# Transparent, Pliable, Antimicrobial Hydrogels for Ocular Wound Dressings
## 3 Results
### Carboxylated Agarose Retards Water Evaporation from Hydrogels  
Wounds are associated with an inflammatory response from the body, which can cause a warm sensation. Hydrogels, due to their high-water content, are particularly well suited to provide a local cooling effect due to the high heat capacity of water. Since water evaporation is an endothermic phenomenon, as the water evaporates, the area under the hydrogel is cooled down, leading to a cooling sensation. To assess the potential cooling effect of the hydrogel, we measured the evaporation rate of water of the three hydrogel formulations (NA, NA75 CA25, and NA50 CA50) by measuring the weight loss over 12 h at 50% humidity. Results indicated that increasing the concentration of CA in the hydrogel formulation resulted in a slower evaporation rate (Figure 3). By replacing 50 wt.% of NA with CA in the polysaccharide composition, we observed a 24.6% drop in the rate of evaporation in 3 mm thick gel samples. The NA samples showed an evaporation rate of 10.0, the NA75 CA25 of 9.2, and the NA50 CA50 of 7.5 mg/cm\({}^{2}\)/hour. This change in evaporation rate could originate from the increased number of potential hydrogen bonds in the CA hydrogel through the introduction of carboxylic acid functional groups. The carboxylic acid functional groups could form hydrogen bonds with the surrounding free water, thus reducing the evaporation rate [27]. While a higher cooling rate is favorable to provide a higher cooling effect, a slower evaporation could maintain the cooling effect for a longer time. For a wound dressing application, a prolonged cooling effect could be beneficial to extend the use of the dressing. Figure 2: (**A**) UV-visible spectra of the three formulations showing a higher transparency in the NA50 CA50 sample. (**B**) Photography of the hydrogel pad showing the opacity of each formulation. NA: native agarose; CA: carboxylated agarose. Applied to post-surgical wounds, the water evaporation phenomenon might be affected by the swelling behavior of the hydrogel in contact with physiological fluids such as sweat or wound exudate. Therefore, the swelling capacity of the three formulations were measured over 12 h (Figure 3B). NA showed the highest swelling capacity at the beginning of the experiment. The NA hydrogels were saturated at 300 min. After 300 min, it was found that the CA blended hydrogels swelled slowly to a final mass increase of 26% +/-5% for the NAS0 CA50 formulation. Increasing the proportion of carboxylated agarose in the formulation results in an increase in swelling capacity after 300 min. An increased swelling capacity may be beneficial for extruding wounds, as it can be expected that hydrogels with an increased swelling capacity can absorb a higher amount of wound exudate, thus helping to keep the wound clean. Elastic Modulus and Swelling Behavior of Blended Hydrogels are Dependent on the Content of Carboxylated Agarose  
The elasticity of the hydrogel blends was characterized by measuring the Young's modulus in elongation. The stress-strain curves (Figure S3) showed that all samples exhibit linear elasticity with minimal plastic deformation prior to break. This suggests that the failure mode of the NA-CA hydrogel blend in elongation is a fast fracture propagation with a negligible post-yield-stress region. Furthermore, we found, as illustrated in Figure 4A, that the Young's moduli of the hydrogel blends dropped linearly as the percentage of CA content increased. When the introduction of CA was increased to 50% of total polysaccharides, a 55% decrease in the material's Young's modulus was observed, making the blended hydrogel twice as elastic as the pure agarose hydrogel (Figure 4A). It has been suggested by [PERSON] et al. [18] that the mechanical properties of agarose hydrogels are dominated by the gel's interaction between the \(\alpha\)-helix secondary structures of the polysaccharide chains leading to closely-knitted crosslinks between polysaccharide chains to form and assemble into a network.