Table 4.

Classification of pH-responsive carrier materials.

Classification	Characteristics	Examples
Microcapsules	Particle size is generally of micro- or nano-scale; divided into a capsule core, capsule wall, and capsule material. Conformation of polyelectrolytes under different pH conditions change, affecting the microcapsule’s diffusion transmittance.	Supramolecular graft polymer, N,N-dialkylaminoalkyl acrylate polymer
Polymer microspheres	Generally, feature acid/alkali groups that can be ionized or associated on a macromolecule skeleton.	Polyacrylic acid (PAA), polymethacrylic acid (PMAA)
Hydrogels	Respond to changes in environmental conditions and have relatively large structures; achieve transition between tight and swelling states. Generally, they contain acidic or alkaline pendant groups but with a wide-swelling pH range.	Polyethylene glycol-b-poly (4-vinylpyridine) (PEG-b-P4VP)
Mesoporous materials	Uniform and adjustable pore size, stable skeletal structure, good biocompatibility, sufficiently large specific surface area, and easily functionalized pore volume surface.	SBA-15, mesoporous silica