Scientists from the Indian Institute of Science Education and Research (IISER) Kolkata and IIT Kharagpur, discovered a new class of materials which can repair themselves when fractured, within a thousand fraction of a second. This research is now published in Science (AAAS) journal. The highly crystalline materials, when broken into pieces, can self-propel and re-join in a blink of an eye, and repair them- selves so precisely that they become indistinguishable from the undisturbed materials. These new materials may find applications in various new age technologies.
Materials applied in technologies constantly undergo mechanical impacts which often damage the devices beyond repair. Therefore, scientists have been searching for self-repairing materials for prolonging the life time of devices without need for periodic external intervention. Wound healing in living tissue and bone has inspired many synthetic self-healing polymers, gels and other soft materials in the last decade or so. However, replicating such repair in crystalline materials has remained a challenge as they are rigid and prevent diffusion of material at the damaged part due to dense and regularly arranged molecules in them.
Prof. C Malla Reddy, who won the prestigious Swarnajayanti fellowship from Department of Science and Technology, Govt of India, in 2015, and his team at IISER Kolkata synthesized a new class of solid materials that with a head-to-tail (positive end-to-negative end) polar arrangement in crystalline state generate opposite electrical potentials at the fracture surfaces when broken. These charges allow an instant recombination and self-repair of the broken crystals without any external intervention. During repair, fractured pieces travel with a honeybee wing-like motion with accelera- tion comparable to diesel cars!
Prof. Nirmalya Ghosh and his team from IISER Kolkata, used a custom designed state-of-the-art polarization microscopic system to probe and quantify the structural order of the piezoelectric self-healing organic crystals with nanomete scale spatial resolution. The researchers show that these crystals, which belong to a general class of piezoelectric materials that can generate electricity under pressure, or vice-a-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 trans- ducers, mechanical sensors, energy harvesters, biomedical implantables, 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.
Prof. Reddy Says ‘since last decade tremen- dous amount of research has been done to find self-healing property in the unnatural polymers, gels and composites which are soft and amorphous in nature. Various strategies have been employed to mimic nature but almost all of them need at least one stimulus such as heat, light, solvent or a chemical healing agent. And universally all materials fail when the broken parts fall apart’. The first author of this paper, Surojit Bhunia from IISER K further added ‘Here, taking cues from nature we explored the inherent piezo- electricity of crystalline materials for finding self-healing property. It is long known that piezoelectricity plays a key role in initiating self-healing in mecha- nically wounded natural biomaterials like bone and collagen. Hence, our discovery in organic materials may provide further insights in to the complex natural materials’.
Prof. Reddy pointed out that ‘Another not 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 crystallo- graphically precise reordering extremely difficult’. Other corresponding author of this paper, Prof. Nirmalya Ghosh, who is also the laureate of the SPIE G G Stokes award in optical polarization 2021, said ‘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. This unique experimental system in combination with a suitable polarization analysis model enabled quantitative assessment and understanding of the self-healing behaviour of the crystals by sensing changes in nano scale structural anisotropy’. Prof. Ghosh added, ‘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’.
Dr. Sumanta Karan from IIT KGP added ‘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’.
Prof. Reddy, Prof. Ghosh, Prof. Khatua and team (Surojit Bhunia, Shubham Chandel, Sumanta Kumar Karan, Somnath Dey, Akash Tiwari, Susobhan Das, Nishkarsh Kumar, Rituparno Chowdhury, Saikat Mondal, Ishita Ghosh, Amit Mondal) hope for realization of technological applications of their novel self-healing piezoelectric materials in near future. The materials in the work are patented by IISER Kolkata.
Prof. Reddy thanks the funding agencies (DST, New Delhi for Swarnajayanti Fellowship (DST/SJF/CSA-02/2014-15) and SERB (No: EMR/2017/005008). Prof. N. Ghosh thanks SERB (No. CRG/2019/005558) and Prof. B. B. Khatua thanks IIT Kgp for funding (132/IIT/EQ-82/MSC/2018). All the team leaders thank their respective institutes for the support.