Cardiac arrhythmias, or heart rhythm disorders, occur when the heart beats too quickly or too slowly. While some cases are not severe, many can lead to heart failure, stroke, or even sudden death. Arrhythmias are often treated with implantable pacemakers and defibrillators that detect and correct abnormal heartbeats using electrical stimulation. However, the rigid nature of these devices can limit the heart's natural movements, injure soft tissues, and cause discomfort and complications such as swelling, perforations, blood clots, and infections. Researchers have now developed the first cardiac implant using graphene, a two-dimensional super-material known for its strength, lightweight, and conductive properties.

Developed by researchers at Northwestern University (Evanston, IL, USA), the graphene "tattoo" implant resembles a child's temporary tattoo but functions like a traditional pacemaker despite being thinner than a single strand of hair. In comparison to the current pacemakers and implanted defibrillators made of rigid materials that are mechanically incompatible with the human body, the new device gently melds to the heart to both sense and treat irregular heartbeats at the same time. In addition to being thin and flexible enough to conform to the heart's delicate contours, the implant is also sufficiently stretchy and strong to tolerate the dynamic motions of a beating heart.

The researchers sought to create a bio-compatible device that could conform to soft, dynamic tissues. After considering various materials, they chose graphene, an atomically thin form of carbon with potential applications in high-performance electronics, high-strength materials, and energy devices. The team was already developing graphene electronic tattoos (GETs) with sensing capabilities that adhere to the skin and continuously monitor vital signs, including blood pressure and electrical activity of the brain, heart, and muscles. However, they needed to explore new methods for using these devices inside the body, directly on the heart's surface.

To achieve this, the researchers developed a new technique to encase the graphene tattoo and adhere it to a beating heart's surface. They encapsulated the graphene in a flexible, elastic silicone membrane with a hole providing access to the interior graphene electrode. They then placed gold tape (10 microns thick) onto the encapsulating layer to serve as an electrical interconnect between the graphene and external electronics used to measure and stimulate the heart. The entire thickness of all layers together is about 100 microns, making it the thinnest known cardiac implant.

In a rat model, the researchers demonstrated that the graphene tattoo could successfully sense irregular heart rhythms and deliver electrical stimulation via a series of pulses without constraining or altering the heart's natural motions. In addition, the technology is optically transparent, enabling the researchers to perform optocardiography - using light to track and modulate heart rhythm - in the animal study. This approach offers a new way to diagnose and treat heart ailments and opens possibilities for optogenetics, a method for controlling and monitoring single cells with light. While electrical stimulation can correct abnormal heart rhythms, optical stimulation provides greater precision, allowing researchers to track specific enzymes and examine particular heart, muscle, or nerve cells.

“One of the challenges for current pacemakers and defibrillators is that they are difficult to affix onto the surface of the heart,” said Northwestern’s Igor Efimov, the study’s senior author. “Defibrillator electrodes, for example, are essentially coils made of very thick wires. These wires are not flexible, and they break. Rigid interfaces with soft tissues, like the heart, can cause various complications. By contrast, our soft, flexible device is not only unobtrusive but also intimately and seamlessly conforms directly onto the heart to deliver more precise measurements.” 

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