How Emerging Nanomaterials are Effective in Bone Regeneration?

Abstract

Bone defects remain challenging due to microenvironment disruption and immune imbalance. Traditional grafts offer structural support; however, they do not actively modulate this pathological niche. This review underscores smart nanomaterial reservoirs for bone regeneration, focusing on their capacity to remodel the hostile microenvironment and restore immune homeostasis. Key pathophysiological barriers, including vascular disruption, macrophage imbalance, and oxidative stress-induced ferroptosis, are identified as therapeutic targets. Additionally, fundamental nanoscale building blocks (rigid inorganic, polymeric, and inorganic piezoelectric materials) are critically compared, with an expanded focus on emerging technologies such as versatile nanofibers and 3D-printable bio-ink formulations. These components can be assembled into smart reservoirs (hydrogels, exosomes, and 3D scaffolds), enabling stimuli-responsive therapeutic delivery (pH, reactive oxygen species, and enzymes). By reprogramming intercellular communication and promoting M1-to-M2 macrophage polarization, these reservoirs restore osteoimmune homeostasis and couple osteogenesis with angiogenesis. Finally, translational challenges and future directions toward dynamically adaptive nanomedicines are discussed.

Introduction

Bone is an essential component of the human skeletal system because it provides critical mechanical support and protects internal organs. However, the integrity of this highly specialized tissue is frequently compromised by various severe pathological conditions, encompassing severe localized infections, neoplastic diseases, traumatic injuries, and metabolic bone disorders.^1,2 In cases of critical-sized bone defects, clinically defined as osseous voids where spontaneous healing is fundamentally impaired, and native regeneration capabilities are limited. Specifically, natural repair mechanisms can restore less than 10% of the original bone volume, often leading to persistent nonunions.³

Conventional interventions mainly rely on autologous or allogeneic bone grafts, which theoretically provide essential osteoconductive and osteoinductive support.⁴ However, their widespread application is limited by several factors, including restricted donor availability, increased surgical complexity at the harvest site, and a high susceptibility to severe postoperative complications such as bacterial biofilm formation, localized thrombosis, and subsequent avascular necrosis.⁵ Metallic implants (eg, titanium alloys and stainless steel) have been utilized as the clinical gold standard for load-bearing defect stabilization.⁶ The inherent mechanical modulus mismatch between rigid metals and natural bone inevitably induces stress shielding, leading to peri-implant osteopenia. Furthermore, the lack of intrinsic osteoinductivity results in poor osseointegration, while continuous physiological loading can trigger corrosion-related heavy metal ion release, exacerbating local tissue toxicity and frequently necessitating complex secondary removal surgeries.⁶

Alternative modalities, including isolated gene therapy and recombinant growth factor delivery, have demonstrated remarkable experimental promise in controlled settings.⁷ However, their clinical translation remains limited due to biosafety concerns, prohibitive treatment costs. Specifically, the rapid in vivo enzymatic degradation of isolated recombinant growth factors necessitates the administration of supraphysiological doses. This not only exponentially inflates treatment costs but also critically elevates the biosafety risk of inducing unregulated ectopic ossification, paradoxical inflammatory cascades, and oncogenic alterations. Recombinant human bone morphogenetic protein-2 (rhBMP-2), despite its potent osteoinductive capacity, has been associated with severe clinical complications including radiculitis, seroma formation, and ectopic bone formation when administered at supraphysiological doses11. To address these delivery challenges, functionalized polymeric nanomaterials have recently emerged as promising carriers for BMP-2, enabling sustained and localized release that mitigates adverse effects while preserving therapeutic efficacy.⁸

Recently, the application of nanotechnology has emerged as a promising strategy in bone tissue engineering (BTE). Nanoparticles (NPs) have gained considerable attention for their ability to enhance osteogenesis, improve mechanical properties, and modulate immune responses. Nanotechnology can improve the mechanical, chemical–physical, and biological properties of scaffolds. Therapeutic nanomaterials, characterized by their ultrafine dimensions spanning from 10 to 100 nm, can structurally mimic the natural hierarchical architecture of native bone tissue while simultaneously interacting with distinct cellular components at the fundamental molecular level.^9,10 Integrating these rigid inorganic nanomaterials,¹¹ polymers,^12,13 and metallic piezoelectric materials¹⁴ into supramolecular systems transforms passive fillers into responsive “smart reservoirs”. In addition to these conventional building blocks, electrospun nanofibers have emerged as indispensable architectural components owing to their inherently high surface-area-to-volume ratio and interconnected porosity, which exquisitely mimic the hierarchical architecture of the native collagen extracellular matrix (ECM) and provide profound topographical cues that govern stem cell elongation and osteogenic differentiation21. Simultaneously, the advent of 3D bioprinting has propelled the development of functional bio-inks—viscoelastic hydrogel formulations laden with bioactive nanomaterials and living cells—that confer exceptional shear-thinning properties for precise spatial printing while maintaining robust mechanical fidelity.¹⁵ However, bone regeneration requires more than a structural support because the local pathological microenvironment plays a major role in healing. This environment is characterized by ischemia, excessive inflammation, and oxidative stress.¹⁶

This review mainly focuses on an emerging paradigm: using smart nanomaterial reservoirs to actively remodel the hostile microenvironment for enhanced bone repair. As illustrated in Figure 1, bone healing requires overcoming a local pathological microenvironment characterized by excessive inflammation, oxidative stress (reactive oxygen species [ROS]), and acidic conditions.^9,17,18 By categorizing fundamental nanoscale building blocks—including rigid inorganic nanomaterials, polymers, and piezoelectric materials^11–14—and their assembly into biomimetic scaffolds and “smart” stimuli-responsive delivery reservoirs such as hydrogels, exosomes, Electrospun Nanofibers, Bio-inks and 3D scaffolds,^19–22 this review emphasizes how these materials can target key pathophysiological barriers. A particular focus is given to their therapeutic functions and osteoimmunomodulatory properties, including scavenging ROS, neutralizing pH, and promoting M1-to-M2 macrophage polarization.^23–26 These processes help orchestrate the “immune dialogue” to couple osteogenesis with angiogenesis, ultimately enabling mature bone regeneration. Finally, we address the challenges and future perspectives for clinical translation.

Figure 1.

Schematic illustration of the bone regeneration process using smart reservoirs. This figure was originally created by the authors based on information synthesized from the literature.^{9,11–14,17–26} (A) Inflammatory microenvironment with elevated ROS, acidic pH, broken vessels, and M1 macrophage infiltration. (B) Smart reservoirs composed of inorganic/polymeric nanomaterials, piezoelectric materials, 3D scaffolds, and exosomes. (C) Therapeutic actions including (i) osteoimmunomodulation via ROS scavenging and pH neutralization, (ii) M1-to-M2 macrophage polarization, and (iii) osteogenesis-angiogenesis coupling. (D) Restored bone with new blood vessels, mature bone matrix, and a high abundance of M2 macrophages.

