**2.5 Classification of zeolite membranes**

Zeolitic membranes are membranes in which the selectivity is due to the zeolitic structure regardless of the membrane constitution and morphology (**Figure 5**). Composite, self-supported, mesoporous, and mixed-matrix membranes can certainly be considered zeolitic if the chemical-physical zeolite characteristics influence the process to which they are applied.

Zeolitic membranes consist of intergrown crystals of sizes in the range of a few nanometers up to several hundred microns. They can be formed in self-supported zeolite membranes, which are very fragile, and therefore, for applications that require the use of pressure gradients, microporous film is grown using permanent inorganic (e.g., ceramic and metal) and organic (e.g., plastic and wood) supports.

Zeolitic membranes can be classified into various categories based on their shared physics-chemical and morphological characteristics, using various classification parameters. Currently, we can identify eight possible classification systems that can coexist and are preferred by scientists depending, for example, on the material application or the feature studied. These systems can thus be identified on the inclusion criterion used:


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

#### **Figure 6.**

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

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].

Zeolitic membranes are membranes in which the selectivity is due to the zeolitic

Zeolitic membranes consist of intergrown crystals of sizes in the range of a few nanometers up to several hundred microns. They can be formed in self-supported zeolite membranes, which are very fragile, and therefore, for applications that require the use of pressure gradients, microporous film is grown using permanent inorganic (e.g., ceramic and metal) and organic (e.g., plastic and wood) supports. Zeolitic membranes can be classified into various categories based on their shared physics-chemical and morphological characteristics, using various classification parameters. Currently, we can identify eight possible classification systems that can coexist and are preferred by scientists depending, for example, on the material application or the feature studied. These systems can thus be identified on

structure regardless of the membrane constitution and morphology (**Figure 5**). Composite, self-supported, mesoporous, and mixed-matrix membranes can certainly be considered zeolitic if the chemical-physical zeolite characteristics influ-

chemical composition of the gels' reaction precursors.

**2.5 Classification of zeolite membranes**

**Figure 5.**

*Classification of zeolite membranes.*

*Zeolites - New Challenges*

ence the process to which they are applied.

the inclusion criterion used:

• Chemical composition

• Membrane morphology

**42**

• Type of crystalline zeolite framework

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


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.

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

Hybrid membranes (mixed-matrix membranes) include membranes consisting 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 inclusions or crystal depositions.

## **3. Adsorption**

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.

Although the zeolitic materials have been well characterized and widely used in chromatographic applications, the analysis of protein adsorption on zeolitic crystals is poorly reported in the literature, and even less numerous are the research activities concerning the zeolitic membranes.

Adsorption is a surface phenomenon characterized by the interaction of a molecular species present in a solution (adsorbate) with the external or internal porous surface of a solid (absorbent). In the thermodynamic sense, most studies have considered adsorption as a reaction, which, of course, is more extensive if the solid material has a high surface area Eq. (1):

$$A + B\_{solid} \leftrightarrow AB \tag{1}$$

biological activity is preserved with immobilization. In fact, it is possible that the interaction of the inorganic matrix with the protein causes its inactivity or functional slowdown as a consequence of structural or conformational changes or steric unavailability of the active site. Therefore, it is evident that a suitable selection of the matrices is essential to obtain immobilized and, at the same time, active biological species. Zeolites have a large surface area and thermal, mechanical, and chemical resistance; therefore, they are well suited to the role of adsorbent supports for biological molecules. The acid/base nature of the material can be modified changing the silicon/aluminum ratio (called SAR) of the framework or introducing different metal atoms into it (creating isomorphic substitution) and varying the silicon/metal ratio by synthesis. Furthermore, it is possible to modify the acidity of zeolites by ionic exchange of the extra-framework cations present in the microporous chan-

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

Most biomaterials used for implants are inert, non-immunogenic, and nontoxic, but devices made with such materials often contain parts that trigger the so-called foreign body reaction, a material rejection complex process still not completely understood and probably related to the presence of histamine and the fibrinogen adsorption onto the implant surfaces [18]. These reactions can produce thrombosis, infections, inflammations [19], formation of fibrotic tissues around implantation, and prostheses. To realize novel active drug-releasing biomaterials, we prepared low-cost, specific drug carrier membranes for innovative biomedical drug delivering materials for implants [20]. In order to achieve this purpose, we synthesized MOR and MFI zeolite nanocrystals and composite membranes using porous stainless steel permanent supports; then we prepared ion-exchanged structures Cu(II)

