Engineers have grown human heart tissue around a mesh of tiny generators; each heartbeat causes the generators to flex, producing a small electrical pulse—all without the use of a battery. The findings were published in the journal Science Advances.
A pacemaker relies on a battery that eventually drains, yet it is located inside an organ—the heart—that never stops working. In a laboratory setting, engineers grew living human heart tissue around a mesh of miniature generators. Every heartbeat flexed these generators, creating a small electrical pulse.
The researchers distributed the generators across the mesh rather than consolidating them into a single unit. According to the team’s calculations, this approach yields more than ten times the energy for a device of this size compared to previously developed energy-harvesting implants. None of this has yet been tested in a living organism.
“Our bodies are 24/7 power plants. Every cell generates its own energy,” says the study’s lead author, Sici Wang of the University of Massachusetts Amherst.
Pacemakers, cochlear implants, brain stimulators, and medical monitors all require a power source. Batteries are bulky, and attempts to make them smaller or more flexible inevitably result in a lower charge capacity.
Some devices worn on the body already operate without batteries—for instance, “smart” footwear that generates power with every step. An internal implant, however, cannot rely on footsteps. “People have long dreamed of a future in which certain electronic devices could expand our capabilities,” said study co-author Jun Yao of the College of Engineering. His laboratory has also created an artificial neuron designed to communicate with living cells.
Other groups have developed devices that convert the movement of a beating heart or a breathing lung into electricity—including a pacemaker powered by the heartbeat itself.
These energy-harvesting devices are typically designed as patches sutured onto an organ or housed within a sealed casing. Such packaging occupies space without generating any energy, and it can also irritate surrounding tissue.
In the body, every cell produces its own energy; thus, the energy supply is distributed throughout the tissue rather than originating from a single point.
“We wanted to shift away from this traditional, centralized paradigm toward something more distributed and based on biological principles,” Yao said.
Wang developed a mesh-like structure made of plastic ribbons, each narrower than a typical human cell. The entire assembly measures approximately 7 by 8 millimeters, with about 78% of its area consisting of open space. Along these ribbons are 64 tiny strips of a ceramic material called lead zirconate titanate, or PZT. Bending a PZT strip generates a voltage.
Thin plastic layers insulate each strip from the surrounding moist tissue, and groups of four strips—connected end-to-end by wires—form a single energy-harvesting unit. Before incorporating any other components, the team tested the strips on a flexible sheet; bending the sheet further resulted in an increase in voltage. When the bending occurred more rapidly, the electric current increased—just as the physics of piezoelectric material would predict. A plastic seal kept the cells submerged in a heart-cell culture medium for over two months, and the cells grown on the mesh remained viable.
The team converted human stem cells into heart muscle cells and seeded them onto the mesh. Over the following days, the cells coalesced into a thin, continuous layer that wrapped around the strips.
The tissue sheet then detached from the culture dish and curled up, primarily in the direction the mesh was designed to bend. Under a microscope, it was clear that most of the mesh was embedded within the tissue rather than simply resting on its surface. The cells filled the gaps in the mesh and also covered the PZT strips.
The tissue began to contract spontaneously, roughly once every two to three seconds. With each contraction, the tissue-harvesting device generated a corresponding electrical pulse. The pulse rose rapidly and decayed slowly—much like the way heart muscle contracts quickly and relaxes slowly.
The research team ruled out the cells’ own electrical activity and chemical drift within the fluid as potential sources of the observed signals. Because the pulse follows every heartbeat, the mesh can also function as a sensor, tracking each beat without the need for an onboard battery.
The initial version did not yield very good results. After accounting for signal loss in the cables and…
According to the measurement equipment, each generator produced about 9 millivolts per beat. During the experiments, the tissue stretched the strips approximately 100 times less than the team had stretched them on a test sheet.
Consequently, the team increased the thickness of the coating specifically where the PZT strips were located, making the PZT layer itself thicker, while the gaps between the strips remained thin. In a bent stack of layers, the material positioned further from the center undergoes greater stretching.
The tissue continued to contract and twist just as vigorously as before. The output voltage increased to approximately 450–750 millivolts. The energy from each pulse was negligible—estimated by the team at about 7 trillionths of a joule—yet each energy generator is roughly the size of a single cell.
Each piezoelectric strip converted motion into electricity with an efficiency of approximately 3.5%. This boost in power was achieved by eliminating bulk material that contributed no energy.
All of this took place within a thin layer of lab-grown tissue. The mesh was not implanted into any animal or human, and the team did not use the generated energy to power any device. Even the more substantial version of the setup failed to meet the team’s own expectations.
The researchers concluded that the cell layer—being so thin—likely lacked a sufficiently strong attachment to bend the strips to the same degree that the tissue itself was capable of bending.
The authors suggest that a thicker, three-dimensional tissue structure within the body might be able to hold the mesh more securely. A multi-layered mesh could potentially harvest energy from deeper layers of an organ, though this has yet to be tested.
“Our bodies tend to reject systems that include bulky batteries, but when a device exists at the cellular level, biocompatibility improves significantly,” said Yao.
The question remains as to how much better the body responds to this mesh compared to a bulky implant. The team proposes two ways to investigate this: either by growing tissue around the mesh in a laboratory and then transplanting it, or by surgically implanting the mesh directly.
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