Where Do We Understand About Pathological Barriers in Bone Regeneration?

The success of bone healing depends on the local pathological microenvironment and coordinated cellular activities within the defect.^27–29 Consequently, developing effective interventions requires deconstructing the multifaceted biological barriers that obstruct skeletal regeneration. These barriers represent key targets for intervention by smart nanomaterial reservoirs. As illustrated in Figure 2, the pathophysiological landscape of impaired bone regeneration can be categorized into three interconnected domains: 2.1 How Does Vascular Disruption Impair the Healing Cascade?;^30–32 2.2 Why is Macrophage Polarization Crucial for Immunological Balance?;^23,24 and 2.3 How Do Metabolic and Oncogenic Microenvironments Disrupt Regeneration?^25,33–35

Figure 2.

Schematic illustration of the pathophysiological barriers to bone regeneration. This figure was originally created by the authors based on information synthesized from the literature.^{23–25,27–35} (A) Vascular disruption and osteocyte-vascular interactions. (B) Macrophage polarization imbalance showing M1 and M2 states. (C) Metabolic and oncogenic microenvironmental disruptions: (i) the metabolic microenvironment involving oxidative stress-induced ferroptosis and osteoblast depletion; (ii) the oncogenic microenvironment involving tumor-driven vicious cycles of bone destruction.

How Does Vascular Disruption Impair the Healing Cascade?

Physiological bone regeneration unfolds through overlapping phases that include an inflammatory stage to clear debris, a reparative phase involving mesenchymal stem cell (MSC) and osteoblast recruitment, and a remodeling period.^36,37 Severe trauma causes vascular and neural damage, impairing blood supply and inducing local hypoxia and nutrient deprivation.³⁸ Recent evidence highlights the indispensable role of osteocyte-vascular interactions during the early stages of repair following severe vascular disruption. The initial inflammatory hematoma temporally restricts blood flow; subsequently, apoptotic osteocytes and infiltrated immune cells must secrete concentrated localized pro-angiogenic factors, including VEGF and cysteine-rich angiogenic inducer 61 (CYR61), to effectively re-establish the vascular network and stimulate endothelial tubulogenesis.³¹ This vascular disruption is critical because angiogenesis, driven by vascular endothelial growth factor (VEGF), enables the delivery of osteogenic precursors, growth factors, and nutrients. A compromised blood supply restricts the transport of cytokines and pro-regenerative factors needed to resolve inflammation and facilitate tissue repair.³⁹

Recent evidence challenges the notion of direct coupling between angiogenesis and osteogenesis. For instance, inhibition of endothelial Notch signaling via Dll4 deletion increases vascular density 1.9-fold without affecting bone healing progression (Figure 3a and b).³⁰ Notably, systemic overexpression of VEGF-A, a central regulator of angiogenesis, increases vascular area by 1.6-fold yet fails to enhance calvarial bone regeneration (Figure 3c and d). These findings indicate that while angiogenesis accompanies bone regeneration, it is insufficient to drive osteogenesis, suggesting that the relationship between vascularization and skeletal regeneration is more complex than previously appreciated. Instead, the presence of functional vasculature capable of delivering osteogenic cues may be more critical than vascular density. This concept is further supported by recent studies demonstrating that controlled magnesium-releasing nanocapsules can dynamically modulate the immune-neural axis to accelerate cranial bone regeneration through enhanced neurovascular integration42, and that harnessing exogenous stimuli with functionally aligned fibrous scaffolds enables spatiotemporal modulation of angiogenesis, MSC homing, and osteogenesis.³²

Figure 3.

Angiogenesis and osteogenesis are uncoupled during calvarial bone healing. (a) Representative immunofluorescence images of calvarial lesions in Control and Dll4ΔEC mice at post-lesion day 14 (PLD14). (b) Quantitative analysis of vessel area and bone area for (a). *** p < 0.001; ns, not significant. (c) Representative images in pLIVE control and pLIVE Vegfa overexpressing mice. (d) Quantitative analysis corresponding to (c). *** p < 0.001; ns, not significant. Reprinted from [Bixel et al, 2024],³⁰ under the terms of the Creative Commons CC BY license.

Why is Macrophage Polarization Crucial for Immunological Balance?

Macrophage polarization dictates regenerative outcomes.⁴⁰ During the healing process, macrophages transition from a pro-inflammatory (M1) to an anti-inflammatory and pro-reparative (M2) phenotype. Persistent M1 over-activation sustains a cytotoxic environment, prolongs inflammation, and leads to chronic nonunion.⁴¹ Conversely, premature or excessive M2 activity can induce pathological fibrous tissue within the defect, resulting in permanent nonunion.^42,43 This dynamic phenotypic transition is temporally regulated during cranial bone healing. Guo et al analyzed macrophage polarization dynamics in a murine calvarial defect model (Figure 4).²³ Figure 4A depicts the experimental timeline and group allocation used in the murine calvarial defect model. During the early post-injury phase (days 3–7), scaffolds that promoted bone healing exhibited a coordinated shift in gene expression from M1 to M2 (Tumor Necrosis Factor-alpha (TNF-α)/Inducible Nitric Oxide Synthase (iNOS) to Arg1/CD206). In contrast, control groups exhibited sustained M1 dominance (Figure 4B). By week 12, immunofluorescence confirmed that defects with successful bone regeneration maintained significantly higher M2 (CD206⁺) and lower M1 (iNOS⁺) populations. This resulted in a 5.4-fold increase in the M2/M1 ratio compared to nonunion controls (Figure 4C–E). These findings visualize that an imbalanced, persistently M1-dominated microenvironment—whether from inadequate resolution or premature M2 dysregulation—leads to chronic inflammation that precludes proper skeletal repair. Recent advances have elucidated that nanostructured biomaterials can alter macrophage polarization by modulating key immune pathways such as NF-κB, MAPK, PI3K-Akt, and JAK-STAT49. Furthermore, early-apoptotic membrane engineering of M2 macrophage-derived nanovesicles has been shown to retain pro-repair properties while reducing inflammatory signaling and enhancing osteogenic activity, establishing donor-membrane state engineering as an effective strategy for osteoimmunomodulatory bone repair.²⁴

Figure 4.

Macrophage polarization dynamics during calvarial healing. (A) Experimental timeline and surgical group allocation in the murine calvarial defect model. (B) Heatmap showing temporal gene expression changes of M1 (TNF-α, iNOS) and M2 (Arg1, CD206) markers on days 3, 7, and 14. (C) Representative immunofluorescence staining for CD206 (green) and iNOS (red) at 12 weeks post-surgery. (D) Quantitative percentage of CD206 positive area. *** p < 0.001. (E) Quantitative percentage of iNOS positive area. *** p < 0.001. Reprinted from [Guo et al, 2025],²³ under the terms of the Creative Commons CC BY license.

How Do Metabolic and Oncogenic Microenvironments Disrupt Regeneration?

