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Abstract:
Developing accurate methods for the assessment of therapeutic protein release from polymer drug delivery systems (microcapsules, microspheres, nanoparticles, 3D-printed systems) is of paramount importance for new formulation development. The most straightforward method for protein release assessment is spectrophotometric analysis of the release medium surrounding the formulation. However, direct spectrophotometric analysis is inapplicable to formulations releasing interfering compounds (co-encapsulated drugs, additives) absorbing light in the same spectrum as proteins. Conventional protein release assays also require frequent release medium sampling and replacement, which reduces their accuracy. We propose a one-step method to assess protein release from core/shell microcapsules eliminating the need for sampling and allowing selective real-time protein quantitation in the release medium. To prevent spectral interferences, released protein is differentiated from interfering compounds by employing a colorimetric protein assay reagent, forming a colour complex selectively with the protein, as the release medium. To eliminate sampling, we employed a continuous flow closed loop set-up, where the release medium is constantly circulating between microcapsule-containing tank and spectrophotometer. A series of colorimetric protein assay reagents (bromocresol green, tetrabromophenol blue, eosin B, eosin Y, biuret) were evaluated in terms of their applicability as the release medium in described system. Only biuret reagent was found compatible with proposed method due to formation of color complex stable over extended period of time and low adsorption to microcapsules. Presented method allowed effective evaluation of albumin release from alginate-polyethersulfone microcapsules with accuracy equal to conventional ‘sample and separate’ technique. Albumin release followed first-order kinetics with plateau reached after 19 h.

Keywords:
protein release, core-shell microcapsule, continuous flow apparatus, colorimetric protein assay, mass transfer coefficient, albumin

Abstract:
Alginate-polyethersulfone microcapsules obtained with the one-stage three-nozzle, electrostatic technique designed by our team can successfully be used to encapsulate biologically active material such as functional proteins, microorganisms, bacteria, fungi and mammalian cells. The paper presents the results of studies on the influence of electric parameters on microcapsule size and internal structure. During the manufacturing process, three different liquids, i.e., aqueous sodium alginate solution, separating liquid and membrane-forming solution, were forced through a three-nozzle head, placed in an electrostatic field. After gelling in a gelling bath, the multi-layer drops appearing at the tip of the three-nozzle head formed microcapsules. The electrostatic field was applied through electric impulses with varied values of: voltage (U), frequency (f) and impulse duration (τ). The results indicated, that an increase of all examined electric process parameters resulted in a decrease in average microcapsule diameter and lower uniformity of batches in terms of size. Average membrane thickness (parameter B) did not change significantly, but along with the increase of all electrical parameters, a significant decrease of membrane thickness at the thickest part (parameter N) was observed. The microcapsules that were the most symmetric in regard to membrane thickness were obtained in the series with variable voltage (U).

Keywords:
alginate, microencapsulation of cells, alginate-polyethersulfone microcapsules, electrostatic technique, immobilization