Aerogels : Tailor-made Carriers for Immediate and Prolonged Drug Release

The potential of inorganic polymeric materials silica aerogels as tailor-made drug carriers is discussed. It is shown that the dissolution rate of poorly soluble drugs can be significantly changed through the adsorption on silica aerogels. Adsorption takes place in supercritical CO2 and allows distribution of the drugs inside the aerogel matrix on the molecular level. The drug concentration in the aerogel is explicitly determined by the temperature, bulk concentration of the drug in the supercritical phase and the properties of the aerogel (density, pore size distribution and surface area). The release rate of the drug depends on the hydrophobicity of the aerogel. In the case of hydrophilic aerogels, an extremely fast release even compared with nanocrystals of drugs is achieved, which is especially advantageous for poorly water-soluble drugs. Hydrophobic aerogels exhibit a slower release which is governed by dif fusion. In addition, the possibility of generating organic microparticles inside the pores of the aerogels by precipitation from supercritical solutions is discussed.


Introduction
Aerogels are low-density solid materials with a fine, open-pore structure.An aerogel is composed of individual primary particles only a few nanometres in size which are linked in a three-dimensional structure.This microstructure causes characteristic properties of aerogel materials: fine pores (5Ҁ100 nm), very high surface areas (200Ҁ1000 m 2 /g) and low densities (0.003Ҁ0.15 g/cm 3 ).Aerogels can be synthesized from silicon oxide (silica aerogels), and also from different organic and inorganic precursors, for example titanium oxide, aluminium oxide, carbon, gelatine, etc. Silica aerogels are usually synthesized by hydrolysis and the subsequent condensation of tetraalkylorthosilicates.The condensation leads to the formation of a gel phase.To convert a gel into an aerogel, the solvent is removed by extraction with supercritical CO 2 or by the direct conversion of the solvent in supercritical state in order to prevent the structure collapse caused by capillary forces [Fricke 1986].The chemistry of aerogel materials is relatively f lexible: their pore size and surface area can be tailored during the synthesis by changing the solvent and catalysts [Rao et al. 1993, Pajonk 1998, Stolarski et al. 1999].Furthermore, different functional groups can be implemented in the structure of silica aerogels by surface modification or by reaction during sol-gel processes [Husing andSchubert 1997, Husing et al. 1998].The hydrophobicity of silica aerogels varies from completely hydrophilic aerogels, whose structure collapses immediately in water, to hydrophobic samples, which f loat on water for many hours without being wetted.
The combination of an extensive range of unusual solid material properties enables the application of aerogels in many different areas of technology.
Overviews about already existing and potential applications of aerogels were given by several authors [Schmidt and Schwertfeger 1998], [Rolison et al. 2001], [Pajonk 2003], [Akimov 2003].At present, an interest has grown in the field of aerogel applications in life sciences.The high porosity of aerogels can be exploited to incorporate pharmaceuticals and biomolecules as was done for other sol-gel materials (e.g.xerogels).Silica aerogels are especially suitable for this purpose, since they are chemically inert and not harmful to the human body.The high temperature resistance of silica aerogels (up to 500°C) allows sterilization by high temperature according to the requirements of pharmaceutical products.Biocatalysis with a lipase enzyme represents one of the advanced silica aerogel research studies.It was shown that the catalytic activity of several non-specific lipases in liquids and supercritical media could be significantly improved by the in-situ encapsulation of an enzyme into the silica aerogel during the sol-gel process [Buisson et al. 2001, El Rassy et al. 2004, Novak et al. 2003].The immobilization of three further enzymes (PGA, thermolysin and chymotrypsin) in silica aerogels was also demonstrated by Basso et al. (2000).Recently, it was shown that proteins (cytochrom c) can be stabilized in an aerogel matrix using the socalled "nanogluing" [Wallace et al. 2003, Wallace et al. 2004], which implies the formation of a stable protein superstructure around gold nanoparticles.The resulting composites show better stability at a greater range of temperatures than previously reported for biocomposite xerogels, and exhibit extremely rapid sensing of the compounds in the gas phase [Wallace et al. 2004].The principal application of silica aerogels as biosensors was