[“Source” Chaqiuhao Series] Conductive testing of flexible wearable electronic device materials

Flexible wearable electronic devices are mainly composed of flexible piezoresistive sensor materials, flexible sensor frames, electrode connections, and signal acquisition and processing circuits. The most important part is the testing of flexible piezoresistive sensor materials. For materials used to manufacture piezoresistive sensors, it is necessary to comprehensively evaluate their electrical, mechanical, dynamic response, environmental stability and other performance indicators to ensure that the materials can meet the needs of practical applications.

For flexible materials, electrical testing requires testing their conductive properties, testing the conductivity and resistivity of the material, and the dispersion and contact performance of the conductive filler in the material. The conductivity and resistivity of the material determine the basic electrical properties of the sensor, affecting the sensitivity and response speed of the sensor. The dispersion and contact performance of the conductive filler determine the overall conductive properties of the material.

For flexible devices, it is also necessary to cyclically test the output voltage of the material under different bending degrees to see if the result is a stable value, so as to evaluate its stability.

Testing solutions for flexible wearable electronic devices

As a testing expert in the field of small signals, Keithley provides a wide range of products to help research on flexible wearable electronic devices. Keithley electrometers are used for accurate high resistance and low current measurements. The 5.5-digit 6514 and 6517B electrometers provide 1fA sensitivity, >200TΩ voltage measurement input impedance, and charge measurement as low as 10fC. Its low noise and drift performance make it an ideal choice for research on flexible materials and their device application testing.

The dual-channel Model 2182A Nanovoltmeter is optimized for stable, low-noise nA-level voltage measurements and reliable, repeatable characterization of low-resistance materials and devices. It offers higher measurement speeds and significantly better noise performance than other low-voltage measurement solutions. It offers a simplified delta mode that can be used to make resistance measurements in conjunction with a reverse current source such as the Model 6220 or 6221.

The Model 6220 DC Current Source and Model 6221 AC-DC Current Source combine ease of use with very low current noise. Low current sources are critical for applications in test environments from R&D to production. With high source accuracy and built-in control features, the Models 6220 and 6221 are ideal for applications such as Hall measurements, resistance measurements using incremental mode, pulse measurements, and differential conductance measurements.

The SMU source meter series from 2400 to 2600 series has high-precision 10nV voltage and 0.1fA current test, with analyzer, curve tracer and IV system functions, at a lower cost. It provides highly flexible 4-quadrant voltage and current source/load, as well as precision voltage and current instruments. This integrated instrument can be used as: precision power supply with voltage and current readback function; true current source; digital multimeter, measuring DC voltage, current, resistance and power with a resolution of 6½ digits; precision electronic load; trigger controller.

Case 1: Soft wearable piezoresistive sensor based on natural rubber using a custom additive manufacturing process

This case study presents soft, wearable piezoresistive sensors made from natural rubber, fabricated using a custom dip-based additive manufacturing process. The study highlights the importance of piezoresistive sensors in monitoring human motion to prevent and treat injuries. By combining natural rubber with acetylene black and using stereolithography-based additive manufacturing, the sensors were able to successfully detect small strains. The study compared sensors produced by mold casting and additive manufacturing, with the latter showing a more uniform distribution of conductive fillers. The sensors exhibited good sensitivity, a wide detection range, and flexibility, making them suitable for monitoring human joint motion. The use of renewable natural rubber and additive manufacturing processes can expand the application of soft, flexible electronics in biomedical devices.

In the dynamic stretching experiment, the researchers used a Keithley 2450 source meter to measure the change in resistance. In the quasi-static stretching experiment, the Keithley 2450 source meter was also used to measure the electrical signal response. In addition, in the finger motion monitoring experiment, the researchers placed the sensor on a 3D printed flexible TPU prototype and measured the electrical signal response when the finger was bent and stretched.

Case 2: Fish scales for wearable, self-powered TENG

This case is a study of a self-powered triboelectric nanogenerator (TENG) based on fish scales. Fish scales are a biomass material with good biocompatibility, making TENG a flexible, wearable and self-powered device with great potential in healthcare and body information monitoring applications. This study highlights the advantages of using fish scales, a natural and degradable material, to develop a self-powered TENG, solving the problem of electronic waste generated by certain polymer-based wearable devices.

In this case, the Keithley 6517B electrometer was used to measure the output voltage characteristics of the fish scale TENG under different pressures, as well as the output voltage and current stability of the fish scale TENG under constant pressure. Under a constant pressure of 50 N, the output current of the TENG remained stable at about 0.18 μA. This test requires the test instrument to have a very high current resolution and extremely low instrument noise floor to measure this extremely small stable value.

Case 3: Large-area roll-to-roll printing of semiconducting carbon nanotube films for flexible carbon-based electronics

This case study describes a versatile roll-to-roll (R2R) printing method for fabricating large-area (8 cm × 14 cm) semiconducting single-walled carbon nanotube (sc-SWCNT) films on flexible substrates such as polyethylene terephthalate (PET), paper, and aluminum foil. The sc-SWCNT films were prepared using high-concentration sc-SWCNT ink and cross-linked poly-4-vinylphenol (c-PVP) as an adhesion layer. The p-type thin-film transistors (TFTs) fabricated based on the sc-SWCNT films exhibit excellent electrical properties, including high carrier mobility, high on-off ratio, small hysteresis and small subthreshold swing, and low operating voltage.

In this case, all electrical measurements of SWCNT TFT and CMOS inverter were performed in air using Keithley 2636B and Keithley 4200 analyzers, with a bias voltage (VDS) of -0.25 V (or 0.25 V) and a scan step of -0.01 V (or 0.01 V). The above instruments were used to measure the electrical properties of the prepared SWCNT thin film transistors, such as carrier mobility, on/off ratio, hysteresis, and subthreshold swing, to evaluate the application potential of the materials in flexible electronic devices.

Case 4: Highly flexible anisotropic magnetoresistive sensor for wearable electronics

This case study describes a study of a highly flexible anisotropic magnetoresistive (AMR) sensor for wearable electronic devices. The researchers developed a flexible AMR sensor by depositing a magnetic metal layer on a Kapton substrate. The sensor exhibited excellent fatigue resistance, and its sensitivity remained at 0.25 Oe-1 even after 500 bending cycles. The sensor showed stable magnetoresistive performance at different bending curvatures (from 1/3 to 1/10 mm-1). The sensor had an AMR ratio of about 1% and a sensitivity of 0.25 Oe-1 in the flat state, demonstrating its potential for wearable electronic applications.

This article uses Keithley 6221 to provide a 10 μA DC current, passing the current along the easy axis through the two electrodes "I+" and "I-". Keithley 2182A is used to connect to the other two electrodes "U+" and "U-" to measure the voltage change caused by the external magnetic field. In this way, the AMR ratio can be obtained.