Supercritical Drying of Alginate Beads for the Development of Aerogel

Article pubs.acs.org/IECR

Supercritical Drying of Alginate Beads for the Development of Aerogel Biomaterials: Optimization of Process Parameters and Exchange Solvents Giovanna Della Porta,*,† Pasquale Del Gaudio,‡ Felicetta De Cicco,‡ Rita P. Aquino,‡ and Ernesto Reverchon† †

Department of Industrial Engineering, University of Salerno, via Ponte don Melillo, 84084 Fisciano (SA), Italy Department of Pharmaceutical and Biomedical Sciences, University of Salerno, via Ponte don Melillo, 84084 Fisciano (SA), Italy

‡

ABSTRACT: Conventional hydrogel drying techniques can often induce collapse and shrinkage of the gel nanostructure. In this work, supercritical drying of alginate hydrogel beads is demonstrated to be a very effective process for aerogels production. The process involves solvent exchange followed by supercritical extraction, which prevents the gel from collapsing, while reproducing an aerogel nanostructure that exactly mimics the original hydrogel. Ethanol and acetone were used as water exchange solvents. Using ethanol, the bead spherical shape and nanostructure were most closely maintained when supercritical drying was performed at 150 bar and 38 °C. In addition, the diameter of the dehydrated beads only decreases by 0.6% compared to the hydrated hydrogels. The use of acetone also generated, at almost all the supercritical drying conditions chosen, aerogel with a uniform internal nanostructure, and the lowest shrinkage of 0.3% was obtained operating at 100 bar and 38 °C. A water/ethanol mixture with a ratio of 2:98 also generated aerogel with a homogeneous internal structure at 100 bar and 38 °C. This mixture could potentially be suitable for applications involving proteins loading into an aerogel.

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INTRODUCTION Polymer gel drying can allow for the production of nanofibrous interconnected devices with a specific 3D shape that can be used as controlled delivery systems for pharmaceutical application1 or as scaffolds in tissue engineering.2 However, using traditional solvent evaporation/extraction, it is difficult to preserve the gel three-dimensional nanostructure due to solvent surface tension that stresses the internal gel nanostructure causing it to collapse. Moreover, traditional processes require long processing times and can result in incomplete organic solvent elimination.3,4 To overcome the traditional process limitations, aerogel production assisted by SC−CO2 has been proposed. The process allows a fast and effective solvent elimination, preserving the original gel nanostructure5,6 because of the near zero surface tension of the CO2 solvent mixture that prevents the structure collapsing during drying. The process is also very fast and yields a complete solvent elimination.7,8 Indeed, supercritical carbon dioxide (SC−CO2) shows a large affinity with almost all the organic solvents, and because of its gas-like diffusivity, it has been proposed for the production of several innovative materials such as porous membranes,9,10 biopolymer microspheres,11,12 or micro- and nano-particles.13−15 Alginates are naturally derived block copolymers composed of β-D-mannuronic and α-L-guluronic acid monomers. These natural polysaccharides are one of the favored formulation excipients in the pharmaceutical industry for their mucoadhesive properties.16 Gelation is obtained through the interaction with divalent or trivalent cations that can cooperatively bind between the guluronic blocks of adjacent saccharide chains creating ionic cross-linking bridges.17,18 Alginate beads have © 2013 American Chemical Society

been investigated in the last few years for the production of controlled drug release formulations,19,20 as well as cell encapsulation in tissue engineering.21 Alginate matrix has also been used to deliver drugs or peptides as encapsulated components, protecting the therapeutic agent by environmental stress/degradation.22,23 Prilling or laminar jet break-up is used as a mild production technique to obtain alginate beads of controlled size and distribution. This technique is based on the break-up of a laminar jet of the polymer solution into near monosized drops by means of a vibrating nozzle device.24 The polymer solution can also contain a solubilized or suspended drug; therefore, when it is dropped into the gelation solution, composite drug-loaded microspheres are obtained. However, alginate beads are in the hydrogel form, and water needs to be removed to stabilize the dosage form when pharmaceutical formulations are produced or to increase the shelf life of the material. Because of the high water surface tension, hydrogels are the most difficult to be converted into aerogels. Several techniques have been proposed in the recent literature such as microwave processing, but they all were only partly successful.25 The water in hydrogel cannot be direct removed by SC− CO2 drying because SC−CO2 shows only a very limited affinity with water; therefore, a solvent exchange step is required. Robitzer et al.26 reported a solvent exchange with ethanol followed by SC drying; the authors investigated only one drying temperature and one pressure condition (74 bar and 31.5 °C) observing a structure shrinkage of about the 20% with respect Received: Revised: Accepted: Published: 12003

