Robotic Manipulation under Harsh Conditions Using Self‐Healing Silk‐Based Iontronics

Abstract Progress toward intelligent human–robotic interactions requires monitoring sensors that are mechanically flexible, facile to implement, and able to harness recognition capability under harsh environments. Conventional sensing methods have been divided for human‐side collection or robot‐side feedback and are not designed with these criteria in mind. However, the iontronic polymer is an example of a general method that operates properly on both human skin (commonly known as skin electronics or iontronics) and the machine/robotic surface. Here, a unique iontronic composite (silk protein/glycerol/Ca(II) ion) and supportive molecular mechanism are developed to simultaneously achieve high conductivity (around 6 kΩ at 50 kHz), self‐healing (within minutes), strong stretchability (around 1000%), high strain sensitivity and transparency, and universal adhesiveness across a broad working temperature range (−40–120 °C). Those merits facilitate the development of iontronic sensing and the implementation of damage‐resilient robotic manipulation. Combined with a machine learning algorithm and specified data collection methods, the system is able to classify 1024 types of human and robot hand gestures under challenging scenarios and to offer excellent object recognition with an accuracy of 99.7%.

circumstance, self-healing components and recoverable circuits are crucial for maintaining stable physical properties of the system and gaining reliable testing results. Due to the steady electrical and mechanical performance before and after the self-healing process, silk-based iontronics can be used as conductive wires in the circuits, resulting in a superior damage resilient ability of sensing system while being scratched by the surrounding sharp obstacles. As a proof-of-concept, the piezoelectric circuit in Figure S5 can maintain the same output after the iontronic wire being cut and then healing together, showing a potential of the circuit to resist external damages.

Components for reconfigurable circuits
On top of damage resistance, self-healable silk-based iontronic film can also serve as tunable components in different reconfigurable circuits, leading to adjustable circuit properties ( Figure S6 and S7). As shown in the inset photographs and results, amplitude and frequency of the output signals can be modulated through the cutting and healing process of the iontronic components, suggesting strong potential for a wide gamut of applications (e.g., modular electronics, selfassembling robots, and customizable consumer electronics).

Composition of the materials
We make supplementary FTIR characterizations of the silk-based iontronic film in order to determine the protein conformation with different compositions. In principle, glycerol molecules act as the plasticizer to replace the incorporated water in protein hydration and to form intensified hydrogen bonds with the peptide matrix, resulting in the initial stabilization of α-helical structures 2 in the films, as opposed to random coil or β-sheet structures ( Figure S3 (a)). Meanwhile, Ca(II) ions form metal-ligand bonds (chelation) with the silk chains, competing with glycerol in a dynamic balance, introducing extensible structures in silk to improve the stretchability ( Figure S3 (a)). Furthermore, as shown in Figure S3 (b), when the concentration of Ca(II) ions increases, the stiff β-sheets decreases and the number of extensible secondary structures increases, such as random coils, resulting in an improved self-healing capability and anti-freezing robustness of the iontronic film.

Self-healing mechanism.
In principle, the self-healing is due to the swollen of silk/Ca(II) matrix caused by water molecules and the reformation of hydrogen and coordination bonds. In detail, when water (humid air) is added onto the fracture parts, silk chains will swell up, and the viscoelasticity of the film increased, thereby leading to the physical fusing of the two separated parts. Moreover, reversible hydrogen bonds will form between the polar groups of silk side chains with both glycerol and the other polar groups of silk side chains. Owing to the dynamic bonding of the intrinsic hydrogen bonds and coordination bonds, small cracks within the film can be healed rapidly when broken bonds form again at the fractured interface.

Comparison of the silk-based iontronics with previous works.
Notably, Ca(II) ions and glycerol have been used separately with silk in previous works, but the performance in both cases was limited. In our article, one of the innovations is to further improve the silk-based iontronics by adding glycerol into the system, which could bring several advantages to the material. First, glycerol molecules act as the plasticizer to replace the incorporated water in protein hydration and to form intensified hydrogen bonds with the peptide matrix, resulting in the initial stabilization of α-helical structures in the films. The formation of these strong hydrogen bonding interactions can enhance energy dissipation during the stretching process and thus increase the flexibility of iontronics film. Thus, in contrast to the previous reported silk/Ca(II) composite, the stretchability of the silk-based film (1000%, RH50) can be greatly improved with the presence of glycerol molecules as shown in the table. Second, with plentiful hydrogen bonds between glycerol and water molecules in our silk/glycerol/Ca(II) composite, the water evaporation rate could be diminished and result in a maintain of stretchability in high temperature (around 700% at 80°C). Thus, compared to the former work, our silk-based iontronics have broader temperature tolerant range. Therefore, our work combines the advantages of glycerol and Ca(II) ions, realizing the synergistic effects of these two additives for the first time.
Furthermore, we also made innovations in applications of the silk-based iontronics. As shown in the table, we report a silk-based iontronic system that can be used on both human skin and robots as a general approach for reliable robotic manipulation under harsh conditions. Thanks to the above mentioning improvements of the material, our silk-based iontronics can be used in harsh conditions, including extreme temperature and sharp objects scratching. Importantly, when coupled with a specified machine learning algorithm, our approach permits accurate human/robotic gesture identification across over 1024 classes, recoverable robotic interaction under challenging temperatures and irrespective of external damage, and grabbed-object recognition with an accuracy of 99.7%.