Inkjet-Printed Ag Nanomaterials based Strain Sensors for Wearable Sensing Applications

Abstract

Of late, the demand of wearable sensors has exponentially risen. For example, strain sensors that can be worn and are skin mountable play a significant role in the areas of human motion detection, healthcare, soft robotics, electronic skin, and sport as well as fitness tracking. It is known that traditional strain sensing devices made of semiconductors and metals exhibit low sensitivities (GF< 2), low strain sensing range (< 5%) and exhibit rigidity which are undesirable characteristics for wearable sensing applications. In order to address these issues, metal nanomaterials such as AgNWs-AgNPs nanohybrids are now employed as active and functional sensing devices for the fabrication of wearable strain sensors. Further, inkjet printing process has been utilized for fabricating wearable devices (together with advantages of wide sensing range and high sensitivity) due to its cost effectiveness and capability of large scale production. The stretchable, wearable, and skin-mountable strain sensors have garnered significant research attention in consumer and medical products which may be attributable to various aspects like cost-effectiveness and ergonomics fuelled by the development in miniaturized electronics, growing consumer awareness for health related issues, and constant need for medical practitioners to obtain quality medical data from patients. Recently, the fabrication of strain sensors with high sensitivity and high stretchability, which can precisely monitor subtle strains and large mechanical deformations exhibited by the human bodily motions, is critical for healthcare, human-machine interfaces, and biomedical electronics. However, a significant challenge still exists i.e. achieving strain sensors with high sensitivity, high linearity, and high stretchability by a facile, low-cost and scalable fabrication technique. Herein, in this research, we achieve AgNWs-AgNPs/Ecoflex based composite strain sensors via inkjet printing technique which precisely deposits functional materials in a rapid, non-contact and maskless approach allowing high volume production. Noteworthily, the fabricated strain sensor display many fascinating features, including high sensitivity (a gauge factor of 96.26), high linearity, a broad strain sensing range greater than ~ 55%, excellent stability and reliability (>1000 cycles), and low monitoring limit (<1% strain). These remarkable features allow the strain sensor to effectively monitor various human motions. This work opens up a new path for fabricating elastomer based strain sensors for wearable electronics. Next, a simple, eco-friendly and scalable fabrication process is presented for manufacturing flexible epidermis-like sandwich structured (i.e.AgNWs-AgNPs conductive network embedded between two slabs of Dragon skin) strain sensors based on AgNWs-AgNPs conductive network incorporated in a Dragon skin elastomeric polymer substrate. The AgNWs-AgNPs conductive network-Dragon skin nanocomposite based strain sensors demonstrate superior sensitivity with a gauge factor of 5.6 at an applied strain range of 40%-60% and a broad sensing range of up to 80%, while exhibiting superior performance and durability for more than 500 stretch-release cycles. The applicability of our high performance strain sensors for the detection of face expressions and large-strain joint motions is demonstrated in this work. Next, we fabricate electronic device comprising of highly conductive AgNWs-AgNPs network embedded onto highly stretchable natural rubber supporting material. When mechanical strains are applied, the disconnection of adjacent AgNWs-AgNPs along with opening-closing of microcracks in a reversible manner results in the variation of electrical resistance of the sensor exhibiting high sensitivity with discernible gauge factors. The inkjet printed strain sensor exhibit ultrasensitivity with a gauge factor of 170.8 coupled with a wide and linear sensing range of over 120%. Moreover, the sensor exhibit fast responsiveness to applied strains, low hysteresis, and remarkable cyclability. We demonstrate that the skin-mounted strain sensor can be used for multiscale sensing to monitor electrical resistance signals ranging from small-scale human face expressions to large-strain human joint motions. Furthermore, functional resistive-type strain sensor based on inkjet printedAgNWs-AgNPs conductive network on stretchable nitrile elastomer supporting material has been fabricated. The novel strain sensor employed disconnection of AgNWs-AgNPs and expansion of porous structure to accommodate the applied mechanical strains. The as-fabricated strain sensor exhibited low contact resistance, high sensitivity (GF~751.07), broad strain sensing range (>60%), fast responsiveness and remarkable long-term durability. The brittle nature of nanowires endows the wearable strain sensor with remarkable sensitivity and the elastomeric supporting material enhances the deformation ability of the strain sensor. Moreover, various demonstrations have been carried out for the detection of small-scale mechanical strains such as expressions of the face and monitoring of large-scale deformations like human joint motions. This study presents a unique sensor which can be useful for multi-scale sensing. The key novelty and contribution of this work is the chemistry for silver nanomaterial deposition with inkjet printing and the systematic testing of dynamic sensing performance of strain sensors under different conditions has been conducted. For example, the critical sensing properties of the wearable strain sensors have been evaluated such as stretchability or strain sensing range, sensitivity, linearity, hysteresis performance, and reproducibility.