April 26, 2013 July 31, 2013 July 31, 2013 July 31, 2013 dx.doi.org/10.1021/ie401335c | Ind. Eng. Chem. Res. 2013, 52, 12003−12009

Industrial & Engineering Chemistry Research

Article

the original one. Quignard et al.27 tested alginate and chitosan hydrogel−aerogel conversion using the same drying pressure and temperature conditions and ethanol/water mixtures for solvent exchange; according to the authors, in these cases, the hydrogel nanostructure was retained. Gavillon and Budtova28 proposed water exchange with acetone or ethanol of aqueous cellulose/NaOH solutions to obtain gels that were dried at 80 bar and 35 °C; for both solvents, shrinkages between 20% and 50% were observed. Recently, alginate gel microparticles were also prepared by SC drying29,30 or loaded with ketoprofen and SC dried using an ethanol/ketoprofen saturated solution as the exchang solvent to avoid drug migration during the solventexchanging step. The authors reported a ketoprofen encapsulation efficiency of 60%.31 Despite the entire attempt proposed in the literature, aerogel shrinkage percentages between 5% and 20% were always observed. Moreover, a detailed study of the operating process condition is still missing. Therefore, the aim of this work is to explore and optimize a systematic alginate aerogels manufacturing starting from alginate beads by SC−CO2 drying. The influence of different exchange solvents such as ethanol and acetone and of process parameters such as pressure, temperature, and processing time will be explored, and their effect on the final aerogels nanostructure will be monitored. Solvent residues in aerogels and shrinking factors will be also considered to find the best SC-drying conditions.

Supercritical CO2 Drying Apparatus. Supercritical CO2 drying was performed in a laboratory apparatus equipped with a 400 cm3 extraction vessel operated in the single-pass mode of passing CO2 through the fixed bed of charged alginate beads. The extracted solvents were recovered using a stepwise depressurization in a separation vessel of 200 cm3 that allowed the continuous discharge of the liquid product. A high pressure diaphragm pump (Milton Roy, model Milroyal B) with a maximum capacity of 5 kg/h was pumped liquid CO2 at the desired flow rate. CO2 was then heated to the extraction temperature in a thermostatted oven (Nova Swiss, model 102STF-60; accuracy ±0.1 °C). The extraction was carried out in semibatch mode Batch charging of alginate beads and continuous flow of CO2 was monitored by a calibrated rotameter (Matheson, model 604) and located after the last separator. Total CO2 delivered during an extraction test was measured by a dry test meter (Sim Brunt, model B10). Temperatures and pressures along the extraction apparatus were measured with thermocouples and Bourdon-tube test gauges, respectively. Pressure was controlled manually by high pressure valves. A description of the apparatus is reported in Figure 1. For each test, the high pressure vessel was charged

