Wetness sensors

Abstract

A wetness sensor includes a self-supporting substrate and an electrically conductive trace carried by the substrate. The trace is patterned to provide at least a portion of a tuned RF circuit, which may be disposed on only one side of the substrate and characterized by an impedance or resistance. The trace is not self-supporting. The substrate is adapted to dissolve, swell, or otherwise degrade when contacted by a target fluid. Such degradation produces a drastic change in the operation of the RF circuit, which can be interpreted by a remote reader as a “wet” condition. Contact of the substrate by the target fluid may change the impedance or resistance of the RF circuit by at least a factor of 5, 10, 100, or 1000, and/or may cause the trace to disintegrate so as to provide the RF circuit with an open circuit, and/or may substantially render the RF circuit inoperative.

1 . A sensor, comprising:
 a self-supporting substrate; and   an electrically conductive trace carried by the substrate and patterned to provide at least a portion of a tuned RF circuit, the RF circuit being characterized by an impedance or resistance and being disposed on only one side of the substrate;   wherein the electrically conductive trace has a thickness of less than 1 micron; and   wherein the substrate is adapted to dissolve, swell, or otherwise degrade when contacted by a target fluid, such that the impedance or resistance of the RF circuit changes by at least a factor of 5.   
     
     
         2 . The sensor of  claim 1 , wherein the substrate dissolves when contacted by the target fluid. 
     
     
         3 . The sensor of  claim 1 , wherein the substrate swells when contacted by the target fluid. 
     
     
         4 . The sensor of  claim 1 , wherein the substrate is or includes a flexible film having a smooth major surface. 
     
     
         5 . The sensor of  claim 1 , wherein the substrate is or includes polyvinyl alcohol (PVA). 
     
     
         6 . The sensor of  claim 1 , wherein the target fluid comprises a polar liquid. 
     
     
         7 . The sensor of  claim 1 , wherein the target fluid comprises water. 
     
     
         8 . The sensor of  claim 7 , wherein the target fluid is or includes one or more aqueous human body fluids. 
     
     
         9 . The sensor of  claim 1 , wherein the electrically conductive trace comprises silver. 
     
     
         10 . The sensor of  claim 1 , wherein the electrically conductive trace has a thickness of less than 500 nanometers. 
     
     
         11 . The sensor of  claim 10 , wherein the electrically conductive trace has a thickness of less than 100 nanometers. 
     
     
         12 . The sensor of  claim 1 , wherein the electrically conductive trace includes an induction coil. 
     
     
         13 . The sensor of  claim 1 , wherein the electrically conductive trace is in intimate contact with the substrate. 
     
     
         14 . The sensor of  claim 1 , wherein the electrically conductive trace has a variable thickness and/or variable width. 
     
     
         15 . The sensor of  claim 14 , wherein the electrically conductive trace has a variable thickness, the thickness variation being associated with a structured interface between the conductive trace and the substrate. 
     
     
         16 . The sensor of  claim 1 , wherein the tuned RF circuit further includes an electrically conductive linking member that connects two portions of the electrically conductive trace. 
     
     
         17 . The sensor of  claim 1 , wherein the impedance or resistance of the RF circuit changes by at least a factor of 100 when the substrate is contacted by the target fluid. 
     
     
         18 . The sensor of  claim 17 , wherein the impedance or resistance of the RF circuit changes by at least a factor of 1000 when the substrate is contacted by the target fluid. 
     
     
         19 . The sensor of  claim 1 , wherein the electrically conductive trace substantially disintegrates so as to provide the RF circuit with an open circuit when the substrate is contacted by the target fluid. 
     
     
         20 . The sensor of  claim 1 , wherein contact of the substrate by the target fluid substantially renders the RF circuit inoperative. 
     
     
         21 . The sensor of  claim 1 , further comprising a skin-compatible adhesive disposed on an outer surface of the sensor. 
     
     
         22 . The sensor of  claim 21 , wherein the adhesive comprises silicone. 
     
     
         23 . An absorbent garment comprising the sensor of  claim 1 . 
     
     
         24 . The absorbent garment of  claim 23 , wherein the absorbent garment includes a liquid-permeable sheet, a liquid-impermeable sheet, and an absorbent material trapped between the liquid-permeable sheet and the liquid-impermeable sheet, and wherein the sensor is disposed between the liquid-permeable sheet and the liquid-impermeable sheet. 
     
     
         25 . A construction article comprising the sensor of  claim 1 . 
     
     
         26 . The article of  claim 25 , wherein the construction article is or includes wall board, insulation, flooring, roofing, and/or a fitting or support structure for a pipe. 
     
     
         27 . A system, comprising:
 the sensor of  claim 1 ; and   a reader configured to remotely assess a condition of the tuned RF circuit.   
     
     
         28 . The system of  claim 27 , wherein the reader is configured for mounting in or on a mobile or stationary support for a person.