# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 3 SHs Formation Methods
### Crosslinking Chemical Methods
The highest SC was observed in hydrogels made from degraded starch, likely due to its lower molar mass, which improves accessibility to crosslinking agents and water. Glyoxal (GYX), a dialdehyde, facilitates the formation of starch-\(\kappa\)-carrageenan hydrogels through hemiacetal bonds, resulting in a stable crosslinking structure at room temperature. The methodology involves two steps: (i) dispersing starch and \(\kappa\)-carrageenan separately in a 10% (\(v/v\)) GYX solution and heating each to 80 \({}^{\circ}\)C for 20 min, and (ii) mixing the polymer dispersions at 80 \({}^{\circ}\)C. This process exemplifies a simple and scalable method for hydrogel preparation, making it suitable for industrial applications such as plant nutrient delivery [93]. Starch can also be chemically modified to enable alternative crosslinking mechanisms, such as thiol-ene click chemistry between starch chains. However, this method involves multiple synthesis steps compared to more conventional approaches. The process includes (i) synthesizing allylic starch in a micellar medium using allyl chloride for 24 h at 45 \({}^{\circ}\)C under agitation [94]; (ii) producing carboxyl succinic starch (St-COOH) via esterification in DMSO at 70 \({}^{\circ}\)C; (iii) obtaining thiolated starch (St-SH) by coupling L-cysteine hydrochloride to the carboxylic acid groups of St-COOH; and (iv) forming SHs by mixing AS and St-SH in PBS, followed by UV irradiation (365 nm, 36 W) for 30 min. The resulting hydrogel demonstrated high biodegradability when exposed to \(\beta\)-amylase at pH 6.7-7.0, which efficiently degraded the starch chains. The development of starch-based polymeric blends also broadens the spectrum of crosslinkers that can be utilized in hydrogel formation. For instance, an alginate/SH hydrogel was synthesized using crosslinkers such as MnCl\({}_{2}\), ZnCl\({}_{2}\), and CaCl\({}_{2}\). In this system, Mn\({}^{2+}\), Zn\({}^{2+}\), and Ca\({}^{2+}\) ions crosslink alginate while simultaneously entrapping starch within the polymeric matrix, thereby increasing the material's hydrophobic domains [95]. A similar strategy was applied to produce a polymeric hydrogel from starch and CS, a natural polycationic polymer known for its biocompatibility, biodegradability, and non-toxicity. However, the high cost of CS limits its widespread industrial application. Combining it with the more economical biopolymer starch enhances the composite material's performance while reducing production costs, making it viable for various industrial processes [96]. In this case, TPP acts as a crosslinker by forming ionic interactions between its negatively charged phosphate groups and the protonated amino groups of CS, effectively entrapping starch within the matrix. This hydrogel system showed great potential for the controlled release of plant growth-promoting bacteria (_Azospirillum brasilense_ Az39 and _Pseudomonas fluorescens_ ZME4). The polymeric matrix successfully released both bacterial strains into natural soils, demonstrating its suitability for agricultural applications [97].