Patients with chronic metabolic disorders such as diabetes mellitus experience additional impairment in localized healing disruptions because their microenvironment is characterized by excessive ROS accumulation.¹⁸ Elevated oxidative stress impairs the osteogenic differentiation of newly migrated MSCs and exacerbates local tissue destruction, forming a vicious cycle of persistent inflammation that prevents stable hard callus formation. Hyperglycemia-driven ROS overproduction impairs osteoblast function and disrupts angiogenic-osteogenic coupling by suppressing the hypoxia-inducible factor-1α and VEGF signaling axes. Consequently, these processes lead to inferior biomechanical properties and increased refracture risk in diabetic patients.¹⁷ Beyond diabetes, osteoporosis represents another pervasive metabolic bone disorder that fundamentally compromises the bone regenerative microenvironment. In osteoporotic bone defects, particularly those induced by postmenopausal estrogen deficiency or senile cellular senescence, the pathological niche is dominated by hyperactive osteoclast-mediated resorption and suppressed osteoblast function. This systemic metabolic dysregulation is further aggravated by a chronic, low-grade inflammatory immune microenvironment featuring abnormal local accumulations of pro-inflammatory cytokines. Consequently, traditional structural grafts often fail in osteoporotic niches because they cannot correct this fundamental immune-metabolic disturbance, necessitating the design of active modulators that can reprogram this imbalanced metabolic microenvironment.²⁵

Ferroptosis, an iron-dependent, lipid peroxidation-driven form of cell death, mediates this oxidative damage in metabolic niches.^44,45 Dysregulated iron metabolism, often via transferrin receptor 1, promotes hydroxyl radical generation through the Fenton reaction.⁴⁶ This cascade depletes glutathione and inactivates glutathione peroxidase 4 (GPX4), the enzyme that protects against lipid peroxidation. The resulting accumulation of lipid peroxides triggers ferroptosis, selectively depleting osteoblasts and compromising bone homeostasis.^45,46

In vivo evidence from diabetic osteoporosis models confirms ferroptosis in diabetes-induced bone loss.³³ Figure 5a schematically illustrates the experimental design and timeline for the STZ&HFD-induced diabetic mouse model, with Figure 5b–r presenting the comprehensive in vivo characterization of diabetes-induced bone loss and ferroptosis activation. The diabetic microenvironment induces pronounced deterioration of bone microstructure, including reduced bone volume fraction (BV/TV), increased trabecular separation, and elevated numbers of empty lacunae and Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive osteocytes (Figure 5c–j). Immunohistochemical and Western blot analyses reveal GPX4 downregulation and Prostaglandin-Endoperoxide Synthase 2 (PTGS2) upregulation in bone tissue (Figure 5k–m), indicating impairment of the GPX4 axis under diabetic conditions. Additionally, malondialdehyde, a terminal product of lipid peroxidation, is elevated in both serum and bone tissue of diabetic mice (Figure 5n and o). Collectively, these findings demonstrate that the hyperglycemic microenvironment triggers osteocyte ferroptosis via GPX4 inactivation and lipid peroxide accumulation. Targeting the ferroptotic axis to restore redox balance and rescue GPX4 expression is a promising therapeutic strategy for osteoporosis and diabetic bone defects. This strategy has been validated by recent research demonstrating that ferroptosis-targeted metal-phenolic nanocomposite hydrogels can integrate ferroptosis inhibition, antioxidation, immunomodulation, and osteoinduction for effective diabetic bone defect repair58. Additionally, dynamic DNA nanoconstruct-integrated hydrogels have been engineered to modulate macrophage polarization and ferroptosis simultaneously, promoting diabetic bone repair through dual-targeting mechanisms.³⁴

Figure 5.

The diabetic microenvironment induces osteocyte ferroptosis in vivo. (a) Experimental design timeline for STZ&HFD-induced diabetic mouse model. (b) Representative gross appearance of control (CTL) and STZ&HFD mice. (c) Micro-CT, X-ray, H&E, and TUNEL staining of bone tissues. (d–j) Quantitative analyses of bone microstructural parameters (BV/TV, Tb.Sp, Tb.N, Tb.Th), empty lacunae, and TUNEL-positive cells. * p < 0.05, ** p < 0.01. (k–m) Immunohistochemistry and Western blot analysis for GPX4 and PTGS2. (n–o) Plasma and bone malondialdehyde (MDA) levels. (p–r) Plasma levels of OCN, free fatty acids, and palmitic acid. Reprinted from [Yang et al, 2022],³³ under the terms of the Creative Commons CC BY license.

In oncological scenarios, the regenerative microenvironment is further disrupted as tumor cells hijack host cells to establish self-sustaining crosstalk.^47,48 This interaction is bidirectional: Tumor cells exhibit osteoblast-like behavior by producing bone matrix proteins while simultaneously secreting cytokines that activate bone-resorbing osteoclasts, driving a vicious cycle of tumor growth and bone destruction.⁴⁹ As illustrated in Figure 6, disseminated tumor cells localized to bone secrete parathyroid hormone-related protein (PTHrP), which stimulates osteoblasts to upregulate receptor activator of nuclear factor-κB ligand (RANKL). RANKL engages its receptor RANK on osteoclast precursors, promoting osteoclast differentiation and activation. Activated osteoclasts resorb bone matrix, releasing a reservoir of immobilized growth factors such as transforming growth factor-β (TGF-β), insulin-like growth factors, and bone morphogenetic proteins (BMPs). These growth factors further enhance tumor cell proliferation and promote PTHrP production. This feed-forward loop establishes a self-sustaining microenvironment wherein skeletal degradation and tumor expansion reinforce each other.

Figure 6.

The “vicious cycle” of bone metastasis. Reprinted from [Mohammad, K., 2025],³⁵ under the terms of the Creative Commons CC BY license.

How Do Nanomaterials Play a Role as Building Blocks?

To provide a systematic overview, the nanomaterial building blocks explored for bone regeneration are summarized in Tables 1–3. These tables categorize materials into rigid inorganic nanomaterials (Table 1), polymeric nanomaterials (Table 2), and inorganic piezoelectric nanomaterials (Table 3). For each category, key physicochemical properties, mechanisms of action, advantages, and limitations are outlined. Rather than providing an exhaustive list, this comparative presentation aims to guide rational material selection for smart reservoir systems discussed in subsequent sections. Recent comprehensive reviews have summarized the advances of various nanoparticles—including metal, polymer, and ceramic nanoparticles—in bone regeneration, highlighting their capacity to enhance osteogenesis, improve mechanical properties, and modulate immune responses.¹⁹

Table 1.

