Biomedical Engineering Reference
In-Depth Information
O
O
R
HO
OH
O
O
O
O
O
O
O
O
R
*
*
n
(a)
O
O
H
2
O
O
O
O
O
R
*
*
n
O
O
R
HO
OH
O
O
HO
OH
HO
OH
OH
H
2
O
HO
OH
(b)
O
1.13
POE
II
: (a) synthesis and (b) hydrolysis.
can be controlled by use of stoichiometry. Hydrolysis initially produces
neutral fragments that do not affect degradation, so there are no autocatalytic
effects. By using alcohols with more than two functional groups (triols, and
so on) it is possible to obtain crosslinked matrices which completely degrade
in small water soluble fragments. POe ii polymers are hydrophobic, water
uptake is limited and thus in physiological conditions they are very stable.
since ortho ester linkages are affected by acids but are stable in base, by
adding a basic compound such as Mg(OH)
2
hydrolysis can be controlled even
if the matrix is in an aqueous environment; Mg(OH)
2
stabilizes the bulk of
the matrix and erosion occurs only on the surface where the base has been
eluted or neutralized. Moreover, mechanical and thermal properties can be
adjusted by using diols with different chain flexibility.
POe iii polymers are synthesized with a triol (whose functional groups
are placed on the first, the second, and the last carbon atom) and a 1,1,1-
triethoxy compound (Fig. 1.14). When 1,2,6-hexanetriol is employed, the
polymer is semisolid at room temperature. Drugs can be incorporated by
simply mixing and the material can be injected. Degradation also occurs
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