Biomedical Engineering Reference
In-Depth Information
view based on primary and secondary crystallization concepts, with the initial step
being the formation of the double-helical structure of the self-aggregates of s-PMMA.
It is not possible at present to further discriminate between these approaches. Finally,
gelation of atactic PMMA (a-PMMA) in 1-butanol and cyclohexanol (Vandeweerdt
et al., 1991 ) can be explained by liquid
-
liquid demixing interfering with a glass
transition, similar to the case of aPS in TD.
8.5
Cryogels of poly(vinyl alcohol) (PVA)
8.5.1
Mechanisms of gel formation
PVA is a water-soluble synthetic polymer with a high degree of hydrolysis (98
99%) and
is considered to be atactic. Thermoreversible PVA physical cryogels, prepared by
repeated cycles of freezing and thawing of an aqueous solution of the polymer, have
captured the attention of both academic and industrial researchers because of their
potential applications in many
-
fields (Inoue, 1975 ; Peppas, 1975 ). Because they are
biocompatible, PVA hydrogels are suitable for a variety of biomedical and pharmaceut-
ical applications (e.g. arti
cial tissues, contact lenses, controlled-release devices for drug
delivery). Freeze/thaw PVA hydrogels show high mechanical strength and good elastic
properties, since they can endure large deformations upon stretching, and recover their
original shape and dimensions on release of this tension.
PVA can be dissolved in water at high temperature (96°C) but, when cooled at room
temperature, solutions (c ~ 10% w/w) do not gel and remain transparent when left in a
sealed tubes for more than 1 month (Ricciardi et al., 2005 ). Elastic properties of cryogel
films are obtained only by subjecting the polymer aqueous solutions to several repeated
freeze/thaw cycles, consisting of 20 h freezing steps at
22°C followed by 4 h thawing
steps at room temperature. PVA cryogels undergo syneresis upon application of repeated
freeze/thaw cycles, expelling some water on their surface. However, they acquire good
mechanical properties, keep a high water content, are stable at room temperature and can
retain their original shape. The elasticity and tenacity of the gel increases with the number
N c of cycles, as indicated in the stress
strain curves in Figure 8.16 . With increasing N c ,
the original cryogel (with a thickness of 1.5 mm) becomes more turbid and almost non-
transparent above N c = 3, suggesting the development of a microphase separated struc-
ture (Yokoyama et al., 1986 ). In parallel with changes of elasticity, crystalline re
-
ections
become more distinct with N c .
Traces of ice crystal growth are left as pores in the cryogel, as suggested by scanning
electron micrograph of a xerogel (solvent-free gel) obtained from 5% w/w cryogel,
shown in Figure 8.17 .
It is seen that the pores, of order 10 μm, are linked together linearly, and it was
established that they are oriented along a direction nearly normal to the freezer plate.
Under freezing, the ice crystals grow along the direction of temperature gradient, the
PVA-rich solution phases being segregated around them, and the gelation proceeds in the
segregated solution phases, which then form the continuous porous gel skeleton.
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