For millions of people who depend on pacemakers, keeping the heart beating at the right rhythm currently means putting medical hardware inside the body.
That could one day change.
Scientists have developed an experimental wearable device that can stimulate the heart using focused ultrasound, potentially eliminating the need for wires threaded into the heart or a pulse generator implanted beneath the skin.
The technology is still firmly in the experimental stage and is not ready to replace conventional pacemakers. But researchers say their findings demonstrate a possible new approach to cardiac pacing — one in which sound waves, rather than electrical leads, could be used to control the heartbeat from outside the body.
The experimental device, developed by researchers from the University of Southern California, the Massachusetts Institute of Technology and collaborating institutions, has been tested in engineered human heart cells and animal models.
In experiments involving rats with abnormal heart rhythms, researchers were able to use ultrasound stimulation to restore a normal sinus rhythm. They also conducted experiments using pig tissue to investigate whether the approach could work at a scale more comparable to the human heart.
The findings, published in Nature Biomedical Engineering, point towards a possible future in which cardiac stimulation could be delivered through a wearable patch rather than an implanted device.
But there is a crucial qualification: this is not yet a pacemaker for human patients.
A Pacemaker That Uses Sound
The device, described by researchers as a non-invasive ultrasound pacemaker (NUP), is designed to sit on the chest and send focused ultrasound into the heart.
That is fundamentally different from how conventional pacemakers work.
Traditional pacemakers use electrical impulses to regulate the heart. A pulse generator is implanted beneath the skin, while electrical leads are positioned in or around the heart to deliver the necessary stimulation.
The experimental system takes a different route.
Instead of sending electricity through implanted wires, it uses sound waves to activate specially modified heart cells.
The approach relies on a technique known as sonogenetics, in which cells are genetically modified so that they respond to ultrasound.
Researchers modified cardiomyocytes — the cells responsible for contracting the heart muscle — so they could produce a mechanosensitive ion channel known as MscL-G22S.
When focused ultrasound reaches these engineered cells, the mechanical force causes the channels to open. Calcium ions then enter the cells.
Calcium plays a central role in cardiac muscle contraction. By manipulating that process, the researchers were able to use ultrasound to trigger activity in the heart cells.
In simple terms, the scientists have created a biological system in which sound can be converted into a signal capable of making specially engineered heart cells contract.
The Challenge of Hitting a Moving Target
Developing a system capable of stimulating the heart from outside the body presents a formidable engineering challenge.
The heart is constantly moving, while the sound waves have to travel through layers of skin, fat and muscle before reaching cardiac tissue.
Simply placing an ultrasound device on the chest is therefore not enough. The system has to determine where the heart is and direct the ultrasound precisely toward the intended area.
The researchers addressed this by combining ultrasound stimulation with imaging.
Their wearable system can identify the relevant region of the heart and focus the ultrasound beam on it. According to the study, the system achieved spatial targeting of less than 1 millimetre and could control stimulation frequencies of up to 9 Hz.
That degree of precision matters.
Cardiac pacing is not simply about making the heart beat faster or causing it to contract. Stimulation must occur at the appropriate location and timing. An ineffective or poorly timed signal could fail to correct an abnormal rhythm and, in the wrong circumstances, could potentially create additional problems.
The experimental system therefore aims to provide controlled and targeted stimulation rather than simply sending ultrasound indiscriminately through the chest.
Restoring Rhythm in Animal Tests
The researchers tested the technology in rats with abnormal heart rhythms.
Using ultrasound stimulation, they were able to restore sinus rhythm — the normal rhythm generated by the heart's natural pacemaker.
The researchers also performed experiments involving pig tissue, an important step because the anatomy and size of a pig heart provide a model that can more closely approximate some of the challenges presented by the human heart.
The technology was also subjected to safety testing in rats during daily activities over an eight-month period, according to the researchers.
These results are encouraging, but they do not establish that the system is ready for human use.
A technology can work in engineered cells or animals and still encounter major obstacles when tested in people.
The Genetic Hurdle
One of the biggest differences between this experimental system and today's conventional pacemakers is that the heart cells being stimulated have to be made responsive to ultrasound.
That requires genetic modification.
The researchers engineered the cardiomyocytes to express the ultrasound-sensitive ion channel that makes sonogenetic stimulation possible.
This means the concept is not simply a matter of replacing an electrical pacemaker with an ultrasound patch.
Before such a system could be used clinically, researchers would have to demonstrate that the genetic modification can be delivered safely and reliably, remains stable over time and does not produce unintended effects.
The researchers investigated aspects of the genetic safety of their approach, but considerably more work would be required before the technology could move into human treatment.
Clinical trials would also have to establish whether the system can reliably control heart rhythms, whether it remains safe over long periods and whether it can perform consistently in people with different body types and cardiac conditions.
Regulatory approval would be another major step.
Why a Non-Invasive Pacemaker Matters
Implanted cardiac devices have transformed the treatment of people with certain heart rhythm disorders. For many patients, they provide life-saving control of abnormal or dangerously slow heart rhythms.
But implantation remains a medical procedure.
Implanted hardware can also create complications involving leads, infection, device failure and other problems that may require additional medical procedures.
A genuinely non-invasive pacing system could potentially change that equation.
Instead of placing a pulse generator under the skin and running leads into the heart, doctors could theoretically use a wearable device to stimulate cardiac tissue from outside the body.
Such a system could be particularly attractive if it could eventually provide the same reliability and precision as implanted devices without requiring the hardware to remain inside the patient.
More Than a Pacemaker
The researchers' approach also points toward another possibility: a wearable cardiac device that does more than stimulate the heart.
Because the system combines ultrasound stimulation with imaging, future versions could potentially be designed to monitor cardiac activity, detect an abnormal rhythm and deliver stimulation when necessary.
In that scenario, a single wearable device could theoretically combine monitoring, detection and treatment.
Such technology would represent a significant shift in the management of some cardiac rhythm disorders.
Instead of a patient receiving an implanted device that remains inside the body, a wearable system could potentially monitor the heart continuously and intervene when necessary.
But that remains a future possibility rather than an established medical treatment.
From Science Fiction to Early Proof
For now, patients should not expect cardiologists to begin replacing conventional pacemakers with ultrasound patches.
The research is an early proof of concept. It demonstrates that focused ultrasound can be used to influence specially engineered cardiac cells and control heart rhythm in animal models.
The difficult question is whether that principle can eventually be translated safely and reliably to humans.
That will require extensive research, human clinical trials and regulatory scrutiny.
Still, the significance of the work lies in the direction it suggests.
For decades, the idea of pacing the heart has been closely associated with electrical signals, implanted generators and wires.
Researchers are now exploring a radically different possibility.
The future of cardiac pacing may not necessarily require a wire travelling into the heart or a battery sitting beneath the skin.
It may, instead, involve a small wearable patch — and sound travelling through the body to tell the heart when to beat.