**3.2 Fabrication via additive manufacturing**

In recent years, interest has increased in the application of additive manufacturing of Mg alloys for its biomedical application. This can allow the obtaining of complex shapes adapted to the patient, since it would be a personalized manufacture. However, and due to the physical properties of Mg and its alloys, the application of additive manufacturing by melting the alloy has not been easy, since the boiling temperature of magnesium is very low (~ 1091°C). Despite this, and due to the manifest interest in the biodegradation properties of Mg motivate its application as biodegradable implants [59], seeking the best combination between resistance to corrosion, wear, mechanical properties and biocompatibility [60].

Another factor influencing the scarce development of additive techniques in Mg alloys is the ease of obtaining coupling parts by injection casting processes or the extrusion capacity of these alloys despite the difficulty imposed by their hcp crystalline structure. In addition, the great reactivity with oxygen that Mg presents must be considered and also limits the application of rapid heating techniques that could cause the combustion of the metal. However, some of the technologies applied to other materials have been applied, by means of specialized teams that require work in protective atmospheres, which ensure the possibility of handling these alloys [61].

The potentially most interesting techniques for the manufacture of magnesium alloys are powder bed fusion (PBF), especially selective laser melting (SLM), widely used in the development of different Mg alloys [62, 63]. Powder bed fusion (PBF) is an AM process in which thermal energy is used to selectively fuse regions of a powder bed [64]. The powder bed contains metal, polymer, or ceramic powder as feedstock. An energy source directed towards the powder bed selectively scans and melts the top layer of the powder bed. The powder bed then lowers and a fresh layer of powder is spread over the melted layer. This process continues until the entire structure has been formed by stacking melted layers of powder.

Another way to use additive manufacturing related to magnesium alloys is to have a porous structure obtained by additive manufacturing as in the case of Perets et al. [65] who obtain the TI-6Al-4 V mesh by SLM and then infiltrate the Mg elemental into the holes. In this way they can obtain structures that do not collapse, although their resistance is not increased. The magnesium will present an accelerated corrosion by galvanic effect and depending on the size of the pores, an osseointegration will be available as it is a very biocompatible set.

Degradation capability of Mg gives a feature of bioactivity in bone formation that leading Balog et al. develop a bioactive metal system compound by structural material for dental implants, via extrusion from a powder mixture of Ti and Mg (4 and 12%) in low temperature [66]. Adding Mg, is possible to obtain a bioactive system and a decreasing of elastic modulus, further promote a good osteointegration due to the Mg resorption and the presence of pores where the bone ingrowth can be formed.

Mechanical properties in Ti parts that receive infiltration of Mg depends on the amount of Mg and matrix used. Studies published by Jiang et al. about infiltration of Mg in a scaffold of Ti-Mg (99,9%) was possible to control density from compaction pressure with volume fractions up to 60% Ti, which confers stiffness similar to those of cortical bone [67]. The use of non-degradable Ti matrices, as described in previous sections, is necessary as a non-degradable support due to its excellent biocompatibility, high resistance to corrosion and excellent mechanical properties. Similarly, efforts have been made to obtain porous Ti by means of spacer techniques [68] or by additive manufacturing processes such as selective laser melting (SLM) [69, 70]. Biodegradable Mg-based alloys are advantageous as fillers for bioactive implants because the release of Mg ions during corrosion *in vivo* is non-toxic and, furthermore, may have a beneficial effect on tissue regeneration and osteoblast response [71]. However, excessive in vivo corrosion of Mg can result in a premature loss of mechanical integrity and it is for this reason that a balance is struck between the structural integrity provided by titanium alloys and the degradation of Mg alloys. The aim is therefore to achieve synergy between both alloys and therefore to improve or control the biodegradation of magnesium through alloys that allow its corrosion rate to be controlled and can adapt it to the rate of bone growth, as proposed by Perets et al. with Mg-2.4% Nd-0.6% Y-0.3% Zr [65] alloy infiltration working in simulated environments.