Our work on the adsorption, and the subsequent release, of a model drug revealed that these zeolite materials are useful to immobilize famotidine (3-[[[2 diathiazolyl]methyl]thio]sulfamoylpropionamidine), a histamine H2 receptor. Furthermore, we evidenced that the synthesized materials, having different types of zeolitic structure and bivalent counter-cations, show different performances suitable to biomedical applications. In fact, the adsorption percentage of on transition metal-containing nanocrystalline zeolites was greater with respect to the as-made materials suggesting that these cations chemically interact with the drug and that

The composition of traditional scaffolds has changed considerably since the end of the 1980s, when the field of tissue engineering was started in a systematic way. This improvement reflects the greater scientific understanding of the needs of the cells in the adhesion and management of their behavior, which are fundamental in tissue engineering applications. The success of a new scaffold is not only based on its mechanical characteristics or on the surrounding chemical environment but also on its detailed chemical surface and topography (in a nanometer scale). These last two characteristics are not so easy to achieve by chemical synthesis for a large

The analysis of cell-substrate interaction is of fundamental importance in order to design biomimetic scaffolds capable of replacing damaged vital organs, or tissues, or

nels, for example, with protons.

**5. Drug delivering zeolite biomaterials**

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

and Zn(II)-containing hydrophilic frameworks.

cupric ions form stable organometallic complexes.

**6. Interaction of zeolite materials and cells**

number of inorganic or polymeric materials.

**45**

The species A dissolved in the solution reacts with the adsorbent B to form AB.

In a thermodynamic equilibrium situation, the Gibbs free energy change tends to zero, whereas the two chemical potentials are equal according to Eq. (2):

$$
\Delta \mathbf{G} = \mu\_{\text{r-l}} - \mu\_l = \Delta \mathbf{G}^0 + RT \ln(\mathbf{K}\_\epsilon) = \mathbf{0} \tag{2}
$$

where ΔG is the Gibbs free energy change, *μs-l* is the chemical potential in the solid–liquid interface, *μ<sup>l</sup>* is the chemical potential in the liquid phase, *R* is the universal gas constant, and *Ke* is the equilibrium thermodynamic constant (Cheng and Zhang, 2014 da Bonilla):

$$\ln\left(K\_{\epsilon}\right) = -\frac{\Delta H^{0}}{RT} + \frac{\Delta \mathbf{S}^{0}}{R} = \mathbf{0} \tag{3}$$

Eq. (3) permits to calculate the adsorption thermodynamic values of Δ*H*° and Δ*S*° plotting *ln*(*Ke*) versus I/T values in the van't Hoff plot. A reasonable physic meaning of *Ke* can be given considering:

$$K\_{\varepsilon} = \frac{\text{activity of occupied sites}}{\text{(activity of empty sites)} \text{ (activity of adsorption in solution)}} \tag{4}$$

$$K\_{\epsilon} = \frac{\frac{q\_{\epsilon}}{q\_{m}}}{\left(1 - \frac{q\_{\epsilon}}{q\_{m}}\right)\frac{C\_{\epsilon}}{C}}\tag{5}$$

and

$$q\_{\epsilon} = \frac{q\_{m}K\_{\epsilon}\left(\frac{C\_{\epsilon}}{C}\right)}{1 + K\_{\epsilon}\left(\frac{C\_{\epsilon}}{C}\right)}\tag{6}$$

This last equation allows to obtain the dimensionless value of *Ke* by plotting the experimental data obtained for *qe* (expressed in moles/grams) versus *Ce* (expressed in moles per liter) and considering the value of *C*° equal to 1 mole per liter.

### **4. Adsorption of proteins and enzymes**

It is known that protein molecules selectively bind to non-biological surfaces such as those of the metals, of carbonate oxides, and semiconductors. Naturally, in order to use these inorganic supports as biomaterials, it is necessary that the protein *Zeolites as Chameleon Biomaterials: Adsorption of Proteins, Enzymes, Foods, Drugs, Human… DOI: http://dx.doi.org/10.5772/intechopen.88422*

biological activity is preserved with immobilization. In fact, it is possible that the interaction of the inorganic matrix with the protein causes its inactivity or functional slowdown as a consequence of structural or conformational changes or steric unavailability of the active site. Therefore, it is evident that a suitable selection of the matrices is essential to obtain immobilized and, at the same time, active biological species. Zeolites have a large surface area and thermal, mechanical, and chemical resistance; therefore, they are well suited to the role of adsorbent supports for biological molecules. The acid/base nature of the material can be modified changing the silicon/aluminum ratio (called SAR) of the framework or introducing different metal atoms into it (creating isomorphic substitution) and varying the silicon/metal ratio by synthesis. Furthermore, it is possible to modify the acidity of zeolites by ionic exchange of the extra-framework cations present in the microporous channels, for example, with protons.

