Biomedical Engineering Reference
In-Depth Information
1.5 SURFACEFUNCTIONALIZATION
The biocompatibility of biomaterials is related to cell behavior in contact with the biomaterial
surface and particularly to cell adhesion to the surface. Surface characteristics of materials, such as
topography (roughness), chemistry, or surface energy, play a key role in the cell adhesion behavior
on biomaterials. The fi rst stage in cell or material interactions involves cell attachment, adhesion,
and spreading. The quality of this fi rst phase will infl uence the cell's capacity to proliferate and to
differentiate (second stage) in contact with the scaffold [224-228].
The process of bone tissue regeneration involves expression of genes and synthesis of proteins
known to be important in the mineralization process. To ensure the
in vivo
biocompatibility of an
implant, it is often necessary to modify the material surface, either physically by surface roughen-
ing, for example, or chemically, such as by attachment of chemically active species [228,229].
To improve the cell-substrate interaction, different strategies including surface modifi cation
have been developed. The functionalized surfaces can control not only the initial protein adsorp-
tion and production, but also the differentiation potential of different cells (such as human stem
cells) [229]. The main strategies developed for surface modifi cation are presented in Sections 1.5.1
through 1.5.4.
1.5.1 P
ROTEIN
A
DSORPTION
This fi rst phase of cell-implant interaction depends on protein adhesion. The proteins currently
used for chemical surface modifi cation of biomaterials are growth factors (or related proteins) and
adhesion proteins (or related peptides). Members of the transforming growth factor-β family are
widely studied: TGF-β1, bone morphogenetic proteins BMP-2, BMP-7, or osteogenic protein OP-1
[230-233]. Among adhesion proteins, RGD-peptides (contain a sequence of the
amino acids: argi-
nine R, glycine G, and aspartic acid D) have a high effi cacy in promoting osteoblast adhesion
[226,234,235].
All these proteins can be adsorbed
in vitro
from the serum containing media or
in vivo
from
biological fl uids. The pH, ionic composition of biological solution, temperature, and the functional
group of proteins are the main factors determining protein adsorption on a specifi c substrate [226].
Another important factor that infl uences the protein adsorption is the surface energy. Positively
and negatively charged substrates adsorb different proteins [226]. In general, proteins that have a
number of positively charged residues are expected to show a high affi nity for the anionic surfaces,
due to electrostatic attractions. By an appropriate pretreatment of the ceramic substrate (e.g., in
phosphate solution), the ionic charges from the surface can be modifi ed, providing an increasing
affi nity of the proteins or other biomolecules [236].
1.5.2 S
ILANE
-M
ODIFIED
S
URFACES
(S
ILANIZATION
T
ECHNIQUE
)
The ability of organosilanes to bond to surfaces arises from the fact that the ethoxy groups (-Si-
(OCH
2
CH
3
)
3
) of aminosilanes, such as aminopropyltriethoxysilane (APTES), form silanols (-Si-
(OH)
3
) in aqueous solution. These silanol groups can then bond covalently to a suitable substrate,
usually inorganic solids displaying appropriate surface chemical groups (such as -OH), thus leaving
terminal (nonbonded) amino groups free to serve as attachment sites for biological modifi ers like
peptides and proteins. Attachment of biological modifi ers to the terminal amino groups can be
accomplished by either of the two processes: physisorption or chemisorption [226,227].
The functional groups such as methyl (-CH3), hydroxyl (-OH), carboxyl (-COOH), and
amino (-NH2) groups are present in many biological molecules and have specifi c physical and
chemical properties that infl uence the cellular process. By silanization, the substrate surface can
be modifi ed in order to immobilize specifi c biomolecules. The inclusion of all these functional
groups, using silane modifi cation techniques, especially on glass surfaces, provides a method that
