Nanogel-Based Precision Bone Regeneration: Rational Design, Biological Barrier Penetration, and Osteoporotic Microenvironment Remodeling

Abstract

The pathogenesis of osteoporosis is primarily associated with impaired communication between osteoblasts and osteoclasts, leading to disrupted bone homeostasis. Conventional biomaterials mainly rely on macroscopic structural support and are limited in their ability to precisely regulate the complex bone microenvironment and overcome biological barriers. As representative soft matter nanoplatforms, nanogels possess unique viscoelastic mechanical properties and adaptive biointerfacial properties, offering new opportunities to overcome physical barriers within bone tissues. Moving beyond previous studies that primarily focused on the passive structural support provided by macroscopic hydrogels, this review presents a micro–nano–bio interfacial perspective to systematically elucidate the roles of nanogels in precision therapy for bone disorders. Considering the spatial constraints of the lacunar–canalicular system (LCS), with canalicular diameters of approximately 100–300 nm, we highlight how the stress relaxation behavior and deformation modulus of nanogels jointly determine their migration and penetration efficiency within confined spaces. Furthermore, nanogels can function as dynamic biomimetic systems that sense alterations in the bone microenvironment and actively regulate immune and metabolic homeostasis within bone tissue. This review summarizes engineering strategies for nanogel development, including optimization of network architectures, hierarchical surface targeting, and multi-responsive drug release mechanisms. In addition, the effects of protein corona formation and biological barriers on the in vivo fate and therapeutic performance of nanogels are critically discussed. Finally, from a translational perspective, we evaluate the therapeutic potential and key challenges of nanogels in precision bone regeneration and propose design principles for next-generation bone-targeted nanomedicines based on mechanical adaptation, biointerface engineering, and intelligent responsiveness.

Graphical Abstract

Introduction

The pathogenesis of osteoporosis is primarily characterized by disruption of the dynamic balance between osteoblasts and osteoclasts during bone remodeling. This imbalance is characterized by insufficient bone formation and excessive bone resorption, ultimately leading to dysregulation of the bone microenvironment.¹ This systemic imbalance not only increases bone fragility but also makes fragility fractures a major complication. As a result, osteoporosis imposes a substantial burden on both patients and society. Traditional rigid filling materials, such as bone cement, can provide immediate mechanical support. However, their effects are mainly limited to the bone surface or bone marrow cavity. They are therefore unable to effectively regulate osteocyte-mediated processes within deep bone tissues.² Therefore, the treatment of osteoporotic bone defects requires innovative strategies that can penetrate deep into lesion sites and precisely regulate cellular behavior.

Compared with conventional rigid nanomaterials, such as hydroxyapatite and mesoporous silica, these rigid materials are more likely to cause a spatial blocking effect within dense bone structures. This limitation restricts their diffusion into the deeper regions of the bone matrix.³ To overcome the mechanical and biofunctional limitations of single-component materials, multi-component hybrid composites integrating inorganic bio-minerals (eg, hydroxyapatite or CaCO₃), carbon nanomaterials (eg, reduced graphene oxide), and polymeric or hydrogel matrices have been extensively engineered to achieve superior fatigue resistance, intrinsically antibacterial properties, and sustained localized drug delivery for bone and dental tissue repair.^4–8 Nanogels have a high water content and a tunable network structure. They exhibit tissue-like mechanical adaptability. This allows them to deform reversibly in confined spaces and achieve penetration across different scales.

Although various soft nanomaterials exhibit favorable biocompatibility and mechanical adaptability, nanogels possess distinct advantages over conventional soft carriers, such as liposomes, micelles, and linear polymer chains, owing to their unique three-dimensional network architecture that enables more stable delivery within complex bone microenvironments.^9,10 Liposomes and micelles primarily rely on molecular self-assembly to maintain their structural integrity. However, their stability can be compromised by factors present in the bone marrow microenvironment, including fluid shear stress, ionic fluctuations, and protein adsorption, thereby limiting their delivery efficiency to deep bone regions.^11,12 In contrast, linear polymer chains exhibit high flexibility but lack a continuous network framework, making it challenging to simultaneously maintain structural stability and cargo protection when navigating through the confined lacunar-canalicular system (LCS). Nanogels overcome these limitations by integrating nanoscale flexibility with the structural integrity provided by crosslinked polymer networks, enabling adaptive deformation under mechanical constraints while preserving the encapsulated drugs or bioactive molecules.¹³ Notably, their tunable viscoelastic properties, including shear-thinning behavior and stress relaxation capacity, allow nanogels to respond to mechanical confinement and undergo reversible deformation, thereby facilitating migration through the bone canalicular network. Therefore, the synergistic integration of structural stability and mechanical adaptability endows nanogels with unique potential as advanced delivery platforms for accessing deep bone microenvironments and provides new design principles for precision bone-targeted therapeutics.

