Mechanical properties of phospholipid coated microbubbles
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Date
27/11/2014Author
Morris, Julia Kathleen
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Abstract
Phospholipid coated, inert gas filled microbubbles (MBs) are currently in widespread
use in medical applications for the enhancement of diagnostic ultrasound images, and
they are promising candidates for use in the area of targeted drug/gene delivery and
uptake. As phospholipid coated MBs were developed for use with diagnostic
ultrasound, their behaviour under acoustic loading is well investigated, however
much less is known about their response to direct mechanical loading, which will
potentially prove important as the range of uses of MBs expands. This is particularly
true of the existing commercially available MB products. In this thesis, atomic force
microscopy (AFM) was used to investigate the mechanical behaviour of three types
of commercially produced phospholipid coated MBs, Definity®, BR14 and
Sonovue®, at small deformations. Force spectroscopy was used to produce force-deformation
(F-Δ) curves showing how the MBs deform under mechanical loading.
Definity® MBs were deformed with tipless cantilevers at high deformations (though
still less than 30% of the initial height of the MB); BR14 and Sonovue® MBs were
probed with both tipless and tipped cantilevers to investigate both whole-bubble
deformation and also shell indentation. BR14 was limited to low deformations;
Sonovue® included both low and high deformations. The F-Δ curves were used to
evaluate MB stiffness and also in combination with up to four mechanical models to
predict the Young’s modulus of the MBs. The suitability of Reissner, Hertz, Elastic
Membrane and De Jong theories for the prediction of the Young’s modulus of the
MBs was explored. In the case of Definity® MBs no correlation between MB size and stiffness was
observed; however an unexpected size dependence was observed in the Young’s
modulus values, possibly due to variations in the thickness of the phospholipid shell.
The membrane stretching component of elastic membrane theory was found to be the
most applicable model on these MBs in this higher deformation regime. However, in
this regime, gas compressibility could play a role and this is not included in the
model.
We studied the mechanical properties of BR14 MBs at very low deformations using
‘soft’ cantilevers. In this regime, gas compressibility should play a minimal role and
there are several mechanical models which may be used. These MBs demonstrated
decreasing stiffness with increasing diameter, and little variation in Young’s modulus
with diameter. Hertz and De Jong theories showed more realistic Young’s modulus
values (compared to other models) with little observable trend.
Sonovue® MBs were used for a more comprehensive study of the small and very
small deformation regimes using ‘soft’, ‘hard’ and tipped cantilevers. They showed
no definitive trend in MB stiffness with MB diameter. Hertz and De Jong theory
were again found to be most suitable. Analysis of curves acquired with tipped
cantilevers indicated that the stiffness of a localised area of the shell membrane is
similar to the overall stiffness of the MB and that the apparent Young’s modulus of
the membrane according to the Hertz theory is also similar to that of the MB as a
whole. Generally, considering all systems, Reissner theory was found to produce large
overestimates of Young’s modulus, exceeding expected values by several orders of
magnitude. Hertz and De Jong theories produced underestimates, though by a much
smaller margin. Elastic membrane theory worked well and produced realistic
Young’s modulus values only at relatively high deformation (the stretching term) in
spite of the fact that gas compressibility is not taken into account. The suitability of
the models is therefore very dependent on the deformation regime of the experiment.
It seems that there is scope for better models at low deformation taking into account
the soft shell of the MB and possibly its specific structure. Precise structural
information of the MB shells does not exist; it is not trivial to attain and should
certainly be a topic of future work with additional instrumentation.