The ability to print soft materials into predefined architectures with programmable nanostructures and mechanical properties is a necessary requirement for creating synthetic biomaterials that mimic living tissues. However, the low viscosity of common materials and lack of required mechanical properties in the final product present an obstacle to the use of traditional additive manufacturing approaches. Here, a new liquid-in-liquid 3D printing approach is used to successfully fabricate constructs with internal nanostructures using in situ self-assembly during the extrusion of an aqueous solution containing surfactant and photocurable polymer into a stabilizing polar oil bath. Subsequent photopolymerization preserves the nanostructures created due to surfactant self-assembly at the immiscible liquid-liquid interface, which is confirmed by small-angle X-ray scattering. Mechanical properties of the photopolymerized prints are shown to be tunable based on constituent components of the aqueous solution. The reported 3D printing approach expands the range of low-viscosity materials that can be used in 3D printing, and enables robust constructs production with internal nanostructures and spatially defined features. The reported approach has broad applications in regenerative medicine by providing a platform to print self-assembling biomaterials into complex tissue mimics where internal supramolecular structures and their functionality control biological processes, similar to natural extracellular matrices.

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processing, and enable numerous inks to be utilized within the same printed construct. However, most soft materials are not widely applicable to such traditional ink-based 3D printing techniques, specifically when necessary physicochemical properties such as certain rheological behavior and crosslinking mechanisms are lacking. Complementary to the ink-based printing approaches described above, the light-based 3D printing modalities, also known as vat polymerization-based 3D printing, are the other technology in place for soft materials in which a photocurable liquid resin stored in a vat (tank) is treated layer-by-layer with either visible or UV light. [7] Typical light-based 3D printing techniques for soft matter include stereolithography, digital light processing, continuous liquid interface production, and two-photon polymerization. [1,7] Despite the superior printing resolution, accuracy, speed, and the freedom to print very complex and delicate parts, the light-based printing techniques suffer from several drawbacks including a limited selection of processable photocurable resins and challenges in realizing multimaterial 3D printing in a single build process. Furthermore, using ink-or light-based printing modalities, it is challenging, if not impossible, to shape soft matters into freeform complex 3D designs, in which the printing can be performed in an omnidirectional manner (not limited to layer-by-layer patterning). Freeform 3D printing removes restrictions on structural complexity, allowing overhanging parts, internal void spaces, and disconnected features to be created. Thus, to overcome the inherent difficulties associated with printing soft matter discussed above and to enable freeform fabrication from an ever-broadening palette of soft materials, liquid-in-liquid 3D printing (LL3DP) approach has emerged as a new class of 3D printing techniques. [8][9][10][11]15,16,22] The LL3DP approaches fall under the category of ink-based 3D printing (Figure 1) since the printing materials (that make up the final part) are printed in the form of an ink as opposed to light-based (vat polymerization-based) 3D printing in which the printing materials are contained in a vat and cured selectively. In LL3DP techniques, while the translation stage moves according to a 3D design, the ink phase is extruded into a meticulously selected second bath phase. [8][9][10][11] The extrusion of the ink within the bath phase prevents the print collapse and ensures the shape fidelity, structural integrity, and necessary feature resolution of the final constructs. [8][9][10][11] By adopting such printing approaches, freeform fabrication Shaping soft materials into prescribed 3D complex designs has been challenging yet feasible using various 3D printing technologies. For a broader range of soft matters to be printable, liquid-in-liquid 3D printing techniques have emerged in which an ink phase is printed into 3D constructs within a bath. Most of the attention in this field has been focused on using a support bath with favorable rheology ...

Associating surfactants involving polar oils have recently been shown to stabilize immiscible liquids by forming nanostructures at the liquid interface and have been used to print soft materials. Although these...

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