**3. Fibroblast cells**

#### **3.1. Human gingival fibroblasts**

#### *3.1.1. Generality*

Fibroblasts are spindle-shaped connective-tissue cells of mesenchymal origin that secretes proteins and especially molecular collagen from which the extracellular fibrillar matrix of connective tissue forms. They have oval or circular nucleus and a little developed cyto‐ plasm giving rise to long prolongation forms [34]. These cells do not have a basal lamina and their surfaces are often in contact with the fibers of the collagen. Their cytoplasm contains a rough endoplasmic reticulum, an important Golgi apparatus, few mitochon‐ dria and a little bit quantity of cytoplasmic filaments. Fibroblasts synthesize enormous quantities of the extracellular matrix constituents. Indeed, the majority part of the extrac‐ ellular matrix components consists of collagen made in the intracellular space where fi‐ broblasts sustain structural modifications.

#### *3.1.2. Gingival tissue*

It's the tissue that surrounds the necks of teeth and covers the alveolar parts of the jaws; broadly: the alveolar portion of a jaw with its enveloping soft tissues [35]. It consists in a pink connective tissue with fibrous collagen surrounded by an epithelial tissue. Its pink col‐ or changes from one person to another, depending on pigmentation, epithelium thickness, its keratinization level and on the underlying vascularization [36]. Fibroblasts are the basic component of the gingival chorion whose intercellular matrix is essentially formed by colla‐ gen and elastin.

#### **3.2. Cell-Biomaterial: Interface and interactions**

#### *3.2.1. Biocompatibility concept*

While a cell is in contact with a biomaterial, many reactions can occur and a sensing phe‐ nomenon will launch between this cell and the biomaterial [37]. Indeed, the cell has a signal network reached as a result of the surface exploration and sensing made in order to verify whether the new environment (biomaterial) is in accordance with its expected physiological conditions necessary for a normal biological activity [38]. Thus, before putting a new materi‐ al in contact with a cell it's of a great importance to choose the corresponding material in such a way that this material obey the cell's norm by not being toxic or injurious and not causing immunological rejection. In one word, this material must be biocompatible.

The biological tolerance of a biomaterial led scientists to regroup the different parameters and mechanisms controlling the interface biomaterial/cell (or tissue) so that they can deduce a concrete and a common definition for biocompatibility concept. Indeed, biocompatibility includes the understanding of the interactive mechanisms relating the biomaterial with its biological environment. Generally, biocompatibility represents the ability of a material to be accepted by a living organism.

In 1987, Williams D.F suggested the following definition «biocompatibility is the ability of a material to be used with an appropriate and suitable reaction of the host for a spe‐ cific application».

According to Exbrayat [39] « biocompatibility is a set of the different interrelations between a biomaterial and its environment, and their biological local or general consequences, imme‐ diate or delayed, reversible or definitive».

Indeed, biocompatibility is a group of networks that liaises between the biomaterial and its environment and takes into account the possible effect of this biomaterial on its environ‐ ment and vice versa. Interactions existing in the interface biomaterial/biological environ‐ ment differ by their intensity and their duration period depending both on the biomaterial and on the tissue in contact.

Characterizing the surface properties of a biomaterial before putting it in contact with a cell seems to be an obligation. This step allows us to know about different parameters and char‐ acters of this biomaterial (topography, roughness, surface energy etc.) in order to find a cor‐ relation with the cell behavior and therefore we can adjust these physico-chemical properties, when making the biomaterial, so that we have a normal and physiological cell behavior in contact with that biomaterial.

#### *3.2.2. Cell adhesion*

component of the gingival chorion whose intercellular matrix is essentially formed by colla‐

While a cell is in contact with a biomaterial, many reactions can occur and a sensing phe‐ nomenon will launch between this cell and the biomaterial [37]. Indeed, the cell has a signal network reached as a result of the surface exploration and sensing made in order to verify whether the new environment (biomaterial) is in accordance with its expected physiological conditions necessary for a normal biological activity [38]. Thus, before putting a new materi‐ al in contact with a cell it's of a great importance to choose the corresponding material in such a way that this material obey the cell's norm by not being toxic or injurious and not

causing immunological rejection. In one word, this material must be biocompatible.

