## 1 Introduction
Parameters, such as pressure and temperature, may inflict changes on a cell's phenotype and thus, they should be carefully managed and their effects assessed [10]. Stem cells, such as human induced pluripotent stem cells (hiPSCs), can play a huge role in understanding SCI due to their ability to self-renew and to differentiate into any cell type in our body [11; 12]. Scientists can design in vitro lab experiments that can directly translate into clinical studies by creating patient-specific tissues [13]. Differentiation protocols for hiPSCs need to ensure the development of a homogeneous neural cell population, as it has been reported that hiPSCs develop into neurons with less efficiency and more variability than embryonic stem cells (ESCs) [11; 14]. Moreover, the use of hiPSC-derived neural progenitor cells (NPCs) limits differentiation to neuron-specific cell fates. [PERSON] et al. reported neuroregeneration and neuroprotection after NPCs transplant in rats with SCI [11]. Another important component to take into consideration for the generation of neural tissue is the bioink to be used for printing. For this study, a novel fibrin-based bioink was used to bioprint NPCs. The use of a fibrin-based bioink promotes neuronal differentiation and survival as it is biocompatible, human derived, contains cell adhesion signaling molecules, and allows the flow of nutrients [15]. In addition, this material will enable neural differentiation as it can mimic the biochemical and mechanical environment of the SC [8]. Being a naturally-derived material, fibrin has a decreased mechanical stability and a slow polymerization process [8; 15]. The polymerization of fibrin starts with the addition of the enzyme, thrombin [16]. Alginate, which polymerizes quickly, can be mixed with fibrin as a second component and make the bioink suitable for bioprinting [8]. To address the challenge of the lack of mechanical stability of fibrin scaffolds, small molecules, such as genipin, can enhance the structural properties of the scaffold and promote neural outgrowth [16]. The cationic polysaccharide, chitosan, is the third component of the novel bioink used to provide mechanical strength to the scaffolds by ionically interacting with alginate and covalently crosslinking with genipin [17]. Inkjet, extrusion, laser-assisted, and stereolithography are the main strategies for bioprinting tissues [18; 19]. However, one of the major challenges faced when using these technologies is the amount of shear stress the cells receive as they are extruded, which leads to low cell viability and lack of long term-functionality of the tissues [20]. The Lab-on-a-Printer(tm) (LOP) technology integrates microfluidic channels with printing technology that allows material processing prior to printing and programmable parameters to obtain specific compositions in the printed materials [21]. In this study, the RX1 tm Bioprinter from Aspect Biosystemstm was used to bioprint NPCs. This bioprinter offers the possibility of printing 3D scaffolds with cell-laden bioinks where programmable patterns can be created [22]. Speed and fiber composition are regulated while printing on a layer-by-layer basis. The RX1 tm uses a LOPtm microfluidic printhead that prints complex and heterogeneous structures with different cell types and materials in a reproducible manner [22]. These characteristics lead to the bioprinting of a precise design with specific micro and macro architectures while the bioink components are evenly distributed in the 3D structure. In the case of SC, it is necessary to mimic precisely the anatomical and physiological characteristics in order to develop a correct model of the disease to perform drug screening. Tuning the aforementioned characteristics will enable the differentiation of NPCs into motor neurons (MNs), one of the main type of neurons affected during SCI and whose functionality needs to be re-established [23].