Controlled Release Of Cargo

Targeted cargo delivery, as described in previous section, can be combined with remotely controlled cargo release. Remote activation is based on an external physical stimulus such as light, ultrasound, or radio frequency, which acts on colloidal nanoparticles present in the walls of the capsules and leads to rupturing/permeation of the walls of the capsules so that the cargo is released from the cavity to the environment. Irradiation of noble metal and magnetic particles with light and radio frequency waves, respectively, leads to local heating as described earlier and subsequent disintegration of the capsule walls. Alternatively, sonication of capsules functionalized with colloidal nanoparticles in their walls leads to mechanical disintegration of the capsule walls. In this way, the cargo can be released from the cavity of the capsules following an external trigger. Ultrasound and radio frequency fields can penetrate tissue well, but they are complicated to focus. For this reason they are favorable for cargo release from capsules deep inside tissue. Visible light, on the other hand, is strongly absorbed by tissue but can be easily focused to micrometer size spots. For this reason, controlled opening of individual capsules inside single cells and subsequent release of their cargo to the cytosol is possible (fig 1) .Short irradiation (<10> not ,very similar35 mW) is sufficient for opening capsules without damaging cells and tissues. While moderate irradiation disintegrates the walls of capsules, releasing molecules from their cavity, higher doses of irradiation can increase the local temperature so far as to destroy surrounding cells, an effect known as hyperthermia. Local accumulation of heatable capsules (i.e. with noble metal and magnetic nanoparticles in their walls which are responsive to light and radio frequency exposure, respectively) in cancerous tissues could be used for induced heating mediated by the nanoparticles involving subsequent tumor deterioration. As only the temperature of cells close to the particles is raised, the heated cells would be selectively killed, without exposing the entire organism to elevated temperatures.

Fig. 1. Controlled-release of cargo with light-responsive capsules. (i) Laser irradiation of noble metal (e.g. Au) nanoparticle-functionalized capsules leads to (ii) local heating of the metal NPs, and (iii) subsequent rupture of the capsule wall.

In summary, nanoparticle functionalized capsules offer the potential to combine all of the above listed concepts into one single system for drug delivery (fig 2). Magnetic nanoparticles in the capsule walls allow direction and accumulation of capsules at the designated target region upon application of external magnetic field gradients. Ligands immobilized on the capsule surface, which are specific for receptor molecules present on target cells, would permit specific and enhanced cellular uptake via receptor-ligand binding. Fluorescent semiconductor nanoparticles in the capsule walls allow monitoring of the transportation and uptake process using fluorescence microscopy. Capsules inside target cells could be opened with a laser pointer by heating gold nanoparticles in the capsule walls which then releases the cargo molecules. Combination of these methods is expected to lead to greater specificity in drug delivery. This procedure could be a great advantage to anticancer therapy, as selective toxicity in tumor cells could be achieved via local chemotherapeutic damage. Nevertheless, to be applicable in clinical practice, several additional hurdles will have to be overcome. First of all, to reach a specific tissue, intravenous administration is often required. Therefore, the capsules should be limited in size as they could obstruct the smallest blood capillaries. From the circulation point of view, the upper limit for an ideal vehicle size should not exceed 200 nm. Secondly, due to their polyionic nature, the capsules tend towards protein adsorption, potentially leading to capsule aggregation in the blood capillaries.

Fig. 2. Target delivery systems. (a) Geometry of polymer capsules loaded with magnetic (black), fluorescent (green), and gold (yellow) NPs in their walls. Ligand molecules are attached to their surface and filled with cargo molecules (grey star-shape). (b) Schematization of targeted local cargo delivery into cells with multifunctional polymer capsules. Capsules allow for delivery to target cells by local accumulation with magnetic field gradients and specific binding to receptors on the target cells, and for controlled release of cargo inside cells upon light-controlled opening of capsules.

Besides the potential use for targeted and controlled release of drugs, cargo delivery with capsules can be seen in the more general concept of triggered local delivery. Caged Ca has been used for decades in biology, in the local and controlled delivery of Ca ions. For this purpose, Ca is introduced into chelating complexes from which it can be released by light-illumination. Noble-metal functionalized capsules could serve the same purpose, although they would allow for light stimulated release of macromolecules from the cavity of the capsules. Stimulated local release of enzymes and DNA would be of particular relevance. Release of enzymes could, for example, trigger the conversion of prodrugs within cells into active drugs. Also short oligonucleotide sequences, such as si-RNA, could be introduced as cargo inside the capsule cavities for gene delivery applications. In particular, such capsules could provide an easy way to improve the current transfection methods used in molecular biology as they would lead to more efficient translational arrest of specific transcripts from RNA inside living cells. In this role, the incorporation of single-stranded antisense oligonucleotides, linear double-stranded and plasmid DNA into polymer nanospheres has been demonstrated.