Rigid Inorganic Nanomaterials for Bone Regeneration

Material Type	Size/Features	Key Mechanisms	Advantages	Limitations	Ref.
Calcium phosphate (CaP) nanoparticles	20–200 nm, spherical/rod, negative	Mimic bone mineral; release Ca2+/PO43– ions; promote osteogenic (↑RUNX2, SP7, alkaline phosphatase [ALP], ColI)	Biocompatible, osteoconductive, biodegradable	Brittle, rapid resorption, variable degradation	[50–52]
Hydroxyapatite(HAP/nHAp)	20–200 nm, variable, negative	Bone mineral mimic; nanotopography promotes osteoblast maturation/mineralization (↑ALP and BMP-2)	Osteoconductive, biocompatible, promotes bone bonding	Slow degradation, brittle, and processing difficulty	[51, 53, 54]
Carbon nanotubes	1–100 nm diameter, tubular, negative	High aspect ratio mimics collagen; mechanical reinforcement; supports cell adhesion; functionalizable	High strength, conductivity, enhances scaffolds	Toxicity concerns, poor dispersion, and unclear biosafety	[55, 56]
Graphene oxide (GO)	Sheet (thickness ~1 nm), negative	Promotes adhesion; functionalizable via oxygen groups; delivers growth factors; osteoinductive	High surface area, versatile functionalization, promotes osteogenesis	Toxicity concerns, slow degradation, and aggregation	[57, 58]
Mesoporous silica nanoparticles	50–300 nm, spherical, negative/tunable	High drug loading; controlled ion/growth factor (GF) release; activates osteogenic signaling	Tunable pores, easy modification, biocompatible	Long-term biosafety unknown	[21, 59]
Bioactive glass nanoparticles	20–200 nm, variable, negative	Release Si4+/Ca2+; form HA layer; promote osteoblast proliferation/collagen secretion	Bone-bonding, osteogenic, degradable	Brittle, processing challenges	[60, 61]
Baghdadite(calcium silicate)	50–500 nm, variable, negative	Release Si4+; form HA layer; promote osteogenesis/angiogenesis	Degradable, osteogenic, releases bioactive ions	Limited in vivo studies, complex synthesis	[61–63]

Notes: ↑indicates upregulation or an increase in the corresponding physiological marker.

Table 2.

Polymeric Nanomaterials for Bone Regeneration

Material Type	Size/Features	Key Mechanisms	Advantages	Limitations	Ref.
Synthetic Polymers
Poly (lactic-co-glycolic acid) (PLGA)/Polycaprolactone (PCL)	100–1000 nm, biodegradable	Controlled drug delivery; tunable degradation	FDA-approved, biocompatible, processable	Inflammatory products, acidic byproducts	[37, 55, 64]
Conductive polymers (Polyaniline (PANI), PPy (Polypyrrole), Poly (3,4-ethylenedioxythiophene) (PEDOT))	Variable, electroactive	Transmit electrical signals; stimulate osteogenesis	Enhances cell signaling, electroactive	Poor biodegradability, potential toxicity	[65]
Piezoelectric polymers (Polyvinylidene fluoride (PVDF), Polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), Poly (L-lactic acid) (PLLA), Polyamides (PAs))	Variable, piezoelectric	Mimic bone electromechanics; stimulate osteogenesis without external power	No external power, flexible, biocompatible	Low piezoelectric coefficient, processing difficulty	[66]
Polyurethane (PU)	Variable, copolymer	Tunable mechanics; reduce stress shielding; incorporate nanoparticles for enhanced osteogenesis	Mechanically tunable, supports cells, and has a magneto-responsive potential	Potential inflammatory degradation	[21, 67, 68]
Polyethylene glycol (PEG) -based hydrogels with Magnetic nanoparticles (MNPs)	Variable, with Fe3O4	Magnetic stimulation promotes osteogenesis/angiogenesis	Non-invasive magnetic control promotes angiogenesis	Limited mechanical strength	[69]
Poly (methyl methacrylate) (PMMA)	Bone cement, 85–110 MPa	Mechanical support; nano-engineered with nHAp, boron nitride, bioactive glass	Clinically established, affordable, strong	Bioinert, non-degradable, exothermic, MMA toxicity	[70–72]
Polyglycolic acid (PGA)/Polylactic acid (PLA)/Polylactic acid-gelatin (PLA-GEL)	Nanofiber membranes	Promote cell proliferation/osteogenesis; nHA enhances osteoinductivity	Biocompatible, biodegradable, processable	Rapid degradation (PGA), hydrophobic (PLA)	[64, 73–75]
Natural Polymers
Chitosan	50–500 nm, positive	Antibacterial, anti-inflammatory, hydrophilic; promotes adhesion/osteogenesis; thermosensitive hydrogels	Biocompatible, biodegradable, mucoadhesive, injectable	Poor mechanical strength requires crosslinking	[67, 76–78]
Other natural polymers (collagen, cellulose, SF, alginate, and fucoidan)	Variable, with nanofillers (nHA, nZr, nSi, and GO)	Mimic ECM; promote adhesion/differentiation; enhance mechanical/biological properties	Biocompatible, biodegradable, abundant	Weak mechanics, batch variability	[21, 78]

Table 3.

Inorganic Piezoelectric Nanomaterials for Bone Regeneration

Material Type	Size/Features	Key Mechanisms	Advantages	Limitations	Ref.
Zinc oxide (ZnO) nanoparticles	5–50 nm, hexagonal, positive	Release Zn2+; antibacterial; piezoelectric; promote MSC osteogenesis; immunomodulatory	Multifunctional (osteogenic, antibacterial, piezoelectric), low cost	Cytotoxicity at high concentration.	[79, 80]
Barium titanate (BaTiO3)	50–500 nm, perovskite	Piezoelectric; generates an electric field; mimics bone impulses; enhances BMSC adhesion/osteogenesis via integrin α5β1	Lead-free, biocompatible, strong piezoelectric response	Processing challenges, brittle	[81–83]
Potassium sodium niobate	Nano-coated, piezoelectric	Generates potentials; activates Ca2+ channels; promotes osteogenesis/angiogenesis via force-electricity coupling	High piezoelectric coefficient promotes cell adhesion/proliferation	Emerging material, limited in vivo data	[84]

In addition to the rigid and polymeric nanoparticles outlined in the tables, electrospun nanofibers and bio-ink formulations have rapidly emerged as indispensable architectural building blocks in regenerative engineering. Electrospun nanofibers, owing to their inherently high surface-area-to-volume ratio and interconnected porosity, exquisitely mimic the hierarchical architecture of the native collagen ECM, providing profound topographical cues that govern stem cell elongation and osteogenic differentiation.²⁰ Simultaneously, the advent of 3D bioprinting has propelled the development of functional bio-inks—viscoelastic hydrogel formulations laden with bioactive nanomaterials and living cells. Nanocellulose and methacrylated gelatin-based bio-inks, for instance, confer exceptional shear-thinning properties for precise spatial printing while maintaining robust mechanical fidelity and cellular hydration post-crosslinking, effectively serving as customizable volumetric reservoirs for patient-specific defect geometries.⁸⁵

How are Nanocomposites and Smart Reservoirs Applied in Bone Regeneration?

