Band structures of passive films on titanium in simulated bioliquids determined by photoelectrochemical response: principle governing the biocompatibility

The band structures and band gap energies, Eg, of passive films formed on titanium (Ti) in simulated bioliquids, Hanks’ solution (Hanks) and saline, were evaluated. Ti was polarized at 0, −0.1, and −0.2 VAg/AgCl, Ef, for 1 h. After polarization, the surfaces were characterized using X-ray photoelectron spectroscopy, and the photoelectrochemical responses were evaluated. The current change during photoirradiation was recorded as a photocurrent transient at each measuring potential, Em, and by changing the wavelength of the incident light. Passive films consisted of a very thin TiO2 layer containing small amounts of Ti2O3 and TiO, hydroxyl groups, and water. During polarization in Hanks, calcium and phosphate ions were incorporated or formed calcium phosphate but not in saline. Calcium phosphate and hydroxyl groups influenced the band structure. Eg was graded in Hanks but constant in saline, independent of Ef and Em. The passive film on Ti behaved as an n-type semiconductor containing two layers: an inner oxide layer with a large Eg and an outer hydroxide layer with a small Eg. In Hanks, Eg was 3.3–3.4 eV in the inner oxide layer and 2.9 eV in the outer hydroxide layer. In saline, Eg was 3.3 eV in the inner layer and 2.7 eV in the outer layer. Calcium phosphate and hydroxyl groups influenced the band structure of the passive film. The Eg of the outermost surface was smaller than that of TiO2 ceramics, which is probably one of the principles of the excellent biocompatibility of Ti among metals

Excellent mechanical and corrosion properties of austenitic stainless steel with a unique crystallographic lamellar microstructure via selective laser melting

We first developed a unique “crystallographic lamellar microstructure” (CLM), in which two differently oriented grains appear alternately, in a 316L stainless steel specimen via selective laser melting technology. The CLM was composed of major 〈011〉 grains and minor 〈001〉 grains aligned along the build direction, which stemmed from vertical and approximately ±45° inclined columnar cells formed in the central and side parts of melt-pools, respectively. The development of CLM was found to largely improve the material properties via the strengthening of the product, simultaneously showing superior corrosion resistance to commercially obtained specimens.Sun S., Ishimoto T., Hagihara K., et al. Excellent mechanical and corrosion properties of austenitic stainless steel with a unique crystallographic lamellar microstructure via selective laser melting. Scripta Materialia, 159, 89. https://doi.org/10.1016/j.scriptamat.2018.09.017

Trabecular health of vertebrae based on anisotropy in trabecular architecture and collagen/apatite micro-arrangement after implantation of intervertebral fusion cages in the sheep spine

Healthy trabecular bone shows highly anisotropic trabecular architecture and the preferential orientation of collagen and apatite inside a trabecula, both of which are predominantly directed along the cephalocaudal axis. This makes trabecular bone stiff in the principally loaded direction (cephalocaudal axis). However, changes in these anisotropic trabecular characteristics after the insertion of implant devices remain unclear. We defined the trabecular architectural anisotropy and the preferential orientation of collagen and apatite as parameters of trabecular bone health. In the present study, we analyzed these parameters after the implantation of two types of intervertebral fusion cages, open and closed box-type cages, into sheep spines for 2 and 4 months. Alteration and evolution of trabecular health around and inside the cages depended on the cage type and implantation duration. At the boundary region, the values of trabecular architectural anisotropy and apatite orientation for the closed-type cages were similar to those for isotropic conditions. In contrast, significantly larger anisotropy was found for open-type cages, indicating that the open-type cage tended to maintain trabecular anisotropy. Inside the open-type cage, trabecular architectural anisotropy and apatite orientation significantly increased with time after implantation. Assessing trabecular anisotropy might be useful for the evaluation of trabecular health and the validation and refinement of implant designs.Ishimoto T., Yamada K., Takahashi H., et al. Trabecular health of vertebrae based on anisotropy in trabecular architecture and collagen/apatite micro-arrangement after implantation of intervertebral fusion cages in the sheep spine. Bone, 108, 25. https://doi.org/10.1016/j.bone.2017.12.012

