Research News
Polymer Particles by Templating of Vesicles
By Jutta Hotz and Wolfgang Meier*
Template-directed synthesis is an elegant approach to
defined nanostructured materials. The basic concept is to
capture the size and shape of a given molecule, particle, or
superstructure such that it is preserved in the newly formed
species even after removal of the template. This concept is
quite commonly used by nature, for example, during
biomineralization.[1] It has, in recent years, also found
increasing interest for the preparation of nanostructured
inorganic solids and polymers using, for example, the
mesostructures of lyotropic liquid-crystalline phases or
microemulsions as a template.[2] The resulting materials are
highly interesting from an applications point of view, e.g., as
supports for catalysts, as adsorption media, or as membranes.
Similarly, vesicles or liposomes, i.e., spherically closed lipid
bilayers, have been used in the synthesis of small inorganic
particles.[3] However, despite their quite interesting hollow
sphere structure, which makes them, for example, an
attractive packaging material for the encapsulation of
colloids or enzymes, they have scarcely been used as a
template for polymer synthesis.
Although polymerization in vesicles is a widely used
method to stabilize the lipid bilayer, for example by
polymerization of synthetic lipids bearing polymerizable
groups such as butadiene, methacrylate, or acrylate moieties,[4] this can be regarded as an immobilization or freezingin of the whole superstructure rather than as a template
polymerization. Namely, one essential feature of the
templating process is that the polymers are formed in or
around vesicles that act solely as matrices and are not directly
involved in the polymerization reaction. During the imprinting process the newly formed polymer usually has to be
crosslinked, in order to be able to retain its shape even after
removal of the template.
Vesicles are basically constituted of three parts available
for the template synthesis: the aqueous core, the lipid±water
interface, and the hydrophobic interior of the lipid bilayer
(see Fig. 1).
It has, for example, been shown that an aqueous solution of
hydrophilic monomers can be entrapped in the water pool in
the interior of the vesicles. Subsequent polymerization
±
[*]
Dr. W. Meier, J. Hotz
Institut für Physikalische Chemie, Departement Chemie
Universität Basel
Klingelbergstrasse 80, CH-4056 Basel (Switzerland)
Adv. Mater. 1998, 10, No. 16
Fig. 1. Schematic representation of the three different parts of a vesicle
available for template polymerization: a) the aqueous core; b) the lipid±water
interface; c) the hydrophobic part of the lipid bilayer.
produced crosslinked poly(acrylamide) gels in the interior
of the vesicles. These hydrophilic latexes were found to
preserve their dimensions and spherical shape after their
isolation.[5]
Polymerization of polymerizable counterions of the lipids,
such as acrylate or methacrylate, at the bilayer±water
interface is a method quite commonly used in the context
of vesicle stabilization.[4] The resulting so-called ªliposomes
in a netº have the advantage that the hydrophobic region of
the bilayer is not directly altered by the polymerization
(which, for example, allows the lipids a certain lateral
mobility in the membrane) yet the vesicle is protected by the
polymer coat. In this way, it is even possible to prepare
unsymmetrical polymer-coated vesicle membranes. Here,
similar to the cytoskeleton of biological cells, the polymerized counterions are fixed at only one side of the bilayer.[4]
However, it seems that these ultrathin spherical polymer
membranes from the lipid±water interface have been
isolated from the templating vesicles in only one case.[6]
The approach involved the formation of small unilamellar
vesicles from a mixture of diallylammonium dihexadecylphosphate and sodium dihexadecylphosphate, polymerization of the counterions by ultraviolet irradiation, removal of
the surfactant, and redispersal of the remaining polymer
hollow spheres. Electron microscopy and entrapment
experiments revealed the presence of 20±200 nm diameter
so-called ªghost vesiclesº, which were indeed closed,
spherical, and relatively porous in character.[6]
It is well-known that hydrophobic substances (e.g., octane)
can be solubilized to a certain degree in vesicle or liposome
suspensions, where they are dissolved in the hydrophobic
region of the lipid bilayers.[7] Obviously, if such substances
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bear polymerizable groups it should be possible to create
hydrophobic polymers in the interior of the bilayers, thus
eventually leading to a spherically closed polymer shell. This
was recognized as early as 1986 by Murtagh and Thomas:[8]
they used mixtures of styrene and divinyl benzene as
