Protein-based Biointerfaces to Control Stem Cell Differentiation - Biointerfaces: Where Material Meets Biology - page 15

general, silane molecules are activated by hydrolysis and then condensation
between Si-OH groups of the silanol and the OH present on the surface
occurs. This activation leads to the exposure of functional groups (from the
silanol molecules) on the ceramic surface. In this way amine, carboxylic acid
or sulfur groups are introduced on the surface material and are often used
for further functionalization reactions through GA or EDC. Though simple,
several problems can occur such as layer stability, silane reactivity towards
water, possible toxicity of free silanes or leachable products, complete hiding
of the bulk material, and potential problem for bioactive ceramics. 94
Besides silanization, the formation of a pD coating layer can be a success-
ful strategy for introducing functional amine groups, useful for other
functionalization steps.
Overall, covalent immobilization procedures require laborious multi-
functionalization work, can produce potentially toxic by-products, and the
stability of linkers and bioconjugates during and after the immobilization
reactions can be an issue. 95
The pros and cons of the covalent bonding method are:

d n 3 r 4 n g | 1

Pros
J Applicable to a broad range of natural or synthetic polymers
J Stable functionalization over time and usually a predictable release
profile
Cons
J Some polymers need to be activated (e.g., grafting reactive groups on
the polymer chain) before functionalization
J By-products from the functionalization step can remain and show
cytotoxicity

.

1.2.1.3 'Linker-free' Approaches
Simplicity and cost-effectiveness, besides biological ecacy, are two of the
factors that remain critically important in the development of new functio-
nalization technologies. The immobilization of biomolecules without
chemical linkers—linker-free—addresses common problems related to
physisorption and chemical linking. Linker-free immobilization was de-
veloped by Kondyurin and is currently used by producers of implantable
devices (LfC Z.o.o). 37,38,96 This approach is particularly useful for polymeric
devices and also has been translated to metallic devices by Chrzanowski. 13 In
this approach biomolecules are immobilized via radicals created at the
interfaces by energetic ion-implantation. Implanted ions disrupted the pri-
mary bonds of the polymer and results in the formation of a carbonized
interface that contains highly reactive radicals, species with unpaired elec-
trons. 97 The radicals are stabilized by delocalization on p-electron clouds of
the condensed aromatic structures. The stability increases with larger con-
jugated areas in the aromatic structure. In addition to the chemical activity
that they bring to the surface, the unpaired electrons typically react with