Conformable thin film-based strain sensing with flexible substrate

1 . A strain sensor comprising:

a flexible substrate; and

a circuit disposed on the flexible substrate, the circuit comprising an inductance to receive an excitation signal, the circuit being configured to generate a radio frequency response to the excitation signal via the inductance;

wherein the circuit comprises an elongated trace coupled to the inductance and configured to bend and stretch longitudinally upon deformation of the flexible substrate, the elongated trace comprising a non-uniformity configured such that the elongated trace deforms and tears at the non-uniformity and exhibits a non-linear increase in resistance as a tensile strain to which the elongated trace is subjected reaches a strain threshold;

wherein the non-linear increase in resistance modifies a characteristic of the radio frequency response of the circuit.

2 . The strain sensor of claim 1 , wherein the non-uniformity comprises a non-uniform composition of the trace.

3 . The strain sensor of claim 1 , wherein the non-uniformity comprises a non-uniform thickness of the trace.

4 . The strain sensor of claim 1 , wherein:

the elongated trace comprises a plurality of metal layers; and

at least one metal layer of the plurality of metal layers is not present at the non-uniformity.

5 . The strain sensor of claim 1 , wherein:

the elongated trace comprises a conduction metal layer and an adhesion metal layer disposed between the conduction metal layer and the flexible substrate; and

the conduction metal layer is not present at the non-uniformity such that the elongated trace has a thickness at the non-uniformity that corresponds with a thickness of the adhesion metal layer.

6 . The strain sensor of claim 1 , wherein the trace is disposed along a curve arising from the deformation of the flexible substrate such that the tensile strain is a hoop strain.

7 . The strain sensor of claim 1 , wherein the characteristic of the radio frequency response is a phase of the radio frequency response generated by the circuit in response to the excitation signal.

8 . The strain sensor of claim 1 , further comprising a further circuit disposed on the flexible substrate, the further circuit comprising an inductor and a capacitor, wherein:

the inductor is configured to receive the excitation signal;

the capacitor comprises a parallel plate arrangement; and

the parallel plate arrangement is configured such that a radio frequency response of the further circuit to the excitation signal is modified by strain arising from further deformation of the substrate and the capacitor.

9 . The strain sensor of claim 1 , wherein the circuit further comprises a capacitance, both the inductance and the capacitance being disposed in series with the elongated trace.

10 . The strain sensor of claim 9 , wherein the inductance and the capacitance are positioned such that the deformation of the flexible substrate does not modify the inductance and the capacitance.

11 . The strain sensor of claim 1 , wherein the flexible substrate comprises a section on which the inductance is disposed and an elongated strip extending outward from the section, the elongated trace being disposed on the elongated strip.

12 . The strain sensor of claim 11 , wherein the elongated trace comprises a conductive loop disposed on the elongated strip.

13 . The strain sensor of claim 11 , wherein the flexible substrate comprises a biocompatible polymer substrate.

14 . A method of sensing hoop strain in connection with an object, the method comprising:

applying a strain sensor around a periphery of the object, the strain sensor comprising:

a flexible substrate; and

a circuit disposed on the flexible substrate, the circuit comprising an inductance, the circuit comprising an elongated trace coupled to the inductance, the elongated trace bending as the strain sensor is applied around the periphery of the object, the elongated trace comprising a non-uniformity configured such that the elongated trace tears at the non-uniformity and exhibits a non-linear increase in resistance as the hoop strain reaches a strain threshold;

directing an excitation signal to the inductance, the excitation signal causing the circuit to generate a radio frequency response via the inductance; and

monitoring the radio frequency response for a change in a characteristic of the radio frequency response to detect the non-linear increase in the resistance.

15 . The method of claim 14 , wherein applying the strain sensor comprises affixing ends of an elongated strip of the flexible substrate to the object, the elongated trace being disposed on the elongated strip.

16 . The method of claim 14 , wherein:

directing the excitation signal comprises generating the excitation signal across a range of frequencies; and

monitoring the radio frequency response comprises evaluating the radio frequency response to detect when the change in the characteristic of the radio frequencies occurs.

17 . The method of claim 14 , wherein the characteristic of the radio frequency response comprises a phase of the radio frequency response.

18 . The method of claim 14 , wherein applying the strain sensor comprises wrapping the strain sensor around a bone such that the hoop strain is indicative of circumferential bone growth.