Additive manufacturing has several different methods, and one of them is the extrusion type shown in Fig. 1 that has gained popularity in bioprinting for tissue construction applications . Several studies have revolved around the creation of hydrogel scaffolds using this kind of printing [2, 3]. For the most part, Three-dimensional (3D) reconstructed computer-aided designs (CAD) models form the basis of depositing the biomaterials . In general, scaffolds prove challenging to reproduce to scale utilizing the CAD models, which is why it can be so critical to determining printing parameters before construction. Three-dimensional (3D) printing parameters using a biomaterial compound in hydrogel form are often described as the ability and reliability of that material to form and maintain a 3D structure that is both sound and reproducible.
These overall printing parameters can profoundly influence other seemingly related factors, like the mechanical characteristics of the printed outcome and the morphology of the scaffolds produced. This, consequently, means that printing parameters influence all cell responses . With this in mind, it is vital to determine and study elements that can affect printing parameters.
This study focuses on extrusion-based three-dimensional (3D) bioprinting of hybrid bioinks and a few critical challenges in this field known as printing parameters. The aim is to isolate and discuss potential factors that can alter printing parameters and identify methods of measurement for it, specifically concerning the printing parameters of hydrogel scaffolds. As it stands, the flow of behavior in biomaterials has been studied, paying particular attention to factors like ink consistency [6, 7], the characteristics of the biomaterials involved , the mechanical aspects of varying hydrogel compounds , and the desired outcome. Despite this, not enough is currently known to draw consistent conclusions regarding the specifics of specialized printing. As a result, the needed parameters for printing scaffolds from a concoction comprised of hydrogels and other biomaterials have yet to be identified. Not enough consideration has been given to printing features that can affect scaffolds made from hydrogel compounds . This study pertains to this issue, and it includes changes in mechanical properties, degeneration, and enlargement over time based on the hydrogel mixture used, including the compounds alginate, alginate-gelatin, alginate-diphenylalanine, and alginate-gelatin-diphenylalanine. This study systematically implements characterizations by which the material flow behavior can be documented, based on both 2D and 3D printing parameters of various hydrogel compounds. These findings are then mapped out to illustrate the relationship between fabricated materials and biomaterials and how they affect printing parameters.
While some studies on the effects of various biomaterials and their printing parameters exist, a clear, definite understanding and ubiquitous definition of printing parameters remain elusive. A basic question like how to explore the connection between other crucial properties that affect printing parameters remains unanswered. A few studies where the flow pattern of particular biomaterials was used to evaluate and determine printing parameters prove to be an example of this [6, 7]. In these studies, the rheological and physical characteristics of the material were the focus of investigation without consideration of other factors , whereas in another study, the only factor considered was the effect of ionic cross-linkers . Some studies went on to focus exclusively on printing parameters and their effect on printing parameters [2, 11]. One study focused on the gelation characteristics during the printing stage to map a structurally and mechanically sound print . Another study focused on gelation factors, enlargement, and the overall printing parameters of varying hydrogel compounds . A separate study, endeavoring to check the analytical methods used, details the printing parameters of materials based on compounds that were analyzed . Kyle et al. went on to report that printing parameters are essentially a matter of rheology, nozzle, and printing parameters like filament and pore dimensions, a geometry which may include the printing angles, and biomaterial composition . Armed with this information, it is clear that considering only one factor in printing parameters while disregarding others is not an effective approach to improving the understanding and practical applications of printing parameters. As indicated, several research studies have analyzed the individual effects on printing parameters and have made small strides to understand the topic further. A clear perception of printing parameters that considers the inter-related conditions which influence it does not currently exist. Rather than focusing on each conditional change exclusively, this study analyzed printing parameters, printing conditions, and rheological properties in detail to systematically map the relationship between printing parameters and the factors that can affect them. As understanding develops, it is crucial to perform more and further in-depth studies to more accurately describe and affirm innovative ways to measure printing parameters. How to measure printing parameters is the critical question and focus of this study .
Alginate is known as one of the few hydrogel compounds used in the bio-manufacturing of scaffolds that are used in tissue construction applications, as recorded in several performed studies [16–23]. A common strategy currently used to refine printing parameters of alginate-based scaffolds', specifically ones constructed using extrusion-based printing, is to blend the pure alginate and some form of hydrogel compounds . Gelatin is specifically combined with alginate, as it is a natural, collagen-derived polymer. Gelatin is considered a cell-friendly environment, making it ideal for mixing this way . Amino acid and amino acid derivatives are compounds that contain both carboxyl and amino types, which prove useful in varying types of peptidomimetic and peptide synthesis . Alternatively, diphenylalanine is either naturally occurring or chemically synthesized amino acids that are non-proteinogenic. Due to a wide range of structural diversity and functionality, these compounds are widely used as building blocks in developing combined libraries. A choice study investigated the characteristics of some cell substrates comprised of natural and chemically synthesized amino acids. The results indicated that the scaffolds with higher water retention rates generally have both of these properties . This ultimately proves that mixing various materials can be one way to control the resulting outcome of the scaffold finely, thus allowing better control over desired functions.