Biomedical Engineering Reference
In-Depth Information
An alternative method of hydrogel preparation without using toxic additives is the freezing-
thawing process in cycles repeated several times [26]. In this process, aqueous solution of PVA is
frozen at
2 and then thawed at room temperature, resulting in the formation of crystal-
lites [27]. Such technological parameters as (i) number of freezing-thawing cycles, (ii) time of each
operation, and also (iii) molecular weight and concentration of PVA determine the fi nal properties of
produced cryogels [28,29]. By cycling freezing-thawing process, the Young's modulus is signifi cantly
increased (up to 20 MPa) [30]. Moreover, after this process, the hydrogel is more thermally stable.
Ku et al. [31] described one of the methods of making the PVA cryogel in detail. In short, about
25% (by weight) aqueous solution of PVA (with
M
n
of above 70,000) was subjected to a series of
freezing (
−
20°C
±
20°C)-thawing cycles, increasing the strength with each cycle. Then the samples were
submerged in a deionized water bath and equilibrated at 37°C. Stammen et al. [32] and Swieszkowski
et al. [33] investigated the mechanical properties of this type of hydrogel at 37°C. Cylindri-
cal cryogel samples were compressed under displacement control at a rate of 508 mm/min until
35% strain was reached. Additionally, the material was tested in tension at a rate of 508 mm/min
until failure. The failure strain of 226% was measured for such a material. Average tensile stress
at failure was about 5 MPa. Unlike many other gels, the PVA cryogel did not soften or swell substan-
tially at the body temperature.
To evaluate an elastic PVA implant for AC replacement, a numerical analysis was performed
using fi nite element method (FEM) [33]. The objective of this study was to investigate the mechani-
cal response of a PVA hydrogel as used for the articular surface of the glenoid component of a total
shoulder arthroplasty. Based on numerical analyses, it was found that replacement of polyethylene
with the hydrogel layer results in the signifi cant reduction in the von Mises (Figure 21.5) and con-
tact stresses together with the growth in contact area. The lower stresses in turn promote fl uid fi lm
lubrication and reduce the wear and implant failure. This study showed a high potential of using
PVA for articular surface in glenoid component.
Recently, Grant et al. [34] proposed an improvement of the PVA hydrogel biocompatibility by
treating its surface with HA to mimic the viscous lubrication layer observed on AC. Such a treat-
ment results in elastic modulus and friction coeffi cient similar to that reported for AC. Further
improvement could be done by the addition of a lipid layer to the PVA hydrogel.
To improve the mechanical strength of hydrogels, multifunctional carboxylic acids taken from
a large group of biologically active compounds present in metabolic pathway of chemical reactions
−
S, Mises
(Ave. Crit.: 75%)
+1.215e
10
0
+1.119e
×
10
0
+1.022e
×
10
0
+9.256e
×
10
−
1
+8.291e
×
10
−
1
+7.327e
×
10
−
1
+6.362e
×
10
−
1
+5.397e
×
10
−
1
+4.433e
×
10
−
1
+3.468e
×
10
−
1
+2.503e
×
10
−
1
+1.539e
×
10
−
1
+5.742e
×
10
−
2
×
FIGURE 21.5
von Mises stresses for PVA-c layer with thickness of 2 mm.
