**2. Traditional biomaterials**

A biomaterial has been defined as any substance (other than a drug) or a combination of substances, synthetic or of natural origin, which can be used for any period of time, in whole or in part of a system that treats, increases, or replaces any tissue, organ, or body function.

In 1992 Black defined biomaterial as "a non-living material used in a medical device, designed to interact with biological systems."

The fundamental requirements of every biomaterial are compatibility with human tissues and possessing all those physical, chemical, and biological characteristics that allow the material to adequately perform the task for which it was designed, such as constituting a resistant support, replacing fabrics lost, and promoting regrowth of damaged tissues.

Today, more and more, research in the field of biomaterials is fueled by the need to find new materials that can last a long time, due to the increase in the average life of the population, the increased need for prostheses even by young people, and the need to reduce the number of revisions that weigh on public health costs. Furthermore, the biological materials deriving from homologous or heterologous transplants have shown important problems: limited availability, need for a further surgical operation, potential transmission of infectious diseases, reduced osteoconductive capacity, and limited ability to be incorporated into the host bone.

found on hydrophobic surfaces. This demonstrates that morphology influences the

*Zeolites as Chameleon Biomaterials: Adsorption of Proteins, Enzymes, Foods, Drugs, Human…*

Another aspect of the morphology that must be considered to control the cellular response is surface topography. Topography, coupled with biochemical and physical signals, regulates cellular functions such as migration, adhesion, morphogenesis, differentiation, and apoptosis. Topography not only allows the systematic study of cell-substrate interactions but can also control cell orientation and morphology, which in turn controls other cellular responses. Therefore the techniques used to create substrate, precise, high-resolution surfaces acquire great importance. Today, topographies are generated with a resolution up to micrometer level, but with the advancement of modeling techniques and technology, the resolution level will reach the nanometer scale like the most in vivo structures (such as the collagen fibrils of

As previously stated, topography can induce changes in cell morphology, thus influencing cellular responses such as proliferation, gene expression, and cellular function. These responses also vary depending on the type of cells being used for sowing. For example, the experimentation conducted with surfaces on which channels have been produced revealed that many cell types tend to line up along the main axis of the channels themselves and that often the organization of the cytoskeletal components and the focal contacts is oriented in the same direction. The degree of cellular alignment in the direction identified by the channels depends in a

complex way on the characteristics of the topographical surface structure. Finally, it has been observed that also the symmetry and regularity of the topographical structure are important properties of the substrate that influence cellular behavior. The results showed that regular topography reduces cell adhesion very markedly, while surface discontinuities have improved cell adhesion. This

properties of the material and therefore its interaction with the cells.

*Type of biomaterials and their biomedical applications.*

*DOI: http://dx.doi.org/10.5772/intechopen.88422*

*2.1.2 Topography*

**Figure 1.**

the basement membrane).

**37**

Biomaterials can be divided into three main types based on the response that they generate in the host tissue: an inert material does not cause a response in the tissue, a bioactive material is integrated by the surrounding tissues, and a degradable material is reabsorbed and incorporated into the surrounding tissue and can even dissolve completely after a certain time. To date, the materials most frequently used in medical applications are metals, typically inert and used for applications subjected to loads, with sufficient fatigue resistance to withstand daily activity; ceramics, used for their hardness and resistance to stress in applications such as joint surfaces, in teeth and in surfaces in contact with bone; and polymers, used for their stability and flexibility but also for low friction in articular surfaces (**Figure 1**).

#### **2.1 Important physics features**

#### *2.1.1 Morphology*

Substrate morphology can influence cell adhesion, influencing the substrate's ability to adsorb proteins and/or altering the conformation of adsorbed proteins.

For example, material roughness affects the adhesiveness of platelets. Blood normally coagulates when exposed to surfaces different from the biological endothelial ones; for this reason various attempts have been made to find a synthetic material that is biocompatible with blood. When the surfaces were tested in a laminar flow cell, it is noted that an added surface roughness led to a decrease in platelet adhesion on hydrophilic surfaces, while an increase in platelet adhesion was *Zeolites as Chameleon Biomaterials: Adsorption of Proteins, Enzymes, Foods, Drugs, Human… DOI: http://dx.doi.org/10.5772/intechopen.88422*

**Figure 1.** *Type of biomaterials and their biomedical applications.*

found on hydrophobic surfaces. This demonstrates that morphology influences the properties of the material and therefore its interaction with the cells.

