DESC:SUSTAINABLE STRAIN SENSORS FOR WIRELESS STRUCTURAL HEALTH MONITORING

TECHNICAL FIELD

[0001]The present disclosure generally relates to the field of structural health monitoring and non-destructive testing. Particularly, the present disclosure relates to biodegradable strain sensors comprising nanocomposite conductive layers on sustainable substrates for real-time wireless detection of mechanical deformations in industrial structures.

BACKGROUND

[0002] Structural systems across industries including civil infrastructure, aerospace, maritime platforms, and biomedical devices undergo continuous degradation from operational stresses, environmental factors, and material fatigue. This deterioration manifests as micro-cracks, strain accumulation, and structural deformation, potentially leading to catastrophic failures if undetected. Traditional inspection methods relying on periodic manual assessments prove inadequate for real-time monitoring of these progressive failure mechanisms.

[0003] Traditional strain sensors further rely on petrochemical-derived substrates like polyimide or silicone, which persist in ecosystems for centuries post-disposal. Conductive materials such as silver flakes or synthetic polymers further exacerbate electronic waste accumulation. Existing sensors also face limitations in sensitivity (gauge factors <10) and durability under cyclic loading

[0004] Modern structural health monitoring (SHM) systems employ strain sensors to detect early warning signs through continuous strain measurement. While these sensors find application in bridges, aircraft components, pipeline networks, and medical implants, conventional designs face critical limitations. Existing sensors predominantly use non-biodegradable substrates and synthetic composites that persist as environmental contaminants. Additionally, their complex manufacturing processes hinder large-scale deployment, while rigid architectures limit conformability to curved surfaces.

[0005] Current technologies fail to address the growing demand for eco-conscious monitoring technologies, particularly in applications requiring temporary implantation or environmentally sensitive deployment. Moreover, the lack of integrated wireless capabilities in many conventional strain sensors complicates data acquisition in remote or hazardous environments. Existing sensor designs also demonstrate insufficient sensitivity for detecting sub-millimeter scale deformations critical in aerospace and medical applications.

[0006] The patent document US8638217 describes a wireless sensor network for structural monitoring that harvests energy from strain or vibration. It employs piezoelectric materials to power sensor nodes deployed on rotating aircraft components or inaccessible locations. The system relies on non-biodegradable substrates and conventional metallic strain gauges. However the disclosed invention uses bacterial nanocellulose or TPU substrates that biodegrade within 180 days, addressing environmental challenges.

[0007] The patent document US11976989 discloses a biodegradable strain sensor using atomic/molecular layer deposition (ALD/MLD) on implantable substrates like screws. However, the ALD/MLD fabrication process requires vacuum conditions and specialized equipment, increasing production costs and complexity. The disclosed invention avoids these limitations through solvent-free screen-printing of graphene-CNT-PLA inks, enabling scalable manufacturing at lower costs than conventional methods.

[0008] There are no existing solutions to combine biodegradability with high strain sensitivity while maintaining cost-affordability. This gap necessitates sustainable sensor architectures that maintain performance parity with conventional systems while enabling environmentally friendly disposal.

[0009] The present invention overcomes these barriers through:

1. Hybrid conductive fillers: Coal-derived graphene and CNTs reduce percolation thresholds while enhancing electrical stability.
2. Screen-printable PLA emulsion ink: Enables direct patterning on flexible substrates without toxic solvents.
3. BLE 5.0 integration: Minimizes power consumption (<10 mW) for multi-year operation.

SUMMARY

[0010] Certain limitations of conventional strain sensing technologies are addressed by the present disclosure, while additional benefits are introduced through novel technical implementations. The disclosed invention provides advancements in material science, device architecture, and system integration to overcome environmental and operational constraints associated with structural health monitoring (SHM) systems.

[0011] A primary objective of the present disclosure is to provide biodegradable strain sensors capable of detecting minute structural deformations with high sensitivity while minimizing ecological impact through sustainable material composition.

[0012] Another objective includes developing a fabrication methodology for strain sensors that combines cost-effective production techniques with precision engineering to ensure consistent performance characteristics across diverse operational environments.

[0013] A further objective encompasses the integration of wireless data transmission capabilities with strain sensing components to enable real-time structural integrity monitoring without requiring physical access to installed sensors.

[0014] These objectives, along with complementary technical advantages, become apparent through detailed examination of the disclosed embodiments, material specifications, and experimental validation data presented in subsequent sections.