The biological tolerance of a biomaterial led scientists to regroup the different parameters and mechanisms controlling the interface biomaterial/cell (or tissue) so that they can deduce a concrete and a common definition for biocompatibility concept. Indeed, biocompatibility includes the understanding of the interactive mechanisms relating the biomaterial with its biological environment. Generally, biocompatibility represents the ability of a material to be

In 1987, Williams D.F suggested the following definition «biocompatibility is the ability of a material to be used with an appropriate and suitable reaction of the host for a spe‐

According to Exbrayat [39] « biocompatibility is a set of the different interrelations between a biomaterial and its environment, and their biological local or general consequences, imme‐

Indeed, biocompatibility is a group of networks that liaises between the biomaterial and its environment and takes into account the possible effect of this biomaterial on its environ‐ ment and vice versa. Interactions existing in the interface biomaterial/biological environ‐ ment differ by their intensity and their duration period depending both on the biomaterial

Characterizing the surface properties of a biomaterial before putting it in contact with a cell seems to be an obligation. This step allows us to know about different parameters and char‐ acters of this biomaterial (topography, roughness, surface energy etc.) in order to find a cor‐ relation with the cell behavior and therefore we can adjust these physico-chemical properties, when making the biomaterial, so that we have a normal and physiological cell

gen and elastin.

*3.2.1. Biocompatibility concept*

accepted by a living organism.

and on the tissue in contact.

diate or delayed, reversible or definitive».

behavior in contact with that biomaterial.

cific application».

**3.2. Cell-Biomaterial: Interface and interactions**

218 Advances in Biomaterials Science and Biomedical Applications

It is well known that during the contact between a cell and a material, information will be transferred from the material surface to the cell and this contact will induce, in return, an alteration to the material. This situation may cause material remodelling [40,22].

Cells adhere to surfaces through adhesion proteins (i.e. fibronectin, collagen, laminin, vitro‐ nectin) using specific cell receptors, called integrins, attached to the cell membrane. Indeed, when fibroblasts grow on a substrate, most of their cell surface is separated from the sub‐ stratum by a gap of more than 50 nm; but at focal contacts, this gap is reduced to 10 to 15 nm. The main transmembrane linker proteins of focal contacts belong to the integrin family and the cytoplasmic domain of the integrin binds to the protein talin, which in turn binds to vinculin, a protein found also in other actin-containing cell junction. Vinculin associates with α-actinin and is thereby linked to an actin filament [1].

Besides their role as anchors, focal contacts can also relay signals from the extracellular ma‐ trix (ECM) to the cytoskeleton. Several protein kinases are localized to focal contacts and seems to change their activity with the type of the substratum on which the rest. These kin‐ ases can regulate the survival, growth, morphology, movement, and differentiation of cells in response to new environment. Figure 5 shows a possible arrangement of these different proteins during a focal contact.

**Figure 5.** Adhesion proteins involved in focal contacts

The formation of focal contacts occurs when the binding of matrix glycoprotein, such as fi‐ bronectin, on the outside of the cell causes the integrin molecules to cluster at the contact site. Fibronectins are associated together by proteoglycans and constitute thins fibers of the extracellular matrix (ECM).

#### *3.2.2.1. Extracellular matrix*

The extracellular matrix (ECM) represents an important element in the processes of cell ad‐ hesion. Indeed, at this level, cell adhesion is under the control of a well defined zone in the cytoplasmic membrane called focal contact. At this zone, filaments of actin are linked to fi‐ bronectin through an intracellular complex of proteins, the adherence complex. The extracel‐ lular matrix (ECM) is made of different proteins such as fibronectins, collagen, laminin, vitronectin [41] and represents the mediator of cell adhesion thanks to its integrins.

