# Multiresponsive Hybrid Microparticles for Stimuli-Responsive Delivery of Bioactive Compounds
## 4 Conclusions  
In the present work, hybrid microparticles with an iron core and amphiphilic polymer shell able to respond to variation in the surrounding pH, the presence of magnetic field, and ultrasound treatment were developed for smart delivery of bioactive compounds. Doxorubicin hydrochloride has been used as a model drug, as it is well known and widely used in the treatment of different cancers. The core-shell microparticles were prepared in four steps: (i) preparation of iron oxide microparticles; (ii) surface modification by the introduction of phenyl carboxyl groups; (iii) conjugation with the amphiphilic CS derivative (PLACA-g-CS); and (iv) drug loading. The core-shell microparticles show a size of around 3 \(\upmu\)m, a positive \(\zeta\)-potential of + 34 mV and excellent stability in the physiological environment.  
The PLACA-g-CS shell allows a higher encapsulation efficiency and a pH-dependent controlled delivery, preserving the magnetic response of the iron core under a magnetic field and limiting the decrease in this response to 20%. The core-shell system demonstrates US-responsive behavior characterized by a reversible change in the spatial displacement of the polymeric chains around the core, causing a release burst with intensity dependent on the power and application time of the US. The in vitro test performed on NIH/3T3 cells demonstrated that the hybrid microparticles in combination with US improve the cytotoxicity of doxorubicin and allow it to last for a longer time compared to the that of the free drug formulation. Moreover, the prepared hybrid system has shown good hemocompatibility, 6-fold greater than that of simple iron oxide microparticles, for concentrations up to 60 \(\upmu\)g/mL.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/12/4324/s1](http://www.mdpi.com/2076-3417/10/12/4324/s1)]([http://www.mdpi.com/2076-3417/10/12/4324/s1](http://www.mdpi.com/2076-3417/10/12/4324/s1)), Figure S1: 1H-NMR spectra of (**A**) PLACA-g-CS and (**B**) CS.  
Conceptualization, A.D.M.; S.S.V. and P.S.P.; methodology, A.D.M. and S.S.V.; investigation, A.D.M. and S.S.V.; data curation, A.D.M. and S.S.V., writing--original draft preparation, A.D.M. and S.S.V., writing--review and editing, A.D.M.; visualization, M.V.B., S.V.K., M.S.Y., A.M.G.; All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Ministry of Science and Higher Education of the Russian Federation, State Project \"Science\" No. WSWW-2020-0011.  
The authors declare no conflict of interest.  
Figure 7: Effect of the surface modification on the hemolysis of the particles at different concentrations. (**A**) Micrographs of NIH/3T3 cells incubated with core–shell microparticles after exposure to US treatment. Values are reported as averages, and error bars represent the SD (n = 3). The micrographs were taken after 24 h of exposure; 50 \(\upmu\)g/mL was the microparticle concentration used. (**B**) Control, only cells. (**C**) Cells incubated with core–shell microparticles and exposed to US treatment at 2 W/cm\({}^{2}\). (**D**) Cells incubated with core–shell microparticles and exposed to US treatment at 20 W/cm\({}^{2}\).