[0015] According to one implementation, the disclosure describes a biodegradable strain sensor apparatus comprising:
• A substrate layer selected from bacterial nanocellulose or thermoplastic polyurethane (TPU) membranes, demonstrating tensile strength between 15-40 MPa and elongation at break exceeding 200%
• A conductive ink formulation incorporating graphene derivatives with sheet resistance below 50 O/sq, applied through screen-printing techniques to achieve uniform thickness of 15±2 µm
• Electrical interface components comprising copper connectors with optional silver epoxy reinforcement, providing stable contact resistance below 0.5 O

[0016] In preferred embodiments, the sensor architecture demonstrates dimensional parameters of 60 mm × 4 mm with active sensing area variations between 30-50 mm². The conductive layer exhibits gauge factors ranging from 0.5 to 1.8 under tensile strain conditions, with resistance changes exceeding 42% during 90° bending deformations. Substrate selection determines specific performance characteristics, where TPU-based sensors show superior flexibility (42% resistance variation vs. 21% for nanocellulose) while maintaining cyclic stability over 1,000 deformation cycles.

[0017] The disclosure further details a fabrication process comprising four critical phases:
1. Substrate Preparation: Optimization of nanocellulose membranes through bacterial synthesis (Komagataeibacter xylinus strain) or TPU sheet formation via solvent casting
2. Conductive Ink Formulation: Homogeneous dispersion of carbon allotropes (graphene, graphite, coal-derived carbon) in polymer matrices achieving viscosity between 12,000-15,000 cP for screen-printing applications
3. Pattern Transfer: High-precision screen printing using 200-325 mesh screens under controlled environmental conditions (25±2°C, 45±5% RH)
4. Device Integration: Thermal curing at 60-80°C for 30-60 minutes followed by connector attachment using low-temperature soldering (<150°C)

[0018] A complementary system architecture enables wireless SHM capabilities through:
• Embedded Bluetooth Low Energy (BLE 5.0) transceivers with 100-meter line-of-sight range
• Custom mobile application software supporting real-time data visualization (sampling rate: 10 Hz) and strain history analytics
• Power management circuits enabling continuous operation for 72+ hours on coin cell batteries

[0019] Experimental validation demonstrates the sensor's capability to detect crack propagation in metallic substrates with 0.2 mm resolution. When bonded to aluminum test specimens, the system recorded resistance changes correlating directly with induced fractures (R²=0.94 linear fit). The TPU variant exhibited enhanced crack detection sensitivity (?R=15% per 0.2 mm crack extension) compared to nanocellulose implementations (?R=8%).

[0020] Key technical advantages of disclosed embodiments include:
• Environmental sustainability through complete biodegradation within 6-18 months under standard soil conditions
• Operational flexibility across temperature ranges (-20°C to +85°C) and humidity extremes (10-90% RH)
• Elimination of sensor retrieval costs through in-situ decomposition post-monitoring
• Compatibility with curved surfaces (radius >5 mm) through substrate elastomeric properties[0017] Implementation scenarios demonstrate particular efficacy in:
• Aerospace structural monitoring (wing deformation analysis during flight cycles)
• Civil infrastructure assessment (bridge joint displacement tracking)
• Biomedical applications (prosthetic limb stress distribution mapping)
• Marine corrosion detection (hull integrity monitoring in saltwater environments)

[0021] The wireless architecture's energy efficiency enables deployment in inaccessible locations, with BLE modules consuming <1.5 mW during active transmission. Data security protocols incorporate 128-bit AES encryption for industrial IoT compliance.

[0022] While the disclosure has been particularly shown and described with reference to preferred embodiments, various modifications in material selection, geometric configurations, and wireless protocols will be apparent to practitioners skilled in structural sensing technologies. Such variations shall be considered within the scope of this disclosure provided they incorporate the fundamental principles of biodegradable substrate utilization, printed conductive patterns, and wireless data transmission as described herein.

BRIEF DESCRIPTION OF DRAWINGS

[0023] Further aspects and advantages of the present disclosure will be readily understood from the following detailed description with reference to the accompanying drawings. Reference numerals have been used to refer to identical or functionally similar elements. The figures, together with the detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages in accordance with the present disclosure, wherein:

[0024] Figure 1 illustrates a block diagram for sustainable strain sensor device assembly, in accordance with some embodiments of the present disclosure.

