Stress-strain curves hydrogel

Practically, tensile samples were allowed to swell in Millipore water for 24h in order to reach the equilibrium prior mechanical testing. In such conditions, only strain due to mechanical deformation, and not to swelling process, was measured. Figure 11 presents typical true stress-strain curves. It comes out that the introduction of PCL segments enliances the overall mechanical properties in comparison to the PDMAEMA hydrogel. 

Figure 13 True stress-strain curves recorded by tensile testing for the swollen PDMAEMA-l-PCL hydrogels atr.t. and containing PCLDMA cross-linker characterized by aMn of1700 ( ), 4300 (o) and 5800 g.mof (d).

Typical compressive stress-strain curves of surfactant-containing hydrophobically modified hydrogels are shown in Fig. 15a, where results obtained from 15 separate HM PDMA hydrogel samples at a state of preparation are presented. The... 

Fig. 15 (a) Typical stress-strain curves of a surfactant-containing HM PDMA hydrogel formed using 2 mol% of C17.3M under compression, represented as the dependences of nominal true stresses (gray dashed curves) on the deformation ratio A SDS = 1 % (wA ). Results of 15 separate tests are shown. Red circles represent the points of failure of the gel samples. The inset shows rrtnie versus A curves of two successive tests conducted on the same gel sample for up to 99.99 % compression, (b) Stress-strain curve of the same HM PDMA hydrogel with 7 % (w/v) of SDS (solid blue curve) and without SDS (solid dark red curve), Co = 15 % (w/v), C17.3M = 2 mol%, NaCl = 0.5 M. From [41] with permission from Elsevier... 

For a deeper understanding of the nature of physical cross-hnks in HM PAAm hydrogels, the gels were subjected to loading and unloading experiments [34]. Figure 20a, b shows the typical stress-strain curves of HM PAAm hydrogels from tensile tests composed of three repeated cycles [34]. The tests were conducted on... 

High-strength PAAc hydrogels formed in oppositely charged surfactant solutions can also be healed by treatment of cut surfaces with surfactant solutions [42]. In Fig. 30, the stress-strain curves of virgin and healed PAAc gel samples... 

Fig. 9 Photographs of (a) fibroin hydrogels and (b) cryogels formed at =50 and —18 °C, respectively, during the compression tests. Csf = 4.2 % EGDE = 20 mmol/g TEMED = 0.10 %. Arrows indicate the direction of piston movement, (c) Stress-strain curves of these gel samples are shown as the dependence of the nominal stress a on the degree of compression. (Erom [50] with permission from the American Chemical Society)...

Fig. 3.16 Tensile stress-strain curves of P(ATC-AAm)/MMT nanocomposite hydrogels. (Ting et al 2015) [Reproduced with pemission of John Wiley and Sons ]...

Figure 1.1 Stress-strain curves from DN gels show much higher strength than conventional gels (Reprinted with permission from Cong, f. P., Katsuyama, Y., Kurokawa, T. and Osada, Y. Double network hydrogels with extremely high mechanical strength. Advanced Materials, 15, 1155-58, Copyright (2003) Wiley-VCH Verlag CmbH).

Fig. 22 (a) True stress (deformation ratio (A) of a surfactant-containing HM PDMA hydrogel from cyclic compression (A < 1) and elongation (A > 1) tests. The tests were conducted with increasing strain, with a waiting time of 7 min between cycles, (b) G filled symbols), G" open symbols), and tan S curves) of a HM PDMA hydrogel shown as a function of the strain fro) at ft) = 6.3 rad s . Sweep tests were conducted in up dark red circles) and down directions blue triangles), as indicated by the arrows Co= 15 % (w/v), C17.3M = 2 mol%, SDS = 7 % (w/v), NaCl = 0.5 M. From [41] with permission from Elsevier... 

The age-related viscoelastic properties of the ocular lens have not been fully characterized. Most of the attempts have been at elucidating only the elastic modulus, since the lens has been treated as an elastic substance (19,26). The process of accommodation however is mechanically analogous to a stress-relaxation experiment, where the stress is allowed to decay at constant strain (refractive power). Hence, the lens is truly viscoelastic. Researchers investigating the viscoelastic characteristics of the lens performed creep-recovery or frequency scan techniques ex-vivo ( 1 8). Ejiri et al. (28) investigated creep properties of a decapsulated dog lens by compression and fitted the time-displacement curve with three Kelvin units. The time constants for the three units were 0.09 s, 7.0 s, and 106 s. The elastic modulus could not be obtained, as the applied stress was unknown due to the aspheric geometry of the lens. In this article, we have investigated the creep behavior of cylindrical disc shaped hydrogels in order to obtain the time constants as well as the elastic modulus of the viscoelastic units. 

Figure 10 shows the curves that represent the viscoelastic response at an applied stress of 5 Pa for the three hydrogels obtained at 37 °C, in a creep test followed by recovery. The creep curves comprise three parts the instantaneous strain, the retardation strain, and the viscous strain. When the applied stress is removed, the recovery process starts, and first the instantaneous strain is recovered, then the retardation one, and finally remains the viscous part. The high elasticity of the hydrogels can be observed, where the reached strain after the stress of 5 Pa was applied for 60 s is very high, and the recovered strain represents 52 % from the maximum value reached by the strain in the creep test. 

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