The clinical problem these materials target is brutal in its simplicity. Infected bone defects combine persistent infection with impaired bone regeneration, and the two pathologies feed each other. When bacteria colonize a defect site, they secrete extracellular matrix components such as polysaccharides and build structurally complex biofilms. These biofilms blunt the host’s immune clearance and physically hinder antibiotics from penetrating the lesion, driving the infection into a chronic, refractory state. The resulting inflammatory storm—dominated by tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-6—suppresses osteoblast function while activating osteoclasts, so bone formation stalls even as bone resorption accelerates. Studies cited in the review highlight a critical mechanism in Staphylococcus aureus osteomyelitis: increased osteoclast activation mediated by RANKL is a major cause of bone loss. Infection-induced inflammation also damages local microvasculature, disrupting oxygen and nutrient supply and blocking mesenchymal stem cells and systemically administered drugs from reaching the wound.

Current clinical practice responds with staged treatment: thorough debridement and antibiotic-loaded bone cement first, bone grafting or bone transport second, once infection appears controlled. The review catalogs why this approach disappoints. Multiple surgeries prolong hospitalization, raise costs, and burden patients psychologically and physiologically. Antibiotic cement releases its payload in an initial burst followed by rapid decay, making it difficult to sustain a stable antibacterial concentration; the early spike risks cytotoxicity, the subsequent drop hinders biofilm clearance, and the erratic pattern encourages bacterial resistance. Bone cement itself offers nothing to promote bone formation, so the residual defect still depends entirely on additional reconstruction. Timing the second stage is a guessing game—graft too early and infection recurs, wait too long and soft tissue contracts and atrophies, raising the risk of nonunion. Extensive defects and repeated debridement during induced membrane technique treatment have been identified as independent risk factors for both nonunion and infection recurrence.

Hydrogels enter as candidates because their properties map remarkably well onto the problem. These water-rich, three-dimensional polymer networks mimic the extracellular matrix, providing a hospitable microenvironment for cell adhesion, migration, and proliferation. Their porous, hydrated structure permits diffusion of oxygen and nutrients, while cross-linking density, polymer composition, and reinforcement strategies can be tuned to match the mechanical demands of bone repair. Crucially for drug delivery, hydrogels can load agents such as vancomycin, rifampin, bone morphogenetic protein-2 (BMP-2), and vascular endothelial growth factor (VEGF) through physical encapsulation, chemical conjugation, or electrostatic interactions. The review emphasizes that these physicochemical parameters are not independent knobs: cross-linking density governs molecular diffusion and release kinetics, degradation must be coordinated with tissue regeneration, and stiffness and porosity shape cell behavior. Therapeutic performance depends on balancing drug release, degradation, mechanical stability, and biological accessibility rather than maximizing any single property.

The core design principle is sequential release: rapidly establish an effective antibacterial concentration early, then sustain osteogenic signals once infection recedes. The biological logic is that premature growth factor delivery offers limited benefit in a highly infected, inflammatory microenvironment, whereas delayed, sustained delivery after inflammation subsides can effectively support cell recruitment, angiogenesis, and osteogenic differentiation. Researchers have pursued this through several architectures. One team constructed an engineered poly(amino acid) hydrogel that loads vancomycin while grafting BMP-2 peptides, achieving synergy between early antibacterial action and later osteopromotion. Encapsulating antibiotics in easily degradable hydrogel microspheres while immobilizing growth factors in the primary network generates a clear release gradient within a single system. Spatially distributed designs go further: a bifunctional scaffold combining a fingolimod-loaded 3D-printed polycaprolactone framework with a vancomycin-loaded aldehyde hyaluronic acid/carboxymethyl chitosan hydrogel provides mechanical support and regenerative cues from the scaffold while the hydrogel fights infection in the pores. Peripheral antibiotic layers can form an antibacterial defense line that restricts infection from spreading inward, while an inner core rich in growth signals promotes bone formation after infection is controlled.

Stimuli-responsive strategies add a layer of intelligence. Infected lesions characteristically show decreased pH, elevated bacteria-related enzymes, and abnormal accumulation of reactive oxygen species, and smart hydrogels can use these signals as triggers for on-demand release. A pH-sensitive silk fibroin/sodium alginate scaffold loaded with teicoplanin and aniline, for example, releases drug faster under alkaline conditions, adapting to the dynamic infection microenvironment. A structural color hydrogel patch reported recently can monitor local pH and temperature in real time and regulate drug release accordingly, hinting at integrated theranostic management of chronic osteomyelitis. Near-infrared photothermal approaches offer externally controlled antimicrobial intervention, with recent analyses suggesting NIR-II wavelengths may outperform NIR-I at deeper infection sites due to greater tissue penetration. The review’s summary table, however, is candid about limitations: stimuli-responsive systems still suffer from insufficient in vivo response thresholds and precision, while cell- and exosome-combined hydrogels face high difficulty in standardized manufacturing and regulation.

