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Bone resorption and possible fracture of host tissue are some consequences resulting from the mismatch between the Young's Modulus of the constituent materials of implants and bone that compromises the reliability of implants for replacing damaged bone tissue. The use of functional graded porous materials presents an interesting approach that could help decrease the Young's modulus while simultaneously mimicking highly hierarchical porosity of the bone structure. However, these structures are more difficult to fabricate than those with homogenous porosity. The design and distribution of this porosity in the implant must ensure the biomechanical and biofunctional balance of the bone tissue it is intended to replace. In this study, Ti radially graded structures were successfully fabricated using Spark Plasma Sintering combined with Space Holder Technique. The effects of temperature on porosity and mechanical properties were thoroughly examined. The results indicated that this processing route allows to achieve good control of porosity, close to the amount of added spacer. Yield stress of 181 MPa and an elastic modulus of 56 GPa were obtained for samples sintered at 800 °C for 5 min under a pressure of 6.3 MPa. These mechanical properties make the structure a viable candidate for replacing human long bones.

Addressing large bone segmental defects remains a profound challenge in orthopaedic surgery, demanding graft designs that mirror patient-specific defects while ensuring vascularization, osteoconduction, and structural integrity. Conventional approaches fall short in achieving this level of customization and functionality. This work leverages fuse filament fabrication (FFF) to fabricate innovative yttria stabilize zirconia graft designs that combine several strategies to tackle these multifaceted challenges. Graft designs ingeniously incorporate dense and porous structures mimicking the intricate architecture of natural bone for improved structural integrity and different ridges and protrusions to enhance the fixation. Additionally, these grafts undergo infiltration with PCL/PVA/AgNPs composite polymers, enriching biocompatibility, structural integrity, and antibacterial properties, fostering conducive conditions for optimal bone growth. Attaining printable filaments with YSZ powders below 50 nm and loads reaching 50 vol% marks a pioneering advancement in bone graft fabrication via Fused Filament Fabrication (FFF). This enabled high densification and low grain size at reduced sintering temperatures, imperative to achieve high mechanical performance. The biodegradation capacity of biopolymers in PBS and their role in the mechanical behaviour of grafts have been assessed, demonstrating the beneficial potential of the multicomponent graft proposed reaching strength values of 90–105 MPa.

The biomechanical performance of dental implants determines their clinical success. In this work, the stress and deformation distribution of a new implant design (dense and porous) was evaluated by the Finite Element Method. Furthermore, the effect of the location of the dental implant in the maxillary zone, as well as the mechanical response in the peri-implant maxillary tissue (cortical and trabecular) is discussed in detail. Before carrying out the computational study of the dental implant, Ti6Al4V cylindrical preforms obtained by conventional powder metallurgy and space-holder technique were characterized, to choose the most appropriate porosity (percentage and size) to achieve the biomechanical and biofunctional balance of the dental implant investigated. The novel porous dental implant under investigation exhibits 40% porosity in the region in contact with the trabecular bone, featuring inclined pores at 60 and 120° with a diameter of 200 μm. Our findings revealed that the cortical bone experienced the highest stress values, whereas the trabecular bone exhibited the highest levels of strain. Notably, the location of dental implants in the maxilla highlighted as the most influential factor affecting the maximum values of von Mises equivalent stress and strain. Furthermore, in the second molar location, the stress and strain levels exceeded the recommended thresholds for maintaining peri-implant bone density. Additionally, the porous implants generated significantly higher levels of stress and strain in the peri-implant trabecular bone than dense implants.

This research work proposes a synergistic approach to improve implants’ performance through the use of porous Ti substrates to reduce the mismatch between Young’s modulus of Ti (around 110 GPa) and the cortical bone (20–25 GPa), and the application of a biodegradable, acrylic acid-based polymeric coating to reduce bacterial adhesion and proliferation, and to enhance osseointegration. First, porous commercially pure Ti substrates with different porosities and pore size distributions were fabricated by using space-holder techniques to obtain substrates with improved tribomechanical behavior. On the other hand, a new diacrylate cross-linker containing a reduction-sensitive disulfide bond was synthesized to prepare biodegradable poly(acrylic acid)-based hydrogels with 1, 2, and 4% cross-linker. Finally, after the required characterization, both strategies were implemented, and the combination of 4% cross-linked poly(acrylic acid)-based hydrogel infiltrated in 30 vol % porosity, 100–200 μm average pore size, was revealed as an outstanding choice for enhancing implant performance.

