Sustained and controlled release of herbal medicines: The concept of synchronized release

Synchronized release, defined as simultaneous release of multiple components while keeping the inter-component ratios steady, is crucial to the design and development of sustained- or controlled-release delivery systems of herbal medicines. A theory of infinite dose dividing is proposed to interpret the rationale behind synchronized release. The key point of the mechanisms is to curb diffusion so as to offset the release divergence caused by differences in physicochemical properties of multiple components. In practice, pH-dependent gradient-release, erosion-controlled and osmotic pump delivery systems have been adapted to attain synchronized release of model herbal medicines. Nevertheless, synchronized release is still in the proof-of-concept stages and should be corroborated by more evidence with more model herbal medicines.