demonstrated by the encapsulation of bacteria Escherischia coli and the subsequent use of the doped aerogel as an aerosol collector to detect bacteriophages [Power et al. 2001].The high adsorption capacity of silica aerogels allows their use for adsorption of pharmaceutical compounds.Since aerogels have an extremely large surface area, it is expected that the drug dispersed or adsorbed in the aerogel can obtain improved dissolution characteristics.Both hydrophobic and hydrophilic silica aerogels were loaded with pharmaceuticals by means of adsorption from corresponding liquid solutions [Schwertfeger et al. 2001].Ambient-dried aerogels with a relatively high density (ρ0.1 g/cm 3 ) were used for this purpose.The authors mixed the aerogels with a solution of the target drug, allowed the mixture to reach equilibrium and then filtered it to get the loaded aerogel.The resulting powder was dried and could be used as a drug delivery system (DDS) [Schwertfeger et al. 2001].Berg et al. (1995) claim a similar procedure for the loading of organic (resorcinol-formaldehyde) aerogels with testosterone adipate and 5-f luorouracil.However, the adsorption of drugs from liquid solutions leads to the partial collapse of the aerogel structure, especially for hydrophilic aerogels.To avoid this, two different methods can be applied: loading aerogels with drugs during the solgel process, as was done with enzymes and proteins as described above, or replacing the liquid solution by supercritical f luids to prevent shrinkage.Lee and Gould (2001) loaded organic aerogels with the drugs methadone and naltrexone by the co-gelling method before supercritical drying.It was proposed to use the resulting aerogel powder as a part of the aerosol for inhalation since the low density of the product allows the particles to be carried by the aerosol stream.It was possible to incorporate 5-f luorouracil into resorcinol-formaldehyde aerogels in this way as well [Berg et al. 1995].Nevertheless, such a kind of loading implies the stability of drugs at the sol-gel conditions (reaction of drugs with chemicals used for sol synthesis should be avoided).Since it cannot be guaranteed in all cases, the second method, i.e. adsorption from supercritical f luids, is worth investigating.Our group has studied the adsorption of different drugs on silica aerogels from supercritical solutions and the subsequent release of drugs [Smirnova et al. 2003 (a), Smirnova et al. 2004, Smirnova andArlt 2004].It was shown that this method allows a homogeneous distribution of the drug in silica aerogel samples.The drying step which is needed if the loading is carried out from liquid solutions is also avoided since CO 2 is removed by simple pressure reduction.The use of hydrophilic aerogels as drug carriers promotes the very fast release of drugs [Smirnova et al. 2004].For one of the studied drugs, griseofulvin, it was demonstrated that the release rate of the drug adsorbed on aerogels is even much faster than that of drug nanoparticles [Smirnova et al. 2005].An additional advantage compared with the nanoparticles is the fact that the drug particles being adsorbed or crystallized in the solid aerogel matrix have a lower tendency to agglomerate and are better protected from the environment.Thus silica aerogels not only allow achievement of a significant acceleration of the dissolution rate of poorly soluble drugs, but they also increase their stability.In this work, we demonstrate the possibility of tailoring the release kinetics of drugs by the properties of the aerogels.Two different methods of loading aerogels with organic substances are described: adsorption and crystallisation from supercritical solutions.
The mixture was then diluted with acetonitrile and poured into cylindrical moulds with a volume of 5 ml and aged for 72 h.The amount of acetonitrile is calculated to ensure that a certain volume of the gel solution and thus the desired target density is achieved.In order to dry the cylindrical gels, the autoclave was previously filled with 60Ҁ80 ml of acetonitrile.The gels were then transferred to the autoclave and the solvent and pore liquid were extracted by supercritical CO 2 during 24 hours at 40°C and 100 bar.The density of the resulting monolith silica aerogels was calculated by weighing a sample and measuring its volume.The surface area and pore size of the samples were determined by nitrogen adsorption (BET method).The aerogels produced in this way were initially hydrophilic.For hydrophobization, silica aerogels were placed in a reactor and heated to 220°C.Methanol vapour was passed through the reactor for 13 h.The resulting aerogels were extremely hydrophobic, and were able to f loat on water for several hours without being wetted.