Although the extracellular matrix generally provides mechanical support to tissues, it serves several other functions as well. Different combinations of ECM components tailor the extrac‐ ellular matrix for specific purposes: strength in a tendon, tooth, or bone; cushioning in carti‐ lage; and adhesion in most tissues. In addition, the composition of the matrix, which can vary, depending on the anatomical site and physiological status of a tissue, can let a cell know where it is and what it should do (environmental cues). Changes in ECM components, which are constantly being remodeled, degraded, and resynthesized locally, can modulate the interactions of a cell with its environment. The matrix also serves as a reservoir for many extracellular signalling molecules that control cell growth and differentiation. In addition, the matrix provides a lattice through or on which cells can move, particularly in the early stages of tissue assembly [42].

Many functions of the matrix require transmembrane adhesion receptors that bind directly to ECM components and that also interact, through adapter proteins, with the cytoskeleton. The principal class of adhesion receptors that mediate cell–matrix adhesion are integrins, a large family of αβ heterodimeric cell surface proteins that mediate both cell–cell and cell– matrix adhesions and inside-out and outside-in signalling in numerous tissues.

#### *3.2.2.2. Adhesion proteins and receptors in fibroblast cells*

Different proteins and their receptors are involved in fibroblast cells adhesion process. The most important and known are fibronectins and their receptors; integrins:

**•** Fibronectins

Fibronectins are dimers of two similar polypeptides linked at their C-termini by two di‐ sulfide bonds; each chain is about 60–70 nm long and 2–3 nm thick. The combination of different repeats composing the regions, another example of combinatorial diversity, con‐ fers on fibronectin its ability to bind multiple ligands [40].

Fibronectins help attach cells to the extracellular matrix by binding to other ECM compo‐ nents, particularly fibrous collagens and heparan sulfate proteoglycans, and to cell sur‐ face adhesion receptors such as integrins. Through their interactions with adhesion receptors (e.g., α5β1 integrin), fibronectins influence the shape and movement of cells and the organization of the cytoskeleton. Conversely, by regulating their receptor-mediated attachments to fibronectin and other ECM components, cells can sculpt the immediate ECM environment to suit their needs.

**•** Integrins

Integrins are the principle adhesion receptors; a large family of αβ heterodimeric cell sur‐ face proteins that mediate both cell–cell and cell–matrix. They are transmembrane pro‐ teins that mediate interactions between adhesion molecules on adjacent cells and/or the extracellular matrix (ECM). They have diverse roles in several biological processes includ‐ ing cell migration during development and wound healing, cell differentiation, and apop‐ tosis. Their activities can also regulate the metastatic and invasive potential of tumor cells. They exist as heterodimers consisting of alpha and beta subunits. Some alpha and beta subunits exhibit specificity for one another, and heterodimers often preferentially bind certain cell adhesion molecules, or constituents of the ECM.

Although they themselves have no catalytic activity, integrins can be part of multimolecu‐ lar signalling complexes known focal adhesions. The two subunits, designated as alpha and beta, both participate in binding.

**Figure 6.** Fibronectin binding to its Integrin receptor (adapted from internet)

Integrins participate in cell-cell adhesion and are of great importance in binding and interac‐ tions of cells with components of the extracellular matrix such as fibronectin. Importantly, integrins facilitate "communication" between the cytoskeleton and extracellular matrix; al‐ low each to influence the orientation and structure of the other. It is clear that interactions of integrins with the extracellular matrix can have profound effects on cell function, and events such as clustering of integrins activates a number of intracellular signally pathways.

#### *3.2.3. Cell adhesion: The physical process*

*3.2.2.1. Extracellular matrix*

220 Advances in Biomaterials Science and Biomedical Applications

stages of tissue assembly [42].