Mimicking the Bone Structure

Bone tissue is a complex and dynamic composite material characterized by a highly organized hierarchical architecture primarily composed of inorganic hydroxyapatite (HAP) crystals embedded within an organic collagen fibril matrix.^1,2 Single-component materials frequently fail to replicate the complex biomechanical and biological demands of the skeletal system.⁸⁶ Advanced nanomedicine approaches facilitate the development of biomimetic constructs that emulate natural osteochondral tissue.²¹ In particular, biomimetic mineralization strategies that replicate the natural process of HAP formation on collagen templates under simulated physiological conditions have emerged as a powerful approach for generating scaffolds that closely mimic native bone composition and nanostructure.^87,88

To reproduce the viscoelastic properties of the bone ECM, Luo et al developed a biomimetic approach using chitin and chitosan whiskers.⁸⁹ Through precise acid hydrolysis and ultrasonic treatment, these nanoscale whiskers form a distinct liquid-crystal phase, structurally and functionally mirroring the dense collagen networks of natural bone.⁸⁹ This liquid-crystalline organization is reminiscent of the cholesteric architecture found in natural bone matrices, where hierarchical ordering contributes to exceptional mechanical performance.⁹⁰ By integrating this supportive liquid-crystalline hydrogel into robust 3D-printed poly(L-lactic acid) frameworks, the resulting hierarchical nanocomposite withstands rigorous mechanical loading. Moreover, this structure provides an optimized 3D microenvironment that vigorously supports host cell proliferation and targeted osteogenesis-angiogenesis coupling.⁹¹

Nano Bone Paste

Clinical scenarios often exhibit irregular geometries, complex osseous voids in which rigid, pre-formed implants fail to establish adequate physical contact with the surrounding host tissue.⁸⁸ Scaffold-free calcium nanomaterials formulated as injectable nanopastes or aqueous nanosuspensions offer a highly adaptable and minimally invasive alternative for treating complex osteoporotic or traumatic bone defects.⁹² These advanced nanoformulations possess significantly higher drug-loading capacities than solid ceramics and macroscopic bioglasses traditionally used in rigid bone scaffolds, allowing for concentrated, localized therapies.²¹ Epple et al developed a bone regeneration paste by coating synthetic CaP nanocrystals with nucleic acids (DNA). Following injection, the nanoparticles are internalized by cells; subsequent CaP dissolution releases DNA that stimulates the formation of BMP-7, which promotes bone formation, and VEGF-A, which stimulates angiogenesis. This process results in enhanced vascularized bone regeneration.⁹³ Such moldable nanocomposites are easily metabolized in vivo, exhibit remarkably faster local dissolution kinetics in situ, and allow immediate, flawless structural conformability within the irregular defect cavity. Moreover, the intrinsic antacid properties of specific alkaline nano-precipitates can help neutralize the acidic microenvironment generated by overactive osteoclasts during excessive bone resorption. Incorporating these therapeutic nanoparticles into adaptable hydrogel networks creates multifunctional composite systems that seamlessly conform to irregular anatomical defects while simultaneously mimicking the natural ECM.⁹² These adaptable reservoirs serve as highly efficient localized drug delivery vehicles, directly acting as nano-drugs to effectively inhibit pathological osteolysis and induce robust, long-term osteogenesis directly at the surgical site.

Nanohydrogels as Smart Reservoirs

While isolated nanoparticles and free biological vesicles possess immense therapeutic potential, their direct systemic or localized administration is often limited by physiological barriers.²² Bare nanoparticles often exhibit inherently limited biocompatibility, potentially triggering severe immune rejection and prolonged inflammatory cascades upon implantation.²² Furthermore, their physical properties prevent the establishment of stable, sustained drug-release profiles. This frequently results in rapid therapeutic clearance or initial burst release that fails to provide long-term biological signaling.^94,95 To overcome these limitations, researchers have explored integrating nanoscale therapeutic agents into hydrogel matrices.⁹⁵ These supramolecular networks act as a highly specialized local “smart reservoir” that regulates release kinetics while providing the indispensable structural stability for firm nanoparticle immobilization within the defect cavity.

Hydrogel networks composed of biomaterials such as hyaluronic acid, gelatin, or alginate closely mimic the natural ECM. These systems provide an adaptable microenvironment that actively supports cellular infiltration, vascularization, and ultimate tissue integration.^96,97 Among these platforms, gelatin methacryloyl (GelMA) hydrogels have attracted particular attention due to their tunable mechanical properties, photocrosslinkability, and inherent cell-adhesive arginine-glycine-aspartic acid (RGD) motifs that promote osteogenic cell attachment and function.^55,98 Stimuli-responsive systems such as hydroxybutyl chitosan-based thermosensitive hydrogels have demonstrated exceptional clinical utility. These dynamic smart reservoirs exist as a fluid sol at room temperature, allowing for minimally invasive injectability that perfectly conforms to complex, irregular bone defect geometries. Upon reaching physiological body temperature, they rapidly undergo a sol-to-gel transition, forming a structurally stable, porous 3D matrix. A recent study verified that delivering osteoinductive agents via this thermosensitive hydrogel significantly accelerated osteoporotic bone regeneration through sustained release kinetics, highlighting its advanced capability to remodel the local metabolic microenvironment and promote synergistic healing.⁹⁹ Moreover, mussel-inspired adhesive hydrogels incorporating alendronate-loaded mesoporous silica nanoparticles have demonstrated the capacity for sustained co-release of silicon ions and alendronate, effectively steering macrophage polarization from the pro-inflammatory M1 phenotype toward the pro-healing M2 phenotype for robust bone regeneration in complex defects.¹⁰⁰

Exosomes as Biological Carriers

Within these intelligent delivery networks, biologically derived nanovesicles known as exosomes have emerged as powerful therapeutic tools. Exosomes are natural extracellular vesicles (30–150 nm) that transport bioactive cargo, including lipids, specific microRNAs, and potent osteogenic growth factors, directly to recipient cells.¹⁰¹

Recent studies have elucidated the precise transcriptomic mechanisms used by these vesicles. For instance, exosomes secreted by adipose-derived stem cells have been demonstrated to modulate macrophage phenotypic switching through the highly concentrated delivery of miR-451a.¹⁰² Upon internalization by macrophages, miR-451a suppresses macrophage migration inhibitory factor (MIF), effectively silencing M1 pro-inflammatory pathways and promoting the pro-regenerative M2 polarization necessary for bone healing.¹⁰³

