(Nanowerk Spotlight) Polymer science has long been at the forefront of developing materials for agricultural applications, but a persistent challenge has been creating effective delivery systems for agrochemicals that don’t contribute to environmental pollution. Traditional polymer-based carriers for pesticides and fertilizers often break down into microplastics, posing risks to ecosystems and human health. This has led to bans on certain types of polymer agrochemical carriers in regions like the European Union. The agricultural industry has thus been searching for biodegradable alternatives that can effectively deliver and release agrochemicals while minimizing environmental impact.
Previous approaches to this problem have focused on developing biodegradable polymers or creating porous structures for controlled release. However, combining both aspects – full biodegradability and highly porous morphology – in a single material has proven difficult. Porous polymer particles offer advantages for controlled substance release compared to solid particles, but engineering degradable versions with the right properties has been an ongoing challenge.
Recent advances in polymer chemistry, particularly in the synthesis of polyphosphoesters, have opened new possibilities. Polyphosphoesters are a class of polymers containing phosphorus in their backbone, which can be designed to degrade under specific conditions. This characteristic makes them promising candidates for environmentally-friendly materials. Concurrently, progress in understanding the self-assembly of block copolymers has enabled the creation of complex nanostructures with precise control over size, shape, and internal architecture.
These parallel developments have set the stage for a potential breakthrough in agrochemical delivery systems. By combining degradable polymers with advanced self-assembly techniques, researchers saw an opportunity to create fully biodegradable, porous particles that could effectively encapsulate and release agrochemicals. This approach aimed to address the dual challenges of environmental persistence and controlled delivery that have long plagued the field.
A team of researchers from the University of Münster, University of Twente, and other institutions has now developed a novel class of fully degradable polymer cubosomes for sustainable agrochemical delivery. Their work, published in Advanced Materials (“Fully Degradable Polyphosphoester Cubosomes for Sustainable Agrochemical Delivery”), represents a significant step forward in creating environmentally friendly carriers for pesticides and other agricultural chemicals.
Synthesis of PEEP-b-PLA with different weight fractions of PLA and the self-assembly of ELA 1–4. a) AROP of rac-lactide with PEEP26 as macroinitiator in the presence of DBU in THF at RT. b) SEC traces of PEEP26 and ELAs 1–4 (measured in DMAc with polymethylmethacrylates as standards). c) Schematic of nanoprecipitation. d-g) ELAs with different block ratios and SEM images: d) ELA 1 (83:17) assembled into mainly vesicular structures, e) ELA 2 (85:15) assembled into vesicles and sponge-like morphologies, f) ELA3 (87:13) yields cubosome-like particles, g) ELA 4 (89:11) results in highly ordered PCs. (Imgae: Reproduced from DOI:10.1002/adma.202406831, CC BY)
The researchers synthesized block copolymers composed of poly(ethyl ethylene phosphate) (PEEP) and polylactide (PLA). These polymers were designed to self-assemble into highly ordered, porous structures called cubosomes when mixed with water. Cubosomes are particles with a complex internal network of water channels, giving them a very high surface area and unique properties for substance encapsulation and release. The cubosomes developed in this study had an average pore size of 19 ± 3 nanometers, which contributes to their distinctive release profile.
A key innovation in this work was the ability to create cubosomes that are fully degradable. Both the PEEP and PLA components of the polymer can break down into benign byproducts – phosphates and lactic acid – under environmental conditions. This addresses a major concern with previous polymer-based agrochemical carriers, which often persisted in the environment as microplastic pollution. Both components of the cubosomes – the poly(ethyl ethylene phosphate) and polylactide – degrade through a backbiting mechanism into these benign substances. This complete biodegradability represents a significant environmental benefit over conventional carriers.
The researchers demonstrated the utility of these cubosomes by loading them with tebuconazole, a common fungicide used in agriculture. They found that the cubosomes could incorporate significant amounts of the fungicide – up to 24% by weight – while maintaining their porous structure. Interestingly, the addition of tebuconazole caused the particles to transform from polymersomes (vesicle-like structures) into cubosomes, enhancing their ability to load hydrophobic substances. This high loading capacity is a crucial feature for practical applications, as it allows for more efficient delivery of active ingredients.
One of the most striking findings was the release profile of the fungicide from the cubosomes. When compared to solid polymer particles containing the same amount of fungicide, the cubosomes released their payload much more quickly and consistently. The porous structure of the cubosomes allowed for a steady, linear release of tebuconazole over about six days, while the solid particles showed a much slower and less complete release. This controlled release behavior is highly desirable for agricultural applications, as it can provide more effective pest control while potentially reducing the total amount of chemicals needed.
The researchers also tested the effectiveness of the fungicide-loaded cubosomes against Botrytis cinerea, a common plant pathogen that causes gray mold. They found that the cubosome formulation was highly effective at inhibiting fungal growth, demonstrating its potential as a practical crop protection tool.
Another important aspect of the study was the investigation of how well the cubosomes adhered to plant leaves. Using grapevine leaves as a model system, the researchers showed that their cubosomes stuck to leaf surfaces much better than solid polymer particles when exposed to simulated rain. After two simulated heavy rain events over a week, about 47% of the cubosomes remained on the leaves, compared to only about 13% of the solid particles.
This improved adhesion is partly due to the lower zeta potential of cubosomes (-12.8 mV compared to -47.17 mV for solid particles), which enhances their ability to stick to leaf surfaces. Zeta potential is a measure of the electric charge on the surface of particles, which affects how they interact with each other and with surfaces. A lower zeta potential indicates that particles are less likely to repel each other and more likely to adhere to surfaces, enhancing their ability to stick to plant leaves.
This improved rain fastness is crucial for agricultural applications, as it means more of the active ingredient stays where it’s needed rather than being washed away into the environment.
The researchers also studied degradation of the cubosomes in detail. They found that the particles began to break down rapidly in alkaline conditions, with the PEEP component degrading first, followed by the PLA. This controlled degradation process ensures that the carrier material doesn’t persist in the environment long after it has served its purpose.
This research represents a significant advance in the field of sustainable agriculture. The development of fully degradable, highly porous polymer particles for agrochemical delivery addresses several key challenges simultaneously. These cubosomes offer improved loading capacity, controlled release, and better adherence to plant surfaces compared to traditional solid particles. At the same time, their complete degradability into non-toxic byproducts addresses growing concerns about microplastic pollution from agricultural practices.
While further research and development will be needed to bring this technology to commercial use, the principles demonstrated in this study open up new possibilities for environmentally friendly agrochemical formulations. The approach could potentially be extended to other types of agricultural inputs, such as fertilizers or plant growth regulators.
As agriculture faces increasing pressure to become more sustainable while still meeting global food production needs, innovations like these degradable polymer cubosomes may play a crucial role. By enabling more efficient and targeted use of agrochemicals while minimizing environmental impact, such technologies could help strike a balance between productivity and ecological responsibility in modern farming practices.
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