# Additive Manufacturing and Chemical Engineering: Looking for Synergies from a Bibliometric Study
## 4 Review of Research Focused on Additive Manufacturing and Chemical Engineering
### Additive Manufacturing and Biofabrication
#### 4.1.2 Bioprinting Materials  
Regarding the materials used for bioprinting, bioinks and biomaterial links can be distinguished. A bioink is defined as a formulation of suitable cells for processing using automated biofabrication technology, which may also contain biologically active components and biomaterials [78]. For clarity and differentiation purposes, biomaterials that can be printed and subsequently seeded with cells after printing, but are not directly formulated with cells, are not classified as bioinks and are named biomaterial inks. Such biomaterial inks may be employed in the fabrication of scaffolds for cell seeding, bioreactors, or implants. For bioinks, cells are an indispensable component of the formulation, which may take the form of individual cells, coated cells, cellular aggregates, or a combination of cells with other materials. In contrast, biomaterial inks use a biomaterial for printing, and cell contact occurs after biofabrication. Biomaterial inks are typically used to produce a rigid scaffold for permanent stabilization or slow degradation of the printed structure, whereas bioinks produce a softer scaffold that can be more rapidly replaced by the deposition of a new extracellular matrix by the embedded cell population [71]. Moreover, hydrogels are three-dimensional polymeric networks that are insoluble in water and exhibit the property of swelling to reach an equilibrium volume without losing their shape. Their capacity to swell in biological conditions renders them an optimal category of materials for tissue engineering. Thanks to their elevated water content, they can imitate the extracellular matrix of native tissues, achieving a closer approximation to natural soft tissues compared to other biomaterials (Figure 11) [79]. These hydrogels can be derived from natural or synthetic sources [80] and a multitude of bioinks have been developed from natural hydrogels, which are well established as suitable materials for three-dimensional cell cultures [81]. These natural materials exhibit excellent biocompatibility and are highly tunable in terms of their physical, mechanical, and biological properties [82] and they often exhibit non-Newtonian and shear-thinning behavior, making them suitable for extrusion-based bioprinting. When compared to natural hydrogels, synthetic hydrogels exhibit greater reproducibility with respect to their physical and chemical properties. They have become a significant alternative for scaffold fabrication, as they can be molecularly tailored with respect to block structures, molecular weights, mechanical strength, and biodegradability [79]. Synthetic hydrogels can only provide structural support and do not inherently promote cellular function, as they require chemical functionalization to integrate bioactivity into their structure [80]. Many synthetic hydrogels exhibit non-Newtonian properties like biopolymeric ones, allowing them to be extrusion-printed. The use of single-component bioinks offers limited value in bioprinting. However, the properties required for optimal printability and print fidelity often conflict with those that enhance cell viability and functions. For instance, natural hydrogels typically exhibit weak mechanical properties, whereas synthetic hydrogels lack bioactivity. Additionally, natural biological tissues are composite materials with anisotropic properties dependent on their components [83]. To address these challenges, various strategies have been developed, including the use of composite hydrogels. By combining favorable biological properties with enhanced structural reinforcement, these composite hydrogels achieve improved printability and precision in bioprinting. Nanocomposites play a significant role in this development. These bioinks aim to mimic native tissues by crosslinking polymers with inorganic or organic nanofilers such as metals, polymers, or ceramics. The presence of these nanoparticles alters the mechanical, physical, and chemical properties of the bioinks, thereby enhancing their structural and functional properties [84].