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MATERIALS AND METHODS Materials. Sodium alginate, European Pharmacopoeia X (MW ≈ 240 KDa, β-d-mannuronic/α-l-guluronic acid ratio 1:2) (Carlo Erba, Milan, IT) was employed as matrix in the preparation of gel beads and was used as received without further purification. Water content (5% w/w) was determined by Karl Fischer titration (Tritomatic KF, Crison Instruments, SA, Barcellona, SP). CaCl2 anhydrous, granular (Sigma-Aldrich, Milan, I), was used in aqueous solution as the cross-linking agent. All other chemicals and reagents were from Sigma Aldrich (Milan, IT) and were used as supplied. Hydrated Beads Preparation. Sodium alginate was dissolved in distilled water at room temperature under gentle stirring for 18 h in order to obtain 100 mL of polymer solution with concentrations of 2.00% (w/w). Alginate beads were manufactured by a vibrating nozzle device (Nisco Encapsulator Unit, Var D; Nisco Engineering Inc., Zurich, CH), equipped with a syringe pump (Model 200 Series, Kd Scientific Inc., Boston, MA, U.SA.) and pumping the drug/polymer solution through a nozzle 600 μm in diameter. The experiments were performed at various volumetric flow rates between 10 and 12 mL/min. The vibration frequency used to break up the laminar liquid jet was set between 250 and 300 Hz with an amplitude of vibration of 100%. The distance between the vibrating nozzle and the gelling bath was fixed at 25 cm. A stroboscopic lamp was set at the same amplitude as the frequency in order to visualize the falling droplets. Drug/polymer droplets were gelled with a 0.3 M CaCl2 aqueous or ethanol solution under gentle stirring. The beads were held into the gelling solution for 10 min at room temperature and then recovered and thoroughly rinsed with distilled water or ethanol. Calcium alginate beads produced using the aqueous solution were treated with ethanol, acetone, or ethanol/water mixtures to displace and substitute water and to be further processed by SC−CO2. Along all this study, alginate hydrogel beads with a mean diameter of 3 mm (±0.3 mm) were used.

Figure 1. Schematic representation of the SC-drying apparatus.

with 50 alginate beads plus 100 mL of the exchange solvent or solvent mixture. After an equilibration of 5 h, the CO2 flow rate was set at 0.6 kg/h. Beads were also dried by convective conventional methods using a tray oven (ISCO mod. 9000, Milan, I) at 105 °C and room conditions of 22 °C and 67% relative humidity. Convective drying runs were stopped when a constant weight was achieved. Bead Size and Morphology. Bead size distribution was measured with an optical microscope (Citoval 2, Alessandrini, Milan, I) equipped with a video camera (CCD-1, JVC, Tokyo, J). The projection diameter was obtained by image analysis (Image J software, Wayne Rasband, National Institute of Health, Bethesda, MD, U.S.A.). At least 40 images were analyzed for each preparation, and the length-number mean diameter was calculated. The average mean diameters and relative standard deviations were calculated for at least three different prilling processes. The beads inner structure images were obtained by electron microscopy using a Carl Zeiss EVO MA 10 microscope with a secondary electron detector (Carl 12004

dx.doi.org/10.1021/ie401335c | Ind. Eng. Chem. Res. 2013, 52, 12003−12009


Article

Zeiss SMT Ltd., Cambridge, U.K.) equipped with a LEICA EMSCD005 metallizator producing deposition of a 200−400 Å thick gold layer. Analysis was conducted at 20 KeV. The samples were cryofractured using two needles after a permanence of 1 min in liquid nitrogen. Split particles were attached to an aluminum stab and covered with gold. The shrinking factor (SF) was defined as 100 minus (mean bead diameter after drying divided by the mean gel bead diameter × 100). Low SF values indicated the absence of a collapsed gel nanostructure.

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RESULTS AND DISCUSSION As discussed in the Introduction, SC−CO2 gel drying by direct extraction is not applicable when water is the solvent embedded in the gel and the solvent exchange steps have to be performed before drying. In this work, different exchange solvents and drying conditions were tested to preserve the original gel internal nanostructure of alginate aerogels. The operating pressure and temperature ranges were chosen by considering the vapor−liquid equilibra (VLE) of the two supercritical mixtures selected, namely, ethanol/CO2 and acetone/CO2, to work above their mixture critical point (MCP).32 Indeed, the works on alginate hydrogel drying reported in the Introduction were performed at pressure and temperature conditions near the pure CO2 critical point. Those authors did not take into account the fact that CO2 solvent mixtures are formed during the drying process. Therefore, supercritical mixture conditions have to be considered in order to evaluate the properties of the processing mixture (surface tension, for example). In Figures 2

