FEASIBILITY OF ADDITIVE MANUFACTURING METHOD FOR DEVELOPING STRETCHABLE &

FLEXIBLE EMBEDDED CIRCUITS

MSME Plan B ProjectAugust 2014

OVERVIEW

• Introduction

• Additive Manufacturing process

• Extruder design

• Material characterization

• Printing functional circuits

• Conclusion

INTRODUCTION

STRETCHABLE ELECTRONICS

Integrated circuits that can be stretched, compressed,

twisted, bent, and deformed

Favorable attributes:

• Conformity

• Lightweight structure

• Shock-resistant construction

CATEGORIES

Foldable electronicsApplications in smart packaging

& paper-based electronics

Embedded electronics that can

stretch with molding processMoldable electronics

Conforming to human anatomy,

bio-integrated devicesConformable

electronics

Consumer electronics: flexible

displays, keypads, smart textilesRollable electronics

Electroative haptic switches;

consumer electronicsMophable electronics

www.idtechex.com/ applications of stretchable electronics

FABRICATION

2 Main Approaches:

• Printed and thin film monolithic circuits

• Islands of rigid electronics with flexible polymer

substrate and interconnects accommodating

elastic strain

MATERIALS

single-walled carbon nanotubes (SWNTs) as conductive dopants

in a rubber matrix

silver nanoparticle ink for direct writing onto a flexible substrate

polyurethane containing spherical nanoparticles deposited by either layer-by-layer assembly or vacuum-assisted flocculation

ionic conductor (hydrogels, etc.)

FABRICATION METHODS

Molded Interconnect Device (MID) technology: rigid or

flexible standard components that are interconnected by

meander shaped metallic wires and embedded

by molding in a stretchable substrate polymer

the chemical vapor deposition (CVD) growth of NWs

followed by their transfer and assembly on flexible

substrates (PDMS, etc.)

conductive composite mat of silver nanoparticles with

rubber fibres (Park et al., 2012)

Aerosol Printing: print multiwall carbon nanotube solution

onto an insulating flexible substrate

(Thompson et al.)

GAP

Current methods of manufacturing:

Fabricated with planar technologies: require

complex, dedicated infrastructure

Prolonged fabrication time

Expensive

To overcome these use of an additive manufacturing

process is proposed:

• Cost-effective and accessible

• Easy-to-use

• efficient free-form fabrication of CAD designs

• Ability to print with biocompatible Silicone

ADDITIVE

MANUFACTURING

ADDITIVE MANUFACTURING

PROCESSES

additive process: successive layers of material are

laid down

Realize complex parts, molds, circuit boards without

requirement of enterprise scale facilities

Capability to print with a wide variety of materials:

plastic, silicone, ceramics, metals, etc.

Opportunity to develop freeform fabrication systems

capable of producing electronics and structures

together for stretchable electronics.

PRIOR WORK

3D printing electronics (Periard et al.):

- printing of both structural layers and

electronic circuitry.

- Fully-functional, rigid electronic

devices.

3D tissue-engineering of living

human organs: (Mironov et al.)

- Cell printer that: print gels,

single cells and cell aggregates.

Printing Bionic organs: (Mannoor et

al.)- additive manufacturing of

biological cells with structural and

nanoparticle derived electronic silicones

PRINTING WITH SILICONE

3D printing with multi-material fabrication:

Silicone based conductive material

Silicone as structural material

Compatibility between silicone and added cells.

• Advantages of silicone: Stretchable, Bio-compatible,

ideal substrate and structural material

• However there is no study on 3D printing of electronics

with silicone on stretchable substrates

GOAL

Design an extrusion method for silicone for

stretchable electronics

Study the material characteristics of conductive

silicone under stretching

Print complete integrated and fully-functional

electronic circuits that are capable of withstanding

stretching and flexion.