[0025] Figure 2 demonstrates real-time crack detection using the strain sensor bonded to a metal surface, in accordance with some embodiments. An artificial crack is propagated perpendicular to the sensor’s length while measuring resistance changes correlated with crack growth at 0.2 mm step sizes.

[0026] Figure 3 depicts a schematic of the low-power Bluetooth (BLE) module integrated with the strain sensor for wireless data transmission to a mobile application, in accordance with some embodiments. The circuit architecture enables real-time strain monitoring and connectivity with external devices.

[0027] Figure 4 shows the fabricated BLE-based electronic circuit for wireless strain data acquisition, in accordance with some embodiments. The circuit includes programming pin components and interfaces with the mobile application for visualization.

[0028] Figure 5 illustrates a block diagram of the structural health monitoring (SHM) system, comprising the strain sensor, data acquisition unit, BLE module, and dedicated mobile application for data analysis, in accordance with some embodiments.

[0029] Figure 6 presents a flowchart of the method for fabricating the sustainable strain sensor, including substrate selection, conductive ink preparation, screen-printing, drying, and electrical connection formation, in accordance with some embodiments

[0030] Figure 7 illustrates comparative resistance change graphs for the strain sensor under 90° bending deformation on different substrates, showing approximately 21% resistance change on bacterial nanocellulose and 42% resistance change on TPU substrate, in accordance with some embodiments.

[0031] Figure 8 presents resistance change measurements recorded during aluminum sheet fracture tests at 0.2 mm step increments, demonstrating the sensor's ability to detect structural failures, in accordance with some embodiments.

[0032] Figure 9 shows stretching and recovery characteristics of the carbon black-based strain sensors, illustrating gauge factors of 0.5 for cellulose substrate and 1.5-1.8 for TPU substrate at 0.2 mm incremental steps, in accordance with some embodiments.

[0033] Figure 10 displays additional views of the BLE-based electronic circuit, including wireless strain data capture via mobile application, in accordance with some embodiments.

DETAILED DESCRIPTION OF THE INVENTION

[0034] Referring now to the drawings, there is shown an illustrative embodiment of the disclosure. It is understood that the disclosure is susceptible to various modifications and alternative forms; specific embodiments thereof have been illustrated by way of example in the accompanying drawings and will be described in detail herein. It will become apparent that the invention may be realized in diverse embodiments while adhering to the core principles disclosed.

[0035] The terms “comprises”, “comprising”, “includes”, or analogous variations thereof, are intended to encompass non-exclusive inclusions, such that a system, apparatus, or method comprising enumerated elements does not exclude the presence of additional elements not expressly listed or inherent to the system, apparatus, or method. For clarity, the phrase “one or more elements in a device preceded by ‘comprises... a’” does not, absent explicit constraints, preclude the inclusion of supplementary components.

[0036] In the subsequent detailed description of the embodiments, reference is made to the accompanying drawings, which form an integral part hereof. The drawings illustrate specific embodiments in which the invention may be practiced. These embodiments are elucidated with sufficient particularity to enable practitioners skilled in the art to replicate the invention. It is acknowledged that alternative embodiments may be employed, and structural or procedural modifications may be instituted without deviating from the scope of the present disclosure. Consequently, the following exposition is not to be construed as restrictive.

[0037] Terms such as “at least one” and “one or more” are employed interchangeably throughout this description. Similarly, “a plurality of” and “multiple” are used synonymously. The terms “structural health monitoring system” and “system” are likewise interchangeable in context.

[0038] The present disclosure relates to sustainable strain sensors for wireless structural health monitoring (SHM) applications. More specifically, the invention provides biodegradable strain sensors fabricated using eco-friendly substrates and conductive nanomaterials, integrated with wireless data transmission capabilities for real-time monitoring of structural integrity across civil, aerospace, automotive, and biomedical applications.

