Reel-to-Reel Fabrication of Smart Sensing Threads

<br/>There has been an emergence of thread based wearable and implantable devices due to their superior flexibility and ability to provide intimate tissue/organ interfaces [1][2]. In prior works, smart threads have been realized for sensing biomarkers of wound healing [1], sensing electrolyte levels in sweat[2][3], monitoring physical activity, and for sensing volatile gasses[2]. Even transistors have been realized on threads[2]. Sensing threads are realized through coating of functional inks using a manual dip-and-dry approach. However this results in non-uniform threads and uneven responses. This abstract addresses a key technological need to enable precise controlled coating of threads for any ink types. This is achieved using a custom built reel-to-reel dip-coating and drying apparatus, which is the focus of this presentation. For a specific case study, we propose the realization of stretchable strain sensing thread with remarkable uniformity (&lt;4% error) over a meter long reel. Strain sensing threads are promising sensors for a wide range of wearable electronics applications. As their resistance depends on the tension, these threads can be used for measuring tension, pressure, and vibrations.<br/>A benchtop reel-to-reel coating system has been designed for uniform coating of any functional inks on any thread type. The platform transfers the thread from one spool to another using two stepper motors driven by microcontroller at user-defined rate. During the transfer, ink is applied using a custom-built 3D printed cartridge. Ink formulation eventually dictates the functionality of smart thread. For proof of principle, we use a carbon ink to realize strain sensing threads The ink-coated thread is then heated and dried in a temperature-controlled heating chamber. During the fabrication process, the tension of the thread is measured simultaneously, and a feedback loop is used to adjust the speed of the motor, so that the tension of the thread remains constant. This way, the thread is coated with consistent electrical properties along its length. Once the carbon layer dries, the carbon particles allow electrons to pass through and make the thread conductive.<br/>To analyze the threads, multiple threads were coated with carbon ink under different tensions. Baseline resistance variation is less than 4% over meter long length. Then the electrical conductivity and strain sensitivity of each thread is measured, which ranges from 0.62 to 2.53 MΩ, a 277% resistance change when stretched and unstretched from 4 to 6 cm. We report multiple utilities for strain sensing threads thus realized, such as monitoring of physical activity, posture, arm/leg/finger movements and pressure. By changing the ink and the cartridge, one can coat other functional inks on threads such as chemoresponsive dyes to realize gas sensors, or semiconducting inks to realize transistors.<br/>References:<br/>[1] Mostafalu, P., Akbari, M., Alberti, K.A. et al., (2016). Microsystems & Nanoengineering, 2:1-10.<br/>[2] Xia, J., Khaliliazar, S., Hamedi, M.M. et al. (2021). MRS Bulletin 46, 502–511<br/>[3] Terse-Thakoor, T., Punjiya, M., Matharu, Z. et al., (2020). npj Flexible Electronics. ;4:18.