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THE PHARMA REVIEW (NOVEMBER 2009)

Aerogels: Novel Drug Carriers

C. Shah, R. Patel, R. Mazumder, S. Bhattacharya, A. Mazumder, R. N.Gupta

Abstract: Pharmaceutical gels are well known as dense networks of fine particles dispersed in suitable media. Based on the types of dispersion phases, they are sub classified as hydrogels, alcogels and organogels, which are dispersions of polymeric materials in water, alcohol, and organic solvents, respectively. Aerogel, having more than 15 entries in Guinness Book of World Record, is a very low-density solid-state gel which is derived by replacing the dispersion phase of a gel with gas. Aerogels can be made with a density only three times larger than that of air. This novel material has many desired properties, such as low thermal conductivity, refractive index, along with thermal stability, large surface area and biocompatibility, which make it a potential candidate as a drug carrier. A striking number of applications have been developed for aerogels like catalysts, thermal insulators, and particle detectors and so on. Pharmaceutical applications of aerogels are being investigated for more than a decade, e.g., as flow enhancing materials, filler, drug carriers and so on.

Introduction
Aerogel, world’s lightest solid material and drug delivery system, was first created by Steven Kistler in 1931. Kistler competed with Charles Learned to see if one of them could replace the liquid inside a jelly jar with gas without causing any shrinkage. Kistler won the bet, and published his findings in 1931. Aerogel is a low-density solid-state material derived from gel in which the liquid component of the gel has been replaced with gas. Aerogels are produced by extracting the liquid component of a gel through super-critical drying. This allows the liquid to be slowly removed without causing the solid matrix in the gel to collapse due to capillary action, as would happen with conventional evaporation. The first aerogels were produced from silica gels. Kistler’s later work involved aerogels based on alumina, chromia and tin oxide. Carbon aerogels were first developed in the early 1980s.

Aerogel is a concept of pulmonary drug delivery from a very light weight and highly transparent polymer material which is also known as “Frozen Smoke”, “Blue Smoke”, or “Solid Smoke”. These names suggest its hazy blue appearance as a solid. It has emerged out as one of the most promising means of drug delivery during the past few years.

Superiority of Aerogel to Aerosol
Porous gels are prepared from a material which is soluble in pulmonary surfactant. The lower the density difference between the floating particle and the air, the higher are the chances the particle will stay afloat at a given level of air motion. Since the aerogel particles are highly porous (up to 95% filled with the air), they are much lighter than a solid particle and they have much better chances of remaining afloat reaching the innermost alveoli of the lungs and settling on the pulmonary surfactant rather than on the mucous membranes along the way. Since human lungs have an equivalent surface area of a tennis court, it is advisable to take advantage of as much of the surface of the lungs as possible for efficient drug delivery. The air suspension characteristics of the micron and submicron size aerogel particles are determined using a small chamber with a paddle fan.

 

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