A 3D-Printed Ultra-Low Young`s Modulus β-Ti Alloy for Biomedical Applications
Among the metal biomaterials used in biomedicine, especially orthopedics, titanium and its alloys have the most suitable characteristics compared with stainless steel and cobalt-chromium alloys because they have high biocompatibility, specific strength and corrosion resistance.
According to the phase composition of titanium alloys, they can be divided into three categories, namely α, β and α+β alloys. Essentially, the microstructure depends on the type and quantity of alloying elements, because isomorphic α-phase stabilizers (Zr, Al, Sn, O, and Si) are preferentially dissolved in the α phase, expanding its phase field, while isomorphic β The stabilizers (H, Mo, W, and V) dissolved in the β phase have the same effect in the β phase field. According to the degree of alloying and the thermomechanical processing path, the balance of α and β phases can be adjusted, so that the strength, toughness and fatigue resistance can be adjusted.
Researchers from the Department of Industrial Engineering of the University of Trento evaluated the potential of metastable β-Ti21S alloys as biomedical parts in the paper "A 3D-Printed Ultra-Low Young`s Modulus β-Ti Alloy for Biomedical Applications".
3D printed human implants. Source: Stryker
Ultra-Low Young`s Modulus
The powder bed-based selective laser melting metal 3D printing technology (L-PBF) can produce nearly completely dense (99.75±0.02%) samples. The material developed by the Department of Industrial Engineering of the University of Trento in Italy consists of only the metastable β phase, with columnar grains oriented along the direction of the building.
The material has Ultra-Low Young`s Modulus (52±0.3 GPa), which has never been reported for this alloy. It has good mechanical strength (σy0.2=709±6 MPa, ultimate tensile strength (UTS)=831±3 MPa) and high tensile strength (21%±1.2%). Without any heat treatment, it It is similar to forged alloys and comparable to heat-treated grade 5 Ti. The good biocompatibility proved by cytotoxicity test proves that it is very suitable for biomedical applications.
Reduce stress shielding effect
The paper pointed out that in addition to Ti-6Al-4V ELI (ASTM F 136), ASTM only standardized Ti-6Al-7Nb (ASTM F 1295) as a biological material. However, one of the main limitations of the α+β alloy is its relatively high Young's modulus E, which is between 110 GPa and 120 GPa. The stiffness mismatch between bone tissues will cause stress shielding effects.
Due to the possibility of using functionally graded porous metal to produce orthopedic implants, AM-additive manufacturing technology has attracted more and more attention. Through metal 3D printing, the goal is to mimic the complex structure of bones to increase the osseointegration of the implant. The main advantage of porous materials is to reduce the mismatch of elastic modulus between bone and implant alloy, reduce stress shielding effect and improve implant morphology, and provide biomaterial anchoring effect for tissue ingrowth.
No heat treatment required
Compared with α+βTi alloy, another advantage of β-titanium alloy related to AM-additive manufacturing is that it can suppress the martensitic transformation in β-Ti alloy. Indeed, the typical high cooling rate of the laser powder bed fusion (L-PBF) AM process causes the α+β alloy to be heat treated before removing the parts from the L-PBF construction platform to obtain fine needles or coarse Layered microstructure. Unfortunately, not all biomedical manufacturers are equipped with vacuum furnaces.
The Department of Industrial Engineering of Trento University uses pre-alloyed β-Ti21S alloy produced by plasma atomization (GKN Hoeganaes Corporation, Cinnaminson, NJ, USA, D10=25μm, D50=41μm, D90=60μm). The chemical composition (weight %) of the powder is 14.6% Mo, 2.8% Al, 2.8% Nb and 0.3% Si, 0.11% O and 0.004% N, with more than Ti.
High elongation at break
Through the metal 3D printer-MYSINT100 L-PBF equipment of Italy SISMA, the laser heat input is maintained between 40 J/mm3 and 90 J/mm3. An argon atmosphere is used, and the thickness of the treatment layer is set to 20 μm.
The final test results show that the commercial powder atomized with gas can successfully produce almost completely dense samples. Even if it solidifies quickly, a completely metastable β structure can be obtained. The Young's modulus obtained is the lowest among β-Ti alloys (52 GPa) reported in the literature, and its mechanical strength is slightly lower than Ti-6Al-4V, which is similar to other β-Ti alloys. High elongation at break indicates good strain adaptability and the possibility of limiting deformation. In addition, the results of biological experiments showed that no cytotoxicity was detected in the experimental and reference samples.
Stress shielding means that when two materials with different moduli are used together, the material with the larger modulus bears greater stress. Take the common implant manufacturing material titanium alloy as an example. When the material is transplanted into the body, because the modulus of the titanium alloy is much greater than that of the human bone, the titanium alloy bears more stress, which will not be conducive to the new Bone growth.
Although orthopedic implants are usually very successful, many patients still face major complications: implant-related infections and implant loosening. With many patients receiving implants for the first time after the age of 60, and the life expectancy of patients increases, this is a major problem for future implants.
The Delft University of Technology in the Netherlands has developed a new titanium implant. The researchers believe that these complications can be prevented by 3D printing porous titanium implants and surface modification. The team used reasonable design principles and additive manufacturing techniques to create orderly interconnections in medical grade titanium alloys. The porous microstructure is very suitable for bone growth.
Teacher Wei Qingsong of Huazhong University of Science and Technology in China has carried out active research on the modification of medical titanium alloy by laser selective melting in situ. It is expected to obtain low modulus and high strength titanium alloy suitable for human bone applications.