|M.Sc Student||Alon Malka-Markovitz|
|Subject||Calcium Phosphate Deposition on Reticulated NiTi Scaffolds|
|Department||Department of Materials Science and Engineering||Supervisors||Dr. Gotman Irena|
|Full Professor Gutmanas Elazar|
|Full Thesis text|
The present research explores the fabrication of strong porous nickel-titanium (NiTi, Nitinol) bone scaffolds by conversion of open-cell Ni foam and explores the possibilities of their surface modification towards enhanced bone compatibility.
Commercially available Ni foams with different pore size, porosity and strut thickness were annealed at 800-1000°C in Ti powder employing a Powder Immersion Reaction Assisted Coating (PIRAC) method. This resulted in formation of Ni-Ti intermetallics of different stoichiometries and, eventually, a single-phase NiTi. Thus obtained NiTi foams were PIRAC nitrided at 900-1000°C in the atmosphere of monatomic nitrogen. The obtained NiTi foams were bioactivated by soaking in 5M NaOH solution or by Biomimetic deposition of Calcium Phosphate (Ca-P) from Simulated Body Fluids (SBF). At each treatment stage, the foams were characterized employing X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) with chemical analysis (EDS) and Auger Electron Spectroscopy (AES). Prototype cylindrical NiTi scaffolds were fabricated by coiling, compression and PIRAC conversion of Ni foams. The scaffolds were tested in compression in an Instron testing machine. Ni ion release from NiTi scaffolds into Ringer’s solution was measured by Inductively Coupled Plasma Absorption Emission Spectroscopy (ICP-AES).
NiTi scaffolds obtained by PIRAC conversion had a regular open cell porosity of 75 to 93 % mimicking the 3D structure of the spongy bone. When treatment was conducted at 900°C, the time required for full conversion was proportional to the square of foam strut thickness. PIRAC-converted NiTi foams exhibited a ductile behavior with no signs of cracking after large deformations in compression. NiTi scaffolds with ~75 % porosity had a compressive strength σy ≈ 35 MPa that was increased by plastic deformation to 150 MPa at 60 % porosity. PIRAC nitriding resulted in the formation of a 1 micrometer thick TiN coating. Upon immersion in saline solution, the initial release rate of Ni from uncoated scaffolds was comparable to the literature data for bulk NiTi, and it decreased with time. Ni release from TiN-coated scaffolds was one order of magnitude lower. Coating with TiN allowed us to successfully grow a bioactive sodium titanate layer or, alternatively, bone-like Ca-P layer on NiTi scaffolds. Both surface modifications should further improve the scaffold osteoconductivity. Preliminary cell culture results suggested that NiTi scaffolds supported the attachment and proliferation of bone cells (osteoblasts).
The above findings make porous NiTi obtained by PIRAC conversion of Ni foams an attractive biomaterial for a number of bone scaffolding applications and warrant continued in vitro and in vivo investigations.