Cracking an egg to release its yolk requires applying external force (like being smacked against the edge of a bowl) to overcome the strength of the eggshell. Similarly, delivering microcapsule-contained therapeutic biomolecules into the human body requires that their containers be broken after they are injected, so that the cargo can be delivered in the right place at the right time. A number of external stimuli can be used to trigger the release of encapsulated molecules, one of the easiest of which is osmotic pressure, as it simply requires the introduction of water to cause the microcapsules to swell and burst. However, in order to create enough inner pressure to break the capsule shell, large amounts of an osmotic agent must be added to the microcapsule to attract the water, and the resulting high burst pressure could damage tissues or cause blood clots.
