Scientists develop the world’s smallest pacemaker

During the recovery period after cardiac surgery, many patients will require a temporary pacemaker. This involves placing electronic leads near the surface of the skin, leading to a box of electronics, the ultimate removal of which is associated with risk of complications. But scientists in the United States have developed a temporary wireless pacemaker that is smaller than a grain of rice, and will eventually be broken down and absorbed - which means that potentially less invasive techniques can be used to place implants in patients. John Rodgers at Northwestern University is behind the device…

John - I guess the best way to think about it is it has a size somewhere between a single grain of rice and a sesame seed. In fact, it's so small, you can load it up into the bore of a syringe and just inject it in almost like a medicine. When it's illuminated with light from the outside of the body, from the skin surface, it turns on the pacemaker, thereby delivering pulses of electrical current to the surface of the heart to provide that cardiac pacing functionality.

Chris - So, the pacemaker is light sensitive and the light rhythm can drive the rhythm of discharge of the pacemaker, which in turn sets the rhythm of the heart.

John - Yeah, exactly. So we have a soft skin interface patch that you put on the surface of the chest, more or less above the location of this millimetre scale pacemaker. And that soft electronics patch has a light emitting diode integrated in it. The electronics then control the pulsatile flashing of that LED and the light that penetrates through the skin, through the tissue and illuminates the cardiac pacemaker then turns it on and off with a frequency that's matching the frequency of the operation of the LED.

Chris - I just want to say: “wow.” Enough light gets through that depth of tissue that you can do that.

John - Yeah, it's pretty interesting. The wavelength that we use is in the near infrared region of the electromagnetic spectrum. If you've ever played around with a red LED or a red laser pointer, you put it on the backside of your finger, your whole finger glows. It's at these wavelengths where tissue absorption properties are quite modest. They're quite low. And mostly what happens with light as it passes through the tissue, it's not absorbed as much as it is just scattered. And that turns out to be a nice thing because it means that we don't have to precisely align the position of our LED to be coincident with the location of the millimetre scale cardiac pacemaker because as light passes through the tissue, it kind of blooms out. And so you have a whole flashlight region of the heart that encompasses the cardiac pacemaker that gets illuminated.

Chris - And is the entire structure light sensitive then? So, it doesn't matter what orientation it's in in the heart tissue, it will see that light and it can detect it and respond to it.

John - It's oriented so that the light sensitive side is sort of pointing toward the skin, although that's not that critical because you have light scattering all around. So, you're basically illuminating a whole volume of tissue. But we do like to have that light sensitive side of the device facing up. And then the other side of the device is providing the electrical contacts that are needed to inject current into the cardiac muscle to initiate a cardiac cycle.

Chris - How does that bit work then? You've got these electrodes, what are they made of? So, how are they generating the electricity? And what's the processing that's going on inside to turn the light signal into those discharges?

John - Yeah, great question. So, it turns out it's the simplest possible battery you could imagine. We use two dissimilar metals for the electrodes. You need a pair of electrodes interfaced in the cardiac tissue to deliver the current pulses. We like to use magnesium and molybdenum or magnesium and zinc. And if you take two dissimilar metals like that and you connect them with an ionically conducting fluid, like the naturally occurring biofluid that's bathing the surface of the heart, that forms a battery. Now, the battery can't discharge unless the electrical circuit is completed. So, you need the backside to be electrically connected as well so that current can flow to balance the chemistry that's going on between those two metal pads. And that circuit is only completed when that photoactivated switch is closed. So, if the device is not illuminated with light, that switch is open. And it works really well for this application because it is simple. And so we can size reduce the entire system and still maintain enough current to stimulate the heart.

Chris - And is there enough current to stimulate a human heart? Because capturing a little tiny mouse heart, that's going to be a lot easier than something much bigger and bulkier.

John - Yeah, that's right. So when we engage in a project like this, animal model trials are kind of an essential aspect. But we progressed with human hearts, not in live human patients, but recently deceased individuals. We were able to demonstrate the ability to capture the cardiac rhythm at a human scale by using those donated hearts from organ donors.

Chris - And the thing completely dissolves away harmlessly.

John - Yes. So we've studied that again in animal models, not in human subjects. We're not there yet. But the materials themselves are not really exotic and just kind of orient people. Magnesium, molybdenum and zinc are all essential minerals for a healthy diet and a healthy metabolism. The photo activated switch is built around a very thin sliver of silicon, which is also a part of a healthy daily diet that doesn't in itself guarantee biocompatibility and harmless dissolution in the body. But I think it sort of makes sense that we're not observing any adverse effects because you need these materials anyway. And I guess if you had enough of these devices, maybe you make a vitamin tablet out of them, but you'd have to add up probably a hundred, you know, I'd have to run the numbers, but kind of on that order to make an impact at a dietary level.

Chris - And were the cardiac team impressed with your endeavors?

John - Well, yeah, I mean, we're an engineering oriented group, but most of what we do is in response to inbound requests from clinical collaborators, cardiac surgeons reaching out and describing this challenge that they have. The kinds of patients who need this temporary pacing span the full range of ages, but it turns out that it is most critically important for pediatric patients who've undergone a cardiac surgery associated with a cardiac defect. And there the ultra-miniaturized geometries are even more important. I mean, smaller is almost always better, but especially for these infants, you know, minimizing the burden, the device load on the body turns out to be a really important consideration. And so that's high on the list of priorities around where this kind of technology kind of moved the needle in terms of patient care.