During surgery, a heartbeat doesn’t just tell whether a person is dead or alive— it can warn of big problems that come up quite suddenly. Keeping watch for subtle irregularities in the heart’s electric activity can help save a patient’s life. But today’s technology can’t give as much detail as doctors want because it degrades too easily—which can cause serious harm to the patient. That’s why John Rogers, professor of materials science and engineering at Northwestern University, collaborated with a team of researchers to develop a new type of sensor that is much safer and more refined, and could likely survive in the body about 70 years.
These paper-thin devices are an array of 396 voltage sensors set in a 9.5- x 11.5-millimeter multilayer flexible substrate that’s meant to attach to the outside of the heart, covering a significant portion of the organ. Previous sensors arrays picked up signals through direct contact between a metal conductor and human tissue, but the new array is covered with an insulating layer of impermeable silicon dioxide. The researchers described the invention this week in Nature Biomedical Engineering.
The array senses these signals when placed on a rabbit heart.Image: Northwestern University
The silicon dioxide is there to solve one of the biggest problems with implanted sensors. Metal conductors corrode and allow biological fluids to leak through, which can lead to a short circuit and have life-threatening impacts, inducing ventricular fibrillation and cardiovascular collapse.
“If water penetrates at any location across the entire area of the device, it’s game over because you’ll get electrical leakage that destroys the entire device,” says Rogers. “It becomes a serious health hazard that would preclude the use of these devices in humans.”
Instead of a direct current running through the sensor, this new model relies on capacitive coupling to read the signals coming from the heart. The heart tissue generates pulses of voltage, which the electrodes sense through the thin layer of silicon dioxide.
“You want this layer to be as thin as possible to enable a strong electrical coupling to the surrounding tissue, but you need it to be thick enough to serve as a robust barrier to water penetration,” says Rogers.
The green layer at top is the silicon dioxide that seals the multilayer device.Illustration: Northwestern University
Researchers have taken this approach before with passive electrodes, but for an active device large enough to monitor a heart and with this many elements, the added insulating layer has often been too thick for the signal to be recorded effectively. Rogers’s team managed to make it just right.
“Individually, none of the concepts were entirely new—there’s been lots of innovating in this area for a long time—but taken together it’s a very significant step,” says Polina Anikeeva, professor of materials science and engineering at MIT. “People have done individual neurons, and small groups of neurons, but not necessarily an entire heart.”
To create a layer thin enough to allow a strong signal through but dense enough to be leakproof, Rogers and his team made as flawless a piece of silicon dioxide glass as they could. Instead of depositing silicon dioxide directly onto the surface of the patch to seal it, a process that could result in an uneven surface, Rogers and his team grew it on a wafer of pure silicon. Growing it this way led to a smooth, leak-free layer of silicon dioxide. They built the other layers of the sensor on top of that and then peeled it off the silicon wafer.
The researchers ran a series of tests to ensure the sensors were flexible enough to adapt to the motion of a beating heart, impermeable enough to prevent leaks, and thin enough to record a strong electric signal. They submerged the sensor in salt water at high temperatures, to mimic the environment of the human body, and calculated that it could persist without leaking for up to seven decades. To prove the sensor’s flexibility and its ability to read an electric signal, they attached it to a beating rabbit heart and recorded the signals it produced.
Eventually, Rogers wants to scale up this technology. With a larger surface area and more nodes, the sensors could get big enough to cover most of the body’s organs. What’s more, the researchers will test whether the sensors can both collect data and deliver energy to an organ, like a pacemaker.
On top of that, a larger, denser version of the sensor array could prove extremely useful in studying the biology and function of organs–specifically the one that remains the most complex and mysterious: the brain.
“The brain has a much higher level of spatial granularity to its electrical function, so moving to a few hundred thousand electrodes in a mapping array could be really interesting in terms of shedding light on the underlying function of the brain,” Rogers says.