Mechanism for formation of Hollow and Granular Silica Aerogel Microspheres from rice husk ash for drug delivery

• Hollow Silica Aerogel Microspheres (HSAMs) were prepared from rice husk ash.
• A mechanism has been proposed for the formation of HSAMs and granular aerogels.
• pH plays a key role in the formation of HSAMs and granular aerogels.
• Release of ibuprofen confirms that HSAMs are suitable for drug delivery.

The present work describes a method for preparation of Hollow Silica Aerogel Microspheres (HSAMs) and Granular Silica Aerogel Microparticles (GSAMs) for drug delivery, from rice husk ash (RHA), an inexpensive source of biocompatible silica. This method involves a two-step sol–gel process for preparation of wet gel microspheres using an improved sol–gel/mineral-oil emulsion (containing dual surfactants), followed by solvent exchange and aging in aqueous ethanol, to replace water in the microspheres with ethanol. It is shown that the amount and concentration of the ammonium hydroxide (NH4OH) solution added during gelation process determine if hollow or granular wet gel particles form. A mechanism for the formation of HSAMs or GSAMs is proposed and validated by experiments. The wet gel particles are subsequently dried with supercritical carbon dioxide at 150 bar and 50 °C to obtain HSAMs or GSAMs. HSAMs afford high loading of drugs as illustrated by loading of 0.47 g ibuprofen per g of HSAM. Further, a fast release of ibuprofen from loaded HSAMs compared to pure crystalline ibuprofen and other loaded forms, indicates that HSAMs produced by the present method are good drug delivery vehicles.