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times as single cells and sometimes as larger
colonies. The biofilm was approximately 50 µm
thick. The laboratory-grown S mutans demonstrated
similar characteristics of aggregated bacteria: interspersed
with water channels and enveloped in a
slime matrix.
Biofilm Growth on Cellulose Ester Membranes
S mutans biofilms grew readily on the cellulose
ester membranes. The biofilms were similar to those
described above and in the literature 16,18 and consisted
of dense aggregates of cell clusters interspersed
with water channels that penetrated to the membrane
filter surface (Figure 4).
The biofilms ranged in maximum thickness
from 20 µm to 80 µm with an average of 45 µm ± 25
µm (mean ± 1 SD) and had a surface coverage of
between 50% and 75%.
Delivery of Fluoride Through Biofilmcolonized
Membranes With and Without
Power Brushing
When the 1100-ppm fluoride solution was
added to the biofilm side of the diffusion chamber,
an increase in fluoride concentration was measured
in the other chamber after approximately 1 minute,
even without power brushing. The increase was due
to Fickian diffusion caused by the difference in fluoride
concentration between the 2 chambers (Figure
5). After 4 minutes, the concentration had increased
from approximately 0.4 ppm to 0.5 ppm. Power
0.9
brushing greatly increased the delivery of fluoride
through the biofilm membrane, with FlexCare causing
the greatest increase over the 4-minute brushing
period. The fluoride concentration in the measurement
side with power brushing from FlexCare
achieved approximately 0.8 ppm, whereas that from
Triumph achieved approximately 0.65 ppm. Without
brushing the mass transfer rate coefficient of fluoride
through the biofilm-colonized membrane was 1.6 ±
0.3 × 10 -5 cm/s (Figure 6). This was increased by 79%
to 2.8 ± 0.2 × 10 -5 cm/s by power brushing with
Triumph and by more than twice as much (129%) to
3.6 ± 0.3 × 10 -5 cm/s by FlexCare (Table). The mass
transfer rate for FlexCare was also 29% more than
for Triumph.
Discussion
This article describes a 2-chamber method to
quantify and compare the delivery (mass transfer) of
fluoride through biofilm-colonized membranes
under various conditions of power brushing and
nonbrushed diffusion. These systems have been previously
applied to study the transport of medically
relevant molecules 22 and tracer salts or nutrients 23,24 ;
however, they have not been used to study the transport
of fluoride through biofilm-colonized membranes
in a dental context. Two-chamber systems
have the advantage that they do not require microscopic
measurement techniques, such as microelectrodes,
microfluorescent dye tracking, 27 or fluorescent
recovery after photobleaching, 22 but have the
0.8
0.7
FlexCare
Fluoride
concentration
(ppm)
0.6
0.5
Triumph
0.4
No brush
0.3
0 50 100 150 200 250
Time (s)
Figure 5—Representative data set showing the rate of fluoride delivery through the biofilm-colonized membrane measured over a 4-minute
period with no brushing (yellow diamonds) or power brushing with either FlexCare (blue diamonds) or Triumph (gray triangles). Power
brushing increased the fluoride delivery, with FlexCare causing the greatest increase in fluoride concentration after the 4-minute period.
20 Compendium / September 2007 Vol. 28, No. 9 (Suppl 1)