Loading of aerogels with drugs
To deposit a drug on a silica aerogel, a weighed amount of the drug and an aerogel sample (0.1Ҁ0.2 g) were separately wrapped in filter paper and placed in an autoclave.Carbon dioxide was added until a desired pressure was reached.The system was stored under constant pressure and temperature until the adsorption equilibrium was reached (24Ҁ72 h).The CO 2 was vented and the loaded aerogel samples were weighed and milled in a porcelain mortar.To determine the drug concentration in the sample, a part of the aerogel powder was dispersed in acetonitrile.The solution was stirred for at least 60 min.to ensure complete dissolution of the drug.The concentration of the drug in acetonitrile was determined using UV-spectrometry (UV-V is spectrometer Specord 200, Analytic Jena).The results were additionally verified by CHN elemental analysis.The drug concentration in CO 2 in equilibrium was calculated as follows: where m drug initial is the amount of drug placed in the autoclave, m drug adsorbed is the mass of drug adsorbed on silica aerogels (as defined by UV and CHN methods), m CO 2 is the mass of CO 2 in autoclave.All samples were characterized by IR spectroscopy and X-ray diffraction spectrometry.For IR measurements, the samples were milled and compressed with KBr in order to obtain thin homogeneous pellets.The absorption spectra (600Ҁ4000 cm Ҁ1 ) were recorded by an IR Spectrometer "Magna System 750".X-ray diffraction patterns of the samples were obtained by means of a Siemens D5000 powder diffractometer with monochromated Cu Kα1 radiation, a flat silicon sample holder, and a position-sensitive detector.

Investigation of drug release kinetics
The assembly for drug-release measurements was designed according to the recommendation for the dissolution test [FIP 1996].It consists of a covered glass vessel, a motor, a metallic drive shaft with a six-bladed agitator, and a cylindrical basket.The sample (drug crystals or loaded aerogel powder) was weighed and placed in the basket.The amount of the drug was chosen so that the final concentration was equal to 10% of the maximal solubility of this drug in 0.1 N HCl (sink conditions).The basket was fixed on the agitator and immersed into the vessel containing 900 ml of 0.1 N HCl at 37°C.The stirring speed was 100 rpm.Aliquots of 2 ml were withdrawn at predetermined time intervals, filtered through a 0.45-µm nylon filter and analysed by UV-spectrometry.

