Wearable multi-sensing double-chain thermoelectric generator

The study addresses the challenge of limited battery life in wearable electronics used for environmental sensing and health monitoring. It explores thermoelectric generators (ThEGs) as promising power sources for wearables due to their continuous DC output and independence from light or mechanical motion, unlike tribo-electric and piezoelectric generators or solar cells. Traditional inorganic bulk ThEGs are rigid and brittle, while organic and organic-composite flexible ThEGs often suffer from low output performance due to high contact resistance. Recent screen-printing approaches using Bi2Te2.7Se0.3 and Sb2Te3 inks on flexible substrates have achieved higher-performance flexible ThEGs. However, most wearable ThEGs only act as power sources for sensor networks and are not themselves sensors. The paper proposes integrating powering and sensing by introducing a double-chain ThEG that can both harvest heat and enable capacitive sensing.

Prior work includes micro energy harvesting technologies for wearables (thermoelectric, triboelectric, piezoelectric, and solar), each with constraints (AC output and motion dependency for TEGs/PEGs; light dependency for solar). Flexible ThEGs based on organic materials have been explored but with limited performance due to contact resistance. Screen-printing of inorganic thermoelectric inks (Bi2Te2.7Se0.3 and Sb2Te3) onto flexible substrates has emerged as a scalable, cost-effective method to fabricate high-performance flexible ThEGs. Nonetheless, reported wearable ThEGs typically serve solely as power supplies; integration with sensing functions remains underdeveloped. This work builds on screen-printed inorganic inks and introduces a double-chain structure enabling simultaneous energy harvesting and capacitive sensing using a silk fibroin dielectric layer.

Thermoelectric inks preparation: Two thermoelectric inks were formulated. A paste synthesis was prepared by sequentially mixing polypropylene glycol diglycidyl ether (PPGDGE) and bisphenol F diglycidyl ether (BPFDGE) (1:1 by weight) to form the liquid epoxy resin (60.4 wt.%), then adding methylhexahydrophthalic anhydride (MHHPA, 38.6 wt.%) as hardener, and 2-ethyl-4-methyl-1H-imidazole-1-propanenitrile (EMIP, 1 wt.%) as catalyst. Bi2Te2.7Se0.3 and Sb2Te3 powders were then mixed with the paste at weight ratios 1:5 and 1:6 to yield n-type and p-type inks, respectively. Silk fibroin solution preparation: Bombyx mori cocoons were boiled in 0.02 M Na2CO3 for 45 min to remove sericin, rinsed five times in deionized water, and dried at 45 °C for 2 h. The fibers were dissolved in 9.3 M LiBr at 60 °C for 4 h. The solution was dialyzed (3.5 kDa MWCO) for 48 h to remove LiBr and then microfiltered through 5 µm filters. The silk fibroin solution was stored below 4 °C to suppress gelation. Device fabrication (DC-ThEG): A 50 µm-thick polyimide (PI) film (60 mm × 35 mm) served as the flexible substrate. Using a 150-mesh pre-patterned screen, n-type Bi2Te2.7Se0.3 ink was screen-printed to form thermoelectric legs and cured at 90 °C for 30 min. A complementary 150-mesh screen was then aligned to print p-type Sb2Te3 legs. Both printed materials underwent thermal annealing at 380 °C for 4 h in N2 to recrystallize and prevent oxidation. Junctions used partially overlapping printed regions of n- and p-type materials (no metal interconnects) to reduce resistance and simplify structure. Two separate chains, each with 5 p–n pairs, formed the double-chain thermocouple configuration (10 pairs total). Silk fibroin solution was dispensed over the inter-chain gap and dried at 45 °C for 30 min to form a sensing membrane bridging the two chains. Tests and measurements: Surface morphology was characterized by SEM (JSM-7600F). A heating plate provided the heat source; hot and cold side temperatures were measured by a thermometer (GM1312). Electrical output voltages and resistances were measured with a digital multimeter (DM3068). A natural cooling protocol was used: starting from ΔT = 60 °C, the hot plate was turned off; measurements at desired ΔT were repeated three times to average results. Capacitor charging was monitored using a digital oscilloscope (DS2302A) and an electrometer (Keithley 6514). For sensing tests of liquid-state water in air, two humidity controllers generated gaseous- and liquid-state water environments; an LCR meter (E4890A) measured real-time capacitance changes between the two printed chains used as electrodes. Device geometry and resistance: Device size 60 mm × 35 mm × ~0.2 mm; leg gap 0.8 mm; leg width 1.5 mm; leg length 22 mm; total series resistance for both chains (10 pairs) 1.75 kΩ. Average printed thicknesses: n-type ~105 µm, p-type ~83 µm.

