Formato de observación

The natural IVD experiences stress relaxation and creep due to fluid flow from the disc after load application. This prevents instantaneous decrease in disc height [178], and provides energy dissipation, shock absorption and even stress distribution to the ver- tebrae after load is applied [227]. Stress relaxation is the decrease in stress over time by holding a specimen at constant strain, whereas creep is the deformation of a material over time under load. Both tests are used to assess viscoelasticity of polymers since viscous flow results in time-dependent response to deformation and stress [228]. However, in IVDs [55, 64] and hydrogels [137], the fluid flow under load plays a role in reducing fluid pressure and increasing compressive deformation through volume reduction. The degree of stress relaxation and creep are examined to determine the suitability of PVA and PVA-NC as a replacement material in comparison to the natural IVD.

4.3.1 Stress Relaxation

As PVA concentration increased, the degree of stress relaxation at 0.25 strain de- creased (Figure 4.17). Hydrogels with higher PVA concentrations had decreased water content, which could have resulted in decreased viscous behaviour. The addition of La- ponite resulted in an increased degree of stress relaxation after one hour when compared to the unfilled 10% PVA control, as shown in Figure 4.18. BC and pBC addition also resulted in increased stress relaxation (Figure 4.19) compared to 10% PVA. The amount of BC or pBC added appears to affect the degree of stress relaxation in the hydrogels – the addition of 0.48% BC had the highest degree of relaxation while 0.25% pBC had the lowest.

Figure 4.17: Stress relaxation at 0.25 strain of unfilled PVA hydrogels in PBS. Increased PVA concentration resulted in decreased stress relaxation.

Figure 4.18: Stress relaxation at 0.25 strain of Laponite-containing 10% PVA-NC hy- drogels in PBS in comparison to 10% PVA. Addition of Laponite resulted in increased stress relaxation.

Figure 4.19: Stress relaxation at 0.25 strain of BC and pBC-containing 10% PVA-NC hydrogels in PBS compared to 10% PVA. Addition of BC and pBC resulted in increased stress relaxation.

After aging in PBS, the amount of stress relaxation decreased in unfilled PVA hy- drogels and did not differ in Laponite, BC and pBC-filled hydrogels (Figure 4.20). The decrease in stress relaxation with aging was most apparent in 15% and 20% PVA hy- drogels.

4.3.2 Creep

Creep data were fitted to the viscoelastic models described in Section 3.4.3.1. The four-parameter models (Equations 3.9 and 3.11) and the 5-parameter exponential rise-to-max equation (Equation 3.13) yielded similar fits. Though the difference was minimal, these models appeared to provide a better fit for the data than the three- parameter-solid model (Equation 3.7) in the initial part of the curve, close to t=0, but did not level off to the same degree towards the end of the test, as shown in Figure 4.21. These differences were also found by Burns when comparing between three and four- parameter viscoelastic models in fitting creep data of human IVDs [63]. For the length of

Figure 4.20: Stress remaining after one hour of stress relaxation at 0.25 strain in unfilled PVA (a) and 10% PVA-NC hydrogels (b) tested fresh in water, and after 7 days of aging in PBS.

the test, the four-parameter models fitted more closely with the shape of the data and thus were used to generate the creep curves for comparisons between PVA and PVA-NC hy- drogels.

Figure 4.21: Creep data of a 10% PVA sample tested fresh in 37 °C water at a constant stress of 0.05 MPa, and fitted with the three-parameter-solid, and the four-parameter Burger’s and Bausch viscoelastic models.

Increasing PVA concentration decreased the amount of creep in the unfilled hy- drogels (Figure 4.22). This is in agreement with stress relaxation results since with a lower degree of relaxation, a smaller increase in strain would be needed to maintain a constant load during creep. Furthermore, since hydrogels with higher PVA concentra- tions were stiffer, lower initial strains resulted from the applied stress of 0.05 MPa.

Adding nanofillers increased creep, and as such, the creep curves of the 10% PVA-NC hydrogels were less flat than the unfilled 10% PVA control. This was also ex- pected from the stress relaxation results. Creep in the Laponite-containing 10% PVA-NC hydrogels was higher than in 10% PVA (Figure 4.23).

Figure 4.22: Creep curves of unfilled PVA hydrogels in 37 °C PBS at a stress of 0.05 MPa for one hour. Increased PVA concentration results in reduction of the initial strain and creep.

Figure 4.23: Creep of Laponite-filled hydrogels compared to 10% PVA in 37 °C PBS at a stress of 0.05 MPa for one hour. Addition of Laponite resulted in increased creep.

BC and pBC addition produced stiffer hydrogels than unfilled 10% PVA, result- ing in smaller initial strains. However, it is apparent from Figure 4.24 that BC- and pBC- filled 10% PVA-NC hydrogels had higher creep strains than 10% PVA that are depend- ent on the amount of pBC added. Although both pBC-filled hydrogels had similar initial strains, 0.4% pBC addition resulted in a larger increase in strain that may be due to a higher water content compared to 0.25% pBC-10% PVA-NC hydrogels.

Figure 4.24: Creep of BC and pBC-filled hydrogels compared to 10% PVA in 37 °C PBS at a stress of 0.05 MPa for one hour. Addition of BC and pBC resulted in increased creep.

Figure 4.25 summarizes the increase in strain after 1 hour in of creep testing be- fore and after aging in PBS. While creep decreased in unfilled 15% and 20% PVA hy- drogels after aging in PBS, 10% PVA and PVA-NC hydrogels either had increases in creep or no change after aging.

Figure 4.25: Percent increase in strain after creep testing at 0.05 MPa stress for one hour. The increase of PVA concentration (a) decreased creep, while the addition of nano- fillers into 10% PVA (b) increased the amount of creep in the hydrogels.