The core mechanism of a miniature sensor on a chip incorporates two layers of silicon that overlay each other separated by the space of 270 nanometers — about 0.005 the width of a human hair. They carry a minute voltage. Vibrations from bodily motions and sounds put part of the chip in flux, making the voltage flux and creating readable electronic outputs. In human testing, the chip has recorded a variety of signals from the mechanical workings of the lungs and the heart with clarity — signals that often escape meaningful detection by current medical technology.
The chip, which acts as an electronic stethoscope and accelerometer in one, is aptly called an accelerometer contact microphone. It detects vibrations that enter the chip from inside the body while keeping out distracting noise from outside the body's core like airborne sounds. If it rubs on skin or a garment, it doesn’t hear the friction but the device is very sensitive to sounds coming at it from inside the body, so it picks up useful vibrations even through clothing.
The detection bandwidth is enormous — from broad, sweeping motions to inaudibly high-pitched tones. Thus, the sensor chip records, simultaneously, fine details of the heartbeat, waves the heart sends through the body, and respiration rates and lung sounds. It even tracks the wearer’s physical activities such as walking. The signals are recorded in sync, potentially offering the big picture of a patient’s heart and lung health. For the study, the researchers successfully recorded a “gallop” — a faint third sound after the “lub-dub” of the heartbeat. Gallops are normally elusive clues of heart failure.
Though the chip’s main engineering principle is simple, making it work and then manufacturable was challenging, mainly because of the extremely tiny scale of the gap between the silicon layers, i.e. electrodes. If the 2 × 2-millimeter sensor chip were expanded to the size of a football field, that air gap would be about an inch wide. That very thin gap separating the two electrodes cannot have any contact — not even by forces in the air in between the layers — so the whole sensor is hermetically sealed inside a vacuum cavity.
The researchers used a manufacturing process called the HARPSS+ platform (High Aspect Ratio Poly and Single Crystalline Silicon) for mass production, running off hand-sized sheets that were then cut into the tiny sensor chips. HARPSS+ is the first reported mass manufacturing process that achieves such consistently thin gaps and it has enabled high-throughput manufacturing of many such advanced MEMS. The experimental device is currently battery-powered and uses a second chip called a signal-conditioning circuit to translate the sensor chip’s signals into patterned readouts. Three sensors or more could be inserted into a chest band to triangulate health signals to locate their sources. Someday a device may pinpoint an emerging heart valve flaw by turbulence it produces in the bloodstream or identify a cancerous lesion by faint crackling sounds in a lung.