Biomedical Engineering Reference
In-Depth Information
not free H
2
O
2
. In this aspect, a high standard of engineering work is
needed to create a microsphere shell that acts as a barrier between
the cells and H
2
O
2
and can immediately decompose the H
2
O
2
once
released to avoidpotential leakage.
The decomposition rate of H
2
O
2
is well known theoretically to
be affected by several parameters such as a change of tempera-
ture, a change of pH, or the existence of impurities that act as a
catalyst.
49
,
50
These affecting variables can be utilized as smart con-
trolling keys for the releasing rate of oxygen once incorporated
within the microsphere. For instance, the use of different catalysts
and the amount added can vary the decomposition rate of H
2
O
2
,
and when these parameters are fixed, a known, controlled releas-
ing rate can be achieved. A more advanced example will be the use
of pH changes from the surrounding to induce the decomposition
rate. This is made possible by first incorporating a pH-sensitive cat-
alyst to the shell of microspheres. Some catalysts lose their activity
oncetransformedintodifferentformatssuchasinacomplexedform
but will resume their activity when free, back to their initial condi-
tion by some physical factors. As mentioned earlier, the acidity of
the cell environment increases under anaerobic conditions due to
the formation of lactic acid. The drop of pH can dynamically induce
dissociation of the complexed catalysts into their free form having
normal activity. When this occurs, H
2
O
2
will be decomposed to gen-
erate more oxygen for the cells' need. As this is a responsive mech-
anism, the level of oxygen can be maintained dynamically at the
required level, notover orunder dosage.
32.3.2
Materials as Building Blocks of Microspheres
In developing smart microspheres, there is a diverse selection of
choices in materials readily to be employed, depending on the
objective, suitability, and intended applications. Different kinds of
materials such as organic (synthetic and biologically derived or nat-
urally obtained) and inorganic (ceramic based) have been investi-
gated and well reported in comprehensive reviews.
1
,
2
,
51
In general,
biologically derived materials have excellent physiological activities
suchasselectivecelladhesion,biodegradability,andlowriskofviral
infection,antigenicity,ordeterioration.Ontheotherhand,synthetic
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