REPORTS
◥MATERIALS SCIENCE
Autonomous self-repair in piezoelectric
molecular crystals
Surojit Bhunia1,2, Shubham Chandel^3 †, Sumanta Kumar Karan^4 ‡, Somnath Dey^5 , Akash Tiwari^3 ,
Susobhan Das^1 , Nishkarsh Kumar^3 , Rituparno Chowdhury^1 , Saikat Mondal1,2, Ishita Ghosh^1 ,
Amit Mondal^1 , Bhanu Bhusan Khatua^4 , Nirmalya Ghosh^3 , C. Malla Reddy1,2
Living tissue uses stress-accumulated electrical charge to close wounds. Self-repairing synthetic
materials, which are typically soft and amorphous, usually require external stimuli, prolonged physical
contact, and long healing times. We overcome many of these limitations in piezoelectric bipyrazole
organic crystals, which recombine following mechanical fracture without any external direction,
autonomously self-healing in milliseconds with crystallographic precision. Kelvin probe force
microscopy, birefringence experiments, and atomic-resolution structural studies reveal that these
noncentrosymmetric crystals, with a combination of hydrogen bonds and dispersive interactions, develop
large stress-induced opposite electrical charges on fracture surfaces, prompting an electrostatically
driven precise recombination of the pieces via diffusionless self-healing.
S
tress-induced accumulation of electric
charges is known to trigger self-healing
in mechanically damaged natural bio-
materials ( 1 ). In synthetic polymers ( 2 – 4 ),
gels ( 5 , 6 ), and composites ( 7 ), various
strategies to mimic nature have been used.
Known self-healing materials, which are gen-
erally soft and amorphous, require nonme-
chanical stimuli such as heat, light, solvent,
or a chemical healing agent, yet universally
all materials fail when the broken pieces are
physically separated ( 3 ). On the other hand,
poor diffusion in densely packed and relatively
hard ordered single crystals, including poly-
mers with crystalline domains ( 2 ), precludes
autonomous healing and makes atomically
precise reordering extremely difficult, even
under long time periods, annealing, mechani-
cal compression, or solution treatment ( 8 , 9 ).
Thus, a challenge remains in materials science
to engineer atomically precise self-healing in
crystalline materials to preserve long-range
order ( 2 , 8 – 10 ).
The desired coupling of self-healing with
crystallinity would enable a number of long-
sought technologies. For instance, many micro-
electronic devices whose function is based
on precision positioning require accurately
oriented, highly crystalline piezoelectrics. Piezo-
electric materials must withstand prolonged
mechanical loading and unloading cycles;
hence, fracture-healing ability is critical to
boost their durability ( 11 ).Only a handful of
reports exist on self-healing in crystalline ma-
terials, with little understanding of the mech-
anisms ( 8 , 9 ).Self-healing of cracks occurs in
hybrid macromolecular ferritin-hydrogel crys-
tals ( 8 ); however, the entire process relies on
a salt gradient in solution. Self-healing is re-
ported in single crystals of soft boronic esters
(with an elastic modulusE<2MPa),using
dynamic covalent chemistry under moist con-
ditions with prolonged contact periods of
~24 hours, yet the macroscopic cracks remain
visible ( 9 ).
We show in organic single crystals an alter-
native approach to self-repair, using the inher-
ent piezoelectric effect in noncentrosymmetric
structures to achieve autonomous self-healing.
Relative to most other known self-healing ma-
terials, these crystals exhibit stiffness and
hardness that are higher by several orders of
magnitude (table S1), and they can autono-
mously recombine even when the fractured
pieces are physically separated. The materials
undergo self-healing within milliseconds with
precise crystallographic ordering, as char-
acterized using atomic-resolution structures
obtained from single-crystal x-ray diffraction
and spatially resolved classical birefringence
measurements.
