A Self-Healing Conductive Ink - Paul Braun Research Group ...

Vol. 24 No. 19 May 15 2012
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D10488

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A Self-healing Conductive Ink
Susan A. Odom , Sarut Chayanupatkul , Benjamin J. Blaiszik , Ou Zhao , Aaron C.
Paul V. Braun , Nancy R. Sottos , Scott R. White , * and Jeffrey S. Moore *
The mechanical durability of conductive materials affects the
performance and lifetimes of devices ranging from electrical
circuits to battery electrode materials. Mismatches in thermal
expansion coeffi cient, Young’s modulus, and Poisson’s ratio
between the conductive materials and packaging materials in
integrated circuits lead to delaminations and fractures both
within conductive pathways and at interconnects.
Dr. S. A. Odom , O. Zhao , Prof. J. S. Moore
Department of Chemistry
Beckman Institute for Advanced Science & Technology
University of Illinois at Urbana-Champaign
405 N. Mathews Ave. Urbana, IL 61801, USA
E-mail: jsmoore@illinois.edu
S. Chayanupatkul , B. J. Blaiszik , A. C. Jackson ,
Prof. P. V. Braun , Prof. N. R. Sottos
Department of Materials Science & Engineering
Beckman Institute for Advanced Science & Technology
University of Illinois at Urbana-Champaign
405 N. Mathews Ave. Urbana, IL 61801, USA
Prof. S. R. White
Department of Aerospace Engineering
Beckman Institute for Advanced Science & Technology
University of Illinois at Urbana-Champaign
405 N. Mathews Ave. Urbana, IL 61801, USA
E-mail: swhite@illinois.edu
DOI: 10.1002/adma.201200196
[ 1 , 2 ]
Fatigue
causes microcracks that can lead to channeling, debonding, and
failure. [ 3–5 ] The repeated lithiation and delithiation of battery
electrode materials upon charging and discharging contribute
to decreased capacitance due to a loss of interparticle connectivity
and conductivity. [ 6–9 ] Numerous approaches have been
used to design circuits and materials to prevent mechanical
failure, but only a few have focused on restoring conductivity
after mechanical damage.
[ 10–15 ]
Autonomic restoration of electrical conductivity may greatly
extend the lifetime of electronic materials. The greatest opportunity
for signifi cant short-term impact may be in devices
in which human intervention is diffi cult and/or costly. For
example, fault-tolerant computer chips are of interest in space
applications in which fi eld-programmable gate arrays
used to self-diagnose and reroute damaged circuits. Redundant
circuitry and integrated sensing add complexity, weight, and
cost to fault-tolerant designs. In battery materials, longevity is
an important concern limiting applications. Improving battery
longevity by repairing mechanical failures in electrode materials
would require complete disassembly of the battery cell to
achieve repair. We are interested in developing general concepts
to restore conductivity in mechanically damaged electronic
materials without external intervention or relying on back-up
circuits.
Adv. Mater. 2012,
DOI: 10.1002/adma.201200196
[ 16 ]
are
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Jackson ,
Recent efforts towards the autonomic restoration of conductivity
have focused on delivering conductive materials to the
site of damage from core–shell microcapsules. [ 10–12 ] Release
of conductive materials has been demonstrated using microcapsules
containing a suspension of conductive carbon nanotubes,
[ 10 ] solutions of precursors to a conductive charge transfer
salt, [ 11 ] and liquid metal alloys. [ 12 ] Conductivity restoration was
demonstrated by manual delivery of core solutions to simulated
cracks [ 11 ] or autonomic delivery to a cracked circuit. [ 12 ] In
this paper, we report a new approach for self-healing: instead
of releasing conductive materials from microcapsules, we utilize
the conductive particles from the conductive ink itself to
heal damage through subsequent particle redistribution. Upon
mechanical damage, solvent released from microcapsules locally
dissolves the polymer binder of the conductive ink, allowing for
particle redistribution and restoration of conductivity upon solvent
evaporation ( Figure 1 a–c).
