Hydrogels are polymeric materials with network linked structure that contains both physical and chemical cross-links. Their particular characteristics are having high water uptake and swelling in aqueous solution . The importance of hydrogels in medical applications was firstly revealed in 1950 . Poly 2-hydroxy ethyl methacrylate (PHEMA) hydrogel was utilized to fabricate contact lenses. This hydrogel exhibited high wettability, elasticity, and biocompatibility. These properties made the PHEMA hydrogel very similar to the natural human body tissues . Afterward, many studies were carried out to develop the synthesis of hydrogels with specified properties, which made them applicable in biomedical researches.
Nowadays, hydrogels have a vast range of applications in the biomedical research area . They have been used to fabricate wound dressings , contact lenses , membranes , drug delivery systems , tissue engineering , artificial skin , biosensors ,etc. The polymerization methods, choosing monomer, cross-linker, solution, and initiator can play an essential role in the final properties and applications of the synthesized hydrogel .
Biocompatibility is a significant feature of hydrogels that depends on surface tension. Surface tension in the intersection of hydrogels and biological tissues is low . Also, due to the high water content, hydrogels' surface is a hydrophilic permeable membrane, which provides biocompatibility [13, 14]. Some hydrogels' smoothness and elasticity decrease the surface friction of hydrogels with surrounded tissues . Despite all of the hydrogels' unique properties, most of them have shown undesirable mechanical properties that can be a substantial problem, especially after swelling in aqueous solutions .
For the first time in 2000, Van Den Bulcke and coworkers synthesized a novel methacrylamide-modified gelatin hydrogel by photo-initiated polymerization of gelatin and methacrylic anhydride . This gelatin methacrylamide and also gelatin methacrylate hydrogels were later introduced as GelMA in literature by different authors . GelMA is a hydrogel with a covalently cross-linked network structure that provides desirable physical and chemical properties . Since its unique properties, GelMA has been considered to be used in numerous biomedical applications such as wound dressings , Connective tissue [21, 22], Cartilage [23–25], Cardiovascular [26, 27], Bone [28, 29], Tooth Bud . Since then, some researchers have used chitosan to develop the properties of GelMA. For instance, Jang Wook used keratin fiber/chitosan . In the other study, Chen and coworkers synthesized GelMA by using chitosan microspheres and used it to treat osteoarthritis . In addition, Jiali Chen bioprinted multiscale composite scaffolds based on GelMA hydrogel and chitosan microspheres for peripheral nerve tissue engineering . Also, Modaresifar utilized chitosan nanoparticles in the structure of GelMA . These syntheses took place in an acidic environment due to chitosan polymer presence, which eventually limited synthesized GelMA in biomedical applications. Acidic solutions made the GelMA a toxic substrate for living cells and growth factors through the residual acidic reagents trapped in the GelMA structure. So in this study, Carboxymethyl chitosan (CMC) was used. CMC can dissolve in various pH environments, and this property is because of carboxymethylation degree. CMC has many biomedical applications such as tissue engineering, wound healing, tissue engineering and drug/gene delivery bio-imaging [35–37]. In recent years, synthesizing methods based on the green chemistry approaches has become a significant focus of researchers. The essence of green chemistry emphasizes the eliminating the usage of toxic and hazardous solvents and reagents to minimize the environmental impact. Therefore, the replacement of toxic solvents with low or nontoxic solvents is highly recommended . Among the conventional green solvents, including water, n-propyl acetate, i-propyl acetate, 1-butanol, and 2-butanol, water is recognized as the greenest bio-based solvent . Therefore, water-soluble polymers have the potential to be synthesized by green methods.
The most commonly used photoinitiator for GelMA synthesis is 2-hydroxy-1-[4-hydroxyethoxy) phenyl]-2-methyl-1-propanone, also known as Irgacure 2959. Heqi et al. evaluated the effect of Irgacure 2959 on cell viability. The results of this study demonstrated the significant cytotoxicity of this photoinitiator at 0,5% (w/v) concentration and more . Additionally, another problem of most conventional UV photoinitiators such as Irgacure 2959 is insolubility in water due to the presence of the aromatic group in their molecule. This issue makes their usage challenging for water-soluble systems. To solve previously mentioned problems, Ikkai et al. were suggested the usage of ammonium persulfate (APS) as a UV photoinitiator. The results of this study demonstrated that the usage of APS as a UV photoinitiator has several advantages, such as non-toxicity and non-necessity to the use of cross-linkers or pH-adjustment additives . Also, the results of another study, which compared the effectiveness of Irgacure 2959 and APS as initiators, showed the better cytocompatibility and rapid gelation rate for APS compared to Irgacure 2959 . Therefore, in this study, we use ammonium persulfate (APS) as a green and reachable photo initiator instead of complicated and expensive initiator such as Irgacure 2959.
As we mentioned previously, modification of gelatin hydrogel by methacrylamide, causes optimized characteristics in this hydrogel for biomedical applications. The study performed by Billiet et al. demonstrated that modification of gelatin hydrogel by acrylamide causes better mechanical properties, higher conversion degree, and faster cross-linking kinetics compared to gelatin methacrylamide hydrogel . Therefore, in this study, we suggested using acrylamide for gelatin hydrogel modification to develop a novel GelMA hydrogel.
In this study, we aimed to develop an innovative green method to synthesize a novel GelMA by using Carboxymethyl chitosan (CMC) which is a water soluble polymer . Therefore, the synthesis process was carried out in a completely neutral environment. Furthermore, using CMC as a part of the hydrogel structure provides unbeatable mechanical properties for the synthesized GelMA. This property makes our novel synthesized GelMA a promising and suitable biomaterial candidate for fabricating sustainable body implants such as artificial vessels, muscles, and ligaments.