Figure 3. Vapor liquid equilibrium diagram for the binary system CO2/acetone: ▲ 35 °C, ● 40 °C adapted from 29 and solvent mixture composition trajectories during drying.

performed at near zero surface tension,33 preserving the internal gel nanostructure. Ethanol Extraction from Alginate Beads: Effect of Operative Parameters. Alginate hydrogels were produced as described in detail in the Materials and Methods section with ethanol as a replacement for water. Preliminary runs were performed at 150 bar and 32 °C, and the drying time was varied between 90 and 180 min to optimize this parameter. The results in terms of shrinking factors (SF), with respect to the diameter of starting wet gel beads, are reported in Table 1. SF Table 1. Shrinking Factor (SF) and Solvent Residue Measured When Ethanol Exchange Plus SC Drying Was Tested P [bar]

150

T [°C] CO2 density[kg/m3]] SF [%] solvent residue [ppm] P [bar]

32 834.94 6.2 478

35 815.30 4.2 235 100

38 794.55 0.6 220

T [°C] CO2 density[kg/m3] SF [%] solvent residue [ppm]

32 749.34 >20 >800

35 710.79 >20 >800

38 662.57 >20 >800

values of 6.2% and 5.9% weres observed, and the nanoporous internal structure was preserved at both processing times (Figure 4a). Therefore, we concluded that a 90 min drying period was enough to extract all of the organic solvent, and this processing time was selected for future experiments. For comparison purposes, the same beads were also air dried for 24 h, and a SF of 28% was observed. The alginate gel internal morphology obtained by air drying is reported in Figure 4b. Comparing the SEM images in Figure 4a and b have been taken at the same magnification. A well-preserved nanostructure is evident only in the first one, whereas a collapsed structure characterizes the air-dried gel. Using an extraction time of 90 min, hydrogel beads were SC dried at different operating temperatures of 32, 35, and 38 °C and at different pressures of 100 and 150 bar. It should be noted that CO2 density is directly connected to its solvent power and increases with pressure, whereas it decreases with increasing temperature (Table 1). When we operated at 100 bar, for all of the temperatures investigated, the beads produced

Figure 2. Vapor liquid equilibrium diagram for the binary system CO2/ethanol: ▲ 35 °C; ● 40 °C); adapted from ref 29 and solvent mixture composition trajectories during drying.

and 3, VLEs are reported for the systems ethanol/CO2 and acetone/CO2 at 35 and 40 °C, respectively. Considering in greater detail the evolution of the mixture composition inside the alginate beads during the SC-drying process, the CO2 solvent mixture contains a larger initial solvent content, and its composition is located on the left of the VLE diagram (red lines in Figures 2 and 3). As the CO2 molar fraction increases due to its continuous diffusion inside the gel, the mixture composition progressively changes and moves to the supercritical region located to the right of mixture critical point (MCP) in the VLE diagram. Only when this process is operated above the MCP is a supercritical mixture formed between CO2 and the organic solvent, and consequently, the SC-drying process can be 12005



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operating at 150 bar and 38 °C and corresponds to a CO2 density of 794 g/cm3. At CO2 densities higher than 800 kg/m3 (operating at 150 bar and at 32 and 35 °C), larger bead shrinkages of 4% and 6% were observed. Solvent residues between 220 and 500 ppm were measured in the aerogels produced. The lowest residue values were found operating at 150 bar and 38 °C. When ethanol is used as the exchange solvent, SC drying performed at 150 bar seemed to be the most appropriate pressure condition. This result also suggests that the high shrinking factor values reported by the several studies mentioned in the Introduction25−30 can be avoided if optimized pressure and temperature conditions are adopted. The aerogels obtained by SC drying also showed a whiter color ( Figure 5). This observation is probably due to a further purification of the original alginate by the SC mixture because of its high solvent power. Indeed, alginate is obtained from brown seaweeds and may contain various contaminants such as polyphenol, endotoxin, and proteins34 that can be co-extracted by ethanol/CO2 supercritical mixtures inducing further product purification. The aerogel internal structure was also observed by FE-SEM confirming gel nanostructure preservation. Acetone Extraction from Alginate Beads: Effect of Operative Parameters. Acetone was also tested as a replacement for water in alginate hydrogels. SC drying was performed at the same operating temperatures and pressures as for ethanol. A summary of the results obtained in terms of SFs is reported in Table 2. At both operating pressures, the beads Table 2. Shrinking Factor (SF) and Solvent Residue Measured When Acetone Exchange Plus SC Drying Was Tested Figure 4. FE-SEM images of the alginate aerogel internal structure obtained by SC drying operating at 150 bar and 38 °C with a CO2 flow rate of 0.60 kg/h for 90 min (a) and by air drying (b). Exchange solvent: ethanol.

