Strong, Christopher
(2023)
Design and development of a novel plant cuticle model using 3D printing.
PhD thesis, University of Nottingham.
Abstract
The agrochemical penetration rate through the cuticle is dictated by wax composition. Cuticle wax composition varies between plant species, resulting in the same agrochemical displaying variable penetration rates as a function of the plant the agrochemical is applied to. Subsequently, methods to maximise cuticle penetration have been identified including surfactant use and the interactions of cuticle waxes, surfactants and agrochemicals has been intensively studied. However, elucidating the contribution of specific waxes and specific mixes of waxes upon these interactions is challenging due to the compositional complexity of cuticle waxes. As such, predictive modelling of these interactions is challenging and models predicting penetration as a function of the cuticle wax and agrochemical physiochemical properties are not possible. Subsequently, agrochemical permeation must be determined experimentally.
This thesis aimed to produce a model of the cuticle waxes comprised of cuticle wax analogs using a 3D printing technique, hot-melt inkjet printing, enabling various cuticle interactions to be investigated and the contribution of specific waxes and wax mixtures to be eluded. Three waxes were selected: dotriacontane, 1-docosanol and stearic acid based on ToF-SIMs analysis of the wax composition of Solanum Lycopersicum epicuticle and intracuticle surfaces. DSC, FT-IR, XRD and a rheometer were used to measure crystallinity, thermal properties, viscosity, eutectic properties, and printability. All tested waxes were printable at 90 °C and presented 50% crystallinity or greater like reconstituted cuticle waxes.
The wax analogs were used as material to 3D print wax films, approximately 50 - 200 μm in thickness, and material characterisation was conducted to analyse the wax powder and printed film. FT-IR showed printing did not impact the film composition; however, FT-RI and XRD showed crystallinity significantly decreased following printing. Contact angle measurements of water and two alcohol ethoxylate surfactant solutions containing 20 and 7 ethoxylate units Synperonic A20 and A7 respectively, were conducted to enable testing of the wax composition’s effect on wetting. Both wax composition and topography significantly changed contact angle, however it is predicted this was driven predominantly by topography. This is because the contact angle on a surface formed in confocal contract with a flat substrate presented no significant change as a function of composition.
Imaging of the films showed surface wax crystalloids mimicking the morphology of cuticle crystalloids. The crystalloids’ structure and distribution were controlled via changing the drops per inch (DPI), substrate temperature and film thickness. Raman Spectroscopy images showed crystalloids predominantly formed at the interface of print tracks, where wax phase separation occurred. It was theorised crystalloids formed because of localised cooling of heterogeneously distributed waxes, that freeze at different temperatures, leading to phase separation and the local purification causing short range ordering and crystalloid formation.
Azoxystrobin penetration through printed films was performed using a Franz cell and azoxystrobin solubilised in water, Synperonic A20 and A7. Aqueous Azoxystrobin permeated 1-docosanol and a 1-docosanol stearic acid binary wax. Interestingly, permeation did not correlate with % crystallinity or sorption rate. However, it matches literature showing cuticles with large alcohol wax compositions permitted rapid permeation. Similarly, the addition of stearic acid to an alcohol wax film enhanced permeation, suggesting wax mixing modifies agrochemicals permeation. This highlights the correlation between wax composition and permeation rate, and that specific waxes and wax mixing induce rapid penetration. However, no azoxystrobin permeated through any other film when solubilised in water, nor when solubilised in A7 or A20. It is theorised excessive portioning into the A7 and A20 micelles prevented permeation through any of the tested films.
Finally, Z. tritici, a fungal spore, attachment to printed wax films was tested to assess the relationship between wax composition and topography on fungal attachment. No correlation was found between surface morphology and attachment. However, a correlation between attachment and composition was observed as attachment was significantly increased on films comprised largely of stearic acid. Similar trends were not observed for 1-docosnaol or dotriacontane. This matched literature showing Z. Tritici preferentially attaches to cuticles containing large quantities of carboxylic acids.
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