Core shell particles for drug delivery

Similarly as surfactant analogues, block copolymers form association colloids in selective solvents. This means that the solvent is poor for the core-forming block and good for the corona  (shell) forming entity. Unless the corona block is very short and the core forming block relatively long, one finds spherical micelles as the preferred aggregate architecture. Micelle formation by polymeric species has various intricacies. Very often the solubility of the copolymer is extremely low so that it is not trivial to find ways to make the micelles. Moreover, once micelles exist, the mobility of chains in the core may be extremely low, in particular when the temperature is below the glass-transition temperature. Then the micelles do not respond to external conditions.  The latter may be advantageous in some applications.

Polymeric micelles composed of a hydrophobic core and a hydrophilic corona are conveniently made by the solvent exchange method. The idea is that the polymer is dissolved in a good solvent for both blocks and then quickly mixed with a poor solvent (e.g., water) for one of the blocks. Especially when the two solvents are mutually fully miscible, the mixing causes a gradual shift from good to poor solvent conditions for the core-forming block. Somewhere in this process the aggregation of the polymers starts. In the early stages the micelles are expected to remain under some thermodynamic control. At  some later stages, however, the micelles freeze into a permanent state with the usual core-shell architecture. When during the formation of the micelles an non-polar active ingredient is present, one can load the polymeric micelles with such compound. Choosing poly(ethyle oxide)  (PEO) as the corona block is of more than average interest, because it is known that particles with a PEO brush have stealth-like properties. This means that they can circulate in the blood for a long time before they are noticed by the defence system of the body. One can choose a biodegradable species for the core-forming block to control the life  time of the micellar objects in the blood stream.

We study the loading capacity, size and stability of polymeric micelles both from an experimental and theoretical point of view. An example for a radial distribution function of a block copolymer is given in the figure.