## 4 Discussions  
The idea of being able to produce organs that are fully functional and have the capacity of integration and restoration of the normal relations from different systems in the human body is an old one. The idea of replicating a human meniscus tries to solve one of the most important challenging pathologies in orthopedic surgery [13]. This study focussed on bioprinting a viable 3D full-scale meniscus seeded with human intrapatellar fat pad mesenchymal-derived stem cells that could provide a positive high percentage of chondrogenic live cells able to secrete matrix through the hydrogel. [PERSON] et al. [14] managed to print aortic valves through a combination of interstitial cells and a mixture of hydrogels consisting of alginate and poly(ethylene glycol)-diacrylate (PEG-DA). Micro-CT studies of the printed organ revealed that the normal architecture was achieved and the constructed organ was able to encapsulate aortic root sinus smooth muscle cells. Other groups [15] managed to produce a human ectopic artificial liver (HEAL), which, after transplantation into mice, presented humanized liver function, human protein synthesis, human drug metabolism, and drug-induced liver injury. [PERSON] et al. [1] implanted in a rodent model a muscle construct built with human muscle progenitor cells. The construction presented myofiber-like structures and after 8 weeks of implantation into a tibialis anterior muscle defect, it provided 82% muscle function recovery. The additive manufacturing method of the printer used in correlation with the low heat of the printer head allows a uniform distribution of cells layer after layer, resulting in an equally loaded structure. The accuracy of cell deposition is dependent on the density of the bioink, speed of printing, and the nozzle diameter [4]. The present study has some limitations. Although the shape of the meniscus was precisely recreated, the bioink was distributed layer by layer in a single manner, but it is well known that the natural structure has different patterns of tissue distribution. These patterns offer great strength to the natural meniscus, thus making it able to resist multiple types of forces like compression, traction, shear, and multiple combinations [9]. Additionally, although the printed structure revealed the presence of extracellular matrix  
Figure 8: **(A)**. Bioprinted meniscus with in vitro cultivated adipose-derived stem cells colored with Aggrecan (**B**). Same Image under 10\(\times\) magnification. formation, the compression test showed significant differences to the native tissue. The hydrogel was used according to the manufacturer's methodology, but they provide it as a universal ink. Some authors [4; 16] have managed to produce ultra-strong hydrogel that has a better performance on mechanical tests, but they have not tested these structures loaded with cells. Another limitation is the shape isolated to be mechanically tested. We chose to compare only a small part of the printed meniscus (shape described) with a homogenous portion of the native meniscus. It has been clearly demonstrated by many authors that the collagen fibers of the human menisci have different patterns of distribution to be able to resist multiple (compressions, shear, torsion) and combined forces [4; 17; 18]. The mid portion of the native meniscus has the most constant consistency of the whole structure and is mostly capable of withstanding axial compression in a normal environment [9]. As the mechanical part was only a secondary objective of this study, we did not perform further tests. Compression strength in combination with infrapatellar adipose-derived stem cells was low compared to the natural meniscus. The construct could be improved using ECM [19], maybe of porcine origin like [PERSON] et al. used, to obtain a stronger construct. Unlike us, they tested porcine origin cells and thus the human ones in contrast to the porcine ECM should be tested in the future [20].