# Whole Organ Engineering: Approaches, Challenges, and Future Directions
## 4 Approaches to Engineering Whole Organs
### Three-Dimensional Bioprinting  
One of the more remarkable technological achievements and advancements in recent history is 3D printing [50; 82; 83]. Ultimately, one of the most impressive feats to arise from 3D printing has been the ability to print biological materials and then have those scaffolds seeded with living cells, which is commonly referred to as 3D bioprinting. This approach includes fundamentals from various fields, including tissue engineering, materials engineering, cellular biology, and advanced robotic printing [53; 84]. Three-dimensional bioprinting is perhaps the most well-known bottom-up approach to tissue engineering in the public eye. Three-dimensional printing as a technology came about in 1981 when [PERSON] published his work on 3D plastic manufacturing using UV-cured photopolymers, which would later be developed into the 3D printing technique stereolithography by [PERSON], which creates a print layer by layer via UV light hardening a solution from which the print is drawn out of [85; 86]. Other 3D printing methods were developed later, including Fused deposition modeling (FDM), which is used in popular consumer plastic printers, in 1988, selective laser sintering in 1986, and selective laser melting in 1995 [87; 88].  
Three-dimensional printing is attractive in whole organ engineering because its ability to generate custom and complex geometries has the potential to translate well into generating personalized organs for patients. The technology was first used in medical applications in the early 2000s for printing hard constructs such as bone implants and prosthetics [89]. Human-scale bone scaffolds have been generated using selective laser melting or sintering of materials like alloys and bioresorbable polymers [90; 91].  
Even more in its infancy than solid construct printing is soft construct printing to replicate soft tissue, done using inkjet, extrusion, or stereolithography (SLA) printing. Inkjet bioprinting uses electric, mechanical, or thermal stress to compress the bioink into a droplet as it is extruded [51].  
Bioinks used in inkjet bioprinters are often composed of cells encapsulated in solubilized biopolymers such as collagen and fibrin that gel after extrusion, allowing for more homogeneous and controllable distribution of cells in an engineered tissue construct than when cells are seeded onto a scaffold, which can contribute to cell behavior better mimicking native functions [53; 59]. Extrusion-based bioprinting utilizes FDM printing technology with a specialized extrusion head, often a syringe tip, to extrude a bioink comprising a structural solubilized biopolymer solution like alginate or collagen, a cell solution, or a combination of the two. Extrusion bioprinting can either use different bioinks as scaffolding and for cell delivery, build the scaffold and extruding cells into the scaffold, or utilize a composite bioink in which cells are laden within a structural biopolymer solution [58; 92]. Similar to inkjet printing, extrusion printing's use of cell-laden bioink that is laid out in predetermined structures results in a more homogeneous and controlled cell distribution that is conducive to cell proliferation, tissue growth, and the recreation of native cell function [58]. SLA bioprinting is performed largely in the same manner as traditional SLA, using a photocurable pool of material from which a platform is lowered with a laser curing the material layer-by-layer [93]. Regardless of the 3D printing technique utilized, bioprinting soft tissue constructs is advantageous because of both the customizability and precision it provides. This precision is highly desirable in producing vascular networks within tissue constructs, as tissue size is otherwise limited by the distance which gas and nutrients can diffuse. The bottom-up approach of 3D bioprinting would allow for this limit to be overcome by generating whole organs with hollow networks built-in so that the construct is vascularized [51].