of differences between the feature values was tested using an unpaired
t-test.
The set of extracted features included: footprint area, average dry
mass density, total dry mass and skewness. Additionally, the cell count
and confluence were determined. The parameters were calculated automatically
using algorithm implemented in MATLAB. The footprint area
can be calculated as the sum of the pixels of the cell region multiplied
by the pixel area. It provides information about the cell spreading and
adherence. The average dry mass density is calculated as a mean of the
dry mass density (see Supporting Information) for each cell region (in
picograms over squared micrometres). Dry mass density quantifies
the mass of the non-aqueous material of the cell [30–33]. In context
with other parameters, average dry mass density reports about the flatness
of the cell. The feature total dry mass quantifies overall dry mass of
the cell (in picograms) and provides information regarding the cell viability.
The skewness is calculated from histogram of the phase values in
the cell region and describes its shape. The Skewness measures symmetry
of the distribution of the phase values from the mean value in the cell
region and therefore provides information about the adherence of
the cell. Values of skewness close to zero report about symmetric distribution
of phase values, which is characteristic for spread and welladhered
cells. Detailed description of these parameters can be found
in the Supporting Information. In addition to already mentioned cell parameters,
cell count in the area of the sample 1 mm2 and the confluence
of the cell culture were determined. The confluence is calculated as a
ratio of the surface covered by cells. The number of cells together with
the level of confluence provides information about the cell proliferation
and cell culture viability.
3. Results
3.1. Chemistry of plasma polymerized layers
Previous experiments with CPA plasma polymerization in pulsed
radio frequency capacitively coupled discharge using glass tubular reactor
showed that the concentration of amine groups is decreasing with
increasing power delivered per monomer molecule [22]. Preliminary
experiments in stainless steel parallel plate reactor confirmed this
trend and three sets of deposition conditions (CPA40, CPA42 and
CPA43 in Table 1) were chosen in order to have different amounts of
amine groups but similar thickness stability of the films. Regardless of
the substrate nature, the deposited CPA plasma polymers uniformly
and homogenously covered the surface. The deposition rate was decreasing
in the row CPA40 N CPA43 N CPA42 (see Table 1), i.e. an increase
of the CPA flow rate and a decrease of the delivered power
were enhancing this parameter. Hence, the CPA plasma polymerization
was carried out in the monomer deficient regime [45,46]. The FT-IR