4‐1 Strain sweep test
4‐1‐1 Typical dynamic mechanical properties for hydrogels
To measure the dynamic mechanical properties of the composite hydrogels, a Rheological Scientific rheometer (ARES instrument) was used to determine the storage modulus (G’) and loss modulus (G”). As shown in Fig. 4‐1, there was a typical change of dynamic mechanical properties for sample with the weight ratios CHC/SAL/glycerol = 1/1.5/0 prepared by exposure to 3 wt% CaCl2 by strain sweep test. The storage modulus (G’) remained constant at low strain but abruptly decreased at strain larger than critical strain ( 1.55%) at which the polymeric system starts to display nonlinear viscoelastic behavior (gel began to breakup in hydrogel behavior). When the strain was larger than the critical strain, there was a swift decrease of the moduli, and the decrease in G’ could be explained in terms of disruption of interactions55. The fact that all sample preparations display G’ values larger than G” at small strains clearly proves the gel character. Then the hydrogel would reach a gel‐liquid transition point ( tanδ ⁄ 1; 17.82% ) indicating a breakdown of gel state to a quasi‐liquid state.
Fig. 4‐1 Typical dynamic mechanical properties of hydrogel with weight ratios CHC/SAL/glycerol = 1/1.5/0 prepared by exposure to 3 wt% CaCl2 using strain sweep (γ 0.01%~100%) measurement.
0.1 1 10 100
0.001 0.01 0.1 1 10 100 1000
G', G''[kPa]
Strain [%]
G' G''
_ 17.82%
_ 1.55%
4‐1‐2 Effect of additive in alginate based hydrogels
In this study, it was crucial to elucidate the role that CHC nanoparticles are playing in the gel behavior, in addition to providing a platform for delivery and protection of various drugs.
To indicate the effect of CHC nanoparticles in alginate based hydrogels, three various CHC concentrations were used in this study, 0, 0.5, and 1 wt% as shown in Fig. 4‐2. The presence of CHC nanoparticles obviously decreased the storage modulus. The modulus value of hydrogels without CHC was almost 2.26 times larger than with 0.5 wt% CHC and 2.87 times larger than with 1wt% CaCl2. In addition, the gelation transition strain ( ) and the gelation transition stress ( ) also varied with the change of the additive concentration. It displayed an increase of the gelation transition strain ( ) and decrease of the gelation transition stress ( ) with the increase of CHC concentration. The values of and for samples without CHC were 5.53 % and 1319.2 Pa, with 0.5 wt% CHC were 10.13 % and 1038.4 Pa, and with 1 wt% were 13.28 % and 967.6 Pa.
Vanvliet found that additives would alter the storage modulus depending on if there are interaction between additive molecule and crosslinking chains or not24. Zhang et al. found that calcium alginate gels formed from solutions to which with high MW dextran had been added displayed a decrease of storage modulus. This was explained by the steric effects of the high MW dextran disturbing the crosslinking structure of the alginate gels. The same explanation seems plausible for the lower storage modulus of the composite gels, as
compared to pure calcium alginate gels25. The CHC nanoparticles inside the gel structure would appear as steric hindrances, separating the alginate chains, resulting in decrease crosslink density and the associated decrease of storage modulus as shown in scheme 4‐1.
Besides, the results of the gelation transition strain ( ) and the gelation transition stress ( ) also supported steric effect. The increase of showed the increasing of affordable deformation and the decrease of demonstrated the decreasing of the crosslinking density. As a result, CHC nanoparticles playing as additives may decrease the dynamic mechanical properties by the steric effect.
Fig. 4‐2 Rheological properties of hydrogels with weight ratios SAL/glycerol = 1.5/0 prepared by exposure to 2 wt% CaCl2 on strain sweep (γ 0.01%~100%) measurements without CHC (a), with 0.5 wt% CHC (b), and with 1 wt% CHC (c).
4‐2 Small deformation test
4‐2‐1 Effect of calcium chloride concentration in gelation medium
Generally, the gelation of an alginate solution can be controlled by the concentration of divalent metal ions. In Fig. 4‐3, the effect of calcium chloride concentration in the gelation medium (1, 2, and 3 wt%) is shown for the sample with the weight ratios CHC/SAL/glycerol = 1/1.5/5, as determined by small deformation test. The results showed that G’ and G” values increased with the increasing of CaCl2 concentration in the gelation medium. The modulus values of hydrogels with 3 wt% CaCl2 in the gelation medium was almost 1.25 times larger
than with 2 wt% CaCl2 and 2.42 times larger than with 1 wt% CaCl2 (comparisons made at γ 0.245%).
