Improving the reliability of pressure ulcer prevention by sensing human bio-signals

Abstract

Pressure ulcers, also known as bedsores, are wounds that form when a person sits or lays in the same
posture for an extended period of time. Continued pressure being applied to the same spot causes
the skin to decay, possibly continuing into underlaying tissue if the wounds are not treated. A common
and proven practice to prevent pressure ulcers from forming is to regularly change the posture of the
patient, so that there is not too much pressure on any one part of the skin.
Momo Medical is creating a sensor system to assist in preventing pressure ulcers. They already
have a prototype using force-sensing resistor (FSR) and piezoelectric sensors to measure changes in
posture of the patient. The prototype is able to detect if a patient has moved enough, whether on their
own or by the nurses, to prevent pressure ulcers. The prototype works, but they need their system to
be more reliable.
The research in this thesis focuses on improving the piezoelectric sensors. Momo Medical uses
the piezoelectric sensors to measure human bio-signals, mainly respiration and heart rate, through the
mattress. Two different printed circuit boards (PCBs) were designed as test set-ups to be able to easily
modify the amplifier to measure the piezoelectric sensors separately. Using these test set-ups two
common piezoelectric sensors were compared, namely lead zirconate titanate (PZT) and polyvinylidene
difluoride (PVDF) sensors. From this comparison it was concluded that the PVDF sensors consistently
have a better signal-to-noise ratio (SNR), but that both the PZT and PVDF sensors were able to measure
human bio-signals effectively. The PVDF sensors did showed even more promising results when
using them in a different mechanical configuration.
In the test set-up the amplifier was also changed to improve the read-out of the sensors. Multiple
iterations on the amplifier design were tested. In the final design a non-inverting amplifier was chosen to
decouple the amplification from the input impedance of the amplifier and a input impedance of 100MΩ
was chosen. Because of the high input impedance in the final design the signal was dampened less
and less amplification was needed, thus reducing noise.