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Stretchable Electronics - shaping electronic circuits -

Prof. Stphanie P. Lacour Lab. For Soft Bioelectronic Interfaces, EPFL

stephanie.lacour@epfl.ch

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What is stretchable electronics?

Video overview https://www.youtube.com/watch?v=jlEIvGzthsk

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Outline

Mechanical concepts Materials Examples of stretchable circuits

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Todays electronic devices

flexible

large-area brittle

trigger foreign body reaction

flat batch

processed

Si technology 18-in. wafer Gen 10: 2850mmx3050mm (2010)

shattered iPhone

Cortical electrode at 1month post-implantation. P. Tresco 2007

Circuits can roll and bend. HP Flexible display

Flex circuits

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Electronics with mechanical freedom

Infineon 2011 power electronics on

40m thick wafer anything thin is flexible

Samsung 2012 flexible OLED display

University of Tokyo 2008

Tactile skin Takao Someya et al.

UIUC 2011 Epidermal electronics

John Rogers et al.

UIUC 2012 Transient

electronics John Rogers et al.

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Background Large Area Electronics

Substrates: Glass Device materials: thin films Technology: planar, thin film processing Applications: Flat Panel Displays (handheld devices,

phones, cameras, monitors, TVs), e-paper

back

plan

e / f

ront

plan

e

Glass carrier: 10m 1mm thick

TFT layer: 1m thick LCD, OLED Display layer

Encapsulation: 1m 1mm thick

circuit diagrams R. Street Adv. Mat. 2009, 21, 1-16

display cross-section 6

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Background - Flexible Electronics

Substrates: Plastic or metallic foils Device materials: thin and thick films Technologies: planar thin-film processing, printing,

electro-plating, hybrid techniques Applications: interconnects, sensors, RF ID tags,

rollable displays, cochlear implants,

Flexible Circuit Technology J. Fjelstad , 2006, 214p.

milFlexCircuit -3M

RF ID tag

All printed TFT on plastic PARC 2009

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On curvatures

A B C

Gaussian curvature: K = 12

A saddle shape: K < 0 B cylinder shape: K = 0 (1 = 0) C spherical shape: K > 0

foil coating

K 0

E 10 GPa bone

E 10-100 kPa skin

Electronic skin wrapped over the fingers

K 0

Electronic tent Orange

K 0 8

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Materials enable applications

Biocompatible + Elastic + Electrically Active + Process compatible

Level of difficulty

Integration of materials with very different properties

Substrates: plastic, elastic Devices: inorganic and/or organic transistors, diodes, sensors, antennas Encapsulation: brittle/plastic composites, elastic

need new design rules that are applicable to all materials

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Pixellated approach structural and physical mismatch

device

IC

IC

device

device

elastic substrate

elastic wiring

top view

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Architecture of stretchable circuitry

Pixellated structure

elastic substrate

rigid platforms hosting devices

elastic wiring

no applied strain 2D stretched

platforms do not deform

elastic wiring stretches

Wagner et al, Phys. E 2004; Lacour et al., Proc. IEEE 2006 11

Stretchable circuits

Stretchable thin-film transistor circuits Imperceptible electronics

Science 325 (2009)

Nature 499 (2013)458

T. Someya

J. Rogers Lacour et al IEEE EDL 25 (2005)

Graz et al APL 98 (2011)

S. Wagner S. Lacour Z. Suo

VDD# Vout#

Vin#

GND#

2mm#

load#TFT#

stretchable#interconnects#

drive#TFT#

J. Vanfleteren

Stretchable microchip

circuits

S. Bauer

Science 344 (2014)

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Deforming a surface

Stress/strain

Elasticity/plasticity

Mechanical failure

Key Mechanical Concepts

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Deforming a surface: Flexibility

Developable surfaces surfaces that can be flattened onto a plane without distortion

cylinder

cone

flat

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Deforming a surface: Stretchability

