Indian scientists discover materials that self-repair mechanical damages

Scientists from the Indian Institute of Science Education and Research (IISER) Kolkata and IIT Kharagpur have discovered a new class of material which, when fractured, can repair themselves within milliseconds.

The highly crystalline materials, when broken into pieces, can self-propel and re-join in the blink of an eye, and repair themselves so precisely that they become indistinguishable from the undisturbed materials. These new materials may find applications in various high-tech applications such as space crafts, to mend themselves.

In our daily use, many materials used in everyday applications often undergo frequent mechanical impacts, which can damage the devices beyond repair. This decreases the life of the equipment and increases maintenance costs. In many cases, like in space crafts, human intervention for restoration is not possible.

As a result, scientists have been searching for self-repairing materials for prolonging the lifetime of devices without the need for periodic external intervention.

Wound healing in living tissue and bone has inspired many synthetic self-healing polymers, gels and other soft materials. However, replicating such repair in crystalline materials has remained a challenge as they are rigid and prevent diffusion of the material at the damaged part due to dense and regularly arranged molecules in them.

Keeping such necessities in mind, researchers from the Indian Institute of Science Education and Research (IISER) Kolkata, teaming up with IIT Kharagpur, have now developed piezoelectric molecular crystals that repair themselves from mechanical damages without need for any external intervention.

Piezoelectric crystals are a class of materials that generate electricity when it undergoes a mechanical impact.

The piezoelectric molecules developed by the scientists called bipyrazole organic crystals recombine following mechanical fracture without any external intervention, autonomously self-healing in milliseconds with crystallographic precision.

In these molecular solids, due to the unique property of generating electrical charges on mechanical impact, the broken pieces acquire electrical charges at the crack junction, leading to attraction by damaged parts and precise autonomous repair.

This research, supported by the Department of Science and Technology, GoI via Swarnajayanti Fellowship to CM Reddy and Science and Engineering Research Board (SERB) research grants, has been published in the journal ‘Science’ recently.

This methodology was initially developed by the IISER Kolkata team led by Prof. C Malla Reddy, a recipient of Swarnajayanti fellowship (2015) given by the Department of Science & Technology, GoI. Prof. Nirmalya Ghosh of IISER Kolkata, a laureate of the Society of Photo-Optical Instrumentation Engineers (SPIE) G.G. Stokes Award in Optical polarization 2021, used a custom-designed state-of-the-art polarization microscopic system to probe and quantify the perfection of the piezoelectric organic crystals. These materials with perfect internal arrangement of molecules or ions are called ‘crystals’, which are abundant in nature.

The IIT Kharagpur’s team of Prof. Bhanu Bhusan Khatua and Dr. Sumanta Karan studied the performance of the new materials for fabricating mechanical energy harvesting devices.

The material may find application in high-end micro-chips, high precision mechanical sensors, actuators, micro-robotics, and so on. Further research into such materials may eventually lead to the development of smart gadgets that self-repair cracks or scratches.

In the past decade, a tremendous amount of research has been done to find self-healing properties in synthetic polymers, gels and composites, which are soft and amorphous in nature,” says Prof. Reddy. “Various strategies have been employed to mimic nature but almost all of them need at least one external stimulus such as heat, light, solvent or a chemical healing agent. And universally all materials fail when the broken parts fall apart.”

Prof. Reddy points out that “another not-yet-accomplished but attractive goal in materials science is to couple the self-healing with crystallinity. Despite the immense potential of highly crystalline materials in numerous high-performance applications related to optical, electrical, energy, biomedical, soft-robotics etc., only a few reports exist on self-healing – with little understanding of the mechanisms.”

“Poor diffusion in the densely packed and relatively hard ordered crystals precludes autonomous healing and makes crystallographically precise reordering extremely difficult,” he adds.

Prof. Bhanu Bhusan Khatua from IIT Kharagpur notes that these crystals, which belong to a general class of piezoelectric materials that can generate electricity under mechanical strain, or vice versa, can heal exceptionally well, and retain their crystalline nature, which is important for many applications.

Piezoelectric crystals have a broad range of applications in precision engineering, including transducers, mechanical sensors, energy harvesters, biomedical implants, etc.

Piezoelectricity studies conducted by Prof. Bhanu Bhusan Khatua and his student, Dr. Sumanta Karan from IIT Kharagpur, evaluated the potential of these self-healing piezoelectric crystals for electricity generation and durability in devices. Taking cues from nature, they explored the inherent piezoelectricity of crystalline materials for finding a self-healing property.

The first author of this paper, Surojit Bhunia from IISER K further adds: “Here, taking cues from nature, we explored the inherent piezoelectricity of crystalline materials for finding a self-healing property. It has long been known that piezoelectricity plays a key role in initiating self-healing in mechanically damaged natural biomaterials, like bone and collagen. Hence, our discovery in organic materials may provide further insights into more complex natural materials.”

“We have used a custom designed state-of-the-art polarization microscopic system at IISER K to probe and quantify the structural order of the piezoelectric organic crystals with nanometer scale spatial resolution,” says Prof. Nirmalya Ghosh, the other corresponding author of this paper, who is also the laureate of the SPIE G G Stokes award in optical polarization 2021. “This unique experimental system in combination with a suitable polarization analysis model enabled quantitative assessment and understanding of the self-healing behavior of the crystals by sensing changes in nano scale structural anisotropy.”

“This new class of optical material exhibiting strong polarization and non-linear optical response and having extraordinary self-healing capability may open the door for a new generation of integrated and miniaturized photonic devices for numerous technological applications in optical sensing, in high precision metrology and in optical nano probing etc.,” he points out.

“Piezoelectric materials require to withstand prolonged mechanical loading unloading cycles in many applications such as transducer, energy harvesters, mechanical sensors, actuators etc.; hence fracture healing ability is critical to boost their durability and reliability,” adds Dr. Sumanta Karan from IIT Kharagpur.