Fatigue Property and Cytocompatibility of a Biomedical Co–Cr–Mo Alloy Subjected to a High Pressure Torsion and a Subsequent Short Time Annealing

In the present study, we evaluated the effects of high pressure torsion (HPT) and subsequent short time annealing processing on fatigue properties and cytocompatibility of the biomedical Co–Cr–Mo alloy (CCM). Before processing, CCM was solution treated (CCMST) to achieve a microstructure composed of coarse single γ-phase equiaxed grains with no internal strain. Through HPT processing, an inhomogeneous microstructure containing both micro- and nano-scaled grains is obtained in CCM specimens, which were named as CCMHPT, accompanied by high internal strain and extensive ε martensite. Following a subsequent short time annealing, a uniform single γ-phase ultrafine-grained microstructure with small local strain fields dispersed forms in CCM specimens, which were named as CCMHPTA. This microstructure change improves fatigue strength in CCMHPT, and further in CCMHPTA, because of the enhanced crack initiation and/or propagation resistance. For cytocompatibility evaluation, the cells cultured on CCMST show an immobilization tendency, while those cultured on CCMHPT exhibit a locomotion tendency. The cells cultured on CCMHPTA have an intermediate pattern. Compared with CCMST, much larger numbers of cells are proliferated in both CCMHPT and CCMHPTA. All these results demonstrate that the CCMHPTA offers an improved fatigue property and a good cytocompatibility. Therefore, it is promising for use in biomedical applications

Crystallographic texture- and grain boundary density-independent improvement of corrosion resistance in austenitic 316L stainless steel fabricated via laser powder bed fusion

Improvement of corrosion resistance of austenitic 316L stainless steel via laser powder bed fusion (LPBF) is currently a prominent research topic; however, the effects of crystallographic texture and the related grain boundary density on the corrosion resistance of LPBF-fabricated parts have not been elucidated. For biomedical applications, crystallographic texture control from a single crystalline-like to randomly oriented polycrystalline microstructure is highly attractive for optimizing the mechanical properties (particularly the Young's modulus) of implants. An investigation of the impacts of crystallographic planes and grain boundaries exposed to the biological environment on corrosion behavior is necessary. 316L stainless steels with different crystallographic textures and grain boundary densities were successfully fabricated via LPBF. The corrosion resistances of the LPBF-fabricated specimens were comprehensively assessed by anodic polarization, dissolution, and crevice corrosion repassivation tests. The LPBF-fabricated specimens showed extremely high pitting potentials in the physiological saline compared with the commercially available counterparts, and importantly, excellent pitting corrosion resistance was observed irrespective of the crystallographic planes and grain boundary density exposed. Moreover, the LPBF-fabricated specimens did not show metastable pitting corrosion even in an accelerated test using an acid solution. The repassivation behavior of the specimens was not affected by LPBF. Such a drastic improvement in the corrosion resistances of the LPBF-fabricated specimens might be attributed to suppression of inclusion coarsening owing to the rapid cooling rate during solidification in LPBF. By using LPBF, the desired crystallographic texture can be introduced based on the desired mechanical properties without concern for corrosiveness.Tsutsumi Y., Ishimoto T., Oishi T., et al. Crystallographic texture- and grain boundary density-independent improvement of corrosion resistance in austenitic 316L stainless steel fabricated via laser powder bed fusion. Additive Manufacturing, 45, 102066. https://doi.org/10.1016/j.addma.2021.102066

Outstanding in vivo mechanical integrity of additively manufactured spinal cages with a novel “honeycomb tree structure” design via guiding bone matrix orientation

