Protein-based Biointerfaces to Control Stem Cell Differentiation - Biointerfaces: Where Material Meets Biology - page 14

presence of different concentrations of citric acid (CA). Results showed that
CA-HA displayed significantly higher anity towards lysozyme. Using the
optimized parameters obtained from lysozyme adsorption (as a model pro-
tein for BMP-2), the results demonstrated that HA particles immobilized
with COO- significantly increased the HA loading capacity for bone mor-
phogenic protein-2 (BMP-2) and prolonged BMP-2 release profile. The
in vitro results showed that the prepared CA-HA was not toxic towards
human osteoblasts and indeed citric ions present on HA surface shifted
the

d n 3 r 4 n g | 1

surface charge towards negative value, which promoted cell

proliferation on HA.
The pros and cons of physisorption are:

Pros
J Applicable to the broad range of natural or synthetic materials
J No pre-treatment of the target molecules is necessary
J Mild conditions
J Usually a very quick and easy approach
Cons
J The coating is not stable for long-term application and molecules
detach quickly from the surface with an unpredictable profile
J Molecules can lose their functionality upon binding

.

1.2.1.2 Covalent Bonding
Covalent immobilization of biomolecules is usually chosen when possible
over physisorption, as the attachment is controlled and stable, and the
kinetics behind biomolecule release can be predicted and calculated. Nu-
merous immobilization protocols have been optimized for different ma-
terials and different cross-linker molecules have been used to graft
biomolecules to specific, surface functional groups. The most popular routes
utilize glutaraldehyde (GA) or carbodiimide (EDC) chemistry to immobilize
proteins and peptides as well as oligonucleotides. 76 GA can be used to ac-
tivate the target surface and attach biomolecules irreversibly on the activated
surface through an amino group (biomolecule)-aldehyde (surface) reaction.
Due to prevalent cytotoxicity effects reported for GA, 92 EDC chemistry has
become very common due to its 'zero-length property'. In other words, EDC
facilitates the reaction between a carboxylic acid group and an amine group,
leaving no residues on the new bond.
In polymers, the functional groups required for GA/EDC coupling re-
actions 93 can be exposed either naturally or by quick surface modification
(e.g., oxygen plasma treatment). For ceramic materials, pre-treatments are
necessary to introduce such functional groups to the surface calcium
phosphate surface. Silanization is the most widely used strategy to introduce
functional groups on inorganic surfaces. 76 Silanization is extremely versatile
in terms of materials to be used with and does not require extreme
conditions (i.e., mild pH, room temperature) or expensive equipment. In