### **5. Drug delivering zeolite biomaterials**

Although the zeolitic materials have been well characterized and widely used in chromatographic applications, the analysis of protein adsorption on zeolitic crystals is poorly reported in the literature, and even less numerous are the research activ-

Adsorption is a surface phenomenon characterized by the interaction of a molecular species present in a solution (adsorbate) with the external or internal porous surface of a solid (absorbent). In the thermodynamic sense, most studies have considered adsorption as a reaction, which, of course, is more extensive if the

The species A dissolved in the solution reacts with the adsorbent B to form AB. In a thermodynamic equilibrium situation, the Gibbs free energy change tends

where ΔG is the Gibbs free energy change, *μs-l* is the chemical potential in the

Δ*H*<sup>0</sup> *RT* þ

Eq. (3) permits to calculate the adsorption thermodynamic values of Δ*H*° and Δ*S*° plotting *ln*(*Ke*) versus I/T values in the van't Hoff plot. A reasonable physic

activity of adsorbate in solution

*qe qm* <sup>1</sup> � *qe qm Ce*

*qmKe Ce C*° 

<sup>1</sup> <sup>þ</sup> *Ke Ce C*°

This last equation allows to obtain the dimensionless value of *Ke* by plotting the experimental data obtained for *qe* (expressed in moles/grams) versus *Ce* (expressed

It is known that protein molecules selectively bind to non-biological surfaces such as those of the metals, of carbonate oxides, and semiconductors. Naturally, in order to use these inorganic supports as biomaterials, it is necessary that the protein

*C*°

to zero, whereas the two chemical potentials are equal according to Eq. (2):

solid–liquid interface, *μ<sup>l</sup>* is the chemical potential in the liquid phase, *R* is the universal gas constant, and *Ke* is the equilibrium thermodynamic constant (Cheng

ln ð Þ¼� *Ke*

*Ke* ¼

*qe* ¼

in moles per liter) and considering the value of *C*° equal to 1 mole per liter.

*Ke* <sup>¼</sup> *activity of occupied sites*

activity of empty sites

**4. Adsorption of proteins and enzymes**

*A* þ *Bsolid* \$ *AB* (1)

*<sup>R</sup>* <sup>¼</sup> <sup>0</sup> (3)

(5)

(4)

(6)

<sup>Δ</sup>*<sup>G</sup>* <sup>¼</sup> *<sup>μ</sup><sup>s</sup>*�*<sup>l</sup>* � *<sup>μ</sup><sup>l</sup>* <sup>¼</sup> <sup>Δ</sup>*G*<sup>0</sup> <sup>þ</sup> *RTln K*ð Þ¼ *<sup>e</sup>* <sup>0</sup> (2)

Δ*S*<sup>0</sup>

ities concerning the zeolitic membranes.

*Zeolites - New Challenges*

solid material has a high surface area Eq. (1):

and Zhang, 2014 da Bonilla):

and

**44**

meaning of *Ke* can be given considering:

Most biomaterials used for implants are inert, non-immunogenic, and nontoxic, but devices made with such materials often contain parts that trigger the so-called foreign body reaction, a material rejection complex process still not completely understood and probably related to the presence of histamine and the fibrinogen adsorption onto the implant surfaces [18]. These reactions can produce thrombosis, infections, inflammations [19], formation of fibrotic tissues around implantation, and prostheses. To realize novel active drug-releasing biomaterials, we prepared low-cost, specific drug carrier membranes for innovative biomedical drug delivering materials for implants [20]. In order to achieve this purpose, we synthesized MOR and MFI zeolite nanocrystals and composite membranes using porous stainless steel permanent supports; then we prepared ion-exchanged structures Cu(II) and Zn(II)-containing hydrophilic frameworks.

Our work on the adsorption, and the subsequent release, of a model drug revealed that these zeolite materials are useful to immobilize famotidine (3-[[[2 diathiazolyl]methyl]thio]sulfamoylpropionamidine), a histamine H2 receptor. Furthermore, we evidenced that the synthesized materials, having different types of zeolitic structure and bivalent counter-cations, show different performances suitable to biomedical applications. In fact, the adsorption percentage of on transition metal-containing nanocrystalline zeolites was greater with respect to the as-made materials suggesting that these cations chemically interact with the drug and that cupric ions form stable organometallic complexes.

