**5.2 Temporary implants**

*Design and Manufacturing*

[3, 94, 95].

**5.1 Additive manufacturing**

ate mechanical response to the desired application [36].

Some studies have used conventional methods for producing scaffolds. However, these methods have no adequate control over pore size and design or interconnectivity [8, 85]. In order to address these problems, since the mid-1980s [86], a new manufacturing type of technology called AM has emerged. Its potential is enormous and overcomes the capabilities of the conventional technologies to produce scaffolds with a complex architecture and with the intention to achieve an appropri-

Nowadays there are several approaches to AM for various applications. The main approaches are fused filament fabrication (FFF), three-dimensional printing (3DP), stereolithography (SLA), and selective laser sintering (SLS). Each process goes through several steps: (i) development of the 3D model through computeraided design (CAD); (ii) the files are stored in standard triangular language (STL) format, which is a CAD file format that supports 3D printing and computer-aided manufacturing (CAM); and (iii) these files are inserted into the input devices to create 3D models in a layer-by-layer process [36]. In addition, there are still two processes where it uses the same principles of layer manufacturing: selective laser melting (SLM) [87–90] and electron beam melting (EBM) [91–93]. Both are used to produce metal scaffolds, although SLM can also process polymers and ceramics

FFF, **Figure 1**, or melt-extrusion is an extrusion-based process and is the simplest 3D printing method (see **Table 3**) [36, 96]. Fine thermoplastic polymers in the form of filaments or granules are cast and extruded through a nozzle that allows

Need for heating in the molding process ➔ degradation of polymer materials

**168**

**Table 3.**

Speed Low cost Simplicity Flexibility

**Figure 1.**

**Fused filament fabrication Advantages Disadvantages**

*Fused Filament Fabrication (FFF) process.*

Poor surface quality

*Advantages and disadvantages of the fused filament fabrication process.*

It is necessary that the scaffolds in bone regeneration be biocompatible, biodegradable, osteoinductive (raising and cell maturation), and osteoconductive (provide a platform for cell growth) [39]. Scaffolds for bone regeneration should meet several specific criteria, such as filling any bone defect, ensuring pore interconnectivity, and having a pore architecture in order to promote bone formation and facilitate the exchange of oxygen bone growth [101–103]. The design of the scaffold can influence both the mechanical properties and cellular behavior [100, 104, 105] as highlighted in **Figure 2**.

A satisfactory bone growth leads to certain requirements. Porosity should be above 50% and pore size between 50 and 400 μm. It is difficult to achieve a "perfect" scaffold for bone regeneration due to pore design and size and a porosity distribution that mimics the native tissue [107, 108]. In the literature, there are no quantitative criteria that specify porosity or pore size or topology for bone regeneration. Porous scaffolds ranging in size from 50 to 500 μm are known to promote cell migration and vascularization, while micropores and nanopores control interaction with proteins and ion exchange with extracellular fluids [19, 109].

#### **Figure 2.**

*Scaffold requirements in terms of response (left) and what should be taken into account (right) (adapted from [106]).*