Mammalian cell-derived exosomes face translational challenges due to low isolation yields, inherent batch heterogeneity, and complex ethical regulations.¹⁰⁴ Consequently, recent research has increasingly focused on plant-derived exosomes (P-Exos). Extracted from abundant natural sources, P-Exos circumvent all mammalian-associated manufacturing hurdles while delivering unique phytoactive metabolites and cross-kingdom regulatory RNAs.^105,106 P-Exos-like nanoparticles from sources such as Epimedium and celery exhibit potent osteogenic activity. Epimedium-derived exosomes loaded into GelMA hydrogels significantly enhanced MC3T3-E1 preosteoblast proliferation, migration, and mineralized nodule formation while reducing senescence markers and promoting angiogenesis. These effects are mediated by activation of the PI3K/Akt signaling pathway and upregulation of RUNX2, TGF-β1, and COL1A1.⁵⁵ Celery-derived exosome-like nanoparticles have been demonstrated to promote BMSCs’ osteogenic differentiation and accelerate mouse cranial bone defect regeneration by inducing glycolysis via the PI3K/LDHA pathway.¹⁰⁷ When encapsulated within intelligent hydrogels, P-Exos consistently demonstrate a strong capacity to upregulate Wnt/β-catenin pathways, inhibit excessive RANKL-mediated osteoclastogenesis, and accelerate the formation of critical H-type blood vessels within ischemic bone defects. A strategy has been proposed a “spatiotemporal programming” framework for engineered exosome delivery via smart scaffolds, systematically integrating cargo engineering strategies (eg, hypoxic/inflammatory priming, genetic modification) with smart biomaterial design (eg, stimuli-responsive hydrogels, core-shell scaffolds) to achieve sequential release aligned with the intrinsic immuno-angiogenic-osteogenic cascade.¹⁰⁸

Artificial Intelligence (AI)-Assisted 3D Printed Scaffolds

AI and machine learning (ML) are increasingly used to optimize the distribution of nanomaterials within reservoirs and predict their release kinetics.¹⁰⁹ While conventional 3D printing technology enables the spatial distribution of nanoparticles within macroscopic scaffolds,¹¹⁰ the field is rapidly advancing beyond simple additive manufacturing through the integration of AI and ML.¹¹¹ AI-driven generative design algorithms can computationally model and predict the optimal architectural parameters of the scaffold, including pore size, interconnectivity, and mechanical stress distribution. These approaches enable the implant to be tailored to the biomechanical demands of the patient’s individual defect.¹¹²

ML frameworks, particularly neural networks trained on finite element simulation data, can now predict scaffold stiffness and porosity solely from geometrical features. This capability enables inverse design optimization to replicate the mechanical gradients between cortical and trabecular bone.¹¹³ For instance, a back-propagation artificial neural network model combined with finite element analysis has achieved high accuracy (R² = 0.9991) in predicting the mechanical response of 3D-printed lattice scaffolds, demonstrating the effectiveness of hybrid approaches for optimizing scaffold architecture.¹¹⁴ AI enables inverse design of scaffolds with patient-specific geometries and mechanical gradients, enhancing fabrication reproducibility and predictive modeling of clinical outcomes. A study developed an integrated computational framework combining finite element analysis (FEA) and artificial neural networks (ANN) to predict the mechanical behavior and degradation kinetics of 3D-bioprinted bone scaffolds. By leveraging this data-driven machine learning approach, the predictive accuracy for scaffold Young’s modulus improved substantially, reducing the relative prediction error to under 5%. This hybrid AI approach enables the precise inverse design of microstructures that perfectly match the patient-specific load-bearing requirements and porosity gradients of the host site.¹¹⁵ Recent research has further demonstrated that 3D-printed PLA scaffolds functionalized with dual small molecule-loaded chitosan nanoparticles containing DIPQUO and GS4012 can effectively support both osteogenic and angiogenic responses104, while 3D-printed neobavaisoflavone-coated scaffolds have been shown to regulate osteoblast/osteoclast activities for enhanced bone regeneration.¹¹⁶

The paradigm is shifting toward 3D printing. By integrating smart shape-memory polymers and light-curable methacrylated resins, 3D-printed scaffolds can dynamically alter their geometric morphology over time in response to specific physiological stimuli (body temperature variations or local pH shifts).¹¹⁷ This dynamic structural adaptability ensures that the implant maintains perfect physical conformability within the defect cavity even as the native bone undergoes continuous remodeling, thereby preventing stress shielding and ensuring uninterrupted mechanical support.¹¹⁸ Recent advances in AI-driven 3D printing technologies are paving the way for next-generation regenerative solutions that combine precise anatomical reconstruction with dynamic responsiveness to the healing microenvironment.¹¹⁴ A dual-immunomodulatory bone scaffold (DIBS) integrating pH-responsive hydrogels with 3D-printed frameworks has recently been developed to sequentially regulate the immune functions of macrophages and MSCs for repairing critical-sized bone defects, representing a paradigm-shifting approach to spatiotemporally programmed bone regeneration.¹¹⁹

Electrospun Nanofibers as Architectural Scaffolds

Electrospun nanofibers architecturally replicate the anisotropic topographical cues of native collagenous ECM.¹²⁰ Aligned fiber topography promotes BMSC osteogenic differentiation via cytoskeletal-nuclear remodeling through the Rap1/YAP pathway,¹²¹ while biomimetic periosteal ECM/PCL aligned nanofiber membranes activate the ITGB1/PI3K/AKT pathway.¹²² Beyond directing stem cell fate, aligned topographies actively dictate macrophage polarization; aligned electrospun scaffolds have been shown to inhibit LPS-induced M1 polarization and promote the pro-reparative M2 phenotype.^120,123 Functionalization with bioactive nanoparticles further amplifies their capacity as smart reservoirs. Electrospun carbon nanofibers decorated with hydroxyapatite nanocrystals exhibit enhanced osteoconductivity and sequester pro-inflammatory cytokines in both in vitro and in vivo models.¹²³ The transition from 2D membranes to 3D constructs—via gas foaming, short nanofiber assembly, or electrospinning–3D printing integration—enables robust vascular and immune cell infiltration.¹²⁰ A one-step gas foaming strategy has been employed to fabricate 3D scaffolds decorated with strontium nanoparticles (3DS-Sr), which function as dynamic reservoirs; sustained Sr²⁺ release downregulates M1 macrophage activity while promoting angiogenesis through HIF-1α/ERK activation and osteogenesis via Wnt/β-catenin.¹²⁰ In vivo, aligned electrospun porous P34HB/lecithin scaffolds achieved a bone volume fraction of 38.7% ± 3.2% at 4 weeks in a rat calvarial defect model, significantly outperforming random fiber controls, with lecithin coating reducing the water contact angle from 112.3° ± 3.5° to 48.6° ± 2.1° to facilitate protein adsorption and immune homeostasis.¹²⁰

