Flexible Ultra-Thin Nanocomposite Based Piezoresistive Pressure Sensors for Foot Pressure Distribution Measurement

Abstract

Foot pressure measurement plays an essential role in healthcare applications, clinical rehabilitation, sports training and pedestrian navigation. Among various foot pressure measurement techniques, in-shoe sensors are flexible and can measure the pressure distribution accurately. In this paper, we describe the design and characterization of flexible and low-cost multi-walled carbon nanotubes (MWCNT)/Polydimethylsiloxane (PDMS) based pressure sensors for foot pressure monitoring. The sensors have excellent electrical and mechanical properties an show a stable response at constant pressure loadings for over 5000 cycles. They have a high sensitivity of 4.4 kΩ/kPa and the hysteresis effect corresponds to an energy loss of less than 1.7%. The measurement deviation is of maximally 0.13% relative to the maximal relative resistance. The sensors have a measurement range of up to 330 kPa. The experimental investigations show that the sensors have repeatable responses at different pressure loading rates (5 N/s to 50 N/s). In this paper, we focus on the demonstration of the functionality of an in-sole based on MWCNT/PDMS nanocomposite pressure sensors, weighing approx. 9.46 g, by investigating the foot pressure distribution while walking and standing. The foot pressure distribution was investigated by measuring the resistance changes of the pressure sensors for a person while walking and standing. The results show that pressure distribution is higher in the forefoot and the heel while standing in a normal position. The foot pressure distribution is transferred from the heel to the entire foot and further transferred to the forefoot during the first instance of the gait cycle.

Keywords: bio-medical applications; flexible sensors; foot pressure distribution; gait analysis; multi carbon nanotubes (MWCNT); nanocomposite sensors; polydimethylsiloxane (PDMS); pressure sensor; wearable sensors.

Figure 1

Schematic representation of fabrication of MWCNT/PDMS pressure sensor.

Figure 2

Raman spectra MWCNT/PDMS nanocomposite films.

Figure 3

Grindometer test for investigate agglomerates formation.

Figure 4

DC resistance of with and without encapsulation of the MWCNT/PDMS pressure sensors over day.

Figure 5

Sensor behavior for forward and reverse force cycle. Inside: Sensor behavior for pressure of 150 to 330 kPa.

Figure 6

Cyclic response for 5000 cycles.

Figure 7

Sensor response on different rate of load.

Figure 8

Drift test for MWCNT/PDMS pressure sensor at different pressure applied on it.

Figure 9

Schematic representation of fabrication of the in-sole with MWCNT/PDMS pressure sensor.

Figure 10

Pressure sensor layout in the sole.

Figure 11

Electro-mechanical behavior of CNT/PDMS pressure sensors implemented in the in-sole.

Figure 12

Positions in stationary phase.

Figure 13

Normalized resistance of sensors during position A.

Figure 14

Normalized resistance of sensors during position B.

Figure 15

Normalized resistance of sensors during position C.

Figure 16

Dynamic response of the pressure sensor during dynamic phase.