As highly hydrated crosslinked networks, the key value of nanogels lies in their deformability and interfacial adaptability, rather than simply their drug-loading capacity. Through network rearrangement, nanogels can be compressed in confined spaces. This enables them to pass through the osteocyte LCS (approximately 100–300 nm in diameter). As a result, they shift from surface deposition to deep penetration into the bone matrix, where they can directly interact with osteocytes and bone marrow immune cells.^14,15

However, penetration ability does not necessarily indicate effective delivery. During deformation, the network structure can be easily disrupted. Under compression or shear, premature payload release may occur. When the crosslinking density is insufficient, this issue becomes more pronounced during bone tissue penetration. Such instability may lead to drug redistribution or abnormal local concentrations. Therefore, there is a structure–property coupling relationship between deformability and structural stability. Its optimization relies on the coordinated regulation of network parameters, rather than improvement of a single factor.

In addition, although ligand modifications such as alendronate can improve bone affinity, nanogels still rapidly form a protein corona after entering biological fluids. The composition of this corona is determined by the local microenvironment rather than the designed surface structure.^16,17 The protein corona may improve the stability of the system. However, it can also mask the original ligands and alter the in vivo distribution pathway.¹⁸ In an immune-active bone microenvironment, an increased proportion of opsonins can enhance clearance by the mononuclear phagocyte system. In this case, nanogels tend to shift from bone targeting to accumulation in the reticuloendothelial system. This reduces the efficiency of effective delivery and may also disturb the local immune state.

Overall, nanogels offer new possibilities for modulating the osteoporotic microenvironment. However, their in vivo performance depends on dynamic changes after exposure to biological systems. Rather than focusing only on penetration ability, it is more critical to maintain stability during delivery. Structural integrity during deformation and prevention of premature payload release are key factors. These remain major scientific challenges that determine therapeutic efficiency.

Although numerous reviews have summarized the applications of macroscale hydrogels in bone tissue engineering, mainly focusing on their bulk mechanical properties and structural roles as scaffolds for bone regeneration,^19,20 a systematic understanding of the dynamic behaviors of soft nanogels at the nano-bio interface and their interactions with the bone microenvironment remains limited. Therefore, this review extends beyond the conventional scaffold functions of hydrogels and focuses on the microenvironment-responsive regulation of bone-targeted nanogels, with particular emphasis on their behavior within complex biological barriers. Specifically, this review discusses how nanogels adapt their structures to confined spaces and overcome physiological barriers, such as the LCS, through deformation-mediated transport. In addition, the role of nanogel viscoelasticity and stress relaxation dynamics in regulating cellular responses and tissue microenvironmental homeostasis is examined. The contribution of protein corona formation and remodeling to the in vivo fate and biological interactions of nanogels is also highlighted. By integrating material architecture, mechanical properties, and biointerface interactions, this review provides design considerations for developing bone-targeted nanotherapeutic platforms and offers insights into the development of adaptive nanobiomaterials for bone regeneration (Figure 1).

Figure 1.

Schematic illustration of engineered bone-targeted nanogel systems integrating adaptive delivery, bone homeostasis regulation, theranostic functions, and clinical translation.

Smart Nanogel Engineering for the Bone Microenvironment

Topological Structure and Mechanical Adaptability

The therapeutic potential of nanogels in osteoporosis arises from their mechanical adaptability, a characteristic derived from their soft-matter properties. Quantitatively, nanogels typically exhibit Young’s moduli ranging from 10¹ to 10³ kPa. This is far lower than the GPa-level rigidity of the bone matrix. It is also clearly different from conventional rigid nanoparticles, such as mesoporous silica and gold nanoparticles.²¹ This mechanical mismatch allows nanogels to act as deformable buffers in confined spaces. They achieve penetration across different scales through network compression and rearrangement, rather than rigid obstruction. Although osteoporosis leads to trabecular bone loosening and expansion of the bone marrow cavity, the osteocyte LCS still maintains a physical size constraint of approximately 100–300 nm.²² At this scale, rigid nanomaterials are prone to geometric trapping. In contrast, nanogels can achieve efficient penetration by reducing their transient modulus and undergoing anisotropic deformation.

Through thermoresponsive polymer design, such as PNIPAM-based networks, nanogels can remain compact during circulation. After reaching bone tissue, changes in the local microenvironment reduce their elastic modulus. This enables deformation-mediated penetration into deep regions of the bone matrix.²³ The penetration ability of nanogels depends not only on particle size. Rather, it is regulated by their elastic reconfiguration capability, which is jointly determined by the effective mesh size (ξ) and crosslinking density.²⁴ By tuning the monomer ratio and crosslinking strategy, nanogels can undergo reversible anisotropic deformation under compressive conditions in the lacuno-canalicular system. At the same time, they can maintain payload stability.

By adjusting monomer composition, crosslinking strategies, and network architecture, nanogels can undergo reversible anisotropic deformation within confined spaces while maintaining effective delivery of encapsulated drugs or bioactive molecules.²⁵ However, optimization of nanogel properties often requires balancing structural stability, degradation behavior, and tissue adaptability. Increasing crosslinking density improves network stability and reduces premature cargo leakage but may restrict molecular diffusion and delay degradation. Conversely, insufficient crosslinking facilitates tissue remodeling but compromises mechanical integrity and causes uncontrolled swelling or drug release. Therefore, crosslinking density should be optimized together with network topology and degradation-responsive linkages to balance penetration, delivery stability, and regenerative remodeling.