**•** Fibronectins

**•** Integrins

The extracellular matrix (ECM) represents an important element in the processes of cell ad‐ hesion. Indeed, at this level, cell adhesion is under the control of a well defined zone in the cytoplasmic membrane called focal contact. At this zone, filaments of actin are linked to fi‐ bronectin through an intracellular complex of proteins, the adherence complex. The extracel‐ lular matrix (ECM) is made of different proteins such as fibronectins, collagen, laminin,

Although the extracellular matrix generally provides mechanical support to tissues, it serves several other functions as well. Different combinations of ECM components tailor the extrac‐ ellular matrix for specific purposes: strength in a tendon, tooth, or bone; cushioning in carti‐ lage; and adhesion in most tissues. In addition, the composition of the matrix, which can vary, depending on the anatomical site and physiological status of a tissue, can let a cell know where it is and what it should do (environmental cues). Changes in ECM components, which are constantly being remodeled, degraded, and resynthesized locally, can modulate the interactions of a cell with its environment. The matrix also serves as a reservoir for many extracellular signalling molecules that control cell growth and differentiation. In addition, the matrix provides a lattice through or on which cells can move, particularly in the early

Many functions of the matrix require transmembrane adhesion receptors that bind directly to ECM components and that also interact, through adapter proteins, with the cytoskeleton. The principal class of adhesion receptors that mediate cell–matrix adhesion are integrins, a large family of αβ heterodimeric cell surface proteins that mediate both cell–cell and cell–

Different proteins and their receptors are involved in fibroblast cells adhesion process. The

Fibronectins are dimers of two similar polypeptides linked at their C-termini by two di‐ sulfide bonds; each chain is about 60–70 nm long and 2–3 nm thick. The combination of different repeats composing the regions, another example of combinatorial diversity, con‐

Fibronectins help attach cells to the extracellular matrix by binding to other ECM compo‐ nents, particularly fibrous collagens and heparan sulfate proteoglycans, and to cell sur‐ face adhesion receptors such as integrins. Through their interactions with adhesion receptors (e.g., α5β1 integrin), fibronectins influence the shape and movement of cells and the organization of the cytoskeleton. Conversely, by regulating their receptor-mediated attachments to fibronectin and other ECM components, cells can sculpt the immediate

matrix adhesions and inside-out and outside-in signalling in numerous tissues.

most important and known are fibronectins and their receptors; integrins:

fers on fibronectin its ability to bind multiple ligands [40].

ECM environment to suit their needs.

*3.2.2.2. Adhesion proteins and receptors in fibroblast cells*

vitronectin [41] and represents the mediator of cell adhesion thanks to its integrins.

Biological systems exhibit electromagnetic activity in a wide frequency range from the static or quasistatic electric field to optical bands. Fröhlich [43] presumed that biological matter has anomalous polarization properties (e.g. induction of great electric dipole after electric field application). Static charge distribution of dipole and/or multipole nature exists (e.g. in protein molecules). Vibrations in biological molecules, therefore, generate an electromagnet‐ ic field [44]. Pokorny et al.[45], assume that the Fröhlich electromagnetic field can be a fun‐ damental factor of cell adherence.

Surface topography is of an important interest in cell adhesion as well as its chemical com‐ position. Indeed, it has been shown that cells adhere and proliferate depending on the sur‐ face roughness and the more the surface is rough the more cell adhesion and proliferation is better [46]. This effect depends on the cell type. For fibroblasts, they line up along the bioma‐ terial surface microstructures and may adapt their shape with uneven surfaces.

Moreover, recent studies had shown that a weak change in the surface roughness may in‐ duce different cell reactions such as change in their shape and their way of adhesion [47, 48].

