Nanoparticulate and Hydrogel Vehicles for Stimuli-Responsive and Sustained Controlled Release of Active Pharmaceutical Ingredients
Most active pharmaceutical ingredients (APIs) reach their target by passive systemic distribution, so the dose required for efficacy at the lesion is set by what healthy tissue can tolerate; conventional dosage forms consequently produce pharmacokinetic profiles that oscillate between toxic peaks and sub-therapeutic troughs. Nanoparticulate carriers (liposomes, lipid nanoparticles, polymeric and inorganic systems, and biomimetic carriers) and hydrogels (natural, synthetic, supramolecular, and microgel-assembled) have emerged as the dominant strategies to address this, increasingly combined as hybrid nanoparticle–hydrogel constructs in which the gel provides locoregional retention and the nanoparticles provide cargo protection, intracellular delivery and stimuli responsiveness. Stimuli-responsive chemistries (pH, redox, enzyme, ROS, hypoxia, temperature, light, magnetic, ultrasound, glucose, and multi-stimuli logic) translate the molecular signatures of a disease into spatiotemporally controlled cargo release. This narrative review consolidates the state of the art (prioritizing 2022–2026) and departs from the conventional carrier-type survey in one respect: the literature is read along an explicit chain—disease cue, sensing chemistry, carrier architecture, release mechanism and kinetics, administration route, and clinical readiness—which exposes a variable that classification by carrier type conceals. Across all three material classes, what governs release behavior is not primarily the carrier chemistry but the identity of the released species (dissolved drug, drug from an embedded nanoparticle, an intact nanoparticle, and a matrix fragment) and the transport step that limits it. This is why power-law exponent analysis developed for dissolved drug fits particulate release poorly, why statistical goodness-of-fit cannot by itself establish a release mechanism, and why carrier class predicts clinical readiness less well than administration route and regulatory product type. Translational hurdles—CMC complexity, regulatory fragmentation, anti-PEG immunogenicity, and the structural mismatch between preclinical promise and clinical efficacy—are critically appraised in light of previously reported <1% delivery efficiency analysis. This review identifies converging strategies that could move stimuli-responsive controlled release from an aspirational outcome to a routine clinical reality.