P [bar]

were light yellow in color (Figure 5) with a nonhomogeneous external shape and in some cases exhibited cavities. We

150

T [°C] CO2 density[kg/m3]] SF [%] solvent residue [ppm] P [bar]

32 834.94 2.45 340

35 815.30 1.28 230 100

38 794.55 0.61 187

T [°C] CO2 density[kg/m3] SF [%] solvent residue [ppm]

32 749.34 0.92 234

35 710.79 0.61 305

38 662.57 0.31 141

produced were spherical and clearly white with a uniform and homogeneous internal nanostructure. Additionally, the SF value was always very low and ranged between 0.6% and 2% with respect to the original bead diameter when operating at 150 bar and between 0.3% and 0.9% when operating at 100 bar. The lowest SF values were always observed at 38 °C. Solvent residues between 350 and 140 ppm were measured. An example of an aerogel internal structure obtained by SC−CO2 drying is also illustrated in the FE-SEM image shown in Figure 6. Therefore, at all the process conditions tested, acetone demonstrated to be a good exchange solvent, and the aerogels obtained were always less shrunk than those obtained using ethanol. Alginate impurities were also always extracted in these cases. Because the MCPs of the ethanol/CO2 and acetone/CO2 systems at the selected temperatures are located at very similar pressures (Figures 2 and 3), the different processing results do not seem to depend on the distance of the operative points from the corresponding MCP but rather on the different affinities of the two organic solvents with SC−CO2. This

Figure 5. Images of aerogel beads obtained by SC drying operating at (a) 100 bar and 38 °C and (b) 150 bar and 38 °C. CO2 flow rate of 0.60 kg/h for 90 min. Exchange solvent: ethanol. Beads mean diameter: 3 (±0.3) mm.

considered these samples not perfectly processed and did not proceed with further morphological analyses by SEM. A possible explanation for the observed behavior is that when operating at 100 bar, for all the temperatures chosen, the binary mixture ethanol/CO2 formed into the beads was too close to its MCP causing a nonuniform extraction. When the SC drying was performed at 150 bar, all alginate beads were spherical and clearly white. A summary of the results obtained at 150 bar and at 32, 35, and 38 °C, in terms of shrinking factors (SFs), is reported in Table 1. The lowest SF value (0.6%) was measured 12006



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tension when the expanded liquid region has been crossed (i.e., the region of the diagram on the left of the MCP). This event can produce the observed lower efficiency in the case of ethanol drying for the drying process at lower pressure. Water/Ethanol Mixtures Drying from Alginate Beads: Effect of Ternary Mixtures Compositions. A possible application of alginate aerogel is to charge the beads with biological active compounds such as enzymes or proteins for pharmaceutical and biomedical applications. In this case, it could be relevant to maintain a small percentage of water in the exchange solvent to prevent the denaturation of these compounds during drying. Expecially in the case of enzymes, it was demonstrated that a small amount of water would prevent their denaturation and preserve the bioactive folding.35 For the SC drying of the ethanol/water mixtures, the operating pressure and temperature conditions were chosen by looking at the ternary mixture diagram shown in Figure 7 and adapted from ref 36. The starting operating points are indicated as red dots in Figure 7, and the red arrows indicate the mixture composition evolution during the extraction. They have been all selected at low water molar fraction, i.e., at an operative pressure of 100 bar and a temperature of 38 °C. Water/ethanol ratios of 10:90, 5:95, 2:98 were chosen. The first two mixtures of water/ethanol, 10:90 and 5:95 w/w, produced wet beads after SC drying with a nonhomogeneous external shape and several cavities (images in Figure 8 b−d are related to the beads dried using a water/ethanol mixture of 5:95). When a mixture of water/ethanol at 2:98 was tested, beads were produced with a well-defined spherical shape, as illustrated in Figure 8 a−c. We hypothesize that the first two mixtures of water/ethanol, 10:90 and 5:95 w/w, produced wet and not well-shaped beads because from those starting points, the ternary mixture

