A few key examples that highlight advantages of controlled release hydrogels for agriculture are described below. The Maynard lab synthesized a trehalose hydrogel for the encapsulation and thermostabilization of the animal feed enzyme, phytase.Phytase is added to pig and poultry feed to increase digestion efficiency, improve feed economical and nutritional value, and prevent pollution by lowering phosphorous excretion into the environment.However, these enzymes are vulnerable to the high temperatures and moisture applied during feed pelleting processes, thus lowering their activity.The study demonstrated the post-synthesis, passive diffusion of phytase within the trehalose hydrogel matrix and subsequent improvement in enzyme activity retention when subjected to a high temperature of 90 °C. Moreover, when the gel was rehydrated, phytase quantitatively released from the gel through passive diffusion within 25 hours, a relevant time frame for the application. By incorporating a known protein stabilizer such as trehalose into the hydrogel scaffold, this study highlights the advantages of polymer tunability. When passive diffusion is the only release mechanism in an environment with unpredictable rainfall, like an agricultural field, the accuracy and efficiency of controlled release systems might be diminished. Stimuli-responsive systems can further enhance the control of the physiochemical properties of the carrier and of the delivery of the active. For example, the Wu lab has made a variety of stimuli-responsive carriers for pesticides including a composite material based on bio-char embedded in calcium-alginate hydrogels.

The carbon-rich bio-char increased the surface area and porosity of the material while the calcium-crosslinked alginate polymers invoked stimuli-responsive sensitivity. Gentian violet,macetas 5 litros a model hydrophilic pesticide, was incorporated into the scaffold in-situ, i.e. during cross-linking with divalent calcium ions. Due to the ionic nature of the system, the hydrogel scaffold demonstrated pH- and ion- sensitivity, causing the active’s release kinetics from the material to differ in aqueous solutions with varying acidity and ionic strength. The study also demonstrated that the hydrophilic hydrogel matrix prevented rapid soil leaching of the water soluble active in acidic, neutral, and alkaline soils. As the pH and ionic concentrations of soil and groundwater can vary significantly, this system is valuable in that it can be tuned for optimal controlled release performance. Due to the complexity of agricultural environments, there are many relevant stimuli for the triggered release of actives including shifts in pH and ionic strength , temperature, enzyme presence, and redox potential. One study of a dual-functional hydrogel demonstrated redox-responsive delivery of agrochemicals followed by soil remediation through heavy metal ion capture in soil.Specifically, they focused on creating a carrier that can release agrochemicals in the low redox potential environment of rice paddy fields. A disulfide-crosslinked carboxymethyl cellulose hydrogel was synthesized via 1-ethyl-3-carboiimide coupling. Model auxin, napthylacetic acid, was loaded post-synthesis because it bears a carboxylic acid and would interfere with the EDC coupling required to cross-link the hydrogel. Another model auxin, 6-benzyladenine, which did not have an amino or carboxyl group was loaded in-situ. The gel reversibly formed a solution in reductive environment due to the cleavage of the disulfide into disconnected thiol moieties. Therefore, with increased strength of the reducing conditions, the release rate of the model auxins also increased.

