Chemistry Reference
In-Depth Information
of air). Consequently, heterogeneous surface reactions during and after plasma pro-
cessing lead to the predominant formation of different oxygen-containing functional
groups like hydroxyls (-OH), carbonyls (
O)O). This
holds even for high concentrations of nitrogen in the discharges (air discharges).
Examples for polymers treated in such a manner are contact lenses, artificial blood
vessels, catheters, and dialyzers [200]. A further application represents the plasma
functionalization of plastic disposables (typically PS or PC) used in clinical or lab-
oratory diagnostics. Especially, cell culture consumables like dishes, titreplates, and
cell culture flasks are regularly plasma treated [213,214]. Usually, the goal of such
treatments is surface activation to overcome insufficient biomolecule attachment on
the untreated bare materials.
Accessary prerequisite for such processes is the avoidance or at least minimiza-
tion of thermal loading, etching, and degradation. High thermal loads leading to
device temperatures near the glass transition point (less than 100
◦
C for many poly-
mers) can affect the mechanical integrity of the devices. Etching processes cause
cleavage of chemical bonds thus splitting polymer molecules into smaller pieces.
Both effects play a role in undesirable polymer degradation, i.e., in the formation of
weak, so-called low molecular weight layers. This degradation is different for distinct
polymer classes. While polyolefines show a rather small tendency for chain scission,
and aromatic rings in the side chains protect polymers like polystyrene from chain
scission, especially photosensitive polymers like polyacrylates, polycarbonates and
polyetherketones are more susceptible to chain scission and degradation. These weak
layers may reduce the adhesion strength of biomolecules and cells undesirably [215].
Additionally, such weak layers contain separate, soluble chains, so-called leachables.
Such
leachables
may induce unwanted protein activation and can impede interfacial
biocompatibility.
=
C
=
O), or carboxyls (-C(
=
8.2.3.4.3 Polymer Treatment in Ammonia Plasmas
Next to treatments in oxygen-containing plasmas, process gas mixtures with ammo-
nia (NH
3
) are most notable. Using NH
3
plasmas, amino groups can be generated
on surfaces. Despite the fact that the obtainable surface densities are low, typically
between 1% and 2% of the carbon surface density [216], and although considerable
concomitant nitrogen- and oxygen-related functionalization occurs, this is of interest
and sufficient for many applications. Chemically, amino groups exhibit a strong basic
character. This provides a potential for well-defined selective covalent coupling, e.g.,
of proteins. As in the case of other plasma functionalization processes, the techni-
cal requirements on processing are not very high. Considering the high toxicity and
reactivity of ammonia, these processes are preferably performed under low-pressure
plasma conditions. Furthermore, it is strongly recommended to ensure a low level
of oxygen contamination in the gas phase since oxygen competes with amino group
precursors for surface radical sites. Reduction of undesired concomitant functional-
ization is alsoprincipally possible by applying very short plasma activation [216,217].
However, the achieved improvement of selectivity of plasma functionalization is at
the cost of amino group density.
It is noteworthy that for most applications, it seems sufficient to have a certain
amount of amino groups available at all. Hence, treatment conditions are chosen
