V) GelMA filling and ultraviolet crosslinking; (v) dissolution of the sacrificial channels; (vi) final construct with hollow channels. (C) Visualization of hollow microchannels endothelialization inside the GelMA construct: (i) linear and (ii) bifurcating microchannels; (iii) CD31 (green) and nuclei (blue) staining on the confluent layer of HUVECs.Int. J. Mol. Sci. 2021, 22,ten ofAs an instance, Wang and colleagues 3D bioprinted a functional cardiac tissue mimicking the native myocardium applying a fibrin-based composite hydrogel as bioink [109]. Effectively, spontaneous and synchronous contraction was observed within the bioprinted model, when positivity to Actn1 (Alpha-actinin-1) and Cx43 (connexin 43) proteins indicated the presence of aligned and coupled cardiac cells. In another study, Zhang et al. [110] developed an anisotropic endothelium layer by mixing a composite alginate bioink, endothelial cells and rat-derived cardiomyocytes. Interestingly, an aligned, spontaneously and synchronously contracting tissue has been generated then this tissue has been MEK2 Gene ID embedded within a microfluidic perfusion bioreactor to make a myocardium-on-a-chip. Park et al. [111] developed the first bioprinted lung-on-a-chip utilizing ECM bioink derived from tracheal mucosa, demonstrating a higher expression of vascular markers, also as adequately induced inflammatory responses useful for studying dust-mite-induced exacerbation in vitro. A further interesting instance is definitely the eye-on-a-chip created by Seo et al. that provided a realistic platform to study dry eye PAK5 medchemexpress disease [112]. In details, these authors applied key human keratocytes and epithelial cells, a 3D cell culture scaffold coupled using a perfusion chamber, a tear channel plus a biomimetic eyelid. Lastly, dry eye disease has been induced by the authors by fixing the frequency of blinking actuation at six times per minute to simulate decreased blinking rates observed in vivo in patients. Interestingly, complicated organs-on-a-chip models of intestine have already been engineered including neighboring channels lined by human microvascular endothelium, immune cells, and pathogenic bacteria. As an example, Shah and coworkers [113] designed a modular microfluidics-based human icrobial co-culture model, HuMiX, to mimic gut microbioma, demonstrating the possibility of constructing a host-microbiome ecosystem containing Caco-2 cells and anaerobic human gut bacteria by a continuous perfusion of culture medium. Homan et al. [114] linked bioprinting to microfluidic technologies for establishing a renal-proximal-tubule-on-a-chip model (Figure 3). This model structure has been made by printing PluronicF127 onto a gelatin ibrinogen matrix at first step, which was then liquefied at 4 C for allowing cell seeding and perfusion. Interestingly, the proximal tubule epithelial cells seeded within the 3D model showed a well-defined polarization and produced kidney-specific cytokines, resembling in vivo scenario. Still now, the investigation on the mechanisms on the myelotoxic strain induced by radiation or drugs is challenging as a result of the inaccessibility of this tissue in vivo. For this goal, Chou et al. [115] created a vascularized bone marrow-on-a-chip technique composed by a channel filled with a fibrin gel, CD34+ cells and bone marrow-derived stromal cells, as well as a parallel channel covered by human vascular endothelium and perfused with culture medium. Because of this program, the authors effectively recapitulated the myeloerythroid toxicity t.