DESIGN

SILICONE SELECTIONKey criteria:

Curing method: RTV/ moisture-accelerated/UV

Curing speed (skin-over time, tack-free time, etc.)

Extrusion rate and viscosity

Conductivity

Material consistency and appearance

Mechanical strength

SS-26S: Moisture accelerated RTV cure, thixotropic paste, non-

corrosive

Silicone Mfg.Stretch

ability*

Chemical

typeAppearance

Viscosity

(cP)

Material

typeCure

Tack-free

time

(min)

Working

time

(min)

at deg. CShore Hardness

(Durometer A)

Conductivity

(S/cm)

SS 261Silicone

Solutions

Non-

corrosive

Silver-Tan (Custom

colors available

upon request)

400000Thixotropic

paste

RTV

(Accelerated

moisture cure)

45 > 24 hrs 115 70 200

SS-26Silicone

Solutions

Non-

corrosive

Silver-Tan (Custom

colors available

upon request)

30000

to

80000

Thixotropic

paste

RTV

(Accelerated

moisture cure)

30 15Room

temp70 200

SS 26SSilicone

Solutions200

Non-

corrosive

Silver-Tan (Custom

colors available

upon request)

500000Thixotropic

paste

RTV

(Accelerated

moisture cure)

30 15Room

temp50 200

* > % Elongation @ break

Material properties (uncured) Cured materialCuring

SYRINGE-BASED EXTRUSION

Syringe and Nozzle selection:

• Inexpensive plastic syringe

• Luer-lok: Easy change of needle

• Substitute material: corn syrup

• Require uniform bead deposition – circular c/s, blunt

tip, shaft size, etc.

EXTRUSION VELOCITY & FORCE

• Desired extrusion velocity: ~5-20 mm/s

For 21 gauge needle (ID=0.5mm) with ½” shaft:

Plunger velocity, vb = 0.1mm/s

Volume flow rate, V = 1.8e-9 m3/s

• From Reynold’s no. Re = 9.79e-7, Flow is laminar

• Calculating pressure drops across all cross-sections of the

syringe using Pouiselle’s law, push force required at the

plunger of a 1ml syringe is:

F = 73.82 N

EXTRUSION HEADMotor selection:

- High torque, High gear ratio, Low speeds

• Linear actuator

• Rotary motor

Syringe tool V1: Motor: L16, Firgelli

peak power point: 175N @4mm/s

gear reduction: 150:1

maximum speed: 8mm/s

Box-type design for syringe

housing

syringe extrusion axis parallel to

the axis of the linear motor:

compact size

Drawback: Hinders loading

unloading of syringe/ material

Syringe tool V2:

Motor force along same direction as

extrusion axis

Mounting point at syringe housing

Easy loading for syringe

Drawbacks:

• Increased length due to co-axial mounting

• Cantilevered weight of motor unit on syringe housing unit

Syringe tool V3:

Phidgets 3257E

12V DC motor with optical encoder

Internal 100:1 gear ratio

3:1 gear reduction using spur gears

Torque: 1.63 Kg-cm @23 RPM

Encoder resolution: 360 CPR

Key criteria for extrusion

control:

Regulating the steady-

state extrusion speed for

the extruder.

Starting and stopping

extrusion-on-demand in-

trajectory.

Constant plunger velocity

control using PID control.

Serial input to set target

speed (with Trajectory-

Based Start and Stop

Method )

EXTRUSION VELOCITY CONTROL

2 3 4 5 6 7 8 9 10 11

x 104

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Time [ms]

Velo

city [

mm

/s]

Extruder velocity control

Target Velocity

Actual Velocity

EXPERIMENTAL SETUP

Trajectory drawing speed: 1.7 mm/s

Pen-up and pen-down speed: 0.4 mm/s

Needle pen-up and pen-down height

(from work surface): 1.0 mm

Corvus arm free travel (non-drawing

trajectory) speed: 30.0 mm/s @ 20.0mm

Motor start time (prior to drawing

trajectory): 0.500 seconds

Motor stop time (in drawing trajectory):

