**2.4 Nanoparticles and porous scaffolds obtained by high-pressure CO2 processing**

In order to obtain microparticles, an experimental apparatus equipped with a high-pressure cell having a capillary nozzle was used. The scheme of this handmade experimental setup is presented in **Figure 2**.

Biopolymeric microparticles were prepared in our laboratory by rapid expansion of high-pressure CO2-chitosan solution in sodium bis-(2-ethylhexyl) sulfosuccinate solution. At pressures higher than 2 MPa, ultrafine particles were formed, while under this value, wires were obtained (**Figure 3**). The Chi/AOT ultrafine particles are instantaneously formed when 2 mL Chi solution 2% (wt/wt), preheated 1 hour at 40°C, is sprayed using CO2 at different pressures into 20 mL AOT 0.03 M aqueous solution through a stainless steel capillary nozzle of 30 mm length and 0.4 mm

**163**

**Figure 2.**

**Figure 3.**

*Experimental setup for RESS technique.*

from aqueous solution [15].

*Synthesis and Functionalization of Nanoparticles in Supercritical CO2*

diameter. The pre-expansion pressure was 1–5 MPa, and the distance from the nozzle tip to surfactant solution interface was of about 20 mm. We observe that with increasing the spraying pressure of polymer, the size of the particles decreases.

*Optical images of CO2-chitosan (chi)/bis-(2-ethylhexyl) sulfosuccinate (AOT) microparticles and wires.*

The microparticles obtained at high pressure are quasi-spherical in aqueous

medium and irregular with many pores and a rough surface after freeze-drying. The morphology of synthesized particles recommends them for possible applications in adsorption of organic and inorganic substances from aqueous medium. The Chi/ AOT microparticles were an effective adsorbent for removal of phenol and o-cresol

Porous alginate matrices were obtained using sub- and supercritical carbon dioxide. Calcium alginate matrices had uniform porous texture generated by highpressure CO2 as foaming agent without co-solvents. Sodium alginate solutions were processed in high-pressure CO2, with freezing. After depressurization, the frozen

*DOI: http://dx.doi.org/10.5772/intechopen.89353*

*Synthesis and Functionalization of Nanoparticles in Supercritical CO2 DOI: http://dx.doi.org/10.5772/intechopen.89353*

**Figure 2.** *Experimental setup for RESS technique.*

*Advanced Supercritical Fluids Technologies*

*Schematic representations of the RESS process.*

co-solvent.

**Figure 1.**

the possible particle aggregation and/or nozzle blockage, using an important amount of carbon dioxide, and low solubility of most pharmaceutical compounds in supercritical CO2. But, the scCO2 solvent power can be increased by using of a

**2.3 Rapid expansion of supercritical solution into a liquid solvent (RESOLV)**

aggregation during the jet expansion. In this process the supercritical solution is depressurized through a nozzle into a collection chamber containing an aqueous solution at room temperature. Different types of water-soluble polymers or surfactants can added to the aqueous medium in order to stabilize the obtained nanoparticle suspension [12]. Biocompatible polymer nanoparticles composed of biodegradable polymers such as polylactic acid (PLA), poly(lactic-coglycolic) acid (PLGA), and poly(ε-caprolactone) (PCL) are drawing a considerable interest in the scientific community because they can be used in medicine as biodegradable support materials and drug delivery vehicles. There are studies focused on the generation of polysaccharide particles. Polysaccharides are biobased polymers used in a lot of domains such as nutrition, energy, health care, and materials science, with large applications in the industry. An example of this family is chitosan, an aminopolysaccharide derived from chitin, being the second most biosynthesized polymer after cellulose. Because chitosan is biocompatible and biodegradable (mucoadhesive with antibacterial and cytocompatible), it can be used in pharmaceutics and biomedical applications, cosmetics, food packaging, agriculture, water treatment, etc. Another polysaccharide is alginate that can be used in several areas like drug delivery, tissue engineering, and wound dressing. For these purposes and due to its essential functional groups (hydroxyl and carboxyl), alginates can be transformed

to hydrogels, porous scaffolds, and micro- and nanoparticles [14].

experimental setup is presented in **Figure 2**.

**2.4 Nanoparticles and porous scaffolds obtained by high-pressure CO2 processing**

In order to obtain microparticles, an experimental apparatus equipped with a high-pressure cell having a capillary nozzle was used. The scheme of this handmade

Biopolymeric microparticles were prepared in our laboratory by rapid expansion of high-pressure CO2-chitosan solution in sodium bis-(2-ethylhexyl) sulfosuccinate solution. At pressures higher than 2 MPa, ultrafine particles were formed, while under this value, wires were obtained (**Figure 3**). The Chi/AOT ultrafine particles are instantaneously formed when 2 mL Chi solution 2% (wt/wt), preheated 1 hour at 40°C, is sprayed using CO2 at different pressures into 20 mL AOT 0.03 M aqueous solution through a stainless steel capillary nozzle of 30 mm length and 0.4 mm

RESOLV represents a variation of RESS. This technique can reduce the particle

**162**

**Figure 3.** *Optical images of CO2-chitosan (chi)/bis-(2-ethylhexyl) sulfosuccinate (AOT) microparticles and wires.*

diameter. The pre-expansion pressure was 1–5 MPa, and the distance from the nozzle tip to surfactant solution interface was of about 20 mm. We observe that with increasing the spraying pressure of polymer, the size of the particles decreases. The microparticles obtained at high pressure are quasi-spherical in aqueous medium and irregular with many pores and a rough surface after freeze-drying. The morphology of synthesized particles recommends them for possible applications in adsorption of organic and inorganic substances from aqueous medium. The Chi/ AOT microparticles were an effective adsorbent for removal of phenol and o-cresol from aqueous solution [15].

