Self-healing materials are smart materials that can intrinsically repair damage leading to longer lifetimes, reduction of inefficiency caused by degradation and material failure. Applications include shock absorbing materials, paints and anti-corrosion coatings and more recently, conductive self-healing materials for circuits and electronics.

1. SELF-HEALING
MATERIALS
Cristina Resetco
Polymer and Materials Science

2. Self-Healing Materials
Motivation: Self-healing materials are smart materials that can
intrinsically repair damage leading to longer lifetimes, reduction of
inefficiency caused by degradation and material failure.
Applications: shock absorbing materials, paints and anti-corrosion
coatings.
Outline
(1) Restoration of Conductivity with TTF-TCNQ Charge-Transfer Salts
(2) Self-Healing Materials with Interpenetrating Microvascular
Networks
(3) Coaxial Electrospinning of Self-Healing Coatings
(4) Nanoscale Shape-Memory Alloys for Ultrahigh Mechanical Damping

3. Self-Healing Materials

4. Self-Healing Materials
a) damage is inflicted on the material
b) a crack occurs
c) generation of a “mobile phase” triggered either by the
occurrence of damage (in the ideal case) or by external
stimuli.
d) damage is removed by directed mass transport towards the
damage site and local mending reaction through
(re)connection of crack planes by physical interactions
and/or chemical bonds
e) after the healing of the damage the previously mobile
material is immobilised again, resulting in restored
mechanical properties
http://www.autonomicmaterials.com/technology/

5. Self-Healing Methods
Material Design

6. Restoration of Conductivity with
TTF-TCNQ Charge-Transfer Salts
A new microcapsule
system restores
conductivity in
mechanically damaged
electronic devices in
which the repairing agent
is not conductive until its
release.
Moore, J. et al. Adv. Funct. Mater. 2010, 20, 1721–1727.

7. Restoration of Conductivity with TTF-TCNQ Charge-Transfer
Salts
Conductive healing agent is generated upon mechanical damage. Two core
solutions travel by capillary action to the relevant damage site before
forming the conductive salt.
The major advantage of this approach is greater mobility of precursor
solutions compared to suspensions of conductive particles.
Tetrathiafulvalene Tetracyanoquinodimethane
tetrathiafulvalene–tetracyanoquinodimethane
Non-conducting Non-conducting Conducting
Moore, J. et al. Adv. Funct. Mater. 2010, 20, 1721–1727.

8. Microcapsule Synthesis
TTF and TCNQ were individually
incorporated into microcapsule cores
as saturated solutions in chlorobenzene
(PhCl), ethyl phenylacetate (EPA), and
phenyl acetate (PA).
Poly(urea-formaldehyde) (PUF)
core–shell microcapsules were
prepared using an in situ
emulsification polymerization in an
oil-in-water suspension.
Figure 1. Optical microscope images from A) an attempt to
encapsulate crystalline TTF-TCNQ salt in PA, B) MCs containing
Electron impact mass spectra of the powdered TTF-TCNQ salt suspended in PA; inset: ruptured MCs
containing powdered TTFTCNQ salt in PA, C) TTF-PA MCs, and
dried microcapsule core solutions D) TCNQ-PA MCs. All scale bars are 200mm.
confirmed the presence of TTF and
TCNQ in the microcapsules.
Moore, J. et al. Adv. Funct. Mater. 2010, 20, 1721–1727.

9. Microencapsulation by in-situ Polymerization
Microencapsulation of DCPD utilizing acid-catalyzed in
situ polymerization of urea with formaldehyde to form
capsule wall.
Brown, E. et al.; J. Microencapsulation, 2003, vol. 20, no. 6, 719–730

10. Damage and Formation of Charge-Transfer Salt
Figure. Microcapsules crushed between two glass slides: A) 50mg
PAMCs; B) 50mg TTF-PA MCs; C) 50mg TCNQ-PA MCs; D) 50mg each
TTFPA and TCNQ-PA MCs.
When mixtures of TTF and TCNQ microcapsules
were ruptured, a dark-brown color was immediately
observed, indicative of the TTF-TCNQ charge-
transfer salt formation.
IR spectroscopy was used to verify charge-transfer
salt formation.
Moore, J. et al. Adv. Funct. Mater. 2010, 20, 1721–1727.

11. Restoration of Conductivity by TTF-TCNQ
Charge-Transfer Salt
Figure 7. I–V measurements of analytes on glass slides
measured between two tungsten probe tips spaced
approximately 100mm apart for neat ruptured TTF-PA, TCNQ-
PA, and TTF-PA:TCNQPA in a 1:1 ratio (wt%) microcapsules.
Moore, J. et al. Adv. Funct. Mater. 2010, 20, 1721–1727.

