# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 3 SHs Formation Methods
### Crosslinking Chemical Methods
A mixture of STMP and sodium TPP has been used to crosslink various starches, including potato, banana, corn, cassava, and breadruit. According to [PERSON] et al. (2020) [84], the introduction of hydrophilic phosphate groups via phosphate diester covalent bonds increases repulsion between phosphate units, thereby reducing the intermolecular forces between starch chains. This structural modification allows for greater water accommodation and uptake. As a result, the enhanced electrostatic repulsion also facilitates water percolation and absorption throughout the starch matrices [85]. Figure 4: Common chemical crosslinkers used in the synthesis of SHs: (**A**) aldehydes, (**B**) epoxides, (**C**) polyphosphates, and (**D**) carboxylic acids. An example reaction is shown for each functional group. Organic acids such as citric acid (CA), malic acid (MA), and tartaric acid have been explored as crosslinking agents for starches. Their incorporation into the polymeric matrix enhances mechanical resistance and reduces permeability. Additionally, these acids contribute to starch hydrolysis, facilitating granule disruption and promoting the formation of a more homogeneous matrix [86]. For example, [PERSON] et al. (2019) developed starch/xanthan gum hydrogels using CA as a crosslinker [87]. Beyond its crosslinking function, CA also exhibits hydrolytic and plasticizing properties. The resulting materials displayed a smooth surface with a slightly yellowish hue, a characteristic commonly reported for starch CA composites [88]. The SC of these hydrogels depends on the proportion of CA used during material formation, which also influences mechanical properties particularly by increasing elongation due to CA's hydrolytic action. CA has also been used to crosslink starch in the presence of glycerol at 75 \({}^{\circ}\)C. Thermal analysis suggests that CA initially reacts with glycerol, followed by the formation of a starch-glycerol-CA complex [89]. Another organic acid, MA, has been evaluated as a crosslinking agent in interpenetrating polymer network hydrogels composed of natural rubber latex (NR) and cassava starch [90]. Tensile strength was found to decrease as starch content decreased. Moreover, starch plays a crucial role in the material's swelling behavior. After 100 h of water contact, the hydroxyl groups in starch facilitate the absorption of a significant amount of water molecules, leading to polymer chain expansion and enabling the material to retain a large volume of water. MA has also been applied to crosslink sago starch with PVA [91], revealing that the swelling behavior of sago starch-maleate-PVA hydrogels depends on precursor composition, pH, and the temperature of the swelling medium. The SW increased as the temperature rose from 25 \({}^{\circ}\)C to 37 \({}^{\circ}\)C, which was attributed to polymer chain expansion at higher temperatures. This expansion enhances the diffusion rate of water molecules into the gel and facilitates their entrapment within the hydrogel network. In addition, pH was shown to influence SC by affecting both the pore volume and water diffusion through the material. The SW was lower at a medium pH of 1.23 (similar to gastric juice) than at pH 6.38 (intestinal fluids) or pH 7.0. This behavior is attributed to the acidic nature of the hydrogel, which contains free carboxyl groups on the polymeric starch chains. At higher pH values, stronger interactions between the hydrogel and the surrounding medium enhance swelling. Furthermore, crosslinking with di- and tricarboxylic acids affects the SC, as observed in starch phosphate hydrogels crosslinked with CA, MA, succinic acid, glutararic acid, and adipic acid, which exhibited swelling capacities ranging from 150 to 185 g/g [92]. However, the starch source also significantly influences hydrogel swelling. This was evident in hydrogels made from waxy maize starch, high-AM starch, degraded starch, and potato starch crosslinked with CA.