Conductive inks are a mixture of conductive particles,
poly mer binders, and dispersing solvents, and they have been
used in the metallization of microcircuits, [ 17 ] solar cells, [ 18 ] large
area electronic structures, [ 19 ] and solder for microelectronics
packages. [ 20 ] More recently, conductive inks have been used to
print fl exible silver microelectrodes, [ 21 ] circuits on curvilinear
surfaces, [ 22 ] and conductive text, electronic art, and 3D antennas
on paper. [ 23 ] With direct screen printing, [ 24 ] ink-jet printing, [ 25 ]
dip-pen nanolithography, [ 26 ] e-jet printing, [ 27 ] and direct
writing, [ 28 ] many traditional processing steps can be eliminated,
including the use of photoresists and etching. Metallic particles
and polymer binder are usually mixed with one or more solvents
that dissolve the polymer binder. We envisioned a healing
concept where the polymer binder remains soluble after circuit
deposition and the conductive components could subsequently
redistribute upon contact with appropriate solvents. Here we
demonstrate that encapsulated solvent delivered to particle/
binder circuits autonomically restores conductivity to mechanically
damaged conductive inks, and that circuits with lines
spaced 200–500 μ m apart do not short circuit during the restoration
process.
We fi rst explored the response of mechanically damaged
conductive inks to a variety of solvents. Inks consisted of silver
particles in a acrylic binder, deposited as lines, on a glass substrate.
Scratching these lines resulted in a complete loss of electrical
conductivity. Subsequently, a drop of solvent was applied
to the scratched region. After solvent evaporation, conductivity
was restored for a subset of the solvents screened. Conductivity
restoration occurrs with a variety of organic solvents that
dissolve the polymer binder of the conductive ink including
xylenes, chlorobenzene, ethyl phenylacetate, and hexyl acetate.
Optical microscopy images of a line of conductive ink before
and after solvent healing ( Figure 2 a and b) demonstrate the
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Figure 1 . Representation of self-healing silver particle sample a) before damage, b) immediately
after damage, showing solvent release from microcapsules, and c) after healing, where
the majority of solvent has evaporated. d) 3D representation of sample used for scratch testing.
e) Top and f) side geometries of samples with dimensions shown. (Samples are not not drawn
to scale.)
Figure 2 . Characterization of conductive ink and microcapsules. Optical micrograph of
scratched silver ink circuits on a glass slide a) before damage and b) after a drop of solvent
was added to the scratch and was allowed to evaporate. c) SEM image of polyurea/polyurea–
formaldehyde (PU/PUF) microcapsules containing hexyl acetate with average diameter of
192 μ m. d) SEM image of silver particles from the commercially available conductive ink.
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ability of solvent to enable reorganization of
the conductive particles. In the healed line, a
scar from the original scratch is visible, yet
an intact conductive pathway was formed.
To extend these screening tests to a fully
autonomic healing system, we included coreshell
microcapsules ( Figure 2 c and Supporting
Information (SI), Figure S1a) to supply solvent
to the conductive ink (see conductive ink
particles in Figure 2 d). Our design consists
of silver particle ink lines deposited onto a
plastic substrate with solvent-fi lled microcapsules
incorporated into a polyurethane layer
deposited atop the silver ink line ( Figure 1 a).
Failure of the circuit via mechanical damage
simultaneously releases solvent from the
microcapsules ( Figure 1 b). By the mechanism
described in the screening tests, released
solvent locally dissolves the poly mer binder,
allowing the immobilized silver particles to
redistribute and form a connected pathway
once the solvent evaporates ( Figure 1 c), thus
restoring electrical conductivity.
We previously reported the preparation
of hexyl acetate microcapsules [ 29 ] and chose
to use this solvent because of its lower toxicity
and affi nity to dissolve the polymeric
binder. We used a procedure optimized for
solvent encapsulation [ 30 ] to prepare the capsules
for this application. After fi ltration and
drying, we sieved the capsules to isolate those
ranging from 180–250 μ m in diameter. The
polyurethane layer, which is deposited over
the ink line, contained hexyl acetate microcapsules
for self-healing specimens or, as a
control, no microcapsules (see Figure 1 d–f
for substrate geometry).
