Flexible thermoelectric energy harvesters using bulk thermoelectric materials and low-resistivity liquid metal interconnectsMehmet C. Ozturk and M. DickeyYasaman Sargolzaeiaval, Viswanath Padmanabhan Ramesh & Taylor Neumann

North Carolina State University

Advanced Self-Powered Systems of Integrated Sensors and Technologies

(ASSIST) Nanosystems Engineering Research Center

1

ASSIST vision

ASSIST’s vision of health and wellness is enabled by its

disrup ve system features

Self-Powered/Ultra Low Power

Wireless

Hassle-Free/Comfortable

Medically Validated

Non-invasive/minimally invasive

Mul -modal health and environmental sensors

Long-term monitoring of personal health & environment enabled by always-on pla orms

Sophis cated picture of health via correla on of mul ple sensors

Enable a pathway towards personalized medicine

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Heat - ThermoelectricMotion - Piezoelectric

ASSISTRoadmap

3

ASSIST Low-Power Sensor Technologies

Long term monitoring of environmental exposures & health

DetectionWheeze / Cough

Critical Data CorrelationOzone / Respiratory &

Cardiac Health

HET 1Asthma Management

ECG PPG VOC Ozone

Critical Data CorrelationFood Intake - Glucose / Lactate

Levels & Uric Acid - Wound Health

HET 2Diet Management &

Wound Healing

Wound Fluid ISF Sweat

Non-Invasive Glucose/Lactate & Wound Health Monitoring

Glucose Lactate Uric AcidpH Breathable Materials

ASSIST Zero-Power Extraction and Transportation Technologies

ASSIST Low-Power Detection Technologies

Critical Data CorrelationMedication Intake & Physiological

Parameters from HET 1.0 & 2.0

Sweat

Non-Invasive Medication Detection

Monitoring of Medication Intake

ASSIST Zero-Power Extraction and Transportation Technologies

ASSIST Low-Power Detection Technologies

ISF

HET 3Medication Adherence

Vigilant ECG (R-R) and Motion

Critical Data CorrelationA-Fib and Activity Identification

Optimized for Low-Power

SAP 1Cardiac Health

ECG AFE

ASSIST Energy Harvesting and Low-Power Circuit Technologies

SoC & RadioSupercapacitorsEnergy Harvesting

SAP 1.0 + HET 1.0

Critical Data Correlation HET 1.0 and Pulsed Transit Time

Blood Pressure

SAP 2Cardiac Health, Blood

Pressure & Asthma

Ozone

ASSIST Energy Harvesting, Sensor & Low-Power CircuitTechnologies

PPGEnergy Harvesting

ECG VOC

SAP 2.0 + HET 2.0

Critical Data CorrelationMetabolic Changes &

Cardiac Health

SAP 3Cardio-Metabolic Health

Ozone

ASSIST Energy Harvesting, Sensor & Low-Power CircuitTechnologies

PPGEnergy Harvesting VOC

ECGBiomarkers < Sweat/ISFBP

Health and Environmental Tracker

Self-Powered Adaptive Platform

Harvesting Heat from the Body

The application imposes large thermal resistances

Flexible thermoelectric generators (TEGs) are desirable:● Conformal to the body

● Better contact with the skin

● Large area harvesting● Simple Integration

● Electrical resistance

● Aesthetics

SkinResistance

ContactResistance

Flexible Heatsink &Small Form Factor

Heat

LateralHeat Spread

Heat Flowthrough Filler

Vertical Heat FlowThrough Polymeric Substrate

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Harvesting Heat from the Body

Key Challenge: Small ΔT across the harvester

SkinResistance

ContactResistance

Flexible Heatsink &Small Form Factor

Heat

LateralHeat Spread

Heat Flow through Filler

Vertical Heat FlowThrough Polymeric Substrate

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Our approach to Flexible Thermoelectrics

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● Bulk thermoelectric materials● Best materials used in rigid TEGs

● No new material development

● Pick-and-Place Tooling● Standard technique

● Flexible packaging● Material Innovations

Ag/AgCl

Ag Nanowires

EGaIn

A flexible approach with a low cost-of-ownership that can rival the performance of rigid TEGs

Eutectic Gallium Indium (EGaIn)Gallium Indium

+ =30 °C 157 °C 16 °C

Liquid• Low viscosity• Low toxicity• Near-zero vapor pressure

Adv. Fun. Mat. 2009

Can be encapsulated by an elastomer

Adv. Mater. 2016

8

TEG Fabrication with EGaIn Interconnects

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TE Leg PDMS

1.31mm

1.45mm

EGaIn

TEG with liquid interconnects

PDMS Encapsulation

Suarez at. Al, Flexible thermoelectric generator using bulk legs and liquid metal interconnects for wearable electronics, Applied Energy, 2017

-505

101520253035404550

0 0.5 1M

easu

red

Pow

er (µ

W)

Air Velocity (m/s)

5°C

22°C

32°C

Testing on the Human Body

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Promising performance despite• No heat spreaders• No heatsink• Thick PDMS encapsulation

Mechanical Testing

10

Mechanical Testing

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10mm 35mm

• Low Resistance - negligible contribution from interconnects• Spikes recovered due to “self-healing” nature of EGaIn

Further Improvements

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Thin Metal Spreaders

Reduction of Filler Thermal Conductivity

Thinner Top and Bottom

Elastomers &Higher Thermal Conductivity

Taller Legs

Printing and Spray Coating of EGaIn

● Printing provides faster and more reliable interconnects on TEGs with large leg count

