2.4.3 Mechanical behaviors and injectability of hydrogels
To achieve optimal wound repair, it is desirable to use materials with
mechanical characteristics similar to the skin, which helps to maintain
structural integrity when the skin tissue is deformed by external force.
Considering the time cost and mechanical strength, 4% and 5% hydrogels
have been chosen for exploration. We used compression stress–strain
assessment to determine the mechanical characteristic of hydrogels
(Figure 3F~H). The mechanical strength of 4% and 5%
hydrogels were both affected by the change of pH. Under neutral
conditions, the Young’s modulus of 4% and 5% hydrogel are 2.88 and
4.00 KPa, respectively, and decreased in alkaline or acidic condition.
We performed strain sweep tests to determine the breakage strains of
hydrogels. The corresponding elastic modulus (G′) and loss modulus (G′′)
under 1 Hz oscillation frequency are shown in Figure 3I and 3J. As the
strain increased, the increase of G′ and G′′ and a cross-over between G′
and G′′ can be observed, indicating that the viscoelastic behavior of
hydrogel shifts from an elastic gel to sol due to extreme deformation
and ultimate breakage. As concentration increases, the breaking strain
decreases due to a higher brittleness. The elasticity of hydrogel meets
the requirements for wound repair.
Injectability is desirable in biomedical application for the localized
administration of the therapeutic hydrogel. During injection, the
hydrogel experiences severe share forces when passing through the narrow
nozzle of a syringe or needle. Therefore, the suitable response of
hydrogel to shear is essential for an applicable injectable hydrogel. To
evaluate the viscoelastic properties of the hydrogels, a frequency-sweep
test was conducted to measure the values of G′ and G′′ for both
hydrogels (Figure 3K, 3L). It was observed that both hydrogels exhibited
a linear response range between 1~10 Hz, affirming their
elastic and stable properties. The values of G′ was significantly higher
than G′′, indicating a solid-like behavior. The frequency-sweep
intersection point indicates a change of the hydrogel from solid to
liquid state at high frequency. Notably, the viscosity of the 4%
hydrogel decreased rapidly with an increase in shear force, indicating
its possible injectability (Figure 3M). As shown in Figure 3N, the
hydrogel mixture was added to a 1 mL single-channel syringe and allowed
to gelation for 12 h, then injected into different pattern with a needle
diameter of 16 G.