Perfil docente para el eficaz desarrollo de las inteligencias múltiples en el aprendizaje

Impact load testing showed that PVA with water content as low as 20% can pro- vide better load damping and shock absorption than UHMWPE, transmitting a lower peak stress upon impact loading [18]. The non-linear concave-up or J-shape stress-strain curves of freeze-thaw PVA hydrogels resemble those of natural tissue in contrast to other synthetic polymers such as UHMWPE [11]. Viscoelastic mechanical behaviour has also been studied through stress relaxation, creep and dynamic mechanical testing. The focus of this section will be on compression properties.

2.4.3.1 Unconfined Compression

In unconfined compression, cylindrical hydrogel samples are typically tested be- tween non-porous platens and allowed to expand laterally while compressed. Investigat- ing the use of Salubria biomaterial for focal cartilage replacement, Stammen et al. [19] tested PVA hydrogels containing 20% and 25% PVA made with 0.9% saline. Samples were equilibrated in water at 37 °C before testing, then compressed to 0.65 strain at strain rates of 100 and 1000%/min, but the authors did not specify if testing was done in air or fluid. Strain rate dependence was apparent at high strains but not at low strains, likely from increasing pressurization with strain. Tangent compressive moduli increased from approximately 2 MPa at 0.3 strain to 20 MPa at 0.65 strain in both hydrogels. Extensibil- ity decreased with increased PVA concentration; the failure stress and strain were 2.1

MPa at 0.6–0.62 strain for the 20% PVA hydrogel and 1.4 MPa at 0.45–0.47 strain for the 25% PVA hydrogel. Strain rate dependence was not found in shear, in which sample volume is conserved, leading to the hypothesis that fluid flow contributed to the compres- sive stress-strain behaviour and strain rate dependence.

Millon et al. [24] tested 10% PVA hydrogels made using 1, 3 and 6 cycles of con- trolled F-T cycling for cartilage replacement or other orthopedic applications. Uncon- fined compression was performed at 1, 10 and 100%/s to 0.45 strain in water at 37 °C. Strain rate dependence was also apparent only at high strains in 6 FTC hydrogels, and a statistical difference in stress at 0.45 strain was only found between strain rates of 1 and 100%/s, which was 0.13–0.16 MPa. Wang and Campbell [23] produced 3–40 wt% PVA hydrogels using the same F-T conditions as Millon et al. Cylindrical sample dimensions were approximated to lumbar intervertebral disc dimensions, which are approximately 4 cm in diameter and 1 cm in height [135]. Unconfined compression was performed at 3 mm/min to approximately 0.25 strain. Tangent moduli was lowest in 3 FTC-3% PVA and highest for 6 FTC-40% PVA, increasing with both number of FTC and PVA concen- tration.

Joshi et al. found that tangent compressive moduli of 6 FTC PVA hydrogels con- taining 84% water and 1–5% PVP did not vary with PVP concentration [58]. Compres- sive moduli were also determined after fatigue cyclic loading to 0.15 strain for up to 10 million cycles at 5 Hz. Tangent modulus decreased by 24% at 0.15 strain, but no change was found at 0.2 and 0.25 strain. Permanent deformation of the hydrogels resulted in a 5% increase in diameter and 17% decrease in height. In Holloway et al., PVA hydrogels containing 1 wt% PVP and 10 to 35 wt% total polymer were compressed at a rate of 100%/min to 0.15 strain in phosphate buffered saline (PBS) at 37 °C [116]. Compressive modulus in the linear portion of the stress-strain curve, between 0.01 and 0.05 strain, did not increase beyond 5 FTCs, and increased with polymer concentration.

Compressive moduli from the above studies are listed in Table 2.4. There are dis- crepancies in describing compressive moduli due to the non-linearity of stress-strain curves and strain rate dependence of PVA hydrogels. Tangent moduli vary to a large de- gree depending on the strain at which they were calculated, and strain rate may have an effect on the apparent stiffness. Using only one portion of the stress-strain curve would not provide a complete representation of hydrogel behaviour. Also, hydrogel composi- tion, F-T cycling parameters, and testing conditions such as temperature and immersion in fluid all influence the compressive behaviour of PVA hydrogels.

Table 2.4: Compressive moduli from unconfined compression of freeze-thaw PVA hy- drogels in the literature.

