Stimuli-Responsive Polymersomes - Intracellular Delivery: Fundamentals and Applications - page 322

Fig. 22 Phase contrast imaging of pore growth with time during a PEG- b -PBD polymersome
bursting under electric field. The first image shows the polymersome prior to application of elec-
tric field ( t < 0). Time t = 0 corresponds to the formation of two pores at both vertical poles, indi-
cated by the vertical arrows in the second image (from left to right). The applied electric field is
in the vertical direction. Time sequence is t = 0.2 sec, t = 0.6 sec, t =1.2 sec and t = 2.9 sec in other
images, where the horizontal double arrows show the growth of the pores. Scale bar = 10 mm.
(Reproduced from ref. Bermudez et al. 2003 )
increasing polyvinyl alcohol (PVA) concentration ( c 0 = 10 wt%) in the continuous
phase through water evaporation. As water evaporates, the PVA concentration in
becomes higher and higher outside of the polymersomes and so water is squeezed
out from the inside of the polymersomes. As a result, the polymersome becomes
smaller, and its wall buckles, as shown in Fig. 23 . This provides a simple trigger for
the release of the encapsulated fluorescent HPTS. By tuning the properties of the
polymersome wall, it might also be possible to adjust the level of osmotic shock
required to break the polymersomes. Alternatively, release can also be triggered by
diluting the continuous phase and thus reducing its osmotic pressure. This simple
triggered release mechanism makes polymersomes a promising candidate for
encapsulation and release of actives.
Ultrasound is gaining attention as a therapeutic tool in addition to its use in diag-
nostics. Recently, Hammer's group (Pangu et al. 2010) reported ultrasonically
induced release from nanosized polymersomes made from PEO- b -PBD copolymers
under ultrasound at 20 kHz. Leakage of a fluorescent dye from vesicle core was
measured to study the permeation. Ultrasound causes significant leakage from the
core above threshold intensity, suggesting that leakage is governed by acoustic cavita-
tion. Size measurements and direct visualization of vesicles show that ultrasound
does not completely rupture them into fragments but causes transient poration
(Fig. 24 ). The extent of leakage inversely depends on membrane thickness and