Biomedical Engineering Reference
In-Depth Information
In a recent study, the preparation of crosslinked carrageenan beads as a control-
led release delivery system was reported. Since
- carrageenan just allowed thermo-
reversible gels, a protocol for an additional crosslinking using epichlorohydrin was
introduced. Low epichlorohydrin concentrations led to unstable and weak beads
with uneven and cracked surfaces. An optimized crosslinker concentration resulted
in smooth and stable gel beads that showed great potential for the application as
delivery systems in food or pharmaceutical products [47].
κ
7.4
Cellulose and Its Derivatives
Cellulose was fi rst described by Anselme Payen in 1838 as a residue that was
obtained after aqueous extraction with ammonia and acid-treatment of plant
tissues [48]. It is a carbohydrate polymer composed of
4) - linked d - glucose.
Cellulose is one of the most common polymers because it is ubiquitous in the
biomass. Its chain length depends on the origin and the treatment of the polymer.
The biosynthesis of cellulose has been described in numerous reviews [49]. Besides
plants as polymer source, it can also be obtained from bacterial production (see
Chapter 5) or from
in vitro
synthesis. Cellulose can be produced either by enzy-
matic polymerization of
β
- (1
→
- cellobiosyl fl uoride monomers or by chemical synthesis,
for example, by cationic ring-opening polymerization of glucose orthoesters. These
approaches are summarized in a review from Kobayashi
et al.
[50] .
The crystal structure of cellulose has been studied intensively. Two modifi ca-
tions of cellulose I were discovered, varying in the character of their elementary
cell, which is either triclinic or monoclinic. Cellulose II is the thermodynamically
most stable structure. More solid and liquid state crystal structures of cellulose
and the fi brillar morphology of the polymer are summarized in the review from
Klemm
et al
. [51] .
Due to its numerous hydrogen bonds, cellulose is insoluble in nearly all common
solvents [52]. For this reason, several cellulose solvent systems have been explored
to enable its chemical modifi cation. LiCl-dimethylacetamide mixtures as well as
tetrabutylammonium fl uoride in dimethyl sulfoxide or metal containing solvents,
for example, cuprammonium hydroxide, have been investigated [53]. Several
chemical derivatizations of cellulose can be realized in order to use cellulose as
drug deliverer or for other medical applications. An overview of these modifi able
functional groups is given in Scheme 7.4. For instance, cellulose can be oxidized
at different positions as well as esterifi ed or alkylated at the primary hydroxyl
group. Especially the last mentioned derivatizations lead to water- and/or organic-
soluble compounds, which can be used for further modifi cations.
The secondary alcohol groups of cellulose can be oxidized to ketones, aldehydes,
or carboxylic functions depending on the reaction conditions. The product is called
oxycellulose and represents an important class of biocompatible and bioresorbable
polymers which is widely used in medical applications. It is known to be hemo-
static, enterosorbent, and wound-healing. Furthermore, oxycellulose can be used
β
