I. Abstract of the Paper’s Core Research

• Metal-carbon nanotube electrodes outperform noble-metal inks in sensitivity.

For devices of the same 5×5 mm size with a 150-μm gap, the m‑CNT exhibits a capacitance change of 26 fF per millimeter of rubber thickness—ten times that of silver electrodes. Linear regression yields an R² of 0.981, and measurements remain stable within the 99% confidence interval, with exceptionally low data dispersion.

• The electrode structure directly determines the strength of the edge electric field sensing.

The smaller the electrode gap, the stronger the longitudinal edge electric field’s penetration; devices with a narrow 150‑μm gap exhibit significantly higher sensitivity than those with a 1‑mm wide gap. Increasing the electrode dimensions can broaden the linear detection range, but at the cost of reduced spatial resolution, allowing for tailored optimization to meet specific detection requirements.

• Conformal measurement on curved surfaces using a flexible substrate

After the Kapton film‑based strain sensor is bent to a 5 cm radius of curvature, its initial‑segment thickness sensitivity is further enhanced; at a thickness of only 4 mm, the signal reaches saturation, making it well suited for nondestructive testing of curved surfaces on arcuate components.

• In‑tire integration enables online tread‑wear monitoring.

A silver‑based underlayer combined with an m‑CNT composite electrode is integrated into the inner liner of automotive tires, leveraging a 487 MHz resonant frequency shift to characterize rubber wear. The internal steel mesh of the tire does not shield the detection electric field, enabling seamless compatibility and synchronous operation with existing TPMS tire‑pressure monitoring systems.

• Fully digital, maskless flexible manufacturing

Aerosols allow for flexible tuning of electrode dimensions and gaps, are compatible with aqueous carbon‑nanotube inks and solvent‑based noble‑metal inks, and can be processed on flexible PI films without thermal damage, enabling rapid prototyping and iterative development of small‑batch sensor arrays.

II. Complete Sensor Fabrication Process and Core Advantages for Aerosol Spraying

(1) Complete Preparation Procedure for Capacitive Thickness Sensors

• Glass/Kapton substrates are ultrasonically cleaned with acetone and isopropyl alcohol, followed by oxygen plasma treatment to enhance ink adhesion.
• Precious-metal ink: 3 mm large nozzle, sheath gas at 200/130 sccm, high-temperature sintering at 200–280°C;
• m-CNT water-based ink: 150 μm fine nozzle, low‑flow atomization, low‑temperature drying at 150°C, and double‑layer stacking to enhance the conductive network.
• Batch printing of gap electrodes with various dimensions (5×5/5×10/10×10 mm) and gap sizes of 1 mm/150 μm;
• Calibration of rubber step‑thickness samples and testing of the in‑tire high‑frequency resonance characteristics.

(II) Five Major Process Advantages (Comparison of Screen Printing, Inkjet, and Photolithography)

• Multi-nano ink universal integrated printing

A single device is compatible with three types of conductive systems—carbon nanotubes, silver, and gold—eliminating the need to retool the production line and enabling rapid comparison of material‑based sensing performance.

• Ultra-fine gap precision molding enhances edge electric field sensitivity.

Aerosol jet focusing enables the stable fabrication of electrodes with a 150 μm narrow pitch, whereas screen-printed lines are too rough to realize structures with minute gaps.

• Compatible with both flexible and rigid dual substrates; in‑curve embedded sensing can be realized.

Non-contact mist‑spraying technology enables molding on both PI flexible films and rigid glass, and can be directly applied to tire surfaces and the interiors of curved workpieces.

• Low-temperature carbon tube printing protects thermosensitive flexible substrates.

Metal‑carbon nanotubes are dried at only 150°C, with no high‑temperature annealing, ensuring that flexible substrates such as PU and PI—prone to deformation—are not damaged.

• Maskless rapid tooling, one-click electrode size adjustment

CAD allows direct modification of electrode dimensions and gap width, eliminating the need to fabricate custom lithography masks; prototype fabrication of multi‑specification sensor structures can be completed in just a few hours.

III. Five Core Technological Innovations of This Study

Innovation 1: Ultra-thin, High-Sensitivity Thickness-Sensing Electrode Based on Metal-Encapsulated Carbon Nanotubes

Innovation 2: Narrow-Gap-Enhanced Edge Electric Field Sensing Design

Innovation 3 Composite Electrode Compatible with Tire Steel Wire Shielding Ring 境

Innovative New Process for Flexible Thickness Measurement on 4-Curved Surfaces

Innovative 5‑field simulation + aerosol additive manufacturing joint development paradigm

IV. Current Industry Challenges and Corresponding Solutions for Overcoming Them

Challenge 1: The silver/gold noble-metal electrode films are thick, causing the electric field to concentrate at the electrode edges, resulting in weak penetration and low sensitivity.

