Polymer network-based nanogels (NGs) and microgels (MGs) have emerged as highly versatile platforms for advanced drug delivery, owing to their tunable architecture, biocompatibility, and responsiveness to diverse stimuli. This review presents a comprehensive and structured analysis of NG/MGs, encompassing their classification based on polymer origin, crosslinking mechanisms, composition, charge, stimuli-responsiveness, and structural architecture. We detail synthesis strategies—including inverse microemulsion and radiation-induced polymerization—and highlight key characterization techniques essential for evaluating physicochemical and functional properties. Emphasis is placed on the design-driven applications of NG/MGs in overcoming biological barriers and enabling targeted therapies, particularly in cancer, inflammation, diabetes, and viral infections. Multifunctional NGs integrating therapeutic and diagnostic capabilities (theranostics), as well as emerging platforms for immunotherapy and personalized medicine, are critically discussed. Finally, we address translational challenges and future directions, including scalable manufacturing, regulatory considerations, and integration with smart diagnostics. This review aims to serve as a foundational resource for researchers and clinicians developing next-generation NG/MG-based therapeutics.

Polymer gel-based triboelectric nanogenerators (TENGs) have emerged as versatile platforms for self-powered sensing due to their inherent softness, stretchability, and tunable conductivity. This review comprehensively explores the roles of polymer gels in TENG architecture, including their function as triboelectric layers, electrodes, and conductive matrices. We analyze four operational modes—vertical contact-separation, lateral-sliding, single-electrode, and freestanding configurations—alongside key performance metrics. Recent studies have reported output voltages of up to 545 V, short-circuit currents of 48.7 μA, and power densities exceeding 120 mW/m², demonstrating the high efficiency of gel-based TENGs. Gel materials are classified by network structure (single-, double-, and multi-network), matrix composition (hydrogels, aerogels, and ionic gels), and dielectric medium. Strategies to enhance conductivity using ionic salts, conductive polymers, and nanomaterials are discussed in relation to triboelectric output and sensing sensitivity. Morphological features such as surface roughness, porosity, and micro/nano-patterning are examined for their impact on charge generation. Application-focused sections detail the integration of gel-based TENGs in health monitoring (e.g., sweat, glucose, respiratory, and tremor sensing), environmental sensing (e.g., humidity, fire, marine, and gas detection), and tactile interfaces (e.g., e-skin and wearable electronics). Finally, we address current challenges, including mechanical durability, dehydration, and system integration, and outline future directions involving self-healing gels, hybrid architectures, and AI-assisted sensing. This review expands the subject area by synthesizing recent advances and offering a strategic roadmap for developing intelligent, sustainable, and multifunctional TENG-based sensing technologies.

Hydrogels have garnered significant attention as multifunctional materials in next-generation rechargeable batteries due to their high ionic conductivity, mechanical flexibility, and structural tunability. This review presents a comprehensive overview of hydrogel types—including natural, synthetic, composite, carbon-based, conductive polymer, and MOF hydrogels—and their synthesis methods, such as chemical crosslinking, self-assembly, and irradiation-based techniques. Characterization tools like SEM, XRD, and FTIR are discussed to evaluate their microstructure and performance. In rechargeable batteries systems, hydrogels enhance ionic transport and mechanical stability, particularly in lithium-ion, sodium-ion, zinc-ion, magnesium-ion, and aluminum-ion batteries. Despite their advantages, hydrogels face challenges such as limited mechanical strength, reduced stability under extreme conditions, and scalability issues. Current research focuses on advanced formulations, self-healing mechanisms, and sustainable materials to overcome these limitations. This review highlights the pivotal role of hydrogels in shaping the future of flexible, high-performance, and environmentally friendly secondary batteries.

Hydrogel-based drug delivery systems are redefining cancer therapy by enabling targeted, biocompatible, and stimuli-responsive treatment strategies. This review examines natural biopolymer hydrogels—such as chitosan, hyaluronic acid, alginate, gelatin, and collagen—and their role in precision oncology. These materials offer biodegradability, tunable properties, and low immunogenicity, supporting controlled drug release. We discuss gelation mechanisms, structural classifications from macrogels to nanogels, and the integration of stimuli-responsive features triggered by pH, temperature, light, and redox conditions within tumor microenvironments. Applications include chemotherapy, radiotherapy, immunotherapy, and phototherapy, using strategies such as passive targeting via the enhanced permeability and retention effect, active targeting via ligand functionalization, and stimuli-triggered release. Cancer-specific implementations in colorectal, gastric, liver, pancreatic, and breast tumors highlight hydrogel versatility. Key challenges include scalability, regulatory compliance, and tumor heterogeneity, while emerging innovations in smart hydrogels, nanotechnology, and personalized medicine offer promising solutions. This review underscores the potential of biopolymer-based hydrogels to advance cancer treatment toward safer, more effective, and patient-centered approaches.

Biopolymer-based hydrogels offer adaptable platforms for localized cancer therapy by regulating administration, tissue retention, payload protection, and therapeutic release. This review critically examines polysaccharide-, protein-, peptide-, and polynucleotide-based hydrogels, relating polymer composition, crosslinking, network architecture, swelling, degradation, and payload interactions to pharmaceutical and biological performance. Physical, chemical, enzymatic, and ionic gelation strategies are evaluated alongside diffusion-, swelling-, affinity-, and degradation-controlled release and their mathematical interpretation. Macrogels, microgels, nanogels, and hybrid systems are compared according to administration route, disposition, and oncology-relevant function. The review further distinguishes local placement, passive accumulation, ligand-mediated cellular interactions, and endogenous or externally triggered release. Applications in chemotherapy, radiotherapy, immunotherapy, hyperthermia, and phototherapy are assessed across colorectal, gastric, liver, pancreatic, and breast cancers. Marketed devices, patented technologies, clinically evaluated systems, and regulatory considerations are also examined. Despite extensive preclinical development, translation remains constrained by polymer variability, incomplete pharmaceutical characterization, sterilization and scale-up challenges, uncertain degradation and clearance, and limited clinically representative evidence. Progress will depend on clinically defined product design, reproducible manufacturing, quantitative disposition and safety assessment, and demonstration of meaningful benefit over existing therapeutic or procedural alternatives.

Hydrogels have garnered significant attention as multifunctional materials in next generation rechargeable batteries due to their high ionic conductivity, mechanical flexibility, and structural tunability. This review presents a comprehensive overview of hydrogel types—including natural, synthetic, composite, carbon-based, conductive polymer, and MOF hydrogels—and their synthesis methods, such as chemical crosslinking, self-assembly, and irradiation-based techniques. Characterization tools like SEM, XRD, and FTIR are discussed to evaluate their microstructure and performance. In rechargeable batteries systems, hydrogels enhance ionic transport and mechanical stability, particularly in lithium-ion, sodium-ion, zinc-ion, magnesium-ion, and aluminum-ion batteries. Despite their advantages, hydrogels face challenges such as limited mechanical strength, reduced stability under extreme conditions, and scalability issues. Current research focuses on advanced formulations, self-healing mechanisms, and sustainable materials to overcome these limitations. This review highlights the pivotal role of hydrogels in shaping the future of flexible, high-performance, and environmentally friendly secondary batteries.