To measure conductivity during damage
and in the initial minutes after damage, we
connected the silver ink line to a Wheatstone
bridge via lead wires. For testing over hours
or days, we measured the resistance using
an ohmmeter. We used scratch damage to
approximate stress-induced cracking. To
mechanically damage the samples, a razor
blade was used to apply scratches to the circuits,
causing all samples to fail electrically.
Scanning electron microscopy (SEM) analysis
(SI, Figure S3) shows that the scratch
extends through the conductive ink and into
the underlying plastic substrate. For concentrations
of capsules in the polyurethane layer
between 10 and 30 wt%, a decrease in sample
resistance was observed within 1–10 min
( Figure 3 a and SI, Figure S4). None of the
control samples without microcapsules
regained conductivity. We tested the conductivity
restoration when the voltage was
not actively monitored, and we found that
Adv. Mater. 2012,
DOI: 10.1002/adma.201200196

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Figure 3 . Conductivity response for conductive ink samples. a) Normalized
bridge voltage ( V norm ) vs. time for a healing sample containing 30 wt%
microcapsules and a control sample with no microcapsules. Scratch applied
at time t = 20 s. b) Resistance of circuits prior to damage, 5 min after damage,
1 h after damage, and 1 week after damage. c) Percent of healed samples
vs. wt% of microcapsules incorporated into the polyurethane healing layer.
(Healing is defi ned as having a normalized bridge voltage of 0.8 or higher.)
Adv. Mater. 2012,
DOI: 10.1002/adma.201200196
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conductivity was restored, whether or not a voltage source was
applied during the healing process.
Although samples containing microcapsules healed within
one hour of the damage event, the process of conductivity restoration
continued on a longer time scale. Therefore, we monitored
the circuit resistance over several days, and the resistance
remained constant after ca. one week. For a representative
sample set ( Figure 3 b), the average resistance of 1.45 Ω after
one week is similar to the original resistance of 0.95 Ω . We
presume the continued repair process is due to continued solvent
evaporation, leading to increased interparticle contact. In
contrast, after one week, the control sample (no microcapsules)
showed no evidence of conductivity restoration.
We investigated the effect of microcapsule concentration in
the polyurethane layer on the percentage of samples in which
conductivity was restored. Initial testing described above was
performed with 30 wt% hexyl acetate capsules, and resulted
restoration in almost 90% of the samples, which we defi ned
as recovery of over 80% of original bridge voltage during initial
testing (within 10 min). At lower capsule concentrations, a
decrease was observed in the percentage of samples in which
a change in resistance occurred ( Figure 3 c), suggesting that
within the tested range of capsule loadings, increased solvent
delivery facilitates the healing process.
Additionally, we monitored adjacent ink lines for short circuits.
We prepared an additional sample type in which a series
of parallel lines with 200–500 μ m separation distance. Simultaneous
scratch damage of these lines initiated loss of conductivity,
and subsequent conductivity restoration of the primary
conductive pathway. However, of the 50 samples tested we did
not observe electrical conduction between neighboring lines.
While this value is larger than those reported in recent printed
conductive ink circuits, which have separation distances of
5–10 μ m, [ 21 ] it is important that at our larger distances do not
result in short circuiting.
In conclusion, we showed that solvent-fi lled microcapsules
autonomically restore conductivity to lines of a conductive
silver ink after scratch damage. Optical microscopy revealed
that the relatively large gaps (ca. 25 μ m) in the conductive ink
are bridged by the reorganized silver particles. We found that a
higher concentration of microcapsules leads to greater percent
of samples that undergo conductivity restoration. It may be possible
to achieve greater success in sample healing by optimizing
the capsule size and volume of solvent released to the damaged
area, and also by varying solvents to change polymer binder solubility
and/or solvent volatility. This result represents the fi rst
example of a self-healing circuit in which the circuit material
itself is used to repair damage. In addition to potential applications
in integrated circuits, a solvent-healing mechanism could
conceivably restore such capacity losses in electrodes fabricated
from a soluble binder. Our system does not require rerouting
to back-up circuitry and does not cause short-circuiting upon
healing. Our result provides a new concept for the realization of
fault-tolerant circuits.
