**1. Introduction**

#### **1.1. Tissue engineering**

Tissue engineering (TE) was first expressed at the NSF "National Science Foundation" workshop in 1987 by Dr. Fung. TE was later described as the application of engineering and life sciences to better understand the correlations between the structure and the function of tissues as well as the development of replacements for the restoration, preservation and/or enhancement of tissue functions [1].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

But it was not until 1993 that Langer and Vacanti gave TE the classical definition of: "an interdisciplinary field that applies the principles of engineering and the life sciences toward the development of biological substitutes that restore, maintain, or improve the tissue function [2]." Various, more or less similar TE definitions can be found in the literature. Moreover, since this is a relatively new field, specific definitions are not always given and may stretch from decellularized matrices to cellular implants.

In tissue engineering, biomaterials must possess appropriate surface properties for better cellmaterial interactions. In addition, biomaterials should possess appropriate bulk properties to function properly in a bio-environment. Therefore, a suitable approach is to select a biomaterial having good bulk properties and enhance its surface properties using a preferential surface treatment [3, 4]. In this way, one can obtain an "ideal" biomaterial with selective surface properties that are decoupled from its bulk properties and avert the need to develop completely new materials which is quite costly and time-consuming.

In the past few decades, tailoring materials surface properties has been extensively performed using various modification techniques such as chemical treatments and etching, ozone treatment, UV radiation, and plasma treatments [5–10].

Plasma surface treatments are most promising due to the speed and uniformity of modification, their chemical flexibility and positive environmental impact [11, 12]. Various types of plasma surface modification technologies have been used to modify materials by incorporating a variety of functional groups on their surfaces; this is done to improve the surface energy, wettability, adhesion, and bioactive response [13, 14].

#### **1.2. Plasma: a brief introduction and historical background**

Plasma is defined as the fourth state of matter in the sequence: solid, liquid, gas, and plasma. The transition between these different respective states can occur by increasing the temperature of the material under consideration.

This state of matter was first described in 1879 by Crookes as "a world where matter may exist in a fourth state." Later, in 1928, this state of ionized gas was eventually given its name "plasma" by Irving Langmuir, when he introduced it in his studies of electrified gases in vacuum tubes [15]. Plasmas can be natural such as lightning, polar light and the stars or man-made. Therefore, without being aware, every person has faced various forms of plasma. Man-made plasma can be generated in laboratories by combustion, flames, lasers or controlled nuclear reactions. But, in the field of plasma polymerization, most plasma are generated and sustained using an electrical discharge.

Plasma is generally formed when gas atoms are subjected to a high enough thermal or electrical energy. Subjected to energy, gas atoms become ions by releasing some of their electrons. Radicals are then created by electron-molecule collisions and bond breaks in molecules. Some excited species will also be created by energy adsorption which will generate photons. This unique mixture of electrons, ions, radicals, photons and neutrals constitute the so-called plasma [16, 17].

Plasmas are classified as thermal "equilibrium" and non-thermal "non-equilibrium" based on the relative temperatures of electrons, ions and neutrals.

In a non-thermal or cold plasma, the electron temperature (≈ 10,000°C) is much higher than the gas temperature (< 200°C), whereas, in a thermal or hot plasma, the electron temperature is very close to that of the heavy particles.

Plasmas used in the field of plasma polymerization are usually cold plasmas since it involves heat-sensitive materials [18, 19].
