## 6 Biomedicine  
In the biomedical field, different types of nanoparticles (NPs) have been developed to improve the diagnosis and treatment of several diseases. Their small dimension makes them suitable to travel through the bloodstream and interact with cells. They have also shown biocompatibility, high charge carrier mobility, high surface-to-volume ratio, and high physicochemical stability [119, 120]. Consequently, by modifying their shape, size, charge, and surface functional groups, the NPs can be loaded with drugs or biomarkers and specifically target certain cells or release the loaded drug in a controlled fashion. The surface-area rate of the NPs is greater than the rate of macroparticles. Subsequently, they allow the incorporation of greater quantities of drugs, targeting agents or biomarkers, to have more area of contact with the cells [121]. Endogenous stimuli, such as pH or enzymes, exogenous stimuli, alternating magnetic fields, or near-infrared light, can also be used to control the drug release [122]. Some NPs have also shown antibacterial or anticancer properties on their own [123, 124], and some also can serve as a diagnostic or monitorization tool. Due to such properties, NPs have been explored to optimize the treatment of severe diseases such as cardiovascular and neurodegenerative conditions and cancer [125, 126, 127, 128]. The main objectives for the development of nanomaterials for cancer treatment applications are [121]:  
* Reduction of toxicity and side effects of conventional drugs by increasing their efficiency;
* Specific binding to target cells;
* Improvement of solubility, stability, tumor aggregation, and half-life of conventional drugs;
* Release of the loaded drug through a stimuli-responsive mechanism;
* Increase the area of interaction for encapsulated drugs or drugs attached to biomacromolecules;
* Overcoming drug resistance by delivering several active agents to specific cellular target sites;
* Overcoming biological barriers;
* Improvement of diagnostic and imaging sensitivity;
* Assessment of drug efficiency in real-time by linking imaging molecules and active anticancer components;
* Contribution to the development of new vaccines;
* Improving the diagnostic and imaging of cancer using smaller devices. Tumor sites are usually poorly vascularized regions. NPs tend to accumulate on the tumor site and increase the retention effect and the permeability, which helps enhance cancer treatments. However, several biological processes can influence the NPs action. Consequently, the features of the NPs, such as dimensions, structure, surface properties, shape, and porosity, must be controlled to achieve the ideal interactions [121]. For instance, one strategy can be reducing the size of the NPs so that they can cross the leaky tumor vasculature but leave them big enough to not leak from the healthy blood vessels. The shape of the NPs affects the uptake of the particles by the cells. However, most studies have used spherical-shaped particles due to challenges in synthesis. The surface chemistry also needs to be carefully engineered considering the type of tumor cells that will be targeted. Some studies indicated that positively charged surfaces increase cellular uptake. However, some studies reported the preference of neutral or negatively charged surfaces for some types of cancer [121]. Drugs are conventionally administered by injection or oral intake. Those pathways have several limitations, such as limited effectiveness and lack of selectivity. For instance, orally taken drugs have poor biodistribution and variable absorption. They can also suffer degradation by the gastric acidic environment together with digestive enzymes [129]. Some anticancer drugs, such as paclitaxel, are not soluble in water which can be overcome by loading the drug onto NPs. Another problem with cancer drugs is the fast elimination through the renal system. NPs can also contribute to enhancing the half-life of the blood circulation of drugs.