#### *3.2.3.1. Forces involved in cell adhesion*

According to Richards [49], cell adhesion to biomaterials is done thanks to focal adhesion sites which represent strict contact sites with the substrate in a so limited space. For fibro‐ blasts, it has been shown the existence of a force called cohesion force responsible for keep‐ ing contact between cells themselves. However, this force is weaker than the adhesion force involved while a cell adheres to a biomaterial. This difference in force level depends on the cell type and on the nature of the biomaterial used for adhesion, and may explain the differ‐ ent ways of cell adhesion and spreading on different surface structures.

#### *3.2.3.2. Surface free energy*

Surface free energy is a thermodynamic measurement which contributes to the interpreta‐ tion of the phenomena occurring in interfaces. It has an important effect on cell adhesion in the way that every change in its value induces the modification of the surface wettability, and therefore cell behaviour will be affected too [50, 51, 52].

Cell-biomaterial interface depends on the physico-chemical properties of the biomaterial and every change in the chemical composition or in the electric charge of the surface will affect its surface free energy.

#### *3.2.4. Parameters involved in cell adhesion*

#### *3.2.4.1. Surface roughness*

Surface roughness has been the subject of many studies as a deciding factor in the process of cell adhesion to biomaterials. Ponsonnet et al.[53] had studied the behaviour of fibroblast cells while adhering to titanium surface with different roughness; they found that cells had adhered to the surface using thin cytoplasmic structures. Indeed, these cells presented a flat‐ tened shape spreading practically over the substrate surface after adhesion to smooth surfa‐ ces. However, on rough surfaces, cell morphology was affected by the surface grooves and they were reoriented by the surface structure.

According to Richards [48], smooth titanium surfaces always increase fibroblasts adhesion and proliferation better than rough surfaces. They suggested that this kind of surfaces should be a better candidate for biological implant thanks to their ability to resist to bacterial infections. Indeed, their weak roughness is unfavourable to the adhesion of bacteria.

#### *3.2.4.2. The electric charge effect*

ic field [44]. Pokorny et al.[45], assume that the Fröhlich electromagnetic field can be a fun‐

Surface topography is of an important interest in cell adhesion as well as its chemical com‐ position. Indeed, it has been shown that cells adhere and proliferate depending on the sur‐ face roughness and the more the surface is rough the more cell adhesion and proliferation is better [46]. This effect depends on the cell type. For fibroblasts, they line up along the bioma‐

Moreover, recent studies had shown that a weak change in the surface roughness may in‐ duce different cell reactions such as change in their shape and their way of adhesion [47, 48].

According to Richards [49], cell adhesion to biomaterials is done thanks to focal adhesion sites which represent strict contact sites with the substrate in a so limited space. For fibro‐ blasts, it has been shown the existence of a force called cohesion force responsible for keep‐ ing contact between cells themselves. However, this force is weaker than the adhesion force involved while a cell adheres to a biomaterial. This difference in force level depends on the cell type and on the nature of the biomaterial used for adhesion, and may explain the differ‐

Surface free energy is a thermodynamic measurement which contributes to the interpreta‐ tion of the phenomena occurring in interfaces. It has an important effect on cell adhesion in the way that every change in its value induces the modification of the surface wettability,

Cell-biomaterial interface depends on the physico-chemical properties of the biomaterial and every change in the chemical composition or in the electric charge of the surface will

Surface roughness has been the subject of many studies as a deciding factor in the process of cell adhesion to biomaterials. Ponsonnet et al.[53] had studied the behaviour of fibroblast cells while adhering to titanium surface with different roughness; they found that cells had adhered to the surface using thin cytoplasmic structures. Indeed, these cells presented a flat‐ tened shape spreading practically over the substrate surface after adhesion to smooth surfa‐ ces. However, on rough surfaces, cell morphology was affected by the surface grooves and

According to Richards [48], smooth titanium surfaces always increase fibroblasts adhesion and proliferation better than rough surfaces. They suggested that this kind of surfaces

terial surface microstructures and may adapt their shape with uneven surfaces.

ent ways of cell adhesion and spreading on different surface structures.

and therefore cell behaviour will be affected too [50, 51, 52].

damental factor of cell adherence.

