Ultrasonic boom

One of the most serious difficulties facing our species is antimicrobial immunity. With microbes becoming increasingly resistant to drugs, we risk a world where medicine can’t keep up. Experts think that by 2050, the phenomenon will kill more people than cancer and cost us more than the combined wealth of the global economy.

In the face of this challenge, scientists and doctors are pushing to find a solution. Apart from developing microberesistant drugs, they are trying to prevent internal infection by keeping machines and equipment clean. Ultrasonic cleaning machines have been an important part of this process for decades. But new technology promises to transform the way they’re used to clean medical instruments in factories and hospitals.

Traditional limitations

Ultrasonic cleaning machines were first developed in the 1950s and their basic make-up has remained practically unchanged ever since. “Current technology on cleaning remains much the same,” says Timothy Mason, professor emeritus at Coventry University. “The baths are boxes, they have transducers and they operate at around 40kHz. That’s the standard ultrasonic cleaning bath, which you can still easily buy now. They haven’t changed much in design since the early days.”

To a certain extent, this is understandable. As Mason says, ultrasonic cleaning baths “work very well” and are capable of good results, especially when combined with the right chemicals. The machines are also smaller. They are no longer the preserve of sprawling factory floors, and are now common in laboratories and dental practices, for instance. These advantages are reflected in the market. By 2021, the ultrasonic cleaning industry is expected to reach $1.8 billion.

But if traditional ultrasonic cleaning baths have their benefits, they are by no means perfect, says Timothy Leighton, professor of ultrasonics and underwater acoustics at the University of Southampton. “The first limitation of an ultrasonic cleaning bath is that it can only fit something if it’s small enough to be put inside,” he explains. “To really clean something large or non-portable, you’d have to dismantle it, carry [the parts] to the cleaning bath and put them in.” This is particularly frustrating in factories, as production lines have to be disrupted to clean instruments properly.

There are other difficulties. Cavitation – the process that forms the cleaning bubbles – is hard to control in a normal ultrasonic bath. This means that some areas might be cleaned better than others. Exacerbating the situation is the fact that the sound field of a bath changes depending on what is put inside – meaning that an object is unlikely to be cleaned uniformly.

Changing the paradigm

Fortunately, rapid progress is being made to overcome these problems. One notable example is StarStream, patented by Leighton and produced by Ultrawave, a Cardiff-based ultrasonics company. A small handheld device with a nozzle at the end, it looks nothing like a typical cleaning bath. But this kind of mobility is exactly what Leighton was aiming for, because it has the potential to make factory and hospital cleaning a lot more efficient. “I wanted to build something that could clean in place,” he explains. “So you could take the cleaner to the device that needed to be cleaned.”

Another development involves increasing the frequency of soundwaves, resulting in a softer clean than traditional cleaning baths. “Rather than having a violent collapse cavitation – the type that’s known to erode ship propellers in an ultrasonic cleaning bath, where the bubbles just implode once and can causing pitting in metals – the bubbles [stay] alive for a long time,” says Leighton. “So instead of collapsing [violently], they just shimmer and this produces very gentle cleaning as a result.”

Not that this approach scrimps on efficiency. For instance, a recent industry test found that StarStream cleaned dirty surfaces 1,000 times better than normal water. At the same time, the new technology works without the help of chemicals or heat. Leighton’s system only uses cold water and can clean well without additives like bleach. This process is helped along by the acoustic waves themselves. They convert the bubbles produced by the system into ‘scrubbing machines’ that burrow into cracks, particularly important in hinged medical equipment among others.

Full stream ahead

All of this has the potential to explode many of the old limitations of ultrasonic cleaning, particularly in manufacturing. Because they dispense with the cumbersome box typical of older ultrasonic machines, devices like StarStream could be seamlessly added to factory production lines, for instance. Its simple design also means that StarStream could save factories a lot of money: all they need is cold water. At the same time, avoiding chemicals would also keep employees safe, Leighton says. These advances “might replace or complement existing cleaning systems; for example, as a pre-wash, allowing the use of chemicals to be reduced and so increasing safety for workers”.

A related development focuses on keeping equipment clean. At the moment, instruments can suffer secondary contamination by sitting in cleaning baths that have been dirtied from previous washes. Pressure washers emit aerosols that settle on nearby objects and people, carrying contaminants with them. But because the latest technology cleans in place, there is no longer risk of devices being infected by what came before. Using fewer chemicals also helps. “You’ll find that no [company] now wants to use any kind of organic solvent,” says Mason. “They’ll all want to use water and detergent, and there are developments [in terms of] the type of detergent used, as you may need a specific type of detergent for cleaning off a nasty mess from things.”

Gentler cleaning is also transforming medical manufacturing. Until now, some delicate medical devices were at risk of damage from standard ultrasonic cleaners. According to a 2009 study by the Materials Information Society, ultrasonic cleaners damaged stainless steel stents and wire catheters before the items had even left the factory. Pressure washers are just as brutal. These issues will be resolved by raising the sonic frequency of cleaners, Mason says.

“You want something that will clean the surface but not damage it,” he explains. “You really cannot use 40kHz because the amount of energy produced by the cavitation bubble collapse will damage the surface a little. [But] you can use ultrasound at a slightly higher frequency than 40kHz to clean medical instruments.”

Not that higher-frequency cleaning is limited to a factory setting. Leighton says it could be useful for expensive hospital equipment such as endoscopes, particularly given they tend to be in short supply. “Imagine you have a hospital that’s got an endoscope,” he says. “They can cost £50,000, so a hospital can’t afford to have too many of them. You could spend days doing a really thorough clean of each endoscope. But because they cost so much, you can’t afford to have them out of action for long. Therefore, you’ve got to clean them effectively and get them back into the theatre. But if you’ve got one patient with, say, HIV, you absolutely must not transfer tissue from one patient to another. I wanted to look at how to clean surgical instruments really quickly and well.”

Fighting infections

New technology could also help patients and hospitals combat infections. Each year, the NHS spends £1 billion fighting wounds in diabetics that cannot be treated by antibiotics. And each year it is forced to perform 6,000 amputations. Similar problems are common around the world, notably in the Middle East. In the United Arab Emirates, for example, until recently, 20% of diabetics who develop foot ulcers end up needing amputations. While normal ultrasonic cleaning baths are too aggressive and bulky to be used on human flesh, new technology is not, Leighton explains. “I’m trying to develop the device into one that cleans wounds,” he says. “[We can] expand that from diabetics to traumatic injuries to burn injuries. All these people have breakages in the skin and as a consequence get infections. We’re talking about using ultrasonics to avoid amputations and save [hospitals] billions [in costs]. The ultrasonic cleaning bath has no place here, because you can’t just put a wound in an ultrasonic cleaning bath, but we’ve shown that StarStream is safe.”

All this can only be good news. With drug-resistant microbes on the increase, versatile ultrasonic machines promise to help keep them at bay. At the same time, they could save medical manufacturers and hospitals a lot of money; 50 years after they entered the mainstream, higher frequencies and greater flexibility are getting ultrasonic cleaning machines ready for the tests of the next century.