### **6. Interaction of zeolite materials and cells**

The composition of traditional scaffolds has changed considerably since the end of the 1980s, when the field of tissue engineering was started in a systematic way. This improvement reflects the greater scientific understanding of the needs of the cells in the adhesion and management of their behavior, which are fundamental in tissue engineering applications. The success of a new scaffold is not only based on its mechanical characteristics or on the surrounding chemical environment but also on its detailed chemical surface and topography (in a nanometer scale). These last two characteristics are not so easy to achieve by chemical synthesis for a large number of inorganic or polymeric materials.

The analysis of cell-substrate interaction is of fundamental importance in order to design biomimetic scaffolds capable of replacing damaged vital organs, or tissues, or

to assist the body's natural healing processes. The ability of a cell to recognize and interact with the substrate represents the first indispensable step, without which processes such as cell adhesion proliferation, migration, and differentiation would not be possible. Therefore the understanding of the mechanisms that determine the early phases of cell-material adhesion, as well as their control, is indispensable for the design of biomaterials. Both the mechanical and biochemical properties of the material determine the efficacy and agreed with which the cells recognize the material.

• High surface area to boost the cell adhesion

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

• Chemical composition appropriate to promote cell differentiation and growth

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

When the adhered cells increase in number, they begin to enter the internal pores of the scaffold. If the porosity and interconnection between the pores are good, the cells grow and colonize the entire scaffold releasing their extracellular matrix. The upper layer of cells consumes more oxygen and nutrients, thus limiting the amount available for the cells that are migrating into the scaffold; the maximum depth at which cells can survive corresponds to the depth of cellular penetration. We studied many types of both self-supported and hybrid PLA-containing zeolitic membranes (MMMs) to study interactions with different types of normal [22] or carcinogenic (MDA-MB-231 [23] and MCF-7 [24]) cells. Initial cell tethering and filopodia exploration are followed by lamellipodia ruffling, membrane activity, and cell spreading. With time endogenous matrix is secreted by the cells, and matrix assembly sites form on the ventral plasma biological membrane. Later, with increased integrin recruitment, these early cell-matrix contacts form anchoring focal complexes at the lamellipodium leading edge that are reinforced intracellularly to form larger focal adhesion plaques upon increased intracellular and/or extracellular tension. The regulation of focal adhesion formation in adherent cells is highly complex and involves both the turnover of single integrins and the reinforcement of the adhesion plaque by protein recruitment. It follows that focal adhesions emerge as diverse protein networks that provide structural integrity and dynamically link the ECM to intracellular actin filaments, directly facilitating cell migration and spreading through continuous regulation and turnover. Furthermore, in combination with growth factor receptors, these adhesive clusters initiate signaling pathways and regulate the activity of nuclear transcription factors and processes crucial to cell growth and differentiation. The adhesion sites act as mechanosensors that form additional contact points with the underlying substratum in response. Preceding focal adhesion reinforcement, a tightly regulated series of temporospatial events occurs, mediating integrin clustering in an anisotropic manner in the direction of force. Our works underlined that the cells of both lines assume a specific morphol-

• Structural parameters appropriate to modulate cellular biosynthesis

ogy under the influence on the major peculiarities of scaffolds.

*Schematic representation of the antimicrobial activity of zeolite scaffolds.*

**Figure 7.**

**47**

The possibility of modifying and controlling surface properties at the micro-/ nanolevel constitutes one of the major breakthroughs, because it opens a whole new range of strategies seeking the desired interaction with the biological environment. In order to prepare a new generation of biomaterials with enhanced properties, a different approach needs to be reached, based on a more fundamental understanding of the way in which the structure of a biomaterial controls its biological activity. The chemical properties influence the surface properties of a material and, consequently, cell behavior. When cells are exposed to a suitable scaffold, a layer of proteins is adsorbed on the scaffold surface within a few milliseconds. Thus cells "see" the layer of adsorbed proteins rather than the actual abiotic surface. The chemistry of the surface of a scaffold can be developed in order to control the adsorption of proteins, which in turn controls cell adhesion. According to the hoped-for result, the chemical characteristics of the surface of a material can be modified to modulate the interactions of cells adherent to the substrate, with consequent influence on morphology, migration, differentiation, proliferation, and cell apoptosis. The effect on cell behavior starts at the point of interaction. Furthermore, the conformation of the surface chemistry also affects the way proteins are immobilized and the adsorption of these on the surface. Starting from this assumption, we designed and prepared various crystalline zeolite scaffolds, which are different depending on the preparation method. It is evident that a porous, crystalline material having an inorganic framework with modulable acidity, hydrophilicity, and pore size constitutes a stable, homogeneous, ion- and solvent-available support. Zeolite membranes symbolize this novel type of chameleonic scaffold.
