A tiny, light-activated pacemaker could make the placement and removal of temporary pacemakers safer for cardiac patients and provide a minimally invasive solution for newborns and children who require temporary pacing after cardiac surgery. The millimeter-scale pacemaker, developed at Northwestern University, has an onboard power supply that is optically controlled. It operates through a galvanic cell — a type of battery formed by the interaction between two metal electrodes and the biofluids surrounding them. When the electrodes come into contact with the biofluids, they form a battery, and the resulting chemical reaction causes the electrical current to flow and stimulate the heart. The pacemaker is paired with a wireless, wearable patch that is placed on the patient’s chest. When the patch detects an irregular heartbeat, it automatically activates an LED, which emits a light pulse that activates the pacemaker. The light flashes on and off at a rate that corresponds to the patient’s normal heart rate. These short pulses penetrate through the patient’s skin, breastbone, and muscles to control the pacing. An external, light-activated switch controls the device’s on-off status. The switch turns on whenever light illuminates the device. When the device receives no light, it remains in an off state. No electrical stimulation is delivered to the heart unless the device is exposed to light. “When the pacemaker is implanted into the body, the surrounding biofluids act as the conducting electrolyte that electrically joins those two metal pads to form the battery,” professor John A. Rogers, who led the device development team, said. “A very tiny, light-activated switch on the opposite side from the battery allows us to turn the device from its ‘off’ state to an ‘on’ state upon delivery of light that passes through the patient’s body from the skin-mounted patch.” The device delivers pulses in the IR, a wavelength that penetrates the body deeply and safely. “Infrared light penetrates very well through the body,” professor Igor Efimov said. “If you put a flashlight against your palm, you will see the light glow through the other side of your hand. It turns out that our bodies are great conductors of light.” The researchers demonstrated the device’s efficacy across a series of large and small animal models and on human hearts from deceased organ donors, at both single-site and multi-site locations. The tiny scale of the pacemaker makes implantation a minimally invasive procedure. Smaller than a grain of rice, the light-activated device can fit inside the tip of a syringe and be injected into the body. Although the device is only 1.8 mm wide, 3.5 mm long, and 1 mm thick, it delivers as much stimulation as a full-sized pacemaker. “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.” (From left) A traditional pacemaker, a leadless pacemaker, and the pacemaker developed at Northwestern University. Courtesy of Northwestern University. When pacing is no longer needed, the bioresorbable pacemaker dissolves in the patient’s body. All its components are biocompatible and dissolve naturally into the body’s biofluids, eliminating the need for surgery. Conventional devices that are temporarily implanted in patients currently require surgery to be removed, which introduces the risk of infection and other complications. “Wires literally protrude from the body, attached to a pacemaker outside the body,” Efimov said. “When the pacemaker is no longer needed, a physician pulls it out. The wires can become enveloped in scar tissue. So, when the wires are pulled out, that can potentially damage the heart muscle.” In addition to functioning as a temporary pacemaker for adults, the miniaturized device is well-suited to newborns and children with congenital heart defects. “Our major motivation was children,” Efimov said. “About 1% of children are born with congenital heart defects, regardless of whether they live in a low-resource or high-resource country. The good news is that these children only need temporary pacing after a surgery. In about seven days or so, most patients’ hearts will self-repair. But those seven days are absolutely critical.” The tiny pacemaker can be placed on an infant’s heart, stimulated with a soft, gentle, wearable device, and nonsurgically removed. “We have developed what is, to our knowledge, the world’s smallest pacemaker,” Rogers said. “There’s a crucial need for temporary pacemakers in the context of pediatric heart surgeries, and that’s a use case where size miniaturization is incredibly important. In terms of the device load on the body, the smaller, the better.” Physicians could use several of the miniaturized devices across the heart to enable synchronized pacing. A different color of light could be used to independently control each device. “We can deploy a number of such small pacemakers onto the outside of the heart and control each one,” Efimov said. “Then we can achieve improved, synchronized, functional care. We also could incorporate our pacemakers into other medical devices like heart valve replacements, which can cause heart block.” Arrays of the millimeter-sized pacemakers could be placed on the frames of transcatheter aortic valve replacement systems, to address risks for atrioventricular block following surgeries. “Because it’s so small, this pacemaker can be integrated with almost any kind of implantable device,” Rogers said. “We also demonstrated integration of collections of these devices across the frameworks that serve as transcatheter aortic valve replacements. Here, the tiny pacemakers can be activated as necessary to address complications that can occur during a patient’s recovery process. So that’s just one example of how we can enhance traditional implants by providing more functional stimulation.” The technology used to build the device could be adapted for a range of additional applications in electrotherapy, such as nerve and bone regeneration, wound therapy, and pain management. The research was published in Nature (www.doi.org/10.1038/s41586-025-08726-4).