3.1 Mechanical properties of synthetic and P. aeruginosabiofilms
Rheometry is commonly used to measure the mechanical properties of
viscoelastic materials. It is also widely used to measure biofilm
mechanical properties, as it is suitable for viscoelastic solids and
non-Newtonian fluids. Averaged properties of a sample are measured,
providing the relationship between force, deformation, and time. Our
goal was to obtain the biofilm mechanical parameters as inputs for the
computational model. As a first step, alginate with embedded microbial
cells was used as a synthetic biofilm, since it is mechanically more
homogenous and it can be formed into a regular geometry. A regular
geometry is desirable for both rheometry analysis and for simulations.
We also tested a homogenized P. aeruginosa biofilm.
In order to confirm the homogeneity of synthetic biofilms, G′ and G′′
from strain sweep test were discussed (Fig. 3). The storage modulus G′,
representing elasticity, and loss modulus G′′, representing viscosity,
were calculated. Averaged G′ and G′′ from synthetic biofilm were 251.3
Pa ± 7.1 Pa and 18.2 Pa ± 5.5 Pa, respectively. A higher G′ compared to
G′′ also revealed that the synthetic biofilm samples were more elastic
than viscous. Error bars in Fig. 3 show the standard deviations from
three different locations of the same sample. The small standard errors
indicated a small magnitude of the bias, which demonstrated that the
alginate is mechanically homogeneous and can provide a simplified
control.
The values of G′ and G′′ for the P. aeruginosa biofilm were 858 ±
36 Pa and 98 ± 12 Pa. These moduli were in the same order of magnitude
as those of the synthetic biofilm sample, which indicates that the
alginate sample was a reasonable surrogate of a real biofilm. Previous
studies on different types of biofilms also suggested a wide range G′
and G′′ from rheometry tests, ranging from
101-103 Pa for both G′ and G′′ (Böl
et al., 2012). The synthetic biofilm and P. aeruginosa biofilm in
this study were within the range of reported data, with a relatively
small standard deviation.
Stress relaxation tests were performed with both synthetic and P.
aeruginosa biofilm samples (Fig. 4). The stress relaxation curve
(black-square markers) demonstrates that after an initial stress
response (elastic response), the viscous response recovers the stress
over time when constant strain was applied on the sample. The shape of
both curves indicated that both samples behaved as typical viscoelastic
materials.
The fitted Maxwell model was also plotted as a green line in Fig. 4. The
residual sum of squares (RSS) were compared from both samples in
nonlinear least-square fitting. The RSS of synthetic biofilm was 45.4
while the RSS of homogenized P. aeruginosa biofilm was 152. The
alginate sample had a better fit than the real biofilm. For a more
accurate fitting, more elements (springs and dashpots) can be added to
the Maxwell model (Peterson et al., 2013).