### *2.1.2 Topography*

contact angle with point of zero charge (PZC) and wettability with silicon/aluminum ratio. All the most advanced applications concern materials that occur in a membrane configuration [5], i.e., having chemical and physical selectivity whether they are pure materials, in mixture, or made of overlapping layers (composites). Zeolites, already in the form of crystals, have selectivity (shape selectivity, hydrophobicity/hydrophilicity), which can be modified by means of chemical

A biomaterial has been defined as any substance (other than a drug) or a combination of substances, synthetic or of natural origin, which can be used for any period of time, in whole or in part of a system that treats, increases, or replaces

In 1992 Black defined biomaterial as "a non-living material used in a medical

The fundamental requirements of every biomaterial are compatibility with human tissues and possessing all those physical, chemical, and biological characteristics that allow the material to adequately perform the task for which it was designed, such as constituting a resistant support, replacing fabrics lost, and

Today, more and more, research in the field of biomaterials is fueled by the need to find new materials that can last a long time, due to the increase in the average life of the population, the increased need for prostheses even by young people, and the need to reduce the number of revisions that weigh on public health costs. Furthermore, the biological materials deriving from homologous or heterologous transplants have shown important problems: limited availability, need for a further surgical operation, potential transmission of infectious diseases, reduced

osteoconductive capacity, and limited ability to be incorporated into the host bone. Biomaterials can be divided into three main types based on the response that they generate in the host tissue: an inert material does not cause a response in the tissue, a bioactive material is integrated by the surrounding tissues, and a degradable material is reabsorbed and incorporated into the surrounding tissue and can even dissolve completely after a certain time. To date, the materials most frequently used in medical applications are metals, typically inert and used for applications subjected to loads, with sufficient fatigue resistance to withstand daily activity; ceramics, used for their hardness and resistance to stress in applications such as joint surfaces, in teeth and in surfaces in contact with bone; and polymers, used for their stability and flexibility but also for low friction in articular surfaces (**Figure 1**).

Substrate morphology can influence cell adhesion, influencing the substrate's ability to adsorb proteins and/or altering the conformation of adsorbed proteins. For example, material roughness affects the adhesiveness of platelets. Blood normally coagulates when exposed to surfaces different from the biological endothelial ones; for this reason various attempts have been made to find a synthetic material that is biocompatible with blood. When the surfaces were tested in a laminar flow cell, it is noted that an added surface roughness led to a decrease in platelet adhesion on hydrophilic surfaces, while an increase in platelet adhesion was

functionalization, ion exchange, impregnation, etc.

device, designed to interact with biological systems."

**2. Traditional biomaterials**

*Zeolites - New Challenges*

any tissue, organ, or body function.

promoting regrowth of damaged tissues.

**2.1 Important physics features**

*2.1.1 Morphology*

**36**

Another aspect of the morphology that must be considered to control the cellular response is surface topography. Topography, coupled with biochemical and physical signals, regulates cellular functions such as migration, adhesion, morphogenesis, differentiation, and apoptosis. Topography not only allows the systematic study of cell-substrate interactions but can also control cell orientation and morphology, which in turn controls other cellular responses. Therefore the techniques used to create substrate, precise, high-resolution surfaces acquire great importance. Today, topographies are generated with a resolution up to micrometer level, but with the advancement of modeling techniques and technology, the resolution level will reach the nanometer scale like the most in vivo structures (such as the collagen fibrils of the basement membrane).

As previously stated, topography can induce changes in cell morphology, thus influencing cellular responses such as proliferation, gene expression, and cellular function. These responses also vary depending on the type of cells being used for sowing. For example, the experimentation conducted with surfaces on which channels have been produced revealed that many cell types tend to line up along the main axis of the channels themselves and that often the organization of the cytoskeletal components and the focal contacts is oriented in the same direction. The degree of cellular alignment in the direction identified by the channels depends in a complex way on the characteristics of the topographical surface structure.

