Highly Stretchable, Directionally Oriented Carbon Nanotube/PDMS Conductive Films with Enhanced Sensitivity as Wearable Strain Sensors

Abstract

Recent interest in the fields of human motion monitoring, electronic skin and human-machine interface technology demand strain sensors with high stretchability/compressibility (ε > 50%), high sensitivity (or gauge factor (GF > 100) and long-lasting electromechanical compliance. However, current metal and semiconductor-based strain sensors have very low (ε < 5%) stretchability or low sensitivity (GF < 2), typically sacrificing the stretchability for high-sensitivity. Composite elastomer sensors are a solution where the challenge is to improve the sensitivity to GF > 100. We propose a simple, low cost fabrication of mechanically compliant, physically robust metallic carbon nanotube (CNT)-polydimethylsiloxane (PDMS) strain sensors. The process allows the alignment of CNTs within the PDMS elastomer, permitting directional sensing. Aligning CNTs horizontally (HA-CNTs) on the substrate before embedding in the PDMS reduces the number of CNT junctions and introduces scale-like features on the CNT film perpendicular to the tensile strain direction, resulting in improved sensitivity compared to vertically-aligned CNT-(VA-CNT)-PDMS strain sensors under tension. The CNT alignment and the scale-like features modulate the electron conduction pathway, affecting the electrical sensitivity. Resulting GF are 594 at 15 % and 65 at 50 % strains for HA-CNT-PDMS and 326 at 25 % and 52 at 50 % strains for VA-CNT-PDMS sensors. Under compression, VA-CNT-PDMS show more sensitivity to small-scale deformation than HA-CNT-PDMS due to the CNT orientation and the continuous morphology of the film, demonstrating that the sensing ability can be improved by aligning the CNTs in certain directions. Furthermore, mechanical robustness and electromechanical durability are tested for over 6000 cycles to up to 50 % tensile and compressive strains, with good frequency response with negligible hysteresis. Finally, both types of sensors are shown to detect small-scale human motions, successfully distinguishing various human motions with reaction and recovery times of as low as 130 ms and 0.5 s respectively.

Stretchable Electronics Based on PDMS Substrates

“…[ 51 ] Even if a fracture happened, the devices are expected to recover its function after self‐healing. [ 51–124 ]…”

“…[ 71–73 ] Currently, many encouraging results have been obtained by utilizing PDMS and the corresponding mature technologies to construct stretchable electronics. [ 42,66,74 ] Herein, we summarize the current advantages in the development of stretchable electronics based on the PDMS substrate. Our discussion starts by reviewing the strategies to fabricate stretchable electronics using PDMS as the substrate.…”

Anisotropic, Wrinkled, and Crack-Bridging Structure for Ultrasensitive, Highly Selective Multidirectional Strain Sensors

“…Resistive-type sensors made by depositing conductive filler networks on flexible and stretchable polymer substrates are promising contenders because of the synergy arising from the conductive nanofillers and stretchable polymer matrices. However, there is always a trade-off between superior stretchability and high sensitivity because of conflicting structural requirements based on different principles [17][18][19][20][21]. The high sensitivity stems from the interruption of conductive paths under a tiny change in strain, while the excellent stretchability requires the sensor to retain the conductive paths even under large deformations [22].…”

Ultra‐Sensitive and Stretchable Ionic Skins for High‐Precision Motion Monitoring

“…These can be attached to human skin for real‐time motion monitoring. Strain sensors can be fabricated using various electrical conductors (e.g., carbon nanotubes, [ 19–22 ] graphene, [ 23–25 ] metal nanowires, [ 26 ] and nanocomposites with elastomers [ 27,28 ] ) and ionic conductors (e.g., hydrogels, [ 29–32 ] ionic gels, [ 33–35 ] and ionic liquids (ILs) in an elastic container [ 36–38 ] ). Electrical conductors were employed in first generation strain sensors because of their considerably high gauge factors (in the range of 20–2000).…”

“…Electrical conductors were employed in first generation strain sensors because of their considerably high gauge factors (in the range of 20–2000). [ 39–42 ] However, electrical conductor‐based sensors suffer from critical limitations when applied in stretchable sensory systems, such as a small strain sensing range, [ 43–46 ] non‐linearity, [ 20,23 ] non‐reversibility, [ 47 ] and optical opacity. [ 48,49 ]…”