Mechanisms of Lacuno-Canalicular Penetration and Stress Relaxation

The ability of nanogels to overcome the spatial confinement of the LCS is mainly determined by the synergy between their microscopic mesh size and stress relaxation dynamics. Based on hindered diffusion principles described by the Renkin equation, the penetration efficiency of nanogels is no longer limited by their apparent hydrodynamic diameter. Instead, it depends on their internal network topology. Murayama et al reported that a fine mesh size of approximately 3.6 nm allows single polymer chains to undergo confined reptation in complex matrices.²⁶ By introducing dynamic covalent bonds, the stress relaxation time (τ) can be tuned. Under fluid shear in bone tissue, nanogels then exhibit a low Deborah number (De ≪ 1) and show fluid-like behavior. This enables efficient penetration under confined conditions.²⁷ In confined channels, the apparent radius of nanogels can dynamically shrink. Internal polymer chains rearrange cooperatively to dissipate deformation stress. As a result, a relatively high partition coefficient can be maintained even under extreme confinement.

Rapid stress relaxation also prevents structural damage and premature payload release under cyclic fluid shear. This improves the distribution density within deep bone tissue.²⁷ Future design should precisely regulate network topology, representative strategies include cellulose nanocrystal reinforcement and PEG diacrylate crosslinking. This can optimize the mechanical response kinetics and enable precise delivery.²⁸

Surface Functionalization and Hierarchical Targeting

Smart nanogels overcome multiple barriers in the osteoporotic microenvironment through a multilevel targeting strategy. Tissue-level targeting: Functionalization with phosphate groups, tetracycline, or bisphosphonates enables selective accumulation in bone tissue.^29,30

Cellular-level targeting: Ligand-receptor interactions are used to direct delivery to specific cell populations. Examples include CD44/CD90 on bone marrow mesenchymal stem cells (BMSCs), osteogenic peptides, and integrin αvβ3 ligands on osteoclasts. This enables precise regulation of bone metabolism.³⁰ It should be noted that protein corona formation may mask surface ligands and affect cellular uptake. However, cell-targeting ligands can still promote cell surface retention and internalization.

Subcellular-level targeting: Nuclear localization signals, mitochondrial targeting peptides, or lysosomal escape motifs can be used for precise subcellular organelle targeting. This improves therapeutic efficiency and reduces nonspecific diffusion or degradation.

In addition, the protein corona can be dynamically remodeled in the pathological bone microenvironment. Endogenous proteins such as ALP and BGP may form an actively programmed protein corona. This can partially compensate for targeting ligands and maintain targeting continuity from tissue to cellular levels.^27,31,32

Multi-Responsive Drug Release

Fracture repair and bone defect healing are accompanied by complex microenvironmental changes, including chronic inflammation, local acidification, hypoxia, and oxidative stress.³³ Conventional passive delivery systems cannot dynamically respond to changes in disease status or local signals. Therefore, microenvironment-responsive nanogels have been developed to improve therapeutic precision. Current responsive nanogels mainly exploit pathological cues including acidity, enzyme activity, and oxidative stress. Acid-responsive systems promote release in inflammatory or resorptive niches, whereas enzyme and ROS-responsive networks enable disease-associated degradation and antioxidant delivery.

Combining multiple responsive mechanisms within a single nanogel platform may further improve disease-specific delivery. Logic-gated strategies provide an additional approach for regulating release behavior. For example, an “AND” logic design requires the simultaneous presence of multiple pathological signals, such as local acidosis and elevated enzyme activity, before drug release occurs. This approach may reduce nonspecific leakage and improve therapeutic selectivity.

However, the clinical translation of these complex responsive systems remains challenging. The integration of multiple signals requires precise control of network structures. In addition, the heterogeneous and dynamic nature of bone tissue may affect the strength and duration of triggering signals. Changes in local blood flow, tissue penetration, and inflammatory status may alter response thresholds and release efficiency. Although dual-responsive systems, such as pH/ROS-responsive nanogels, have been widely investigated, multi-signal responsive platforms are still under early development. Future studies should combine programmable polymer networks, biological recognition elements, and in vivo monitoring technologies to improve nanogel adaptability in complex bone microenvironments (Figure 2).

Figure 2.

Engineering principles of smart nanogels for precision regulation of the osteoporotic microenvironment.

Mechanisms of Nanogels in Regulating Bone Remodeling Balance

Osteogenic Regulation

During the repair of osteoporotic bone defects, recruiting endogenous BMSCs to lesion sites is essential for in situ bone regeneration. The SDF-1/CXCR4 signaling axis plays a central role in stem cell recruitment. However, its poor stability and rapid degradation in vivo limit direct application.^34,35 Nanogels serve as stable delivery platforms for SDF-1. They enable SDF-1 encapsulation and sustained release, thereby prolonging its biological activity. This approach activates downstream signaling pathways such as PI3K/Akt signaling pathway and effectively promotes BMSC migration.^34,36 Further optimization strategies include environment-responsive designs, such as pH-sensitive or ultrasound-responsive systems. Co-delivery with osteogenic factors, including BMP-2, anti-miR-138, or OPG, can also be applied. This enables a staged strategy involving initial recruitment followed by osteogenic differentiation.^37–42

