Flexible Distributed Pressure Sensing Strip for a Urethral Catheter

Abstract

A multi-sensor flexible strip is developed for a urethral catheter to measure distributed pressure in a human urethra. The developed sensor strip has important clinical applications in urodynamic testing for analyzing the causes of urinary incontinence in patients. There are two major challenges in the development of the sensor. First, a highly sensitive sensor strip that is flexible enough for urethral insertion into a human body is required and second, the sensor has to work reliably in a liquid in-vivo environment in the human body. Capacitive force sensors are designed and micro-fabricated using polyimide/PDMS substrates and copper electrodes. To remove the parasitic influence of urethral tissues which create fringe capacitance that can lead to significant errors, a reference fringe capacitance measurement sensor is incorporated on the strip. The sensing strip is embedded on a catheter and experimental in-vitro evaluation is presented using a bench-top pressure chamber. The sensors on the strip are able to provide the required sensitivity and range. Preliminary experimental results also show promise that by using measurements from the reference parasitic sensor on the strip, the influence of parasitics from human tissue on the pressure measurements can be removed.

Keywords: capacitive sensors; catheter pressure sensors; in vivo force sensors; in vivo sensors; instrumented catheter; parasitic capacitance; urethral sensors.

Fig. 1

Foley catheter inserted into urethra.

Fig. 2

Instrumented catheter with sensors.

Fig. 3

Instrumented catheter with sensors (top view).

Fig. 4

Mechanical model of a capacitor.

Fig. 5

Mechanical model of a capacitor's electrode.

Fig. 6

Top electrode is assumed to deflect linearly due to applied pressure.

Fig. 7

Flexible capacitive sensor layers.

Fig. 8

Electrode fabrication steps.

Fig. 9

Fabricated electrodes on translucent flexible substrate.

Fig. 10

Fabrication process of flexible pressure sensor.

Fig. 11

Three fabricated designs for top electrode: (a) spring shaped, (b) elliptical: a dashed rectangle is drawn to show the etched window in PDMS, (c) rectangular. PDMS cavity can be seen in (b) and (c).

Fig. 12

(a) top: 4 degree of freedom aligner to move and align top and bottom electrodes, (b) bottom: backlight that is used for aligning top and bottom layers.

Fig. 13

PCB interfaced to sensor showing major electronic components.

Fig. 14

(a) Foley catheter, (b) cross sectional view of the catheter.

Fig. 15

Schematics of the sensor on the catheter.

Fig. 16

(a) top: assembled sensor on catheter, (b) bottom: the flexible sensor is bent.

Fig. 17

Pressure chamber, it can apply pressure in [0,5] psi range.

Fig. 18

5 times, P = 0.1 (psi) sensors’ response from C₁ to C₉.

Fig. 19

Sensor strip response (a) response of the 9 rectangular sensors to P = [0.15 0.3 0.5 0.8 1.2 1.5 2 3] (psi), five tests. (b) response of the 9 elliptical sensors to P = [0.15 0.3 0.5 0.8 1.2 1.5 2 3] (psi), five tests.

Fig. 20

Sensors’ response and the response of the reference electrode.

Fig. 21

Human body's effect on capacitance (5 tests).

Fig. 22

Covering the sensor with more tissue does not change the capacitance.

Fig. 23

Uncompensated signal (blue), reference signal (red) and cleaned signal (black).