- The double-chain ThEG (DC-ThEG) integrates energy harvesting and multi-functional capacitive sensing using the two printed thermoelectric chains as electrodes bridged by a silk fibroin layer. - Flexibility and uniform printing quality were demonstrated; SEM showed compact, well-distributed surfaces. - Thermal response: applying a 90 °C heat source led to hot side T1 rising from 25.2 °C to 86.5 °C and cold side T2 rising from 25.2 °C to 36.5 °C. - Electrical performance: open-circuit voltage increased linearly with ΔT; VOC ≈ 151 mV at ΔT = 50 °C. Output power reached ≈ 13 µW at ΔT = 50 °C. - Load characteristics: at fixed ΔT, output power versus load resistance exhibited a maximum near the matched load of ~1.8 kΩ; at fixed load, power increased with ΔT. - Device parameters: overall resistance 1.75 kΩ (10 pairs, two chains in series); dimensions 60 mm × 35 mm × 0.2 mm; leg gap 0.8 mm; leg width 1.5 mm; leg length 22 mm; printed thicknesses n-type ~105 µm, p-type ~83 µm. - Energy storage and application: using a series-parallel switching circuit, the DC-ThEG charged twenty-two 2200 µF capacitors to 3.3 V, sufficient to drive a commercial calculator using harvested biothermal energy. - Sensing: the silk fibroin layer enabled detection of liquid-state water droplets in air (as distinct from water vapor) via capacitance changes, owing to differential absorption by silk fibroin; due to the linear dependence of silk fibroin dielectric constant on temperature, the device also shows potential for temperature sensing. - The powering and sensing functions operated compatibly under each other’s working conditions, and the device exhibited notable mechanical and electrical output stability.

The double-chain configuration enables two key capabilities: (1) thermal energy harvesting via screen-printed Bi2Te2.7Se0.3/Sb2Te3 thermocouples and (2) capacitive sensing by using the two independent chains as electrodes bridged by a silk fibroin dielectric. Partially overlapped printed junctions reduce interconnect resistance and simplify the architecture, contributing to improved device performance and manufacturability. The linear VOC–ΔT relationship and a matched load near 1.8 kΩ confirm predictable thermoelectric behavior suitable for power management. The device’s ability to charge capacitors to 3.3 V and power a calculator demonstrates practical energy utilization from human body heat. For sensing, silk fibroin’s selective absorption of liquid-state water droplets versus vapor yields distinct capacitance responses, enabling detection of airborne liquid water. Additionally, the temperature-dependent dielectric constant provides a pathway for temperature sensing. Experiments showed that harvesting and sensing can be conducted concurrently without significant interference, supporting the feasibility of integrated, self-powered wearable sensing platforms. The use of scalable screen printing on flexible PI underscores potential for low-cost, mass fabrication of multifunctional wearable systems.

This work introduces a wearable, screen-printed double-chain thermoelectric generator that concurrently harvests thermal energy and performs capacitive sensing using a silk fibroin inter-chain layer. The device achieves VOC ~151 mV and ~13 µW output at ΔT = 50 °C, exhibits a matched load of ~1.8 kΩ, and demonstrates practical energy storage to 3.3 V to power a commercial calculator using human body heat. The silk fibroin layer enables detection of liquid-state water in air and indicates potential for temperature sensing. The double-chain architecture with overlapped printed junctions reduces resistance and simplifies integration, while the flexible PI substrate supports wearable applications. This integration of powering and sensing is a step toward all-in-one self-powered microsystems. Future work could focus on optimizing thermoelectric leg geometry and materials for higher power density, refining silk fibroin sensing layers for enhanced selectivity and sensitivity, and expanding the sensing modalities within the same platform.