Chengyi Xu, Ph.D., a researcher at the University of Alabama at Birmingham, is imagining a future without stethoscopes.
Xu, an assistant professor in the Department of Mechanical and Materials Engineering, is working on technology that could one day enable ultra-sensitive, ultrathin wearable sensors to capture tiny acoustic signals from a person’s body and transmit the data to a readable device.
Although the technology is admittedly a long way from being introduced in a clinical setting, Xu says his group’s research has uncovered potential technological advances that could lead to a variety of scientific applications. The group’s findings were recently published in the scientific journal Science Advances.
Good Vibrations
“Traditionally, when a person has a respiratory illness, the sound coming from the lungs changes,” Xu said. “It’s a tiny acoustic signal, but a doctor can hear it through a stethoscope. The same is true for the heart. There is a very small displacement that creates sound called ‘mechano-acoustic signals.’ Determining if those sounds are normal or abnormal is the first step towards making a diagnosis.”
Of course, there are many devices that can detect those same acoustic signals and transmit them to a readable device, but Xu says these technologies are often not ideal for continuous, long-term, and non-intrusive monitoring. “Today, patients typically need to visit a clinic, where physicians use a stethoscope to listen for subtle abnormalities in these weak acoustic signals. That approach provides only brief snapshots and cannot continuously track disease onset, progression, or recovery with high precision. It also depends heavily on the clinician’s experience and hearing ability, which can vary substantially. Although electronic stethoscopes can record and upload data, they are still not well suited for prolonged wear because their bulk and rigidity can compromise comfort and restrict mobility.”
In order to make such a device worthwhile, Xu says, “The electronic device we need instead is a sensor with ultrahigh sensitivity to capture the tiniest abnormalities, while also being ultrathin and ultralow-profile--ideally like a second skin--so patients can wear it comfortably for extended periods. To do this, we need to do two different things: One, we need to develop an electronic material that can support this type of ultrasensitive, ultralow-profile sensors; and two, we need to engineer the mechanism that will allow us to combine this material with a functional device. So, there are material, mechanical, and system-level integration aspects that would all have to come together.”
Establishing a Base Material and Proof of Concept
Xu and his collaborators (from Stanford University and the Honda Research Institute, USA) are currently working on the first step of that process, developing a material and studying the fundamental mechanism of how it behaves. The material is made up of an atomically thin ribbon network made of molybdenum disulfide (MoS2). “Each ribbon is only 2-5 microns wide, Xu said. “In contrast, a human hair is nearly ~100 microns in diameter, so this is 20 to 50 times smaller in width. In thickness, the ribbon is only 0.8 nanometer—basically a single layer of atoms (that is why MoS2 is called a 2D material).”
Despite its tiny size, Xu and the team have been able to control the growth of this material into unique ribbon networks and transfer them onto ultrathin, soft elastomer substrates. “The material is intrinsically bendable,” Xu said. “And when it is bent, it modulates how charge carriers (essentially electrons) flow through the network, and this effect produces a record-high change in electrical resistance, explaining why it can respond to tiny strains. Meanwhile, the distinct network architecture enables exception mechanical robustness under repeated bending cycles compared with traditional film-based sensors.”
As a proof of concept, Xu’s team moved to validate the use of this new material in dynamic sensing applications. “We wanted to see whether an ultrathin soft sensor like this could detect the tiny mechanical signals involved in everyday human activities,” Xu said. The researchers demonstrated its function across three key frequency ranges: slow signals such as breathing and airflow, mid-range vibrations generated by touch and contact, and higher-frequency signals associated with the human voice. Its high sensitivity also enabled the decoupling of complex mixed signals. “That means one platform could potentially monitor everything from respiration to touch interactions to speech, all in a skin-like format.”
The next step will be translating this technology for the continuous measurement of human physiological signals, which requires further optimization of device performance under real-world operation conditions. That includes improving long-term sensing stability, developing portable signal acquisition and power management systems, and designing encapsulation strategies to enhance durability during extended wear.
That process, Xu says, will not be a rapid one. He first began this research through a project funded by the Honda Research Institute (USA) when he was a research staff scientist at Stanford University. He hopes to attract additional funding partners to help advance the technology at UAB, but says there is still exciting work to be done at both fundamental materials science and device levels. “Already, we are seeing that our single-ribbon layer network on a soft substrate shows very high sensitivity under small mechanical strain, and it outperforms other state-of-the-art materials,” he said, “That opens the door to a new generation of skin-like imperceptible electronics that could one day listen to the body, respond to the environment, and merge sensing into our everyday life.”