Free thiol groups of the reduced polymer could complex to metal ions Cu2+ and Hg2+ in contaminated soil or the gel could be regenerated upon oxidation of the thiols into disulfides. Due to their topology, films and coatings can be used for a number of specialty agricultural applications such as for fertilizer coatings or mulch.Rychter et al. created a degradable hydrophilic film made of PLGA-PEG-PLGA for the delivery of herbicides with varying water solubility, metazachlor and pendimethalin.The herbicides were cast with the polymeric film, and the hydrophilicity of the carrier matrix as well as the water solubility of the herbicides determined the release rate of the herbicides. The researchers enhanced the hydrophilicity of the film by increasing the percent of PEG, boosting the hydrolytic and enzymatic degradation rate of the polymer matrix, thus releasing the active ingredients at a faster rate. Moreover, metazachlor, which is relatively water soluble, released at a higher rate than pendimethalin, which is more lipophilic. Polymeric films have also been applied as seed coatings to help fortify seeds with microbes, enhance germination and development, and prevent seed infection.Coatings have been applied as multi-functional matrices, providing benefits beyond seed and crop productivity. In particular, Xu et al. demonstrated the use of a biopolymer-based seed coating for improved agrochemical delivery, seedling germination, and following growth.The matrix comprised of nanofibers embedded within a hydrophilic, biodegradable gelatin- and cellulose acetate-based coating and was used to carry and release copper, an important micro-nutrient and antimicrobial. By coating the seed, copper delivery was concentrated around the vicinity of the seed, allowing for more effective protection from a fungal pathogen. Furthermore, the authors demonstrated that the copper release kinetics were modulated based on the film composition and wettability, demonstrating the importance of physiochemical properties and interactions in agricultural systems.Alternatively, bulk polymeric materials can be pressed into pellets and utilized as carriers in this form. Boyandin et al. created pellets made of poly composites with PEG, PCL, and wood powder for the immobilization of hydrophobic herbicide, metribuzin.P3HB and PCL are hydrolytically and enzymatically degradable polymers while PEG is hydrophilic, so including the different polymers varies the physiochemical properties of the pellets.

As a result, the release of metribuzin was controlled by varying the composition of the pellets with faster release rates in the PEG composites over PCL and wood formulations. The smaller dimensions of micro- or nano- materials have unique features that bolster the targeting and overall efficacy of controlled release formulations. Particles with very small sizes, less than 100 nm, have potential to be taken up through plant roots or leaves and then diffused into plant vasculature.One study noted that plant cell walls have specific pore diameters of approximately 30 nm, and, as a result, aimed to create nanoparticles which were £ 30 nm.Also owing to their small size, and therefore higher radius of curvature, micro- and nano- carriers also have higher surface areas which enhances their interactions with surrounding environments, improving their molecular adsorption and surface reactions. For instance, targeting ligands and coatings that adhere to leaves can be included on the periphery of carriers, ultimately preventing active cargo from being washed away from hydrophobic plant surfaces.Additional benefits of the small size and high surface area of micro- and nano- materials include increased porosity, better diffusion of actives through the carrier, higher loading capacity of actives, and better responsiveness to external stimuli.Here, “micro-” or “nano-” are referred to as they were identified in their manuscript. Typically, this means diameters of 100 nm to 1000 nm for nanocarriers and several micrometers for microcarriers.Microgels/nanogels and microspheres are dense polymeric matrices just like bulk hydrogels, but are distinguished as they are colloidal networks. Generally, they allow for higher chemical functionality, are more porous,macetas de 30 litros and have better responsiveness than their macro-scale counterparts.Meurer et al. demonstrated many advantages of micro-gels through the application of a bis-crosslinked poly micro-gel for the foliar delivery of a micro-nutrient, Fe3+.Ionic micro-nutrients are generally water soluble and therefore experience insufficient effectiveness after rainfall or irrigation as they are prone to leaching with water. The micro-gels provided a scaffold which selectively swelled in acidic media; the poly hydrochloride component of the micro-gel is a polyelectrolyte, causing the matrix to swell in low pHs as electronic repulsions between the cationic polymer and protons in the acidic media occur. The gels were further modified with a ligand that strongly chelates Fe3+ ions, 2, 3-dihydroxybenzoic acid, to improve the uptake and release kinetics of the micro-nutrient. Additionally, the micro-gels were externally modified with anchor peptides post-synthesis where the peptides promoted adhesion to the waxy surfaces of plant leaves. When tested on iron-deficient cucumber leaves, the micro-gels demonstrated excellent delivery capabilities. Overall, the system highlights micro-gel tunability and modularity with multiple chemistry components collectively aiding in the delivery of a hydrophilic fertilizer. Microgels/nanogels and microspheres have been further tuned by using other degradable matrices to accelerate active release rate,coating their surfaces with polymeric shells to further slow agrochemical release,incorporating polymers with high ultraviolet absorptivity to protect a photosensitive herbicide,98 using stimuli-responsive PNIPAAm to create near-infrared and temperature-controlled release of an insecticide,and including moieties for the specific capture and sustained release of a signaling molecule.Like the bulk materials described previously, these materials are made of dense polymeric matrices and have demonstrated efficacious interactions with both hydrophilic and hydrophobic actives. Amphiphilic systems have also been developed in agriculture and offer more specificity towards active ingredients due to their ability to self-assemble in aqueous solutions. Amphiphilic carriers can be made from various systems, including block copolymers, random copolymers, combinations of homo-polymers, or inclusion complexes added to polymers. Generally, these materials rely on self-assembly; hydrophobic cores are able to sequester water insoluble bio-active agents while a hydrophilic outer shell aids in steric stability, solubility, and controlled release.

