To detect and treat certain heart-rhythm disorders—particularly arrhythmias like atrial fibrillation—the most advanced technologies today use flexible integrated electronics placed on or inside the heart. But blood and other bio-fluids can leak through the electrodes’ thin films, degrading them and potentially causing adverse health effects. To make these technologies suitable for long-term electrophysiology, researchers have created an ultrathin, biocompatible dielectric layer to encompass flexible electronics. Described recently in Nature Biomedical Engineering, the new leakage-free devices can provide accurate maps of the heart’s electrophysiological activity using capacitive sensing rather than through direct current flow.

“We have the technology that can last for up to 70 years and solve real cardiac problems,” says study co-lead author Igor Efimov, a biomedical engineer at the George Washington University. “It can be applied directly to the heart and sense excellent signals, allowing for very high resolutions of arrhythmia and precise diagnostics.”

A more advanced form of electrocardiograms (in which the heart’s electrical activity is measured from the surface of the skin), electrophysiology is routinely conducted to help detect and treat heart-rhythm disorders, which can be dangerous. Atrial fibrillation (literally quivering heartbeat), for example, can lead to blood clots, stroke, and heart failure. There are some 3 to 5 million people in the United States alone with atrial fibrillation and drugs can only help about 15% of them, Efimov says. For everyone else, electrophysiology mapping combined with ablation (surgical removal, such as by heat energy) is necessary.

To perform an electrophysiological exam, doctors insert a long catheter into the groin area and thread it up to the heart; next, they insert an electrode through the catheter and use it to take multiple readings of heart muscle activity. These individual readings are then synthesized on a computer to create a map of the heart’s electrical activity. “But the current techniques are ineffective and take a lot of time,” Efimov says.

More recent technologies, such as those previously developed by Efimov’s co-lead author John Rogers of Northwestern University, take a more holistic approach. These technologies use flexible integrated electronics applied directly to the heart for multiplexed sensing, allowing a map to be generated from hundreds of simultaneous measurements. But this approach requires electrical pads to be in direct physical contact with heart tissue, with electrical signals being passed through openings in the underlying electronics. Blood can penetrate through the polycrystalline metal films of the electrodes, causing the metal to corrode and hardware to fail. This issue makes the devices unsuitable for human use, especially for use in the long-term electrophysiological readings necessary for people with issues like congenital heart problems.

To avoid this problem, Rogers thought to encapsulate the electrodes in a barrier layer that prevents bio-fluids from penetrating into the sensitive electronics. To create the new device, Rogers and his colleagues began by grinding a silicon-on-insulator wafer down to 200 µm thickness. They defined 792 silicon metal-oxide semiconductor transistors into the wafer, using a series of deposition, etching, and photographic patterning steps to create dielectric and metal layers for the interconnects and sensing electrodes. They bonded a polyimide layer on top of the electronics to create a thin, flexible system, with the buried oxide layer of the wafer serving as the capacitive interface and encapsulation layer. In all, the system has 396 multiplexed capacitive sensors, each consisting of two underlying silicon nanomembrane transistors; one transistor in each pair connects from its gate electrode to a metal pad. The entire top surface of the system is covered with a thermally grown 200-nm-thick layer of silicon dioxide.

“Interestingly, you usually think of glass as something rigid and brittle, but if you make it few micrometers [thick], it’s very flexible—almost as much as paper,” Efimov says. “It makes a good protective layer that insulates the electronics from fluids but does not affect capacitive sensing capabilities.”

In experiments, the researchers found that the current-leakage levels of their device were four orders of magnitude smaller than other flexible-electronics technologies. And its operational lifetime was two to three orders of magnitude longer. When tested ex vivo on rabbit hearts, they found they could produce high-definition spatiotemporal electrophysiology maps by plotting signals from all 396 nodes as a function of time. The devices also worked well in in vivo tests of a canine heart, and they could even be used for the high-speed electrophysiology of other small organ systems, specifically a rat’s auditory cortex. Furthermore, the new capacitive high-density sensing device succeeded where previous flexible passive electrode arrays failed (due to insufficient spatial density), mapping and modeling the heart activity patterns of ventricular fibrillation.

The new technology is “very important for long-term electrophysiological monitoring,” says Dae-Hyeong Kim, a biomedical engineer at Seoul National University who was not involved in the research. “The current technology is using capacitive sensing to measure electrophysiological signals, and thereby all electrodes can be fully encapsulated by polymeric layers. This strategy dramatically enhances the protection of flexible electronics even under long-term implantation.”

The new devices will allow doctors to quickly identify the source of a patient’s ventricular fibrillation or other arrhythmia, and then treat it with ablation techniques, Efimov says. The devices may also improve implantable pacemakers and defibrillators, which have just a few electrodes that only provide low-resolution data and sometimes result in inappropriate therapy, he adds.

Read the article in Nature Biomedical Engineering.