Mechanical properties of hydrogels

Since the stress-strain curve of hydrogels (and tissues) is non-linear in the elastic portion, even at low strains, a polynomial fit was used for the data, and the stiffness of materials was determined according to method described in Section 4.2.4.6. Moreover, instead of giving only the second-order elastic constants for the materials, the stiffness as a function of strain was shown in order to represent the material behavior in a wider strain range. The representative compressive stress as a function of the deformation strain curves, and the stiffness as a function of the strain curves of the HA-PVA- and AL-PVA-based

hydrogels (and midbrain tissue) are shown in Fig. 5.10 (a) and (b), respectively, whereas for GG-HA- (and GG-Ca, midbrain tissue, heart tissue) and HA-HA-based hydrogels are shown in Fig. 5.10 (c,e) and (d,f), respectively.

The stress-strain curves (Fig. 5.10 (a,c,e)) show that all the hydrogels were initially resistant to deformation, and when the load was increased, they became progressively stiffer, leading finally to the fracture of the hydrogels. The HP2 and AP hydrogels had the lowest fracture strains (50% to 57% strain) compared with the others (over 60% strain). The stress-strain curves of the HA-PVA- and AL-PVA-based hydrogels behaved similarly to midbrain tissue at lower strains, but at higher strains the curve of the midbrain tissue became steeper showing no clear fracture. In addition, it was noted that at lower strain values, HP4 hydrogel was stiffer than midbrain tissue, but at higher strains (over 20% strain) the opposite was the case. Altogether, the stress-strain curves of the brain tissue and the HA-PVA- and AL-PVA-based hydrogels were in the same range at lower strains and showed similar elastic behavior, whereas at higher strains in the fracture area the difference was clearer. The results of HA-HA-based hydrogels showed that the hydrogels fractured at 55% to 70% strain range. Due to a higher fracture strain, the HH2 hydrogel was considered to be more elastic compared with the HH2C and HH1 hydrogels, whereas the col I-containing HH2C hydrogel was the least elastic. The control GG-Ca hydrogel fractured at a lower strain (around 30% strain) compared with the GG-HA-based hydrogels (over 55% strain). When the stress-strain curves of GG-HA-based hydrogels and tissues were compared, the GG-HA-based hydrogels showed a similar elastic behavior to brain tissue at lower strains.

The stiffness-strain curves (Fig. 5.10 (b,d,f)) showed that the stiffness was strain dependent and quite constant at around 20% strain with all the hydrogels. After that, the stiffness increased more or less depending on the sample, and at a specific point started to drop. This drop occurred sooner for the HP2 and AP hydrogels when compared with the others. The stiffness decreased when the polymer concentration of the hydrogel decreased (HP1a and HP1b hydrogels). At low strain values, the stiffness of HA-PVA-based hydrogels was higher when the molecular weight of HALD-component was lower (HP3 and HP4 hydrogels) or its DS% was higher (HP2 and HP4 hydrogels). In addition, the stiffness of the AP hydrogel was similar to the lowest HP1b hydrogel. At lower strain values, the stiffness of the HP1 hydrogel was the most similar to midbrain tissue. The stiffness of GH2 and GH4 hydrogels was fairly constant up to 20% to 30% strain, after which it increased rapidly, leading eventually to the fracture of hydrogel. The stiffness of GH1 and GH3 hydrogels was fairly constant until 40% strain. Both hydrogels started to fracture at around 60% strain. For different hydrogels, the drop in the stiffness value (indicating fracture) occurred at different strains. The drop in stiffness value occurred sooner for the GH2 hydrogel and the control GG-Ca gel than the others. In general, GG-HA-based hydrogels can be considered more elastic when compared with the control GG-Ca gel. Brain tissue, on the contrary, showed constant stiffness until 20% strain, after which it increased with no clear fracture. The constant area of heart tissue was even shorter, after which the stiffness increased rapidly with no clear fracture. According to Fig. 5.10 (d), GH1, GH2 and GH3 hydrogels were shown to behave in a fairly similar way to brain tissue from 0% to 15% strain. After that, the stiffness of the brain tissue increased more steeply with no sign of a fracture, whereas the stiffness of hydrogels slowly increased, and they fractured. On the other hand, the stiffness of the hydrogels was shown to be too low at low strains to mimic heart tissue.

presented in Table 5.2. The HP1b hydrogel showed a statistically significant difference p< 0.01 with respect to the HP1, HP1a, HP2, HP3, and HP4 hydrogels. The HP4 hydrogel showed significant difference p< 0.05 with respect to the HP1a, HP3 and AP hydrogels and midbrain, and p< 0.01 with respect to HP1 and HP1b hydrogels. Moreover, the AP hydrogel showed significant difference p< 0.05 with respect to the HP1, HP2, HP3, and HP4 hydrogels. No statistically significant differences were detected between the other HA-PVA- and AL-PVA-based hydrogels or with respect to midbrain tissue. Overall, the second-order elastic constants of HA-PVA- and AL-PVA-based hydrogels were relatively low (0.9 kPa to 5.1 kPa) and similar to midbrain tissue, which could indicate their suitability for neural application. In particular, the HP1 hydrogel showed the most similar elastic behavior (and second-order elastic constant) compared with midbrain tissue at lower strains. The HH2C hydrogel with the highest second-order elastic constant showed a significant difference (p< 0.05) with respect to the HH1 hydrogel. Otherwise, there were no statistically significant differences between the HA-HA-based hydrogels. GH3 hydrogels showed a significant difference (p< 0.05) with respect to the GH4 hydrogel, control GG-Ca gel, and heart tissue. Heart tissue showed a relatively significant difference (p= 0.0571) with respect to GH1 and GH2 hydrogels, control GG-Ca gel, and midbrain tissue. Other- wise, there were no statistically significant differences detected between the GG-based hydrogels or with respect to midbrain and heart tissues. All p-values (tested also between different gel types) are collected in Appendix I (Table 1.). The hydrogels (and tissues) ar- ranged according to decreasing second-order elastic constant: (heart>)HH2C>HH2(>GG- Ca)>GH4>HH1>HP2>HP4>GH2>HP3>HP1>GH1≈HP1a>GH3(>brain)>AP>HP1b.