Chemistry Reference
In-Depth Information
elemental composition can be found. However, after some days of exposure to air,
the oxygen content reaches several percent and continues to increase for weeks
and months.
Ammonia plasmas are commonly applied for polymer surface modification, and
a number of research results have been published with regard to the improved
understanding of the plasma processes [217] as well as on numerous biomedical
applications [216]. The list of respective application examples in Table 8.11 provides
just a glimpse of the width and variety of the field.
Note that ammonia plasmas can be applied not only to typical hydrocarbon
polymers, but also to fluorinated polymers and to very sensitive resorbable polymers
like poly(3-hydroxybutyrate (PHB)) and polylactides. Together with the improved
cell culture outcome, this makes that type of surface modification an ideal candidate
for demanding prospective new applications in tissue engineering, implants, devices,
and consumables.
Besides direct cell culture applications, amino-functionalized polymer surfaces
are frequently utilized as chemically reactive platforms for the covalent immobi-
lization of biologically active molecules in aqueous solutions. Well established and
therefore most common are carbodiimide catalyzed reactions between the surface
amino groups and carboxylic groups on the molecules to be immobilized, which lead
to the formation of interfacial amide bonds [207,220].
8.2.3.4.4 Carboxylated Surfaces
Another chemical functional group with relevance to biomedical applications is the
carboxylic acid (-COOH) group. Carboxylic acid groups possess a strong nucle-
ophilic character and hence a high chemical reactivity. They allow the generation
of negatively charged surfaces and the creation of ionic or covalent chemical bonds.
CO
2
plasma treatment is reported to be specially useful for functionalization with car-
boxylicacidgroups.However,apartfromproducingcarboxylgroups,CO
2
-containing
plasmas generally create other functional groups containing C and O such as hydroxyl
groups, aldehydes, ketones, esters, and (from postplasma reactions) hydroperoxides
as well [207]. Also, insertion of carboxyl groups is not very high, typically in the
range of some percent of surface carbon bonds. It is, therefore, not much different
from oxygen plasma treatment. This can be seen as a reason why very often plasma
polymerization is preferred for generation of carboxylic groups. Regardless of the
complications of deposition, the outcome of plasma polymerization, e.g., of acrylic
acid, can be much better in terms of density and selectivity of functional group
generation.
It should be mentioned that an argumentation exists, which suggests to pre-
fer substrates with carboxylic groups for the very popular carbodiimide-catalyzed
covalent coupling procedure of proteins. Briefly, carbodiimide activates carboxylic
groups on the protein that can react with amino groups on the same protein or another
protein rather than with an amino group on the polymer surface, thereby causing
crosslinking and oligomeric protein agglomerates. Thus, not all molecules found on
the surface may be interfacially covalently linked and biological function may be
compromised [207].
