Mitsuharu Todai

Control of Oriented Extracellular Matrix Similar to Anisotropic Bone Microstructure

Bone microstructure is dominantly composed of anisotropic extracellular matrix (ECM) in which collagen fibers and epitaxially-oriented biological apatite (BAp) crystals are preferentially aligned depending on the bone anatomical position, resulting in exerting appropriate mechanical function. The regenerative bone in bony defects is however produced without the preferential alignment of collagen fibers and the c-axis of BAp crystals, and subsequently reproduced to recover toward intact alignment. Thus, it is necessary to produce the anisotropic bone-mimetic tissue for the quick recovery of original bone tissue and the related mechanical ability in the early stage of bone regeneration. Our group is focusing on the methodology for regulating the arrangement of bone cells, the following secretion of collagen and the self-assembled mineralization by oriented BAp crystallites. Cyclic stretching in vitro to bone cells, principal-stress loading in vivo on scaffolds, step formation by slip traces on Ti single crystal, surface modification by laser induced periodic surface structure (LIPSS), anisotropic collagen substrate with the different degree of orientation, etc. can dominate bone cell arrangement and lead to the construction of the oriented ECM similar to the bone tissue architecture. This suggests that stress/strain loading, surface topography and chemical anisotropy are useful to produce bone-like microstructure in order to promote the regeneration of anisotropic bone tissue and to understand the controlling parameters for anisotropic osteogenesis induction.

Effect of Phase Stability on Some Physical and Mechanical Properties in β-Ti Single Crystal for Biomedical Applications

The electron-atom ratio (e/a) dependence of the appearance of the lattice modulation and physical properties in β-phase Ti-xNb alloys (x = 28, 30, 34 and 40) were investigated by using some physical properties measurements, compressive test and transmission electron microscope observations (TEM observations), focusing on the β-phase stability. The microstructure, physical properties, deformation mode depend on the e/a ratio which is closely related to the β-phase stability in Ti-Nb alloys. The e/a ratio is defined by the average electrons per atom in free atom configuration. Athermal ω-phase is suppressed in Ti-30Nb alloy single crystal with low e/a ratio. The Ti-30Nb alloy single crystal also exhibits a lattice modulation and low Debye temperature. These results imply that the β-phase stability in β-phase Ti alloys decreases with decreasing the e/a ratio and are related to the softening of elastic stiffness, c′. Consequently, a decrease in the e/a ratio leads to the softening of c′ and a significant reduction in modulus along the [100] direction in β-phase Ti alloys single crystal. In fact, the Young’s modulus along [100] of the Ti-15Mo-5Zr-3Al alloy (wt.%) single crystal with low e/a ratio exhibits as low as 45 GPa, which is comparable to that the human cortical bone. That is, controlling the e/a ratio is an ultimate strategy to develop the future superior biocompatible implant materials with extremely low Young’s modulus and good deformability.

Martensitic transformation from incommensurate state with nano-scale domain structure in a Ti–42Ni–8Fe (at.%) alloy under a compressive stress

This article reports a martensitic transformation from an incommensurate state with a nano-scale domain-like structure in a Ti–42Ni–8Fe alloy. The martensitic transformation is suppressed at a temperature above 4.2 K under zero stress, but occurs under a compressive stress. The product phase is most likely to be the R-phase, but there is a large temperature hysteresis of about 87 K.

Unusual dynamic precipitation softening induced by dislocation glide in biomedical beta-titanium alloys

Softening of metallic materials containing precipitates during cyclic deformation occurs through dissolution of the precipitates, because the to-and-fro motion of the dislocation causes dissolution of the precipitate particles by cutting them. Here, however, we found the completely opposite phenomenon for the first time; a “dynamic precipitation softening” phenomenon. In a Ti-35Nb-10Ta-5Zr body-centered cubic structured β-Ti alloy single crystal developed for biomedical implant, the to-and-fro motion of the dislocation “induced” the selective precipitation of the ω-phase whose c-axis is parallel to the Burgers vector of the moving dislocation, which led to the significant cyclic softening of the crystal. The formation of the ω-phase is generally believed to induce significant hardening of β-Ti alloys. However, the present results suggest that this is not always true, and control of the anisotropic features of the ω-phase via control of crystal orientation can induce unusual mechanical properties in β-Ti alloys. The unique anisotropic mechanical properties obtained by the cyclic-deformation-induced oriented ω-phase formation could be useful for the development of “single-crystalline β-Ti implant materials” with advanced mechanical performance.

ω Phase Transformation and Mechanical Properties in Binary Zr-Nb Biomedical Alloy

The athermal ω phase transformation, magnetic susceptibility and deformation behavior of Zr-xNb alloys (x = 10 and 14) for use in medical devices subjected to magnetic resonance imaging (MRI) were investigated using electrical resistivity measurements, transmission electron microscopy observations and compression tests. The alloy with x = 10 exhibited a positive temperature coefficient in the electrical resistivity curve and the presence of an athermal ω phase at room temperature. On the other hand, the alloy with x = 14 showed an anomalous negative temperature coefficient (NTC) in the resistivity curve. Similar NTCs also appear in β-Ti alloys, which is interpreted as the growth of an athermal ω phase and the appearance of lattice modulation. The ω phase and diffuse satellites, which are possibly related to lattice modulation, were confirmed in the Zr-14Nb alloy at room temperature. The volume fraction of the athermal ω phase and the appearance of lattice modulation are related to the operating deformation mode and Young’s modulus. Thus, controlling the ω phase transformation in Zr-Nb alloys is key to developing medical devices that can be used in MRI.

Position of Incommensurate Satellites Appearing in Ti-Ni Based Shape Memory Alloys

We have investigated electron diffraction patterns of a Ti-44Ni-6Fe alloy exhibitng a negative temperature dependence in electrical resistivity below Tmin = 210 K. The electron diffraction patterns taken near Tmin show diffuse satellites at gB2 + * when the zone axis is [111] and [001]. For both the beam directions, the value ζ is slightly smaller than 1/3. On the other hand, the satellites are missing when the zone axis is [110]. This means that the incommensurate phase has a modulated structure with the propagation vector * (ζ~1/3) and the displacement direction is one of <110> which is vertical to the propagation vector. This modulation is obviously the consequence of the phonon softening of TA2-branch with the propagation vector near * (ζ~1/3). In addition to the satellite at gB2 + * (ζ~1/3), satellites appear at gB2+* with ζ = 1/2 when the zone axis is [001] and rod-like steaks appear in <112>* direction when the zone axis is [110]. However, these satellites and rod-like streaks do not show clear temperature dependence, suggesting they are not directly related to the phonon softening of TA2-branch.