Finally, it has been observed that also the symmetry and regularity of the topographical structure are important properties of the substrate that influence cellular behavior. The results showed that regular topography reduces cell adhesion very markedly, while surface discontinuities have improved cell adhesion. This

shows that the substrate topography is important for cell adhesion and therefore for cell-substrate interaction.

liquid-vapor interface, obtained by virtual tangent drawing along the vapor-liquid interface. Water molecules are not able to form hydrogen bonds with the hydrophobic support; therefore, they form hydrogen bonds between them generating a more ordered structure with less entropy. Water molecules on a polymeric surface reorganize around proteins, causing the irreversible unfolding and adsorption of native proteins on the substrate surface. The proteins present in the serum can act as surfactants, or they can lower the surface tension of a liquid; the hydrophobic domains interact with the substrate and the hydrophilic domains form hydrogen bonds with the water molecules, thus facilitating the wettability of the surfaces. This involves a release of ordered water molecules, which is energetically favorable due to the increase in entropy, known as hydrophobic effect. In general, proteins are preferentially adsorbed on hydrophobic surfaces, mediated by their hydrophobic domains. Instead of seeing the underlying surface, the cells see the layer of proteins adsorbed on the surface of the substrate, which then modulate cell

*Zeolites as Chameleon Biomaterials: Adsorption of Proteins, Enzymes, Foods, Drugs, Human…*

*DOI: http://dx.doi.org/10.5772/intechopen.88422*

Polymeric biomaterial surface charge affects the adsorption and the unfolding of proteins on its surface. Unlike wettability, the driving force for protein unfolding on a charged surface is electrostatic interaction, not hydrophobic interactions. Protein unfolding depends on the net charge that proteins and cells encounter on the surface in the cell culture medium. Many proteins have a net negative surface charge, which promotes their adsorption on a positively charged surface.

We can imagine that the adaptability of zeolitic materials to interact with dif-

Zeolites used as biomaterials can be distinguished according to the origin: natural, artificial, and synthetic. It should be noted that the crystallized structures by means of hydrothermal reactions in the laboratory under controlled conditions have, at the same time, higher crystallinity, purity, and reproducibility of chemical composition, zeolitic structure, dimensions, morphology, and distribution of the pore and channel system. The choices of chemical parameters, in the synthesis and in the pre- or posttreatments, always have repercussions on the macroscopic chemical-physical characteristics of prepared materials, such as the hydrophobicity, the point of zero charge (PZC), and the presence of the various types of ions, present in the form of exchangeable cations or clusters. We can analyze these different characteristics by gradually shifting our analysis from the microscopic

Zeolites are bi-functional materials having both Lewis and Brönsted acidity. These two types of acidity are not independent of each other but are closely related

ferent biologically active molecular species and with different environments containing cells makes them (within the vast field of biomaterials) entirely comparable to chameleons (in the animal kingdom). In fact, it can be imagined that just as the complex specialized organization of cells can produce a color change in chameleons (by acting on well-defined physical parameters), the changing complex chemical organization in zeolite framework can interact with proteins, enzymes, cells, and foods (by modifying the preparation methods). The chameleon-like characteristics of the zeolite membranes as biomaterials can be inferred from the various

applications reported in the literature and are highlighted in **Figure 2**.

atomic field to the macroscopic membrane field.

**2.4 Synthesis and characterization of zeolite biomaterials**

adhesion.

**39**

*2.2.2 Surface charge*

**2.3 Zeolites as biomaterials**

## *2.1.3 Stiffness*

Stiffness of a material is measured with the modulus of elasticity or Young's modulus. It is important to have sufficient substrate stiffness for the anchordependent cells to adhere to the surface. It is fundamental for the characterization of the interactions that modulate intracellular signaling pathways and cellular events, from gene expression to cellular locomotion. In fact, cell movement can be guided by manipulation of substrate stiffness characteristics. It has been shown how the mechanical properties of the matrix influence the differentiation of stem cells. Moreover, the proliferation and cellular mobility varied as the stiffness of the substrate varied. In particular, different types of substrates with different stiffness seeded with NSPC2 cells showed that the optimal stiffness for proliferation was 3.5 kPa, while for neuronal differentiation, it is less than 1 kPa [6].

#### *2.1.4 Crystallinity*

By controlling the amorphous-crystalline microstructure of the surface layer of the substrate, it is possible, for example, to improve the compatibility of blood surfaces. Surfaces with different degrees of crystallinity were tested, and an increase in the adhesiveness of the platelets was noticed on substrates that had less crystallinity. The particular amorphous-crystalline surface microstructure also modified the denaturation of adsorbed proteins. For example, the particular amorphous-crystalline microstructure of apolar surfaces such as propylene (with 55% surface layer crystallinity) has been shown to reduce platelet activity [7].

