The badge goes into sleep mode when the sensor detects that the badge has been taken off a live person, but is kept “awake” by muscle tremor, gross bodily movement, or even vibration from the pulse. The standard sensor can reveal detail of heart “sounds” when held against the chest using a light elasticated strap. A variant of this component detects vital signs when a “smart badge” (equipped with RF telemetry) is worn by a staff member or patient and enables a periodic signal to be transmitted for tracking or location purposes. The film element bends as the mass stands still, leading to extremely high voltage sensitivity (around 1 V/g). The Minisense 100 is a standard component that employs a mass-loaded cantilever design with rigid PCB material forming the clamp and allows the mounting of the connection pins. The added mass causes the sensor to respond inertially under the influence of acceleration. As an example, Figure 2 shows the response of the sensor while an object was gripped then released repeatedly, showing an amplitude of around 3V open-circuit or around 250 με. Flexing or rotating the wrist could generate much higher amplitude signals, especially when a lower frequency limit was selected on the preamplifier. The output of approximately 130 mV pk-pk corresponds to an open-circuit voltage of around 100 mV pk-pk, that in turn, can be interpreted as a dynamic strain of roughly 8 με. This signal was recorded while the hand was at rest. It is also extremely durable, capable of withstanding hundreds of millions of flexing cycles, and shock resistant.Įxploiting the properties of piezo film as a “dynamic strain gauge,” an element can be easily bonded directly onto the skin (e.g., on the inside of the wrist). TE manufactures a general purpose sensor that already has pressure-sensitive adhesive on one side, but the adhesive is not rated for biocompatibility, so for short-term trials, a patch of 3M 9842 (thin polyurethane tape with adhesive coating) was fixed on the skin and the piezo film sensor applied on top. Figure 1 shows a pulse signal, obtained using a charge amplifier with low frequency limit set to 1 Hz and a sensitivity of 1 mV/pC. It is obviously a technology of great interest where available power is limited (it can even generate tiny quantities of power in some configurations). The film sees only time-varying changes in strain, with a frequency response that may start from as low as 0.1 Hz. Piezo film is also lightweight, thin, and highly flexible, and requires no external power in order to function. This unique combination of properties enables a wide range of medical applications to be addressed where very low-level mechanical signals must be detected. This can prove to be a positive advantage in cases where the sensor may be placed under varying levels of pre-load. When a strip of piezoelectric PVDF polymer film (piezo film) is stretched, it generates an electrical signal (charge or voltage) between upper and lower electrode surfaces, proportional to the amount of elongation. Piezoelectric materials are generally thought of as responding to pressure, but in the case of piezo film, the geometry of the element means that very high stress levels can be achieved in the cross-section of the film when very low force is applied in the longitudinal direction, while the same level of force applied over a larger area to the surface of the film would generate far lower stress. Therefore piezo film demonstrates phenomenal sensitivity to dynamic strain, typically in the region of 10 to 15 mV per microstrain (ppm change in length) for 28 μm thickness PVDF. The phrase “dynamic” strain is used because electrical charge generated by a change in strain leaks away into the electrical circuit connected to the film, and so static conditions of strain cannot actually be detected.
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