● Spray coating of encapsulation provides a much thinner encapsulation 13

Flexible TEG with 256 legs & Spray Coated Encapsulation

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0

0.05

0.1

0.15

0.2

0.25

0.3

0 20 40Ther

mal

Con

duct

ivity

(W/m

K)

Volume percentage

Alumina doped PDMS

Al2O3, AlN, and BN Doped PDMSIncreasing the thermal conductivity of the encapsulating layer

153X enhancement can be achieved in the thermal conductivity of PDMS

0.00

0.10

0.20

0.30

0.40

0.50

0 20 40Ther

mal

Con

duct

ivity

(W/m

K)

Volume percentage

AlN doped PDMS

Material Thermal Conductivity Powder size

Al2O3 Powder ~35 W/mK ≤10 µm

AlN Powder ~150 W/mK ~10 µm

BN Powder ~30 W/mK ~1 µm

0

0.1

0.2

0.3

0.4

0.5

0 20 40Ther

mal

Con

duct

ivity

(W/m

K)

Volume Percentage

BN doped PDMS

Output Power – Device characterization on the wrist – Impact of time on the wrist

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0

50

100

150

200

0 0.5 1O

utpu

t pow

er (µ

W)

Air velocity (m/s)

Room Temperature (22 °C)

Immediate

1 minute

5 minutes0

20406080

100120

0 0.5 1

Out

put P

ower

(µW

)

Air Velocity (m/s)

Outdoor (~32 °C)

Immediate

1 minute

5 minutes0

50

100

150

200

0 0.5 1

Out

put P

ower

(uW

)

Air Velocity (m/s)

Refrigerator (1 °C)

Immediate

1 minute

5 minutes

3X improvement in performance with• Thin PDMS encapsulation• Al2O3 doped PDMS top/bottom layers

Still… No heatsink or heat spreaders

Time on body: Time on body: Time on body:

Device characterization on the wristVoltage, Temperature Differential and Power

32°C

22°C

1°C

0

5

10

15

20

25

30

35

40

0 0.2 0.4 0.6 0.8 1M

easu

red

Load

Vol

tage

(mV)

Air Velocity (m/s)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.2 0.4 0.6 0.8 1

Tem

pera

ture

Diff

eren

ce (°

C)

Air Velocity (m/s)

32°C

22°C

1°C

22°C

1°C

32°C

020406080

100120140160180200

0 0.2 0.4 0.6 0.8 1

Out

put P

ower

(µW

)

Air Velocity (m/s)

Benchmarking (w/o Air Flow)

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0.05 0.178

1

0.224 0.25 0.084

33.2

4.5

00.5

11.5

22.5

33.5

44.5

5

Outp

ut P

ower

(µW

/cm

2 )

Author/Temperature (°C)

PDM

S +

Dis

pens

ing

prin

ting

of T

E

legs

, Yon

seiU

nive

rsity

(Kor

ea)/2

012

Flex

ible

fabr

ic+

Dis

pens

ing

prin

ting

of

TE l

egs

Yons

eiU

nive

rsity

(Kor

ea)/2

014

CN

T TE

legs

/ No

enca

psul

atio

nTe

xas

A&M

Uni

vers

ity/2

014

PDM

S +

Dis

pens

ing

prin

ting

of T

E le

gsYo

nsei

Uni

vers

ity (K

orea

)/201

5

CN

T TE

legs

/ No

enca

psul

atio

nTe

xas

A&M

Uni

vers

ity/2

014

PDM

S +

Dis

pens

ing

prin

ting

of T

E le

gsYo

nsei

Uni

vers

ity (K

orea

)/201

5

Mul

ti st

age

TEG

/ PD

MS+

El

ectro

chem

ical

dep

ositi

on

of T

E le

gs/T

ohok

oU

nive

rsity

(Jap

an)/2

017 ASSIST

Tamb = 15oC

Tamb = 22oC

Tamb = 1oC

Further Improvements – COMSOL simulation

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Low Thermal Conductivity Elastomer

Air bubbles

Aerogel Particles

Aerogel Islands

A low thermal conductivity elastomer between the legs can increase the power by ~ 2X

Graphene/EGaIn Doped Elastomer● Our approach was elastomer

doping with high thermal conductivity particles

● We have tried● Al2O3, AlN, BN, Graphene and

EGaIn

● Best results were obtained with EGaIn + Graphene Doping

● 0.85 W/mK (5.6X improvement)

● We can increase the encapsulating thickness to 200 um without paying significant penalty0

2

4

6

8

10

12

0 0.5 1

Outp

ut p

ower

(uW

/cm

2 )

Fill Factor

50um PDMS50um 1W/mK elastomer100um 1W/mK elastomer200 um 1 W/mK elastomer

3D COMSOL Modeling(Natural Convection)

Graphene percentage

PDMS + Graphene

PDMS + EGaIn+ Graphene

2.2% 0.602 W/mK 0.838 W/mK

4.3% 0.652 W/mK 0.853 W/mK

6.3% Cannot measure 0.871 W/mK

Improved TEG design

P N P N P N P

Low Thermal ConductivityElastomer

NanocompositeBiTe Legs

Low-ResistivityEGaIn Interconnects

High Thermal ConductivityElastomer

TitaniumGold

Copper Heat Spreader

Spray CoatedThin Elastomer

Conclusions

● EGaIn provides ● Printable, low-resistivity, stretchability

● Self-healing, reliability

● Room-temperature bonding to TE legs

● There is still much room for improvement● Thermal management - Device packaging and materials

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There is potential for producing flexible TEGs that rival the performance of rigid TEGs

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