During the design of scaffolds for in vivo implantation, crystallinity can also influence the biodegradability of the scaffold and consequently the cellular response. The crystalline region is in fact more resistant to water infiltration and therefore delays the degradation of the biomaterial. For example, the adhesion, proliferation, and morphology of human chondrocytes of articular cartilage tested as a function of the crystallinity of various degradable polymers. The results suggested that cell proliferation is slower on crystalline polymers than amorphous polymers. This highlights the interesting dynamics between cell and substrate depending on the crystallinity of the material.

A variation in crystallinity can also affect surface roughness, on a nanometric scale. Osteoblasts seeded on polymeric substrates having different crystallinity and their number were measured using fluorescence microscopy. The results showed that the proliferation rate was greater on the smooth regions of the substrates, while it was smaller on the rough regions; a decreasing monotonic variation of proliferation as a function of roughness was observed. The critical roughness above which there is a significant reduction in the proliferation rate is 1.1 nm. It has therefore been shown that the cells respond directly to the topography of the substrate, as they are sensitive to nanometric variations in the substrate topography.

#### **2.2 Important chemical features**

#### *2.2.1 Wettability*

Wettability of a solid surface is a measure of its hydrophobicity and hydrophilicity. It concerns to the ease liquid phase spreading on a solid surface, which, for polymeric materials, is generally evaluated by contact angle measurements. Contact angle represents the angle formed by the intersection of liquid-solid and

*Zeolites as Chameleon Biomaterials: Adsorption of Proteins, Enzymes, Foods, Drugs, Human… DOI: http://dx.doi.org/10.5772/intechopen.88422*

liquid-vapor interface, obtained by virtual tangent drawing along the vapor-liquid interface. Water molecules are not able to form hydrogen bonds with the hydrophobic support; therefore, they form hydrogen bonds between them generating a more ordered structure with less entropy. Water molecules on a polymeric surface reorganize around proteins, causing the irreversible unfolding and adsorption of native proteins on the substrate surface. The proteins present in the serum can act as surfactants, or they can lower the surface tension of a liquid; the hydrophobic domains interact with the substrate and the hydrophilic domains form hydrogen bonds with the water molecules, thus facilitating the wettability of the surfaces. This involves a release of ordered water molecules, which is energetically favorable due to the increase in entropy, known as hydrophobic effect. In general, proteins are preferentially adsorbed on hydrophobic surfaces, mediated by their hydrophobic domains. Instead of seeing the underlying surface, the cells see the layer of proteins adsorbed on the surface of the substrate, which then modulate cell adhesion.

#### *2.2.2 Surface charge*

shows that the substrate topography is important for cell adhesion and therefore for

Stiffness of a material is measured with the modulus of elasticity or Young's modulus. It is important to have sufficient substrate stiffness for the anchordependent cells to adhere to the surface. It is fundamental for the characterization of the interactions that modulate intracellular signaling pathways and cellular events, from gene expression to cellular locomotion. In fact, cell movement can be guided by manipulation of substrate stiffness characteristics. It has been shown how the mechanical properties of the matrix influence the differentiation of stem cells. Moreover, the proliferation and cellular mobility varied as the stiffness of the substrate varied. In particular, different types of substrates with different stiffness seeded with NSPC2 cells showed that the optimal stiffness for proliferation was

By controlling the amorphous-crystalline microstructure of the surface layer of the substrate, it is possible, for example, to improve the compatibility of blood surfaces. Surfaces with different degrees of crystallinity were tested, and an increase in the adhesiveness of the platelets was noticed on substrates that had less crystallinity. The particular amorphous-crystalline surface microstructure also modified the denaturation of adsorbed proteins. For example, the particular amorphous-crystalline microstructure of apolar surfaces such as propylene (with 55% surface layer crystallinity) has been shown to reduce platelet activity [7]. During the design of scaffolds for in vivo implantation, crystallinity can also

influence the biodegradability of the scaffold and consequently the cellular response. The crystalline region is in fact more resistant to water infiltration and therefore delays the degradation of the biomaterial. For example, the adhesion, proliferation, and morphology of human chondrocytes of articular cartilage tested as a function of the crystallinity of various degradable polymers. The results suggested that cell proliferation is slower on crystalline polymers than amorphous polymers. This highlights the interesting dynamics between cell and substrate

they are sensitive to nanometric variations in the substrate topography.