During osteogenic differentiation, the Wnt/β-catenin signaling pathway serves as a key regulatory mechanism. Its reduced activity contributes substantially to impaired osteogenesis in osteoporosis.⁴³ Nanogels can deliver small-molecule agonists or Li⁺ to locally stabilize β-catenin. This promotes the expression of osteogenic genes such as Runx2. Bone-targeting modifications can further improve accumulation efficiency.⁴⁴ BMP-2 is a potent osteogenic factor. Encapsulation within nanogels can protect BMP-2 from degradation and enable sustained delivery. This reduces the risk of ectopic bone formation caused by high doses. It can also be combined with scaffold materials to guide new bone formation.^45,46

In addition, nanogels can regulate the metabolic state of BMSCs. They help counteract aging-related mitochondrial dysfunction and oxidative stress. By delivering NAD⁺ precursors or antioxidant enzymes such as SOD and CAT through site-specific release systems in a mildly acidic environment, redox homeostasis in the bone marrow microenvironment can be restored. This improves stem cell function at the metabolic level and helps reverse the decline in osteogenic potential.^47,48

Osteoclast-Related Inhibition

The core pathology of osteoporosis is excessive osteoclast activity. The RANK/RANKL/OPG axis represents the central regulatory system governing osteoclast activation.^49–53 RANKL binds to RANK and promotes osteoclast differentiation and activation. In contrast, OPG acts as a decoy receptor and competitively binds RANKL, thereby inhibiting bone resorption. In osteoporosis, the RANKL/OPG ratio is increased, leading to dominant bone resorption. For osteoclast inhibition, nanogels provide multilevel intervention strategies. On one hand, OPG or OPG-mimetic molecules can be introduced on the surface to neutralize RANKL. Alternatively, RANK-binding competitive peptides can block the RANKL–RANK interaction. These strategies enable localized suppression of osteoclastogenic signaling. They can also be combined with osteogenic factors to balance bone formation and resorption.^54–57 On the other hand, cationic nanogels can deliver siRNA to target RANKL or Cathepsin K at the gene level. This effectively suppresses osteoclast differentiation. Intervention in upstream signaling pathways can also inhibit osteoclast activity. For example, suppression of FASN can reduce c-Fos and NFATc1 signaling, thereby alleviating bone loss.^58–61

However, optimizing responsiveness while maintaining in vivo stability remains challenging. Responsive nanogels can therefore achieve lesion-specific drug release. ROS-responsive nanogels can release bisphosphonates or anti-inflammatory agents in regions with elevated ROS levels. This enables precise inhibition of osteoclast activity while reducing off-target effects. Further optimization of material design is still required. Premature release under physiological ROS levels should be avoided, and in vivo stability must be maintained.^62,63

Coordinated Regulation of Osteogenesis and Osteoclastogenesis

An ideal nanogel system should possess spatiotemporal programmability. It should function not only as a drug carrier but also as a regulator of the bone microenvironment. Through the combined effects of biochemical signaling and physical topological cues, nanogels can help restore bone homeostasis. Their multilevel responsive structures enable stage-specific regulation during bone remodeling. In the early stage, surface charge interactions or receptor competition can rapidly block RANKL signaling and suppress excessive osteoclast activity. Subsequently, matrix metalloproteinase (MMP)-mediated degradation gradually releases osteogenic factors. This creates a sequential strategy of “first inhibition, then activation”, which more closely resembles the dynamic coupling between bone resorption and bone formation during physiological bone remodeling.⁶⁴ At the molecular level, the incorporation of bioactive nanocomponents, such as layered double hydroxides (LDHs), can synergistically activate key osteogenic signaling pathways, including p38 MAPK, ERK/MAPK, FGF, and TGF-β pathways. This markedly enhances osteoconductivity and osseointegration, thereby promoting efficient new bone formation.^64,65 In addition, nanostructured ceramic materials possess a high specific surface area and strong surface catalytic activity. These properties allow them to mimic the inorganic phase of natural bone and promote osteoblast proliferation and differentiation at the molecular level.⁶⁶

In addition to chemical release, the physical topology of nanomaterials can also regulate the osteoimmune microenvironment. For example, pollen-like mesoporous silica nanoparticles (PMSNs) possess a unique surface structure that directly influences macrophage behavior. They can upregulate CD28 receptor expression and inhibit ERK phosphorylation, thereby achieving phagocytosis-independent immune regulation.⁶⁷ This physically triggered mechanism suppresses macrophage polarization toward the M1 pro-inflammatory phenotype. It reduces the expression of inflammatory factors such as IL-6 and CXCL11, while increasing remodeling-related factors including mCSF and MMP9. As a result, a favorable immune microenvironment is created for the recruitment of osteogenic precursor cells, helping to maintain the dynamic balance between osteoclast inhibition and reparative osteogenesis (Figure 3).

Figure 3.

Mechanisms of nanogels in regulating the balance between osteogenesis and osteoclastogenesis.

