Inkjet-Printed Controllable Textile Strain Sensors for Human Motion Detection

Flexible strain sensors have been attractive recently to detect and monitor human motion detection for an entertaining purpose like VR or AR, and medical purpose like rehabilitation. However, current strain sensors are not feasible to integrate with wearable platforms due to the complicated manufacturing and embedding issues. Electronic textiles (e-textiles) can be an attractive alternative to conventional rigid and bulky sensors because of intrinsic textile properties such as flexibility, stretchability, breathability, washability, and high durability under extreme deformations. This work presents a novel technique for developing textile strain sensors by only inkjet-printing of metal-organic decomposition silver ink on knit fabrics along with different directions (warp, weft, bias).<br/><br/>For material selection, we printed a 5×20 mm conductive pattern along with bias direction on knit fabrics having different tightness. The gauge factor increased from 0.16 to 19.7 according to the increase of the tightness factor of the fabrics from 2.35 to 3.46. This is related to the ink penetration depth depending on the tightness factor. Also, yarn-level structural changes while stretching showed direction-dependent behavior. This affected the relative resistance change and gauge factor because the number of contact points between conductive paths on the yarns changed during deformation. Stretching along with weft direction showed the lowest gauge factor for all fabrics, and there was a drastic resistance increase or instant off-state along with warp direction. Stretching along with bias direction combining both warp and weft direction showed a reasonable gauge factor. Also, the gauge factor of the strain sensors can be controlled by printing paths. The increase in the number of printing paths made the ink more penetrated and showed a lower gauge factor. This means that the printed pattern can work as a strain sensor or interconnect by arranging pattern directions and printing paths. Also, the knit-based textile strain sensors have a wide range of sensing performance up to 60 % due to high flexibility as knit fabric’s own property.<br/><br/>For a use-case scenario in wearable technologies, we fabricated a full arm sleeve with two parts of printed conductive paths, including the strain sensor along with bias direction and interconnect along with weft direction, then connected to a readout circuit and Bluetooth data transmission system. The strain sensor-embedded armband showed the resistance change that can be translated to the strain by elbow motion. This allows for a full range of elbow motion to monitor the movement without having bulky equipment attached to the arm and should remain comfortable and breathable without hindering the effectiveness of the fabric. Also, we simulated the armband prototype using CLO 3D, which is commercial particle-based 3D garment simulation software, to predict the sensing performance of the developed sensors. Due to the facile process of directly inkjet-printing on fabrics, the developed textile strain sensors can be easily integrated with wearable devices for human motion detection. Thus, they have huge potential in practical and manufacturable applications.