BACKGROUND CONTEXT: Therapeutic devices for spinal disorders, such as spinal fusion cages, must be able to facilitate the maintenance and rapid recovery of spinal function. Therefore, it would be advantageous that future spinal fusion cages facilitate rapid recovery of spinal function without secondary surgery to harvest autologous bone. PURPOSE: This study investigated a novel spinal cage configuration that achieves in vivo mechanical integrity as a devise/bone complex by inducing bone that mimicked the sound trabecular bone, hierarchically and anisotropically structured trabeculae strengthened with a preferentially oriented extracellular matrix. STUDY DESIGN/SETTINGS: In vivo animal study. METHODS: A cage possessing an anisotropic through-pore with a grooved substrate, that we termed “honeycomb tree structure,” was designed for guiding bone matrix orientation; it was manufactured using a laser beam powder bed fusion method through an additive manufacturing processes. The newly designed cages were implanted into sheep vertebral bodies for 8 and 16 weeks. An autologous bone was not installed in the newly designed cage. A pull-out test was performed to evaluate the mechanical integrity of the cage/bone interface. Additionally, the preferential orientation of bone matrix consisting of collagen and apatite was determined. RESULTS: The cage/host bone interface strength assessed by the maximum pull-out load for the novel cage without an autologous bone graft (3360±411 N) was significantly higher than that for the conventional cage using autologous bone (903±188 N) after only 8 weeks post-implantation. CONCLUSIONS: These results highlight the potential of this novel cage to achieve functional fusion between the cage and host bone. Our study provides insight into the design of highly functional spinal devices based on the anisotropic nature of bone. CLINICAL SIGNIFICANCE: The sheep spine is similar to the human spine in its stress condition and trabecular bone architecture and is widely recognized as a useful model for the human spine. The present design may be useful as a new spinal device for humans.Ishimoto T., Kobayashi Y., Takahata M., et al. Outstanding in vivo mechanical integrity of additively manufactured spinal cages with a novel “honeycomb tree structure” design via guiding bone matrix orientation. Spine Journal, 22, 10, 1742. https://doi.org/10.1016/j.spinee.2022.05.006

Innovative design of bone quality-targeted intervertebral spacer: accelerated functional fusion guiding oriented collagen and apatite microstructure without autologous bone graft

BACKGROUND CONTEXT: Although autologous bone grafting is widely considered as an ideal source for interbody fusion, it still carries a risk of nonunion. The influence of the intervertebral device should not be overlooked. Requirements for artificial spinal devices are to join the vertebrae together and recover the original function of the spine rapidly. Ordered mineralization of apatite crystals on collagen accelerates bone functionalization during the healing process. Particularly, the stable spinal function requires the ingrowth of an ordered collagen and apatite matrix which mimics the intact intervertebral microstructure. This collagen and apatite ordering is imperative for functional bone regeneration, which has not been achieved using classical autologous grafting. PURPOSE: We developed an intervertebral body device to achieve high stability between the host bone and synthesized bone by controlling the ordered collagen and apatite microstructure. STUDY DESIGN: This was an in vivo animal study. METHODS: Intervertebral spacers with a through-pore grooved surface structure, referred to as a honeycomb tree structure, were produced using metal 3D printing. These spacers were implanted into normal sheep at the L2–L3 or L4–L5 disc levels. As a control group, grafting autologous bone was embedded. The mechanical integrity of the spacer/bone interface was evaluated through push-out tests. RESULTS: The spacer with honeycomb tree structure induced anisotropic trabecular bone growth with textured collagen and apatite orientation in the through-pore and groove directions. The push-out load of the spacer was significantly higher than that of the conventional autologous graft spacer. Moreover, the load was significantly correlated with the anisotropic texture of the newly formed bone matrix. CONCLUSIONS: The developed intervertebral spacer guided the regenerated bone matrix orientation of collagen and apatite, resulting in greater strength at the spacer/host bone interface than that obtained using a conventional gold-standard autologous bone graft. CLINICAL SIGNIFICANCE: Our results provide a foundation for designing future spacers for interbody fusion in human.Matsugaki A., Ito M., Kobayashi Y., et al. Innovative design of bone quality-targeted intervertebral spacer: accelerated functional fusion guiding oriented collagen and apatite microstructure without autologous bone graft. Spine Journal 23, 609 (2023); https://doi.org/10.1016/j.spinee.2022.12.011