Figure 6. FE-SEM image of alginate aerogel internal structure obtained by SC drying using acetone as exchange solvent and operating at 100 bar and 38 °C with a CO2 flow rate of 0.60 kg/h for 90 min.

evidence can also be deduced from VLE diagrams in Figures 2 and 3, in which the area of the two-phase region (at the same temperature) is very different for the two systems. Ehanol/CO2 exhibits a larger biphasic region than acetone/CO2, showing that acetone and CO2 have larger compatibility (i.e., affinity). Another possible explanation is related to the composition of the solvent CO2 in the beads and can be described with a composition path starting from pure solvent to practically pure CO2 at the end of the drying process, as indicated by the red lines in Figures 2 and 3. When the drying process has been performed at lower pressure (100 bar), it is possible that in the case of ethanol/CO2 the system has maintained a higher surface

Figure 7. Images and FE-SEM images of aerogel beads obtained by SC drying operating at 40 °C and 100 bar with a CO2 flow rate of 0.60 kg/h for 90 min and using as exchange solvent ethanol/water at different ratios: 98:2 (a−c) and 95:5 (b−d). Beads mean diameter: 3 (±0.3) mm. 12007



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CONCLUSIONS In conclusion, supercritical drying of alginate hydrogel beads was demonstrated to be a very effective process for alginate aerogels production. The analysis of VLEs of the CO2 solvent mixtures was decisive in selecting the most appropriate supercritical drying conditions. In addition, bead shrinkage was reduced to a minimum of about 0.3% with respect to 5− 20% of the values found in the literature. A water/ethanol mixture with a ratio of 2:98 was also found to be effective as an exchange mixture before the SC drying. In this case, the presence of water may prevent the denaturation of the bioactive compounds loaded into the beads during the solventexchanging step.

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Figure 8. Vapor liquid equilibrium diagram for the ternary system CO2/ethanol/H2O at 40 °C and 100 bar, adapted from ref 32. The starting operating points at different water/ethanol compositions are indicated as red dots. I is the one phase region; II is the two phase region. The arrows indicate the evolutions of the solvent mixture composition during supercritical drying.

AUTHOR INFORMATION

Corresponding Author

*Tel: +39 089964104. Fax: +39 089964056. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Bhattarai, N.; Li, Z.; Edmondson, D.; Zhang, M. Alginate-based nanofibrous scaffolds: Structural, mechanical, and biological properties. Adv. Mater. 2006, 18, 1463. (2) Drury, J. L.; Mooney David, J. Hydrogels for tissue engineering: Scaffold design variables and applications. Biomaterials 2003, 24, 4337. (3) Del Gaudio, P.; Colombo, P.; Colombo, G.; Russo, P.; Sonvico, F. Mechanisms of formation and disintegration of alginate beads obtained by prilling. Int. J. Pharm. 2005, 302, 1. (4) Bellich, B.; Borgogna, M.; Carnio, D.; Cesàro, A. Thermal behavior of water in micro-particles based on alginate gel. J. Therm. Anal. Calorim. 2009, 97, 871. (5) Daniel, C.; Alfano, D.; Venditto, V.; Cardea, S.; Reverchon, E.; Larobina, D.; Mensitieri, G.; Guerra, G. Aerogels with a microporous crystalline host phase. Adv. Mater. 2005, 17 (12), 1515. (6) Reverchon, E.; Pisanti, P.; Cardea, S. Nanostructured PLLAhydroxyapatite scaffolds produced by a supercritical assisted technique. Ind. Eng. Chem. Res. 2009, 48 (11), 5310.

