**Background:** Hydrogels are three-dimensional network polymers capable of swelling and storing significant amounts of water (at least 10% of total weight) while maintaining structural integrity through physical and chemical crosslinking. First coined by Wichterle and Lim in 1960, hydrogels possess flexibility resembling actual tissue due to their high water content. They contain hydrophilic functional groups (-COOH, -NH2, -OH, -CONH, -CONH2, -SO3H) and undergo reversible volume phase transitions in response to chemical stimuli (dissolved ions, pH, chemical composition) and physical stimuli (magnetic field, electric field, pressure, temperature, light intensity). Natural and synthetic polymers including polyacrylamide (PAA), polyvinyl imidazole (PVIm), polyethylene glycol (PEG), polyvinyl alcohol (PVA), and polyvinylpyrrolidone (PVP) have been utilized as hydrogel-based sensors. The advent of additive manufacturing (3D printing) has enabled new possibilities for manufacturing geometrically complex structures from digital models, though geometric complexity and resolution remain open issues.
**Methods:** This narrative review synthesizes literature on hydrogel classification, 3D printing techniques, and applications. Hydrogels are classified into natural hydrogels (cellulose, starch, chitosan, peptides/proteins), synthetic hydrogels (PVA, PEG, PAM), functional polymers (stimuli-responsive), nanocomposite hydrogels (incorporating metal nanoparticles, carbon nanotubes, graphene), conductive hydrogels (ECoHs), and photoresponsive hydrogels. Four primary 3D printing techniques are described: (1) Inkjet laser 3D printing, which uses acoustic, thermal, or electric fields to print bioink in a layer-by-layer approach with droplet sizes between 1 and 300 pL and deposition rates of 1 to 10,000 droplets per second, requiring bioink viscosity less than 10 mPa/s; (2) Extrusion-based 3D printing, which uses mechanical or pneumatic energy to extrude bioink through a nozzle, accommodating viscosities between 30 mPa·s⁻¹ and approximately 6 × 10⁷ mPa·s⁻¹; (3) Digital Light Processing (DLP), which uses a digital micromirror device (DMD) to create light patterns with resolution of 20–200 μm; and (4) Stereolithography (SLA), which uses a laser beam for photocrosslinking with resolution of 100 μm and viscosity requirements of 1 to 300 mPa/s.
**Key Results:** The review presents specific experimental findings from cited studies. Huang et al. demonstrated that Au/BC hydrogels triggered higher ALP activity with increased calcium nodules in hBMSCs compared to neat BC hydrogels, with mRNA expression levels of Runx2, COL1, OCN, and OPN significantly upregulated. Wang et al. developed 3D-printed TCP/OP/PLGA scaffolds with collagen I coating that exhibited 98% live cells after 3 days of incubation and significant ALP⁺ levels after 7 days. Jacus et al. fabricated ECH nanocomposite structures with superior electrical conductivity of 875 S cm⁻¹ and good biocompatibility with human mesenchymal stem cells. For conductive hydrogels, reported conductivities include: Chitosan MA with Graphene NS at 0.0025 S/cm; GelMA with DNA-coated MWCNT at 24 ± 1.8 S/cm; PEGDA with MWCNT-NH2 at 2.21 ± 0.121 mC·cm⁻²; PANI with Phytic acid at 0.23 S/cm; and PNIPAM-L-CNT at 0.016–0.02 S/cm. For water purification, Yang et al. synthesized CMC hydrogel beads that exhibited higher absorption efficiency of Pb²⁺, Cu²⁺, and Ni²⁺. Bandyopadhyay et al. developed bacterial cellulose and guar gum-incorporated PVP-CMCH hydrogel film that kept blueberries fresh for 15 days with 80% degradability within 28 days.
**Clinical Implications:** 3D-printed hydrogels show tremendous potential for bone and joint tissue repair, requiring stronger and harder materials than cartilage with pro-angiogenic properties. Nerve guidance conduits (NGCs) fabricated from natural and synthetic materials (alginate, agarose, chitosan, degradable polyurethane) using extrusion printing can connect proximal and distal ends of defective nerves. For craniofacial applications, bioceramic scaffolds (HA/TCP composites) lack strength and dimensional stability, while copolymer PLGA shows outstanding osteoconductivity with Young's modulus resembling real bone. Polyetherketoneketone (PEKK) demonstrates superior mechanical strength and biocompatibility for craniofacial implants. Key limitations include: hydrogel mechanical strength (compressive, tensile, shear thinning), sterility and antibacterial capacity, nozzle clogging in inkjet and extrusion methods, cell viability concerns (must exceed 85% for inkjet printing), and the challenge of vascularization for tissue constructs beyond 100–200 μm. The review concludes that stimuli-based (pH and temperature) hydrogels in biosensor applications remain relatively scarce, and large-scale production of 3D-printed hydrogels for bioremediation requires further improvement.