Loading of aerogels with drugs
The loading of hydrophilic aerogels with different drugs was studied.The maximal possible loading that could be achieved by adsorption from a saturated drug solution in supercritical CO 2 at given experimental conditions (180 bar, 40°C) is presented in Table 1.The concentration of every drug was chosen so that the bulk CO 2 phase was saturated with the corresponding drug.Aerogels with a density of 0.03 g cm Ҁ3 were used.
The loading of aerogels with drugs depends on different process parameters.One of the most important is the concentration of the drug in the bulk phase (supercritical CO 2 ), which is limited by the solubility of the drugs in supercritical CO 2 .In many applica-m drug initial Ҁm drug adsorbed m drug initial Ҁm drug adsorbed ѿm CO 2 tions, a high loading of the carrier with the drug of interest is important.As can be seen in Table 1, hydrophilic aerogels can adsorb a relatively large amount of ketoprofen, miconazol and ibuprofen.In contrast, dithranol and griseofulvin show an extremely low affinity for the aerogels because of their poor solubility in the supercritical gas.There are several methods that allow a higher solubility to be attained: higher pressure can be used in order to reach a higher CO 2 density and thus a better solubility; an entrainer such as ethanol or acetone can also be applied.
To observe the possible changes of the drug structure during the loading procedure, the identity of the drugs was analysed using X-ray analysis and IR spectroscopy.It was found that all drugs adsorbed on silica aerogels exhibited an amorphous structure.As an example, the XRD patterns of ketoprofen (which exhibits a good adsorption on the aerogels) and griseofulvin (poor adsorption on the aerogels) adsorbed on silica aerogels are given in Fig. 1 in comparison with the patterns of original drugs.Both griseofulvin and ketoprofen show several diffraction peaks typical for crystalline powder.In contrast, no corresponding diffraction peaks could be found in drug-aerogel formulations (the only peak, at 28 degrees in all patterns, comes from the silicon sample holder), leading to the conclusion that no crystallites of either drug are present.Also, the SEM pictures taken for all drug-aerogel formulations confirm this statement.
The amorphisation of ketoprofen was additionally proven by IR spectroscopy [Smirnova et al. 2004].The IR spectra of drug-aerogel formulations were recorded and compared with that of the original drugs in their crystalline form and of a simple mixture of ketoprofen crystals and untreated silica aerogel powder (Fig. 2).The characteristic absorption bands of ketoprofen (717, 1455 and 1655 cm Ҁ1 ) appear in the spectra of drug-aerogel formulations (Fig. 2A).Furthermore, all spectra show the acid dimer peak at 1697 cm Ҁ1 and the peak at 1654 cm Ҁ1 attributed to the benzoyl carbonyl group [Florey 1981].One can see that the benzoyl carbonyl peak in the case of the aerogel formulation is broader and the acid dimer peak is much smaller than the corresponding peaks in the physical mixture (Fig. 2B).This is in agreement with the results of Gupta et al. (2003), who observed similar changes of the IR spectrum of ketoprofen adsorbed on magnesium silicate.The corresponding changes of the spectra are associated with the amorphisation of ketoprofen during the adsorption process [Gupta et al. 2003].Except for the changes caused by amorphisation, no further changes in the IR spectrum of ketoprofen and all other drugs after the loading procedure were found.The chemical nature of the respective drugs seems not to change during the  Not only the loading conditions, but also the properties of the aerogel itself influence the drug loading and their further release from the aerogel matrix.
To prove this, one of the studied drugs, ketoprofen, was adsorbed on different aerogel samples and both adsorption and drug release processes were studied as a function of the aerogel properties.The aerogels were synthesized as discussed before and their properties are given in Table 2.A part of the aerogel samples was hydrophobized before the adsorption experiments took place.The corresponding samples were labelled "S1pb-S4pb".The structural properties (density, pore size) were not significantly inf luenced by hydrophobization.
Fig. 3 shows the BJH pore size distribution of aerogel samples.All samples show the presence of both micro-and mesopores.In the mesoporous range, a broad distribution is found in aerogels S1 and S2 centred at approximately 40 nm and 35 nm, respectively, whereas samples S3 and S4 have a narrow distribution centred at around 32 nm.
The adsorption of ketoprofen on these aerogel samples was studied at three different concentrations of ketoprofen in the bulk phase.All other parameters (pressure, temperature, loading time) were kept constant.In order to get an idea of how the drug could be arranged on the aerogel's surface, the size of the ketoprofen molecules was estimated by quantum chemistry methods at 402.5 Å 2 (Hyperchem program).Taking into account BET surface areas of the aerogel samples, the number of adsorption layers could be estimated.Clearly, the real orientation of drug molecules on the aerogel surface is unknown, so the calculation made in this way is no more than a rough estimate, but it helps to understand the adsorption process.An estimated monolayer calculated in this way is given in all figures.As can be seen from  CO 2 (0.001 wt %), ketoprofen molecules are adsorbed equally, independent of the aerogel density.These loading values lie in the range of the estimated monolayer values (see Fig. 4A).At larger bulk concentrations of ketoprofen in CO 2 , the loading starts to increase with increasing aerogel density.This can be explained by an increase in the specific surface area of the aerogel with increasing density (see Fig. 4B).
However, if the surface area was the only factor which inf luences the loading, the plotting of the areanormalized adsorption (loading divided by the surface area) against density would show a straight line.Since this is not the case (see Fig. 4C), another factor must inf luence the adsorption as well.The pore size distribution plays an important role in the adsorption process.As seen in the mesoporous region increases in the range S1S2 S3ϷS4.The area-normalized adsorption increases in the same sequence.The adsorption in large mesopores is probably more effective than in smaller ones due to better diffusion.A similar effect was observed by Shen et al. (2003), who studied the adsorption of dyes on different activated carbons.They showed that the samples with a larger mesopore content exhibit a higher adsorption rate and a larger loading compared with activated carbons with a smaller mesopore content.Aerogels S3 and S4 have a similar mesopore content and a similar surface area, thus similar loadings may be expected.The same effect is observed if ketoprofen is adsorbed on hydrophobic aerogels (Fig. 5), although the values of loading themselves are much lower than those of hydrophilic aerogels at a similar bulk concentration of ketoprofen in CO 2 .
The decrease of the loading values compared with hydrophilic samples should be due to the lack of OH groups in hydrophobic aerogels.Furthermore, in contrast to hydrophilic aerogels, the loading of hydrophobic aerogels depends on the aerogel's density even at a very low ketoprofen concentration where the monolayer is still not achieved.Assuming the adsorption is favoured by the interactions of ketoprofen with the OH groups on the aerogel surface, one can say that the hydrophobic aerogel with the same surface area provides less active sites for adsorption.Therefore, the effective surface area available for adsorption is smaller.So the values of the estimated monolayer have different meanings for hydrophobic and hydrophilic aerogels.The surface of hydrophobic samples is saturated with drugs faster than that of hydrophilic samples, so actually the monolayer should be reached at smaller concentrations of ketoprofen.