[0039] The invention pertains to strain sensors utilizing biodegradable substrates and conductive nanocomposites for structural health monitoring. The sensors detect mechanical deformations through piezoresistive effects and transmit data wirelessly via integrated low-power electronics, addressing environmental concerns associated with conventional non-degradable sensors

Sensor Architecture and Fabrication

[0040] Figure 1 illustrates a block diagram for sustainable strain sensor device assembly, in accordance with some embodiments of the present disclosure. The assembly comprises a biodegradable substrate (130) selected from bacterial nanocellulose or thermoplastic polyurethane (TPU), a screen-printed conductive ink layer (140) deposited on the substrate, and electrical connections (120) formed on the conductive ink layer to enable strain measurement. The device further integrates a Bluetooth Low Energy (BLE) module (300) configured to wirelessly transmit strain data to a mobile application (350). The block diagram outlines the interconnection between the substrate, conductive layer, electrodes, and wireless transmission components, providing an overview of the functional architecture of the sustainable strain sensor.

[0041] In some embodiments, the conductive ink comprises graphene, graphite, carbon, or coal-derived graphene, formulated into a viscous suspension for screen-printing. The printed sensor strip 140 possesses dimensions of 60 mm×4 mm60 mm×4 mm, optimized for strain distribution and signal linearity. Electrical connections 150 are formed using copper wires bonded with silver epoxy, ensuring minimal contact resistance. Post-printing, the sensor is cured in an air oven at 120°C for 30 minutes to stabilize the conductive layer.

[0042] Figure 2 demonstrates real-time crack detection using the strain sensor 200 bonded to a metal surface 210, in accordance with some embodiments. An artificial crack 220 is introduced at the midpoint of the metal substrate 210 and propagated perpendicular to the sensor’s longitudinal axis via the linear actuator. As the crack extends in 0.2 mm increments, the sensor’s resistance change correlates with crack growth, enabling precise detection of structural flaws. The TPU-based sensor exhibits a resistance variation of up to 42% under 90° bending, while the nanocellulose variant shows a 21% change, as quantified by Biologic impedance analyzer.

Fabrication Methodology

[0043] BNC films are cultured using Komagataeibacter xylinusin Hestrin-Schramm medium for 7 days, yielding 4.3 g/L cellulose with 97.5% crystallinity. TPU membranes are solvent-cast from pellets dissolved in dimethylformamide, achieving 300% elongation at break.

[0044] Coal-derived graphene is synthesized via fluid dynamics exfoliation of anthracite, producing flakes with 20 wt% graphene, 5 wt% CNTs and 75 wt% PLA dissolved in ethyl lactate.

[0045] The mixture is ball-milled (200 rpm, 4 hrs) to achieve viscosity = 12,000 cP, optimal for screen printing.

Sensor Assembly

[0046] The ink is deposited through a 140-mesh stencil onto substrates, forming 60 mm × 4 mm traces (thickness: 15 µm). Curing at 120°C for 30 minutes crosslinks PLA chains, reducing sheet resistance from 250 O/sq to 50 O/sq. Copper electrodes are bonded using silver epoxy (EPO-TEK H20E-PFC), cured at 120°C for 15 minutes.

Wireless Data Transmission Sy stem

[0047] Figure 3 depicts a schematic of the BLE module 300 integrated with the strain sensor 310, in accordance with some embodiments. The module 300 comprises a microcontroller 320 (e.g., ESP32) configured to sample resistance data at 10 Hz, an analog-to-digital converter (ADC) 330 with 12-bit resolution, and an instrumentation amplifier. The system operates at 3.3 V, consuming 18 mA during active transmission. Real-time strain data is wirelessly transmitted to a mobile application 350, which visualizes temporal resistance variations and triggers alerts upon exceeding predefined thresholds.

[0048] Figure 4 illustrates the fabricated BLE-based electronic circuit 400, including programming pins 410, voltage dividers 420, and a Li-ion battery pack 430. The PCB layout minimizes noise interference, achieving a signal-to-noise ratio (SNR) of 58 dB during dynamic strain measurements. The mobile application interface 440 displays resistance trends, historical data, and enables calibration for specific substrate materials.

[0049] Additional perspectives of the BLE-based electronic circuit are presented in Figure 10, including demonstrations of wireless strain data transmission to the mobile application interface. The figure illustrates real-time capture of resistance variations in response to applied bending forces, validating the system's capability for continuous structural health monitoring. The circuit's compact form factor enables integration with various structural components while maintaining signal integrity during dynamic loading conditions.

[0050]The BLE module (Nordic nRF52840 or ESP32) operates at 2.4 GHz with a 1.3 km range, consuming 5.3 mA during active transmission. The onboard 12-bit ADC samples strain data at 100 Hz, sufficient for detecting sub-millimeter cracks in metal surface.

