טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentZorn Gilad
SubjectSurface Modification and Bio-Functionalization of Low
Modulus Ti-Nb and TiNi Alloys Employing Self
Assembled Monolayers (SAMs)
DepartmentDepartment of Materials Science and Engineering
Supervisors Professor Elazar Gutmanas
Dr. Irena Gotman
Full Thesis textFull thesis text - English Version


Abstract

 Titanium alloys are widely used as orthopedic implant materials due to their excellent corrosion resistance; good biocompatibility and relatively low elastic modulus that minimizes stress-shielding-related bone resorption. Still, the inability of Ti alloys to control bone cell behavior and to directly bond to bone prevents the creation of a contiguous interface capable of carrying stresses normally occurring at the site of insertion.

In the present research, biofunctional surface modification of two Ti alloys with unusually low elastic moduli - β-Ti45Nb and NiTi (Nitinol) via covalent attachment of phosphonate-based self-assembled monolayers (SAM) followed by immobilization of RGD-peptides simulating the cell-binding ability of extra-cellular matrix proteins, has been studied.

To produce SAM coats, the substrates with reproducible surface chemistry and roughness scale were immersed in non-aqueous solutions of different phosphonic acids and heated at up to 120ºC for ~ 20 h. Hexadecylphosphonic acid (HDPA) having a non-functional CH3 tail group was used to optimize the coating procedure.

Thorough characterization of monolayers and the underlying surfaces was performed employing contact angle measurements, Variable Angle Spectroscopic Ellipsometry (VASE), Infrared Spectrometry (FTIR), High Resolution X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). Certain features of SAMs were studied on atomically smooth model surfaces produced by sputtering Ti-Nb or TiN on silicon wafers.

Surface analyses of HDPA-treated anodized Ti45Nb and nitrided+anodized NiTi verified the presence of a well-ordered phosphonate-anchored SAM. The existence of covalent bonding between the phosphonate anchors of HDPA and the oxide substrate was confirmed using the original XPS approach based on comparing the high resolution oxygen signals from the unbound and metal-bound HDPA molecules. Applying the same SAM-attachment procedure to PIRAC TiN-coated NiTi alloy produced a grossly disordered organic coat, whereas anodic oxidation of TiN-coated NiTi allowed for a much more successful HDPA self-assembly. The difference in phosphonate attachment capability of the oxidized and nitrided surface was explained based on their different ability to regenerate surface hydroxyl groups acting as catalysts in the process of SAM formation.

The developed SAM attachment procedure was used to attach a phosphonate-anchored alkyl molecule having an electrophilic chloroacetyl functionality - CAUDPA - to Ti45Nb alloy, and a cell-binding RGDC peptide was subsequently immobilized on CAUDPA-treated Ti45Nb via reaction of the chloroacetyl tail with the thiol group of the peptide terminal cystein. Enhanced adhesion of bone cells on RGDC-grafted Ti45Nb surfaces was observed in cell culture studies, confirming the feasibility of the developed peptide immobilization procedure.