Biomedical Engineering Reference
In-Depth Information
The hydrogel prepared from scleraldehyde with a low degree of oxidation by
crosslinking with diamines can be represented by a network composed of ran-
domly oriented triple helices interlinked at the sites where the aldehyde groups
are present [185] .
The fi rst industrial application of scleroglucan was in oil recovery, where it
showed better pH and temperature stability than xanthan. In watered-out reser-
voirs, where seawater pressure is no longer suffi cient to recover the oil, the addi-
tion of scleroglucan to improve the viscosity of the feed water can improve the
process signifi cantly. Additionally, scleroglucan lubricates the drill and controls
the backpressures created during drilling [179]. In the fi eld of pharmaceutics,
numerous studies are published on applications of scleroglucan both in its native
form and as derivatives. Hydrogels obtained by different crosslinking agents are
suitable for a release modulation from various dosage forms. Sustained release
and environment-controlled delivery systems that can be obtained from crosslinked
scleroglucan represent a challenging fi eld of applications [186]. There are numer-
ous additional applications using the superb rheological properties and the high
stability of scleroglucan ranging from food and cosmetics to paints and ceramic
glazes. In most of these cases, scleroglucan is a competitor of xanthan [187].
7.13
Xanthan
Xanthan is a polysaccharide with a cellulose backbone of
4) linked d - glucose.
Every second glucose is substituted at position 3 with a side chain consisting
of
β
- (1
→
- d - mannose. The terminal
mannose moiety is partially substituted with a pyruvate, coupled as an acetal at
positions 4 and 6. The internal mannose usually bears an acetate group at position
6. The degree of pyruvate substitution varies between 30% and 40%, whereas
60-70% of the internal mannose units are acetylated. The molecular weight of
xanthan is about 1,000,000 g mol
− 1
(Scheme 7.13 ).
Because of the glucuronic acid moieties and the partial pyruvate substituents in
the side chain, xanthan is an ionic polysaccharide and its polyelectrolytic character
in water was studied in detail [188].
The polymer is produced by the fermentation with the bacteria
Xanthomonas
campestris
[189] ,
Xanthomonas phaseoli
[190] , and
Xanthomonas jugIandis
[191] and
other
Xanthomonas
species with an annual production of approximately 30,000
tons. The pyruvyl and acetyl content of xanthan isolates depends on the fermenta-
tion conditions and bacterial strain [192]. As shown for
X. campestris
, the produc-
tion, composition, and viscosity of the xanthan synthesized by this strain are
infl uenced by the fermentation time and nutrient exhaustion in batch culture and
by the dilution rate in continuous culture. The specifi c rate of xanthan synthesis
is maximal during exponential growth, although some xanthan is also formed
during the stationary phase [189]. Under optimized conditions, a production of up
to 22 g/L xanthan can be reached in a stirred tank fermentor [193]. As the product
β
- d - mannose - (1
→
4) -
β
- d - glucuronic
acid - (1
→
2) -
α