Advanced Applications of Nanogels for Coordinated Bone Remodeling

Sequential Release Systems

Based on the concept of spatiotemporal programming regulation, sequential release systems represent an important engineering strategy for nanogels. These systems have shown significant therapeutic advantages in complex pathological conditions such as postmenopausal osteoporosis. An ideal treatment for osteoporosis should mimic the natural process of bone repair. Excessive bone resorption should first be rapidly suppressed to create a favorable environment for new bone formation. This should then be followed by sustained promotion of osteogenesis. Owing to their programmable release capabilities, nanogels can achieve this sequential therapeutic strategy. By regulating material composition, structure, and responsiveness, different drugs can be delivered with temporal and spatial precision, thereby restoring bone homeostasis.

For postmenopausal osteoporosis, several bone-targeted nanosystems have achieved combined inhibition of osteoclast activity and promotion of osteogenesis. For example, alendronate-modified PLGA nanocarriers co-loaded with 17β-estradiol and vitamin D3 showed efficient bone targeting. These systems synergistically activated the PI3K/AKT/mTOR pathway, improved bone mineral density and trabecular structure, and reduced uterine side effects.⁶⁸ Citrate-modified pH-sensitive polymers can be used for estrogen delivery. In the acidic microenvironment generated by bone resorption, these systems rapidly release estrogen, thereby inhibiting osteoclast activity and reducing systemic side effects.⁶⁹ Bone-targeted liposomes loaded with Icariside II can regulate the adipogenic–osteogenic balance of BMSCs. This effect is associated with the Akt/GSK-3β/β-catenin signaling pathway.⁷⁰ Locally functionalized implants, such as dual drug-loaded PLGA-coated titanium implants or implants co-functionalized with zoledronic acid and teriparatide, have enhanced bone formation and implant fixation in animal models.^71,72

These studies indicate that nanogel-based sequential release systems can precisely control drug kinetics. They enable a strategy of first inhibiting osteoclast activity, then promoting osteogenesis. This provides a new approach for precision treatment of osteoporosis.

Regulation of Type H Angiogenesis

Osteoporosis is often associated with impaired coupling between vascular and bone metabolism. In particular, a reduction in type H vessels affects the recruitment and differentiation of osteogenic precursor cells. Nanogels can serve as delivery platforms for pro-angiogenic factors such as VEGF and PDGF-BB, supporting the restoration of vascular-bone coupling.^73,74 VEGF is a key angiogenic factor in bone repair. Similar growth factor delivery strategies have demonstrated the feasibility of restoring angiogenic signaling in regenerative tissues.⁷⁵ PDGF-BB promotes pericyte recruitment and stabilizes newly formed blood vessels. The combination of VEGF and PDGF-BB can generate a functional vascular network that more closely resembles native type H vessels. Multicompartment or layered nanogel designs can enable the temporal and spatial controlled release of growth factors.⁷⁶ By responding to the acidic or high-ROS microenvironment associated with bone resorption, nanogels can achieve spatiotemporally specific release of pro-angiogenic factors in regions with active osteoclast activity. Such spatiotemporal regulation mediates the dynamic coupling between bone resorption and bone formation and promotes the coordinated reconstruction of angiogenesis and bone regeneration.

The formation of type H vessels depends on Notch signaling and local gradients of growth factors. Nanogels can maintain spatial gradients of PDGF-BB and VEGF, thereby promoting endothelial cell migration and vascular stabilization.⁷⁷ In addition, the viscoelasticity of nanogels provides not only structural support but also regulates endothelial cell behavior through mechanotransduction. Studies have shown that matrices with moderate viscoelasticity can activate the Notch and Hippo-YAP/TAZ signaling pathways, thereby promoting endothelial cell proliferation, migration, and tube formation.⁷⁸ Nanogels act not only as delivery carriers but also mimic the function of the extracellular matrix. This supports the concept of nanogels as biomimetic regulators. It highlights their dual biomimetic capability in mechanical and biological signal regulation.

Theranostic Integration

Theranostic integration combines diagnosis and therapy within a single nanoplatform. It enables real-time monitoring and simultaneous intervention in the progression of osteoporosis. NIR-II (1000–1700 nm) fluorescence imaging offers deep tissue penetration and a high signal-to-noise ratio. It is suitable for monitoring osteoclast activity and new bone formation. Nanogels can load NIR-II probes that respond to acidic microenvironments or bone-specific enzymes, such as tartrate-resistant acid phosphatase. A decrease in fluorescence intensity may indicate reduced osteoclast activity. Probes sensitive to alkaline phosphatase activity in osteoblasts or to newly formed bone mineral deposition can be used to track bone formation progress.⁷⁹ This system provides continuous, in situ feedback on therapeutic effects, which can help optimize drug dosing and rehabilitation strategies. Compared with conventional DXA measurements, theranostic integration enables dynamic monitoring and improves the management of osteoporosis in community healthcare and chronic disease settings.^80,81 Combined with NIR-II imaging technology, nanogels enable visualized delivery. By designing enzyme-responsive fluorescent probes, a dark-to-bright signal can be triggered in the presence of Cathepsin K. This allows real-time monitoring of osteoclast activity. It also provides early warning and improves prevention and treatment outcomes (Figure 4). To provide a comprehensive overview, recently reported nanogel systems for precision bone regeneration, including their chemical composition, loaded payloads, targeting mechanisms, and biological outcomes, are summarized in Table 1.