Drug release from silica aerogels
In our previous studies, it was shown that the release of the drugs adsorbed on hydrophilic silica aerogels is much faster than that of the crystalline drug form [Smirnova et al. 2004;Smirnova and Arlt 2004;Smirnova et al. 2003 b].This is demonstrated in Fig. 6 for the drug ketoprofen.Experimental points were fitted with two common dissolution models ("First order model" and "Peppas model" [Stricker 1989]).
Several effects play a role in release enhancement.Firstly, the specific surface area of ketoprofen is significantly enlarged due to the adsorption on silica aerogel.Secondly, the hydrophilic silica aerogel rapidly collapses in water.The reason for this collapse are capillary forces which are exerted by the surface tension when liquid water enters a nanometre-scale pore of the aerogel.As a result, the solid silica backbone is fractured completely and the aerogel loses its solid integrity.So the drug molecules adsorbed as single molecules on the aerogel network are immediately surrounded by water molecules, and thus dissolve faster.Finally, as discussed above, the drug adsorbed on the aerogel does not have a crystalline structure, so no energy is needed to destroy the crystal lattice of the drug, as in the case for dissolution of the crystalline form of the drug.Only the energy of the desorption of drug molecules from the silica surface (enthalpy of desorption) should be considered.Going by this mechanism, the release rate of the drug should not depend on the physical properties of the hydrophilic aerogels such as density or surface area.This suggestion was confirmed experimentally, as shown in Fig. 7.The dissolution rate of ketoprofen adsorbed on hydrophilic aerogels does not change with the density of the hydrophilic aerogel.
A different effect is observed if ketoprofen is adsorbed on hydrophobic silica aerogels (Fig. 7).At the beginning of the dissolution process, the drug dissolves from the surface of the hydrophobic aerogel (burst effect) and then later diffuses from its pores.Because the structure of hydrophobic aerogels is much more stable in water than that of hydrophilic samples, the release process from hydrophobic aerogels is governed to a greater extent by the slower inf lux of water, which leads to a slower release.Hydrophobic aerogels of higher density (0.08 g/cm 3 ) and thus smaller pore size show a slightly slower release rate than those of lower density (0.03 g/cm 3 ), as given in Fig. 7.It is therefore not only the loading of aerogels with drugs but also the drug release rate that can be inf luenced by the properties of silica aero-gels in this case.These findings allow the release rate to be adjusted to some extent according to the desired application by adjusting the properties of the aerogels.

