Biomedical Engineering Reference
In-Depth Information
acetic anhydride selectivity. Green and co-workers (2000) prepared silicon-containing
composites from starch by adding SiCl 4 using supercritical CO 2 as the carrier medium.
Characterization of the resulting composites showed that 12-15% silicon, in the form of
SiO 2 , was homogeneously dispersed throughout the starch matrix.
Duarte and co-workers (2009a) used supercritical fluid technology to prepare starch-
based, porous matrices for use as scaffolds for tissue engineering. This technique (referred
to as immersion precipitation) involves extracting polymer solutions in organic solvents
with supercritical CO 2 . Since the solvent chosen to dissolve the polymer is soluble in
supercritical CO 2 , whereas supercritical CO 2 is a non-solvent for the polymer itself, a dry,
porous polymer structure is formed in the process. The polymers used in this study were
commercial blends of starch and poly-L-lactic acid (50:50 and 30:70 by weight); the solvents
used were dichloromethane and chloroform. Process variables such as polymer concentration,
temperature, and CO 2 pressure were examined. This technique was also used to prepare
porous starch-poly(L-lactic acid) composite scaffolds loaded with inorganic particles
(Bioglass) for use in bone tissue engineering (Duarte et al ., 2009 b). A similar method was
used to obtain a dry, porous structure from a commercial 30:70 wt-% blend of corn starch
and poly(
-caprolactone) that was dissolved in chloroform at a concentration of 15% (Duarte
et al ., 2010). The effect of pressure (80-150 bar) and temperature (45 and 55 °C) on the
morphologies of the scaffolds was studied by SEM and micro-computed tomography.
Mechanical properties of the scaffolds were also studied, as well as the adhesion, morphology,
viability and proliferation of cells.
A biomass conversion system using supercritical water was developed by Saka and
Ueno (1999) for the hydrolysis of cellulose and starch to glucose and levoglucosan. The
treatment temperature was 500 °C and the pressure 35 MPa. Water in this supercritical
state can function as an acid and thus hydrolyze the polysaccharides to produce low
molecular weight sugars. Although the major subject of this study was cellulose, corn
starch, which is easier to hydrolyze, was examined for comparison. Cellulose was liquefied
by the supercritical water treatment in about ten seconds, whereas starch was liquefied in
about five seconds. Glucose yields from cellulose II, cellulose I and starch were 48%,
32% and 33%, respectively. Water under supercritical conditions has also been used to
degrade biomass into gaseous products such as hydrogen, CO 2 , and hydrocarbons such as
methane. This process has been commonly referred to as gasification and is receiving
increased attention due to the current interest in the use of hydrogen as a fuel. Antal and
co-workers (2000) used this technique to treat biomass feedstocks including corn starch
and potato starch gels, wood sawdust suspended in a corn starch gel, and potato wastes.
The organic material vaporized when the feedstock was rapidly heated in a tubular flow
reactor to temperatures above 650 °C at pressures above the critical pressure of water; a
packed bed of carbon in the reactor catalyzed the gasification reaction. The gaseous
mixture formed in the process was composed of hydrogen, CO 2 , methane, carbon
monoxide, and traces of ethane; the composition of the mixture was influenced by the
peak temperature of the reactor and the condition of the reactor wall. High yields of gas
(>2 l/g) with a high content of hydrogen (57 mol-%) were observed at the highest
temperatures. A thermodynamic model was developed by Tang and Kitagawa (2005) to
predict the equilibrium composition of product mixtures. D'Jesús and co-workers (2005)
studied the effect of KHCO 3 addition, particle diameter, and the dry matter content of
aqueous dispersions on the gasification of corn starch and corn silage with supercritical
water. Treatments producing hydrogen, CO 2 , and hydrocarbons ranging from CH 4 to
C 4 H 10 were carried out at a maximum temperature of 700 °C. In a subsequent study
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