Integrated technology of casting and 3D printing

Casting is a traditional manufacturing method. Preparation of complex structures by 3D printing are often involved to remove support materials. Mold forming contributes to removing these materials and mass produce. By combining 3D printing and traditional manufacturing technologies, various vascular scaffolds can be designed and prepared. Extrusion-based 3D printing requires rheological properties of print materials. For sacrificial materials, carbopol as a sacrificial gel to fabricate vascular scaffolds was introduced [48]. The sacrifice material possessed shear thinning properties and was extruded by an extrusion-based 3D printer to create personalized vascular structures. In this process, frames with another material were initially printed. Next, sacrifice structures were printed with carbopol and then casting materials were poured to overlay the printed carbopol. After the casting materials were cured and the sacrificial material was removed, scaffolds were prepared (Fig. 9A). As the traditional casting technology has been widely studied in mechanical discipline and molds are easy to be prepared using 3D printing, now it has become one of the main methods to prepare vascular scaffolds because of the research progress of sacrificial materials. Nie et al. [136] reported an integrated method, mold casting and inkjet 3D printing and fused deposition modeling (FDM) 3D printing, to fabricate vascular scaffolds. An inkjet 3D printer was used to print ultrafine fiber networks of vascular channels. Hydrogels were then casted into fiber networks to form hydrogel sheets. Hydrogels were cured at 4 °C. After the removal of fiber materials, scaffolds with internal vascular networks were formed by hydrogel sheet bonding using UV curing (Fig. 9B). The printing accuracy of vascular scaffolds depends on the printing resolution of fibrous structures. Multi-scale vascular networks can be manufactured in this way. But the entire process is complex and requires twice-crosslinking, leading to damaged scaffolds after demolding. For demolding, Lv et al. [99] reported in-depth analysis of demolding methods. Separable molds were prepared by inkjet 3D printing, and hydrogels were casted and cured by UV curing. Compared to integral molds, separable molds reduced surface contact during demolding and achieved perfect demolding. Skylar-Scott et al. [137] prepared a heart model including a left anterior descending (LAD) artery and a diagonal branch, using integrated technology of UV-assisted 3D printing, casting, and embedded 3D printing, as shown in Fig. 9C. First, a mold for the heart model was fabricated by UV-assisted 3D printing. Then the model was filled with cardiac tissue matrixes by casting. Finally, the artery and diagonal branch were printed by embedded 3D printing, respectively. This process contributes to the successful preparation of such complex organs because the configuration of organs and the formation of a vascularized network are guaranteed.

Preparation processes of vascular scaffolds by the integrated technology of casting and 3D printing: A Preparation of vascular scaffolds by the integrated technology of casting and extrusion-based 3D printing. Reproduced with permission [48]. Copyright 2019, IOP Publishing. B Preparation of vascular scaffolds by the integrated technology of casting and EHD 3D printing and FDM 3D printing. Reproduced with permission [136]. Copyright 2020, The Royal Society of Chemistry. C Preparation of cardiac tissues by the integrated technology of UV-assisted 3D printing to prepare a cardiac mold, casting to fill with matrix, and embedded 3D printing to fabricate vascular networks. Reproduced with permission [137]. Copyright 2019, Skylar-Scott et al. D Preparation of vascular scaffolds by corrosion casting with solution perfused into blood vessels. Reproduced with permission [138]. Copyright 2016, Elsevier

Overall, casting techniques involving 3D printing can enable the preparation of complex molds and vascular scaffolds. There are individualized differences in vascular morphology of patients in clinical treatments and researches. For individualized scaffolds, the preparation process is quite simple, because 3D models can be easily obtained through parametric design or reverse engineering. However, regardless of what 3D printing processes, scaffold modeling is required. Mold preparation by corrosion casting may help solve the problem. The principle of corrosion casting is similar to reverse engineering. Corrosion casting can directly extract structures of vascular scaffolds. Compared with traditional structure modeling and preparation methods, structures obtained by corrosion casting are more bionic. The preparation of vascular scaffolds using corrosion casting was reported by Huling and workers [138], as shown in Fig. 9D. The process began with the perfusion of PCL solution and acetone into native kidney tissues for enough days until acetone evaporation. Then, vascular corrosion cast was achieved after the dissolution of tissues by sodium hydroxide. Next, the PCL cast was dip-coated with collagen. Vascular scaffolds were finally prepared by the crosslinking of collagen and dissolution of PCL cast. In this method, vascular corrosion cast obtained from solution perfused into blood vessels is regarded as the process of scanning points cloud data in reverse engineering. The process of obtaining a vascular scaffold is seen as the process of reconstructing and manufacturing 3D model based on points cloud data. Usual vascular scaffolds are designed and manufactured according to the shape of original blood vessels. But limited by printing accuracy of 3D printing and conventional manufacturing technologies, this conventional method can only achieve the preparation of simple structures, or the preparation of scaffolds but requires the extra introduction of additional processes such as bonding. Besides, increased processes may lead to inaccurate structural accuracy and pollution issues. Structures such as configuration or sacrificial models of scaffolds can be fabricated by corrosion casting, paving an avenue for preparation of bionic vascular scaffolds.