Using flexible conducting polymers and novel circuitry patterns printed on paper, researchers have demonstrated proof-of-concept wearable thermoelectric generators that can harvest energy from body heat to power simple biosensors for measuring heart rate, respiration or other factors.
Above – Electrical conductivity is measured for a thermoelectric polymer film in the laboratory of Shannon Yee at the Georgia Institute of Technology. (Credit: Candler Hobbs, Georgia Tech).
Because of their symmetrical fractal wiring patterns, the devices can be cut to the size needed to provide the voltage and power requirements for specific applications. The modular generators could be inkjet printed on flexible substrates, including fabric, and manufactured using inexpensive roll-to-roll techniques.
Thermoelectric generators, which convert thermal energy directly into electricity, have been available for decades, but standard designs use inflexible inorganic materials that are too toxic for use in wearable devices. Power output depends on the temperature differential that can be created between two sides of the generators, which makes depending on body heat challenging. Getting enough thermal energy from a small contact area on the skin increases the challenge, and internal resistance in the device ultimately limits the power output.
To overcome that, Menon and collaborators in the laboratory of Assistant Professor Shannon Yee designed a device with thousands of dots composed of alternating p-type and n-type polymers in a closely-packed layout. Their pattern converts more heat per unit area due to large packing densities enabled by inkjet printers. By placing the polymer dots closer together, the interconnect length decreases, which in turn lowers the total resistance and results in a higher power output from the device.
“Instead of connecting the polymer dots with a traditional serpentine wiring pattern, we are using wiring patterns based on space filling curves, such as the Hilbert pattern – a continuous space-filling curve,” said Kiarash Gordiz, a co-author who worked on the project while he was a Ph.D. student at Georgia Tech. “The advantage here is that Hilbert patterns allow for surface conformation and self-localization, which provides a more uniform temperature across the device.”
The new circuit design also has another benefit: its fractally symmetric design allows the modules to be cut along boundaries between symmetric areas to provide exactly the voltage and power needed for a specific application. That eliminates the need for power converters that add complexity and take power away from the system.
“This is valuable in the context of wearables, where you want as few components as possible,” said Menon. “We think this could be a really interesting way to expand the use of thermoelectrics for wearable devices.”
So far, the devices have been printed on ordinary paper, but the researchers have begun exploring the use of fabrics. Both paper and fabric are flexible, but the fabric could be easily integrated into clothing.
“We want to integrate our device into the commercial textiles that people wear every day,” said Menon. “People would feel comfortable wearing these fabrics, but they would be able to power something with just the heat from their bodies.”
With the novel design, the researchers expect to get enough electricity to power small sensors, in the range of microwatts to milliwatts. That would be enough for simple heart rate sensors, but not more complex devices like fitness trackers or smartphones. The generators might also be useful to supplement batteries, allowing devices to operate for longer periods of time.
Among the challenges ahead are protecting the generators from moisture and determining just how close they should be to the skin to transfer thermal energy – while remaining comfortable for wearers.
The researchers use commercially-available p-type materials, and are working with chemists at Georgia Tech to develop better n-type polymers for future generations of devices that can operate with small temperature differentials at room temperatures. Body heat produces differentials as small as five degrees, compared to a hundred degrees for generators used as part of piping and steam lines.
“One future benefit of this class of polymer material is the potential for a low-cost and abundant thermoelectric material that would have an inherently low thermal conductivity,” said Yee, who directs the lab as part of the Woodruff School of Mechanical Engineering. “The organic electronics community has made tremendous advances in understanding electronic and optical properties of polymer-based materials. We are building upon that knowledge to understand thermal and thermoelectric transport in these polymers to enable new device functionality.”
Organic materials can be printed into thermoelectric (TE) devices for low temperature energy harvesting applications. The output voltage of printed devices is often limited by
(i) small temperature differences across the active materials attributed to small leg lengths and
(ii) the lower Seebeck coefficient of organic materials compared to their inorganic counterparts.
To increase the voltage, a large number of p- and n-type leg pairs is required for organic TEs; this, however, results in an increased interconnect resistance, which then limits the device output power. In this work, we discuss practical concepts to address this problem by positioning TE legs in a hexagonal closed-packed layout. This helps achieve higher fill factors (∼91%) than conventional inorganic devices (∼25%), which ultimately results in higher voltages and power densities due to lower interconnect resistances. In addition, wiring the legs following a Hilbert spacing-filling pattern allows for facile load matching to each application. This is made possible by leveraging the fractal nature of the Hilbert interconnect pattern, which results in identical sub-modules. Using the Hilbert design, sub-modules can better accommodate non-uniform temperature distributions because they naturally self-localize. These device design concepts open new avenues for roll-to-roll printing and custom TE module shapes, thereby enabling organic TE modules for self-powered sensors and wearable electronic applications.
In the last decade, the organic TE community has largely focused on developing new materials and improving the TE properties of polymers. Despite having zT values comparable to inorganics for low grade energy harvesting, the demonstrated device prototypes have shown low performance. This indicates that device design and module engineering also play a crucial role in realizing practical devices. Given the solution processability of organic materials, we are not restricted to utilizing fabrication techniques developed for inorganic semiconductors, and we can instead leverage the unique properties of organic materials to design high performance modules. In this work, we suggest that by positioning n- and p-type legs in a closed-packed hexagonal layout and wiring them based on a Hilbert space-filling curve pattern a better module performance can be achieved. The closed-packed layout increases the fill factor, and the fractal nature of the Hilbert curve pattern allows for tessellation into sub-modules for load-matching to a variety of end applications. Furthermore, the tessellations are naturally more tolerant to non-uniform temperature distributions as sub-modules are localized in a Hilbert mapping, which is beneficial for wearable electronics. These concepts can be readily implemented in devices that are based on the existing organic TE materials via R2R printing techniques for large-scale deployment at a low cost for energy harvesting applications. Although the discussions were mainly directed toward printable organic TE modules, it should be noted that the hexagonal packing and the Hilbert interconnect wiring extend to printable inorganic TE modules as well