0.100 seconds

PRINTING PARAMETERS

MATERIAL

CHARACTERIZATION

INTERFACING WITH SILICONE TRACES

3 methods:

1. Header pin connection

2. custom-prepared electrodes/components with

steel mesh contact interfaces

3. Direct contact/insertion into silicone bead: The traces are printed first, the

component placed onto

printed pads (at required

location) on the substrate

Connectivity of conductive silicone traces to smt components:

v/s

DMA TESTING

Objectives:

study conductive behavior of SS-26S

material in an embedded silicone

environment observe performance characteristics

of the material under a uniaxial strain

Experiment conducted at 7-SIGMA

• Q800 DMA: non-contact linear drive

• Max. force 18N

• Force resolution of 0.00001N

• Strain resolution of 1nm• Force ramp: 2N/min

• Sampling rate: 10Hz

TA Instruments Q800 DMA

Keithley 6487 Picoammeter

RESULTS

0 5 10 15 20 25 30 35 40 452

3

4

5

6

7

8

9

10

Strain [%]

Resist

anc

e [O

hms]

Resistance v/s Strain

Unstretched test sample

Repeat elongation 1

Repeat elongation 2Sample:Total trace length = 37.6mmTrace width = 1mmTrace height = 1mm

Slope = 0.58 Ω/%strain

0 5 10 15 20 25 30 35 40 450.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

Strain [%]

Cond

uctivi

ty [S/cm

]

Conductivity v/s Strain

Δ = 0.01 S/cm

PRINTING CIRCUITS

TEST CIRCUIT

Voltage divider circuit

+5V regulated power

supply

R1 =10 ohms (fixed

resistance)

R2 = RSi (Resistance of

the Silicone trace)

8-pin ATtiny85

programmed using

Arduino as ISP

Serial communication

via TX pin out to

Bluetooth module

(Sparkfun HC-06)

SOME EXAMPLES

TESTING RIG1) Camera housing

2) HD camera for

tracking elongation of

sample

3) Cable out to

computer USB 3.0

4) Tripod for mounting

the camera housing

5) linear translation stage

with a mounting

platform

6) Rotary control for

linear slide

7) mounting plate on

linear slide platform

8) slender clamps to

constrain and grip the

test sample

9) Stretchable circuit

sample

ELONGATION TRACKING

• Computer vision algorithm: HSV color

detection of markers on sample using Slider GUI

• Separation in pixels determined

• Data integrated to serial output from

circuit with timestamp

CIRCUIT UNDER ELONGATION

Limit test:

Direct contact interface: smt LED and 1206 package resistor with

conductive silicone trace

LED worked till 60% strain. Contact lost due to delamination from

component lead

At original length @60% strain

CIRCUIT UNDER ELONGATION

RESULTSResistance computed from output

voltage of voltage divider

0 1 2 3 4 5 6 7 8 90

2

4

6

8

10

12

14

16

18

20

Strain [%]

Re

sis

tan

ce

(O

hm

s)

Resistance v/s strain

0 0.5 1 1.5 2 2.5

x 105

50

100

150

Volta

ge [

mV

]

Voltage and strain vs. time

0 0.5 1 1.5 2 2.5

x 105

0

5

10

stra

in [

%]

Time [ms]

Reference Voltage

Output Voltage

Strain

0 0.5 1 1.5 2 2.5

x 105

0

10

20

Re

sist

an

ce [

Oh

ms]

Resistance and strain vs. time

0 0.5 1 1.5 2 2.5

x 105

0

5

10

stra

in [

%]

Time [ms]

Resistance

Strain

RESULTS3 cycles of stretching

to 8.7% elongation

0 1 2 3 4 5 6 7

x 105

0

5

10

15

20

25

28

Res

ista

nce

[Ohm

s]Resistance and strain vs. time

0 1 2 3 4 5 6 7

x 105

0

5

10

stra

in [%

]