The utilization of porous biomedical implants featuring a bimodal microstructure has garnered substantial interest within the scientific community. This study delves into the intricate interplay between processing parameters, microstructural attributes, and the tribo-mechanical performance of titanium grade 4, showcasing its potential to serve as implants to address compromised cortical bone tissue. The investigation meticulously examines the impact of milling duration (10 and 20 h), proportion of milled powder (50 and 75 wt%), and the volume fraction of space-holding agents (40–60 vol% NaCl) on the resulting characteristics of the bimodal microstructure, which plays a crucial role in achieving optimal biomechanical equilibrium. The Vickers microhardness, conventional and instrumented (P-h curves), and the wear behavior (ball-on disk) are discussed in terms of bimodal microstructure distribution, particle size and porosity level inherent to the fabrication conditions (mechanical milling + space-holder + hot-pressing). In general terms, milling time and milled powder fraction were the most influent parameters on the final properties of the materials. With the processing route used, the achieved microhardness values and wear behavior are comparable with those obtained by means of surface modifications or alloys. The Young's moduli obtained were in the range of 30–50 GPa, which could help to reduce the shielding phenomenon, while presenting a good mechanical resistance and wear behavior. In light of these findings, the fabricated specimen, composed of 75 wt% milled powder subjected to a 10-h milling duration, supplemented by a 60 vol% fraction of NaCl, emerges as a prime candidate manifesting superior biomechanical equilibrium. This judicious configuration exhibits a promising trajectory for its application in bone replacement endeavors.

The use of β-Titanium alloys to fabricate metal implants with Young's modulus that resembles bone tissues is presented as an alternative to commercially pure titanium or α-Titanium alloys, although it is still necessary to introduce proper implant porosity to reach the Young's modulus of cortical bones. In this work, porous samples were fabricated by loose sintering (0 MPa) and compared to samples manufactured at 1000 MPa, both sintered under the same conditions. Raw powders and sintered samples of the β-Titanium alloy, Ti35Nb7Zr5Ta, were characterized in detail in terms of both physicochemical and microstructural properties. Moreover, the tribo-mechanical behavior of sintered samples was evaluated by performing ultrasound technique, instrumented micro-indentation (P-hcurves), and scratch tests. The bio-functional behavior was studied by impedance spectroscopy and contact angle measurements. The results allowed the evaluation of the limits of conventional powder metallurgy (percentage of porosity, size, and morphology of pores), as well as the influence of the porosity and chemical composition to achieve a better biomechanical and bio-functional behavior that would guarantee bone requirements. The Ti35Nb7Zr5Ta alloy showed relatively high electrical impedance values compared to commercially pure titanium, indicating an improved bio-corrosion behavior. Furthermore, wettability measurements indicated that porous disks fabricated by loose sintering exhibit higher hydrophilicity, often associated with a better antibacterial response.

The porous substrates of commercially pure titanium have been coated with a novel bilayer of bioactive glasses (BGs), 45S5 and 1393, to improve the osseointegration and solve the stress-shielding phenomenon of titanium partial implants. The porosity of the substrates and the scratch resistance and bioactivity of the coating have been evaluated. Results are discussed in terms of stiffness and yield strength of the substrates, as well as the chemical composition, thickness, and design of the bioglass coating (monolithic vs bilayer). The role of the pores was a crucial issue in the anchoring of the coating, both in porosity percentage (30 and 60 vol %) and in pore range size (100–200 and 355–500 μm). The study was focused on the adhesion and infiltration of a 1393 bioglass layer (in contact with a porous titanium substrate), in combination with the biofunctionality of the 45S5 bioglass layer (surrounded by the host bone tissue), as 1393 bioglass enhances the adherence, while 45S5 bioglass promotes higher bioactivity. This bioactivity of the raw powder was initially estimated by nuclear magnetic resonance, through the evaluation of the chemical environments, and confirmed by the formation of hydroxyapatite when immersed in a simulated body fluid. The results revealed that the substrate with 30 vol % of porosity and a range of 355–500 μm pore size, coated with this novel BG bilayer, presented the best combination in terms of mechanical and biofunctional properties.

Titanium and its alloys are considered interesting materials for endosseous implants. However, they still present drawbacks related to their in-vivo behavior that can be overcome by coatings, such as apatite. This work focuses on the deposition of apatite coatings on commercially pure titanium (grade II) substrates previously pre-calcified. The influence of the temperature used in the thermal treatment on the microstructure and tribo-mechanical surface properties was analyzed. The coatings were structurally and chemically characterized and their tribo-mechanical behavior was evaluated. The nano-apatite coatings were only formed on surfaces with successive treatments in NaOH and CaCl2solutions. In addition, scratch tests showed that after the heat treatment the nano-apatite coatings had high bond strength to the substrate.