3D-Printable Bio-Inks for Patient-Specific Regeneration

3D bioprinting enables patient-specific deposition of cells and biomaterials into anatomically customized constructs. A breakthrough is the thermoreversible ionic-covalent entangled (TRICE) bioink—comprising gelatin and κ-carrageenan—which mimics bone ECM, supports >92% cell viability, and achieves a compressive modulus of ~33.2 MPa when combined with calcium phosphate ink, representing 200-fold greater strength than hydrogel-only constructs.¹²⁴ Mechanistically, TRICE activates MAPK/ERK-mediated osteogenesis through enhanced focal adhesion and prevents stress-shielding-induced inflammation via reversible ionic crosslinks; in rabbit femoral condyle models, these scaffolds promoted de novo bone formation within 8 weeks.¹²⁴ Nanomaterial incorporation provides stimuli-responsive functions. Alginate/methyl cellulose bio-inks with 2% (w/v) UiO-66 MOF nanocrystals upregulate Runx-2, COL-I, and OCN expression, while the porous architecture of UiO-66 enables localized ROS scavenging to protect cells from oxidative damage.¹²⁵ Concentration optimization is critical: in GelMA/gelatin/nanocellulose bio-inks, 1% (w/v) nanohydroxyapatite supports osteogenic protein production (Runx-2, COL-I, DMP1, OCN), whereas 5% (w/v) impairs photocrosslinking, reduces viability, and limits differentiation.¹²³ In situ bioprinting—directly depositing bio-inks into defect sites during surgery—represents the clinical frontier; a pivotal study applied a standard 3D bioprinter to perform in situ bioprinting over critical-sized cranial defects in live rabbits under anesthesia, demonstrating successful bone regeneration.¹²⁶ Silk-gelatin bio-inks further activate canonical Wnt/β-catenin and Indian hedgehog pathways during MSC osteogenesis,¹²⁷ while nanoengineered ionic-covalent entanglement (NICE) bioinks combine nanocomposite reinforcement with ionic-covalent entanglement to print cell-laden constructs with high structural fidelity, maintaining cell viability over 120 days.¹²⁸

What are the Release Mechanisms of Smart Reservoirs During Bone Regeneration?

The principal advantage of these supramolecular networks lies in their capacity to respond to pathological changes in the bone defect microenvironment.¹²⁹ Advanced triggerable nanomaterials are designed to be highly responsive to internal physiological stimuli or external physical triggers. These systems enable highly synchronized, on-demand release of therapeutic medications at the time and location where they are biologically required.

pH-Stimuli-Responsive Release

Pathological bone defects, particularly those with bacterial infections or tumor-induced osteolysis, are characterized by mild acidity from metabolic byproducts such as pyruvate and lactate, alongside hypersecretion by overactivated osteoclasts.¹³⁰ This localized acidification can be exploited by pH-responsive smart reservoirs to trigger therapeutic release. Calcium-based nanomaterials, functioning essentially as alkaline precipitates, can be integrated into these hydrogels to actively neutralize the acidic microenvironment, thereby suppressing pathological osteoclast activity and shifting the localized milieu back toward physiological homeostasis.¹³¹ Liu et al developed a resorbable guided bone regeneration membrane (PCL/DEX@Ca-Zol) based on a pH-sensitive zeolite imidazole framework that encapsulates calcium ions, zoledronic acid, and dexamethasone. This system is engineered to release its therapeutic components under acid-triggered conditions present within bone defects. This system synergistically regulates the bone microenvironment through cascading effects by neutralizing pH, enhancing osteogenic differentiation and mineralization, and inhibiting osteoclast activity in a single coordinated manner, ultimately achieving enhanced bone regeneration with minimized side effects. A structure-evolving bone bioadhesive (PG/Sr-ALN) has recently been developed to enable temporally programmed osteoimmune modulation, wherein pH-responsive Sr²⁺ release promotes M2 macrophage polarization in the early inflammatory stage for osteoporotic bone defect repair.¹³²

ROS-Stimuli-Responsive Release

In patients with chronic metabolic disorders such as diabetes mellitus, the bone healing microenvironment is compromised by hypoxia and excessive ROS accumulation, which impair stem cell differentiation and exacerbate tissue destruction.¹⁷ As the hydrogel network scavenges ROS, it undergoes controlled structural degradation, subsequently releasing crucial repair factors. This process also induces immunomodulatory shifts, including increased population of CD206-positive M2 macrophages.¹³³ This highly coordinated ROS-responsive degradation eradicates the primary oxidative threat. Furthermore, it upregulates osteogenic pathways, effectively breaking the vicious cycle of inflammation and oxidative stress to substantially enhance bone regeneration.¹⁷ A representative example involves a bimetallic metal-organic framework-derived nanoenzyme loaded with alendronate and Mg²⁺ to regulate the hyperglycemic microenvironment and restore osteogenesis/osteoclast homeostasis.¹³⁴ The incorporation of Mn atoms into the Co₃O₄ structure optimized the electronic configuration, significantly enhancing superoxide dismutase- and catalase-like activities for efficient ROS scavenging.¹³⁴ Encapsulation of this nanoenzyme within a glucose-responsive borate ester-based hydrogel enabled successful cranial defect regeneration in diabetic rats by alleviating glucose-ROS-induced inflammation while promoting angiogenesis and osteogenesis.¹³⁴

Enzyme-Stimuli-Responsive Release

Enzyme-responsive architectures are engineered to respond dynamically to the distinct histopathological alterations characterizing the advancing stages of tissue regeneration.¹³⁵ An enzyme-responsive, biodegradable scaffold incorporating nanodendrimers was engineered via click chemistry.¹³⁵ Its polyphosphoester backbone is specifically cleaved by ALP—an enzyme upregulated during bone formation—ensuring that scaffold degradation aligns with the pace of new tissue generation.¹³⁵ In vitro, the material supported stem cell differentiation and degraded in response to ALP, while in vivo rat models confirmed successful bone regeneration and scaffold integration.¹³⁵ Another study developed hyaluronic acid-based hydrogel microspheres incorporating ZnO nanoparticles for treating infectious bone defects.¹³⁶ These microspheres exhibit enzyme-responsive behavior, with antimicrobial peptide release significantly enhanced in the presence of hyaluronidase, an enzyme overexpressed at infected sites. The system demonstrated antibacterial effects against Staphylococcus aureus. It promoted osteogenic differentiation in vitro, while in vivo validation in rat skull defects confirmed enhanced bone regeneration, angiogenesis, and anti-inflammatory responses.¹³⁶

How Do Smart Reservoirs Affect Osteoimmunomodulation and Bone Homeostasis?