The compound 3,3′,5,5′-tetramethyl-4,4′-
bipyrazole ( 1 ; Fig. 1A) is synthesized in gram
scale and crystallized from methanol solu-
tion at ambient conditions (see supplementary
materials). The needle-shaped single crystals,
with typical length ~1 to 2 mm and width/height ~0.1 to 0.2 mm (Fig. 1A), fractured via
propagation of a linear crack when subjected
to a three-point bending test (Fig. 1B). When
the force was withdrawn, a strong attractive
force between the fracture surfaces caused the
two fragments to rejoin (movies S1 to S4). The
fragments self-propelled and reassembled with
a perfect alignment to rapidly self-heal (movies
S1 to S4) on a time scale of 1 to 2 ms (Fig. 1C,
fig. S1, movie S4, and supplementary text). The
crack generation and self-healing could be re-
peated multiple times in a crystal (Fig. 1B and
movie S5). The fractured pieces of crystal 1
could recombine even when they were phys-
ically separated at distances as large as ~50mm
(Fig. 1D and movie S6). The self-actuated pieces
traveled with an acceleration of ~2.2 m/s^2 (Fig.
1, C and D, fig. S2, and movie S7).
In ideal cases where self-healing is efficient,
the cracks disappeared completely and were
not observable even by polarized optical mi-
croscopy (Fig. 1B), scanning electron micros-
copy(SEM;Fig.1E,fig.S3,BandC,andfig.S4B),
dark-field imaging microscopy (Fig. 1F and fig.
S5B), and atomic force microscopy (AFM; Fig.
1G). Such perfectly self-healed (hereafter, neat-
ly healed) crystals were indistinguishable from
the pristine (as-grown) crystals (Fig. 1B, panels 2,
4, 6, and 8). Submillimeter cracks, generated
by uniaxial compression, also healed rapidly
(fig. S6 and movie S8). The repeatability of the
self-healing cycles depended on the way in
which the force was applied and the landing
direction of the two pieces. The neatly healed
crystals remained intact over a period, and
their behavior was comparable to that of the
pristine crystals.
In some cases, efficient self-healing was hin-
dered by obstruction of debris and nonlinear
cracking (fig. S4, C and E), which caused dif-
ficulty in perfect closure of the fractured sur-
faces even after the complete withdrawal of
external stress. In such cases, cracks were visi-
ble on the crystals (hereafter, imperfectly healed
crystals; see Fig. 1B, panel 10, and fig. S4, C to E).
Although cracks were visible, the imperfectly
healed crystals could still support their own
weight when lifted by holding at one end
(movie S7).
Examination of pristine samples by single-
crystal x-ray diffraction (SCXRD) revealed that
the solid sample 1 is a monohydrate form of the
bipyrazole, crystallizing in a tetragonal unit cell
with polar space groupP 43. The asymmetric
unit contains four bipyrazole molecules, each
with a cocrystallized ordered water molecule
(Z′= 4) (see fig. S7). The crystals grow along
thecaxis with two crystallographically equiv-
alent side faces, (100) and (010) (Fig. 2, A and
B). The water molecules form an infinite one-
dimensional (1D) zigzag chain via O–H···O
cooperative hydrogen bonds parallel to the
crystal length orcaxis (Fig. 2A and fig. S7A).
The adjacent parallel water chains are bridgedSCIENCEsciencemag.org 16 JULY 2021•VOL 373 ISSUE 6552 321
(^1) Department of Chemical Sciences, Indian Institute of Science
Education and Research Kolkata, Nadia 741246, West Bengal,
India.^2 Centre for Advanced Functional Materials, Indian
Institute of Science Education and Research Kolkata, Nadia
741246, West Bengal, India.^3 Department of Physical
Sciences, Indian Institute of Science Education and Research
Kolkata, Nadia 741246, West Bengal, India.^4 Materials Science
Centre, Indian Institute of Technology Kharagpur, Kharagpur
721302, India.^5 Institute of Crystallography, RWTH Aachen
University, 52066 Aachen, Germany.
*Corresponding author. Email: [email protected]
(C.M.R.); [email protected] (N.G.)
†Present address: NAM, EPFL, 1015 Lausanne, Switzerland.
‡Present address: Pennsylvania State University, University Park,
PA 16802, USA.
RESEARCH