Ultrasound Wearable Pacemaker Developed by MIT Engineers

The engineering here bridges mechanical engineering, biomedical engineering, and genetics, a combination that would have seemed unlikely even a decade ago.

Start with the physics. Ultrasound consists of pressure waves at frequencies above 20 kHz, beyond human hearing. Medical ultrasound typically operates between 1 and 15 MHz. At these frequencies, waves propagate through soft tissue with relatively little attenuation, which is why ultrasound imaging works so well for scanning organs deep inside the body. The pacemaker sticker operates at the lower end of this range, where penetration depth is greatest, allowing the pulses to reach the heart through several centimetres of chest wall.

The transducers themselves are piezoelectric elements, crystals that deform when voltage is applied and vibrate at the desired frequency. Students who have covered piezoelectricity in A-level Physics will recognise the inverse piezoelectric effect at work. The sticker embeds an array of these elements into a hydrogel adhesive layer that Zhao's group has refined over several years. The hydrogel serves two purposes: it bonds strongly to skin, and its acoustic impedance closely matches that of human tissue, minimising reflections at the skin surface and ensuring maximum energy transfer into the body.

The biological innovation, sonogenetics, borrows from optogenetics, a Nobel Prize-winning technique that makes neurons fire in response to light. Optogenetics requires fibre-optic cables implanted in the brain. Sonogenetics replaces light with sound, which penetrates tissue without any implant.

The team modified a specific mechanosensitive ion channel, a protein embedded in the cell membrane that acts as a gate. Normally, these channels respond to membrane stretching. The genetic modification lowers their activation threshold, so that the mechanical vibrations from ultrasound are sufficient to force them open. When they open, calcium ions (Ca2+) rush into the cell down their concentration gradient, a process A-level Biology students will recognise as facilitated diffusion through gated channels. The influx of calcium triggers the contractile machinery of the cardiac myocyte, the actin-myosin sliding filament mechanism, producing a heartbeat.

The closed-loop concept the team is pursuing adds another layer of engineering. Ultrasound imaging uses the same transducers in receive mode, listening for echoes rather than transmitting pulses. By alternating rapidly between transmit and receive, a single sticker could image the heart's motion, detect when the rhythm drifts, and deliver a corrective pulse within milliseconds. This is feedback control applied to biology, a concept familiar from A-level control systems in Design and Technology.

The clinical pathway involves a form of gene therapy using adeno-associated virus (AAV) vectors, the same delivery system approved for treatments such as Luxturna (for inherited blindness) and Zolgensma (for spinal muscular atrophy). The AAV carries the genetic instructions for the modified ion channels into cardiac cells, where they integrate and begin producing the sensitised channel proteins. This is a one-time procedure, and the modified channels remain functional for the life of the cell.

Manufacturing the sticker at scale presents its own challenges. Each piezoelectric element must be precisely positioned and acoustically coupled to the hydrogel layer. The electronics must be miniaturised to a wearable form factor while maintaining sufficient power output to drive the transducers. The team's current prototype uses an external battery pack, but future versions could integrate flexible batteries or energy-harvesting circuits.