Deformable surfaces Surfaces that can conform complex shapes once or many times Surfaces that can expand and relax reversibly

stretched inflated

flat

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Stress

Mechanical stress and strain

)Paorm/N(areaload)(stress 2=

%100LL

lengthoriginallengthinchange)(strain ==

5.00;z

y

z

x 0; compressive strain < 0

Poisson ratio

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Mechanical strain

strain () = change in lengthoriginal length

=LL

100%

R0 h

bend

surface bending strain = = h2 R0

tensile strain > 0

compressive strain < 0

h/2

neutral plane

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Stress (strain) curve

stre

ss

strain

plastic

elastic

linear regime

permanent deformation

tensile strength

)Pa(E

= Youngs modulus

fracture

unload

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Typical Stress(strain) curves

strain (%)

0 0.05 0.1 0.15 0.2 1 5 25

stre

ss (M

Pa)

0

50

100

150

200

250

300

350

400

450 Brittle (glass-like) materials SiNx, SiO2, Si High stress, low fracture strain

Plastic deformation metals, stainless steel, polyimide High stress, higher fracture strain

SiNx SiOx

Si

stainless steel

polyimide

metal

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rubber

Mechanical Failure

Brittle behaviour

Crack propagates

Semiconductors Dielectrics Metal oxides

Ductile behaviour

Film necks

Metals

Z. Suo, Harvard University, http://www.seas.harvard.edu/suo/

stre

ss

strain

brittle ductile

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Substrates Elastomers

Device materials Brittle materials

Encapsulation Materials Plastics or elastomers

Materials for Stretchable Electronics

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Selection of substrates

Application-dependent RF ID tag OLED display implantable interface

Physical form dependent Lightweight conformable rollable stretchable

Manufacturing process dependent Batch R2R

PARC flex photosensors

Polyonics

Holst Centre LG Display

GlobalSolar Michigan State Univ.

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Main substrate types for stretchable applications

Polymer substrates flexible, optical transparency, rolls or spinable poor thermo-mechanical stability, rough surface, not elastic

Elastomeric substrates reversible stretchability, spinable, moldable poor thermal stability thus process compatibility, chemical susceptibility

Biodegradable substrates stiff enough for manipulation, degrades quickly cannot process directly of these materials

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Plastic substrates PolyEthylene Terephthalate (PET)

PolyEthylene Naphthalate (PEN)

Thermoplastic polymer Widely available Low cost

Polyester polymer More recent material Very good barrier properties

Teonex

Mylar

Polyimide (PI)

Thermoplastic Very good thermal stability Kapton

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Elastomeric substrates

Elastic polymers Long polymer chains cross-link during curing

Covalent cross-linkage

Natural and synthetic materials Rubber Silicones, acrylics, polyurethanes Biodegradable materials (silk, gels)

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PDMS Sylgard 184 Dow Corning

Polydimethylsiloxane 2-part polymer to mix in weight ratio E = 0.5-3MPa, ~ 0.5 Coef. of thermal expansion:

CTE = 310ppm/C Transparent Dielectric constant: 2.65

Preparation Casting, spin-coating Molding

Biocompatible grades

0 5 10 15 20 25 30-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

stre

ss (M

Pa)

s tra in (% )

E=2MPa

Silicon: Glass: Polyimide:

E = 160GPa E = 70GPa E = 3GPa

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Paper as a substrate

Packed cellulose fibers (+fillers) Thickness range: 0.1mm Light weight (10s g/m2) Opaque to translucent

Hygroscopic material Good electrical insulator ( [1010 - 1014.cm]; 1.3-4 @ 1MHz)

Often coated with protective materials R2R compatibility Low-cost, recyclable, biodegradable

Tobjork et al Adv. Mat. 2011 23 1935 Zschieschang et al Adv. Mater. 2011 23 654

http://www.handprint.com/

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Biodegradable substrate

From the drug-delivery field or natural materials Thermoplastic polyesters PLGA, Ecoflex (BASF) Potato/corn starch based materials Caramelized glucose Silk (water-soluble and enzymatically degradable)

Bettinger et al Adv. Mat. 2010 22 651 Irimia-Vladu et al Adv. Funct. Mater. 2010 20 4069

PLGA degradation over time in vitro

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Active device materials

1. Inorganic materials materials from the microelectronics and MEMS fields In thin film formats (

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Inorganic/organic brief comparison

Property Silicon Pentacene

Atomic bonding Covalent bond Molecular bond

Crystal symmetry Single crystal form Face-centered cubic

Amorphous, polycrystalline

Dimensionality (electronic prop.)

isotropic anisotropic

Deposition techniques High temperature High costs (epitaxy, etc.)

