Basic Study of Autologous-Bone-Replaceable Artificial Bone Fabrication With Porosity Distribution Using Electrolysis

Recently, the use of bioresorbable materials (e.g., β-tricalcium phosphate (β-TCP)) has enabled the development of autologous-bone-replaceable artificial bones that are degraded and resorbed, i.e., replaced with autologous bone, when placed inside the human body for a sufficiently long duration. Although such autologous-bone replaceability requires high porosity of the artificial bone to promote the ingression of blood vessels and cells, the high porosity reduces the mechanical strength, which leads to disadvantages such as possible fracture after bone substitution surgery. One solution to this problem is to optimally arrange low-porosity portions for mechanical strength and high-porosity portions for autologous-bone replaceability in solid artificial bones. Commercially available artificial bones typically have fixed shapes such as a rectangular parallelepiped or cylinder. The use of recent solid freeform fabrication technologies, however, has enabled solid artificial bones with various shapes to be customized for individual medical cases. In this paper, the authors propose a solid freeform fabrication method for autologous-bone-replaceable artificial bones with a porosity distribution. A β-TCP porous artificial bone can be fabricated by placing a slurry consisting of β-TCP powder, water, a peptization reagent and a frother in a mold, drying it to form a solid shape and then sintering it. This β-TCP slurry contains ammonium polyacrylate as the peptization reagent, which is an electrolyte, and ammonia, hydrogen and oxygen gases are produced from its electrolysis. The authors conceived the idea of controlling the foaming of the β-TCP slurry by electrolysis, and of designing and implementing a fabrication system consisting of a fine nozzle with a microscrew for extruding β-TCP slurry as a filament and electrodes for controlling the electrolysis of the slurry. Using this system, we can fabricate a solid shape by drawing two-dimensional sections with the slurry filament and stacking each section, and at the same time vary the porosity by controlling the electric current applied for the electrolysis of the slurry. Using the experimental system, three β-TCP porous samples (approximately 18mm × 18mm × 9mm) of high (71.8%), medium (59.5%) and low (54.6%) porosity are successfully fabricated by applying electric currents of 20mA, 10mA and 0mA, respectively. Then a β-TCP porous sample (approximately 40mm × 10mm × 10mm) with a gradient porosity distribution (from 72.3% to 56.1%) is successfully fabricated by varying the electric current from 0mA to 20mA in a continuous fabrication process. From these results, the authors confirm the efficacy and potential of the proposed approach.