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.