Schematic illustration of rhamnolipid-coated double-emulsion nanoparticles containing doxorubicin and erlotinib (RL-NP-DOX-ERL) for efficient drug delivery to tumor tissue and combination chemotherapy. a Structure of RL-NP-DOX-ERL formed by W/O/W double emulsion. b Drug delivery and combinational therapy using RL-NP-DOX-ERL