Stretchable Thermoelectric Fibers Using CuI Nanoparticle Networks for Multi-Sensing Wearable Electronics

Recently, the field of biomedical engineering and robotics has witnessed wearable sensor that is able to imitate functions of human skin. Wearable sensor is required the characteristics such as flexibility and stretchability with capacity that converts external stimulate including temperature, pressure, displacement, pulse, electrocardiograph (ECG) signals, tactile sensations, to electrical signals. Recently, thermoelectric materials that transform reversibly the temperature gradient into an electrical output voltage have been applied to detect changes in temperature through output voltage and current.<br/>To applied wearable sensor, there are several strategies as followings: (1) structural innovations of inorganic thermoelectric materials, (2) materials innovations of organic thermoelectric materials. Although inorganic materials have high figure of merit (zT) and high electrical conductivity, inorganic materials are difficult to deform mechanically due to brittleness. To solve the limitation of rigidity, thermoelectric device consisted with inorganic materials designed by kirigami/origami structure. However, this structure not only has limitation of deformation but also is too complicated to integrate in wearable device. In case of organic thermoelectric materials, the thermoelectric performances are poorer than that of inorganic thermoelectric materials. To improve thermoelectric performances, organic materials are chemically doping. However, the efficiency and stability of chemical modification are always unsatisfactory.<br/>Herein, we fabricated stretchable thermoelectric copper iodide nanoparticles (CuINPs) embedded fibers. CuINPs, which is promising and ecofriendly thermoelectric materials, are densely formed inside and outside of the polyurethane (PU) fiber using the chemical reaction methods. This method consists of two steps: (1) the fiber immersed in Cu precursor to absorbed Cu ions in the PU matrix, (2) the absorbed fiber reacted with I precursor to synthesis CuINPs. CuINPs embedded fiber achieves a power factor of 18.31 µWm⁻¹K⁻²with electrical conductivity of 5.99 S/cm and Seebeck coefficient of 175.8 µVK⁻¹ at room temperature. Moreover, the resistance of the fiber increased with increasing tensile strain. The Seebeck coefficient of the fiber constantly remained with changing strain of fiber. We demonstrated a sensing system that can detect pressure and temperature changes simultaneously. This facile CuINPs embedded fibers will pave the way for applying inorganic TE materials in a stretchable form in wearable electronics.