Associative Liquid‐In‐Liquid 3D Printing Techniques for Freeform Fabrication of Soft Matter

Abstract

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 ...

A dive into the bath: embedded 3D bioprinting of freeform in vitro models

“…48 For further insights on the specific rheological requirements of support baths, readers are referred to detailed reviews on this topic. 49,50…”

Hybrid Biofabrication of Heterogeneous 3D Constructs Using Low-Viscosity Bioinks

“…Bioprinting is a rapidly advancing field that aims to recreate a complex physiological microenvironment by the controlled spatial deposition of specific cells and biomaterials for in vivo regenerative and in vitro modeling purposes. , Elaborate microscale constructs of increasingly intricate details can be fabricated by applying newly developed synthetic hydrogels of augmented tunability and advanced bioprinting techniques, supporting the creation of complicated designs in microextrusion bioprinting. − However, hydrogels that support comprehensive cellular bioactivities, including cell adhesion, migration, proliferation, and differentiation, are innately soft and exhibit low shape fidelity and mechanical stability. ,− While natural mammalian hydrogels are biologically favorable in recreating physiological environments, their poor printability and tunability often signify that the high versatility and resolution of the bioprinter are lost in the fabrication process. − To print complicated microscale constructs with natural hydrogels, the bioink composition can be modified by increasing the hydrogel concentration or supplementing higher-viscosity materials. ,− However, the adjustment can interfere with cellular function and reduce the biologically favorable features of the cytocompatible hydrogel. ,,, Recent advancements in in-bath bioprinting, which incorporates cross-linking agent-infused granular and liquid support baths to provide mechanical support for natural hydrogels, can also be used to improve the overall structural stability of the printed constructs. ,− However, in-bath bioprinting requires additional steps to generate and remove the cross-linking bath, and the delicately printed hydrogel construct can lose its structural integrity during the transfer out of the printing bath and may shrink or disintegrate during cell culture, resulting in variable size and shape. ,, …”

“…11,14−17 However, in-bath bioprinting requires additional steps to generate and remove the crosslinking bath, and the delicately printed hydrogel construct can lose its structural integrity during the transfer out of the printing bath and may shrink or disintegrate during cell culture, resulting in variable size and shape. 11,17,18 To address this challenge, a relatively new field of hybrid biofabrication, or convergence bioprinting, has emerged. Hybrid biofabrication is a multidisciplinary approach to creating complex biological constructs, which integrates multiple technologies, materials, and processes.…”

Fabrication of Bijels via Solvent Transfer‐Induced Phase Separation using Liquid‐in‐Liquid Printing

“…These approaches rely on the association between the printing and bath phases (in the case of "associative liquid-in-liquid printing" techniques) and/or the favorable rheological behavior of the bath (i.e., "embedded extrusion 3D printing"). [30][31][32][33][34][35][36] Using these platforms, various soft matter systems with novel structural and mechanical features have been created and shaped into complex designs without being limited to layer-by-layer printing manner. [32][33][34][35][37][38][39][40] However, there has not been any report on creating relatively complex bijel constructs using either liquid-in-liquid 3D printing or any other type of freeform fabrication methods.…”

“…[30][31][32][33][34][35][36] Using these platforms, various soft matter systems with novel structural and mechanical features have been created and shaped into complex designs without being limited to layer-by-layer printing manner. [32][33][34][35][37][38][39][40] However, there has not been any report on creating relatively complex bijel constructs using either liquid-in-liquid 3D printing or any other type of freeform fabrication methods.…”