Time [ms]

Resistance

Strain

0 1 2 3 4 5 6 7 8

x 105

0

50

100

145

Vol

tage

[mV

]

Voltage and strain vs. time

0 1 2 3 4 5 6 7 8

x 105

0

5

10

stra

in [%

]

Time [ms]

Reference Voltage

Output Voltage

Strain

OBSERVATIONS

SOD-323F packageZD contacts: 0.4 mm x 0.25 mm, (height 0.25mm)

1206 packageResistor contacts: 1.6 mm x 0.4 mm, (height 0.55 mm)

Contact Separation

at electrode when

relaxing from high

strain

• Sufficient contact for

conductive pathway

for 1206 package

• Delamination from

diode electrode

upon relaxation from

high strain: Loss of

electrical signal due

to lack of contact

• Isolate component

from strain to

maintain electrical

contact under elongation

CONCLUSION

Additive manufacturing process

Designed and tested custom extrusion module for

printing circuits with conductive silicone on

flexible substrates

Constant velocity controlled printing

implemented

Testing and Performance characteristics

Tested conductive properties of SS-26S under

uniaxial strain

Studied interfacing methods

Printed integrated fully-functional circuits and

tested performance under strain

FUTURE WORK

Improve interfacing with

components:

a) Use of non-conductive

mesh for strain relief

b) Use of multi-durometer skin with higher durometer

islands for optoelectronic components

Design of integrated curing module with dual-cure

(UV + moisture) for increased speed.

build and testing of a complete ‘skin-like’ device with

embedded devices custom fit to anatomical

geometry using Corvus.

AKNOWLEDGEMENTS

Dr. Tim Kowalewski, Advisor

John O’Neill: Corvus programming

Rod Dockter II: Computer vision guru

Tim Zalusky: PID implementation

Special thanks to Wade Eichhorn, Dave Winters and

Jim Norris of 7-SIGMA Inc., Minneapolis

Thank You!

The University of Minnesota is an equal opportunity educator and employer.

Syringe: 3mlNeedle size: 25GShaft length: 5/8”Force: HIGH

Syringe: 1mlNeedle size: 21GShaft: 1/2” (blunt)Force: LOW

Syringe: 1mlNeedle size: 20GShaft: 1-1/2”Force: LOW

Syringe: 12mlNeedle size: 20GShaft: 1-1/2”Force: VERY HIGH

Syringe: 1mlNeedle: 30G, 1/2”Force:NEEDLE BEND 10mm

10mm

0 5 10 15 20 25 30 35 40 450.04

0.06

0.08

0.1

Strain [%]

Cond

uctivi

ty

[S/cm

]

Conductivity v/s Strain

0 5 10 15 20 25 30 35 40 452

4

6

8

10

Strain [%]

Resist

anc

e

[Ohm

s]

Resistance v/s Strain

Unstretched test sample

Repeat elongation 1

Repeat elongation 2

Δ = 0.01 S/cm

Total trace length = 37.6mmTrace c/s = 1mm x 1mm

Slope = 0.58 Ω/%strain

0 1 2 3 4 5 6 7 8 90

5

10

15

20

Strain [%]

Res

ista

nce

[Ohm

s]

Resistance v/s strain

Elongation

Constant strain

Relaxation

0 0.5 1 1.5 2 2.5

x 105

50

100

150

Volta

ge [m

V]

Voltage and strain vs. time

0 0.5 1 1.5 2 2.5

x 105

0

5

10

str

ain

[%

]

Time [ms]

Reference Voltage

Output Voltage

Strain

0 0.5 1 1.5 2 2.5

x 105

0

10

20

Resi

stance

[O

hm

s]

Resistance and strain vs. time

0 0.5 1 1.5 2 2.5

x 105

0

5

10

stra

in [%

]

Time [ms]

Resistance

Strain