Cr ystallisation of organic compounds in silica aerogels
Adsorption on the aerogel surface is not the only way to load aerogels with chemicals.Another possibility is the precipitation of drug crystals or particles in the pores of aerogels.Precipitation might lead to much higher loadings than the adsorption.The formation of inorganic nanocrystals in silica aerogels was studied by several research groups [Yao et Lorenz et al. 1998], however, the formation of drug nanoparticles in an aerogel matrix has not been reported.Since the typical pore size of the aerogel varies from 2 to 50 nm, it can be expected that the crystal growth is limited by the pore size and leads to the formation of nanoparticles inside the aerogel, as was observed for the wet silica gels [Yoda et al. 2001, Sanz et al. 2000].The final product, silica aerogels with drug nanoparticles inside, would have several advantages.First, drug nanoparticles inside the pores of the aerogel are less sensitive to oxidation and less reactive than original drug nanoparticles.Second, the agglomeration of drug nanoparticles Ҁ the main problem of nanoparticles in general Ҁ is prevented in this case because the particles are separated from each other.So the aerogels might be used as a "packing material" for drug nanoparticles.The drug "packed" in this way can be applied directly with the carrier.Also in this case, the release kinetics can be regulated by the hydrophobicity the aerogel (both immediate and prolonged release can be achieved).
The first precipitation experiments were carried out with a model substance Ҁ naphthalene and supercritical CO 2 .The experimental procedure is identical to that of the adsorption experiments, but the flow rate during the pressure release is significantly higher: from 350 to 4000 l/h (calculated at 1 bar, 25°C).Fast expansion is needed to achieve a high supersaturation of the solution and thus to initiate the cr ystallisation.Several types of particles are thereby formed: (a) in the bulk phase inside the autoclave (cr ystallisation from bulk supercritical CO 2 ), (b) in the vessel in which the expanding gas is flowing, and (c) inside the aerogel (target product).In the case of (a), large hexagonal naphthalene crystals were obtained (Fig. 8), similar to those obtained by Tai et al. (1995) during bulk crystallisation from supercritical CO 2 .The size of the crystals depends on the supersaturation and varies from 100 µm to 8 mm.
In the case of (b), spherical particles of approximately 100 µm were obtained.The particles precipitated inside the aerogel (case c), as shown in Fig. 9. Particle size varies from 100 nm to 2 µm.As soon as the particle size is larger than the average pore size, we conclude that in the case of naphthalene precipitation, the particle growth leads to the partial destruction of the aerogel pores.As expected, the particle size decreases with increasing expansion rate.The concentration of naphthalene in aerogel samples varies from 30 to 77 wt %.
These first results demonstrate the principal possi-bility of producing micro-and nanoparticles of organic substances inside the aerogel body by precipitation from supercritical solutions.In our further investigations, the precipitation process must be studied systematically in order to optimize the process parameters such as pressure, temperature and depressurization rate.Later on, the experiments will be extended to the drug substances.

Conclusions
In this work, the possibility of controlling the dissolution rate of poorly soluble drugs by adsorption on silica aerogels with different properties is demonstrated.The loading procedure (adsorption from supercritical gas) allows the homogeneous distribution of the drugs inside the aerogel matrix on the mol-  Aerogel Density: 0.06 g/mL Magnification: 50 X ecular level.The resulting formulations are stable and no chemical degradation of drugs is observed.The drug concentration in the aerogel is explicitly determined by the temperature, bulk concentration of the drug in the supercritical phase and the properties of the aerogel.For a given drug (ketoprofen), the loading increases with the increasing surface area and with the volume of the mesopores of the aerogel, whereas hydrophilic samples have higher loading rates than hydrophobic ones in all cases.The release rate of the drug from the drug-aerogel formulations is significantly inf luenced by the hydrophobicity and pore size of the aerogels.In the case of hydrophilic aerogels, an extremely fast release Ҁ even compared with nanocrystals Ҁ of drugs is achieved, which is especially advantageous for poorly water-soluble drugs.This effect is based on the collapse of the structure of hydrophilic aerogels in aqueous solutions due to the surface tension inside the pores.Hydrophobic aerogels exhibit a slower release which is governed by diffusion, since they are more stable in water.Thus, it is possible to tailor the release kinetics of drugs by changing the aerogel's properties.Furthermore it is shown that organic microparticles can be generated inside the pores of the aerogels by precipitation from supercritical solutions.This technique makes it possible to protect the particles from agglomeration.As a model substance, naphthalene led to a ver y high loading of the aerogel (50Ҁ70 wt %).Further investigations of drug precipitation in silica aerogels and the inf luence of the process parameters thereof are under way.

FlurbiprofenFig. 1 X
Fig. 1 X-ray diffraction patterns of a) silica aerogel and drug-aerogel formulations; b) crystalline ketoprofen and griseofulvin

Fig. 4 Fig. 5
Fig. 4 Adsorption of ketoprofen on hydrophilic aerogels: (A) Loading as a function of density; (B) Loading as a function of aerogels' surface area; (C) Area-normalized loading as a function of density of the aerogel

Fig. 6 Fig. 7
Fig.6 Release kinetics of different ketoprofen formulations in 0.1 N HCl at 37°C

Fig. 8 Fig. 9
Fig. 8 Naphthalene crystals obtained from the bulk solution

Table 1
Loading of hydrophilic aerogels from saturated solutions in CO 2 at 180 bar, 40°C

Table 2
Characteristics of silica aerogels

Table 2 ,
the pore volume in