Study PVA [%] Sol- vent Additive FTC Test

environment Strain rate

Modulus [MPa] (strain)

Stammen

2001 [19] 20, 25 0.9% saline – Unknown Water, 37 °C, equilibrated prior to testing

100,

1000%/min Tangent: 2.08–3.02 (0.3), 16.51–19.06 (0.6) Millon

2009 [24] 10 Water – 1, 3, 6 Water, 37 °C 100%/s Tangent: 0.039–1.18 (0.45)

Wang 2009 [23]

3–40 Water _– 1, 3, 6 Water, 37 °C 3 mm/min Tangent:

0.001–1.303 (0.05), 0.001–2.117 (0.20) Joshi 2006 [58] 11–15 Water 1–5% PVP 6 PBS, 37 °C 100%/min Tangent: 0.23 (0.15), 0.4 (0.25) Holloway

2011 [116] 10–35 Water 1% PVP 6 PBS, 37 °C 100%/min Linear: 0.070–0.801 (0.01–0.05)

2.4.3.2 Stress Relaxation, Creep and Dynamic Mechanical Properties

Viscoelastic properties such as stress relaxation, creep and viscous damping may be desirable in a biomaterial for IVD applications, since these properties are found in the natural IVD. PVA hydrogels also display viscoelastic behaviour. In stress relaxation tests, 25% PVA hydrogels decreased to 45% of the initial stress after 24 hours at 0.2 compressive strain [19], while 10% PVA also relaxed to approximately 45% of the initial stress after 1 hour at 0.45 strain [24]. Wang and Campbell [23] performed stress relaxa- tion for only 30 seconds at 0.25 strain, and the degree of relaxation attained was less than 20% for all compositions. Their study found that rate of stress relaxation increased with

the stiffness of the material, increasing with both polymer concentration and number of FTCs. The degree of stress relaxation was found to decrease with increasing polymer concentration in PVA hydrogels by Kobayashi [136]. However, the degrees of stress re- laxation in PVA hydrogels are much lower than in the natural IVD, which decreases to less than 10% of the initial stress after only 30 minutes [55]. Compressive stress relaxa- tion data was successfully fitted to a double exponential decay function by both Millon et al. [24] and Wang and Campbell [23], as was previously done for tensile stress relaxation [11, 13].

Stauffer and Peppas [114] examined creep in 15 wt% PVA with 2–5 FTCs by ap- plying stress for 15 s. However, only final strain was plotted against applied stress, and creep curves were not shown. Final strains of 2 FTC hydrogels were 0.10 to 0.18, in- creasing linearly with applied stress up to 3.5 MPa, while over the range of applied stress, 5 FTC hydrogels had approximately the same final strain of 0.085. Wang and Campbell [23] applied a compressive stress of approximately 0.2 MPa for 30 s in their creep tests. The increase in strain during creep was found to be highest in hydrogels with low poly- mer concentrations. For 15–40 wt% PVA hydrogels with 6 FTCs, the increase in strain ranged from 1.8 to 2.5%, but neither the initial nor final strain was specified. Creep data in this study was fitted with the three-parameter-solid model used in IVD creep.

PVA hydrogels were simulated by a poro-viscoelastic model consisting of the three-parameter-solid model for the solid phase and a fluid phase that can be exuded dur- ing creep [137]. Creep strains of 0.1–0.2 were obtained experimentally for hydrogels containing 40–60% PVA after 8 hours at 0.5 MPa stress, and both initial and final strains decreased with increasing PVA concentration. The poro-viscoelastic model successfully modelled creep behaviour in these hydrogels. The solid polymer phase and flow of the fluid phase both likely contributed to viscoelastic properties.

Dynamic properties of PVA hydrogels have been investigated in shear. The stor- age modulus tended to increase with increasing frequency of loading, but this effect ap- peared to be small for frequencies up to 10 Hz [29, 138], and decreased with increasing

cyclic strains, which may have been due to polymer deformation that was not immedi- ately recovered between loading cycles [119]. Tan δ decreased with increased loading frequency, indicating more elastic behaviour with faster loading [29]. PVA hydrogels can be characterized as highly elastic with tan δ values on the order of 0.01, compared to at least 0.1 in rubber, since the large proportion of water in hydrogels results in low inter- nal friction [139]. By increasing PVA concentration from 15% to 20% in hydrogels con- taining 3% HA, shear storage modulus increased while loss modulus remained similar, thus decreasing tan δ [29]. This indicates an increase in elastic behaviour with higher polymer content.

In general, mechanical properties are dependent on the structure of PVA hy- drogels, as stiffness increases with polymer concentration and F-T cycling. Viscoelastic properties correlate with polymer concentration and modulus of the hydrogels. Under compression, the role of water flow and permeability also appear to be important in load bearing, strain rate dependence, stress relaxation and creep in PVA hydrogels.