ABSTRACTIrradiated fuel pellets present radial gradient porosity. CeO2has been proven as a surrogate material to understand irradiated mixed oxide (MOX) due to its similar structural and mechanical properties. A novel compaction device was developed to produce CeO2cylindrical pellets with controlled radial porosity. Three blends of CeO2with different binder contents (0.5, 3 and 7.5 vol.% of ethylene-bis-stearamide, EBS) were prepared and used to obtain three different porosities for the core, intermediate and outer rings of pellets, respectively. Different compaction pressures were employed in each region to get the intended porosities. The whole pellet was subjected to a heating rate up to 500 °C to remove the EBS binder. Finally, a pressureless sintering step was performed at 1700 °C for 4 h. A microstructural characterization was performed in the three areas, including grain size and porosity. Mechanical properties like hardness, fracture toughness and tribo-mechanical response, as scratch resistance, were also determined. Pellets fabricated from this device have shown microstructural and mechanical properties with a good correlation to those of irradiated nuclear fuel.

Commercially pure titanium (cp Ti) is typically accepted as one of the best in vitro and in vivo bone replacement biomaterial, due to its excellent balance between biomechanical and biofunctional properties. In that context, the aim of this work is to prove the hypothesis of a simultaneous solution to certain specific limitations of cpTi, which can often compromise the reliability of implants: (i) stress-shielding phenomenon, and (ii) a deficient biointerface with bone, which reduces the osseointegration. Porous samples of cp Ti, grade IV, were obtained by space-holder technique (50 vol.% NH4HCO3, 800 MPa, at 1250 oC during 2h, under high vacuum), to produce a good balance between Young ́s Modulus and yield strength. Different types of porous samples were manufactured by considering different size particles ranges of NH4HCO3: 100-200μm, 250-355μm and 355-500μm. Afterwards, they were coated with a PEEK/45S5 bioactive glass composite by electrophoretic deposition, to be finally sintered at 350oC for 1h. The coatings homogeneity, infiltration efficiency, adhesion and cracking, were studied in order to establish correlations with processing conditions (time of deposition, applied voltage, composition, concentration and stability of the colloidal suspension). Detailed structural characterization of the coatings was performed (SEM and XRD), besides the contact angle and contact profilometry testing. Additional mechanical and chemical insights were achieved by evaluating both the tribo-mechanical (instrumented microindentation and micro-scratch testing) and electrochemical behaviors (potentiodynamic polarization and in vitro corrosion tests in SBF). All these results allowed us to determine the optimal balance of properties for a porous substrate (space holder of 250-355μm) with a coating obtained for 65 V, 2 min, 6 mm (distance between electrodes), 10 g/L bioactive glass and 20 g/l PEEK. The high adhesion estimated between the bioactive/biopolymer coatings and the porous titanium substrates (excellent infiltration) suggest that this new biocomposite is a good candidate for load-bearing applications.

The manufacture of graded materials has gained an enormous interest during the last decade due to the diversity of industrial and biological materials systems that require or are actually designed to implement that criterion; those natural or artificial materials offer multiple possibilities of applications. In this work, a novel uniaxial and sequential compaction device has been successfully designed and fabricated, in order to obtain samples with three different layers; this new device is suitable for both conventional and non-conventional powder metallurgy (PM) techniques. In addition, this device allowed us to use different combinations of powders and space-holder particles, irrespective of their nature, sizes, morphologies and proportions. It has no restriction about applying different compaction pressures for every layer, which may result in increasing or decreasing porosity. This compaction device is especially powerful if the aim is obtaining samples with radial graded porosity for biomedical applications (replacement of cortical bone involved in different joints and dental restorations) and nuclear applications (mimicking burnt used nuclear fuel). Specifically in this work, different samples with radial graded porosity were fabricated and then microstructurally and mechanically characterized: i) Commercially pure titanium (CP Ti) samples, starting from blends CP Ti with 20 vol.%, 40 vol.% and 60 vol.% of Sodium Chloride (NaCl) as space holder, which were placed in core, intermediate and external layers, respectively; processing conditions were 800 MPa of compaction pressure and 1250 °C for 2h in high vacuum of sintering; and ii) CeO2 samples, starting from blends CeO2 with 0.5 vol.%, 3.0 vol.% and 7.5 vol.% of Ethylene Bis Stearamide (EBS) as space holder, which were placed in core, intermediate and external layers, respectively; processing conditions were 460 MPa in external layer and 700 MPa in core and intermediate layers of compaction pressure, and 1700 °C during 4h in static air of sintering. This new device has proved to have unique advantages for solving problems of structural integrity in conventional PM manufacturing in a simple, economic and reproducible way.