222 Advances in Biomaterials Science and Biomedical Applications

*3.2.3.1. Forces involved in cell adhesion*

*3.2.3.2. Surface free energy*

affect its surface free energy.

*3.2.4.1. Surface roughness*

*3.2.4. Parameters involved in cell adhesion*

they were reoriented by the surface structure.

In the majority of the studies carried out about biomaterials made from polyelectrolyte film, as in our case, the electric charge effect is in proportion with the thickness of the film built and depends on the charged functional group of the polyelectrolyte used [54].

For Andrade [25], the notion of the nature of an electric charge is important to be mentioned but its effect is not significant and doesn't induce an efficient change on surface wettability. However, it has been shown that a better adhesion of cells was observed on negatively charged polyelectrolyte [55]. In reality, most of the existed cells and their corresponding ad‐ hesion proteins are negatively charged. Nevertheless, this charge can be without any effect in the case when functional groups become able to control cell adhesion mechanism by their hydrophilic or hydrophobic character as it will be shown later in this text. Dubois [56] pre‐ sumed that an electric charge trapped within an insulating biomaterial, none associated to a particular chemical group, is able to affect its biological environment. Moreover, Maroudas [57] revealed the dependence of cell adhesion and spreading on a solid surface on the sur‐ face charge of the substrate.

#### *3.2.4.3. Chemical composition*

The different chemical components of a biomaterial must be studied and known before to start investigating cell adhesion to that biomaterial. Therefore, this step is fundamen‐ tal for concluding about the biocompatibility of a given biomaterial and its effect on cell adhesion [58].

The wettability of a surface depends on the chemical composition of the material and each change than can occur at this level will disturb cell adhesion process [59]. Besides the effect of the biomaterial, the adhered cell type plays an important role in adhesion. Indeed, for the same biomaterial surface, different cell reactions were observed for two types of cells [60]; this kind of biomaterial seems to be biocompatible with one cell type but not tolerated by the other cell type.

According to Marmur [61], most of the materials in the nature are rough and heterogeneous and contact angle may change along the contact line with a value depending on the rough‐ ness and heterogeneity level.

#### *3.2.4.4. Surface hydrophilicity and hydrophobicity*

Contact angle measurement allows us to calculate surface free energy [62]. It also allows knowing about the polar or non polar nature of the interactions at the interface liquid/ solid. Moreover, one can deduce from it the hydrophilic or the hydrophobic character of a surface [63].

A study about polyelectrolyte films found that hydrophobic interactions on a surface in‐ duce the adsorption of proteins and stabilise the complex formed [64]. Indeed, it has been proved that myoglobin or lysozymes are able to adhere to polystyrene sulfonate (PSS) and form many layers. However, this adhesion was not possible when using an‐ other surface having the same electric charge as PSS but with a hydrophilic character. The electrostatic interactions between the protein complex and this hydrophilic surface were easily destructed after water rinsing. Thus, surface hydrophilicity and hydrophobic‐ ity are a determinant parameter for substrate wettability on account of the rearrange‐ ment of the functional groups at the surface of a biomaterial in contact with a cell [65, 66, 67]. Indeed, it has been shown that fibroblast cells adhere and proliferate better on biomaterials with a moderate hydrophilicity [68, 69].

Andrade [66] presumed that, in the case of deformable materials, an elasticity model of 3.5 105 dyn/cm2 is necessary for avoiding contact angle change. A roughness below 0.1 μm has a negligible effect on contact angle. Most of the materials holding over than 20 to 30 % of wa‐ ter present a receding contact angle (θr), in water, near zero because of the hydrophilic char‐ acter which dominates the interface in these conditions. The same author estimated that the majority of polymers have a changeable volume which can be the reason for contact angle change: this change is depending on the duration of the contact with water, on the nature of the liquid and on the temperature of measurement. Non existent contact angle hysteresis may be due to the duration of contact between the material and the liquid which is shorter or longer than the measurement time needed for recording contact angle change. Therefore, surface hydrophilicity and hydrophobicity depends on the volume blowing of the material, on the diffusion phenomenon and on the mobility and reorientation of the molecules on the material surface.

