## 1 Introduction  
Nowadays, most of the research efforts are focused on the development of metallic biomaterials for bone replacement. Among all biomaterials, it is widely known that titanium and its alloys are the candidates with the best in vitro and in vivo behaviour. However, the stress-shielding phenomenon remains a concern in their use for biomedical applications. The stress shielding is associated with the mismatch between the Young's modulus of bone tissue and titanium (cortical bone around 20-25 GPa and titanium 110 GPa) [1], which causes bone resorption and eventual fracture of the host cortical bone surrounding the implants [2, 3]. The design and manufacturing of implants with lower stiffness materials could be a solution for this problem [4] and several works conducted aiming to develop scaffolds with a suitable balance between mechanical and biofunctional behaviour [5; 6; 7]. Currently, there are different methods to reduce the stress shielding, such as: (i) Polymer matrix compounds. An example of this kind of biomaterial is the HAPEX(r), composed by 40% hydroxyapatite and 60% HDPE (High Density Polyethylene), although it is not used for load-bearing applications due to their limited mechanical properties [8]. (ii) Metastable \(\beta\)-titanium alloys. These materials have lower moduli (55-90 GPa) and have been in development since the 1990s. Even the low moduli monolithic Ti alloys are significantly stiffer than bone [9]. Their medical use is also conditioned for their low wear resistance and limited strength. Important advances in new Ti-Nb-Ta-Mn alloys represent a promising way to solve problems related to stress shielding but still the reduced strength of the samples is considered a concern [10]. (iii) Porous materials. Up to 34 processing routes to fabricate porous materials have been reported [11; 12; 13]. The objective of many of these methods is the manufacturing of titanium foams [14; 15; 16; 17; 18; 19; 20], in which the porosity percentage must be controlled with the aim to reduce the implant stiffness without any undesirable influence on the mechanical properties [21]. In this context, the limitations in controlling the quantity, size, distribution and morphology of the pores by conventional routes should be considered. Furthermore, the high cost, and the great difficulty in obtaining reproducibility and versatility of the new processing routes (laser sintering, ion beam milling, field-assisted sintering technology (FAST), etc.) should be also contemplated. On the contrary, both the powder-metallurgy processing and the space-holder technique provide a suitable route to obtain Ti porous structures [21]; they also have a remarkable advantage because they are an economical and non-toxic methods, without a toxic agent that can affect cellular functions. From a powder-technology point of view, porous titanium could be produced by several methods [17; 18; 19; 20; 21; 22; 23; 24; 25]. The performance of porous titanium via conventional Powder Metallurgy (PM) offers flexibility and it is also a cost-effective alternative. Among the different techniques, there are interesting manufacturing processes where low compaction pressures are employed because of higher porosities and a lower Young's modulus can be obtained. Loose sintering (LS) is an attractive method to produce porous specimens. In this process, compaction pressure is not applied. In this way, specimens produced by this technique have higher porosity than specimens fabricated via conventional PM. Despite that, there are not so many works where the porous titanium could be produced at low compaction pressure [1]. In order to solve the limitations of conventional PM, space holder techniques help to control porosity parameters such as pore morphology and percentage [1; 15; 26; 27; 28; 29; 30; 31]. In addition, the effective material response can be determined experimentally or numerically.