(ethanol/water/CO2) generated into the beads falls into the miscibility hole of the ternary system and an incomplete and nonhomogeneous extraction is obtained. On the contrary, when the mixture of water/ethanol at 2:98 w/w was tested, the miscibility hole was bypassed, i.e., spherical beads were produced with a homogeneous internal structure (FE-SEM image reported in Figure 9). In this case, the internal gel nanometric network was retained, and a SF value of about 2% was measured. Therefore, these conditions were also explored for bovine serum albumin (BSA) loading into alginate aerogel beads as the protein model system. Exchanging water with a water/ethanol mixture (ratio 2:98) and then SC drying the beads at the conditions reported above the 30% w/w of encapsulation efficiency was monitored for the bioactive protein.

Figure 9. FE-SEM image of the aerogels internal structure obtained operating at 100 bar and 38 °C for 90 min. Exchange solvent: ethanol/water (98:2). 12008



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(7) Cardea, S.; Sessa, M.; Reverchon, E. Supercritical CO2 assisted formation of poly(vinylidenefluoride) aerogels containing amoxicillin as controlled release device. J. Supercrit. Fluids 2011, 59, 149. (8) Caputo, G.; De Marco, I.; Reverchon, E. Silica aerogel-metal composites produced by supercritical adsorption. J. Supercrit. Fluids 2010, 54 (2), 243. (9) Reverchon, E.; Adami, R.; Cardea, S.; Della Porta, G. Supercritical fluids processing of polymers for pharmaceutical and medical applications. J. Supercrit. Fluids 2009, 47, 484. (10) Cardea, S.; Pisanti, P.; Reverchon, E. Generation of chitosan nanoporous structures for tissue engineering applications using a supercritical fluid assisted process. J. Supercrit. Fluids 2010, 54, 290. (11) Della Porta, G.; Reverchon, E. Continuous supercritical emulsions extraction: A new technology for biopolymer microparticles production. Biotechnol. Bioeng. J. 2011, 108 (3), 676. (12) Falco, N.; Reverchon, E.; Della Porta, G. Continuous supercritical emulsions extraction: Packed tower characterization and application to poly(lactic-co-glycolic acid) plus insulin microspheres production. Ind. Eng. Chem. Res. 2012, 51, 8616. (13) Campardelli, R.; Adami, R.; Della Porta, G.; Reverchon, E. Nanoparticle precipitation by supercritical assisted injection in a liquid antisolvent. Chem. Eng. J. 2012, 192, 246. (14) Adami, R.; Reverchon, E. Composite polymer-Fe3O4 microparticles for biomedical applications produced by supercritical assisted atomization. Powder Technol. 2012, 218, 102. (15) Reverchon, E.; Caputo, G.; Correra, S.; Cesti, P. Synthesis of titanium hydroxide nanoparticles in supercritical carbon dioxide on the pilot scale. J. Supercrit. Fluids 2003, 26 (3), 253. (16) George, M.; Abraham, T. E. Polyionic hydrocolloids for the intestinal delivery of protein drugs: Alginate and chitosanA review. J. Controlled Release 2006, 114, 1. (17) Thu, B.; Bruheim, P.; Espevik, T.; Smidsrød, O.; Soon-Shiong, P.; Skjåk-Bræk, G. Alginate polycation microcapsules: I. Interaction between alginate and polycation. Biomaterials 1996, 17, 1031. (18) Zheng, H. Interaction mechanism in sol−gel transition of alginate solutions by addition of divalent cations. Carbohydr. Res. 1997, 302, 97. (19) Benoit, J. P.; Faisant, N.; Venier-Julienne, M. C.; Menei, P. Development of microspheres for neurological disorders: From basics to clinical applications. J. Controlled Release 2000, 65, 285. (20) Del Gaudio, P.; Russo, P.; Lauro, M. R.; Colombo, P.; Aquino, R. P. Encapsulation of ketoprofen and ketoprofen lysinate by prilling for controlled drug release. AAPS PharmSciTech 2009, 10, 1178. (21) Chayosumrit, M.; Tuch, B.; Sidhu, K. Alginate microcapsule for propagation and directed differentiation of hescs to definitive endoderm. Biomaterials 2010, 31, 505. (22) George, M.; Abraham, T. E. pH sensitive alginate-guar gum hydrogel for the controlled delivery of protein drugs. Int. J. Pharm. 2007, 335, 123. (23) Gong, R.; Li, C.; Zhu, S.; Zhang, Y.; Du, Y.; Jiang, J. A novel pHsensitive hydrogel based on dual crosslinked alginate/N-[alpha]glutaric acid chitosan for oral delivery of protein. Carbohydr. Polym. 2011, 85, 869. (24) Brandenberg, H.; Widmer, F. A new multinozzle encapsulation/ immobilization system to produce uniform beads of alginate. J. Biotechnol. 1998, 63, 73. (25) Aquino, R. P.; Auriemma, G.; D’Amore, M.; D’Ursi, A. M.; Mencherini, T.; Del Gaudio, P. Piroxicam loaded alginate beads obtained by prilling/microwave tandem technique: Morphology and drug release. Carbohydr. Polym. 2012, 89 (3), 740. (26) Robitzer, M.; David, L.; Rochas, C.; Di Renzo, F.; Quignard, F. Nanostructure of calcium alginate aerogels obtained from multistep solvent exchange route. Langmuir 2008, 24, 12547. (27) Quignard, F.; Valentin, R.; Di Renzo, F. Aerogels materials from marine polysaccharides. New J. Chem. 2008, 32, 1300. (28) Gavillon, R.; Budtova, T. Aerocellulose: New highly porous cellulose prepared from cellulose-NaOH aqueous solutions. Biomacromolecules 2008, 9, 269.

