The earliest medical implants were almost comically rustic. In 1958, the first pacemaker ever installed in a human patient failed after just three hours, perhaps because it had been assembled in the mould of an empty can of Kiwi shoe polish; nor were the replacements much better. Arne Larsson, the patient lucky enough to be fitted with that first tin, agonised through 26 different pacemakers.

From these faltering early days, pacemakers and their cousins have settled into respectable middle age. Electrical implants now play a vital role across medical life, whether in fighting heart disease, paralysis or depression. This is shadowed by the impressive size of the global implantables market, which is forecasted to jump to $116 billion by 2022, an annual growth rate of 7.1% compared with 2016.

Still, many modern implantables endure their own problems, especially when it comes to size and longevity. Researchers work hard to find solutions, of course, but many soon realise that the human body is too complicated to rely on a single technology. No wonder scientists are now mixing different techniques – and finally expelling the memories of clunky 20th-century devices.

Fire the battery

Electrical implants have long relied on the humble battery. To a certain extent, this is understandable. Like their counterparts in toys or watches, batterypowered implants work well for long periods and tend not to break.

But as associate professor in mechanical engineering at the University of Utah Shad Roundy explains, the stakes are far higher in the medical field. “A battery might last several years, but then you need to have an intervention to replace it. Some last five years. That sounds great, until you realise you have to get surgery.”

Size is another drawback. For pacemakers to last even five years, the battery needs to be fairly large. These days, the average pacemaker is about the size of a US 50-cent piece and three times as thick, which might not seem huge until you realise it has to be physically placed inside the human body. Though scientists are experimenting with squashing implantables down, they are limited by the power source. “As we move to smaller devices, we need higher power and energy densities than batteries can provide,” Roundy explains. To make matters worse, battery-powered implantables are not wireless, meaning the energy source has to sit awkwardly by the device – or else subject the patient to a spaghettidinner of cables under their skin.

All this is especially frustrating when you consider how common implantables are. Around three million people are currently walking around with pacemakers by their hearts, and 600,000 are installed each year. That is just the start – implantable medical devices have been used by about 6% of people across the developed world, a figure that rises to 10% in the US. This covers everything from neurostimulators, designed to help the brain fight dementia, to cochlear hearing aids.

Make waves

Scientists started hunting for alternatives to battery-run implantables almost as soon as the technology was invented. As far back as the 1960s, researchers channelled Nikola Tesla and tried powering pacemakers via radio rays, sent from external coils. The project ultimately failed – the temperatures involved were dangerously high – but it launched a half century of study into how to power medical implants wirelessly.

Indeed, that botched first attempt at wireless power eventually developed into the most widespread technique of modern times. Known as inductive coupling, it involves pairing two coils together magnetically, so that changing the current in one induces voltage in the other. Not that the coils are actually touching, meaning that implants can be powered from chargers outside the body. Altogether, Roundy says, inductive coupling offers “high efficiency and good power density”, especially when the coils are large and close together.

These advantages, to say nothing of the lack of wires, make inductive coupling an increasingly popular choice for larger, accessible devices, such as pacemakers. But how does that square with the trend towards smaller implants, especially ones in deeper parts of the body.

As Roundy notes, here is where the hiccups begin. “The problem with inductive coupling is that the frequency at which you have to operate goes up as the size of your coil goes down,” he says. “As the frequency at which you have to operate increases, the absorption of radio-frequency rays in the tissue goes up.”

This, in turn, sparks further challenges. The first is that energy efficiency collapses, as rays get absorbed into bodily tissue rather than powering the device. The second is even more serious. Taking in too many high-frequency radio rays can spark tissue heating and other health worries. To put it another way, crank up an inductive coupler too high and you risk doing more harm than good, even if the machine itself hums along nicely.

“The ultrasound transmitter needs to be in good contact with your skin… This is why, when you go and do ultrasound imaging, they put that gel on your skin. You really do need intimate contact with the skin, and from an application point of view, that can be a problem.”

Overcoming these handicaps has prodded scientists towards another technique: ultrasound. In some ways it works like inductive coupling. Ultrasonic waves are fired from an external source to an implant inside the body, which then converts the energy into electricity. But the big advantage of ultrasound is that it can be run at lower frequencies than electromagnetic waves. That not only allows for smaller devices and cuts inefficiency – fewer waves are gobbled up by tissue – it also improves safety.

$166 billion
Forecasted size of the global implantables market by 2022.
Allied Market Research

Ironically, lower frequencies can ultimately make ultrasound even more robust. Whatever the technology, the FDA curbs the amount of power that can be fired into the body, but ultrasound has bigger limits than electromagnetic waves and inductive coupling. That can potentially result in “higher power densities for ultrasound”, Roundy says. It helps, too, that the maximum power allowed for ultrasound is a straightforward number – 720mW per square centimetre. In contrast, Roundy explains, the limit for electromagnetic radiation depends on a “bunch” of graphs. “The safety regulations are fairly complicated.”

People across the developed world have used implantable medical devices.
Implantable Neural Prostheses 2

Get under your skin

Ultrasound-powered implants offer several advantages over inductive coupling, a fact that is attested by the excited noises coming from scientists across the US. “How implants powered by ultrasound can help monitor health,” cheered Stanford after a team at the university built a device to measure bodily functions. “Ultrasoundrechargeable batteries can extend the time between replacements considerably, reducing healthcare costs and patient concerns,” agreed Leon Radziemski of his research at Piezo Energy Technologies, a Tucson-based company.

Yet Roundy is keen to emphasise that ultrasound is far from perfect. “The transmitter needs to be in good contact with your skin,” Roundy explains. “This is why, when you go and do ultrasound imaging, they put that gel on your skin. You really do need intimate contact with the skin, and from an application point of view, that can be a problem.” More fundamentally, ultrasound waves are unable to penetrate thick tissue, which would prove to be a challenge if the implant is buried under the skull or other bones.

These problems feed into a broader perception of technology as a saviour. “People want to look for technology that can solve all their problems,” Roundy says. “That is not likely to happen.” Instead, Roundy suggests that progress is gradual and often involves mixing techniques piecemeal.

As an example, Roundy highlights the work of Michel Maharbiz, a professor at Berkeley who united inductive coupling and ultrasound into a single project. “He used inductively coupled coils to go through the skull and then used ultrasound to go deep into the brain. It was a two-stage process – electromagnetism to go through the bone and then ultrasound.”

Roundy is similarly excited about other advances in the field, especially when it comes to linking devices. By using ultrasound to communicate between implants, doctors could one day monitor “the same condition in different locations” at once. “That data could then be aggregated at one point in the body, where it would transmit out to your mobile phone, or perhaps your care provider has a reader.”

Roundy admits all that sounds “pretty futuristic” – but then imagine announcing the pacemaker to someone in 1950. As Arne Larsson could have told contemporary scientists, success takes patience. Though he suffered through over two-dozen pacemakers, Larsson ultimately died peacefully in 2001, 43 years after that first, momentous, operation. Modern implant doctors should therefore take solace, even if they leave the shoe polish boxes outside the hospital gates.