Some materials are able to go out of shape in contact with a liquid depending on their me‐ chanical properties and on their relaxation time and temperature. So, what characterizes a polymer is its chemical composition, roughness, mobility, wettability, surface free energy and its electric charge [70].

#### *3.2.4.5. Surface wettability: Contact angle hysteresis*

Contact angle hysteresis is the result of contact angle change between the surface we are characterizing and another ideal surface physico-chemically homogeneous. It's the direct re‐ sult of a different sensitivity to the wettability process of heterogeneous surfaces. According to Rupp et al.[71], the receding contact angle value (θr) is under the control of the small hy‐ drophilic particles of the surface which are able to disturb or to delay the non wettability process. Indeed, when the hysteresis remains constant after many immersion and emersion cycles it's called thermodynamic (or true) hysteresis. However, in the opposite case, it's called kinetic hysteresis (see Figure 7).

Thermodynamic hysteresis is due to the surface roughness and heterogeneity. Nevertheless, kinetic hysteresis is caused by the adsorption mechanisms (due to the liquid phase), surface polar group's reorientation and surface deformation [24].

A study about polyelectrolyte films found that hydrophobic interactions on a surface in‐ duce the adsorption of proteins and stabilise the complex formed [64]. Indeed, it has been proved that myoglobin or lysozymes are able to adhere to polystyrene sulfonate (PSS) and form many layers. However, this adhesion was not possible when using an‐ other surface having the same electric charge as PSS but with a hydrophilic character. The electrostatic interactions between the protein complex and this hydrophilic surface were easily destructed after water rinsing. Thus, surface hydrophilicity and hydrophobic‐ ity are a determinant parameter for substrate wettability on account of the rearrange‐ ment of the functional groups at the surface of a biomaterial in contact with a cell [65, 66, 67]. Indeed, it has been shown that fibroblast cells adhere and proliferate better on

Andrade [66] presumed that, in the case of deformable materials, an elasticity model of 3.5

Some materials are able to go out of shape in contact with a liquid depending on their me‐ chanical properties and on their relaxation time and temperature. So, what characterizes a polymer is its chemical composition, roughness, mobility, wettability, surface free energy

Contact angle hysteresis is the result of contact angle change between the surface we are characterizing and another ideal surface physico-chemically homogeneous. It's the direct re‐ sult of a different sensitivity to the wettability process of heterogeneous surfaces. According to Rupp et al.[71], the receding contact angle value (θr) is under the control of the small hy‐ drophilic particles of the surface which are able to disturb or to delay the non wettability process. Indeed, when the hysteresis remains constant after many immersion and emersion cycles it's called thermodynamic (or true) hysteresis. However, in the opposite case, it's

Thermodynamic hysteresis is due to the surface roughness and heterogeneity. Nevertheless, kinetic hysteresis is caused by the adsorption mechanisms (due to the liquid phase), surface

 dyn/cm2 is necessary for avoiding contact angle change. A roughness below 0.1 μm has a negligible effect on contact angle. Most of the materials holding over than 20 to 30 % of wa‐ ter present a receding contact angle (θr), in water, near zero because of the hydrophilic char‐ acter which dominates the interface in these conditions. The same author estimated that the majority of polymers have a changeable volume which can be the reason for contact angle change: this change is depending on the duration of the contact with water, on the nature of the liquid and on the temperature of measurement. Non existent contact angle hysteresis may be due to the duration of contact between the material and the liquid which is shorter or longer than the measurement time needed for recording contact angle change. Therefore, surface hydrophilicity and hydrophobicity depends on the volume blowing of the material, on the diffusion phenomenon and on the mobility and reorientation of the molecules on the

biomaterials with a moderate hydrophilicity [68, 69].