Titanium and its alloys are well known as one of the best in-vitro and in-vivo bone replacement metallic biomaterial due to its excellent balance between biomechanical and biofunctional properties. The selective laser melting (SLM) method has a lower cost and shorter manufacturing time than the conventional routes used in the fabrication of titanium alloys. In this work, Ti6Al4V sheets were manufactured by SLM (LM samples) and subsequently annealed for stress relief at 750 oC for 10 min (LM-A samples). SEM, XRD and contact profilometry measurements were carried out to characterize the elemental composition, phases and surface morphology of different samples. A micro-tribo-mechanical evaluation was also performed by micro-indentation and scratch tests. The resulting surface was rough (Ra = 9.1 ± 0.5 μm) for all samples, showing protuberances with spherical morphology. For annealed samples, an oxide layer composed of rutile and Al2O3 was observed that increased the micro-hardness of the surface in LM-A sheets. However, after removing this oxide layer, the micro-hardness of the LM-A sheets was reduced when compared to LM samples as a result of the stress relief. A direct relationship between Vickers micro-hardness and scratch resistance was always observed. Therefore, LM-A sheets showed higher scratch resistance at low loads (oxidized surface effect) than LM samples, but lower resistance at high loads (bulk effect).

This work proposes an innovative strategy that combines two well-known easy and economical preparation techniques: powder metallurgy based space-holder (SH) technique to provide porous biocompatible Ti substrates with balanced biomechanical behaviour (for cortical bone tissue substitution) promoting bone ingrowth and biocoating infiltration, and electropolymerization to coat these substrates with the PPy-AgNPs composite conductive polymers improving its corrosion resistance, biocompatibility with enhanced antibacterial activity. The deposited PPy based coatings present a cauliflower like structure well adhered to the porous substrates. The macroporosity of and rough inner pore surface of Ti SH substrates are responsible for the superior adhesion of the conductive polymer comparing to typical denser substrates obtained by powder metallurgy or forging. The corrosion protection properties of the coatings were investigated by OCP and PA in PBS media to simulate possible implant conditions, revealing improved corrosion resistance for the composite coatings. The bioactivity of the coatings was evaluated by immersion tests, revealing the formation of Hydroxyapatite after 90 days immersion in PBS. In both PPy and PPy-AgNPs composite coatings, a displacement of the polarization curves to more noble potentials and a decrease in the current density, indicated that the coating’s protective character is maintained after 90 days immersion in PBS. The antibacterial activity was assessed by using the Kirby-Bauer disk-diffusion method against Staphylococcus aureus (ATCC 25923). The inhibition halo increased from 5.5±0.4mm for the bare substrate to 8.2±0.6mm for the PPy coated substrate and to 12.5±0.7mm for the PPy-AgNPs coated porous Ti. This feature associated to the improved corrosion protection and biocompatibility would significantly contribute to the success of the potential use of porous Ti implants by SH technique envisaging substitution of small damaged bone tissues for example in tumors.

In this research work, the fabrication of biphasic composite implants has been investigated. Porous, commercially available pure Ti (50 vol % porosity and pore distributions of 100–200, 250–355, and 355–500 μm) has been used as a cortical bone replacement, while different composites based on a polymer blend (gelatin and alginate) and bioactive glass (BG) 45S5 have been applied as a soft layer for cartilage tissues. The microstructure, degradation rates, biofunctionality, and wear behavior of the different composites were analyzed to find the best possible coating. Experiments demonstrated the best micromechanical balance for the substrate containing 200–355 μm size range distribution. In addition, although the coating prepared from alginate presented a lower mass loss, the composite containing 50% alginate and 50% gelatin showed a higher elastic recovery, which entails that this type of coating could replicate the functions of the soft tissue in areas of the joints. Therefore, results revealed that the combinations of porous commercially pure Ti and composites prepared from alginate/gelatin/45S5 BG are candidates for the fabrication of biphasic implants not only for the treatment of osteochondral defects but also potentially for any other diseases affecting simultaneously hard and soft tissues.