Delayed Release from Hermetically-Sealed Aqueous Capsules by Modulating the Thin Waxy Shell

Biopolymer hydrogel capsules are typically created through physical crosslinking of polymer chains. They are environmentally friendly and non-toxic, making them very useful in different industries. However, hydrogel capsules are often unsuitable for drug delivery since they cannot keep small molecules encapsulated for long times without leaking. In an effort to resolve this problem, our lab has designed capsules with a hydrogel core and a thin shell of paraffin wax around the core.^[1] The wax shell keeps hydrophilic solutes hermetically sealed, i.e., there is no leakage for a long period of time (months). The barrier properties of the wax shell arise because it is an extremely hydrophobic material with a melting point well above room temperature (~ 60°C).<br/><br/>While a hermetic seal is useful, there is still a need for solutes encapsulated in the capsule to be released at the appropriate time. For this purpose, we have improved the above capsule design to allow for: (a) a hermetic seal for a period of time; followed by (b) a slow and sustained release of the solute thereafter. The key to our new design is to add a low-melting additive to paraffin wax. Using this design, we can store hydrophilic solutes (such as hormones and therapeutics) in the capsules for multiple days at room temperature without any release (i.e., the solute remains hermetically sealed over this period). After this period, the solute is released at a slow rate out of the capsule. The time delay before solute release can be tuned through several variables, including the concentration as well as the molecular nature of the additive. Since the thin waxy shell is completely biocompatible, this approach could have a potentially significant impact on the delivery of many hydrophilic solutes, including drugs, agrochemicals and cosmetic agents.<br/><br/><br/>[1] J. P. Goertz, K. C. DeMella, Benjamin R. Thompson, Ian M. White and <i>S. R. Raghavan</i><br/>Responsive capsules that enable hermetic encapsulation of contents and their thermally triggered burst release.<br/><i>Materials Horizons</i>, 6, 1238 (2019)