Formulating Biologicals: Encapsulation and Stability

In an earlier article for Agropages [ “The Challenges of Biopesticide Formulation”, J. Bullock, Agropages 2019 Formulation & Adjuvant Technology, May 2019 http://news.agropages.com/News/NewsDetail---30891-e.htm. ] we described the different types of biological actives used in agriculture and described some of the challenges which these actives can present the formulator. Chief amongst those challenges is stability – in the case of natural extracts such as peptides and proteins there can be chemical or physical mechanisms for instability and in the case of microbials, biological stability can be an issue. Additionally, the stability of the product to degradation by UV light can also be important and challenging. One of the ways in which formulators can protect the active against degradation is by using microencapsulation technologies. As well as improving stability, microencapsulation can be used control the rate of release of an active or to reduce the exposure of the user to the active for reasons of safety.

Microencapsulation technologies are well-established in the field of conventional chemical pesticides and to an extent these technologies can be adapted for biological actives. The most common technology for microencapsulation is probably interfacial polymerisation. This method is best suited to an active ingredient which is soluble in a non-aqueous or oil phase, or in cases where the active itself is a liquid which is immiscible with water[ See for example “New Trends in Crop Protection Formulations 2013”, A. Knowles, Informa Agra 2013.]. The first step is to make up a non-aqueous phase which typically contains the active, a solvent or oil and a water-immiscible monomer. This phase is then mixed into the aqueous phase which contains (in addition to water) emulsifiers and a second monomer which is soluble in the aqueous phase. An emulsion is formed – usually by high-shear mixing - with droplets of the desired size distribution. The two monomers can react with each other at the only place where they meet, i.e. at the surface of the emulsion droplets between the two phases. The end result of this process is that the active and the solvent form the cores of micro-particles which are encapsulated in a polymer shell. These capsules are dispersed in water and formulated as a suspension or, if desired, the water may be removed in a subsequent step to produce a dry powder consisting of capsules which can be redispersed in water during the application.

Microencapsulation by interfacial polymerisation is popular with formulators of conventional chemical pesticides because it allows a great deal of flexibility to tune properties such as rate of release by varying parameters such as the droplet size distribution, the thickness of the capsule walls, the monomer types and the polymerisation conditions. In principle, interfacial polymerisation can be used for biological actives – and will have the same advantages - but care needs to be taken as the monomer chemistry (typically isocyanate) is highly reactive. In the case of biochemical extracts, if the active does not react with the monomers, this technology can be considered, but in the case of living microbial actives, the risk of the chemistry interfering with the organism is high and the technology is less likely to be suitable. A further possible disadvantage for those wishing to make “all natural” claims in the biologicals market is that the monomers and shell polymers are synthetic chemicals.

Another method of microencapsulation is spray encapsulation, which typically uses spray drying equipment. The starting point for this formulation is typically a water-based dispersion (if the active is a solid) or emulsion (if the active is a liquid). A film forming polymer is added to this formulation which is then atomised and sprayed into the tower of the spray dryer. Heated air is used to dry the droplets as they fall such that a dry powder or fine granule is formed, consisting of the active and formulants (e.g. dispersing agents) surrounded by a capsule wall consisting of the film-forming polymer. This method of microencapsulation does not involve any reactive chemistry, therefore the risks of chemically degrading the active are lower, making it potentially a good option for sensitive actives such as biological extracts or event living microorganisms. The final formulation is a dry granule with a low water activity, which can help to keep microorganisms in a dormant state until the product is applied in contact with water. It is also possible to add nutrients for the microorganism to the formulation prior to drying, such that when the product is activated by water the microorganisms can use these nutrients to grow.

A third method of microencapsulation is complex coacervation. Here the active is suspended or emulsified into a polymer (often gelatin). A second polymer (often gum arabic) is added and the pH of the system is reduced. This in turn reduces the solubility of the second polymer causing it to precipitate around the particles of polymer 1 and the active. When cooled, the mixture forms a complex of two polymers encapsulating the active. To make the polymer shell more permanent, a cross-linking agent is added. The end product is therefore an aqueous suspension of microcapsules. This is another potentially suitable method for bio-actives, as the chemistry is relatively “mild” and bio-derived polymers can be used as the capsule-forming material. One “watch-out” is however the cross-linking agent which could potentially interfere chemically with the active ingredient.

In addition to these more established methods of microencapsulation, there has also been a push towards using naturally derived capsules for biological actives. One method has been commercialised by Eden Research under the name of Sustaine[ “Sustaine: A Novel Microencapsulation Technology”, F. Leynk at the Biopesticide Summit, Swansea 2020 and via http://www.edenresearch.com/our-technology.]. The technology uses natural yeast cells (which are no longer active) as the encapsulant. In the example given by Eden, an active ingredient such as terpene can be absorbed into the cell via the cell pores. Under dry conditions these pores remain closed but in contact with water (e.g. after application) the pores open up to allowed controlled and sustained release of the active.

Microencapsulation technologies are also used in industries such as pharmaceuticals, food and cosmetics as well as in agricultural products. These other industries therefore offer a potential pool of novel ideas which could potentially be translated for use with biopesticides. One interesting naturally-based technology has been developed by Sporomex[ Sporomex – Applications http://www.sporomex.co.uk/applications.] and uses deactivated pollen cells as the encapsulant. The cells are porous and according to Sporomex this feature allows for a delayed release of the contents of the capsule, which can be further modified by a co-encapsulation process.

In summary:

The nature of biological actives means that microencapsulation can provide options to protect the active or to control its release.

Microencapsulation methods used for conventional pesticides can be adapted for biologicals but they can involve reactive components or ingredients which are not naturally derived.

Methods using naturally derived capsules have interesting future potential for biological actives in agricultural uses.