(29) Alnaief, M.; Alzaitoun, M. A.; García-González, C. A.; Smirnova, I. Preparation of biodegradable nanoporous microspherical aerogel based on alginate. Carbohydr. Polym. 2011, 84, 1011. (30) Alnaief, M.; Smirnova, I. In situ production of spherical aerogel microparticles. J. Supercrit. Fluids 2011, 55, 1118. (31) Del Gaudio, P.; Auriemma, G.; Mencherini, T.; Della Porta, G.; Reverchon, E.; Aquino, R. P. Design of alginate-based aerogel for nonsteroidal anti-inflammatory drugs controlled delivery systems using prilling and supercritical-assisted drying. J. Pharm. Sci. 2013, 102 (1), 185. (32) Chiu, H. Y.; Lee, M. J.; Lin, H. Vapor-liquid phase boundaries of binary mixtures of carbon dioxide with ethanol and acetone. J. Chem. Eng. 2008, 53, 2393. (33) Sun, Y.; Shekunov, B. Y. Surface tension of ethanol in supercritical CO2. J. Supercrit. Fluids 2003, 27, 73. (34) Dusseault, J.; Tam, S. K.; Ménard, M.; Polizu, S.; Jourdan, G.; Yahia, L.; Hallé, J. Evaluation of alginate purification methods: Effect on polyphenol, endotoxin, and protein contamination. J. Biomed. Mater. Res., Part A 2006, 76, 243. (35) Laudani, C. G.; Habulin, M.; Knez, Z.; Della Porta, G.; Reverchon, E. Lipase-catalyzed long chain fatty ester synthesis in dense carbon dioxide: Kinetics and thermodynamics. J. Supercrit. Fluids 2007, 41 (1), 92. (36) Durling, N. E.; Catchpole, O. J.; Tallon, S. J.; Grey, J. B. Measurement and modelling of the ternary phase equilibria for high pressure carbon dioxide−ethanol−water mixtures. Fluid Phase Equilib. 2007, 252, 103.

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Supercritical Drying of Alginate Beads for the Development of Aerogel

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