## 4 Discussion
The Carbopol-embedded printing approach was also instrumental for creating ossicle models with alginate-based bioinks, alone or loaded with MSC, the cells being able to undergo osteogenic differentiation via chondrocytes [21] (given the endochondral nature of ossicles' embryogenesis [31]) either upon in vitro cultivation or directly when bioprinted in the alginate hydrogel [22]. This finding also argues for the good cell-supporting properties of calcium-crosslinked alginate, including the maintenance of viability for the time needed by the cells to differentiate in an osteogenic medium [22]. In vitro, the degradation of alginate varies, depending on its crosslinking [22] and molecular composition, between two [22] and four [32] weeks, i.e., long enough to support the osteogenic and chondrogenic differentiation of MSC, respectively. Considering that the solid ceramic implants are brittle and when tested in the clinic had a high failure rate [5], we integrated the ceramic models with a hydrogel, as a biological material with viscoelastic properties to counteract the brittle nature of the ceramic materials. The penetration of a ceramic construct with large-enough pores by a hydrogel in vitro and/or by cells in vivo, although not immediately providing a mechanical benefit (or on the contrary), may actually lead in the long term to a better accommodation of the model to the surrounding tissues. For this reason, we also tested the printing with CPC of models with pores in the hundreds-of-microns range, by using a CAD-generated print toolpath. These porous models could be printed in a meaningful size; however, all the anatomical shape details could not be retained through this method yet. This limitation was mainly due to the fragility and brittleness of these tiny porous models, which made small anatomical details prone to breaking during solidification and manipulation. To aid in the handling of the small and fragile porous models, the models were coated with crosslinked alginate. This result suggests that alginate embedding may improve the mechanical properties of these fragile constructs and temporarily prevent their brittle fracture until they are further solidified via in vitro ossification by the scaffold-colonizing cells. Moreover, the incorporation of the porous ceramic in the alginate solution followed by physical crosslinking with either CaCl\({}_{2}\) or BaCl\({}_{2}\) also facilitated the microCT analysis of the constructs. This was due to the high atomic mass of calcium and barium, which increased the hydrogel's contrast [18], making it visible simultaneously with the solid ceramic scaffold. This is another original contribution of this paper to the field of biofabrication. Among the other limitations of this study is the lack of exploration of the physical properties of the printed objects as well as of the cultivation of cell-containing ossicle models for osteogenic differentiation. The majority of our work was conducted based on visual observations, seeking to attain both a good model reproducibility in shape and size and an acceptable printing resolution. Admittedly, the resolution of the printing for these models remains a challenge, due to the small size of the ossicles. However, the shape of the constructs may even not need to be strictly similar to their CAD, as shown by others, who needed to slightly adjust the shape of 3D printed implants to accommodate the patient's ear, even when the CAD was made based on a specific individual's CT-scan [25]. In the next phase of our research, we will cultivate in the long term the cell-containing ossicle models, both with and without a CPC porous scaffold, for in vitro osteogenic differentiation. By monitoring the time-dependent changes in the construct dimensions, we will seek to determine the optimal initial cell density and the proportion of porous ceramic needed to adequately maintain the initial shape of the ossicles' soft component.