A review of high internal phase Pickering emulsions: Stabilization, rheology, and 3D printing application

High internal phase Pickering emulsion (HIPPE) is well-known for its extremely high-volume fraction of internal phase, which leads to flocculated yet deformed emulsion droplets and resultant rheological behaviors such as shear-thinning property, viscoelasticity, and thixotropic recovery. Together with the properties inheriting from regular emulsion systems, including large interfacial area and well-mixture of two immiscible liquids, the HIPPEs with such rheological behaviors have been emerging as building blocks to construct scaffolds with designed structures and programmable functions using an extrusion-based three-dimensional (3D) printing technique, making 3D-printed HIPPE-based scaffolds attract widespread interest from various fields such as food science, biotechnology, environmental science, and energy transfer. Herein, the recent advances in the preparation of suitable HIPPEs as 3D printing inks for various applied fields are reviewed. This work begins with the stabilization mechanism of HIPPEs, followed by introducing the origin of the rheological behaviors and strategies to adjust rheology to prepare more eligible HIPPEs as printing inks. Then, the compatibility between extrusion-based 3D printing and HIPPEs as building blocks was discussed, followed by a summary of the potential applications using 3D-printed HIPPE-based scaffolds. Finally, limitations and future perspectives on preparing HIPPE-based materials using extrusion-based 3D printing were presented.