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Researchers Develop Flow-Casting Method for Bioactive Coating of 3D Printed Porous Titanium Implants

We often see titanium used in 3D printed implants for humans, and even for animals. When compared to implants that are more dense, porous titanium (PT) implants, with custom porosity and structure, have mechanical properties far more similar to natural bone, which helps reduce stress-shielding effects and facilitate nutrient transport and bone tissue ingrowth during regeneration.

But, metals based in titanium (Ti) are naturally bioinert, meaning they don’t initiate a response when introduced to biological tissue; this can lead to lowered bone-to-implant contact and possible rejection. So for bone tissue engineering, it’s important to modify the surface of metallic implants with coatings, whether it’s something like a diamond or bioactive glass (BG).

A collaborative group of researchers from Zhengzhou University and Northwestern Polytechnical University in China, recently published a paper, titled “A Facile Flow-Casting Production of Bioactive Glass Coatings on Porous Titanium for Bone Tissue Engineering,” about their work in developing a flow-casting method to rapidly coat PT with BG coatings.

Flow-casting production of homogeneous BG coatings on PT scaffolds.

The abstract reads, “Additive manufacturing enabled the fabrication of porous titanium (PT) with customized porosity and mechanical properties. However, functionalization of PT surfaces with bioactive coatings is being challenged due to sophisticated geometry and highly porous structure. In this study, a facile flow-casting technique was developed to produce homogeneous 45S5 bioactive glass (BG) coatings on the entire surface of PT. The coating weight as a function of BG concentration in a BG-PVA slurry was investigated to achieve controllable coating yield without blocking macropore structure. The annealing-treated BG coating not only exhibited compact adhesion confirmed by qualitative sonication treatment, but also enhanced the mechanical properties of PT scaffolds. Moreover, in-vitro assessments of BG-coated PT cultured with MC3T3-E1 cells was carried out having in mind their potential as bioactive bone implants. The experimental results in this study offer a simple and versatile approach for the bio-functionalization of PT and other porous biomedical devices.”

Fluorescent images of cells cultured on bare (a,c,e) and BG200‐coated (b,d,f) Ti scaffolds after 2 and 4 days.

Bioceramics, like TiO2 and BG, have been used before as coatings in order to, as the paper puts it, “functionalize bioactive character to inert bone scaffolds.” There have been several methods used in the past to produce bioceramic coatings on dense bone implants, but because porous ones have more complex structures, it’s hard to coat the entire surface without blocking the porous, interconnected structure.

The team used SLM technology to 3D print PT samples on a Renishaw AM 250, then rinsed them with demineralized water and ethanol to remove residual HF before letting them air-dry. Then, a sample was dipped in a slurry of BG-PVA, before being moved to a rotation platform. Using compressed nitrogen force during rotation, the slurry’s solid content was gradually but completed flow-casted onto the surface of the struts, which as the researchers explained in the paper resulted in “the formation of a homogeneous BG coating on the entire PT surfaces without blocking the macropore structure.”

“The key parameters of the flow-casting process were varied to obtain homogeneous BG coatings,” the researchers wrote. “In addition, microstructural, mechanical, and biological properties of BG-coated PT were characterized having in mind the promising potential of BG-functionalized PT in bone tissue engineering.”

(a) The weight of BG‐PVA coating as a function of BG concentration in slurry; the inset shows the increased white content on the coated PT with increasing BG concentration (* p < 0.05, ** p < 0.01, *** p < 0.001), (b) axial and radial observation of BG200‐coated PT after annealing, (c) XRD patterns of raw BG and annealed BG powder using the same annealing protocol.

While developing the facile flow-casting method, the team determined that by varying the concentration of BG, they could specifically tailor the coating yield in order produce homogeneous coatings on the PT substrates that wouldn’t block the interconnected macropores. Additionally, after an annealing treatment, the BG200-coated PT showed a strengthened elastic modulus, and also “exhibited excellent coating adhesion,” which are good signs for bone implants.

SEM images of the inner surface morphologies of (a-c) bare Ti scaffold and (d-f) BG200-coated Ti scaffold, (g) cross-sectional images of BG200 coatings at different magnifications.

“Although the annealed BG coating exhibited nonuniform thickness varying from 2 µm to 6 µm, in-vitro biological evaluation confirmed an improved osteoblast activity, probably owing to the bioactivity of the BG phase,” the researchers concluded. “Summarizing, flow-casting of BG coatings, possibly combined with other bioactive components or functional molecules, suggested a simple and effective approach for the biofunctionalization of PT or other porous devices, which will be advantageous for designing optimal scaffolds for biomedical applications.”

Co-authors of the paper are Haiou Yang, Qijie Zhu, Hongfei Qi, Xianhu Liu, Meixia Ma, and Qiang Chen.

