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

COMMUNICATION
2
www.advmat.de
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.
wileyonlinelibrary.com
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
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