UC Davis

Complex coacervation is a phase separation of a mixture into two immiscible liquid phases mainly due to electrostatic interactions between two oppositely charged polymers. Microencapsulation by complex coacervation, though highly effective and achievable at the bench-scale, is challenging to scale-up because of the complexity and high-cost of the process. Thus, I developed a novel complex coacervation process (herein referred to as the ‘CoCo process’) that combines the coacervation, shell hardening and drying steps into one step by spray drying. During spray drying, the base vaporizes upon atomization, lowering the pH of the atomized droplets and inducing the two oppositely charged polymers to associate by complex coacervation. Rapid moisture removal force tightens associations between the polymers, leading to formation of water-insoluble microcapsules that are collected at the outlet of the spray dryer. The CoCo process overcomes the commercialization barrier and appears as a promising technique to encapsulate various cargo for a wide range of applications. This work investigated how to control the barrier properties of matrix microcapsules formed by the CoCo process to stabilize the bioactive components and control the release of the bioactive components for various applications. First, as a proof concept, the potential of the CoCo process to encapsulate volatile oil was investigated by encapsulating D-limonene using gelatin and alginate as matrix building components, and succinic acid and a volatile base in the formulations to modulate the pH. Here I defined a metric termed as the extent of complex coacervation (ECC) to assess the extent to which all polymers within the particles participate in complex coacervation and it was defined as the fraction of polymers that do not solubilize from the CoCo particles when the spray dried powders are suspended in water. Insoluble CoCo particles were produced without chemical cross-linking, with extent of complex coacervation of 75 ± 6% for D-limonene loaded CoCo particles with 82.7 ± 3.6% of D-limonene retained during spray drying (Chapter 2). Second, to understand how to control the barrier properties of the matrix, I investigated how the formulation variables including succinic acid and gelatin content influenced the extent of complex coacervation and how the extent was related to the barrier properties of the CoCo matrix to protect D-limonene from volatilization in dry powders and control the release of D-limonene in aqueous environments. The CoCo powders formulated with 4% gelatin, 0.5% alginate, either 0.5% or 0.75% succinic acid demonstrated enteric release of D-limonene with 18.0 ± 3.9% ~ 26.3 ± 6.4% of D-limonene release in simulated gastric fluid (SGF) and 58.2 ± 6.4% ~ 71.3 ± 3.4% of D-limonene release in simulated intestinal fluid (SIF) and 7.2 ± 1.0 ~ 7.7 ± 0.5% of D-limonene release in water. The matrix also provided robust protection for volatile compounds during spray drying, where ~78% D-limonene was retained and followed by 2-8% loss during subsequent 4-month storage at room temperature. This study demonstrated controlling barrier properties of gelatin-alginate CoCo powders using the novel CoCo process. For controlling the release of cargo in aqueous media, the extent of complex coacervation was important, where the higher extents of complex coacervation were achieved by increasing the gelatin concentration (increasing gelatin to alginate ratio) in the formulation. For retaining the cargo during spray drying and subsequent storage, controlling the extent of complex coacervation was not important (Chapter 3). Third, the latex polymer was added to the CoCo formulation to investigate how the incorporation of the polymer in the CoCo microcapsules influenced the barrier properties of the CoCo matrix. The effect of the latex polymer in the CoCo microcapsules was cargo-related. The CoCo microcapsules amended with a latex polymer-ethylcellulose were markedly less efficient at retaining D-limonene during spray drying. The volatile retention of D-limonene was 19.7% in the microcapsules with 0.25 parts ethylcellulose and 1 part CoCo polymers, compared to 77.7% of D-limonene retention in the CoCo microcapsules. The ethylcellulose in CoCo microcapsules also accelerated the release of D-limonene from 9.4% to 25.2% in water in 2 h and from14.1% to 25.2% in SGF in 30 min and slowed the initial release of D-limonene from 58.4% to 35.7% in SIF in 5 min (Chapter 4). The CoCo process was used for peptide encapsulation to facilitate oral delivery of therapeutic peptides. Five peptides - semaglutide, liraglutide, GLP-1, gonadorelin acetate, oxytocin acetate were used as model peptides. Promising enteric release of semaglutide and liraglutide was achieved with 0.25 parts latex polymers (e.g. ethylcellulose and polyvinyl acetate phthalate)-1.0 part CoCo polymers. Only 12.3 ± 0.7% of semaglutide and 24.0 ± 0.5% of liraglutide was released in SGF in 2h, while more than 88% of peptides was released in SIF. Peptides with more charges and side chains could enable more interactions between peptides and the matrix, leading to better protection for peptide in SGF (Chapter 5). Finally, the CoCo process was used to encapsulate bromelain, an enzyme mixture extracted from stems and fruits of the pineapple plant, and to explore the capability of the CoCo process to maintain the proteolytic activity of bromelain. Bromelain was not only the cargo but also incorporated as the wall material. Full bromelain activity recovery in bromelain CoCo powder was achieved using the CoCo process with approximately 40% protein coacervated with alginate (Chapter 6). Overall, the work demonstrated the potential of the CoCo process to microencapsulate different types of cargos and how the formulation development overcame the challenges related to the application of bioactive compounds.