12. Optimization of Precursor Concentration
Moore, J. et al. Adv. Funct. Mater. 2010, 20, 1721–1727.

13. Self-Healing Materials with Interpenetrating Microvascular
Networks
Key advances in direct-write assembly:
Two fugitive organic inks possess similar
Healing strategy mimics
viscoelastic behavior, but different temperature-
human skin, in which a minor
dependent phase change responses.
cut triggers blood flow from
the capillary network in the
underlying dermal layer to
the wound site.
Hansen, C. et al. Adv. Mater. 2009, 21, 1–5.

14. Direct-Write Assembly with Dual Fugitive Inks
(a) Epoxy substrate is leveled for writing
(b) Wax ink (blue) is deposited to form one network
(c) Pluronic ink (red) is deposited to separate networks
(d) Wax ink is deposited to form 2nd microvascular network
(e) Wax ink vertical features are printed connecting to both
networks
(f) Void space is filled with low viscosity epoxy
(g) After matrix curing, pluronic ink is removed
(h) Void space from previous pluronic network is re-infiltrated with
epoxy
(i) Wax ink from both microvascular networks is removed
(j) Networks are filled with resin (blue) in one network and
hardener (red) in the second network
Hansen, C. et al. Adv. Mater. 2009, 21, 1–5.

15. Repeated Repair Cycles
Once a crack contacts the microvascular network, epoxy resin and
hardener wick into the crack plane due to capillary forces.
Healing Efficiency
Hansen, C. et al. Adv. Mater. 2009, 21, 1–5.

16. Coaxial Electrospinning of Self-Healing Coatings
Healing agent encapsulated in a bead-on-string structure
and electrospun onto a substrate.
Advantages
Park, J. et al. Adv. Mater.
2010, 22, 496–499

17. One-Step Coaxial Electrospinning Encapsulation
Spinneret contains two
coaxial capillaries
Two viscous liquids are fed
through inner and outer
capillaries simultaneously
Electro-hydro-dynamic
forces stretch the fluid
interface to form coaxial
fibers due to electrostatic
repulsion of surface charges
Park, J. et al. Adv. Mater. 2010, 22, 496–499

18. Core–Shell Bead-on-String Structures
Figure. SEM images of a) the core–shell bead-on-string morphology and b) healing agent released from the
capsules when ruptured by mechanical scribing. c) Fluorescent optical microscopic image of sequentially spun Park, J. et al. Adv. Mater.
Rhodamine B (red) doped part A polysiloxane precursor capsules and Coumarin 6 (green) doped part B capsules. 2010, 22, 496–499
d) TEM image of as-spun bean-on-fiber core/sheath structure.

19. Self-Healing after Microcapsule Rupture
Self-healing by
polycondensation of
hydroxyl-terminated
PDMS and PDES
crosslinker catalyzed
by organotin.
Park, J. et al. Adv. Mater. 2010, 22, 496–499

20. Self-Healing by Polymerization
Figure. SEM images of scribed region of the self-healing sample after healing a) 458
crosssection and b) top view of the scribed region on a steel substrate.
Figure 2. Control and self-healing coating samples that were stored under ambient Park, J. et al. Adv. Mater.
conditions for 2 months after 5 days salt water immersion. 2010, 22, 496–499.c

21. Nanoscale Shape-Memory Alloys for
Ultrahigh Mechanical Damping
Nanoscale Pillars of shape-memory alloys exhibit
mechanical damping greater than any bulk material.
San Juan, J. et al. Nature Nanotech., Vol. 4, 2009.

22. Dissipation of mechanical energy by reversible
transformation between Austenite and Martensite
due to stress.
San Juan, J. et al. Nature Nanotech., Vol. 4, 2009.

23. Size Effect of Cu-Al-Ni Nanopillars
Figure. SEM image of Cu–Al–Ni pillar, mean
diameter of 900 nm.
Cu-Al-Ni pillars were produced (1) Stabilization of austenite by
by focused ion beam (FIB) elimination of martensite nucleation
micromachining of surface sites
sections of Cu-Al-Ni crystals. (2) Stabilization of martensite by
small pillars that relieve elastic energy
San Juan, J. et al. Nature Nanotech., Vol. 4, 2009. at the surface by crossing the entire
specimen

24. Comparison of High Damping Materials
Merit index = E1/2 ΔW/πWmax
W – dissipated energy per stress-release cycle
ΔW- maximum stored energy per unit volume
E – Young’s modulus San Juan, J. et al. Nature Nanotech., Vol. 4, 2009.

25. What is Next ?
Go Nano !