hydrophobic monomers, which allowed them to prepare
crosslinked hydrophobic polymer structures in the membranes. No separate monomer phase could be detected and
the vesicle dimensions remained constant within experimental accuracy during the swelling and subsequent
polymerization.[8,9] From this it was concluded that the
polymerization really occurred in the interior of the bilayer
and under retention of the vesicle structure. Kinetic studies
on a similar system revealed that the free-radical polymerization (of alkylacrylates) in vesicles is a very fast reaction:[10]
the monomers are present locally, within the bilayer, at a
rather high concentration and, perhaps, also have a favorable
orientation for the chain reaction. The kinetics are, in
contrast to emulsion or microemulsion polymerization,
similar to a homogeneous phase polymerization.[10]
The lipid bilayer, during polymerization, imprints its
exceptional geometry (namely, a typical thickness of only
about 5 nmÐit is possible to realize coherent bilayers with
diameters up to several hundred micrometers!) on the
resulting polymer networks. Such a hydrophobic polymer
framework in the interior of the lipid bilayer offers quite
interesting properties with respect to the mechanical
stabilization of lipid bilayers. One essential point is that the
lipid molecules are not covalently attached to this polymer
framework. Therefore, their lateral mobility, the phase
behavior, and even the permeability of such membranes
have been found to be not significantly changed by the
presence of the polymer.[8,9] An interesting possibility for
studying the properties of these polymer-stabilized membranes in more detail is to use planar lipid bilayers, so-called
black lipid membranes (BLMs), as model systems.
It is obvious that polymerization of hydrophobic monomers should also be possible in BLMs. In this way polymerstabilized membranes can be produced with diameters of up
to 0.5 mm and a thickness of only 4±5 nm.[11] Rupture of such
BLMs can be induced by carefully applying single electric
field pulses across the membrane. The critical voltage
causing the breakdown of the membrane provides information about the membrane's mechanical stability.[11] Indeed, it
has been shown that a hydrophobic polymer framework in
the interior of the membrane leads to a considerable
mechanical stabilization of the bilayer, increasing the critical
voltage by a factor of about 5.[12]
Another highly interesting property of these systems can
be observed during such experiments: the polymer scaffolding enables self-repair of the membrane. Even macroscopic
defects in such bilayers, e.g., induced by an electric field
pulse, can reseal and a tightly closed membrane is formed
again.[12] In contrast to conventional BLMs, where rupture is
governed by surface tension and lipid±lipid interactions,[11] in
the polymer-network-containing membranes an additional
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Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998
elastic contribution acting against pore growth has to be
taken into account. Extension of a defect in the membrane
requires also the polymer framework to be torn. The network
chains can, however, store elastic energy to a considerable
degree prior to rupture. Once the extension has stopped, the
energetically unfavorable pore reseals, due to the lateral
mobility of the lipid molecules.
In the context of membrane stabilization as well as with
respect to possible applications of the resulting polymer
structures it is essential to know more about the structure and
the behavior of this polymer framework in the interior of the
membrane. Only if the polymer network is homogeneously
distributed within the lipid bilayer can one really speak of a
template polymerization. Then, the crosslinking polymerization within the membrane leads to the formation of a
quasi-two-dimensional polymer network. Using vesicles or
liposomes as a template, hollow spheres of polymer are
formed, with an inner and outer radius directly controlled by
the vesicle. Such spherically closed polymer shells are
interesting from both an applications and an academic point
of view. They offer, for example, new possibilities for
encapsulation purposes, as nano- or microreactors, or can
be used as model systems for two-dimensional polymer
networks.[13]
Indeed, various evidence had already been presented for
the homogeneous distribution of the polymers within the
lipid bilayers, e.g., an enhanced lifetime of polymercontaining vesicles,[8±10] their stabilization against lysis, or
even the possibility of visualizing them using atomic force
microscopy in the liquid tapping mode (which is not possible
with unpolymerized vesicles due to their fragility).[14]
However, it was only very recently that the structure of the
polymer itself could successfully be visualized.