angle represents the angle formed by the intersection of liquid-solid and

A variation in crystallinity can also affect surface roughness, on a nanometric scale. Osteoblasts seeded on polymeric substrates having different crystallinity and their number were measured using fluorescence microscopy. The results showed that the proliferation rate was greater on the smooth regions of the substrates, while it was smaller on the rough regions; a decreasing monotonic variation of proliferation as a function of roughness was observed. The critical roughness above which there is a significant reduction in the proliferation rate is 1.1 nm. It has therefore been shown that the cells respond directly to the topography of the substrate, as

Wettability of a solid surface is a measure of its hydrophobicity and hydrophilicity. It concerns to the ease liquid phase spreading on a solid surface, which, for polymeric materials, is generally evaluated by contact angle measurements. Contact

depending on the crystallinity of the material.

**2.2 Important chemical features**

*2.2.1 Wettability*

**38**

3.5 kPa, while for neuronal differentiation, it is less than 1 kPa [6].

cell-substrate interaction.

*Zeolites - New Challenges*

*2.1.3 Stiffness*

*2.1.4 Crystallinity*

Polymeric biomaterial surface charge affects the adsorption and the unfolding of proteins on its surface. Unlike wettability, the driving force for protein unfolding on a charged surface is electrostatic interaction, not hydrophobic interactions. Protein unfolding depends on the net charge that proteins and cells encounter on the surface in the cell culture medium. Many proteins have a net negative surface charge, which promotes their adsorption on a positively charged surface.

#### **2.3 Zeolites as biomaterials**

We can imagine that the adaptability of zeolitic materials to interact with different biologically active molecular species and with different environments containing cells makes them (within the vast field of biomaterials) entirely comparable to chameleons (in the animal kingdom). In fact, it can be imagined that just as the complex specialized organization of cells can produce a color change in chameleons (by acting on well-defined physical parameters), the changing complex chemical organization in zeolite framework can interact with proteins, enzymes, cells, and foods (by modifying the preparation methods). The chameleon-like characteristics of the zeolite membranes as biomaterials can be inferred from the various applications reported in the literature and are highlighted in **Figure 2**.

Zeolites used as biomaterials can be distinguished according to the origin: natural, artificial, and synthetic. It should be noted that the crystallized structures by means of hydrothermal reactions in the laboratory under controlled conditions have, at the same time, higher crystallinity, purity, and reproducibility of chemical composition, zeolitic structure, dimensions, morphology, and distribution of the pore and channel system. The choices of chemical parameters, in the synthesis and in the pre- or posttreatments, always have repercussions on the macroscopic chemical-physical characteristics of prepared materials, such as the hydrophobicity, the point of zero charge (PZC), and the presence of the various types of ions, present in the form of exchangeable cations or clusters. We can analyze these different characteristics by gradually shifting our analysis from the microscopic atomic field to the macroscopic membrane field.

#### **2.4 Synthesis and characterization of zeolite biomaterials**

Zeolites are bi-functional materials having both Lewis and Brönsted acidity. These two types of acidity are not independent of each other but are closely related to each other, and both participate in the formulation of the total acidity and hydrophilicity of the zeolitic material. Lewis acidity is linked to the presence and relative concentration of trivalent aluminum atoms within the framework (and of other chemically equivalent atoms); therefore, it strongly depends on the so-called SAR ratio. Hydrophobic zeolites are not very acidic (like silicalite-1 or silicalite-2), while hydrophilic zeolites (like zeolite Y or zeolite A) or isomorphically substituted (with cation 3<sup>+</sup> ) have a high value of Lewis acidity (**Figure 3**).

The second type of acidity is linked to functional silanol groups and more complicated to analyze. We must distinguish between Brönsted acidity of the single bonds (electronically influenced by the neighboring atoms) and the total acidity (depending both on the chemical composition and on the reaction environment used for the synthesis). As a first approximation, we can consider Brönsted total acidity concerning the outer surface of the crystals so it is probably the most important type in interactions with species larger in size than that of zeolite pores such as human cells (**Figure 4**).

Finally, an extremely important characteristic is point of zero charge (PZC), which is the pH value at which the zeolitic membrane is electrically neutral.

*Zeolites as Chameleon Biomaterials: Adsorption of Proteins, Enzymes, Foods, Drugs, Human…*

Alimentarius Commission for their use as fertilizer (Codex Alimentarius Commission, 2016) and EFSA experts as flavoring material or food storage adjuvant (EFSA J

Only a natural type of fibrous zeolite is considered dangerous and erionite [8, 9].