RT-low temperature Low costs (spin-coating, evaporation)

carrier mobility cm2/V.s

10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103

amorphous films

polycristalline films Molecular crystals

silicon a-Si:H polycryst. cryst.

orga

nics

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Thin is Flexible

1-atom thick materials Carbon Nanotubes CNTs and Graphene

http://ipn2.epfl.ch/CHBU/NTbasics1.htm 32

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Biocompatible, biodegradable organic materials

semiconductors

dielectrics

substrate

Advanced Functional Materials, 2010, 20 4069 33

Biodegradable, biocompatible inorganic materials

Mg, MgO, ZnO Degrade in water or biofluids into metal hydroxydes e.g. Mg(OH)2, Zn(OH)2

J. Rogers group, Small April 2013 34

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Liquid metals

Electrically conductive metallic fluid encapsulated in microchannels (Mercury) EGaIn: 3.104S/cm Low melting point: 17C In elastic conduits

Patternable Self-healing

Advanced Functional Materials, 23(18), 2308-2314 Advanced Functional Materials, March 2013

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Deported/stiffening contacts

Electrical Probing

R. Carta et al. Sens. Act. A 2009 online stretchable section

stiffened contact

Contacts do not deform

Contacts stretch along

stretchable contact pads

5mm

S.P. Lacour, EPFL

Compliant contacts

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Results from the lab. LSBI, EPFL

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relaxed!

stretched!

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0-60 -50 -40 -30 -20 -10 0

VDS (V)!

I DS (

A)!

VGS = 0,-20V!

VGS = -40V!

VGS = -60V!appl = 12.6%"

Stretchable circuits

Applied Physics Letters, 2011, 98, 124101 Journal of Applied Physics, 2014, 115, 143511.

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The soft-to-hard challenge

PDMS"

stretched PDMS = 20%"

Cross-sections

non deformable or stiff layer"

PDMS"stretched PDMS = 20%"

sharp peak strain" >> 20%"

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stretch

stretchable substrate

device island

elastic interconnect

On uniform stretchable substrate

Al2O3 disk Au film Al2O3 disk Au film

before stretching at 20% strain after stretching

Applied Physics Letters, 2013, 102, 131904

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embedded stiff platform

stretchable substrate

device island

elastic interconnect

stretch

D

t d

S

g h

Rigid embedded platforms

Al2O3 disk Au film

embedded SU8 platform

Al2O3 disk Au film

embedded SU8 platform

before stretching at 20% strain after stretching

Applied Physics Letters, 2013, 102, 131904

PDMS SU8

S=4D

100m 50m

D

S=4D

100m 50m

D

SU8 P-PDMS

D

PDMS SU8

S=4D

100m 50m

D

d

G

UV Cr mask sample cross-section

SU8

in P

DM

S Sl

oped

SU

8 in

PD

MS

SU8

in P

-PD

MS

D d D

G=2mm

Optimizing further

A. Romeo et al. Proc. of the SPIE 2014 (in print)

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Engineered Substrate- Strain Profile

A. Romeo et al. Proc. of the SPIE 2014 (in print)

Conformable and stretchable transducers

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Stretchable electronics new combinations of very different materials Micro/nanofabricated electronic circuits embedded in

ultra-compliant matrices Design of the enabling mechanical architecture for

stretchable circuitry Strain in semiconductors < 0.3% Elastic interconnects; devices on rigid platforms

An opportunity and many remaining challenges Exploring the science and technology for biomimetic

man-made interfaces.

Summary & Outlook

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Acknowledgment

Laboratory for Soft Bioelectronic Interfaces, EPFL Ivan Minev, Katherine Musick, Tero Kulmala, Aaron Gerrat, Swati Gupta Cdric Paulou, Amlie Guex, Alessia Romeo, Arthur Hirsch, Hadrien Michaud

Zhigang Suos lab, Harvard University Qihan Liu