# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 3 SHs Formation Methods
### Crosslinking Physical Methods  
SHs crosslinked by physical methods are reversible and can be modified by environmental conditions [98]. These materials rely on non-covalent interactions such as hydrophobic forces, polyelectrolyte complexation (PEC), electrostatic interactions, and hydrogen bonding [99], thereby reducing the reliance on toxic crosslinkers typically used in chemical methods. However, while physically crosslinked SHs exhibit water insolubility, their overall stability is generally lower compared to those formed via chemical crosslinking [100]. In aqueous systems, starch granules are readily disrupted by heat, initiating the gelatinization process and enabling polymeric interactions with other components. Upon cooling (within a range of 4-75 \({}^{\circ}\)C), starch undergoes retrogradation, during which the polymer chains realign and recrystallize, forming a structured material within a mold.  
Alternatively, SHs can be prepared in organic alcohols, resulting in the formation of alcogels. When subjected to freeze-drying to remove water vapor, the resulting material is termed a cryogel, characterized by a nanoporous structure with a high surface area [101]. However, this structural behavior is highly dependent on the starch source and the AM-to-AP ratio [102]. Such behavior is also observed in chemically modified starches. For instance, polyvinyl acetate (PVA) has been grafted onto starch via radical copolymerization, forming a hydrogel crosslinked by physical interactions. In this case, the SC of the starch-g-PVA hydrogel can be modulated by adjusting the length of the grafted PVA branches [103]. The crosslinking occurs primarily through hydrogen bonding between hydroxyl (-OH) groups, as seen in hydroxyethyl starch (HES)-PVA hydrogels, where FTIR analysis revealed that the physicochemical properties depend on the polymer ratios [104]. The hydrogel properties were strongly influenced by HES concentration, which affected both pore size and pore area distribution. In another example, CS--a natural polysaccharide--forms a stable polymeric blend with HES. The miscibility of this blend is attributed to hydrogen bonding between the reactive groups of CS and hydroxyethyl starch, resulting in improved compatibility and stability [105].  
PEC hydrogels represent another class of physically crosslinked materials formed through ionic interactions. These hydrogels are created by the electrostatic attraction between oppositely charged polyelectrolyte chains, resulting in materials with notable swelling capabilities. [PERSON] et al. (2009) developed PEC hydrogels using \(\kappa\)-carrageenanan as the counter-polyion to starch modified with 2-hydroxy-3-(N,N,N-trimethylammonium) propyl groups, which confer a polycationic character [106]. Swelling tests showed that the hydrogel maintained structural integrity, exhibiting rapid swelling within the first hour, reaching an SC of 380%. To further enhance the performance of PEC SHs, starch was modified with CM groups, enabling complex formation with CS. The resulting PEC from starch-carboxymethyl (SCM) and CS exhibited a prolonged disintegration time, indicating improved stability [107]. However, the swelling behavior of the SCM-CS hydrogel depends on the properties of the swelling medium. At pH 1.2, the hydrogel demonstrated greater swelling compared to pure water. This enhanced swelling at an acidic pH is attributed to the protonation of functional groups within the polymeric matrix, which promotes electrostatic repulsion and facilitates water absorption [108].