Experimental Section
Silver ink circuits were prepared using SPI Conductive silver paint, which
was painted onto acrylic substrates. Acrylic substrates were cut from a clear
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cast acrylic sheet 0.15 cm thick, purchased from McMaster-Carr, catalog
number 8560K175. Clear Flex 50, a two-part polyurethane elastomer,
was purchased from Smooth On, Inc. Hexyl acetate, urea, resorcinol,
and formalin were purchased from Aldrich Chemical Co. Ethylene-maleic
anhydride copolymer (Zemac-400) powder with an average molecular
weight of 400 kDa (Vertellus) was used as a 2.5 wt% aqueous solution.
The commercial polyurethane prepolymer, Desmodur L75, was purchased
from Bayer Material Science and was used as received.
Microcapsule Preparation : Microcapsules were prepared with slight
modifi cations using our previously published procedures .
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[ 29 , 30 ]
Similarly,
100 mL of distilled water was placed in a 600 mL beaker, along with
25 mL of 2.5 wt% ethylene co-maleic anhydride as a surfactant. The
beaker was placed in a temperature-controlled water bath equipped
with a mechanical stirring blade (40 mm diameter), which was brought
to 400 rpm. To the aqueous solution was added the solid wall-forming
materials: urea (2.50 g), ammonium chloride (0.25 g), and resorcinol
(0.25 g). Afterward, the pH was raised from 2.7 to 3.5 by addition of
NaOH (aq). Desmodur L75 (4 g) in hexyl acetate (60 mL) was added
to the stirring solution, creating an emulsion. After 10 min, 6.33 g of
formalin solution was added, and the temperature was increased to
55 ºC. The reaction proceeded under continuous stirring for 4 h after
which the reaction mixture was allowed to cool to room temperature.
The microcapsules were fi ltered the next day using a Buchner funnel,
washing with water, and were dried under air for 24 h before sieving.
Microcapsule Analysis : After isolation via fi lteration, microcapsules were
sieved to collect those with diameters ranging from 125–180 μ m. Optical
micrographs of dried capsules in mineral oil on glass slides were taken
using a Leica DMR Optical Microscope. Images of dried capsules were
obtained using SEM (FEI/Philips XL30 ESEM-FEG) after sputter coating
with a gold-palladium source. Thermogravimetric analysis (TGA) was
performed on a Mettler-Toledo TGA851 e , calibrated by indium, aluminum,
and zinc standards. A heating rate of 10 ºC min − 1 was used, and
experiments were performed under nitrogen atmosphere from 25–450 ºC.
For each experiment, approximately 5 mg of sample was used.
Wheatstone Bridge Circuit : The conductive metal in each specimen
acts as one resistor in an unbalanced constant voltage Wheatstone
Bridge circuit. The voltage source is a BK Precision DC Power Supply
(model 1710). The voltage gauge and voltage source are monitored by
LabVIEW DAQ. Scratch damage was applied using a Corrocutter with
razor blades at forces ranging from 28–30 N. A new razor blade was
used for each measurement to ensure consistency in the integrity of the
blade for each sample.
Circuit Fabrication : Each circuit was 3 mm wide and ranged from
20–50 μ m in thickness, as measured by profi lometry. Each test sample
contained three circuits to allow for testing of multiple samples at at
the same time. Using a polydimethylsiloxane (PDMS) mold, a 1 mm
thick fi lm of polyurethane elastomer containing 0, 10, 20, or 30 wt%
microcapsules was deposited on top of the silver particle circuits. Wires
were then attached to the circuits using copper pads and solder.
Sample Imaging : To obtain cross-sectional images, a diamond saw was
used to cut thin slices of the circuits; our cuts were made perpendicular
to the direction in which the scratch was applied. The samples were
analyzed by SEM with analysis in refl ectance mode. Not only are the
conductive ink and capsule-containing polyurethane damaged, but the
scratch also extends into the supporting polymer substrate.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Received: January 13, 2012
Published online:
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Adv. Mater. 2012,
DOI: 10.1002/adma.201200196