**3. Most promising techniques to fabricate Ti-Mg alloys**

Ti alloys can be fabricated using as alloying element Mg to create porosity, leading to formation of biocompatible scaffolds with lower elastic modulus by metallurgical and additive manufacturing process. Thus, the stiffness of Mg-based implants can be more easily tailored to match that of bone, which reduces the risk of stress shielding, a phenomenon that will be discussed in the next section.

## **3.1 Fabrication via powder metallurgy**

One of the most conventional techniques to create porous metallic materials is by the powder metallurgy. Conventional manufacturing of Ti alloys via powder metallurgy involves: (1) blending of the powder to achieve a uniform particle size distribution and, if needed, mixing of the Ti powder with the required alloying elements; (2) shaping of the powder blend (this can be achieved via different manufacturing methods where the simplest and cheapest is cold uniaxial pressing); and (3) solid-state sintering (i.e. heat treatment at high temperature, below the melting point of Ti) generally performed under vacuum [46]. This technique is cost effective, allows for better control of powder size and introduction of desirable pores. When it comes to pore size and shape, these are related to the size of the starting powder, its shape, its size and the shape of the spacer used to promote porosity. Among all of them, Mg is a good candidate in the manufacture of Ti scaffolds, as its solubility in Ti is low. Furthermore, Mg2+ increases osteoconductivity [47] and does not present any type of biomedical inconvenience, such as toxicity.

The main works carried out using Mg are for Ti-Al-V alloys [48]. Wen et al., (2001) reported that Mg foam prepared with a porosity of 50% showed a compressive strength of 2.33 MPa, with Young's modulus of 0.35 MPa, respectively [49]. Zhuang et al. [50] also evaluated the mechanical properties of porous Mg manufactured by powder metallurgy and the scaffolds with porosity from 36 to 55% showed a Young's modulus value in the range of 3.6 to 18.1 GPa, closer to that of natural bone [50]. They also investigated the effect of porosity on biodegradation. In their study, it was reported that materials with greater porosity degraded more quickly, due to greater interconnectivity and surface exposure, conditions that maximize chemical reactions. Although Mg materials have a low elastic modulus, mechanical resistance and corrosion are limiting factors for their use [51]. However, the use of Mg together with