While in a dioctadecyldimethylammonium bromide/polystyrene system cryogenic transmission electron microscopy
investigations clearly show that a phase separation occurs,
leading to a parachute-like structure, [15] in a similar
dioctadecyldimethylammonium chloride/alkylmethacrylate
mixture confocal laser scanning microscopy (CLSM) and
scanning electron microscopy (SEM) investigations provide
evidence for a homogeneously closed polymer hollow sphere
morphology.[16] The differences between the two systems
perhaps arise from the fact that the alkyl chain milieu within
the bilayer is a poor solvent for polystyrene while the
poly(alkylmethacrylate)s have a better compatibility. Another point may concern the relative monomer concentrations: while the alkylmethacrylates in one investigation[16]
were applied in a molar ratio of lipid to monomer that was
smaller than 1:1, in another, using styrene,[15] this ratio was
1:2. A similar phase separation within the bilayer, leading to
morphologies analogous to those reported by Jung et al.,[15]
has been observed upon solubilization of large amounts of
toluene (up to a lipid-to-toluene ratio of »1:4) in dipalmitoylphosphatidylcholine vesicles.[17]
As already mentioned before, one essential feature of the
templating process is that the newly formed polymer has to
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Adv. Mater. 1998, 10, No. 16
Research News
be able to retain the characteristics of its template even after
its isolation. This could indeed be shown in the case of the
crosslinked poly(alkylmethacrylate)s.[16] CLSM, SEM, and
light-scattering investigations clearly prove the hollow
sphere morphology of the pure polymer particles. In this
context giant vesicles with diameters up to several hundred
micrometers have been used as templates in order to be able
to directly visualize the structural details of the resulting
polymer particles (see Figs. 2 and 3).
Fig. 2. Confocal laser scanning micrograph of a crosslinked poly(butylmethacrylate) hollow sphere [16]. The length of the bar corresponds to 50 mm.
Although the isolated particles shrink considerably after
extraction from the lipid membrane, they preserve their
spherical shape with a rather low surface roughness. Their
final size has been found to be directly proportional to that of
the original vesicles.[16] A similar observation was made by
Jung et al.,[15] who also showed that each vesicle contains only
one polymer particle, the size of which is directly correlated
to that of the templating vesicle. Apparently only the
monomer solubilized in the bilayer is polymerized and
intervesicular exchange plays only a minor role.
The contraction of the isolated polymer particles is,
however, not too surprising. The polymer is obviously forced
in the interior of the membrane into a nearly twodimensional conformation. After it is liberated from the
templating bilayer the associated polymer chains of the
polymer network can gain entropy by adopting a threedimensional conformation. To realize this it is unavoidable
that such closed spherical shells shrink. Simultaneously the
thickness of the polymer shell increases considerably. These
considerations qualitatively explain the observed contraction of the pure polymer particles. Nevertheless, it is so far
not fully understood why the polymers preserve their
spherical shape even in the dry state: for polymer hollow
spheres of the given geometry (ratio of diameter to shellAdv. Mater. 1998, 10, No. 16
Fig. 3. a) Scanning electron micrograph of a crosslinked poly(butylmethacrylate) hollow sphere. The length of the bar corresponds to 50 mm [16].
b) Scanning electron micrograph of a hemispherical fragment of a polymer
hollow sphere obtained by shearing the polymer particles (diameter » 250 mm;
shell thickness » 300 nm). The length of the bar corresponds to 50 mm [16].
