Introduction to Electrospinning in Agricultural Applications

Mini farm in a city park.

High surface area and ease of incorporation of active ingredients have prompted some researchers to investigate using electrospun nanofibers in agricultural application. Another advantage of electrospinning as compared to other fiber spinning technique is that electrospinning-based nanofiber dispenser can be made portable and therefore can be used in farms if it is proven to be cost effective and efficient for agricultural applications. Active ingredients that have been incorporated into electrospun nanofibers include pheromones and pesticides.

Pest control

A method of controlling insect pests is to use pheromones to lead them to traps. Electrospun nanofiber with its large surface area is able to effectively release the pheromones for this purpose. Bansal (2010) showed that it is possible to electrospin nanofibers incorporated with pheromone for plant protection against Lobesia botrana (grape vine moth). However, this study used a complex emulsion mixture containing 1 wt% oligolactide /pheromone/Brij S20 dispersion/16 wt% polyhexyleneadipate-block-methoxypolyethyleneglycol dispersion/polyethylene oxide for electrospinning. Bisotto-de-Oliveira et al (2014) incorporated Trimedlure (synthetic attractant) into electrospun nanofibers for controlling medfly population. Ethylcelluose, polyethylene glycol (PEG- PCL), polyvinyl acetate- vinyl pyrrolidone and polycaprolactone were each blended with Trimedlure using organic solvents and electrospun to form nanofibers. Up to 10% w/v of the active ingredient was added. Their studies on the field showed that the chemically treated membrane is effective in attracting the flies.

An electrospinning setup by Bioinicia [Video of FLUIDNATEK LE-50].

Given that different polymers may be electrospun to give fibers, selection of the polymers for encapsulation of pheromones need to be tested to find the best combination in attracting the targeted pests. Parameters such as post-spinning pheromone loading factor and pheromone release rate needs to be considered. As shown by Bisotto-de-Oliveira et al (2015) in their encapsulation of synthetic sex pheromone of Grapholita molesta, the amount of pheromones in electrospun fibers were different in polyvinyl acetate/polyvinyl pyrrolidone (PVAc/PVP), polycaprolactone (PCL) with THF and PCL with DCM. Such difference in load retention may be due to the lost of pheromone to the surrounding during electrospinning.

Fungi protection

Fungi are another major class of disease causing organisms for agricultural crops. An advantage of electrospun membrane is that its high porosity allows the plant to breathe although actual application method needs to be tailored to the specific requirement. Esca is a disease of grapevine caused by Phaeomoniella chlamydospora and Phaeoacremonium aleophilum. Spasova et al (2019) tested the use of electrospun cellulose acetate/polyethylene glycol (CA/PEG) containing 5-chloro-8-hydroxyquinolinol (5-Cl8Q) for the protection of grapevine against P. chlamydospora and P. aleophilum fungi. Using agar culture, the electrospun CA/PEG membrane loaded with 5-Cl8Q showed inhibitory effect on P. chlamydospora and P. aleophilum fungi while neat electrospun membrane has no effect on them. This inhibitory effect was sustained over a 96 hrs period. Drug release test showed an initial burst release with 83% of the load released in the first 30 mins and this allows the minimum inhibitory concentration (MIC) to be reached quickly.

Electrospinning has been tested as a fungicide treatment for seeds. Castaneda et al (2014) used blending to incorporate commercially available fungicide (Vitavax Thiram SC-200 and Carbex 500) into ethyl cellulose before electrospinning over rice seeds. The results showed an improvement of rice germination to about 95% with the nanofiber coating, which is 8% more than the negative control (rice without coating). Using nanofibers to encapsulate the fungicide may allow different chemical additives to be used together through separate nanofibers.

Fertilizer Application

Fertilizer application is an area which electrospun fibers have been tested for. Fertilizers have been added into the solution for electrospinning into fibers. Depending on the polymer carrier and the structure of the fibers, the release rate of the fertilizers may be tailored [Castro-Enriquez et al 2012; Kampeerapappun and Phanomkate 2013]. A network of fibers is much less likely to be washed away than particles. This will reduce the amount of fertilizer loss by the farmer and at the same time prevent pollution of the waterways due to fertilizer run-offs. In the dry season, a layer of fertilizer loaded fibers electrospun directly over the top soil may also be used to hold down the soil with the seed in place while waiting for rain to arrive in an exposed dry field. Seeds coated with fertilizers loaded electrospun fibers have been shown to facilitate germination [Krishnamoorthy et al 2017].

Pollution/Contamination control

Agriculture with its use of fertilizers, pesticides, herbicides and fungicides to improve yield of the crops is often a source of pollution and may sometimes be detrimental to the crops itself. Herbicides while meant to reduce growth of weeds, may act against seed germination or seedlings if the water source is contaminated with it. Pesticides and fungicides residues are always a concern for consumers while excess fertilizers washed into the streams and rivers may disrupt its ecosystem.

