# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 3 SHs Formation Methods
### Crosslinking Chemical Methods  
AM and AP contain hydroxyl groups that can react covalently with crosslinkers such as glutaraldehyde (GA), epichlorohydrin (EH), sodium trimetapphosphate (STMP), and bi- or tricarboxylic acids (Figure 4) [75]. Corn starch was modified by crosslinking using GA [76]. The degree of substitution (DS) was found to be influenced by the pH of the reaction medium, increasing under alkaline conditions [76]. FTIR analysis revealed that in acidic environments, the hydroxyl band of the crosslinked starch shifted to higher frequencies, suggesting a decrease in hydrogen bonding. In contrast, this spectral shift was not observed when the crosslinking occurred in an alkaline medium [76]. GA has also been used in polymeric blends, such as starch/polyvinyl alcohol (PVA) hydrogel films [77]. The material formation involves three steps: (i) gelatinizing the starch at 70 \({}^{\circ}\)C for 1 h, (ii) mixing the PVA aqueous solution, gelatinized starch, and GA for 2 h, and (iii) drying the material at room temperature. The hydrogel swelled by approximately 250% in water and showed no dependency on pH, as both starch and PVA contain no ionizable functional groups. Recently, other research has reported the use of liquid ionic compounds, such as BMIM-BF4 (1-Butyl-3-methylimidazolium tetrafluoroborate), to enhance the plasticity and generate porosity within the hydrogel, increasing the swelling capacity (SC) up to 300% [78]. Another alternative for starch crosslinking is EH, an epoxide organochlorine that requires basic aqueous conditions, typically NaOH at 40 \({}^{\circ}\)C [79]. Studies have shown that the swelling power of EH-crosslinked starch decreases with increasing concentrations of both the crosslinker and the polymer. Furthermore, SC also diminishes with rising salt concentrations. This behavior is attributed to a reversal of polarity on the starch backbone, which elevates the ionic strength within the gel, creating osmotic pressure and limiting fluid infiltration. Additionally, this method can lead to imperfections in the gel due to micro-heterogeneities, which cause entanglements and micro-aggregations of the crosslinker within the hydrogel network [79]. EH has also been used to crosslink starch in the presence of humic acid [80], following the methodology proposed by [PERSON] et al. (1989) [79], thereby enhancing the material's properties and expanding the range of starch applications. On the other hand, starch can be phosphorylated through reaction with inorganic phosphate compounds, such as STMP, forming covalent phosphate diester bonds that act as crosslinks within the starch network [81]. Approximately 60-70% of the total P is bound to the C-6 position of the starch anhydroglucose units, with the remainder attached at the C-3 position. The rheological behavior of these hydrogels depends on both temperature and NaOH concentration, as these factors influence the stability of the diester phosphate bonds. An increase in NaOH concentration promotes hydrolysis of the diester linkages, reducing viscosity by disrupting the entanglement points within the gel matrix. The crosslinking of corn starch with STMP has also been reported by [PERSON] and [PERSON] (2006) using reactive extrusion (REX) at 130 \({}^{\circ}\)C [82]. The crosslinking mechanism involves the ring opening of STMP, followed by radical grafting to form starch tripolyphosphate (TPP). The reaction is completed when the phosphate end group reacts with another starch polymer, resulting in a covalent crosslinked bond [83]. STMP has also been applied in the (REX) of starch-carboxymethyl (SCM) cellulose to form hydrogel films at different pH levels. These films demonstrated higher swelling capacities compared to pure starch films. Moreover, the grafted carboxymethyl (CM) cellulose improved the mechanical properties of the resulting hydrogels. At higher pH values, the ionization of carboxyl and phosphate groups further increased the swelling degree due to enhanced electrostatic repulsion [83].