Electronic stethoscopes, pulse sensors and BCG devices all do one thing: turn small mechanical vibration on the body surface into an electrical signal. Piezoelectric film has an underrated advantage here — its acoustic impedance is about 2.6× that of water, while piezo ceramics are roughly 11×. Signal transfer between film and skin is inherently more efficient.
Deployment scenario
The film becomes a 20×20 mm to 60×60 mm patch with medical adhesive backing, placed at the left sternal border (heart sounds), the carotid or radial artery (pulse) or the throat (voice vibration). A flexible FPC runs to a button-sized acquisition module (charge amplifier, ADC, BLE) recording for hours to days.
For clinical and out-of-hospital use it can be a single-use patch (low cost, no cross-infection) or a reusable textile-integrated version sewn into underwear or a wearable vest.
Achievable performance
The first (S1) and second (S2) heart sounds are clearly resolvable in the 20–200 Hz band, the basis for identifying valvular events; pulse waveforms yield heart rate and heart-rate variability; chest displacement from respiration is stably extracted at 0.1–0.5 Hz. Unlike microphones, piezoelectric film does not couple to air and is far less affected by ambient noise; unlike accelerometers it is thinner, more conformal and less prone to losing contact when posture changes.
Comparison of technical routes
| Aspect | PVDF film | Electret / MEMS microphone | MEMS accelerometer |
|---|---|---|---|
| Impedance match | ≈ 4 MRayl, close to tissue | Matched to air, mismatched to skin | Not applicable |
| Ambient noise | Barely affected by airborne sound | Picks up ambient noise directly | Highly affected by motion artefacts |
| Thickness & conformity | Tens of micrometres, conforms to skin | Needs a rigid cavity; thick | Rigid package; edges lift |
| Power | Passive | Requires bias supply | Requires supply |
| Limitations | Pyroelectric interference at low frequency; needs high-impedance front-end | Ambient and wind noise | Static attitude error; high power |
