It seems pretty logical: Hearts beat, movement is energy, and electricity produced off the movement could power an implantable medical device such a pacemaker.
But over the decades, there has never been an energy-harvesting strategy that really met medical device designers' needs--until now. Enter flexible electronics pioneer John Rogers, PhD, of the University of Illinois-Champaign with a super-thin silicone-encased, bendable energy harvester that can be affixed to a beating heart.
Rogers and colleagues have placed the device on the surface of hearts beating inside anesthetized cows and sheep, demonstrating that it can produce up to 1.2 ?W/cm², according to a paper Rogers and his colleagues recently published in the Proceedings of that National Academy of Sciences.
A millionth of a watt or two is not a lot, but it is enough electricity to power a pacemaker, Rogers told MPMN on Thursday. Embed more flexible components inside the silicone, and it might even become a complete pacemaker.
|Rogers and colleagues have placed the energy harvester on the surface of hearts beating inside anesthetized cows and sheep, demonstrating that it can produce enough electricity to power a pacemaker.|
The energy harvester, though, still needs long-term trials to show it can hold up inside animals, Rogers acknowledges. Human trials could be a decade or more away in the United States.
Still, it is now tantalizingly possible that medical device designers will one day be able to power their creations using the natural movements of the patient using the device--including heartbeat-powered pacemakers and potentially defibrillators if flexible batteries are packaged in.
That means much tinier, less invasive devices for the hundreds of thousands of heart disease patients presently using pacemakers and defibrillators.
The challenges that Rogers and his research partners had to overcome over two and a half years, though, demonstrate why it is only now that powering a device off a beating heart seems even remotely possible.
Rogers, who co-founded Cambridge, MA-based flexible electronics maker MC10 and other ventures, has plenty of experience creating bendable silicon circuitry and other flexible electronic. He suspected that the field might have an answer for power generation because previous schemes, such as harvesting energy from blood flow or taking advantage of temperature changes, relied on watch-like devices with moving parts.
"I think anything that involves a moving part is going to be extremely challenging," Rogers says. He thinks the need for moving parts is what has caused medical device designers to rely on batteries versus power generation.
Rogers latched onto the idea of instead using a piezoelectric substance, a material that generates electricity when it is moved, to create an energy harvester. He ended up opting a crystalline, ceramic compound called lead zirconate titanate, called PZT for short, because it is already commonly used.
"We prefer a solid-state type of construction that eliminates any moving components inside the device," Rogers says.
Three major challenges then occupied much of Rogers' and his colleagues time:
They needed to produce PZT layers that were actually flexible.
The answer was similar to what Rogers latched onto for flexible silicon circuitry: Make it really thin. The New Yorker recently described the years Rogers and colleagues spent years figuring out how to bake thin silicon circuits on a silicon base or substrate, chemically wash away the silicon surface underneath the circuit, and then use a specially designed rubber stamp to ever-so-gently transfer the thin circuit away to a flexible base. The same type of thing had to be done with PZT, which is also baked on a silicon substrate and then transferred off so it could be embedded inside the silicone.
Rogers thinks it is really this transference process--this ability to remove circuits and other electrical components from the silicon they are baked on in a reasonable, reliable way--that is enabling the power generator and many other flexible electronic innovations.
Power generation needed to be super efficient.
A beating human heart only produces 2 to 3 watts of energy, and Rogers knew the energy harvester would only be able to harvest a tiny fraction of that if it was to avoid damaging the heart. These were PZT layers embedded in silicone and integrated with rectifiers and millimeter-scale batteries. Research partners at Northwestern University ran countless computer simulations to figure out the optimal layering of PZT that would produce the most energy. "We worked hard with our theoretical partners at Northwestern to put together a model, experimentally validated, that really captures all the quantitative details about how current is produced when you build a device of that type with any materials combination," Rogers says.
It also needed to avoid damaging the heart.
The more external pressure the energy harvester places on the heart, the more electricity it will produce. But it is also more likely to cause arrhythmias as other damage that defeat the purpose for having the device there in the first place. "It requires very careful attention to how the device is interacting with the organ it is mounted on," Rogers says.
A good start was to make sure the energy producer put less pressure on the heart than the natural pericardium encasing it. Next were the open heart experiments on the live cows and sheep conducted at the University of Arizona. As a result of the experiments, Rogers suspects it is safe to produce a few microwatts off a beating heart, though not much more.
More electricity might be possible off lung movement, abdominal movements or even muscle movement. But with the heart, a few microwatts is a safe range if designers are to avoid disrupting the beating of the heart, according to Rogers.
"A heart is kind of a critical organ, so we have to be careful. ... You could push it, but you have to do careful studies. ... I don't think you want to be anywhere near where bad things might start to happen," Rogers says.
|See Princeton University's John McAlpine discuss bionic nanomaterials on the Center State at MD&M West, February 10-13 in Anaheim, CA.|
For Rogers and his colleagues, the next step with the energy generators will be to implant them for half a year inside living cows and sheep and see how well they hold up.
Batteries inside some of the tiny, next-generation leadless pacemakers such as St. Jude Medical's Nanostim or Medtronic's Micra can last for 10 to 13 years, so the flexible electronics power generator will have to last even longer, Rogers says.
"We need to do the longer term survivability on the animals. ... By definition it needs to last for more than 10 years," Rogers says.
An actual energy generator operating on the surface of a human heart is probably a decade away.
"I don't have a lot of concerns with the device itself. It's really how the device is interacting with the body ... the biological responses. ... You have to be conservative on those types of things," Rogers says.