In the removal of herbicides contaminants, Palvannan et al (2014) tested functionality and performance of laccase immobilized on electrospun zein/polyurethane nanofiber. The first step of the process is to electrospin zein/polyurethane nanofiber. Laccase was subsequently immobilized on the nanofiber through activation using glutaraldehyde and cross-linking in laccase solution. Retention of the functionality of laccase immobilized on the nanofiber surface was verified by the degradation of chloroxuron with up to 25 reuse cycles.

Affluent from animal farms is also another source of water contamination. These contaminants may be naturally occurring from the animals but due to the larger number of animals in a given area, it may lead to a higher concentration of the discharge which may affect other animals in the river system if it is left unchecked. Pule et al (2015) tested the ability of a colorimetric probe, based on gold nanoparticles incorporated into electrospun polystyrene nanofiber to detect 17β-estradiol concentration in water. This substance is a natural steroidal oestrogens which affects fish gender and reproduction in high concentration. Their study found that the probe possessed high sensitivity with naked-eye detection limit of 100 ng/ml. Upon contact with 17β-estradiol, the probe turns pink while for other substances such as p,p'-DDE, deltamethrin, 4-tert-octylphenol, nonylphenol and cholesterol , the probe changed to brownish color. This makes the probe sufficiently sensitive and precise in detecting 17β-estradiol.

Seed Development Research

The importance of understanding seed development has led to research using various methods to understand environmental stimuli on root growth. In the use of transparent microfluidic channel for studying root growth, one main challenge is to regulate the temperature on seed development. Jiang et al (2012) have demonstrated the effectiveness of using electrospun polyethylene oxide fibers membrane to control the temperature reaching the seed. Comparing the insulation property of electrospun membranes with cast membranes, the former was able to provide much better insulation for the same thickness. The level of insulation can be controlled easily by varying the duration of fiber deposition which directly impacts membrane thickness. With this, they were able to show the growth rate variation of Arabidopsis seeds at different temperatures.

Bacteria and microbes introduction

Bacteria and microbes are important players in the soil ecosystem. In some economically important crops, the presence of certain bacteria and microbes are essential for their health. Electrospun nanofibers may be used for the encapsulation of microbes to improves it viability and applications. Damasceno et al (2013) used electrospun polyvinyl alcohol (PVA) nanofibers to encapsulate rhizobia, an economically interesting bacterium found in legumes. The PVA matrix potentially protects the rhizobia from environmental stress such as temperature and dehydration. Comparing the viability of rhizobia encapsulated in PVA nanofiber and the negative control (unprotected rhizobia), significantly more rhizobia remains viability after 48 h of storage for those encapsulated in PVA nanofiber. Study was also carried out to determine whether the encapsulation of the bacteria in PVA nanofiber has a negative impact on the number of nodules present in soybean when applied. There is no significant difference in the number of nodules formed between rhizobia encapsulated in PVA nanofiber and the positive control (unprotected rhizobia) over a 30 days period. Therefore, encapsulation of rhizobia in PVA nanofibers is a viable method for storage and delivery of the bacteria. De Gregorio et al (2017) immobilizes plant growth promoting rhizobacteria (PGPR), Pantoea agglomerans ISIB55 and Burkholderia caribensis ISIB40 in electrospun PVA fibers for coating of soybean. After 30 days of storage, germinated soybean was successfully colonized by the bacteria. With P. agglomerans ISIB55, an increased germination, length and dry weight of the root was recorded. While with B. caribensis ISIB40, an increased leaf number and dry weight of the shoot was recorded.

Drug Delivery

Electrospun fibers have been tested for drug delivery in various medical applications. A similar strategy may be employed for farm animals. Karuppannan et al (2017) tested the potential use of electrospun progesterone-loaded zein fibers for estrus synchronization of bovines. In their study higher concentration of progesterone was found to disrupt fiber production due to increased viscosity. However, even at the lowest tested concentration, the release of progesterone can be sustained over a week with 87% of the hormone released. With higher progesterone concentration, the half-life of the hormone release increases accordingly.

In-vitro pollen germination

Pollen germination in vitro is useful for determining pollen viability and other in vitro tests. The medium for supporting pollen germination typically comes in the form of liquid or agar gel. Electrospun membrane may be used as a physical support for pollen germination on liquid medium. Bodhipadma et al (2016) showed that with appropriate electrospun membrane thickness, pollen germination of A. hexapetalus may be enhanced when used with liquid medium. Two materials, polyvinylidene fluoride (PVDF) and polylactic acid (PLA) were electrospun. When the membranes were thin at 8 µm, germination of more than 65% were recorded compared to less than 60% for liquid only medium and less than 50% for agar gel medium. However, it is important to note that at greater membrane thickness (18 µm), the germination dropped significantly to less than 3%. This is probably due to the thick membrane blocking access of the pollen to the liquid medium. In terms of pollen tube length, with electrospun membrane and liquid medium, more than 500 µm length was recorded compared to just 390 µm and 140 µm length for agar gel and liquid only media respectively.

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Content [Agriculture]