The tiny pacemaker sits next to a single grain of rice on a fingertip. [Image: John A. Rogers/Northwestern University]
Many patients—including the 1% of children born with congenital heart defects—need temporary pacemakers after having heart surgery or as a result of other cardiac complications, either while waiting for a permanent pacemaker or to maintain a normal heart rate during the healing process. But the current protocol for implanting and removing these temporary devices can be invasive and potentially cause damage to the heart muscle.
Now, researchers at Northwestern University, USA, have designed a light-activated pacemaker smaller than a grain of rice that eliminates the need for additional surgeries (Nature, doi: 10.1038/s41586-025-08726-4). The device can be inserted with a syringe, then dissolves when it’s no longer needed.
Healing the smallest hearts
During a heart surgery, surgeons typically stitch electrodes onto the heart to be used as a temporary pacemaker. Wires protrude from the patient’s chest and connect to an external pacing box that delivers a current to the muscle to control the rhythm. When pacing is no longer needed, physicians must surgically remove the electrodes, potentially causing infection, tissue damage, bleeding or blood clots. “The wires can become enveloped in scar tissue,” explains study co-lead Igor Efimov. “So when the wires are pulled out, that can potentially damage the heart muscle.”
Seeking to create a less-invasive alternative, in 2021 Efimov and co-lead John A. Rogers developed their first bioresorbable dissolvable pacemaker. The device, which was about the size of a quarter and powered by near-field communication protocols, was successful in pre-clinical animal studies. But cardiac surgeons asked the researchers to make the device smaller, so it would be better-suited for pediatric patients.
“Our original pacemaker worked well,” Rogers said. “It was thin, flexible and fully resorbable. But the size of its receiver antenna limited our ability to miniaturize it. Instead of using the radio frequency scheme for wireless control, we developed a light-based scheme for turning the pacemaker on and delivering stimulation pulses to the surface of the heart.”
When the wearable device (left) detects an irregular heartbeat, it emits light to activate the pacemaker. [Image: John A. Rogers/Northwestern University]
Infrared light switch
The resulting device measures just 1.8 mm in width, 3.5 mm in length and 1 mm in thickness, and can be injected with a syringe. The researchers believe this makes it the world’s smallest pacemaker. In addition to introducing the optical control mechanism, the researchers further reduced the size of the device by using a simple galvanic cell to supply power. It uses two different metals as electrodes that, when in contact with surrounding biofluids, form a battery. The resulting chemical reactions cause the electrical current to flow to stimulate the heart.
The tiny pacemaker is paired with a separate small, soft, wireless device worn on the patient’s chest that continuously captures echocardiograms to monitor heart rate. When the wearable device detects an irregularity, it automatically activates an LED that flashes a pulse of near-infrared light, with a wavelength of about 850 nm. Illuminating the pacemaker acts as an “on” switch. It decreases the resistance of the pacemaker’s phototransistor by several orders of magnitude, closing the circuit and discharging the battery formed by the pacing electrodes and the adjacent cardiac tissue to deliver electrical stimulation to the heart.
Syncing up
The team said its experimental studies demonstrated effective pacing in mouse, rat, porcine, canine and human cardiac models at both single-site and multi-site locations. The researchers also believe that physicians could distribute numerous pacemakers across the heart, thanks to their tiny size, and use different colors of light to independently control a specific device. This would enable more sophisticated synchronization and allow different areas of the heart to be paced at different rhythms.
“The heart requires a tiny amount of electrical stimulation,” Rogers said. “By minimizing the size, we dramatically simplify the implantation procedures, we reduce trauma and risk to the patient, and, with the dissolvable nature of the device, we eliminate any need for secondary surgical extraction procedures.”