# Additive Manufacturing of Bone Scaffolds Using PolyJet and Stereolithography Techniques
## 1 Introduction  
Although natural human bones have characteristics that allow them to regenerate and heal naturally, these characteristics are often not effective for large bone defects and injuries resulting from tumor resections, old age, and traffic accidents. These challenges in orthopecels pose significant risks to the health and quality of life of individuals [1, 2]. Bone grafts such as autografts, allografts, and xenografts act as bone substitutes and are used in cases where a bone is unable to heel the defects caused by severe damage, trauma, deformity, or tumor [3]. Although natural bone grafts are biocompatible, osteoconductive, and osteoinductive, there are still limitations associated with their use, such as anatomical differences, lack of availability, risk of infection, and site morbidity [4, 5, 6]. To overcome the limitations associated with natural bone grafts, synthetic scaffolds have been developed. The materials used in the synthetic scaffolds are divided into four generations: first generation, composed of metals and alloys; second generation, composed of ceramics and polymers; third generation, composed of composites; fourth generation, composed of smartmaterials [7]. In comparison with natural bone grafts, synthetic materials have drawbacks, including the manufacturability of complex structures [8], uncontrolled permeability [9], unmatched mechanical properties, high density (for metals), ion release from metals [10], the friability of ceramics [11], the low strength of polymers [12], and the uncontrollable degradation of composites [13]; however, smart nanoparticle-based composite materials can be used to control the degradation rate [7]. Synthetic bone scaffolds can be fabricated using a variety of methods, which are divided into two main categories, namely conventional and additive manufacturing techniques. The conventional techniques used for the fabrication of bone scaffolds are gas foaming, solvent casting, particle leeching, freeze drying, and electrospinning techniques. The conventional techniques also have shortcomings, including insufficient pore interconnectivity, inadequate pore size, and porosity and structure controllability [9]. Additive manufacturing technologies have led to noteworthy progress in the development of synthetic scaffolds for bone tissue engineering, as they provide defined pore interconnectivity, pore size, and porosity, as well as structural controllability. Additive manufacturing techniques used in bone tissue engineering include stereolithography (SLA), fused deposition modelling (FDM), selective laser sintering (SLS), and selective laser melting (SLM) [14; 15]. According to ISO 52900, additive manufacturing fabrication is defined as a manufacturing process that employs an additive manufacturing technique, whereby successive layers or units are built up to form model. Synthetic bone scaffolds are not structurally equivalent to natural human bone because of the complex and heterogeneous physiological structures of native tissues. The literature is particularly focused on the fabrication of simplified synthetic scaffolds instead of reproducing the internal microarchitecture of natural human bone, which would make synthetic scaffolds functionally equivalent in terms of their mechanical properties and architectural parameters for regeneration and repair. In this manner, additively manufactured synthetic scaffolds have been fabricated with well-characterized transport and mechanical properties using unit elements with different architectural parameters, such as the porosity, pore size, and pore shape [16]. The porosity, pore size, and pore shape of a synthetic scaffold are critical design parameters that not only affect the permeability, which can result in cell death, but which also affect the mechanical properties, which can result in insufficient load-bearing capacity. The parameter that mainly affect the transport and mechanical properties of synthetic bone scaffold is porosity [17].