Thermal Energy Harvesting with Thermoelectrics for Self-powered Sensors: With Applications to Implantable Medical Devices, Body Sensor Networks and Aging in Place
- Author(s): Chen, Alic
- Advisor(s): Wright, Paul K
- et al.
This work examines the feasibility of applying thermoelectric generators as power sources for implantable applications. Thermoelectric design principles, manufacturing methods and novel materials are foundational aspects of the work.
Rapid advancements in the field of biomedical engineering has led to the vast number of implantable medical devices developed within the last few decades. As implantable medical devices provide more functionality, sufficient energy storage while maintaining compactness becomes challenging. The lifetime of implanted medical devices will often be much shorter than the expected lifespan of patients, adding risks and costs to the patient in the form of additional surgical procedures. A perpetual power source that extends the longevity of implantable devices still remains elusive. This presents opportunities for solid-state thermal energy harvesting with thermoelectric energy generators (TEGs) that scavenge waste heat, the most abundant source of energy from the body.
Thermoelectric energy generators (TEGs) provide solid-state energy by converting temperature differences into usable electricity. Since the fat in the human body provides thermal insulation, the largest temperature differences (typically 1-5 K) are found in the highest fat regions of the body. Bioheat transfer modeling shows that the optimal placement of TEGs for energy generation is in the abdomen under high convective conditions. Based on average 100 µW (at 1 V) input power requirements of implantable medical devices, thermoelectric and heat transfer design theories suggest a need for high aspect ratio thermoelectric elements in high density arrays to take advantage of the low temperature differences in the fat layer.
In order to maximize power output, traditional thermoelectric device designs must be abandoned and a planar TEG device design is proposed as an effective and scalable method for implantable medical applications. Dispenser printing was then shown as a scalable and repeatable manufacturing method for depositing thick-film thermoelectric materials in the fabrication of planar TEGs. The use of printed fabrication methods led to the development and synthesis of novel printable composite thermoelectric materials. The thermoelectric properties of the printed thermoelectric materials were analyzed and carefully characterized as a function of temperature. The maximum dimensionless figure of merit (ZT) at 302K for an n-type Bi2Te3-epoxy composite was 0.18 when cured at 250°C, while the ZT of a p-type Sb2Te3-epoxy composite cured at 350°C was 0.34.
A 50-couple TEG prototype with 5 mm x 640 µm x 90 µm printed element dimensions was fabricated on a polyimide substrate with evaporated metal contacts. The prototype device produced a power output of 10.5 µW at 61.3 µA and 171.6 mV for a temperature difference of 20K resulting in a device areal power density of 75 µW/cm2. The results of the work are promising and alternative methods to improve the performance of future devices are proposed. While the initial focus of this work was specific to the field of biomedical devices, the technologies that have been developed are applicable to other fields involving energy harvesting. The prospective impact of this work ultimately paves the path towards the advanced healthcare system of the future based on integrated autonomous wireless systems for the needs of "aging in place" or "aging at home" technologies.