# Erythrocytes and Nanoparticles: New Therapeutic Systems
## 1 Introduction  
Chemotherapeutic drugs that are conventionally used in clinical trials show several drawbacks, e.g. side effects and lack of targeting. Nanotechnology can offer innovative methods to enhance the efficiency of such drugs. The loading of therapeutic agents into nanostructures can increase the cellular targeting, allowing lower therapeutic doses [1]. The most important challenges faced in nanomedicine are represented by drug delivery targeting to regions of interest while keeping the loaded NPs hidden under the radar of the immune system [1; 2; 3; 4; 5]. Several nano-delivery systems were developed for drug delivery purposes--and their biocompatibility, non-immunogenicity, non-toxicity and biodegradability have led to great therapeutic applications of these structures. Depending on their composition, all of these nano-systems can be classified into different categories: polymeric NPs, e.g., dendrimers, micelles, nanogels and protein NPs; nonpolymeric NPs, e.g., carbon nanotubes, nano-diamonds, metallic NPs and quantum dots. ; lipidic NPs, e.g., liposomes; and solid lipid NPs. Red blood cell (RBC) exploitation could represent an optimal solution in the search for ameliorative solutions in the design of novel functional nanomaterials that achieve the desired therapeutic effects while avoiding reticuloendothelial system (RES) clearance. Red blood cells have attracted great attention in the field of bio-delivery for their unique biological and biophysical properties. As tissue oxygen delivery cells, RBCs are the most abundant cells of the human body. After differentiating from erythroblasts in the bone marrow, they are released into the blood and survive in the circulation for a period that may vary between 70 and 140 days after which they are removed by the RES. In adults, RBCs correspond to approximately 70% of all cells by number, thus representing the most extended cellular surface accessible to blood [6]. The specialized structure of the membrane combines internal viscosity with cytoskeleton elasticity; this gives RBCs high deformability and allows their passage through the capillaries, as well as granting them resistance to shear stress. Additionally, an RBC membrane is characterized by several proteins that impart many vital functions, e.g., an active role in systemic metabolism, participation in redox homeostasis, important modulation of innate immunity, responsiveness to neurotransmitter stimulation or metabolism, release of macrovesicles and uptake and metabolism of circulating endogenous or exogenous bioactive molecules [7]. These features--along with their biocompatibility, low immunogenicity and high persistence--make RBCs an attractive platform for nanomedicine, in particular for drug encapsulation and delivery. In particular, the circulation time of loaded drugs is enhanced after this process thanks to the advantages of RBCs, which allow better pharmacokinetics. The most commonly used method to load drugs inside RBCs is the hypotonic method [8; 9]. The creation of a hypotonic environment around the cells allows their swelling and the opening of temporary pores in the cellular membrane, thus allowing the drug/nanomaterials to penetrate and accumulate in the RBCs. Subsequently, a hypertonic solution is added into the cell suspension in order to release the previously created pores and to restore the original cellular size [10]. The swelling and resealing process used to encapsulate ergos into RBCs is the critical point--necessary for successful drug loading and the preservation of membrane biocompatibility. These factors are essential when the goal is to prevent a reduction in their circulation time [11]. In effect, although widely used, this method leads to a certain degree of change in cell morphology and membrane structure which results in hemolysis, thus impairing biocompatibility of prepared RBC carriers and, consequently, their use in vivo [10]. Another key problem could be the incompatibility of different blood groups.