Structural Health Monitoring System

[0051] Figure 5 presents a block diagram 500 of the SHM system, in accordance with some embodiments. The system integrates the strain sensor 100, the BLE module 300, and the mobile application 350. Processed data is transmitted to the BLE module 300, which relays it to the mobile application 350 for Fourier transform analysis, peak detection, and anomaly reporting.

Performance Characterization

[0052] The TPU-based sensor demonstrates a gauge factor of 1.5–1.8 under uniaxial tensile strains up to 15%, while the nanocellulose variant exhibits a gauge factor of 0.5, as quantified by a Biologic SP-300 potentiostat. Hysteresis tests reveal <5% deviation over 1,000 bending cycles, attributable to the viscoelastic recovery of the substrate. Fatigue resistance is validated through 10,000 stretch-release cycles at 2 Hz, with <8% degradation in conductivity.

[0053] Figure 7 illustrates the comparative resistance change characteristics when the fabricated sensors are subjected to 90° outward bending. The strain sensor utilizing the bacterial nanocellulose substrate exhibits an approximate resistance change of 21%, while the TPU substrate-based sensor demonstrates a significantly higher resistance change of approximately 42% under identical bending conditions. This differential response highlights the superior flexibility and strain sensitivity of the TPU substrate, making it particularly suitable for applications requiring detection of subtle structural deformations.

[0054] Wireless transmission tests confirm reliable data streaming within a 30 m line-of-sight range, with packet loss rates below 0.1% in interference-free environments. The mobile application 350 implements adaptive filtering to distinguish mechanical strain from ambient vibrations, achieving a detection accuracy of 92.4% in bridge simulation trials.

[0055] Figure 8 presents quantitative resistance measurements during controlled fracture tests. When attached to aluminum test specimens subjected to tensile loading, both sensor variants demonstrate distinctive resistance change patterns as fractures propagate at 0.2 mm increments. The TPU-based sensor exhibits a more pronounced resistance variation compared to the nanocellulose variant, corresponding to the findings illustrated in Figure 7. These results validate the sensor's capability to detect incipient structural failures before catastrophic fracture occurs.

[0056] Figure 9 depicts the stretching and recovery characteristics of the fabricated strain sensors. The TPU substrate-based sensor demonstrates a gauge factor range of 1.5-1.8 during incremental stretching at 0.2 mm steps, with consistent resistance-strain relationships maintained through multiple deformation cycles. In contrast, the bacterial nanocellulose substrate-based sensor exhibits a lower gauge factor of approximately 0.5 under identical testing conditions. The differential performance is attributed to the distinct viscoelastic properties of the substrate materials, with TPU offering enhanced mechanical compliance and strain transfer efficiency. These measurements, obtained using a Biologic impedance analyzer, confirm the sensor specifications detailed in paragraph [0046].

Method for Structural Health Monitoring

[0057] Figure 6 outlines the method 600 for fabricating and deploying the strain sensor, in accordance with some embodiments. The process initiates with substrate selection 610, followed by ink preparation 620 and screen-printing 630 Post-curing and electrical connections 640 are established. For SHM applications, the sensor is adhesively bonded to the target structure, and the BLE module is activated for continuous monitoring.

[0058] In operational use, the BLE Module (300) samples resistance data at 10 Hz, applying digital filters to isolate structural deformation signatures. Anomalies are flagged when resistance deviations exceed three standard deviations from baseline readings, prompting automated alerts via the mobile application 350. Post-deployment, the biodegradable substrate degrades within 180 days under ambient conditions, eliminating manual removal costs.
The strain sensor comprises:

• Substrate: 0.1 mm thick bacterial nanocellulose (BNC) or thermoplastic polyurethane (TPU) membrane, selected for biodegradability (90% degradation in soil within 60 days) and tensile strength (15–25 MPa).
• Conductive layer: Screen-printed ink containing 20 wt% coal-derived graphene (flake size: 2–5 µm), 5 wt% CNTs, and 75 wt% PLA binder. The ink exhibits sheet resistance of 50–80 O/sq after curing at 80°C for 30 minutes.
• Electrodes: Copper foil (35 µm thickness) bonded using silver epoxy, providing stable electrical contacts under 150% strain.