Challenge 2: Traditional printed electrodes have large gaps, making it impossible to detect minute thickness variations.

Challenge 3: Detecting the electric field shielded by metal cord layers embedded in automobile tires.

Challenge 4: Lithography processes are limited to planar rigid substrates and cannot integrate curved components.

Challenge 5: The ink is single‑component, making it difficult to simultaneously compare the sensing performance of multiple conductive materials.

V. Future Diversified Industrial Application Scenarios

• In-vehicle Tire Safety Online Monitoring

• Industrial Composite Material Coating Thickness Inspection

• Non-invasive thickness sensing of medical soft tissues

• Online Quality Control for Lithium-Ion Battery Separator and Packaging Film Production Lines

• University-level IoT Sensor Research and Development Platform

Figure 1. Schematic illustration of the structure and operating principle of the printed sensor as a noninvasive material-thickness detector. (a) Cross-sectional schematic of the printed sensor electrodes in the test configuration, showing the edge electric field induced by perturbations in the overlying material. (b) Schematic of the measurement process: a 1‑mm‑thick rubber layer is placed directly on top of the printed electrodes, and the CMU is used to monitor changes in capacitance. (c) Comparison of sensitivity when measuring rubber thickness using three different inks—silver nanoparticles, gold nanoparticles, and carbon nanotubes—presented as normalized capacitance values as a function of rubber thickness. The sensor has dimensions of 5 × 5 mm, with a gap of 150 µm.

Table I. Summary of Results. For the silver nanoparticle‑based sensor with 5 × 5 mm electrodes and a 1 mm gap, the variation in capacitance as a function of rubber thickness was derived from experimental data distinct from those shown in Figure 2. This discrepancy arises from certain setup differences between the data presented in the table and those included in Figure 2, which give rise to unrealistic fluctuations.

Figure 2. Effect of printed electrode size and spacing on sensitivity. (a) Normalized capacitance change as a function of rubber thickness for three different electrode sizes (electrode spacing = 1 mm), with the inset showing the corresponding dimensions in millimeters; (b) Normalized capacitance change as a function of rubber thickness for two different electrode spacings (electrode size = 5 × 5 mm).

Figure 3. Sensitivity and consistency of identical silver nanoparticle sensors printed on different substrates as the rubber thickness varies within an 8‑mm range. The substrates include glass slides, flat Kapton films, and curved Kapton films with a radius of curvature of 5 cm. The sensor geometry is 5 × 5 mm, with a gap of 150 µm.

Figure 4. Comparison of sensitivity and surface morphology for three distinct nanomaterial inks—gold nanoparticles, silver nanoparticles, and single-walled carbon nanotubes. All sensors consist of electrodes with dimensions of 5 × 5 mm and a pitch of 150 μm. (a) Scanning electron microscopy image of the single-walled carbon nanotube film used as the electrode in the planar sensor architecture; (b) Plot of capacitance (in pF) versus rubber layer thickness (in mm), with 99% confidence intervals depicted as shaded bands; the sheet resistance of the single-walled carbon nanotubes is also superimposed; (c) and (d) Scanning electron microscopy images and sensing data for the silver nanoparticle‑based electrode sensor; (e) and (f) Scanning electron microscopy images and sensing data for the gold nanoparticle‑based electrode sensor.

Figure 5. Electric field distribution in the planar sensor, as computed using COMSOL. The edge electric field lines vary with the thickness of the rubber coating on the electrode surface. In the simulation, the rubber thicknesses used are (a) 2 mm and (b) 5 mm. (c) shows the sensor structure with a thicker electrode and a 2‑mm‑thick rubber coating, clearly illustrating the electric field distribution at the edges of the planar electrodes.

Figure 6. Reflection signals measured with a vector network analyzer after increasing the rubber thickness of an automotive tire. The sensor was mounted on the tire liner, which has a thickness of approximately 1.5 cm, and a 1‑mm‑thick rubber layer was superimposed over the tread (at the sensor’s location) to simulate changes in tread depth. (a) shows a photograph of the sensor. The reflection‑signal amplitude was recorded at 487 MHz, near the resonant frequency. Each rubber thickness was tested ten times; Figure (b) presents the 99% confidence intervals. Figure (c) displays the full‑band spectrum scan from 100 MHz to 1 GHz for cases with 0 mm and 8 mm of added rubber, along with a local magnification at the most sensitive frequency.