Micro-encapsulation of Essential Oils for AntimicrobialFunction and Mosquito Repellency

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the isoelectric point in order to prevent aggregation. Electrophoresis is one of the meth-

ods used to calculate the zeta potential by measuring the electrophoretic mobility of the

microcapsules in a medium (Gray et al. 2016). A study on chitosan coated microemul-

sions (CH-MEs) found that the zeta potential of CH-MEs was increased after being coated

with chitosan solution but decreased when surfactant concentrations increased. This was

led to develop CH-MEs with optimal stability and acceptable physicochemical behaviour

(Kesavan et al. 2013).

12.3.3

Release of the core material

The release of the core material from the microcapsule is measured by using gentle ag-

itation to achieve a well dispersion and then the changes in the solute concentrations over

time are measured either in intervals or continuously. The release kinetics and the diffu-

sion coefficient can therefore be calculated (Gray et al. 2016). The release rate of an active

ingredient from the microcapsule depends on different factors including: the wall mate-

rial, the core material itself, the morphology and geometry of the particle, the degree of

cross-linking, the conditions (e.g. pH, temperature, ionic strength) and the method of mi-

croencapsulation (da Silva et al. 2014; Dima et al. 2016). A study on chitosan-encapsulated

menthol microcapsules observed that the amount of crosslinkers used (TPP) had an effect

on the release time of menthol, and generally the higher the TPP concentration, the slower

the release time. At 1% w/w TPP, 95% of menthol was release within 60 h, whilst at 15%

w/w TPP used, only 38.3% was released at 60 h (Nuisin et al. 2013).

It is important that the release of the core material occurs at the appropriate time and

place. A study by Nuisin et al (2013) shows that the release rate is mainly related to interac-

tions between the core and wall material (da Silva et al., 2014). The pH of the microcapsule

environment can also have an effect on the release rate of the core material. The drug con-

taining alginate-pectin microcapsules showed higher drug release percentages in acidic pH

1.2 compared to an alkaline pH 8.2, with maximum drug release of 75.6% and 42% re-

spectively (Jaya et al. 2009). The characteristics and the quantity of the core material can

also affect release rate. Dürrigl et al. (2011) found that their calcium loaded microparti-

cles, showed greater drug release with a higher drug load and a 2:1 drug: polymer ratio.

Release studies are conducted using mathematical models that describe the different ways

that molecules are transported across the capsule wall. Larger microcapsules are more

suited to controlled release applications, because there is a reduced protection of the core

material by the wall material and therefore the release rate of the core material is improved

(Dong et al. 2011).

12.4

ANTIMICROBIAL ACTIVITY AND MOSQUITO REPELLENCY OF EN-

CAPSULATED ESSENTIAL OILS

Essential oils (EOs) are aromatic natural products typically extracted from plant matter

by distillation (Georgiev et al. 2019). EOs have been a subject of interest as alternative

antimicrobial agents, because they convey broad spectrum antimicrobial activity against

bacteria, fungi and viruses (Tariq et al. 2019; Winska et al. 2019). For example, cinnamon