Figure 4.

Advanced applications of nanogels in regulating osteogenesis- osteoclastogenesis coupling.

Table 1.

Representative Bone-Targeted Nanogel/Carrier Systems for Bone Disease Therapy and Regeneration

Nanogel/ Carrier Design	Therapeutic Cargo	Bone Targeting & Responsive Release	Biological Mechanism	Therapeutic Outcome	Disease Model/ Application	Ref
Alendronate- functionalized PLGA nanogel	17β-Estradiol and Vitamin D3	Active bone targeting through alendronate- mediated chelation with hydroxyapatite; sustained cargo release	Activates PI3K/Akt/mTOR signaling to promote osteogenic activity and bone formation	Increased bone mineral density and improved bone remodeling	Osteoporosis	[68]
Citrate-modified pH-responsive polymer nanogel	Estrogen	Citrate-mediated affinity toward bone mineral; accelerated release under acidic osteoclast resorption microenvironment	Local estrogen delivery suppresses osteoclast activation and excessive bone resorption	Reduced osteoclast activity and preservation of bone mass	Osteoporosis	[69]
Hierarchical multicompartment nanogel	VEGF and PDGF-BB	ROS/acidic microenvironment-responsive sequential release with spatially controlled delivery	Reconstructs type H vessels and enhances angiogenesis–osteogenesis coupling	Promoted vascular regeneration and bone formation	Bone defect regeneration	[76,77]
Alendronate- modified mPEG-PLGA nanogel (AL-mPEG-PLGA)	Astragaloside (AS)	Alendronate- mediated hydroxyapatite targeting; enhanced bone accumulation and controlled release	Suppresses inflammatory bone resorption by reducing IL-6, CTX-1, and TRACP-5b; enhances osteogenesis	Improved trabecular microarchitecture, increased BMD, and enhanced oral bioavailability	OVX-induced osteoporosis	[82]
Alendronate- conjugated PLGA-mPEG nanogel (ALN-PLGA-mPEG)	Rifapentine (RPT)	Bone matrix targeting via bisphosphonate– hydroxyapatite interaction	Provides localized antimicrobial activity and reduces systemic drug exposure	Increased bone drug accumulation and reduced systemic toxicity	Bone tuberculosis	[83]
P5CD/TPP/ Ada-S-Ada supramolecular nanogel	Photosensitizer- based functional platform	Dual pH/ROS- responsive structural disassembly; acidic activation and ROS-triggered collapse	Activates photodynamic activity through 1O2 generation and microenvironment-responsive degradation	Provides a versatile platform for controlled delivery of poorly soluble cargos	Stimuli- responsive bone therapy platform	[84]
GelMA/phenylboronic acid-modified gelatin dynamic nanogel-like hydrogel	Cell-free bioactive scaffold	Shear-thinning injection, self-healing dynamic bonds, and tissue adhesion	Viscoelastic matrix regulates integrin β1/YAP signaling and activates Smad2/3 and β-catenin pathways	Enhanced BMSC migration, osteochondral differentiation, cartilage regeneration, and subchondral bone repair	Osteochondral defect regeneration	[85]

Nano-Bio Interface and in vivo Fate

Bone-Specific Protein Corona

After entering the bloodstream, nanogels rapidly adsorb plasma proteins on their surface within milliseconds to seconds, forming a dynamically evolving protein corona. This interface is not static. It undergoes a transition from a soft corona to a hard corona. Its composition and exchange kinetics determine the in vivo fate of nanogels, including targeting behavior, cellular uptake pathways, and biodistribution. In osteoporosis, the serum protein profile changes systemically. Proteins related to bone turnover, such as alkaline phosphatase (ALP) and bone Gla protein (BGP), may become enriched during the early stage of corona formation. This disease-driven adsorption pattern provides a basis for reverse engineering of nanogel interfaces. Instead of fully preventing protein adsorption, a more promising strategy is to actively regulate corona composition.

The physicochemical properties of nanogel surfaces, including zeta potential, hydrophilicity/hydrophobicity, and functional group distribution, determine protein adsorption selectivity and kinetics. For example, a moderately negative charge and balanced hydrophobicity can preferentially enrich apolipoproteins or fibronectin. These proteins not only affect circulation time but also promote adhesion and osteogenic differentiation of BMSCs through integrin receptors such as α5β1. Therefore, design strategies are shifting from anti-fouling surfaces toward selective pre-adsorption and function-oriented corona engineering.

Zwitterionic polymers can form a highly hydrated layer that reduces nonspecific adsorption of opsonins and decreases clearance by the mononuclear phagocyte system. In addition, precise surface functionalization, such as phosphate and carboxyl groups, can enrich bone-affinity proteins like osteopontin. This enables endogenous protein-mediated spontaneous bone targeting.⁸⁶ However, the protein corona may also weaken active targeting strategies. Disease-related serum proteins can mask surface ligands and reduce receptor-binding efficiency. Therefore, in secondary cell-targeting design, the competition between ligand density and corona coverage should be carefully considered. This is important to maintain multivalent binding capability.