Another use of zeolites, with good results, is to add them as additives for animal feed. The integration of zeolites in animal feed has been studied on different animals (sheep, calves, pigs, etc.), and the results obtained have shown that these substances allow preventing diseases. Moreover, the use of zeolite in animal nutrition improves the assimilation of nutrients and therefore, consequently, promotes

In recent years, zeolitic membranes have been studied as new biomaterials used for biomedical applications. These membranes, in fact, are considered as an ideal support for the immobilization of biological molecules not having the limitations

Numerous biological molecules were adsorbed and immobilized on zeolite crystals and membranes. These species include cytochrome c [11], bovine serum albumin (BSA) [12], glucose [13], uremic toxins [14, 15], nitrosamines [16],

An important characteristic of zeolitic membranes is that the basic/acid nature of the material can be modified by varying the Si/Al ratio or by introducing different metals (Me) into the crystalline structure and changing the Si/Me ratio [12]. Cytochrome c is a water-soluble electron carrier that is efficiently immobilized onto zeolitic membranes, but the composition of the membrane is an important factor that influences the immobilization performance. We also studied adsorption of BSA protein [12] on FAU, BEA, and MFI zeolite crystals synthesized under hydrothermal conditions and membranes showing that the chemical composition

FDA (Code of Federal Regulations, April 2017). They are also widely used in agriculture as fertilizers because they are declared nontoxic by IARC (IARC, Lyon,

It can induce, if breathed, tumors (pleural and peritoneal mesothelioma) and pathologies of the respiratory system, caused by its microfibrous morphological

Vol. 68, 5061997). Furthermore, they have been approved by the Codex

11: 3155, 2013) and use as feed additives (EFSA J11: 3039 2013).

*Schematic summary of the Brönsted acidity for zeolitic biomaterials.*

*DOI: http://dx.doi.org/10.5772/intechopen.88422*

an increase in weight of the animal itself [10].

associated with traditional polymeric membranes [11].

characteristics.

**Figure 4.**

and catalase [17].

**41**

Zeolites have been approved and defined as safe for human consumption by the

**Figure 2.** *Application fields of zeolite biomaterials.*

#### **Figure 3.**

*Scheme of Lewis and Brönsted acidity of single bonds. The substitution of tetravalent Si atoms in the lattice with the trivalent Al atoms generates local negative charges, which are then compensated by extra-framework cations. The charge compensation by protons results in strong Brönsted acid sites.*

*Zeolites as Chameleon Biomaterials: Adsorption of Proteins, Enzymes, Foods, Drugs, Human… DOI: http://dx.doi.org/10.5772/intechopen.88422*

#### **Figure 4.**

to each other, and both participate in the formulation of the total acidity and hydrophilicity of the zeolitic material. Lewis acidity is linked to the presence and relative concentration of trivalent aluminum atoms within the framework (and of other chemically equivalent atoms); therefore, it strongly depends on the so-called SAR ratio. Hydrophobic zeolites are not very acidic (like silicalite-1 or silicalite-2), while hydrophilic zeolites (like zeolite Y or zeolite A) or isomorphically substituted

) have a high value of Lewis acidity (**Figure 3**). The second type of acidity is linked to functional silanol groups and more complicated to analyze. We must distinguish between Brönsted acidity of the single bonds (electronically influenced by the neighboring atoms) and the total acidity (depending both on the chemical composition and on the reaction environment used for the synthesis). As a first approximation, we can consider Brönsted total acidity concerning the outer surface of the crystals so it is probably the most important type in interactions with species larger in size than that of zeolite pores

*Scheme of Lewis and Brönsted acidity of single bonds. The substitution of tetravalent Si atoms in the lattice with the trivalent Al atoms generates local negative charges, which are then compensated by extra-framework*

*cations. The charge compensation by protons results in strong Brönsted acid sites.*

(with cation 3<sup>+</sup>

*Zeolites - New Challenges*

**Figure 2.**

**Figure 3.**

**40**

*Application fields of zeolite biomaterials.*

such as human cells (**Figure 4**).

*Schematic summary of the Brönsted acidity for zeolitic biomaterials.*

Finally, an extremely important characteristic is point of zero charge (PZC), which is the pH value at which the zeolitic membrane is electrically neutral.

