Biodegradable Seed Encapsulant for Aerial Reforestation using UAV’s

The use of Uncrewed Aerial Vehicles (UAVs) is the future of reforestation practices, with the potential to enhance targeted delivery, increase efficiency, reduce costs, and provide a flexible system capable of operating in challenging environments. Current technologies often employ seed balls, which are spherical objects generally consisting of clay, soil, and other additives to support the growth and germination of seeds. However, while these can be effective, they face an array of challenges, including timely manufacturing and transport, inaccurate delivery, susceptibility to environmental disturbance, limited support for long-term establishment, and vulnerability to pests, all of which greatly limit their efficacy. The use of hydrogels was identified as a novel encapsulant for UAV dispersal and may provide distinct advantages over commercial counterparts including surface adherence to improve targeted delivery and providing greater support for seeds through a nutrient-rich hydrogel matrix as a seed encapsulant. Previous research into hydrogel coatings and applications has focused on, coating the seed with a thin layer of material, applying the hydrogel to the soil before delivery, application onto established seedlings. Whereas in this study, research focused on encapsulating the entire seed in a nutrient rich encapsulant. Investigating the encapsulants shall greatly enhance understanding of the viability of hydrogel based encapsulants in a reforestation setting and assess the entire usability of the chosen natural and biodegradable materials in terms of production, deployment, germination and growth across a range of species. Hence, the project assesses the development and application efficacy of biodegradable hydrogel seed encapsulants capable of being deployed via UAVs for applications in environmental reforestation. The research was therefore divided into four areas, which formed the basis for the investigation and structure of this investigation. Firstly, assessment of the encapsulant’s requirements, including both seed and mechanical requirements, as well as material analysis to justify research pathways. Secondly, provision of encapsulant formulations to enable deployment and growth, this includes assessment of mechanical properties to determine attributes and suitability to achieve the aims of the project. Third, performance of encapsulant (i) upon deployment via UAVs for reforestation and/or (ii) upon automated deployment in agricultural applications. Lastly, simulation of field assessment of encapsulant performance in laboratory environment which will include identified native as well as common agricultural seeds. Through rheological, flow rate index, compression, and deployment tests, various hydrogel encapsulants successfully met the preparation and deployment criteria. The extended stabilisation times of many PHM encapsulants was crucial in determining efficacy. Initial encapsulants (Chapters 3 and 4) using psyllium husk mucilage (PHM) required measuring the stabilisation point of the material, with yield stress as the defining analysis. Adding dextran altered stabilisation time and reduced yield stress, while sodium alginate increased gelation time but enhanced yield stress. Therefore, encapsulants containing sodium alginate (ALG) did not meet processing window requirements and were not used in the growth and germination testing phase. Other key findings included the assessment of extrusion force (EF) for PHM samples namely P-2, D-4.1, and D-5.1. Dextran included samples (D-4.1 and D-5.1) demonstrated reduced extrusion force, enhancing feasibility for aerial deployment. The research identified D-4.1 and D-5.1 as effective mechanical modifiers, with D-5.1 achieving a yield stress of 156 Pa after 24 hours, outperforming other samples. The chapter also established extrusion force requirements for scaled-up models, determining maximum forces of 119N, 128N, and 137N for P-2, D-4.1, and D-5.1, respectively. These findings contribute to the selection of optimal material combinations for future applications in biodegradable gel systems. Unlike the PHM encapsulants, the bentonite-based encapsulants (Chapters 5 and 6) had no stabilisation time concerns, as ALG and carboxymethylcellulose (CMC) could be transported in solubilised form. Defining clay concentration was crucial, with rheology used to determine a minimum viscosity/yield stress for the materials. The study concluded that a minimum of 50 w/v% bentonite clay in a 1 w/v% polymer solution (CMC/ALG) was necessary for extrusion. Deployment testing also concluded that an encapsulant containing 50 w/v% bentonite clay successfully met the deployment criteria. Additional additives were also prepared in the form of CMC-Citric Acid (CA) particles, prepared by the dehydration esterification process, and ALG(-CMC) microbeads prepared in a w/o emulsion with calcium chloride. These additives were prepared for the purpose of water and nutrient retention, in which similarly developed particles have shown great success in soil additive applications. The study mainly focused on swelling ratio (SR) in which the microparticles far outperformed the CMC-CA particles. Moreover, the CMC-CA process often produced uneven films even at reduced temperatures (40°C), resulting in material losses due to uneven crosslinking, which again favoured the use of microparticles. It is important to note though that research has produced varying results on the effectiveness of particles given their size, and as such larger particles (CMC-CA) might have a greater prolonged effect on plant growth than microbeads despite lower SR. However, the microparticle size could be increased by altering the polymer or surfactant (Tween® 80) concentration or changing the process to extrusion based which would be more environmentally friendly as there is no oil waste phase. The study concluded that the optimal ALG-CMC microbead was developed using 100mL of 4% w/v polymer solution containing an ALG-CMC ratio of 1:1, in 500mL oil phase containing 1% v/v Tween® 80. These particles had an 80% inclusion size of 196μm to 577μm and had an average SR of 48.4 which was significantly higher than beads without the inclusion of CMC (19.6). Growth and germination testing concluded that the developed encapsulants were unsuitable for Acacia Stenophylla at drier conditions (50% Field Capacity (FC)). However, at 90% FC, the use of the bentonite-CMC encapsulants improved the development of seedlings. In the case of Cymbopogon refractus seeds, both PHM and bentonite-based encapsulants were not usable without further development. The PHM gel was entirely unsuitable resulting in seedling death due to gel degradation, and CMC showed poor germination and growth. However, for A. stenophylla growth the P-2 encapsulant at both 50% and 90% FC improved both total mass and germination rates compared to C-Surface. At 90% FC, P-2 also showed comparable root, shoot, and leaf development to C-Planted, indicating its potential for use in high-moisture environments with fast-growing species. Despite extensive testing, future studies could overlook the agricultural seeds if the focus is reforestation as no correlation was observed between agricultural and non-agricultural seeds. For agricultural seeds, it was concluded that seed selection had a greater impact on encapsulant effectiveness than soil moisture. For beans, the PHM-DEX blend was the best encapsulant for growth, while for cucumbers, the bentonite-CMC encapsulant with water retention beads and fertilisers (CMC-AB) performed best. However, encapsulant performance was affected by soil moisture, with PHM encapsulants ineffective at 50% FC for cucumbers but comparable to CMC-AB at 90% FC. This indicates that encapsulants must be optimised for specific conditions to maximise seed usage efficiency. Moreover, encapsulants largely reduced germination for all seeds, which was concluded to be a result of the encapsulant creating a barrier that prevented emergence. As such, despite certain improvements in growth parameters, the overall results suggest limited justifiability for large-scale agricultural use of both the developed ALG/CMC and PHM gels without overcoming the barrier properties of the gel. Therefore, it was concluded that while hydrogel encapsulants were effective in an agricultural setting, they performed poorly for arid/semi-arid species, with germination largely impeded by the encapsulants, particularly for smaller seeds. The research indicates that while hydrogels have great potential for creating a material that can be easily prepared on-site and effectively deployed from a 3m height, even onto hard surfaces, their effectiveness in promoting germination and growth was rather poor. However, the inclusion of hydrogel bead additives resulted in significant improvements in growth, suggesting that future investigations addressing the highlighted challenges may hold potential for success.