## 3 Complex Fluids in the Human Body for Blood Flow  
The elastic properties of some human fluids like blood, saliva or vitreous humor plays an important role in their flow behavior. As it was mentioned previously, at the micro-scales (typical scales from the human circulatory system), the elastic effects are enhanced even at low Reynolds numbers [7,54,55]. The complex rheological behaviour of whole blood has been studied and continues to be investigated using new technological advances [56,57]. It is important to point out that conventional macroheological techniques have certain limitations as they require a large amount of material, they typically operate at low frequency, and they measure motions over relatively large length scales (mms). On the contrary, microrheology (a branch of rheology defined as the science of deformation and flow of materials working on micron length scales [58]) has overcome these limitations, enabling the characterization of materials in situations wherein traditional rheometers are difficult to use, i.e., when the material is available only in very low quantities (<1 mL).  
Microrheology methods typically use embedded micron-sized probes to locally deform a sample. It can be classified in two techniques: _passive_ or _active_ microrheology. The first one is based on the observation of a thermally excited particle on order of a micron, suspended in the solution under investigation. The second technique is based on the response of a single micro-bead to an external force field. Both techniques directly relate the bead trajectories to the viscoelastic properties (i.e., viscoelastic moduli \(G^{\prime}\) and \(G^{\prime}\)). Microrheology has important advantages in comparison with conventional rheology: the need for small sample volumes, measurement acquisition times on the order of seconds, and the use of small colloidal particles theoretically extends the accessible frequency range by shifting the onset of inertial effects to the MHz regime [59]. [PERSON] et al. [60] used microrheological techniques to measure the viscous and elastic properties of human blood, as the conventional rheology did not offer enough sensibility for the measurement. Nevertheless, if the probe size is much smaller than the particle size in the fluid, the microrheological properties may be totally different from the bulk fluid behavior. [PERSON] et al. [61] performed nanorheological measurements in blood using chiral magneto-plasmonic particles. They were able to provide the dynamic viscosity of bovine blood plasma even in the presence of red blood cells with different hematocrit level.  
For this reason, a complete rheological characterization of the main human fluids and a thorough study of their flow dynamics when a microbot travels through characteristic geometries of the human body will provide a new insight for the development of an efficient motion of these microbots based on their optimized morphology.  
The function of a human circulatory system is to transport blood around the body in which the main organ is the heart, and other main parts of the circulatory system include the arteries, arterioles, capillaries, venules, veins and blood. The lungs also play a major part in the pulmonary circulation system. There are three circulatory processes occurring simultaneously within the body. Firstly, systemic circulation carries blood around the body, pulmonary circulation carries blood to the lungs and coronary circulation provides the heart with its own supply of blood. For this reason, for any potential work to be developed by the microbot, knowing the characteristics of the human circulatory system is crucial.