With regard to this study, the storage modulus of the gels prepared using gelation media with different Ca2+ concentrations varied greatly. According to rubber elasticity theory the correlation between storage modulus and the network crosslink density can be described
by the equation56:
G gRTN (4‐1)
where G is the network equilibrium shear modulus; g is a constant, nearing 1.0 for incompressible materials; R is the gas constant; T is the absolute temperature; N is the number of elastically active network chains per unit volume for a network. Although this theory of rubber elasticity is based on the concept of an entropy‐driven restraining force
counteracting the deformation of the polymer network, it can be applied to provide an indication of network structure from shear modulus behavior57, 58. Segeren et al. found several features of the behavior of alginate gels forming by Ca2+ to be consistent with rubber elasticity theory59. Thus, by the guidance of the theory of rubber elasticity, it can be concluded that increasing Ca2+ concentration in gelation medium resulted in the rise of crosslinking density with the corresponding increase in storage modulus (Fig. 4‐3).
Fig. 4‐3 Rheological properties by the small deformation test (γ 0.015%~7%) for samples with weight ratios CHC/SAL/glycerol = 1/1.5/5 and varying CaCl2 concentration in the gelation medium.
4‐2‐2 Effect of glycerol concentration
Acidification of the gelation solution may change the rheological properties, too. In Fig.
4‐4, the G’ and G” values under small deformation test is shown for samples prepared with varying glycerol contents (fixed weight ratios CHC/SAL = 1/1.5 and 3 wt% CaCl2 in the gelation medium). The G’ and G” values increased with the increasing of glycerol concentration. The G’ values of 10% glycerol was 1.18 times larger than if prepared with 5%
glycerol and 1.75 times larger than gels prepared without glycerol (comparisons made at γ 0.245%).
In this study, the addition of glycerol resulted in an increase of storage modulus. This is in contrast with what is previously reported for pure calcium alginate gels. Zhang et al. found that calcium alginate gels formed from solutions to which with low molecular weight (MW) dextran or glycerol had been displayed increased viscosity of the gel‐liquid phase. However, no significantly change was observed in the storage modulus25. It seems likely that in the composite gel system the glycerol act as a hydrogen bonding connector between CHC nanoparticles and alginate chains, as well as between CHC nanoparticles. This hydrogen bridging will act crosslinking, and thus increase the storage modulus of the gels.
Fig. 4‐4 Rheological properties by the small deformation test (γ 0.015%~7%) for samples with weight ratios CHC/SAL = 1/1.5 and varying glycerol content, prepared by exposure to 3 wt% CaCl2.
0.1 1 10 100
0.01 0.1 1 10
G', G''[kPa]
Strain [%]
G' (0% G''(0%
G' (5% G''(5%
G' (10% G''(10%
_ 1.55%
4‐2‐3 Cohesion energy of composite hydrogels
The critical strain ( ) can correlate to the gel cohesion energy, according to:
(4‐2) where Ec is the cohesion energy and is the storage modulus at critical strain.55 The cohesion energy is connected to the energy from the network structure formed by physical crosslinks between the polymer chains. The results of various CaCl2 in gelation medium and glycerol concentration are showed in Table 4‐1. The cohesion energy is increasing with the increasing of storage modulus at critical strain. All samples shown in Fig. 4‐3 and Fig. 4‐4 had the same critical strain, indicating similar gel structures. Given that all samples had the same critical strain, the calculated cohesion energy was only altered by the storage modulus of the samples. The values (20~40 10 ⁄ ) calculated for the samples mentioned above are extremely high compared to that of CHC nanoparticles, which forms a gel only through hydrophobic forces and hydrogen bonding (48 ⁄ ).
