Formulation of Next-Generation Multicompartment Microcapsules by Reversible Electrostatic Attraction.

The combination of several active substances into one carrier is often limited due to solubility, stability and phase-separation issues. These issues have been addressed by an innovative capsule design, in which nanocapsules are assembled on the microcapsule surface by electrostatic forces to form a pH-responsive hierarchical capsule@capsule system. Here, melamine-formaldehyde (MF) microcapsules with a negative surface charge were synthesized and coated with a novel MF-polyethyleneimine (PEI) copolymer to achieve a positive charge of ζ=+28 mV. This novel coating procedure allows the electrostatic assembly of negatively charged poly-l-lactide (PLLA, ζ=-19 mV) and poly-(lactide-co-glycolide) (PLGA, ζ=-56 mV) nanocapsules on the microcapsule surface. Assembly studies at pH 7 gave a partial surface coverage of PLLA nanocapsules and full surface coverage for PLGA nanocapsules. The pH-responsive adsorption and desorption of nanocapsules was shown at pH 7 and pH 3.

Thermally induced release from polymeric microparticles with liquid core: the mechanism.

Herein we demonstrate how the volatility of a liquid can be manipulated by enclosing microdroplets of the liquid into thin polymeric shells. In this way, composite core-shell microparticles consisting of 80 wt% of a liquid core material and 20 wt% of a polymer can be made 150 °C more stable than the individual core component. The thermal stability of the composite microparticles is found to be determined by the boiling point of the core material and the average particle size, while the role of the particle shell thickness is much less relevant. Two mechanisms responsible for the release of the core material from the microparticles at elevated temperatures were resolved: (1) thermally induced degradation of the shell and (2) diffusion of the core material through the polymeric shell boosted by the increased inner pressure.

Effect of Modifying the Membrane Surface with Microcapsules on the Flow Field for a Cross-Flow Membrane Setup: A CFD Study.

In this study, the attachment of microcapsules on the membrane surface and its influence on the flow field for a cross-flow membrane setup are investigated. The microcapsules were placed on the top layer of the membrane. The overall purpose of this modification was the prevention of membrane biofouling. Therefore, in a first step, the influence of such a combination on the fluid flow was investigated using computational fluid dynamics (CFD). Here, different properties, which are discussed as indicators for biofouling in the literature, were considered. In parallel, different fixation strategies for the microcapsules were experimentally tested. Two different methods to add the microcapsules were identified and further investigated. In the first method, the microcapsules are glued to the membrane surface, whereas in the second method, the microcapsules are added during the membrane fabrication. The different membrane modifications were studied and compared using CFD. Therefore, virtual geometries mimicking the real ones were created. An idealized virtual geometry was added to the comparison. Results from the simulation were fed back to the experiments to optimize the combined membrane. For the presented setup, it is shown that the glued configuration provides a lower transmembrane pressure than the configuration where microcapsules are added during fabrication.

Towards Microcapsules with Improved Barrier Properties.

Microencapsulation is the generic term for numerous technologies, which are often used when the release rate of an active substance in a medium has to be controlled and/or contact between the active substance and the medium has to be prevented. This is achieved by wrapping the tiny particles or droplets of the active substance (capsule core) with a thin layer, or membrane, of another material (capsule shell). The permeability of the membrane determines whether, how fast and under which conditions the active material will be released and/or the components of the medium will enter the inner part of the capsule. Insofar as application is concerned, premature release of an active substance from microcapsules during storage is a very common problem. Prevention of diffusion of an active component or components of the outer medium through the capsule membrane is a complex challenge, which so far cannot be considered as solved. This review briefly covers the theoretical aspects of release kinetics from microcapsules and discusses how such parameters as capsule average size, capsule shell thickness as well as the chemical composition of active material and medium can influence the release profiles. All theoretical considerations are based on the dissolution-diffusion mechanism classically used for the explanation of diffusion trough flat membranes/films. In the second part of the manuscript it is discussed, which strategies have been used for the improvement of the barrier properties of microcapsules up to date and to which extent those strategies were successful.