(Nanowerk News) Researchers led by Northwestern University and the University of Texas at Austin (UT) have developed the first cardiac implant made of graphene, a super two-dimensional material with extremely strong, light and conductive properties.
Similar to children’s temporary tattoos, these new graphene “tattoo” implants are thinner than a hair but still function like a classic pacemaker. But unlike today’s pacemakers and implantable defibrillators, which require hard, rigid materials that are mechanically incompatible with the body, these new devices gently fuse with the heart to simultaneously sense and treat irregular heartbeats. The implant is thin and flexible enough to fit the delicate contours of the heart and elastic and strong enough to withstand the dynamic movement of the beating heart.
After implanting the device into a mouse model, the researchers showed that the graphene tattoo managed to sense an irregular heart rhythm and then deliver electrical stimulation via a series of pulses without restricting or changing the heart’s natural movement. Even better: The technology is also optically transparent, allowing researchers to use an external optical light source to record and stimulate the heart through the device.
The study was published in the journal Advanced Materials (“Graphene biointerface for the diagnosis and treatment of cardiac arrhythmias”). This marks the thinnest cardiac implant known to date.
“One of the challenges for today’s pacemakers and defibrillators is that they are difficult to attach to the surface of the heart,” said Igor Efimov of Northwestern, senior author of the study. “Defibrillator electrodes, for example, are basically coils made of very thick wires. This cable is not flexible, and breaks. Rigid interfaces with soft tissues, such as the heart, can cause various complications. Instead, our soft and flexible device is not only unobtrusive but also intimately and seamlessly adjusts directly to the heart to provide a more precise measurement.”
An experimental cardiologist, Efimov is a professor of biomedical engineering at Northwestern McCormick School of Engineering and professor of medicine at Northwestern University’s Feinberg School of Medicine. He led the research with Dmitry Kireev, research associate at UT. Zexu Lin, Ph.D. candidate in Efimov’s lab, was the paper’s first author.
Known as cardiac arrhythmias, heart rhythm disturbances occur when the heart beats too fast or too slow. While some cases of arrhythmias aren’t serious, many cases can lead to heart failure, stroke, and even sudden death. In fact, complications associated with arrhythmias claim an estimated 300,000 lives each year in the United States. Doctors generally treat arrhythmias with implantable pacemakers and defibrillators that detect abnormal heartbeats and then correct the rhythm with electrical stimulation. While these devices save lives, their rigid nature can restrict the heart’s natural motion, injure soft tissues, cause temporary discomfort and lead to complications, such as painful swelling, perforation, blood clots, infection and more.
With these challenges in mind, Efimov and his team set out to develop bio-compatible devices that are ideal for adapting to soft and dynamic tissues. After reviewing various materials, the researchers settled on graphene, an atomically thin form of carbon. With its extremely strong structure, light weight and superior conductivity, graphene has the potential for many applications in high-performance electronic devices, high-strength materials and energy devices.
“For reasons of bio-compatibility, graphene is very attractive,” said Efimov. “Carbon is the basis of life, so it is a safe material that is already being used in a variety of clinical applications. It is also flexible and soft, which works well as an interface between electronics and mechanically active organs and is gentle.”
Hit the hitting target
At UT, study co-authors Dimitry Kireev and Deji Akinwande have developed graphene electronic tattoo (GET) with sensing capabilities. Flexible and lightweight, their team’s e-tattoos adhere to the skin to continuously monitor the body’s vital signs, including blood pressure and the electrical activity of the brain, heart, and muscles.
However, while e-tattoos work well on the surface of the skin, Efimov’s team needed to investigate a new method for using these devices inside the body — directly to the surface of the heart.
“This is a completely different application scheme,” said Efimov. “Skin is relatively dry and easy to reach. Obviously, the heart is in the chest, so difficult to access and in a wet environment.”
The researchers developed an entirely new technique for wrapping graphene tattoos and attaching them to the surface of a beating heart. First, they encased the graphene inside a flexible, elastic silicon membrane — with holes punched to provide access to the interior graphene electrodes. Then, they gently placed a gold band (10 microns thick) onto the encapsulation layer to serve as the electrical link between the graphene and the external electronics used to measure and stimulate the heart. Finally, they put it in the liver. The overall thickness of all coatings as a whole measures about 100 microns.
The resulting devices are stable for 60 days in an actively beating heart at body temperature, which is comparable to the duration of temporary pacemakers used as a bridge to permanent pacemakers or rhythm management after surgery or other therapy.
Taking advantage of the device’s transparent nature, Efimov and his team performed optocardiography – using light to track and modulate the heart’s rhythm – in animal studies. This not only offers a new way to diagnose and treat heart disease, this approach also opens up new possibilities for optogenetics, a method for controlling and monitoring single cells with light.
Electrical stimulation can correct abnormal heart rhythms, whereas optical stimulation is more precise. With the light, researchers can track certain enzymes as well as interrogate certain heart, muscle or nerve cells.
“We can essentially combine electrical and optical functions into a single biointerface,” said Efimov. “Because graphene is optically transparent, we can actually read it, which gives us a much higher reading density.”