Glycerol conc. (%)
under 3% Ca2+ (%) ⁄ Ca2+ conc. (wt%)
under 5% glycerol (%) ⁄
0 1.55 20.1 0.06 1 1.55 13.3 0.07
5 1.55 29.7 0.05 2 1.55 24.2 0.08
10 1.55 35.7 0.04 3 1.55 29.7 0.05
Table 4‐1 Critical strain and cohesion energy of CHC/SAL (weight ratio = 1/1.5) gels under
4‐3 Strain step test
One highly interesting characteristic that was shown for CHC/SAL composite hydrogels was shear‐reversible gelation. The sample with the weight ratios CHC/SAL/glycerol = 1/1.5/0 prepared by exposure to 3 wt% CaCl2 also showed this special characteristic that it rapidly recovered its dynamic mechanical properties after a high shear strain induced structural breakdown, this phenomenon is known as thixotropy. This interesting characteristic is clearly illustrated in Fig. 4‐5. When high shear strain was applied, with the corresponding high shear
stress (γ 100% and ω 10 ⁄ ), the G’ values decreased from 182 ⁄ to 2.11 ⁄ resulting in a quasi‐liquid state (tanδ 5.0). However, when the amplitude
was decreased (γ 0.01%) at the same frequency, G’ instantly recovered its initial value and the system came back a quasi‐solid (gel) state (tanδ 0.19). Compared to gels formed purely by alginate or chitosan55, the composite gels displayed a rapidly recovery of their storage and loss modulus after shear induced breakdown of the network structure. This knowledge is based on the fact that time dependence of reaggregation after following the shear‐induced deaggregation of the aggregates occurs at filtering through neutral filters with pore size comparable to the dimension of the aggregates or at flowing of sodium alginate solution through small nozzle when spray drying is applied60.
Fig. 4‐5 Stress induced shear reversible properties of sample with weight ratios CHC/SAL/glycerol = 1/1.5/0 prepared by exposure to 3 wt% CaCl2 in continuous step strain measurements.
0.1 1 10 100
0 200 400 600 800 1000
G', G''[kPa]
Time [s]
G' G''
4‐4 Self‐healing test
Hydrogels exhibiting shear‐reversible gelation can recover their mechanical properties after a shear‐induced breakdown, and could potentially be utilized in biomedical applications such as injectable gels for drug delivery. Under the large shear during injection the gels would be in a quasi‐liquid state and thus exhibiting flow. However, after injection the shear forces would be absent and the gels would recover to the original quasi‐solid state9, 61‐63. Self‐healing hydrogels such as copolypeptide hydrogels have low mechanical properties (G’
values 1 ) and require more than one hour for storage modulus to recover its initial value9, 64, 65. Most polymer hydrogels formed by covalent bonds are usually brittle and lack the ability to self‐heal66. A non‐covalent approach using dendritic macromolecules as binders in clay nanosheets – sodium polyacrylate hydrogels has been reported67.
Two different color (blue and translucency) of composite hydrogels with weight ratios CHC/SAL/glycerol = 1/1/20 were prepared by exposure to 2 wt% CaCl2 (Fig. 4‐6a). The healed composite hydrogel was strong enough to hold when suspended horizontally (Fig. 4‐6b) or vertically (Fig. 4‐6c). The gels were reported to have high mechanical strength, rapid shear recovery capability and self‐healing behavior, and very easy preparation procedure. From the excellent shear recovery displayed by our composite gels, it was reasoned that our dual‐structure gel possibly could be self‐healing as well. Indeed, the investigated samples did show a self‐healing behavior as seen in Fig. 4‐6. The healing bridge was strong enough to
hold when suspended horizontally (Fig. 4‐6b) or vertically (Fig. 4‐6c). To acquire this feature, the freshly cut surface should supply for the active function groups on the sectioned surface.
The self‐healing was dependent on relatively high glycerol content in the composite gels; at low glycerol concentrations the self‐healing was not observed. Thus, we suggested that the self‐healing can be attributed to the increased hydrogen bonding within the gels in the presence of glycerol, as supported by the rheological measurements.
Fig. 4‐6 Photographs illustrating the self‐healing properties for composite hydrogels. Here for samples colored by Trypan blue and non‐colored samples with the same weight ratio CHC/SAL/glycerol = 1/1/20 prepared by exposure to 2 wt% CaCl2 (a). A bridge constructed by connecting two gels of different color can be suspended horizontally (b) and held vertically (c).