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Power (μW);
Number of thermocouples .
1500
1000
500
0
n/2
l /t
0.001 0.01 0.1 1 10 100
Length of thermocouple legs (mm)
interface area of 10 × 10 μm 2 is assumed (fixed) while t
decreases to 2 μm at l = 1 μm. (On practice, this is done by
using microelectronic technologies [2].) At l = 1 μm, the
resistance still increases by a factor of 4 as compared with
1.5 cm-long thermopile. If the contact resistance could be
decreased to 10 Ω μm 2 , it would allow reaching over 200
μW at l = 10 μm (i.e., about 64% of the power obtainable at
l = 15 mm), and 180 μW at l = 3 μm. Calculations show that
the thermal resistance of the thermopile and the temperature
drop on it stay almost the same over the whole range for a
thermocouple length, Fig. 5, unless the enlargement of
electrical contacts is performed at l < 10 μm. This
enlargement also adversely affects the parasitic thermal
resistance R empty at l < 10 μm, Fig. 5, and demands reoptimizing
the thermal resistance of a thermopile using the
equation of thermal matching. To compensate for some
decrease of ΔT at l < 10 μm due to enlargement of electrical
contacts, a number of thermocouples, equal to n/2, is
increased, Fig. 6. The ratio l / t shown in Fig. 6 demonstrates
that the length of thermocouple legs in the range of 5-10 μm
allows reaching relatively high power using microelectronic
technologies. The related research is ongoing and 6 μm-tall
structures have been already fabricated, Fig. 7 [3]. However,
the structures are based on poly-SiGe that shows far worse
thermoelectric properties than BiTe used in the modeling
above, i.e., its factor Z is about 0.05-0.1, at least by a factor
of 10 less. Therefore, one more step is required for reaching
the calculated targets, i.e., development of the similar
technological process, but based on materials with higher Z.
P
7-9 October 2009, Leuven, Belgium
100
VI. APPLICATION OF THERMAL MATCHING IN
WEARABLE MEDICAL DEVICES
10
1
0.1
Fig. 6. Optimum number of thermocouples, l / t ratio and maximum power
in a wearable TEG of 3 cm × 3 cm × 1.7 cm size at a contact resistance of
100 Ω μm 2 between semiconducting legs and metal interconnects.
Al
p-SiGe
n-SiGe
3 μm
Fig. 7. Arcade thermopile: the design and fabricated thermopile-like test
structure with 6 μm-tall released air bridges [3].
l / t ratio
Small-size thermopiles available on the market do not fit
the requirements for the thermocouple leg length coming out
of the modeling of an optimal thermopile because their l / t
ratio is much smaller than needed. An appropriate aspect
ratio then can be obtained by stacking thermopiles on top of
each other [4, 5]. This increases R TEG
to the required R TEG,opt
and hence allows reaching the maximum of output power.
Before fabricating prototypes of body-powered medical
devices, the principles of thermal matching have been
verified on watch-size wrist TEGs [4, 5]. Then, one of such
TEGs has been used for powering a wireless pulse oximeter
(SpO 2
sensor) [6], Fig. 8a. The device non-invasively
measures oxygen content in arterial blood. A TEG used in
this device provides a power of about 200 µW on average
with usual variations within the 100-600 µW range.
Typically, battery-powered pulse oximeters existing on the
market consume above 10 mW. Therefore, before making it
powered from the human body, a power consumption of
electronics has been reduced by a factor of 10 3 . As a result,
the electronics module and 2.4 GHz wireless link together
consume 62 µW. The device is battery-less and operational
up to 25-26°C. At higher ambient temperatures, the power
shortages are expected. If this happens, the device switches
into a sleeping regime for a while. During sleep, its power
consumption is extremely low and the device wakes up again
upon collecting enough charge in the supercapacitor.
Another example of body-powered battery-less devices is
an electroencephalography (EEG) system-in-a headband [7]
consuming 0.8 mW. The main challenges in creation such
complex system powered by the wearer’s heat are (i)
lowering power consumption of biopotential readout while
maintaining the signal quality and (ii) real-time data
transmission. In the above pulse oximeter, the signal
processing is performed onboard, so that power consumed
by the transceiver is minimal. In EEG, on the contrary, realtime
brain waveform should be transmitted, so the radio
consumes large power, and a large-size TEG is required.
Therefore, it is designed as 10 sections of 2×4 cm 2 size on a
stretchable headband, Fig. 8b. Radiators on the outer side of
thermopiles ensure effective thermal matching of the TEG
with the environment. The TEG is designed for indoor use at
typical ambient temperatures maintained in hospital wards.
At 22°C, it produces about 30 µW/cm 2 , i.e., close to the limit
of power on people at this temperature. There is however a
drawback of such high power generation: at lower ambient
temperatures, the heat flow rapidly exceeds the sensation of
discomfort and the device turns into uncomfortably cold
object. (For example, at 19°C, the TEG already produces
3.7 mW, but sensation of cold becomes too annoying.)
To avoid sensation of cold, another version of the EEG
system, a headphone-like EEG diadem, Fig. 8c, has been
provided with a hybrid thermoelectric-photovoltaic energy
harvester, [7]. This device is comfortable down to 7°C.
The electrocardiography (ECG) system-in-a-shirt, Fig. 8c,
unlike above devices, is powered from a secondary battery
[8]. The battery is constantly recharged using the wearer’s
body heat. The power consumption is 0.5 mW, so about
0.7 mW are required from the TEG. Fourteen 6.5 mm-thin
©EDA Publishing/THERMINIC 2009 99
ISBN: 978-2-35500-010-2