Traditional gelatin is produced from animal origin, and it has been made from bones, cartilage, tendons, ligaments, and skin of animals such as cattle, pigs, fish, or chickens. As aforementioned, gelatin is derived from the partial hydrolysis of collagen protein presented in the previous sources [40]. Thus, due to the process of obtaining it, it may also be referred to as hydrolyzed collagen, hydrolyzed gelatin, or collagen peptide after it has undergone hydrolysis. After this process, the final product is colorless and soluble.

Gelatin is one of the most used ingredients with non-toxic and biodegradable characteristics in food and non-food industries for many purposes: for promoting gelation, stabilizing, thickening, emulsifying, and film forming [40]. Biodegradability is today an important issue since the sustainability of each product needs to exist; otherwise, the product might have problems with its future use because sanitary treatment is expensive and delimitation of space is a large problem for cities and governments. Even after obtaining the gelatin or the desired product from it, it has been demonstrated to have biodegradable characteristics, and it can contribute to producing new materials with high biodegradability potential inside the human organism [41] and in the environment [42,43].

Gelatin is used for giving consistency and viscosity to the hydrogels and, thanks to its aforementioned biodegradable and renewable nature, it can be an important source of sustainable hydrogels. Nowadays, the use of gelatin for the synthesis of hydrogels for multiple purposes depends on its structure and texture characteristics, and the applications range from the biomedical or biotechnological areas [2,3,30,44] to the most variable water and waste treatments [15,16,27,36,45]. Thus, gelatin-based hydrogels are versatile materials, which makes them very interesting. Some useful aspects of gelatin-based hydrogels employed for all the applications together with some others such as alimentary or environmental ones are displayed in Table 1.

As shown in Table 1, the wide applicability of hydrogels has been studied. In the biomedical field, for instance, purposes such as tissue engineering [3,30], cell culture scaffolding for bacterial growth and further human uses [48], dental pulp regeneration [49], capsules for drug delivery [4], and the treatment of myocardial infarction [31] have been investigated. Gelatin hydrogels are usually permeable to nutrients and oxygen, which enhance the survival rate of cells and their biological functions [48]. So, hydrogels with different compositions combining gelatin with other polymers could be useful for biomedical applications.

Gelatin is a protein that can easily biodegrade in the environment but has some stability limitations at high temperatures since once it has been dissolved, long durations at temperatures above 40 °C can promote protein denaturation. Moreover, gelatin is soluble in water, and this can also be a problem when synthesizing neat gelatin materials. As shown in Figure 1, gelatin is usually made from repeating units of glycine-X-Y. The high amount of the amino acids proline (12%), hydroxyproline (10%), and hydroxylysine (0.5%) make gelatin particularly special [50]. Depending on its origin, the content of proline and hydroxyproline might vary; in fact, fish gelatin is known for having a lower concentration of these amino acids compared to that coming from mammals. This affects negatively the gelling ability of fish gelatin, leading to a decrease in the gelation and melting temperatures together with a worsening of its mechanical properties, among others [51]. Thus, in order to synthesize more stable and resistant gelatin-based hydrogels, this biopolymer has been combined with many other polymers such as polyvinyl alcohol [28], alginate dialdehyde, alginate dialdehyde-gelatin reinforced with bioactive glass nanoparticles [3], dialdehyde carboxymethyl cellulose-dextrin [30], graphene oxide and laponite [31], collagen [49], chitosan [5], dopamine grafted, 1,4-phenylenebisboronic acid and graphene oxide [11], and oxidized alginate (OA) reinforced by silicon carbide nanoparticles (SiC NPs) and cross-linked with N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) [52]. Figure 1 shows the chemical structure of different compounds used for the synthesis of gelatin-based hydrogels.