Biomedical Engineering Reference
In-Depth Information
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structure. Based on their ionic charges, hydrogels may be
classified (
Ratner and Hoffman, 1976; Brannon-Peppas
and Harland, 1990
) as (i) neutral hydrogels, (ii) anionic
hydrogels, (iii) cationic hydrogels, or (iv) ampholytic
hydrogels. Based on physical structural features of the
system, they can be classified as (i) amorphous hydrogels,
(ii) semicrystalline hydrogels, or (iii) hydrogen-bonded
or complexation structures. In amorphous hydrogels, the
macromolecular chains are arranged randomly. Semi-
crystalline hydrogels are characterized by dense regions
of ordered macromolecular chains (crystallites). Finally,
hydrogen bonds and complexation structures may be
responsible for the three-dimensional structure formed.
Structural evaluation of hydrogels reveals that ideal
networks are only rarely observed.
Figure 3.2.5-1A
shows
an ideal macromolecular network (hydrogel) indicating
tetrafunctional cross-links (junctions) produced by co-
valent bonds. However, in real networks it is possible to
encounter multifunctional junctions (
Fig. 3.2.5-1B
)or
physical molecular entanglements (
Fig. 3.2.5-1C
) playing
the role of semipermanent junctions. Hydrogels with
molecular defects are always possible.
Figures 3.2.5-1D
and 3.2.5-1E indicate two such effects: unreacted func-
tionalities with partial entanglements (
Fig. 3.2.5-1D
) and
chain loops (
Fig. 3.2.5-1E
). Neither of these effects
contributes to the mechanical or physical properties of
a polymer network.
The terms ''cross-link,'' ''junction,'' or ''tie-point'' (an
open circle symbol in
Fig. 3.2.5-1D
) indicate the con-
nection points of several chains. These junctions may be
carbon atoms, but they are usually small chemical bridges
[e.g., an acetal bridge in the case of cross-linked poly-
(vinyl alcohol)] with molecular weights much smaller
than those of the cross-linked polymer chains. In other
situations, a junction may be an association of macro-
molecular chains caused by van der Waals forces, as in the
case of the glycoproteinic network structure of natural
mucus, or an aggregate formed by hydrogen bonds, as in
the case of aged microgels formed in polymer solutions.
Finally, the network structure may include effective
junctions that can be either simple physical entangle-
ments of permanent or semipermanent nature, or or-
dered chains forming crystallites. Thus, the junctions
3.2.5 Hydrogels
Nicholas A. Peppas
Hydrogels are water-swollen, cross-linked polymeric
structures containing either covalent bonds produced by
the simple reaction of one or more comonomers, physical
cross-links from entanglements, association bonds such
as hydrogen bonds or strong van der Waals interactions
between chains (
Peppas, 1987
), or crystallites bringing
together two or more macromolecular chains (
Hickey
and Peppas, 1995
). Hydrogels have received significant
attention because of their exceptional promise in bio-
medical applications. The classic topic by
Andrade
(1976)
offers some of the best work that was available
prior to 1975. The more recent topic and other reviews
by
Peppas (1987, 2001)
addresses the preparation,
structure, and characterization of hydrogels.
Here, we concentrate on some features of the prepa-
ration of hydrogels, as well as characteristics of their
structure and chemical and physical properties.
Classification and basic structure
Depending on their method of preparation, ionic charge,
or physical structure features, hydrogels maybe classified
in several categories. Based on the method of prepara-
tion, they may be (i) homopolymer hydrogels, (ii) co-
polymer hydrogels, (iii) multipolymer hydrogels, or
(iv) interpenetrating polymeric hydrogels. Homopoly-
mer hydrogels are cross-linked networks of one type of
hydrophilic monomer unit, whereas copolymer hydro-
gels are produced by cross-linking of two comonomer
units, at least one of which must be hydrophilic to render
them swellable. Multipolymer hydrogels are produced
from three or more comonomers reacting together (see
e.g.,
Lowman and Peppas, 1997
, 1999). Finally, inter-
penetrating polymeric hydrogels are produced by pre-
paring a first network that is then swollen in a monomer.
The latter reacts to form a second intermeshing network
