In Conclusion

Multifunctional polyelectrolyte capsules fabricated by the LbL assembly technique possess remarkable properties, even though they are held together by electrostatic attraction. In particular, this very general assembly mechanism allows for the integration of virtually all different types of charged nanoscale objects into and on top of their walls. Introduction of nanoscale objects introduces functionality (such as fluorophores, magnetic particles, and local heat sources) and specificity (such as biomolecular ligands). The cavity of the capsules can be filled with cargo, which is to be released at designated targets or with active molecules, as for example for multiplexed sensing. Compared with other carrier systems, capsules can be functionalized at two distinct compartments, walls and cavities, which introduces flexibility for the interference-free introduction of multiple functionalities.

It has to be clearly pointed out that the individual concepts outlined in this article for use with capsules have previously been suggested for other types of carrier systems. Magnetic drug targeting, receptor-ligand based targeting, hyperthermia, and photo-induced release of cargo are concepts well known in medical research. The novelty of the capsules as a carrier system lies in the possibility of combining all these different strategies into one single object and thus provide true multifunctionality.

Most of the concepts mentioned in this article have been experimentally demonstrated on cell cultures and the experimental data can be found in the references. In particular, a proof of principle for a microcapsule-based system for biospecific target delivery and for local analytic detection in small volumes in cell cultures has been successfully realized by several groups. Though several groups have started with experiments on animals, enormous hurdles still have to be overcome before applying polymer capsules to clinical practice. Of particular importance will be the prevention of capsule aggregation in blood vessels, the control of capsule clearance by the organism, and the synthesis of highly biocompatible capsules.