# Erythrocytes and Nanoparticles: New Therapeutic Systems
## 2 Bio Hybrid Carrier Using RBC  
Three types of nano/bio hybrid carriers arising from RBCs can be classified (Figure 1): whole red blood cells which have incorporated drug loaded NPs (carrier into carrier); NPs coated with RBC membrane; and NERs, which can be considered NPs derived from RBC.  
The design and production of NERs could reduce the micrometer-sized RBC membrane to nanometer-sized while maintaining its interesting biological and biophysical properties. The biomedical application of NPs can be hindered if they are recognized by immune system cells--in particular, macrophages, which prevent NPs from reaching their final destination. Additionally, the activation of the complement system, which leads to inflammation and side effects, can be a hindrance. The encapsulation of NPs into RBCs is a camouflage tactic against the immune system [23; 24]. Several clinical trials were performed by successfully loading doxorubicin [25] or valproate [26] into NPs and subsequently encapsulating them into RBCs. For example, using this method, [PERSON] and colleagues [27] reported an increase of the half-elimination time of doxorubicin from the bloodstream, compared to the administration of NPs alone. This external camouflage increases the efficiency and safety of NPs--even if these systems are limited by an increase in the stiffening and sensibilization of erythrocytes to damage by osmotic, mechanical and oxidative stress [28; 29]. The employment of a carrier into carrier has a great advantage over classical RBC carriers with respect to preventing the rapid and uncontrolled escape of the encapsulated therapeutic agent due to passive drug diffusion out of the RBCs. Recently, researchers have designed a cellular hitchhiking strategy. This new alternative method can improve blood pharmacokinetics and the vascular delivery of NPs while simultaneously avoiding uptake by tissues such as the liver or spleen. A recent work showed chitosan-based nanogels (loaded with the antiepileptic drug sodium valpro) to have high cell-escaping characteristics, especially during the encapsulation procedure [26]. These NPs were prepared through an ionotropic gelation, mediated by the interaction of the polycation chitosan with the polyanion sodium tripolynphosphate. Subsequently, using a hypotonic dialysis method, these nanocarriers were encapsulated into RBCs and showed a good profile of slow release of the agent after the administration of the carrier into carrier [26].  
Another recent study [30] reported the encapsulation of two different molecules--5(6)-carboxyfluorescein and rhodamine (labeled dextran)--into RBCs using the hypotonic osmotic method, followed by incubation with gold NPs. The release of these molecules was triggered by exposure to a near-infrared laser which led to pore formation, allowing the cargo to be released from the RBC membrane. In a similar study, the stimulus was a magnetic field: doxorubicin was loaded into the RBC and chlorine e6 (Ce6) onto poly(ethylene glycol) (PEG)-coated iron oxide NPs; in turn, the NPs were conjugated onto the surface of Doxorubicin (Dox)-loaded erythrocytes. A site-specific and continuous Dox and Ce6 delivery toward tumor tissue occurred when an external magnetic field was applied [31]. Through this approach, this carrier into carrier (based on RBCs and magnetic NPs) blends the photodynamic therapy of chlorine with the chemotherapeutic effect of Dox, leading to the inhibition of tumor growth. Another application of carrier into carrier is diagnostic; recently, supermagnetic iron oxide NPs, used as contrast agents for magnetic resonance imaging, were encapsulated into RBCs under hypoosmotic conditions to overcome the fast phagocytosis to which superparamagnetic iron oxide nanoparticles (SPIONs) are subjected if administrated alone [32].