Micelles have been extensively applied for the protection of agricultural chemicals and biological actives.Through the self-assembly of amphiphilic block copolymers in aqueous solutions, micelles sequester hydrophobic cargo within their lipophilic core, while enhancing cargo solubility with a hydrophilic outer shell. Moreover, like micro-gels and microspheres, micelles can be externally modified for specific targeting capabilities. For example, a block copolymer of hydrophilic and hydrophobic blocks, maleimide-conjugated tetra and poly, respectively, formed micelles through core electrostatic interactions with plasmid DNA for exogenous gene delivery to plant cells.The maleimide end group was present on the hydrophilic outer shell of the micelle, facilitating post-modification of the micelle with cysteine containing, cationic peptides. These cell penetrating peptides facilitated transfer of plasmid DNA within plant cells. By using a micelle carrier for the DNA, instead of binding the peptide directly or pre-modification, destructive electrostatic interactions between anionic DNA and cationic peptides were alleviated, thus increasing gene delivery efficiency. Stimuli-responsive micelles have also been created for the release of internalized hydrophobic cargo. For example, Ye et al. created an amphiphilic chitosan-nitrobenzyl conjugate which assembled into a micelle in aqueous conditions and sequestered hydrophobic herbicide, Diuron.Upon exposure to light irradiation, the micelle transformed into a nanocapsule as the hydrophobic, photoresponsive nitrobenzyl moiety was released when exposed to sunlight. The herbicide could then passively diffuse through the shell carrier for controlled delivery. The authors observed high encapsulation efficiency through the micelle formation and delivery specificity in that Diuron did not release without a photo-trigger. b-Cyclodextrin was utilized as a core to create micelle-like, thermo- and pH- responsive star polymers with poly-block-PNIPAAm.These polymers were applied for the capture and delivery of a antimicrobial agent, crystal violet. The anionic poly block was functionalized closest to the host molecule core, allowing for core-sequestration of the cationic antimicrobial active. The PNIPAAm shell collapses into a globular state above its lower critical star polymer core. On the other hand, the shell hydrates when below its LCST, allowing for release of crystal violet. This temperature-responsive release could prevent adverse effects from heat, including heat-induced plant diseases. Finally, the amphiphilic nature of the system allowed for its cuticular penetration and, with the aid of a non-ionic surfactant, translocation to plant compartments such as leaves, roots, and stems. The nanoparticles resemble micelles in that they are created with amphiphilic block copolymers, however they are also are not prone to dissociation in dilute solutions above the critical micelle concentration and are less polydisperse due to precise conjugation of the polymers to the cyclodextrin core.Some systems utilize amphiphilic random co-polymers rather than micelles to sequester water insoluble cargo. Random copolymers with hydrophilic and hydrophobic segments will self-fold and aggregate in water due to discrepancies in polarity along the polymer chain. These simplified systems can be preferred for agricultural applications over other nanodelivery systems due to their low cost and facile preparation.The Sumerlin group reported the synthesis of an amphiphilic, pH-responsive polysuccinimide -based nanoparticles for the targeted delivery of non-polar agrochemicals into plants.Particularly, they envisioned nanoparticle delivery into the plant phloem, which aids in transport of nutrients and photosynthates and is susceptible to phloemlimited pathogens. The phloem environment is slightly alkaline, compared to the slight acidic surrounding plant tissue. Thus, a biodegradable, alkaline-responsive system was created with PSI which hydrolyzes into poly in basic conditions.