Hydroxypropyl methylcelluloseHPMC), as a key excipient in sustained release formulations, mainly regulates drug release through a physical gelation mechanism. When the formulation comes into contact with body fluids, HPMC surface rapidly hydrates, forming a dense gel layer. This layer has the following functions:

1. Diffusion barrier: The high viscosity of the gel retards inward penetration of water and the outward diffusion of drugs, reducing the release rate.

2. Erosion control: The gel layer gradually erodes over time, and the drug slowly released along with the degradation of the matrix, with the release rate positively correlated with the viscosity (e.g., K4M, K15M, etc. and concentration of HPMC.

3. pH stability: HPMC can gelate stably under different pH conditions in the gastrointestinal tract, making it suitable for pHsensitive drugs.

4. Tailored release: By adjusting the degree of substitution (the ratio of hydroxypropyl to methoxy groups) or combining with other excipientssuch as ethyl cellulose), zero-order, pulsatile, or targeted release can be achieved.

In addition, the non-toxic, film-forming, low-cost characteristics of HPMC make it the preferred material for oral sustained release tablets, matrix tablets, and hydrophilic gel systems, widely used in formulations such as antibiotics and drugs.