Mechanical and biological properties are two aspects of the most fundamental requirements for optimal bone tissue engineering (BTE) scaffolds. Nonetheless, it has been difficult for existing fabrication strategies to prepare BTE scaffolds with both superior mechanical and biological compatibility. Inspired by the hierarchical materials in Nature, BTE scaffolds with hierarchical porous structures can potentially improve mechanical efficiency and biological functions. In addition to an interconnected macroporous (>100 μm) structure, the incorporation of micropores (<50 μm) can facilitate cell attachment and promote bone tissue ingrowth. 

With this in mind, this study reports the fabrication of hierarchical porous hydroxyapatite (hpHA) scaffolds by direct ink writing (DIW) of emulsion inks. As an extrusion-based 3D printing technique, DIW allows customizable design and accurate control of printed structures at a macroporous scale. The microporosity and micro-pore morphology of the struts can be further controlled via the formulation of the emulsion inks. We introduce two types of gelling additives, poly(ethyleneimine) (PEI) and Pluronic® F-127, respectively, into particle-stabilized emulsions and fabricate hierarchical hpHA scaffolds by DIW. We discover that the two gelling additives can lead to distinctive microstructures due to their different gelling mechanisms. The 3D printed scaffolds in this study exhibit tunable and comparable mechanical properties with those of the cancellous bone. The introduction of an additional microporous structure is also found to greatly improve the in vitro cell attachment, spreading, and proliferation. Hence, the hierarchical porous hydroxyapatite scaffolds show great potential in BTE applications, and the results reported herein will be useful for future optimization of BTE scaffolds.