Scalable and facile synthesis of stretchable thermoelectric fabric for wearable self-powered temperature sensors

Abstract

Wearable sensor systems with ultra-thinness, light weight, high flexibility, and stretchability that are conformally in contact with the skin have advanced tremendously in many respects, but they still face challenges in terms of scalability, processibility, and manufacturability. Here, we report a highly stretchable and wearable textile-based self-powered temperature sensor fabricated using commercial thermoelectric inks. Through various combinations of poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS), silver nanoparticles (AgNPs), and graphene inks, we obtained linear temperature-sensing capability. The optimized sensor generates a thermoelectric voltage output of 1.1 mV for a temperature difference of 100 K through a combination of PEDOT:PSS and AgNPs inks and it shows high durability up to 800 cycles of 20% strain. In addition, the knitted textile substrate exhibits temperature-sensing properties that are dependent upon the stretching directions. We believe that stretchable thermoelectric fabric has broader potential for application in human-machine interfaces, health-monitoring technologies, and humanoid robotics.

Fig. 1. Schematic illustration of a stencil-printing method, structural characteristics of the stretchable substrate, and material analysis of thermoelectric inks. (a) Fabrication process of the sensor with stencil-printing technique. First, the N-leg (n-type thermoelectric material) was printed onto the textile substrate, and then the P-leg (p-type thermoelectric material) was printed with an overlapped area of 4 mm², using polyimide patterned masks for both the N-and P-legs. (b) Images of the multi-axis stretchable knitted fabric. (c) SEM images of a pristine knitted fabric illustrated with the knit structure configuration of a loop head and two loop legs (left). Additional SEM images show an enlarged surface with the composite of PEDOT:PSS doped with DMSO (top right) and silver nanoparticles (AgNPs) (bottom right) on knitted fabric. Scale bars are 200 μm. Raman spectra of AgNPs (d), graphene (e), and PEDOT:PSS doped with DMSO (f).

Fig. 2. Thermoelectric properties and output characteristics of the temperature sensor. (a) Principle of the thermoelectric temperature sensor. (b) Output Seebeck voltage and (c) Seebeck coefficient of various thermocouples on the knitted fabric.

Fig. 3. Schematic diagram of the corresponding deformations of knitted loops and thermoelectric performance of the temperature sensor in relation to stretching deformation. (a) Knitted loops allow elongated, widened, and distorted geometry. Output Seebeck voltage of thermocouple (fabricated with AgNPs and PEDOT:PSS) when (b) elongated, (c) widened, and (d) distorted. Seebeck coefficient of thermocouple (fabricated with AgNPs and PEDOT:PSS) when (e) elongated, (f) widened, and (g) distorted.

Fig. 4. Temperature sensor characteristics as a function of stretching cycles at 20%. (a) Output Seebeck voltage (b) Seebeck coefficient.

Fig. 5. (a) Schematic layout and (b and d) optic images of the wearable 5 × 5 temperature sensor array, showing that the surface of the array is locally touched by fingers on the specific areas. (c and e) Output Seebeck voltage mapping and corresponding temperature distribution.