Polyanhydride Microcapsules Exhibiting a Sharp pH Transition at Physiological Conditions for Instantaneous Triggered Release

Photoactivated Cyclic Polyphthalaldehyde Microcapsules for Payload Delivery

Microcapsules with a cyclic polyphthalaldehyde (cPPA) shell and oil core were fabricated by an emulsification process. The low ceiling temperature cPPA shell was made phototriggerable by incorporating a photoacid generator (PAG). Photoactivation of the PAG created a strong acid which catalyzed cPPA depolymerization, resulting in the release of the core payload, as quantified by 1H NMR. The high molecular weight cPPA (197 kDa) yielded uniform spherical microcapsules. The core diameter was 24.8 times greater than the cPPA shell thickness (2.4 to 21.6 μm). Nonionic bis(cyclohexylsulfonyl)diazomethane (BCSD) and N-hydroxynaphthalimide triflate (HNT) PAGs were used as the PAG in the microcapsule shells. BCSD required dual stimuli of UV radiation and post-exposure baking at 60 °C to activate cPPA depolymerization while room temperature irradiation of HNT resulted in instantaneous core release. A 300 s UV exposure (365 nm, 10.8 J/cm2) of the cPPA/HNT microcapsules resulted in 66.5 ± 9.4% core release. Faster core release was achieved by replacing cPPA with a phthalaldehyde/propanal copolymer. A 30 s UV exposure (365 nm, 1.08 J/cm2) resulted in 82 ± 13% core release for the 75 mol % phthalaldehyde/25 mol % propanal copolymer microcapsules. The photoresponsive shell provides a versatile polymer microcapsule technology for on-demand, controlled release of hydrophobic core payloads.

Microballoons: Osmotically-inflated elastomer shells for ultrafast release of encapsulants and mechanical energy

HYPOTHESIS

Microcapsules with osmotically-inflated elastic shells exhibit an ultrafast release of encapsulants while mechanically stimulating the microenvironments, akin to popping balloons.

EXPERIMENTS

To prepare elastic shells with uniform thickness and size, monodisperse water-in-oil-in-water (W/O/W) double-emulsion drops are produced in a capillary microfluidic device. The polydimethylsiloxane (PDMS)-containing oil phase is thermally cured to create the elastic shell. The elastic shells are inflated by pumping water into the lumen in hypotonic conditions. The inflated microcapsules produced undergo mechanical compression, and their release properties are studied.

FINDINGS

By controlling the osmotic pressure difference, Microballoons are inflated into a diameter of 200 μm - 316 μm and shell thickness of 7.8 μm - 0.7 µm, respectively. The inflated shell pops due to mechanical failure when subjected to mechanical stress above a certain threshold, resembling a balloon. During popping, the stretched shell rapidly retracts to the original uninflated state, resulting in an ultrafast release of encapsulants from the lumen within a millisecond. This process converts elastic potential energy stored in the shell into mechanical energy with substantial power. The microballoons mechanically stimulate the local environment, leading to the direct and rapid release of encapsulants. This has the potential to improve absorption efficiency.

A unified thermodynamic and kinetic approach for prediction of microcapsule morphologies

HYPOTHESIS

Microcapsule formation, following internal phase separation by solvent evaporation, is controlled by two main factors of thermodynamic and kinetic origin. Morphology prediction has previously focused on the final thermodynamical state in terms of spreading conditions, limiting the prediction accuracy. By additionally considering kinetic effects as the emulsion droplet evolves through the two-phase region of its ternary phase diagram during solvent evaporation, this should enhance prediction accuracy and explain a wider range of morphologies.

EXPERIMENTS

Dynamical interfacial tensions, and thereby spreading coefficients, during the formation of poly(methyl methacrylate) and poly(d,l-lactic-co-glycolic acid) microcapsules were measured by first establishing the boundaries and tie-lines of the ternary system in the emulsion droplets. Kinetic effects during the formation were investigated by varying the solvent evaporation rate and hence the time for polymer shell formation equilibration. The theory was validated by comparing predicted morphologies to microscopic snapshots of intermediate and final morphologies.

FINDINGS

The proposed theory explained both intermediate acorn and core-shell morphologies, where a late transition from acorn to core-shell produced microcapsules containing highly off-centered cores. By considering the kinetic factors, the formulation could be altered from yielding kinetically frozen acorns to core-shell and from yielding multicore to single core microcapsules.