitals should process instruments through an ultrasonic cleaner, followed by a double-entrance washer disinfector, before the instruments enter the cleanroom. Although many hospitals are still only using washer disinfectors, the disadvantage these units have over ultrasonics is their cleaning process. The spray system is similar to that of a dishwasher, but a simple spray-clean cannot always penetrate hard-to-reach areas of the instruments. Because of how ultrasonic cleaners work, the process of cavitation means that the instruments will undergo a more rigorous cleaning than when using a spray-cleaning system.
This is not to say that ultrasonic cleaners should replace washer disinfectors, as they do not carry out a thermal disinfection process. However, a surgical instrument that has undergone an ultrasonic cleaning cycle will be free from proteins, meaning the washer disinfector cycle can be shortened to just carry out the disinfection stage without the time-consuming spray-clean cycle. It is also worth noting that pockets of surface debris can act as a heat shield for pathogens, but an ultrasonic cleaner would remove these pockets so that all areas are disinfected. The cavitation bubbles can penetrate the difficult-to-clean areas on instruments, such as hinged mechanisms, screw threads and serrated edges. Debris often becomes lodged in these places during surgical procedures and can be difficult to remove. It has been scientifically proven that reprocessing surgical instruments above a temperature of 35°C can further the problem by baking on proteins. All washer disinfectors operate at a temperature above this as thermal disinfection is carried out, while most ultrasonic cleaners will carry out their cleaning process at a lower temperature, thus resulting in a reduction of the risk of proteins still being evident.
Figure 2. Fully validated ultrasonic lumen cleaner. Photo courtesy of Ultrawave, Ltd.
Medical device technology has prompted the design and manufacture of ultrasonic cleaners that have the ability to clean hollow lumen instruments for use in hospital cleanrooms (see Fig. 2). Different units are available depending on the type of instrument to be cleaned. There are a number of units currently on the market designed specifically for cleaning items such as rigid scopes and cannulated instruments. These units contain internal ports to which hollow instruments can be connected. As the cleaning cycle is running, cleaning fluid is pumped through the ports and down the internal channel of the instrument. Because the item is completely submerged in the cleaning fluid, the process of cavitation actually occurs inside the instrument. The sound waves will pass through the metal skin of the instrument, subjecting the inside of the instrument to the same cleaning action as the outside. This is what differentiates ultrasonic cleaners from the other methods that are available.
Ultrasonic cleaners can also be incorporated into a laboratory cleanroom. In this situation, it is crucial that items used within the contamination-free environment remain completely clean. Many of the instruments in this environment are subjected to varied uses so it is important to ensure that no residues are left from previously contained substances. Although hand-cleaning of instruments such as test tubes and beakers can be done relatively effectively, this method is no substitute for ultrasonics because microscopic cavitation bubbles can penetrate smaller surface valleys than a scrubbing brush can. It’s also standardized and does not vary depending on who is scrubbing. Sustained exposure to the ultrasonic action during the cleaning process means that even difficult contaminants are gradually broken down and lifted off. Certain items, such as pipettes, can be difficult to completely decontaminate because of narrow channels. Residue can become lodged in these channels, but using an ultrasonic cleaning process will ensure that these small, delicate items are cleaned thoroughly, even removing particles in the micron range without causing any damage.
Ultrasonic cleaning can also be incorporated into cleanrooms where the manufacturing of microchips or hard disk drives takes place. It is vital that during the production stage of these intricate items the surface areas are not subjected to any potential contaminants such as dust. If this occurs, it could result in defective products. Thus, it is necessary for these items to be thoroughly cleaned before the final stages of manufacture. By subjecting it to an ultrasonic cleaning cycle, the surface of the hard drive or chip will be clean from all particles that may potentially cause a problem once sealed and in use. It is vital that the cleaning process is both thorough and gentle in order to ensure that items such as these do not become damaged. In applications like these, high-frequency ultrasonic cleaning would be the most appropriate process to adopt. As was previously explained, the cavitation bubbles that form within higher-frequency ultrasonic baths are smaller and implode with less force and so the risk of surface damage will be further reduced.
As technology continues to advance in the medical market, both in terms of the procedures and the instrumentation used, it will be necessary for the cleaning procedures used to advance in parallel.
The main advances in ultrasonics will be the methods by which the cleaning is validated. In other words, it will not be the cleaning process itself that evolves, rather it will be how this process is both recorded and operated. Modern technology now allows for machines to be touch-screen operated and, although this is currently only available from a handful of companies, it looks destined to be widely available within the next three years. Coupled with this is the requirement that all cleaning within hospitals be validated and traceable in order to ensure that each instrument has gone through the correct reprocessing channels en route to final sterilization. Each stage of the cycle (e.g., water temperature, cycle time, and who ran the cycle) can be recorded and validated in order to confirm that it was completed successfully. Any failures in the cleaning cycle are recorded, making it an easily traceable process-something that is now a vital part of the hospital reprocessing procedure.
Although these advancements will be predominantly aimed at the medical market, they will become useful to those operating in other industries. The ability to validate that the cleaning process has been completed successfully should prove highly beneficial to those in both scientific and industrial applications.
Edward Mant is a marketing assistant at Ultrawave Ltd. in Cardiff, Wales. Mant received his BA in marketing and business from the University of Glamorgan in 2000. Since graduating, Mant has worked in a variety of marketing sectors including a marketing/PR agency and a government-funded Consultancy. Throughout his marketing career, Mant has built a large portfolio of articles published in national and international magazines, and in local and national press, covering subjects such as training, recruitment, new products and various services. He can be reached via e-mail at [email protected]
1. Carfrung W.A., A. Brunwick, D.M. Nelson, et al. “Effectiveness of ultrasonic cleaning of dental instruments,” American Journal of Dentistry, pp. 152-6, June 1995.
2. Jamie Lewis, Ultrawave PhD student. Study title: “On the role of reflections and standing waves in ultrasonically induced cavitation and cleaning intensity: A simulated and practical approach.”