Porous alginate matrices were obtained using sub- and supercritical carbon dioxide. Calcium alginate matrices had uniform porous texture generated by highpressure CO2 as foaming agent without co-solvents. Sodium alginate solutions were processed in high-pressure CO2, with freezing. After depressurization, the frozen

samples were ionically cross-linked with calcium ions with and without glycerol. The effects of the presence of glycerol as plasticizer, carbon dioxide pressure, temperature, and processing time (20 minutes and 5 hours) on the structure of the obtained calcium alginate matrices were investigated. The porosity of alginate matrices increased with CO2 pressure, processing time, and glycerol adding. The plasticizer, glycerol, improves mechanical properties and texture for scaffolds.

**Poly(ԑ-caprolactone)** is another polymer with potential application in the biomedical field due to its properties such as good solubility, low melting point (59–64°C), and very good blending compatibility. PCL is suitable for controlled delivery of drug because it has high permeability for several drugs and excellent biocompatibility and it can be completely eliminated from the body. Due to the low melting point and good rheological and mechanical properties, PCL can be used as a biomaterial in cardiovascular and bone tissue engineering.

Microparticles of poly(ε-caprolactam) were obtained by rapid expansion in water of the PCL polymeric solution (with a cosolvent, methanol or dimethylformamide) saturated with high-pressure carbon dioxide, in experimental setup presented in **Figure 3**. The solution was denoted by P1 – PCL in methanol (1 g/ mL) and P2 – PCL in dimethylformamide (1 g/mL). The working temperature was 70°C and pressure 6.5 and 8.5 MPa. In **Table 1** and **Figure 4**, values for pressure are expressed in bar. Depending on the nature of the cosolvent, temperature, and pressure, poly(ε-caprolactam) particles of spherical shape and variable dimensions (particles diameter 1.4–6.7 μm) were obtained.

The morphological changes of PCL solutions in sub- and supercritical carbon dioxide and the addition of cosolvent were revealed by Boethius microscopy.

The interactions between CO2 and the carbonyl groups in the PCL molecules led to the lower melting temperature of the P1 and P2 samples treated with sub- and supercritical CO2 (**Table 2**).

The presence of high-pressure CO2 influences the stretching vibration in the group C〓O (carbonyl). For P1, once the carbon dioxide pressure increases, the maximum absorption of the carbonyl group moves to higher values than P2 where the maximum absorption decreases with increasing pressure. This displacement reaches a limit at the highest pressures as a result of increased carbon dioxide mobility. The integral area of the carbonyl peak decreases linearly with increasing pressure for the P1 system (data consistent with those in the literature) and varies

**165**

*Synthesis and Functionalization of Nanoparticles in Supercritical CO2*

**System Melting point (0**

*Structural changes of PCL were evidenced by the FTIR technique (***Figure 4***).*

nonlinearly for the P2 system. It has been observed that with the decrease of carbon dioxide pressure, the frequency of the C-O-C stretching vibration moves to higher values. At higher temperatures (70°C) and higher pressures (6.5 and 8.5 MPa), the polymer melts and recrystallizes after expansion in a semicrystalline or amorphous state, resulting in melting temperatures below PCL. For the samples treated at high pressures, we observe a shift of the maximum attributed to the crystallinity to smaller wave numbers, so a decrease of the crystallinity produces a decrease of melting temperature for the PCL samples treated under high-temperature and

PCL 60.3–61.9 White crystals P1-25-1 55.8–57.5 White crystals P2-25-1 57.1–58.9 White crystals P1-70-85 55.5–57.6 Amorphous P1-70-65 58.7–59.8 White crystals P2-70-85 58.4–60.3 White crystals P2-70-65 57.0–59.3 Amorphous

**C) Observations**

In the last decade, drugs loaded in porous biodegradable polymeric foams have found to have important applications in tissue engineering and delivery systems. These polymeric porous scaffolds with an open-pore structure can ensure and increase seeding, attachment, growth of cells, extracellular matrix production, vascularization, and tissue growth. Supercritical CO2 is an excellent choice to produce impregnated polyester foams in a one-step process creating porosity, without residual solvent in the products. After polymer scaffold degradation, obtained tissue would not contain synthetic polymer. The rate degradation of the scaffold should be similar or slower than the rate of tissue formation; therefore, it is impor-

