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John O | October 2017

Circuit design demonstrates possibility of wearable thermoelectric generators


Engineers at the Georgia Institute of Technology (Atlanta) have used flexible polymers and novel circuitry patterns printed on paper to demonstrate the potential for wearable thermoelectric generators that harvest energy from the body to power biosensors.

 


Electrical conductivity is measured for a thermoelectric polymer film in the laboratory of Shannon Yee at the Georgia Tech. (Candler Hobbs/Georgia Tech)

 

According to a report from Georgia Tech, the symmetrical fractal wiring patterns that the engineers developed can be sized to provide the power necessary for specific applications to function. The generators would be inkjet onto flexible substrates, including fabric, and manufactured with 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,” the article explained.

 

It continued, “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.”

 

Engineers overcame these challenges by designing a device with thousands of closely-packed dots made of alternating p-type and n-type polymers. The packing density decreases the interconnect length, lowering total resistance and allowing higher power output. The dot pattern was based on the Hilbert pattern, a continuous space-filling curve, one researcher explained.

 

“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,” the article noted. “That eliminates the need for power converters that add complexity and take power away from the system.”

 

Researchers have printed on paper but are now looking into fabrics that could be used in clothing. The devices can power small sensors that require microwatts or milliwatts, which is enough for a heart rate sensor, but not enough to power fitness trackers or smartphones.

 

The research was recently published in the Journal of Applied Physics. The abstract stated:

 

“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.”

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