Microscopic Cleaning for Clean Rooms

The inclusion of an ultrasonic cleaning system can mean that the efficacy and efficiency of a clean room can be greatly improved.

Clean Rooms are, by very definition, environments which must remain clean at all times and the level of cleanliness required depends on the process within. For example a class 100 clean room used in hard disk drive manufacture can contain no more than 100 particles larger than 0.5 microns in a given cubic foot. The most common methods of cleaning items en-route to a clean room are those of washer disinfectors or, surprisingly, a simple handwash procedure. There is however an alternative method which has the ability to clean contaminates to the micron level and has been proven to be more effective than handwashing alone. Here we explore the process of ultrasonic cleaning and its application in the clean room environment.

Clean rooms are an essential part of many businesses worldwide and a diverse range of uses. These can vary from microchip manufacture and hard drive cleaning through to pharmaceutical manufacture and surgical instrument decontamination.

An area that is still under some discussion is the means of how best to ensure items used within the clean room or entering a clean room are completely clean in order to avoid jeopardising the operating environment. This is especially the case for medical devices and surgical instruments as it is vital these are completely free from contamination prior to sterilisation and use.

Ultrasonic cleaning has been around for over forty years but its use within a clean room environment is still relatively sparse, which seems somewhat strange when you consider the cleaning efficacy it achieves. This may be down to 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 piezo ceramic transducers. These resonant structures are made up of a number of parts; a front mass (the area which is bonded to the tank), 2 piezo ceramic rings (PZT composite), berrillium 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 (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.

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 it causes these bubbles to expand, creating a vacuum within them. They continue to grow until a size is reached where they cannot support their own density, which causes them to implode: the phenomenon called cavitation. During this implosion extreme temperatures and forces are achieved. The implosion process for each bubble will only last for nanoseconds but at any one time there will be millions of these microscopic implosions occurring within even the smallest ultrasonic tank. As they implode the surrounding fluid rushes in to fill the gap left by the bubble which creates a cleaning action similar in effect of 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. Due to the implosions being microscopic in scale the action is very gentle and easily lifts contamination off the surface area of the item immersed in the liquid.

Although ultrasonics is a highly effective cleaner 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, which have been created to maximise 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. Although increasing the temperature it should be noted 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 should be noted that for surgical instrument decontamination in the UK the solution should not be 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 increase the cleaning time. This is significantly less than other methods which are 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 they can be incorporated into the clean room process. Which of these is applicable will depend on the type of clean room.

The first of these is to have the ultrasonic cleaner as part of the cleaning process prior items entering the clean room environment. All items within a clean room need to be free from contamination in order not to compromise the clean room operation. By using ultrasonic cleaning products items entering the said environment will be free from unacceptable debris, thus minimising the risk of contaminants entering the clean room.

The second instance where an ultrasonic cleaner can be incorporated into the clean room cycle is to have one present in the clean room itself. Although the products within a clean room 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 scientific instrumentation used within the clean room. Although items such as these are often removed for cleaning it would be far more logical to clean them within the clean room. This would eradicate the possibility of them being exposed to contaminants after the cleaning process and prior to re-entry to their operating environment. By installing an ultrasonic cleaner the items will be cleaned under clean room conditions, thus remaining free from potentially hazardous particles.

Since the UK suffered from a vCJD (Creutzfeldt-Jakob disease) scare the government 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 sterilisation supply departments, or CSSD's, in many key hospitals and it is here that ultrasonic cleaners are increasing in numbers. This is because in order to ensure surgical instruments are completely decontaminated, prior to undergoing their normal disinfection and sterilisation procedure, the use of an ultrasonic cleaner will produce better debris removal results than other methods which are available such as washer disinfectors. Throughout the world more and more hospitals are having CSSD's installed in order to reprocess their surgical instruments. These CSSD's 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 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 they enter the clean room. 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 now used. Because of how ultrasonic cleaners work the process of cavitation means that the instruments will undergo a more rigorous clean 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 which has undergone a cycle in an ultrasonic cleaner will be free from proteins, meaning the washer disinfector cycle can be shortened to just carry out the disinfection stage but without the time consuming spray clean cycle. It is also worth noteing 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 have the ability to 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.

Medical device technology has prompted the design and manufacture of ultrasonic cleaners for use in hospital clean rooms, which have the ability to clean hollow lumen instruments. Different units are available depending on the type of instrument to be cleaned. Currently on the market there are a number of units for cleaning items such as rigid scopes and cannulated instruments. Designed specifically for cleaning these instruments these units contain internal ports where 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. As the item is completely submerged in the cleaning fluid it means that the process of cavitation actually occurs inside the instrument. The sound waves will pass through the metal skin of the instrument meaning the inside of the instrument is being subjected to the same cleaning action as the outside. It is this that differentiates ultrasonic cleaners from the other methods which are available.

Ultrasonic cleaners can also be incorporated into a laboratory clean room. 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 the microscopic cavitation bubbles can penetrate smaller surface valleys than a scrubbing brush. It's also standardised and does not vary depending on who is scrubbing. Sustained exposure to the ultrasonic action during the cleaning process means even difficult contaminants are gradually broken down and lifted off. Certain items, such as pipets, can be difficult to completely decontaminate because of the narrow channels. Residues can lodge themselves in these channels but by using an ultrasonic cleaning process it will ensure that these small delicate items are cleaned thoroughly even removing particles in the micron range without incurring any damage.

Ultrasonic cleaning can also be incorporated into clean rooms 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 being manufactured. It is necessary for these items to be thoroughly cleaned before the final stages of manufacture. By subjecting them to an ultrasonic cleaning cycle the surface of the hard drive or chip will be clean from all particles which may potentially cause a problem with their operation once sealed and in use. With items such as these it is vital that the cleaning process is both thorough and gentle, in order to ensure that they do not become damaged. In applications like this high frequency ultrasonic cleaning would be the most appropriate application to adopt. As was previously explained the cavitation bubbles which form within higher frequency ultrasonic baths are smaller and implode with less force and so the risk of surface damage occurring 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 terms of ultrasonics will be the methods by which the cleaning is validated. In other words it will not be the cleaning process itself that evolves, it will be the means of 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 for all cleaning within hospitals to be validated and traceable in order to ensure that each instrument has gone through the correct reprocessing channels en-route to its final sterilisation. Some units on the market today already have this incorporated into the design and it is these units that are pushing the boundaries of ultrasonic technology at the present time. The validation has the ability to record and validate every stage of the cycle from water temperature, cycle time and who ran the cycle in order to confirm it was completed successfully. Any failures throughout the cleaning cycle are recorded on the final printout 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 be sure to become useful to those operating in other industries. Having validation that the cleaning process has been completed successfully should prove highly beneficial to both those in scientific and industrial applications.

As featured in CleanRooms magazine, Jan 2006.

1) http://www.webopedia.com/TERM/C/clean_room.html
2) Carfrung WA, Brunwick A, Nelson DM, et al (June 1995) �Effectiveness of Ultrasonic cleaning of dental instruments� Am.J.Dent, p: 152-6
3) 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."