# 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
Commonly used inorganic nanomaterials for developing nanocomposite hydrogels include metal nanoparticles (gold or silver), mineral nanoparticles (natural silicate clay, laponite, or layered double hydroxides as examples), magnetic nanoparticles (typically based on magnetite), carbon nanoparticles (graphene and derivatives like graphene oxide) and polymer nanoparticles such as nanocellulose [85]. Polymeric hydrogels prepared from blends of two natural and synthetic polymers are frequent. Among these, interpenetrating polymer networks are particularly notable, being unique crosslinked polymer blends where at least one network is synthesized/crosslinked in the presence of the other. These can be divided into double networks and ionically-covalently crosslinked networks. While double networks exhibit extremely high mechanical strength and toughness (superior to single-component networks of either constituent polymer and comparable to cartilage and rubbers), ionically-covalently crosslinked networks serve to dissipate energy from applied loads with covalently crosslinked polymer networks that maintain the hydrogel's elasticity [86]. Researchers with expertise in chemical engineering have been involved in the formulation of more sustainable hydrogels, bioinks, and biomaterial inks. The optimal composition of natural polymers, including silk fibroin, gelatin, and alginate, in a bioink for bioprinting in bone tissue engineering was investigated [87]. Through meticulous parameter optimization, the bioink achieved the highest print accuracy with 4%  
Figure 11: Examples of bioprinted hydrogels (Adapted with permission from Ref [80]. 2017, Elsevier). demonstrated excellent biocompatibility and enhanced alkaline phosphatase activity in osteoblast-like cells when supplemented with alendronate. The use of natural materials derived from plant and animal sources for the production of biomaterial inks has been explored, considering xanthan gum [88], fish scale particles [89], or powdered fenugreek (an annual plant cultivated worldwide as a semiarid crop, especially in the Indian subcontinent) [90]. The addition of xanthan gum enabled further tuning of biodegradability and an improvement in gelatin methacryloyl printability and shape fidelity. Fish scale as a natural collagen and hydroxyapatite source contributed to the biomaterial ink properties for bone engineering applications, providing additional stiffness and osteoinductivity to the hydrogels. Composite scaffolds using polycaprolactone, silk fibroin, bovine bone, and fenugreek powder exhibited enhanced mechanical durability and improved capacity to generate hydroxyapatite in vitro. A highly controllable method to optimize the printability of hydrogels based on internally crosslinked polysaccharides without the need for additives or post-printing treatments has been developed [91]. By introducing pH as a further parameter to be controlled, multiple pH-dependent crosslinking kinetics were possible without varying hydrogel composition. In addition, the results indicated that cells survived and proliferated following bioprinting, which can also interact and reorganize the hydrogel microstructure. In addition to the composition of biomaterials, modification of the bio-printing process itself also influences the properties of the obtained elements. The effects of various printing process parameters (including the air pressure for dispensing, dispensing head movement speed, and crosslinking conditions) on bioprinted Schwann cell-encapsulated scaffolds using composite hydrogels of alginate, fibrin, hyaluronic acid, and/or RGD peptide were investigated [92]. Regulating these parameters, mechanically stable scaffolds with fully interconnected pores were obtained, where the performance of Schwann cells was improved in terms of viability, proliferation, orientation, and ability to produce laminin.