[0059] The fabrication methodology, detailed in Figure 6, involves four primary stages:
1. Substrate Selection: Bacterial nanocellulose or TPU membranes are selected for their biodegradability, mechanical flexibility, and compatibility with screen-printing processes.
2. Conductive Ink Preparation: Graphene or carbon-based inks are homogenized to achieve optimal viscosity (1200–1500 cP) for uniform deposition.
3. Screen-Printing: A 200-mesh stainless steel stencil deposits the ink onto the substrate, forming a 15 µm-thick conductive trace.
4. Electrical Integration: Copper electrodes are affixed using conductive epoxy, and the sensor is integrated with a Bluetooth Low Energy (BLE) module 300 for wireless data transmission.

Industrial Applications

[0060] The sensor’s flexibility and wireless capability enable deployment on curved surfaces in aerospace components (e.g., aircraft wing joints), civil infrastructure (e.g., bridge girders), and biomedical devices (e.g., prosthetic limb sockets). In automotive testing, the sensor monitors chassis flexure during collision simulations, transmitting data to cloud platforms for finite element model validation. Maritime applications include corrosion detection in offshore platforms, where the sensor’s waterproof encapsulation withstands saline immersion.

[0061] The disclosed invention represents a significant advantage in structural health monitoring by merging sustainable materials with advanced wireless telemetry. By eliminating non-biodegradable components and simplifying fabrication, the system achieves cost reductions of 40–60% relative to commercial foil gauges.

[0062] Civil Infrastructure: Sensors bonded to steel bridge girders detect microstrains from traffic loads, transmitting data to municipal maintenance teams via LTE-M networks.

[0063] The invention provides a scalable solution for eco-friendly structural monitoring, combining biodegradable materials, high sensitivity (GF >1.5), and wireless connectivity.

[0064] The performance characteristics documented in Figures 7-10 substantiate the claimed gauge factor ranges and resistance change percentages specified in the preceding paragraphs. Notably, the 42% resistance variation observed in TPU-based sensors during 90° bending tests corresponds directly to the specifications outlined in claim 4, while the measured gauge factors of 0.5 for nanocellulose and 1.5-1.8 for TPU substrates validate the performance metrics described in paragraphs [0015] and [0046]. These experimental results confirm that the disclosed strain sensor technology achieves the sensitivity and response linearity required for precision structural health monitoring across diverse application environments.

,CLAIMS:We Claim,
1. A sustainable strain sensor device (100) for structural health monitoring comprising:

a biodegradable substrate (130) selected from bacterial nanocellulose or thermoplastic polyurethane (TPU);
a conductive ink layer (140) screen-printed on said substrate with a thickness of 10-20 µm, comprising at least one material selected from graphene, graphite, carbon, or coal-derived graphene;
electrical connections (120) formed on the conductive ink layer, and
a Bluetooth Low Energy (BLE) module (300) integrated with said conductive ink layer, configured to wirelessly transmit strain data to a mobile application (350).

2. The device as claimed in claim 1, wherein said conductive ink layer (140) exhibits a resistance change between 35–50% under bending angles of 80–100°.

3. The device as claimed in claim 1, wherein said thermoplastic polyurethane substrate (130) demonstrates a gauge factor of 1.5–1.8 under uniaxial tensile strains of 10–20%.

4. A method (600) for fabricating a sustainable strain sensor, comprising:
selecting a biodegradable substrate (610) from bacterial nanocellulose or TPU membranes;
preparing a conductive ink (620) by homogenizing graphene or carbon-based materials to 1200–1500 cP viscosity;
screen-printing (630) said ink onto said substrate;
curing (640) the printed sensor at 100–150°C for 20–40 minutes; and
integrating a BLE module (300) for wireless data transmission.

5. A structural health monitoring (SHM) system (500) comprising:
the strain sensor (100) as claimed in claim 1;
a BLE module (300) operatively connected to said sensor; and
a mobile application (350) implementing adaptive filtering to distinguish mechanical strain from ambient vibrations.

6. A method for real-time crack detection in a structure, comprising:
bonding the strain sensor as claimed in claim 1 to a metal surface;
propagating the crack perpendicular to the sensor’s longitudinal axis; and
measuring resistance changes in the sensor correlated with crack growth.

7. The method as claimed in claim 6, wherein the strain sensor exhibits 35–50% resistance change on a TPU substrate and 15–25% resistance change on bacterial nanocellulose during crack propagation.

8. The strain sensor device as claimed in claim 1, wherein the biodegradable substrate degrades within 6 – 18 months under ambient environmental conditions.