The molecular story the review tells is one of interdependence rather than simple addition. Early antibiotic delivery does more than kill bacteria—it removes the biological constraints that infection imposes on regeneration. As bacterial load falls, pro-inflammatory cytokines decline, releasing osteoblasts and mesenchymal stem cells from inhibition, while neutrophils and M1 macrophages reduce their continuous output of reactive oxygen species and proteolytic enzymes that would otherwise damage new bone matrix and the delivery system itself. In the regenerative phase, BMP-2 activates Smad pathways, promoting Runx2 and Osterix expression, alkaline phosphatase activity, matrix secretion, and mineralized nodule formation, while VEGF drives endothelial recruitment and neovascularization that supply oxygen and nutrients to forming bone. Some antibiotics at sub-inhibitory concentrations may even assist osteogenesis by inhibiting matrix metalloproteinases or easing inflammation. Meanwhile, macrophages shift from a pro-inflammatory toward a pro-reparative phenotype, releasing interleukin-10, VEGF, and platelet-derived growth factor. The review cautions that the M1/M2 classification is a simplified framework—in vivo macrophages show substantial plasticity and adopt intermediate states—and that recent biomaterial strategies increasingly aim to coordinate macrophages, stem cells, and endothelial cells together rather than targeting a single phenotype.

Evidence supporting these concepts spans in vitro assays, animal models, and early large-animal work. In vitro evaluation combines antibacterial testing against S. aureus, E. coli, and drug-resistant strains with osteogenesis assays using alkaline phosphatase activity, Alizarin Red staining for calcified nodules, and expression analysis of RUNX2, osteopontin, and osteocalcin; some studies now use bacteria-host cell co-cultures to better simulate the infected environment. In vivo models typically inoculate S. aureus into radius, femur, or tibia defects in rabbits or rats, then track repair with X-ray, micro-CT quantification of new bone and trabecular parameters, histological staining, immunohistochemistry for CD31 and osteocalcin, and quantitative bacterial counts. Notably, an engineered poly(amino acid) hydrogel with vancomycin and grafted BMP-2 peptides showed effective infection control and enhanced bone regeneration in a rabbit radial model, supporting the feasibility of single-stage treatment. A thermosensitive chitosan hydrogel loaded with vancomycin nanoparticles sustained release for up to 14 days while promoting bone repair, and a dual-network hydrogel built through chlorogenic acid-magnesium coordination combined with genipin crosslinking inhibited drug-resistant bacteria including methicillin-resistant S. aureus. In a more clinically oriented comparison, antibiotic-loaded hydrogels were tested against poly(methyl methacrylate) cement in single-stage revision surgery in an ovine model of chronic MRSA implant-associated infection, and a pH-responsive bisphosphonate hydrogel controlled MRSA while preserving cortical bone integrity in an osteoporotic fracture-related infection model.

The review is equally clear that laboratory success has not yet translated into the clinic, and it identifies the obstacles with unusual specificity. Release profiles remain prone to early antibiotic burst and growth factor inactivation; growth factors are fragile and often lose biological activity through insufficient stability or overly rapid release. Clinical infections frequently involve polymicrobial communities, mature biofilms, and drug-resistant organisms—conditions that standardized animal models with single bacterial strains and uniform defects fail to reproduce, along with the variability in host immune status, comorbidities, and defect characteristics seen in patients. Evaluation across studies is heterogeneous, with different strains, inocula, endpoints, and simplified in vitro release conditions that ignore the dynamic pH, enzymatic activity, and protein-rich environment of real infected bone. Long-term biosafety questions—drug residues, degradation products, ectopic osteogenesis, systemic toxicity, and abnormal immune responses—remain unresolved. And as systems incorporate more drugs, nanoparticles, cells, and proteins, reproducible manufacturing, sterilization, storage stability, and batch-to-batch consistency become progressively harder to guarantee.

The review’s conclusion is measured but forward-looking: the most translatable hydrogels may not be the most functionally complex ones, but those that strike the right balance among antibacterial efficacy, immunomodulation, osteogenic activity, safety, and manufacturing controllability. Future priorities include smart stimuli-responsive delivery, combinatorial antibacterial strategies such as antimicrobial peptides and phage-delivering hydrogels, staged immunomodulation exemplified by a sequential system that releases calcium ions early to promote M1 polarization and infection inhibition, then sustained zinc ions to guide macrophages toward a pro-reparative phenotype, and standardized preclinical validation in large animals and comorbid models. Integration with 3D bioprinting matched to patient imaging data, stem cells, exosomes, and bioactive small molecules could expand the platform further. For now, bifunctional hydrogels remain a preclinical promise—but one whose central insight, that infection control and bone regeneration are biologically interdependent processes that a single well-timed material could orchestrate, is reshaping how researchers think about healing the infected skeleton.

Subject of Research: Antibiotic and growth factor-loaded hydrogels for infected bone defect repair

Article Title: Antibiotic and growth factor-loaded hydrogels for infected bone defect repair: mechanisms of spatiotemporal delivery and cellular microenvironment regulation

Article References: Chen, L., Zhang, X., Luo, L., Lan, J., Deng, X., Wen, Y., & Li, L. (2026). Antibiotic and growth factor-loaded hydrogels for infected bone defect repair: mechanisms of spatiotemporal delivery and cellular microenvironment regulation. Frontiers in Cell and Developmental Biology, 14, 1996917. https://doi.org/10.3389/fcell.2026.1996917

Image Credits: AI Generated

DOI: 10.3389/fcell.2026.1996917

Keywords: infected bone defect, hydrogel, antibiotic delivery, growth factors, sequential release, bone regeneration, biofilm, osteomyelitis, immunomodulation, angiogenesis, 3D bioprinting, stimuli-responsive materials