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MICROENCAPSULATION: FUNDAMENTALS, METHODS
AND APPLICATIONS
DENIS PONCELET
ENITIAA, Rue de la Géraudière BP 8225, 44322 Nantes Cedex 3, France,
e-mail: poncelet@enitiaa-nantes.fr
Abstract. Microencapsulation is widely use in industry but remains relatively
unknown from the public. The reason is that microcapsules are not an end-
product, but generally a technique to overcome process limitations. Micro-
encapsulation allows immobilization, protection, release and functionalisation of
active ingredients. Despite the high diversity of methods, this paper proposes a
classification and description of the main technologies to produce microcapsules.
Keywords: microencapsulation, immobilization, controle release
1. Introduction
In the last few years, one could see the development of commercial products
based on microcapsules. However, microencapsulation has been widely used in
industry for several decades. The principle of encapsulation is very old. If
biochemistry is a principle of life, nothing would have been possible without its
integration in membrane bound structures (cells, mitochondria...). Without
immobilization and spatial organization of biochemical reactions in an internal
volume and through the membrane would not be possible. The high efficiency
of, for example ATP production, would not be possible.
Figure 1. Multi-core microcapsules mimic biological
cells and are sometimes called artificial cells.
(Coletica®)
23
–
J.P. Blitz and V.M. Gun’ko (eds.), Surface Chemistry in Biomedical and Environmental Science, 23 34.
©2006 Springer.
24 DENIS PONCELET
By developing encapsulation methods, scientists and engineers mimic nature
to obtain innovative structures to isolate, protect, release and functionalize active
1
ingredients. However nature is not so easy to mimic, and what humans have
developed are still inferior to what biological cells offer.
Encapsulation is used in many industrial and scientific domains. It is not
surprising to find then diverse definitions and terminology, often directed to a
specific field. However, a generic and functional definition could be
“Entrapment of a compound or a system inside a dispersed material for its
immobilization, protection, controlled release, structuration and functiona-
lization.”
This definition is more oriented to objectives than on the structure of the
microcapsules. It includes a very large number of systems starting from hollow
molecules such as cyclodextrin, to large solid microsphers of 2 to 3 mm. It
proposes a product-oriented approach, a solution that limits debate around
terminologies.
If we look a little more deeply into this definition, the first question is which
type of system could we encapsulate? This could range from small molecules
(some try to encapsule water) to quite complex ones (peptides, drug, DNA). It
could be a mix of these molecules, or complex structures like viruses,
protoplasts or even complete biological cells. Inside the capsules, the active
system could be in the form of a solution, a suspension or an emulsion.
Which type of structures could represent microcapsules? The “true”
microcapsule is a liquid core surrounded by a membrane. However, many
different structures are included under the term “microcapsules” or “nano-
capsules” (Figure 2). At the smallest scale, one could use hollow molecules inside
of which the active ingredient could be fixed. At a larger scale, more or less
complex molecular assemblies could form nanocapsules, or nanospheres, or
lipidic structures like liposomes. For sizes less than a few micrometers, one talks
of nanoencapsulation. For larger sizes, one finds hydrogel beads, solid
microspheres, and microcapsules. For sizes greater than 1 mm, some talk about
macroencapsulation. Encapsulation could also include agglomeration of fine
particles or the coating of solid particles. Finally, some include emulsions if they
are stable enough to fit the above definition.
Figure 2. Examples of microcapsule structures.
MICROENCAPSULATION OVERVIEW 25
Parallel to the structural complexity, a large number of technologies exist to
produce microcapsules, which is a field unto itself.
2. Why Encapsulation?
Since encapsulation is costly, the requirement must first be justified. We can
classify five categories for the objectives of encapsulation.
• Immobilization or entrapment. To limit contact between certain parts of a
system. If some ingredient must be separated, encapsulation of this
ingredient and release only upon rupture of the microcapsules fills this
objective. The entrapment of a flavor could create a sustained aromatic
effect, or to control the release at a specific time (such as during cooking).
Immobilization of batteries or enzymes allows continuous processing while
avoiding washout.
• Protection. If some ingredients are fragile and need to be protected from
their environment. For example, vitamins or polyunsaturated fatty acids are
denaturized by oxygen. Many biological cells are sensitive to shear. Some
drugs and probiotics are destroyed during gastric transit. When incorporated
in microcapsules, all these systems will be protected to some extent against
the chemical, physicochemical and mechanical environmental conditions.
However, the problem may be reversed. Incorporation of iron in food
promotes oxidation of fatty acids. A number of industrial additives may
reduce the performance of the material itself. In this case, it is more efficient
to encapsulate the minor ingredients (iron, additives). Encapsulation could
then be used to protect the environment from the use of some products. Most
industrial enzymes are sold in an encapsulated form to avoid allergic and
professional health problems.
• Controlled release. For practical use the active ingredient must be
released. A drug must be delivered with well defined kinetics. Sometimes it
is not the encapsulated ingredient that is released but a by-product. This is
the case when the encapsulated product is an enzyme or a catalyst.
Encapsulation may have the objective to limit release, but in some cases to
make it more rapidly available. A typical example is an instant powder
consisting of aggregates made of fine particles that are insoluble, in a very
soluble matrix.
• Structuration. Homogeneous mixing of a small liquid volume with a high
volume of powder constitutes a real challenge. Microencapsulation allows
converting this liquid in powder and facilitating this operation. dosage forms
for pharmacy applications are readily obtained by microencapsulation. By
26 DENIS PONCELET
coating brown sugar, a quite aggregative powder, with crystalline sugar,
one gets a flowing powder.
• Functionalisation. Finally, microencapsulation may be used to develop
new functions such as regulating biocatalyst activity by controlling the
membrane permeability through pH changes. Microcapsules may also offer
a marketing function such as giving specific “metallic” aspects to functional
food to differentiate them from food and medication.
The diversity of applications is very broad and even microencapsulation is
already largely used in industry, one could expect a strong development in the
next decade.
3. How to Make Capsules
Many applications from a variety of fields for diverse objectives have led to
many methods of encapsulation. Moreover, terminology varies from domain to
domain. The same technology may have different names in different fields.
Figure 3 tries to offer an approach where most technologies fit in an
unambiguous way.
Step Active Incorporation in microcapsule core
1 In liquid in solid
(solution, melting, emulsion, suspension) (agglomeration,
absorption ...)
Mechanical & engineering
liquid in air Dispersion Agitation +
2 liquid in liquid Spraying
Prilling Spraying Emulsion Coating/agglomeration
Microemulsion
1 2 3 4 5
Stabilisation
3 Polymerisation Coacervation Solidification
Gelification Coalescence Evaporation (drying)
Chemistry Physicochemistry Physics
Figure 3. Technologies of encapsulation.
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