After heart surgery, many patients require temporary pacemakers either to regulate the heart rate while waiting for a permanent pacemaker or to support normal heart rhythm during recovery. The current standard of care involves surgeons attaching electrodes to the heart muscle during surgery. Wires from these electrodes exit the chest and connect to an external pacing device, which delivers current to control the heart's rhythm. Once the temporary pacemaker is no longer necessary, the electrodes are removed. However, this process can lead to complications such as infections, dislodgement, tissue damage, bleeding, and blood clots. In response to these challenges, engineers have developed a pacemaker small enough to fit inside a syringe, allowing it to be non-invasively injected into the body. This pacemaker is designed for temporary use, particularly benefiting newborn babies with congenital heart defects and those with small, fragile hearts.

Developed by engineers at Northwestern University (Evanston, IL, USA), this pacemaker is smaller than a grain of rice and is paired with a soft, flexible, wireless, wearable device. This device mounts on the patient's chest and controls the pacemaker. When it detects an irregular heartbeat, the wearable emits a pulse of light, activating the pacemaker. These light pulses penetrate the patient’s skin, muscles, and breastbone to control the pacing. The pacemaker dissolves once it is no longer needed, as all components are biocompatible and dissolve harmlessly in the body’s biofluids, eliminating the need for surgical removal. This innovative device's effectiveness has been demonstrated across large and small animal models and human hearts from deceased organ donors, with findings published in the journal Nature.

This research builds on earlier work by the team, which first developed a dissolvable device for temporary pacing. In that earlier study, the team introduced a flexible, lightweight, bioresorbable pacemaker that removed the need for bulky batteries, rigid hardware, and wires. The concept of bioresorbable electronics, which provides therapeutic benefits before harmlessly dissolving like absorbable sutures, was also explored. By adjusting the composition and thickness of these devices, the team could control their functional lifespan before dissolution. The original dissolvable pacemaker worked well in pre-clinical animal studies, but cardiac surgeons suggested making the device smaller for non-invasive implantation and use in smaller patients. The device initially relied on near-field communication (NFC), the technology used in smartphones for payments and RFID tags, which required an integrated antenna. Instead of NFC for wireless control, the team created a light-based approach to turn on the pacemaker and send stimulation pulses to the heart. This innovation allowed them to drastically reduce the device's size.

Additionally, the researchers rethought the power source. Rather than using NFC, the new pacemaker operates using a galvanic cell, a simple battery that converts chemical energy into electrical energy. The pacemaker uses two different metals as electrodes that, when in contact with biofluids, form a battery. The resulting chemical reactions produce the electrical current needed to stimulate the heart. The researchers utilized an infrared light wavelength that can safely and deeply penetrate the body. If the patient’s heart rate drops below a certain threshold, the wearable device detects this and automatically activates an LED light. The light flashes on and off at a rate that matches the normal heart rate. Despite its tiny size—measuring just 1.8 millimeters in width, 3.5 millimeters in length, and 1 millimeter in thickness—the pacemaker delivers the same level of stimulation as a full-sized pacemaker. Due to its miniature size, physicians can place multiple pacemakers across the heart. By using different wavelengths of light, individual pacemakers can be independently controlled, enabling more sophisticated synchronization than traditional pacing methods. This system can pace different areas of the heart at different rhythms, potentially helping to terminate arrhythmias in special cases.

“We have developed what is, to our knowledge, the world’s smallest pacemaker,” said Northwestern bioelectronics pioneer John A. Rogers, who led the device development. “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.” 

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