Slow and Sustained Release of Hydrophilic Solutes from Capsules

Abstract

Capsules can be used to deliver solutes encapsulated in their aqueous core, including drugs. However, small, hydrophilic solutes tend to leak out of typical capsules in a matter of minutes. Solute release can be completely prevented if the capsule shell is made of a hydrophobic solid like paraffin wax. The goal of this study is to achieve release profiles between these extremes 3⁄4 i.e., release of solutes in a slow and sustained manner over a period of days. For this, we modify the wax-shell design above and include a nonionic surfactant (from the Brij series) along with the wax. Most of our studies have been done with Brij-C10, which has a polyethylene glycol (PEG) head and a hexadecyl (C16) tail. We show that capsules with a shell of 80/20 wax/Brij can release model dyes from the aqueous core over 20+ days. Such slow release has not been reported previously. The release rate can be tuned via the concentration of Brij in the shell (the higher the Brij, the faster the release) as well as the chemistry of the Brij (i.e., the size of the surfactant head or tail). The sustained release occurs because the Brij-bearing shell has microchannels through which the solute can permeate. Our approach can be used to slowly release many solutes, including dyes, drugs, and reactive reagents (such as H2O2). The simplicity and versatility of this approach make it highly suitable for controlled release applications across the pharmaceutical, agrochemical, and cosmetics industries.