**2. Plasma modification of surfaces**

The ability of non-thermal plasmas to dramatically modify surfaces properties has been known for over 25 years. Plasma treatments allow surface modification of polymeric materials without altering their bulk properties [20–22]. These plasma processes can be categorized into 3 major types of reaction: plasma activation, post-irradiation grafting (briefly discussed in the next sections), and plasma polymerization (the focus of this chapter).

#### **2.1. Plasma activation**

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

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

In the past few decades, tailoring materials surface properties has been extensively performed using various modification techniques such as chemical treatments and etching, ozone treat-

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,

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 tempera-

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

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

Plasmas are classified as thermal "equilibrium" and non-thermal "non-equilibrium" based on

of electrons, ions, radicals, photons and neutrals constitute the so-called plasma [16, 17].

matrices to cellular implants.

70 Recent Research in Polymerization

new materials which is quite costly and time-consuming.

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

the relative temperatures of electrons, ions and neutrals.

ment, UV radiation, and plasma treatments [5–10].

adhesion, and bioactive response [13, 14].

ture of the material under consideration.

using an electrical discharge.

In plasma activation, surface modification is done by exposure to non-polymer forming plasmas. The active species in the plasma can bombard the polymeric surface and break covalent bonds thus leading to radical formation. These radicals can subsequently react with other species in the plasma to form functional groups. In this way depending on the selected plasma gas, different functional groups such as carbonyl, carboxylic acid, hydroxyl, and amine functional groups can be added on the surface thus making it more hydrophilic [23–25]. It is believed that radical species rather than ions or electrons are most important in this type of modification [26].

#### **2.2. Plasma polymerization**

Observations of organic compounds formed in a hydrocarbon based plasma discharge dates back to 1874 [27]. These deposits were considered to be undesirable by-products. However, in the 1960s [28–30], studies of plasma polymerization started and were completed by considerable advances in polymer science [31]. Plasma polymerization was defined as "the formation of polymeric materials under the influence of plasma" [31]. Nevertheless, the real potential of plasma polymerization was not uncovered until only the past two decades. Nowadays, plasma polymerization is known as a very valuable surface modification technique.

During the process of plasma polymerization, high energy electrons as well as UV will ionize the precursor molecules [32–34]; this leads to radicals which are highly unstable and reactive species that will interact and bond with one another and deposit on the substrate thus forming a coating on its surface. Plasma will also lead to bond breaks on the substrate surface thus creating radicals. These will interact with the precursor's radicals acting as anchor sites which enhances the plasma polymerized coating stability.

During plasma polymerization, two processes occur simultaneously: - ablation (removal of surface molecules) and – polymerization (surface monomer deposition). These two processes are in competition and their interaction and co-existence in plasma is well known [35].

Plasma polymerization is very complex and versatile since various parameters such as discharge power, treatment time, precursor type and concentration can affect the physic-chemical characteristics of the deposited coating. Moreover, different reactive species can be formed in the plasma depending on the used dilution gas (e.g., helium, argon, air or nitrogen), which also affects the characteristics of the coating [25, 36–38].

Advantages of plasma polymerization include the following:


Nevertheless, plasma polymerization also presents several disadvantages:


However, despite its disadvantages and focusing on its numerous advantages, plasma polymerization has rapidly developed during the past decades and is now used for various applications.

#### *Plasma polymers*

Plasma polymers are markedly different from conventional polymers. Conventional polymers have a well-defined structure of repeating units that corresponds to the used monomer. Whereas, plasma polymers are crosslinked, randomly structured deposits obtained from the fragmentation and recombination of monomers within an electric discharge.

During plasma polymerization, the active species fragments the organic precursor (monomer), thus creating radicals that can recombine both in the plasma and on the substrate surface forming a crosslinked so-called plasma polymer coating/film on the substrate surface.

As to the film chemical structure, during this process, partial loss of functional groups occurs (fragmentation) in a system/process dependent way. Moreover, not all radicals will react and some will be trapped in the plasma polymer network [41]. As a consequence, the elemental composition of plasma polymers differs from that of conventional polymers prepared from the same monomer. For example, the elemental composition of conventionally polymerized polyethylene (C<sup>2</sup> H4 )n is equal to that of the monomer (C<sup>2</sup> H4 ); however, in plasma polymerized ethylene the hydrogen concentration is lower (radical formation by -H bond scission) and oxygen is incorporated in the plasma polymer (by reaction with the formed radicals).

Hence, the material obtained from plasma polymerization is very different than that obtained by conventional polymerization of the same monomer [39].

#### **2.3. Post-irradiation grafting**

Plasma polymerization is very complex and versatile since various parameters such as discharge power, treatment time, precursor type and concentration can affect the physic-chemical characteristics of the deposited coating. Moreover, different reactive species can be formed in the plasma depending on the used dilution gas (e.g., helium, argon, air or nitrogen), which

2) Good adhesion to the substrate material and deposition is independent on the structure or

4) Various precursors can be chosen which leads to a vast array of surface functionalization (monomers used do not have to contain a double bond for the polymerization to proceed)

5) Many process parameters can be used thus providing great diversity of surface modifications

2) Scaling up and converting it into a continuous process could present some technical challenges

4) It is hard to predict the exact surface characteristics of the deposited plasma polymer es-

However, despite its disadvantages and focusing on its numerous advantages, plasma polymerization has rapidly developed during the past decades and is now used for various applications.

Plasma polymers are markedly different from conventional polymers. Conventional polymers have a well-defined structure of repeating units that corresponds to the used monomer. Whereas, plasma polymers are crosslinked, randomly structured deposits obtained from the

During plasma polymerization, the active species fragments the organic precursor (monomer), thus creating radicals that can recombine both in the plasma and on the substrate surface form-

3) The specific roles of each plasma component are difficult to separate and analyze

6) Everything in the coating range of the plasma can become part of the coating

fragmentation and recombination of monomers within an electric discharge.

ing a crosslinked so-called plasma polymer coating/film on the substrate surface.

3) Relatively good chemical stability and physical durability of the coatings

Nevertheless, plasma polymerization also presents several disadvantages:

also affects the characteristics of the coating [25, 36–38].

6) The obtained coatings are more or less uniform

pecially when complex molecules are used 5) Coating multi-functionality can also be an issue

1) Ultra-thin film formation

72 Recent Research in Polymerization

type of the substrate

1) System dependency

*Plasma polymers*

Advantages of plasma polymerization include the following:

Surface modification via polymer coatings is also frequently done by surface grafting methods which are often referred to as "plasma-induced graft (co)polymerization." This is a two-step process. In the first step, the surface is exposed to air plasma or subjected to an ozone treatment which creates peroxide groups at the surface. Other non-oxygen containing plasma can also be used (e.g., Ar or He plasma) followed by atmosphere or O2 exposition; created radicals will then form peroxides and hydroperoxides. In the second step, the formed functionalities are used to initiate a polymerization reaction by contact with the monomer molecules. Each functionality being a potential initiating site, the number of created (hydro)peroxides has a significant effect on the surface grafting density.
