April 3, 2025 – In the U.S. at any given time, thousands of adults and hundreds of children are on waiting lists for a lifesaving heart transplant, facing waits of six months or longer. Some will die in the interim, including one out of every eight children on the list. Worldwide, about 20 people die per day while waiting for any organ.
A promising advance from scientists in Germany offers a potential solution – a "heart patch" made from stem cells that could provide a bridge to transplantation, or maybe even a permanent fix.
These patches of heart muscle – made from adult human stem cells – can be sutured onto a patient's heart in a minimally invasive surgery to help it pump, the journal Nature reported in January.
Each patch contains lab-grown cardiac tissue made of up to 200 million cells embedded in a collagen hydrogel. The patch, called engineered heart muscle, or EHM, was proven to work in rhesus macaques. The first human to get a patch was a 46-year-old woman in 2021, helping her survive until she could get a heart transplant.
The patches were then used on 15 other patients with severe heart failure, with more data on those cases expected by the end of the year.
The innovation represents a step forward in the larger quest to solve the heart transplant crisis. About 50,000 people worldwide have end-stage heart failure, but only 5,000 heart transplants are performed annually because of the shortfall of donor hearts. At the same time, more than 6 million Americans live with heart failure, and that number is only expected to grow as the population ages. That makes the "bridge-to-transplant" development important, researchers say. Clinical trials may reveal whether this could one day end the need for a transplant altogether.
The Patch and Other Treatments
What's remarkable about the heart muscle patches is they can be engineered from induced pluripotent stem cells (iPSCs), as opposed to embryonic stem cells, which present ethical issues. iPSCs are reprogrammed from adult stem cells to become any cell type in the body – in this case, cardiomyocytes in the heart.
Arrhythmia or tumor growth can occur when pluripotent stem cells are put into tissue, as the organ may reject the new cells, or the heart may develop issues with its own electrical system. But that didn't happen with the new heart patch.
As to whether these patches can be brought up to scale, pharmacologist Sian Harding, PhD, was certain they could. "I've made these patches myself," said Harding, an emeritus professor of cardiac pharmacology at Imperial College London, and author of The Exquisite Machine: The New Science of the Heart. She estimated a cost of about $15,000 "to make a really solid piece of tissue about the size of your palm that would cover up the heart damage from a heart attack." That's not unreasonable for a heart implant, she said.
Right now, the most common treatments for people with heart failure other than transplant include medications, mechanical devices, bypass operations, and lifestyle changes.
ACE inhibitors help relax blood vessels and decrease blood pressure. Beta-blockers block the effects of adrenaline and noradrenaline on the heart to decrease heart rate, blood pressure, and heart contraction force. Sodium-glucose transporter-2 (SGLT-2) inhibitors like Jardiance lower blood sugar levels and reduce the risk of heart attacks or strokes.
These medications are generally well-tolerated and effective, but they can become less effective over time or cause adverse effects. They may also not be effective enough for those who certainly will need a transplant at some point in the future.
Meanwhile, mechanical devices like left ventricular assist devices – or LVADs, for short – have been shown to be lifesaving but are invasive. LVADs are implanted during open-heart surgery and take blood from the left ventricle of the heart and pump it into the aorta, where it can then be pumped throughout the rest of the body. This improves blood flow and reduces heart failure symptoms.
"In the United States right now, the main things we rely upon are mechanical assist devices for pump failure, whether that be as a therapy for heart failure by itself, or as a mechanism to get them towards transplant later on in life," said Sounok Sen, MD, a cardiologist and assistant professor of medicine at Yale School of Medicine.
LVADs have been proven to improve quality and quantity of life, although there are trade-offs, said Sen. Patients need to accept living with a pump and understand they will have to carry equipment around with them, along with batteries and controllers. Plus, patients may still develop right-sided heart failures.
LVADs "require lines that go into the body and need to have an external battery pack, which you've got to carry around with you, and charge up," Harding said. "You can get chronic infection from the lines."
Solving the Transplant Shortage
The biggest barriers to heart transplantation in the United States include the maze of treatments and operations available, late referral for heart transplantation, and a disorganized organ transplant system. In the U.S., about 3,000 adults and 400 children are on waiting lists for a heart transplant. More than 100,000 patients are awaiting transplants of any kind, and more than 28,000 donated organs go unused because of flaws in the system.
Every day, an average of 13 people die in the United States awaiting organ transplants of all kinds. According to Sen, health care professionals are up against a host of conditions that encompass heart failure, such as coronary disease and arrhythmia, so patients feel lost as to what to do.
"One of the challenges we face is that there are many treatment options for patients with cardiovascular disease and heart failure," said Sen. "Patients can oftentimes find themselves in a maze of different options, where transplantation is not even one of them." As a result, many patients are referred late for consideration of transplantation, potentially losing years of life.
Spreading the word about transplantation would help. "I don't think it's one element that can be fixed," said Sen. But in the U.S., there's plenty of room to improve the number of organ donors and improve the organ donation system. "I think we have a lot of opportunities to improve access to solid organs … heart transplantation, lung transplantation, kidney, liver, et cetera," he said.
For one thing, the organ donor system here is "opt-in": People must actively choose to become organ donors by registering with their state's donor registry or indicating their wishes on their driver's license. In contrast, England, Scotland, and Wales operate an "opt-out" system; it's assumed you'll donate unless you say otherwise.
If the United States switched to an opt-out system, it could be a big step in easing the shortage.
Said Sen, "If I had to pick one thing we can show our communities, it's the kind of amazing transformation that organ donation does to a single person in that community. We can look at data and wait list times, and that's objective and compelling, but at the end of the day, we're impacting people one life at a time."
