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Poster
PI-013
Calibration and Accuracy of a Wearable Pressure Sensor.
Introduction: For the Wound and Lymphedema communities, a Wearable Pressure sensor (WPS) -- to evaluate dressings and teach practitioners best wrapping techniques -- could be transformational. To adopt such an innovation, the proposed WPS must be clinically accurate. The goal of this poster is to describe the accuracy of such a Wearable Pressure Sensor.Methods:
The wearable pressure sensor (WPS) includes a proprietary capacitive-based sensor (Reference 1). Sensor information is read by a system on a chip (SOC), which transmits pressure information by Bluetooth to an iPhone. This sensor is thin, flexible, and allows excellent stability and battery life ( >2 months). The WPS has a dedicated iPhone beta test App. This App has a novel pressure gauge display and in addition shows pressures from multiple sensors simultaneously (Reference 2).
Calibration: We used a validated (Reference 3) pneumatic sensor (2 cm dia. balloon) as the gold standard (GLD-STD)^. The WPS is calibrated against the GLD-STD. A large blood pressure cuff applies pressure of 0-110 mm Hg low-to-high in 10 mm Hg increments. An WPS algorithm calculates pressure curves from the GLD-STD relative to WPS. Leg model: Empirically, a human leg provides the best mechanical stress-strain characteristics for calibration and garment testing. The first author used his left leg for all measurements.
Garment testing: There are 4 levels of compression: Stocking (5-10- or 10-20-mm Hg), plus 0, 1,2, or 3 “E” size tubular bandages. Deviation of a WPS reading from known pressure is described as +/- % ERROR, as follows:
+/-% ERROR = 100*absolute value (WPS PRESSURE - GLD-STD PRESSURE)/GLD-STD PRESSURE
We used 9 calibrated wearable sensors: Three sensors for 2 tests; one for 3 tests; 5 for 4 tests.
Results:
%ERROR of the WPS relative to GLD-STD is as follows:
Compression level (mm Hg)
+/- % ERROR
Standard deviation
n
0-9
22%
0.3%
2
10-19
14%
3.5%
4
20-29
9%
3.0%
7
30-39
8%
3.0%
3
Discussion:
For this work, accuracy is defined as +/-%ERROR of a test sensor off-target, with the “bulls eye” being the reading of a “gold standard” sensor. Empirically, error of +/-15% is deemed adequate for clinical use. For the Wearable Pressure Sensor, this level of accuracy occurs at >20 mm Hg compression. Most compression garments and dressings exert >20 mm Hg. Accuracy improves as pressure increases. At the low end, pressure of