Biomimetic Nanostructures Incorporated with Synthetic Melanin for Advanced Soft Robot Materials

Soft robotics opens up new opportunities in a variety of fields, including biotechnology, agriculture, manufacturing, and defense, by replacing traditional rigid robots in complex scenarios. Soft robots are designed to imitate the flexibility and adaptability found in natural systems. Moreover, the materials employed in soft robotics typically require durability, with the necessity of biocompatibility, contingent upon the desired performance. Keratin, a fibrous protein comprising a major biomatrix in hair, skin, nails, wool, and feathers, can be an ideal candidate for soft robotics. The natural keratin-based materials offer biocompatibility and attractive mechanical properties, such as high tensile strength, hardness, toughness, and flexibility. However, materials developed from extracted keratin from natural sources often exhibit inferior mechanical properties compared to their natural counterparts, limiting their application in soft robotics.<br/>In natural keratin-based biomatrix, melanin is commonly observed, and its catechol groups are known for their contributions to self-healing, enhanced mechanical properties, and chemical stability, in addition to their primary role as pigments. Polydopamine (PDA) has garnered attention as an artificial melanin due to its similar chemical structure and properties to its natural counterpart. The customizable design parameters of melanin-like PDA particles (MLPPs) make them even more intriguing than natural melanin. In this study, we introduced MLPPs into the keratin biomatrix to investigate the relationship between MLPP design parameters and the mechanical properties of the biomatrix. Specifically, we synthesized MLPPs using PDA alone or by coating PDA onto various particles and then incorporated these MLPPs into the keratin matrix. We correlated the mechanical properties of the biomatrix with PDA coating thickness, particle sizes, and concentrations of embedded MLPPs. We anticipate that the improved mechanical properties of keratin-based materials achieved through the integration of MLPPs, along with the multifunctional capabilities offered by diverse MLPPs, will not only render them suitable for soft robotics but also broaden their potential applications, including light harvesting, 4D printing, color-switching, and metal scavenging.