Some examples of polymers used as porous scaffolds were poly(D,L-lactide) (PLA) and poly(D,L-lactide-co-glycolide) (PLGA), respectively, and poly(methyl methacrylate), PMMA, PMMA/poly(ε-caprolactone), PCL, etc. Such polymers are harmless to the growing cells and can be removed from the organism by normal metabolic pathways. They also can be used in other in vivo applications, such as

Encapsulation of drugs within colloidal-sized polymeric matrix is largely used to improve the sustained release, reduce the side effect of the drugs, and increase the bioavailability of the drug from the pharmaceutical formulations. Polymeric beads for the controlled release of the drugs are most often used as the best solution due to

The special properties of scCO2 made it a good transport vector for solid matrix impregnation. This process depends on the partition of interest substance between the supercritical fluid phase and matrix (such as porous polymers) used for impregnation. First the substance is mixed with high-pressure carbon dioxide and then is

*DOI: http://dx.doi.org/10.5772/intechopen.89353*

high-pressure conditions.

**Table 2.**

resorbable sutures [16].

**3. Drug encapsulation using scCO2**

*Melting point of high-pressure samples of PCL.*

tant to elucidate the mechanism of this degradation process.

easy preparation procedure and high drug-loading efficiency.

**Figure 4.** *FTIR spectra for PCL, P1, and P2 and different temperatures and pressures.*

**System Melting point (0 C) Observations** PCL 60.3–61.9 White crystals P1-25-1 55.8–57.5 White crystals P2-25-1 57.1–58.9 White crystals P1-70-85 55.5–57.6 Amorphous P1-70-65 58.7–59.8 White crystals P2-70-85 58.4–60.3 White crystals P2-70-65 57.0–59.3 Amorphous *Structural changes of PCL were evidenced by the FTIR technique (***Figure 4***).*

*Synthesis and Functionalization of Nanoparticles in Supercritical CO2 DOI: http://dx.doi.org/10.5772/intechopen.89353*

### **Table 2.**

*Advanced Supercritical Fluids Technologies*

samples were ionically cross-linked with calcium ions with and without glycerol. The effects of the presence of glycerol as plasticizer, carbon dioxide pressure, temperature, and processing time (20 minutes and 5 hours) on the structure of the obtained calcium alginate matrices were investigated. The porosity of alginate matrices increased with CO2 pressure, processing time, and glycerol adding. The plasticizer, glycerol, improves mechanical properties and texture for scaffolds. **Poly(ԑ-caprolactone)** is another polymer with potential application in the biomedical field due to its properties such as good solubility, low melting point (59–64°C), and very good blending compatibility. PCL is suitable for controlled delivery of drug because it has high permeability for several drugs and excellent biocompatibility and it can be completely eliminated from the body. Due to the low melting point and good rheological and mechanical properties, PCL can be used as

Microparticles of poly(ε-caprolactam) were obtained by rapid expansion in water of the PCL polymeric solution (with a cosolvent, methanol or dimethylformamide) saturated with high-pressure carbon dioxide, in experimental setup presented in **Figure 3**. The solution was denoted by P1 – PCL in methanol (1 g/ mL) and P2 – PCL in dimethylformamide (1 g/mL). The working temperature was 70°C and pressure 6.5 and 8.5 MPa. In **Table 1** and **Figure 4**, values for pressure are expressed in bar. Depending on the nature of the cosolvent, temperature, and pressure, poly(ε-caprolactam) particles of spherical shape and variable dimensions

The morphological changes of PCL solutions in sub- and supercritical carbon

The presence of high-pressure CO2 influences the stretching vibration in the group C〓O (carbonyl). For P1, once the carbon dioxide pressure increases, the maximum absorption of the carbonyl group moves to higher values than P2 where the maximum absorption decreases with increasing pressure. This displacement reaches a limit at the highest pressures as a result of increased carbon dioxide mobility. The integral area of the carbonyl peak decreases linearly with increasing pressure for the P1 system (data consistent with those in the literature) and varies

The interactions between CO2 and the carbonyl groups in the PCL molecules led to the lower melting temperature of the P1 and P2 samples treated with sub- and

dioxide and the addition of cosolvent were revealed by Boethius microscopy.

a biomaterial in cardiovascular and bone tissue engineering.

(particles diameter 1.4–6.7 μm) were obtained.

*FTIR spectra for PCL, P1, and P2 and different temperatures and pressures.*

supercritical CO2 (**Table 2**).

**164**

**Figure 4.**

*Melting point of high-pressure samples of PCL.*

nonlinearly for the P2 system. It has been observed that with the decrease of carbon dioxide pressure, the frequency of the C-O-C stretching vibration moves to higher values. At higher temperatures (70°C) and higher pressures (6.5 and 8.5 MPa), the polymer melts and recrystallizes after expansion in a semicrystalline or amorphous state, resulting in melting temperatures below PCL. For the samples treated at high pressures, we observe a shift of the maximum attributed to the crystallinity to smaller wave numbers, so a decrease of the crystallinity produces a decrease of melting temperature for the PCL samples treated under high-temperature and high-pressure conditions.