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Custom 3D Printed CT-Bone Graft Implants Coming to Japan and Europe

We first heard of innovative CT-Bone technology three years ago, when Dutch company Xilloc reached an agreement with Tokyo-based Next21 K.K., the creator of CT-Bone, to bring 3D printable bone into hospitals in Europe. Back in 2001, Next 21 K.K. collaborated with the University of Tokyo and RIKEN on developmental research into the technology, which uses 3D printing to make synthetic bone grafts out of calcium-deficient HA material.

Now, after receiving an approval for manufacturing and marketing medical devices from the country’s Ministry of Health, Labor and Welfare (MHLW), the company is announcing formal approval for a new technology to 3D print synthetic bone grafts, which can both fuse and be assimilated into a patient’s existing bone.

There are currently four different types of existing bone grafts for patients with different kinds of bone defects and deformities: Autograft and Allograft (the most common), Synthetic Bone graft, and Xenograft. Custom synthetic graft materials are shaped from a heated and sintered block of material with machine tools, and is hard for natural bone tissue to absorb, which could lead to inflammation.

Autograft, which is the foremost transplant method in Japan, requires an additional surgery in order to remove a piece of bone from the patient’s leg or hip, so patients have to go through a second invasive procedure and deal with the potential risks, like damage and infection, from extended exposure. Allograft from a bone bank is the most common in the US and Europe, but as it’s harvested from cadavers, there are potential infectious and ethical conundrums to consider. Additionally, it can be hard to find a cadaver bone that’s the appropriate size and shape to match a patient’s original bone.

But, 3D printing makes it possible to reproduce the shape of the original bone with 0.1 mm accuracy, and CT-Bone also uses a curing treatment method to help with recrystallization. This the technology, as Next21 K.K. puts it, “most suitable for molding biomaterial like a bone graft.”

CT-Bone does not use a sintering process to increase mechanical strength like other synthetic bones or 3D printed ceramics do, so it actually becomes physiologically activated; this helps the material in the custom implant fuse and assimilate to the patient’s existing bone much more quickly.

While most typical bone implants are made from material like titanium or PEEK, or even cut and re-positioned bone from the patient, CT-bone is a 3D printable, calcium phosphate implant that’s actually converted into real bone by the patient’s own body.

After a CT-scan, Next21 K.K.’s biomedical engineers work with the surgeons to create a patient-specific implant (PSI), which can incorporate porosity and match the patient’s anatomy perfectly, which helps facilitate bony ingrowth and good bone-to-implant contact. It only takes a few months post-implantation for CT-Bone to unify with the patient’s existing bone.

Thanks to a subsidy from the New Energy and Industrial Technology Development Organization (NEDO), the company completed a pre-clinical study for CT-Bone, titled “Computed tomographic evaluation of novel custom-made artificial bones, “CT-bone”, applied for maxillofacial reconstruction” and performed with support from the National Institute of Biomedical Innovation, Health and Nutrition (NIBIOHN). Co-authors include Yuki Kanno from the University of Tokyo, Takashi Nakatsuka with Saitama Medical School, Hideto Saijo, Yuko Fujihara, and Hikita Atsuhiko from the university, Ung-il Chung with the university’s Graduate Schools of Engineering and Medicine, and Tsuyoshi Takato and Kazuto Hoshi with the university.

The abstract reads, “We fabricated custom-made artificial bones using three-dimensionally layered manufacturing (3D printing) process, and have applied them to patients with facial deformities. We termed this novel artificial bone the “CT-bone”. The aim of the present study was to evaluate the middle-and long-term safety and effectiveness of the CT-bones after transplantation.”

CT-Bone grafts were implanted into 23 sites on 20 patients with facial bone deformities and then evaluated through the use of CT scans post-op, minimally for one year and then maximally for seven years and three months after transplantation.

According to the paper, “No serious systemic events due to the CT-bone graft were found during the observation period (1 year postoperatively). In 4 sites of 4 patients, the CT-bones were removed due to local infection of the surgical wounds at 1-5 years postoperatively. Compatibility of the shapes between the CT-bone and the recipient bone was confirmed to be good during the operation in all of the 20 cases, implying that the CT-bones could be easily installed onto the recipient sites. During the CT evaluation (<7 years and 3 months), no apparent chronological change was seen in the shape of the CT-bones. Sufficient bone union was confirmed in 19 sites. The inner CT values of the CT-bones increased in all the sites. The longer the postoperative period, greater increases in the CT values of the CT-bones tended to be observed.”

Next21 K.K. plans to commercialize CT-bone in the Japanese market, and initiate export to other Asian countries. Having already reached a license agreement with Xilloc for local manufacturing and sales of CT-Bone in the EU, the company will also expand sales to Europe.

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[Images provided by Next21 K.K.]