thickness »1000:1) one would rather expect the system to
collapse like a deflated balloon. A similar contraction under
retention of the shape has, however, also been observed upon
polymerization of giant butadienic lipid vesicles.[18]
The extent of this contraction depends sensitively on the
crosslinking density of the polymer network structures. The
shrinkage increases with increasing crosslinking density,
showing hereby a scaling behavior similar to that known for
branched polymers upon variation of the number of their
branch units.[16] For the highest crosslinking densities the
particles contract to about 1/10 of the original size of the
templating vesicles. Consequently, the shell thickness of the
pure polymer hollow spheres increases by a factor of up to
about 100, i.e., from significantly smaller than 5 nm (which is
the typical overall thickness of the templating lipid bilayers)
in the interior of the membrane to about 200 nm for the pure
polymer.[16]
Although this technique of direct templating of vesicles or
liposomes has scarcely been used up to now in polymer
synthesis, this method offers interesting new possibilities for
the formation of defined nanostructured polymers. While the
overall morphology of the resulting polymers is determined
Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998
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Research News
by the shape and size of the templating vesicles, the polymer
framework can be simply modified by conventional chemical
reactions. Such functionalized nanocapsules can serve, for
example, as containers that are able to selectively bind
certain substances in their interior, which implies their
suitability for application in separation processes or drug
delivery.
±
[1] For a review see, e.g., L. Addadi, S. Weiner, Angew. Chem. Int. Ed. Engl.
1992, 31, 153.
[2] See, e.g., M. Antonietti, R. Basten, S. Lohmann, Macromol. Chem. Phys.
1995, 196, 441. D. M. Anonelli, J. Y. Ying, Curr. Opin. Colloid Interface
Sci. 1996, 1, 523. C. G. Göltner, M. Antonietti, Adv. Mater. 1997, 9, 431.
[3] J. H. Fendler, Membrane-Mimetic Approach to Advanced Materials,
Springer, New York 1994.
[4] H. Ringsdorf, B. Schlarb, J. Venzmer, Angew. Chem. 1988, 100, 117.
[5] V. P. Torchilin, A. L. Klibanov, N. N. Ivanov, H. Ringsdorf, B. Schlarb,
Makromol. Chem. Rapid Commun. 1987, 8, 457.
[6] S. L. Regen, J.-S. Shin, J. Am. Chem. Soc. 1984, 106, 5756.
[7] M. Abe, K. Ogino, H. Yamauchi, in Solubilization in Surfactant
Aggregates (Eds: S. D. Christian, J. F. Scamehorn), Marcel Dekker,
New York 1995.
[8] J. Murtagh, J. K. Thomas, Faraday Discuss. Chem. Soc. 1986, 81, 127.
[9] J. Kurja, R. J. M. Nolte, I. A. Maxwell, A. L. German, Polymer 1993, 34,
2045.
[10] N. Poulain, E. Nakache, A. Pina, G. J. Levesque, J. Polym. Sci. 1996, 34,
729.
[11] M. Winterhalter, K.-H. Klotz, R. Benz, in Electromanipulation of Cells
(Eds: U. Zimmermann, G. A. Neil), CRC Press, Boca Raton, FL 1996,
Ch. 3.
[12] M. Winterhalter, W. Meier, unpublished.
[13] G. Gompper, D. A. Kroll, Curr. Opin. Colloid Interface Sci. 1997, 2, 373.
[14] J. D. Morgan, C. A. Johnson, E. W. Kaler, Langmuir 1997, 13, 6447.
[15] M. Jung, D. H. W. Hubert, P. H. H. Bomans, P. M. Frederik, J. Meuldijk,
A. M. van Herk, H. Fischer, A. L. German, Langmuir 1997, 13, 6877.
[16] J. Hotz, W. Meier, Langmuir 1998, 14, 1031.
[17] E. Brückner, H. Rehage, presented at the ªBunsentagungº, Münster,
Germany, May 1998.
[18] M. Dvolaitzky, M. A. Guedeau-Boudeville, L. LØger, Langmuir 1992, 8,
2595.
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Adv. Mater. 1998, 10, No. 16