Zeolites have been approved and defined as safe for human consumption by the FDA (Code of Federal Regulations, April 2017). They are also widely used in agriculture as fertilizers because they are declared nontoxic by IARC (IARC, Lyon, Vol. 68, 5061997). Furthermore, they have been approved by the Codex Alimentarius Commission for their use as fertilizer (Codex Alimentarius Commission, 2016) and EFSA experts as flavoring material or food storage adjuvant (EFSA J 11: 3155, 2013) and use as feed additives (EFSA J11: 3039 2013).

Only a natural type of fibrous zeolite is considered dangerous and erionite [8, 9]. It can induce, if breathed, tumors (pleural and peritoneal mesothelioma) and pathologies of the respiratory system, caused by its microfibrous morphological characteristics.

Another use of zeolites, with good results, is to add them as additives for animal feed. The integration of zeolites in animal feed has been studied on different animals (sheep, calves, pigs, etc.), and the results obtained have shown that these substances allow preventing diseases. Moreover, the use of zeolite in animal nutrition improves the assimilation of nutrients and therefore, consequently, promotes an increase in weight of the animal itself [10].

In recent years, zeolitic membranes have been studied as new biomaterials used for biomedical applications. These membranes, in fact, are considered as an ideal support for the immobilization of biological molecules not having the limitations associated with traditional polymeric membranes [11].

Numerous biological molecules were adsorbed and immobilized on zeolite crystals and membranes. These species include cytochrome c [11], bovine serum albumin (BSA) [12], glucose [13], uremic toxins [14, 15], nitrosamines [16], and catalase [17].

An important characteristic of zeolitic membranes is that the basic/acid nature of the material can be modified by varying the Si/Al ratio or by introducing different metals (Me) into the crystalline structure and changing the Si/Me ratio [12].

Cytochrome c is a water-soluble electron carrier that is efficiently immobilized onto zeolitic membranes, but the composition of the membrane is an important factor that influences the immobilization performance. We also studied adsorption of BSA protein [12] on FAU, BEA, and MFI zeolite crystals synthesized under hydrothermal conditions and membranes showing that the chemical composition

#### **Figure 5.**

*Classification of zeolite membranes.*

and structure of zeolitic membranes influences protein adsorption kinetics. In our work the acidity of zeolite structures was modulated considering several frameworks and MFI structures (having isomorphous vanadium atoms incorporated into the crystalline structure), and Si/Al, Si/V, and Al/V ratios were varied changing the chemical composition of the gels' reaction precursors.

• Pore size

**Figure 6.**

alumina membranes.

**3. Adsorption**

**43**

inclusions or crystal depositions.

• Type of crystallization (**Figure 6**)

*DOI: http://dx.doi.org/10.5772/intechopen.88422*

*Schematic representation of different types of zeolite membrane crystallizations.*

• Metal-containing frameworks

• Origin (natural, artificial, synthetic)

Classification based on the membrane chemical composition includes inorganic, composite, and hybrid membranes. Inorganic zeolitic membranes are solid membranes made of zeolitic crystals and/or inorganic materials such as metals, oxides, amorphous silica, and ceramic particles. These membranes are very stable and resistant to mechanical stress at high temperatures and pressure gradients.

*Zeolites as Chameleon Biomaterials: Adsorption of Proteins, Enzymes, Foods, Drugs, Human…*

Composite membranes are constituted by a superposition of different materials layers, which can be evidenced by an orthogonal section to the surface like zeolite/

Hybrid membranes (mixed-matrix membranes) include membranes consisting

The possibility of modulating the specific characteristics of zeolites in a membrane configuration using inorganic zeolitic membranes in biotechnological applications such as molecular separations, enzymatic membrane reactors, protein chips, drug delivery, etc. is an attractive perspective that would offer remarkable potential applications. Naturally, the selection of materials suitable for specific applications cannot ignore the study of the interaction between the biological species and the crystalline inorganic support and therefore the understanding of adsorption.

of zeolitic crystals dispersed in a polymeric film. These membranes have great tensile strength and great elasticity but low thermal and mechanical resistance as well as poor aging stability. They are prepared easily and quickly by means of

• Synthesis methodology

Obtained results shows that zeolite Y surface adsorbs largest amounts of BSA and its percentage of adsorption increases with temperature and depends on the pH of the solution used, being absolute maximum in correspondence of protein pI value. The adsorption difference between the various types of zeolite also depends on the type of hydrothermal crystallization within the inorganic support [12].
