Initial attachment of osteoblast-like cells on functionalized surfaces coated with calcium phosphate

Tissue engineering techniques which combine synthetic grafts with molecules and cells are considered as viable long term solutions for bone tissue repair and reconstruction procedures. The self SAM technology enables modeling of surface functional groups on biomaterials, while the concept of ion-selective precipitation reaction causes formation of Ca-P coating on these functionalized surfaces. Hydrophilic surfaces such as -COOH or -OH end groups have very powerful induction capability for the heterogeneous nucleation of hydroxyapatite-like layer, while nucleation could be prohibited on an - NH 2-terminated surface. Using the SAM technique we grafted three different organosilanes onto silicon wafers to yield -NH2, -COOH and -OH functionalized surfaces respectively. The surfaces were characterized by contact angle measurements, ellipsometry, FTIR, SEM/EDX and RBS. Ca-P coating was formed on the SAMs by immersion in a simulated physiological fluid (pH 7.4, at 37°C). FTIR showed dependence of the morphology of the Ca-P coating on both the type of surface functionality and on the duration of coating. The spectrum exhibited P-O and C-O absorption bands characteristic of a carbonated calcium hydroxyapatite with stronger P-O bands on -OH surface. MC3T3-E1 osteoblast-like cells were used in bioassays to study the effect of the end groups on the initial cellular attachment and alkaline phosphatase expression on Ca-P coated surfaces. Cell assays showed increased cellular attachment on the Ca-P coated-OH SAM, after cells were seeded for 1h, compared to the -NH 2 and -COOH-terminated surfaces, while up to 5x alkaline phosphatase activity was shown on Ca-P coated surfaces. At a low temperature therefore, the end groups of SAMs act as effective nucleation sites to induce formation of a biomimetic apatite coating, in a one-step biomineralization process. The most potent apatite forming surface, the -OH SAM surface, demonstrates that surface -OH groups are important to induce bone bioactive behavior at surfaces.