In the bone regeneration microenvironment, a bone-specific protein corona may also provide functional amplification effects. For example, osteopontin (OPN) contains phosphorylated domains that mediate osteoclast adhesion and participate in bone remodeling signaling. Selective enrichment of such proteins can enhance nanogel retention on bone surfaces and strengthen cell interactions. However, a thick or nonspecific corona may increase particle size, limit diffusion, and even trigger abnormal immune responses. Future designs should shift from passive adaptation to protein corona toward active programming of the corona. By tuning interfacial energy, polymer chain conformation, and adsorption kinetics, precise control over corona composition and function can be achieved.

Intracellular Transport and Degradation

After lysosomal escape, the intracellular trafficking and metabolic fate of nanogels determine both therapeutic efficacy and long-term safety. Once released into the cytoplasm, nanogels and their cargo, including drugs, genes, or proteins, undergo complex metabolic processing. Biocompatibility and degradability of the material are key design factors. Biodegradable systems, such as PLGA or gelatin–siloxane nanogels, can gradually break down into small molecules. These degradation products are then eliminated through normal metabolic pathways.^87,88 Poorly degradable materials may accumulate in organs such as the liver and spleen over time. This can induce chronic inflammation or oxidative stress.⁸⁹ Metabolomics studies have shown that nanocarriers and their payloads can affect protein synthesis, energy metabolism, and antioxidant defense systems. This suggests that even with bone-targeted delivery, systemic and intracellular metabolic disturbances should still be evaluated.^90,91 An ideal nanogel should balance efficient targeting with safe clearance. For example, amphiphilic self-assembled particles can disassemble into small molecules after exerting their therapeutic effect. These degradation products can then be excreted through the kidneys, thereby reducing the risk of chronic toxicity.⁹² In addition, the metabolic behavior of PEGylated materials, such as M-PEG₆-OH being mainly excreted through the kidneys and undergoing hydroxyl oxidation, can provide useful guidance for improving the safety of nanogels.⁹³

In summary, a systematic understanding of protein corona formation, intracellular trafficking, metabolic transformation, and final clearance of nanogels in the osteoporotic microenvironment is essential. It links therapeutic potential with long-term clinical safety. Through rational design of chemical composition, crosslinking density, and stimulus-responsive degradation, nanogels can achieve bone-targeted drug delivery. They can also release therapeutic agents specifically in acidic or enzyme-rich microenvironments. In normal physiological conditions, they remain stable or gradually degrade into non-toxic products, which are safely eliminated through metabolic pathways (Figure 5).

Figure 5.

Nano-bio interface interactions and in vivo fate of nanogels in the osteoporotic microenvironment.

Clinical Translation, Challenges, and Future Perspectives

Overcoming Biological Barriers

The therapeutic efficacy of nanogels in bone regeneration and osteoporosis treatment depends on their ability to effectively penetrate bone tissue. The bone–blood barrier regulates the exchange of substances between bone tissue and the bloodstream and represents a major physical obstacle for nanodelivery systems.⁹⁴ Poor blood supply may lead to insufficient drug concentration at lesion sites. For example, in the treatment of bone tuberculosis, anti-tuberculosis drugs may fail to eliminate residual mycobacteria. This can increase the risk of nonunion.⁹⁵

Nanogel penetration across the bone–blood barrier relies on both passive diffusion and active transport. The bone-blood barrier shares certain structural features with other selective vascular barriers, including tight junction regulation and transporter-mediated exchange. It features tight junctions and specific transport mechanisms.⁹⁴ In the treatment of multiple myeloma, nanocarrier systems must overcome the bone marrow endothelial barrier to target blood vessels.⁹⁶ Possible strategies include designing nanogels that recognize receptors on bone marrow endothelial cells. Another approach is the “cell hitchhiking” strategy, such as stem cell homing or neutrophil chemotaxis, to deliver nanocarriers to lesion sites. Optimization of particle size, surface charge, and surface modification is also important.

Studies have shown that 20 nm iridium nanoparticle aggregates exhibit higher transport efficiency than 80 nm aggregates in blood circulation and secondary organs.⁹⁷ Surface ligand engineering, such as P-selectin modification or fucosylation, can activate caveolin-1-dependent transcytosis pathways. This enables active transport across biological barriers.⁹⁸ Bone-affinity ligand modification, such as bisphosphonate functionalization, may enhance accumulation in mineralized tissues.⁹⁹

Cell-mediated delivery strategies can also be applied in fetal interventions. For example, lipid nanoparticles targeting the CD45 receptor have been used for gene editing of fetal hematopoietic stem cells.¹⁰⁰ Overall, relying solely on particle size or surface modification is insufficient to fully overcome the bone–blood barrier. More promising future strategies may involve combining cell-mediated delivery with the use of physiological transport pathways to achieve coordinated trans-barrier transport.

