Wearable multi-sensing double-chain thermoelectric generator

Wearable electronics are vital for IoT applications like environmental sensing and health monitoring. However, limited battery life necessitates frequent recharging. Micro-energy harvesting techniques offer a solution, including thermoelectric generators (ThEGs), triboelectric generators (TEGs), piezoelectric generators (PEGs), and solar cells. While TEGs and PEGs require movement and solar cells need light, ThEGs offer continuous DC output, making them promising for wearable applications. ThEGs function based on the Seebeck effect, generating current from a temperature difference. For wearable use, flexibility, stability, and high performance are crucial. Screen printing offers a cost-effective mass-production technique to address this need but most wearable ThEGs only act as power sources, not sensors. This research presents a wearable DC-ThEG to overcome these limitations by integrating energy harvesting and multi-functional sensing capabilities. The screen-printing technique is employed to create two chains of thermocouples, while a silk fibroin layer is introduced to provide the sensing functionality.

The literature review highlights the increasing importance of wearable electronics and the limitations imposed by short battery life. Several micro-energy harvesting techniques have been explored, each with its limitations. TEGs and PEGs produce AC and require movement, while solar cells require light. ThEGs, operating on the Seebeck effect, produce continuous DC current from temperature gradients, making them suitable for wearable applications. However, existing wearable ThEGs often lack flexibility or sufficient performance due to the use of rigid materials or high contact resistance. The use of organic or organic-based composite thermoelectric materials has been proposed to enhance flexibility, but their low power output remains a constraint. Screen printing offers a cost-effective mass-production method but is seldom combined with sensing capabilities. Therefore, research focuses on improving the integration of ThEGs with sensing functions to create a truly self-powered and multifunctional wearable device.

The study involved the preparation of n-type (Bi2Te2.7Se0.3) and p-type (Sb2Te3) thermoelectric inks using a liquid epoxy resin base, MHHPA hardener, and EMIP catalyst. Bi2Te2.7Se0.3 and Sb2Te3 powders were added to create the respective inks. A silk fibroin solution was prepared by boiling Bombyx mori cocoons in sodium carbonate to remove sericin, dissolving the silk fibers in lithium bromide, and then dialyzing and purifying the solution. The DC-ThEG was fabricated by screen printing the n-type and p-type thermoelectric inks onto a polyimide (PI) film substrate to form two separate chains of thermocouples. The silk fibroin solution was then applied to the gap between the chains. The device's surface morphology was characterized using SEM, and thermoelectric characteristics were tested using a heating plate and digital multimeter, measuring the open-circuit voltage and output power at varying temperature differences. Capacitance measurements using an LCR meter were performed to assess the sensing capabilities in different humidity conditions, simulating gas and liquid states of water. Finally, the charging process of capacitors was characterized by a combination of a digital oscilloscope and an electrometer to verify the ability of the DC-ThEG to power commercial electronics.

The fabricated DC-ThEG demonstrated excellent flexibility and uniformity based on photographic, optical microscope, and SEM images. The device exhibited a high open-circuit voltage of ~151 mV at a temperature difference of 50 °C and an output power of 13 µW. The output power showed a rising trend with increasing temperature difference and an optimal load resistance of around 1.8 kΩ. By connecting the device to a switching circuit, 22 capacitors could be charged to reach a 3.3 V output, sufficient to power a commercial calculator. The DC-ThEG successfully demonstrated the ability to detect the presence of liquid-state water in the air by monitoring the capacitance change of the silk fibroin layer which responds to the dielectric constant of water. The linear relationship between the dielectric constant of silk fibroin and temperature suggests potential as a temperature sensor. The combined functionality of energy harvesting, liquid water detection, and temperature sensing was confirmed through experimental comparisons.

The results demonstrate the successful integration of energy harvesting and multi-functional sensing in a flexible and wearable device. The high open-circuit voltage and ability to power a commercial calculator show the potential for practical applications. The novel double-chain configuration enhances the device's functionality without significantly affecting its power density. The use of silk fibroin as a sensing material is advantageous due to its biocompatibility and responsiveness to environmental factors. The findings address the challenge of developing self-powered and multifunctional wearable devices for various applications. The successful demonstration opens exciting opportunities for creating all-in-one microsystems.

This study successfully demonstrated a novel wearable DC-ThEG that integrates energy harvesting and multi-functional sensing capabilities. The device exhibits high performance, flexibility, and cost-effectiveness due to the screen-printing fabrication technique and the utilization of a silk fibroin sensing layer. Future research directions could explore optimizing the thermoelectric material composition and device design to enhance efficiency and expand sensing capabilities. Investigating different biocompatible materials for sensing layers could further broaden the applications of this technology.

While the DC-ThEG demonstrates promising results, limitations exist. The current power output may not be sufficient for all wearable electronics. Further optimization of the device design and material selection is needed to improve power generation efficiency. The long-term stability of the silk fibroin layer under various environmental conditions requires further investigation. The study's scope mainly focuses on humidity and temperature sensing; exploring other sensing modalities could enhance the device's versatility.