# Transparent, Pliable, Antimicrobial Hydrogels for Ocular Wound Dressings
## 1 Introduction  
Surgical procedures on the eye or its surrounding such as couloplastic or extraocular muscle surgeries are followed up by the application of a wound dressing. Several types of dressings can be used, but transparent ones have the advantage of allowing for wound osculation without removing the dressing while providing non-obstructed vision for the patient [1]. Current clinical practices use dressings composed of agar and polyacrylamide polymer [2], cotton gauges to deliver pharmaceuticals to the eye [3], and polyurethane-based drug eluting dressings [4]. Hydrogel dressings, because oftheir high content of water, produce a cooling effect on the wound, helping to reduce the warm feeling associated with inflammatory tissues [5]. Often, in addition to the dressing, the wound care is accompanied by the application of antibiotics, which can help reduce wound infection and support wound closure [6]. However, for a transparent wound dressing that releases an antimicrobial molecule to be adequate for ocular wounds, it needs to be pliable to follow the intricate eye anatomy, yet mechanically robust to remain in place for a long period [7].  
In recent years, the possibility of manufacturing high-performance and multi-purpose wound dressings with non-woven materials has been widely explored. Such materials include chitosan [8; 9; 10], alginate [11; 12], and bacterial cellulose [13]. Due to the range of unique properties, agarose, a polysaccharide sourced from seaweed, has found uses in biomedical [14], cosmeceutical [15], and food applications [16]. One of the most useful properties of agarose is its capability to form physically crosslinked hydrogels that are biocompatible and inert [17]. Agarose hydrogels exhibit hysteresis in their phase behavior, that is, they are soluble at temperatures above 90 \({}^{\circ}\)C while they form a gel below 40 \({}^{\circ}\)C [18].  
Blending can be used as a technique to manipulate hydrogel properties for various applications. Successful examples of this include hydrogel blends of agarose-methylcellulose for the control of the gelation time and temperature [19], agar-poly(vinyl alcohol) for control of the mechanical properties [20], and gelatine-hydroxyethyl cellulose for control over drug release rate [21].  
Agarose can be chemically modified through a controlled carboxylation of the polysaccharide backbone (Figure 1A) to obtain carboxylated agarose (CA) and gain precise control over the mechanical properties of the modified hydrogel by inducing a switch of the secondary structure from \(\alpha\)-helical to \(\beta\)-sheet [22]. Blending native agarose (NA) with different amounts of CA, leads to gels with predictable shear moduli ranging from 0.1 to 100 kPa [23].  
While blending offers considerable industrial advantage for the industrial manufacturing of hydrogels with broad mechanical properties, neither NA nor CA possess intrinsic anti-microbial properties. However, based on their good molecular transport properties [24], here, we attempted to exploit NA-CA hydrogel blends to formulate wound dressings capable of carrying antimicrobial agents and providing continuous topical drug delivery for over ten hours.  
In this study, we investigated three hydrogel blends using a total solids content of 2 wt.% consisting of NA-to-CA ratios ranging from 100% to 50%: NA100, NA75 CA25, and NA50 CA50. The hydrogel blends were formulated with the two different antimicrobial molecules, cefazolin and doxycycline (Figure 1B,C), and their transparency, water evaporation, mechanical properties, and drug release capabilities were characterized in vitro. The biological properties of the formulated hydrogels were then tested against two common bacteria strains, _Escherichia coli_ and _Staphylococcus aureus_, associated with eye infections.  
Figure 1: Chemical structures of (**A**) native and carboxylated agarose, (**B**) cefazolin, and (**C**) doxycycline.