**5. Conclusions**

The present study indicates that, for all culture conditions, μPs biofunctionalized with anti-HER2 antibodies were more efficiently internalized by cells expressing HER2 than μPs biofunctionalized with a non-specific antibody. Moreover, in fluidic culture conditions (simulating the blood stream), the specificity of targeting was still more useful for μP internalization, because the increase in internalization was higher for cells overexpressing HER2. Overall, the results presented emphasize the importance of targeting not only for directing μPs to the appropriate cells but also for achieving reasonable internalization rates, thereby opening the door to the use of microparticles as carriers for drug delivery.

**Author Contributions:** Conceptualization: E.I., C.N., T.G., and L.B.; Data curation: I.M.-E., J.S., and L.B.; Formal analysis: I.M.-E., J.S., and C.N.; Funding acquisition: C.N.; Investigation: I.M.-E.; Project administration: C.N.; Supervision: L.B.; Validation: E.I., C.N., T.G., and L.B.; Writing—original draft: I.M.-E. and L.B.; Writing—review & editing: I.M.-E., E.I., J.S., C.N., T.G., and L.B.

**Funding:** This research was funded by the Spanish Ministerio de Ciencia e Innovación (MAT2014-57960-C3-3-R and MAT2017-86357-C3-3-R) and the Generalitat de Catalunya (2017-SGR-503). I.M.E. Thanks to the Spanish Ministerio de Ciencia e Innovación for a predoctoral grant.

**Acknowledgments:** The authors wish to thank the Servei de Microscòpia at the Universitat Autònoma de Barcelona.

**Conflicts of Interest:** The authors declare no conflict of interest.
