Self-Healing Semiconducting Polymers for Uses in Soft Robotics via Coordination Chemistry

Robots are increasingly assisting humans in performing various tasks. The requirement for safe physical human-robot interactions has led to the emergence of the field of soft robotics. Typically, soft robots consist of flexible and deformable materials such as elastomeric polymers, which have an inherent compliance comparable to biological tissues (10⁵ to 10⁹ Pa), greatly enhancing human interaction safety.^{1, 2} Their intrinsic softness and compliance further endow them with features such as resilience to impacts and collisions due to shock absorbance, making soft robots an ideal candidate for handling delicate tasks in uncertain and unstructured environments.³<br/>Despite the many advantages of soft materials, their usage in robotics does present challenges in terms of damage resistance. Synthetic soft polymers are highly susceptible to fatigue, overload and cuts, tears and perforation by sharp objects arising from uncontrolled working environments, which would induce irreversible damage to the material, massively reducing the functional performance of robots.³ Inspired by a powerful feature of living organisms, an economic and ecological solution to this vulnerability is to construct future soft robotic systems out of self-healing semiconducting polymers, permitting the healing of microscopic and macroscopic damages as well as recovering material functionalities.<br/>Various strategies have been applied to synthesise self-healable semiconducting polymers via non-covalent interactions such as hydrogen bonding, π-π stacking, host-guest interactions and ionic interactions.^{5, 6} Multiple advantageous features are associated with metal-ligand complexes, in general, most coordination complexes can chelate spontaneously, resulting in easily processable materials.⁷ However, achieving high mechanical strength within self-healing semiconducting polymers remains a crucial challenge. Due to the broad variety of possible metal and ligand combinations, the coordination bonds are highly tuneable, with accessible bond energies ranging from strong covalent to weak supramolecular forces such as van der Waals.⁸ By tuning the coordination bond strengths, the presence of metal-ligand complexes in the polymer matrix could result in a synergistic effect on mechanical toughness and self-healing efficiency, offering new insights into the design of self-healing soft robotics.<br/>Herein, we describe a new self-healing semiconducting polymer system based on metal-ligand coordination between transition metal ions (Cu²⁺, Zn²⁺) and bipyridine moieties incorporated in the cyclopentadithiophene-derived (CPDT) conjugated polymer backbone. Drastic changes to the polymer structure have been observed through spectroscopic measurements with varying ratios of bipyridine. Upon addition of the metal ions, a noticeable change in viscosity implied successful cross-links among polymer chains. Thermal analysis has revealed the stability temperature of the polymers to be as high as 400°C, giving the materials the potential to withstand extreme environments. In this presentation, we will discuss the self-healing and mechanical properties of the polymers from a coordination chemistry point of view, probing an understanding of the healing mechanism. Through the findings of our research, we hope that this creative combination of self-healing materials with robotics can present many opportunities for the future development of soft robotics.<br/><br/>Reference:<br/>1. D. Rus and M. T. Tolley, <i>Nature</i>, 2015, <b>521</b>, 467-475.<br/>2. E. Roels; <i>et al., </i><i>Adv. Mater.</i>, 2022, <b>34</b>, 2104798.<br/>3. S. Terryn; <i>et al.,</i> <i>Mater. Today</i>, 2021, <b>47</b>, 187-205.<br/>4. S. Y. Wang and M. W. Urban, <i>Nat. Rev. Mater.</i>, 2020, <b>5</b>, 562-583.<br/>5. Y. L. Rao; <i>et al.,</i> <i>J. Am. Chem. Soc.</i>, 2016, <b>138</b>, 6020-6027.<br/>6. H. Park; <i>et al.,</i> <i>Nat. Commun.</i>, 2023, <b>14</b>, 5026.<br/>7. C. H. Li and J. L. Zuo, <i>Adv. Mater.</i>, 2020, <b>32</b>, 1903762.<br/>8. H. C. Wu; <i>et al.,</i> <i>Adv. Funct. Mater.</i>, 2021, <b>31</b>, 2009201.