Universality and versatility of surfactant self-assembly in liquid-in-liquid 3D printing of soft matter*

Despite low viscosity and lack of required mechanical properties in soft materials, the newly emerged liquid-in-liquid 3D printing approach has enabled the fabrication of predefined architectures from these otherwise non-printable materials. In a specific group of liquid-in-liquid 3D printing, surface active components such as surfactants have proved to be the enabling factor for printing stable aqueous structures within a hydrophobic surface-active oil phase. Furthermore, the underlying ternary phase diagram that is established using both experimental and simulation techniques reveals that the geometrical transition in nanostructures (from micellar to lamellar) has enabled the stabilization of printed constructs. In this work, the versatility and universality of this printing platform are illustrated through the use of various surfactant classes (cationic, ionic, nonionic, and zwitterionic). The constructs 3D printed using all these surfactants not only show relatively high complexity in design and structural characteristics but also demonstrate other practical features such as perfusibility and self-healing properties. Furthermore, using both experimental (small-angle X-ray scattering and interfacial/bulk rheometry) and computational (mesoscale simulation) techniques, the underlying mechanism (i.e., morphology transition at the liquid-liquid interface) for all systems with different surfactants is explored. Lastly, various printing patterns are achieved by taking into account the dynamics of the printing system and tuning printing parameters. The implications of this work lie in the freedom to use various surfactants with different molecular structures and properties, which in combination with practical features and complexity of prints, opens up new opportunities for liquid-in-liquid 3D printing techniques in various fields, including bioelectronics, tissue engineering, and drug delivery systems.