Macrophage Polarization

The contemporary evaluation of BTE success is no longer solely determined by direct osteogenic capacity. Instead, it increasingly depends on coordinated modulation of the local immune microenvironment. Following severe skeletal injury, the immediate influx of immune cells establishes an acute inflammatory phase, during which recruited macrophages predominantly exhibit a pro-inflammatory M1 phenotype responsible for clearing apoptotic cells and pathological debris.¹³⁷ However, prolonged M1 predominance maintains a hostile inflammatory local environment that hinders the biological transition toward the crucial repair phase, frequently resulting in delayed healing or chronic nonunion. Engineered smart reservoirs strategically intervene during this fragile temporal window by executing sustained and localized release of specific immunomodulatory agents.²⁶ To orchestrate this transition, recent experimental designs have utilized liposome-based nanocarriers for targeted microenvironmental delivery. A study utilizing clodronate liposomes demonstrated that precisely depleting hyperactive M1 macrophages in the acute inflammatory phase preserves the cytokine-mediated coordination of early osteogenesis and angiogenesis, highlighting the necessity of temporal immunomodulation in extraction socket healing and bone regeneration.¹³⁸ Li et al developed an exosome-releasing hydrogel that promoted macrophage polarization from the pro-inflammatory M1 phenotype to the pro-regenerative M2 phenotype.¹⁰² The underlying mechanism involves exosomal miR-451a targeting MIF to regulate macrophage phenotype.¹⁰² In a rat calvarial defect model, this approach achieved near-complete bone regeneration with 95% defect coverage.¹⁰² An alternative immunomodulatory strategy based on inorganic nanomaterials has demonstrated remarkable efficacy in reshaping the compromised osteoporotic microenvironment.¹³⁹ Calcium-aluminum layered double hydroxide nanosheets functionalized with calcein were displayed to physically neutralize local acidity while simultaneously inducing M2 macrophage polarization via activation of the c-Maf signaling pathway.¹³⁹ This targeted cellular reprogramming triggered abundant secretion of anti-inflammatory cytokines IL-10 and TGF-β, facilitated recruitment of regulatory T cells, and suppressed TH17-mediated inflammatory cascades. In an osteoporotic rat model, this approach increased BV/TV from 6.2% to 10.7%, validating the therapeutic potential of combined acid-neutralization and immune-regulation for osteoporosis reversal.¹³⁹ MSC-mimicking nanovesicle-embedded bio-adhesive hydrogels have recently been developed for dual immunomodulation and osteogenesis, demonstrating matrix metalloproteinase (MMP)-responsive release that attenuates MMP activity and regulates the immune microenvironment for maxillofacial bone regeneration.¹⁴⁰ Additionally, a nanoengineered coating with dual antioxidant and immunomodulatory functions on titanium implants has been shown to promote osteoregeneration in osteoporotic conditions.¹⁴¹

Reprogramming Intercellular Communication

Bone remodeling depends on a finely regulated dynamic equilibrium between osteoclast-mediated bone resorption and osteoblast-mediated bone formation.^142,143 This equilibrium is maintained through complex intercellular communication networks within the bone marrow niche, involving coordinated interactions among skeletal cells, immune populations, and vascular components.¹⁴⁴ Smart nanomaterial reservoirs actively reprogram cellular communication networks. By deploying bioactive nanovesicles and localized therapeutics that mimic paracrine signaling, these systems can restore disrupted intercellular crosstalk.^145,146

Recent advances have demonstrated that exosomes derived from bone marrow MSCs and other bone-resident cells play a pivotal role as mediators of intercellular communication within the bone microenvironment. These nanoscale vesicles traverse the extracellular space to transport complex combinations of functional proteins, lipids, and nucleic acids directly into specific target cells. This material transfer significantly regulates skeletal metabolism and effectively modulates intercellular communication circuits that have been severely disrupted by trauma or metastatic tumor cell invasion. Notably, exosomes participate in several key regenerative processes, including immunomodulation, osteogenesis-angiogenesis coupling, and the osteoclast-osteoblast balance, underscoring their central role in coordinating the multicellular network required for effective bone regeneration.¹⁴⁷ Exosomes derived from Krt14⁺Ctsk⁺ periosteal cells have demonstrated the capacity to promote both osteogenic differentiation of BMSCs and angiogenesis of endothelial cells, illustrating the multifunctional nature of exosome-mediated intercellular signaling in bone regeneration.¹⁴⁶ When incorporated into degradable polymeric matrices or supramolecular hydrogel, these released exosomes and accompanying engineered nanoparticles can simultaneously target endothelial cells and osteoprogenitors, thereby stimulating osteogenesis-angiogenesis coupling.¹⁴⁸ Controlled degradation of these functionalized nanocomposites releases potent pro-angiogenic factors, such as VEGF, alongside critical osteogenic signals, facilitating the formation and maturation of vascular networks capable of supplying indispensable oxygen and nutrients to newly forming bone callus. Furthermore, specialized bioactive elements integrated into these reservoirs, such as copper- and manganese-doped borosilicate nanoparticles, have demonstrated a dual capability to enhance the autonomous self-regulation of stem cell osteogenic differentiation while enhancing the proliferation of endothelial precursors required for vascular regeneration.¹⁴⁹ By bridging the physical and biochemical communication gaps between the immune system, the regenerating vascular network, and the bone matrix, these smart spatiotemporal delivery systems effectively restore the pathological uncoupling caused by severe disease and promote coordinated tissue regeneration. Biomimetic nanoparticles decorated with EphB4 have recently been developed to enhance osteoclast targeting and therapeutic effect in osteoporosis through interfacial nanoengineering that translates complex receptor-guided biological interactions into stable, high-performance nanotherapeutics.¹⁵⁰

The Future Prospective

Despite their regenerative potential, the clinical translation of advanced nanomaterials is hindered by concerns about long-term biocompatibility and systemic toxicity.¹⁵¹ The biological safety profile of engineered nanoparticles is intricately determined by a complex interplay of specific physicochemical characteristics, including core dimensions, morphological shapes, inherent surface charge, and dynamic aggregation states within physiological fluids.¹⁵² High cellular uptake of metallic nanoparticles can trigger dose-dependent cytotoxicity.¹⁵³ To mitigate these risks, surface engineering strategies are employed to mask the reactive nanoparticle core, reducing immune recognition while preserving therapeutic efficacy.^154,155

A key trend is the development of theranostic nanoplatforms, which integrate diagnostic imaging with targeted therapy within a single construct. By labeling exosome membranes and hydrogel matrices with fluorophores, magnetic coatings, or bioluminescent markers, researchers can monitor biodistribution, cellular uptake, and degradation in real time, providing essential pharmacokinetic data for regulatory approval.^156–158 For instance, accumulation of calcium-rich nanomaterials generates high-attenuation computed tomography (CT) signals through targeted tissue calcification, assisting surgeons in verifying tumor-free margins during bone sarcoma resection.^155,159 As diagnostic resolution and molecular profiling capabilities advance, future efforts will prioritize personalized nanomedicines tailored to each patient’s pathological microenvironment and genetic landscape.¹⁶⁰

In conclusion, the deployment of nanomaterial reservoirs—ranging from 3D bioprinted bio-inks and structural nanofibers to stimuli-responsive nanohydrogels—represents a profound paradigm shift. These intelligent systems successfully overcome the inherent biological and mechanical limitations historically associated with traditional metallic implants and structural grafts. By orchestrating the “immune dialogue” and correcting complex metabolic dysregulations like diabetes and osteoporosis, such advanced nanomedicines seamlessly bridge the critical physiological gap between early inflammation resolution and long-term mature bone matrix formation. Resolving the prevailing multidisciplinary challenges surrounding standardized manufacturing, clinical tracking, and rigorous long-term biosafety validation will inevitably propel these engineered smart reservoirs into clinical practice, resulting in a therapeutic approach for the functional regeneration of critical bone defects.

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Disclosure

The authors declare that they have no competing interests.