Stimulus triggered release of actives from composite microcapsules based on sporopollenin from Lycopodium clavatum

Smart drug delivery systems (SDDSs) are a paradigm shift in drug delivery, particularly in microencapsulation technology where the drug is released in response to an internal and/or external stimulus. In this study, a smart microencapsulation platform was developed using three different types of stimuli triggered release of a model active (rhodamine 6G) from sporopollenin from Lycopodium clavatum. Triggers were based on pH-, thermal- and near infrared light-sensitive polymer composition. Carbopol nanogel and methylcellulose were used as responsive aqueous polymers for pH and thermally triggered release, respectively. Methylcellulose loaded with active and gold nanoparticles was used for photothermal triggered release. The formulations were encapsulated into the empty cores of sporopollenin microcapsules using the compressed tablet technique. The pH triggered release of the active was achieved above pH 7, which was mediated by the swelling of the carbopol nanogel that forced its way out of the elastic trilite scars of the sporopollenin. Results from measuring the active fluorescent intensity and its content over time confirmed the crucial role of carbopol in the pH triggered release. For the thermo-sensitive polymer methylcellulose, it was found that the release of the active from methylcellulose loaded sporopollenin was triggered upon heating the system reaching 90% whereas it levelled out at 40% for methylcellulose-free sporopollenin. The maximum amount of active was released at around 55 °C, where the sol-gel transition of the methylcellulose starts. Photothermally triggered release using near infrared (NIR) laser revealed that the amount of active released from sporopollenin loaded with both gold nanoparticles and methylcellulose was approximately four times higher than that from sporopollenin loaded with methylcellulose/active only, confirming the key role of gold nanoparticles in the release via photothermal heating of the polymer formulation.