**1. Introduction**

Nanomedicine is a relatively new discipline that arises from the intersection between nanotechnology and medicine. It is based on the control of matter at the nanometer scale for applications in the field of human health. The use of materials in this range has been a great advance for the pharmacology by modifying fundamental properties of the drugs such as solubility, diffusivity, half-life in the bloodstream and drug release and distribution profiles [1–4]. Although the production and use of nano-sized matter dates from hundreds of years [5, 6], nanomedicine as a modern interdisciplinary science was first established at the end of the last century. Many authors consider the beginning of nanotechnology in the famous lecture of the physicist and Nobel laureate Richard P. Feynman in 1959 for the American Physical Society entitled: "There's Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics" [7]. In it, Feynman presented a futuristic vision of technology that leads towards the atomic scale and towards the final limits established by physical laws. Revolutionary ideas were put forward, such as reducing the integrated circuits of a computer to diameters between 10 and 100 atoms. To understand the scope of his predictions, suffice it to remember that, at the time of presenting these ideas, a computer occupied an entire room if not several. However, the word (or the prefix) nano was not mentioned even once in his presentation, Feynman stuck to describing the miniaturization of machines and its possible applications. The honor of having coined the term "nano" is awarded to Norio Taniguchi, for his presentation "On the basic concept of nanotechnology" in 1974 [8].

It is important to emphasize that the term nanotechnology applied to the study of nanoparticles simply consists in renaming the study of colloidal dispersions, in which field the contributions of renowned scientists such as Michael Faraday stand out, who in 1857 disseminated the first synthesis of gold nanoparticles and other metals [9]. In the paper, Faraday reveals his amazement at the changes in the optical properties of metallic colloidal dispersions. These properties were later explained in


#### **Table 1.**

*FDA-approved nanomedicines for drug delivery. Adapted from reference [16] with permission of Springer Nature.*

#### *Nanoparticles as Drug Delivery Systems DOI: http://dx.doi.org/10.5772/intechopen.100253*

1908 by Gustav Mie who would give a solution to Maxwell's equations for particles with a finite volume [10]. In 1925, Richard Zsigmondy would be awarded the Nobel Prize in Chemistry for his demonstration of the heterogeneous nature of colloidal dispersions [11]. His contributions in methodological terms have become fundamental for the study of modern colloidal chemistry and nanotechnology.

In 1981, Eric Drexler [12] proposed what is now known as a bottom-up approach, where atoms are self-assembled to create higher-order structures. It contrasts with the approach proposed by Feynman, who conceives the beginning of nanotechnology from a top-down approach, building smaller and smaller machines that, ultimately, are used to manipulate matter with atomic precision. In particular, the bottom-up approach is of great interest in nanoparticle synthesis, where self-assembly properties, the product of natural chemical and physical interactions between molecules, can be exploited to produce defined characteristics. It is this concept that opens a wide range of possibilities towards the synthesis of nanoparticles with a wide variety of functionalities. From this point of view, nanoparticle engineering is based on "programming" with predetermined instructions the self-assembly of atoms or molecules in such a way that the desired nanoparticles are the final product.

Different authors see the paths of nanotechnology and medicine intertwined in 1986 when Matsumura and Maeda [13] observed that an anticancer protein bound to polymeric nanoparticles exhibited greater accumulation in tumor tissues than in healthy tissues. This discovery led to the theory of enhanced permeability and retention (EPR) as a consequence of tumor physiology and the size of nanoparticles (<200 nm), which are capable of penetrating tumor cells due to their reduced size and, at the same time, being retained [13]. The discovery lays the foundation for the development of different theories on targeted delivery via passive transport to tumor tissues and a large cascade of advances in the design of drug nanocarriers. In 1995, the first liposome-based nanostructure for the delivery of doxorubicin, an important anticancer drug, was approved by the FDA (Food and Drug Administration, USA) under the trade name Doxil® [14]. Since then and until April 2016, more than 50 nanomedicines of different kinds have been approved by the FDA and this is expected to be only the beginning of the near future [15]. **Table 1**, which has been adapted from reference [16], summarizes the FDA-approved nanomedicines used for drug delivery up to date. At the time of writing this chapter, one of the most conservative market capitalization estimates that the value of all nanomedicines is comprised of \$ 47.5 billion and is expected to rise to \$ 164 billion by 2027 driven by the crisis SARS-CoV-2 [17].
