## 1 Introduction  
Extrusion-based bioprinting is an additive manufacturing (AM) technique used for various tissue engineering applications (Figure 1) [1]. Many studies have been carried out to create hydrogel scaffolds using this technique [2, 3]. Generally speaking, computer-aided design (CAD) is used to deposit biomaterials [4]. However, scaffolds are rarely fabricated exactly according to the CAD model. That is why the printability index is important as an element showing the difference between the scaffold design (typically in a CAD model) and the printed scaffold. Three dimensional (3D) printability of a hydrogel biomaterial is defined as the ability of a hydrogel to form and maintain a reproducible 3D structure with dimensional integrity. Although the range of accuracy for extrusion-based machines is in the order of a micron, there is still a challenge when it comes to shaping the fidelity and printability of scaffolds bio-fabricated using the extrusion-based technique. The printability can influence other interrelated factors, such as the morphology and mechanical properties of scaffolds. Consequently, it can affect cell response [5], and it is well-accepted that the mechanical properties of scaffolds can influence cell faith [6]. Hence, it is important to study elements that can influence printability. Although there are a few studies on the printability of different biomaterials, the real picture and definition of printability remain unclear and there are fundamental questions about how to map the relationships between printability and other interrelated factors such as biomaterial and fabrication. For example, in some studies, the flow behavior of biomaterials was considered to evaluate printability [7; 8]. In these studies, only the physical and rheological characteristics of materials were investigated [9]. In another study, the influence of ionic crosslinkers on printability was investigated without considering other factors [10]. In some studies, only printing parameters were investigated as critical factors influencing printability [2; 11]. In another study, the gelation properties of materials during the printing process were studied to achieve a mechanically stable structure [12]. [PERSON] et al. studied gelation time, swelling, and the printability of various groups of hydrogels [13]. In another study, analytical methods were implemented to check the printability of materials [14]. [PERSON] et al. reported that printability is a matter of rheology, biomaterial composition, nozzle variables, pore and filament dimensions, geometry, and printing angle [15]. Hence, considering only one of the factors is not a systematic approach to improving printability. As mentioned, different studies have specifically investigated the effect of some factors on printability. However, there is no clear picture of printability considering the interrelated factors influencing printability. In this study, rheological properties, printing parameters, and printing conditions were investigated systematically to map the relationship between these parameters and printability, rather than considering each factor individually. As such, more studies should be performed in this area to define and establish novel approaches to define and measure printability. The key question of this study related to how to measure printability [15]. Alginate is one of the hydrogels used for the biofabrication of scaffolds used for tissue engineering applications, as reported in numerous studies [16; 17; 18; 19; 20; 21; 22; 23]. Specifically, one of the approaches to improve the printability of alginate scaffolds fabricated by extrusion-based bioprinting method is to mix alginate with other types of hydrogels [24]. Gelatin is one of the hydrogels usually mixed with alginate. Gelatin is a natural polymer derived from collagen and it has a cell-friendly environment and this is one of the reasons for mixing alginate with this biomaterial [12].