Loading of doxorubicin into surface-attached stimuli-responsive microgels and its subsequent release under different conditions

This study highlights the use of chargeable stimuli-responsive microgels for modification of rather hydrophobic model surfaces and reveals the optimal conditions, which are to be applied for enhanced microgel adsorption onto such surfaces. The highly efficient surface modification is achieved when the microgel is adsorbed in its uncharged and sufficiently dehydrated (hydrophobized) state. After subsequent charging, the adsorbed microgels can be loaded with a bioactive payload of opposite charge, wherein a negatively charged microgel acts as a capacious container hosting a considerable amount of a positively charged drug (doxorubicin). The loaded doxorubicin can be gradually eluted through washing, this process being facilitated if the eluent contains a low molecular weight salt, thereby manifesting that binding of the drug to the microgel is of pronounced electrostatic nature. Furthermore, the release of doxorubicin from the surface-attached microgels can also be induced by linear synthetic polyelectrolytes, which could be, as shown for the first time in this work, any polyions, both cationic and anionic ones. Hence, polycations compete with the drug for binding to the surface-attached anionic microgel, while polyanions compete with the microgel for binding of doxorubicin. Such competitive interactions in ternary systems (microgel - drug - linear polyion) result in displacement of doxorubicin from the surface-attached microgels, regardless of whether the polymer-competitor is oppositely or even (rather peculiarly) similarly charged with respect to the microgel. Thus, this study illustrates means, which can be used to control escape of bioactive payloads from oppositely charged microgels, and this knowledge can be exploited for development of uptake and release systems for various applications (e.g., in medicine or agriculture) and prognosis of their functioning in complex aqueous environments, which can contain inter alia various (bio)macromolecular species.