224 Advances in Biomaterials Science and Biomedical Applications

105

material surface.

and its electric charge [70].

*3.2.4.5. Surface wettability: Contact angle hysteresis*

called kinetic hysteresis (see Figure 7).

polar group's reorientation and surface deformation [24].

**Figure 7.** Immersion and emersion loops showing the two types of hysteresis: (A): thermodynamic hysteresis and (B): kinetic hysteresis. The sample is repeatedly immersed in the liquid leading to typical hysteresis loops. From each loop, wettability parameters (advancing and receding contact angle or wetting tension) can be calculated

Contact angle hysteresis is often assigned to the surface roughness and heterogeneity. Actually, a study made by Lam et al. [26], have shown that hysteresis is related to the molecules' mobility, the liquid diffusion and the surface swelling. These authors had ob‐ served a close dependence between the liquid molecules size and the liquid/material con‐ tact duration. Liquid resorption and retention are the direct causes of hysteresis. However, as the liquid surface free energy is higher that that of the material; therefore the liquid retention into the material will increase the material surface free energy and thus reduces the receding contact angle (θr). Indeed, liquids having smaller molecular chains (or smaller molecular weight) diffuse faster into the polymer surface leading to an important decrease in contact angle.

According to Shananan et al.[72], contact angle hysteresis is related to the polymer polarity. Indeed, when a polymer gets in touch with a polar liquid (water), it orients its mobile polar groups on the surface in order to increase the interfacial water/polymer energy and there‐ fore decreasing the system surface free energy. In the other hand, when the polymer is con‐ tact with a non polar liquid, its functional groups conserve their state and will not reorient. These authors assumed the existence of two parameters behind hysteresis: the intrinsic po‐ larity of the material and the mobility of its polar groups on the surface. Nishioka et al.[73], had observed that the advancing contact angle hysteresis is under the control of surface sites more hydrophobic than those controlling the receding contact angle hysteresis.

The contact angle hysteresis observed on hydrophilic and hydrated polymers is due to the polar groups' orientation on the interfaces polymer/liquid and polymer/air. This reorienta‐ tion represents the polymer reaction to every environmental change (air, liquid). The reced‐ ing contact angle (θr) depends on the contact duration with water, the environment temperature and on the glass transition temperature (Tg) of the material itself. Each material has its own glass transition temperature (Tg) allowing a defined molecular mobility suffi‐ cient for an important rearrangement [74].

#### **3.3. Conclusion**

The concepts of solid surfaces assumed that the surfaces in question were effectively rigid and immobile. Such assumptions allow one to develop certain models and mathematical re‐ lationships useful for estimating and understanding surface energies, surface stresses, and specific interactions, such as adsorption, wetting, and contact angles. It is assumed that the surfaces themselves do not change or respond in any specific way to the presence of a con‐ tacting liquid phase, thereby altering their specific surface energy [75]. Although such as‐ sumptions are (or may be) valid for truly rigid crystalline or amorphous solids, they more often than not do not apply strictly to polymeric surfaces.

In contact with condensed phases, especially liquids, surface relaxations and transitions can become quite important leading to a possible dramatically change in the interfacial charac‐ teristics of a polymer with possibly important consequences in a particular application. And since the processes are time-dependent, the changes may not be evident over the short span of a normal experiment. For critical applications in which a polymer surface will be in con‐ tact with a liquid phase, such as implant device for biomedical application, it is not only im‐ portant to know the surface characteristics (e.g., coefficient of friction, adhesion, adsorption)under normal experimental conditions but also to determine the effects of pro‐ longed (equilibrium) exposure to the liquid medium of interest. It is therefore important for biomedical as well as many other applications that the surface characteristics of a material of interest be determined under conditions that mimic as closely as possible the conditions of use and over extended periods of exposure to those conditions, in addition to the usual characterizations.