AI-Assisted Customized Design

Osteoporosis is characterized by highly heterogeneous pathological changes. Different types of osteoporosis, such as postmenopausal osteoporosis (PMOP) and glucocorticoid-induced osteoporosis (GIOP), are associated with distinct alterations in the bone microenvironment.¹⁰¹ Therefore, designing nanogel systems with disease-specific mechanical properties, degradation behaviors, and drug release profiles is important for achieving precision therapy. Recently, artificial intelligence (AI) and machine learning approaches have provided new strategies for the rational design of nanogels. By integrating multidimensional parameters, including polymer composition, crosslinking density, network topology, and environmental responsiveness, AI models can identify relationships between material structures and functional properties.^102–104 These models can also assist in predicting nanogel swelling behavior, degradation kinetics, and drug release profiles.

Previous studies have demonstrated that machine learning models trained on diverse polymer datasets can predict hydrogel properties. For example, algorithms such as random forest (RF) and support vector regression (SVR) can predict equilibrium swelling ratios and degradation behaviors based on parameters including monomer composition, crosslinking density, and temperature.^105–107 Deep neural networks (DNNs) and transfer learning approaches have also been applied to simulate complex drug release kinetics and optimize polymer network designs with specific performance requirements.^108,109 By establishing nonlinear relationships between network architectures and mechanical properties, such as shear-thinning behavior and viscoelastic stress relaxation, machine learning models can facilitate the screening of candidate nanogel systems and reduce reliance on traditional trial-and-error optimization.

AI-assisted nanogel design may provide additional opportunities for osteoporosis treatment. For GIOP, AI models could incorporate nanoscale alterations in the bone matrix induced by glucocorticoid exposure to simulate nanogel mechanics and drug release behaviors. This approach may support the development of gel systems with both osteogenic and bone-protective functions.^110,111 In addition, AI approaches may optimize the loading ratio and sustained-release profiles of antiresorptive agents, such as denosumab, to achieve more stable and prolonged therapeutic effects.^112,113 Furthermore, by integrating clinical parameters, including age, sex, underlying diseases, and glucocorticoid exposure, AI models may assist in personalized treatment design and predict potential adverse effects, thereby improving therapeutic safety.^114,115

Despite these advantages, several challenges remain in AI-driven nanogel design. First, the limited interpretability of complex deep learning models remains a major concern. The “black-box” nature of these models makes it difficult to clarify the relationships between material parameters and biological outcomes. Second, the lack of high-quality, standardized, and shareable datasets restricts model development. Variations among experimental systems may also reduce model generalizability. Finally, although AI can narrow the candidate space for nanogel screening, computational predictions still require systematic validation through in vitro and in vivo experiments. Therefore, experimental verification remains an essential step before AI-assisted nanogel design can be translated into clinical applications.

From Passive Repair to Active Regulation in Precision Medicine

Osteoporosis treatment is undergoing a major paradigm shift. Traditional passive repair strategies mainly rely on calcium supplementation, vitamin D, or single-target drugs. However, these approaches fail to restore the dysregulated bone microenvironment and are often associated with side effects.^116,117 Nanogel systems can sense pathological microenvironments and perform multiple biological functions. In this way, they enable active regulation and help restore the dynamic balance between osteoblasts and osteoclasts.²⁷ For example, engineered cell membrane-coated nanogels can simultaneously neutralize RANKL and release PTH 1–34 in acidic microenvironments. This achieves synergistic effects in bone-targeted delivery, osteoclast inhibition, and osteogenesis promotion. The programmable properties of nanogels allow them to penetrate physiological barriers, precisely deliver therapeutic agents, and actively regulate local cellular behavior and signaling pathways, such as Wnt/β-catenin signaling. As a result, they can guide bone regeneration from the molecular to the cellular level.^118–120

In the future, precision bone regeneration will rely on the integrated evaluation of individual genetic background, metabolic characteristics, and the microbiome. For example, the gut microbiota and its metabolites can influence bone metabolism, providing new opportunities for microbiome-based interventions.^121,122 In secondary osteoporosis, etiology-specific precision therapies are especially important.¹²³ The long-term goal is to develop closed-loop systems with sensing, decision-making, and execution capabilities. These systems would not only deliver therapeutic agents, but also respond dynamically to changes in the bone microenvironment, such as fluctuations in pH or enzyme activity. They could then adjust drug release rates in real time and maintain bone tissue in a repair-supportive dynamic balance.

Key Barriers and Challenges in the Clinical Translation of Bone-Targeted Nanogels

Although preclinical studies have demonstrated the potential of nanogels for regulating the bone microenvironment and improving targeted delivery, their translation from laboratory research to clinical applications remains challenging. Major obstacles include large-scale manufacturing, quality control, regulatory evaluation, and commercialization.^124,125

First, scalable production and good manufacturing practice (GMP) compliance remain critical issues. Nanogels are often prepared using small-batch methods under laboratory conditions, whereas clinical applications require stable, reproducible, and scalable manufacturing processes. Key properties of nanogels, including particle size distribution, crosslinking degree, network structure, and surface functionalization, can be affected by preparation conditions and may result in batch-to-batch variations.¹²⁶ Therefore, standardized manufacturing processes combined with precise fabrication technologies, such as microfluidic systems, are needed to ensure consistent critical quality attributes (CQAs) of nanogels.

Second, regulatory evaluation of nanogels remains complex due to their diverse compositions, surface modifications, and multifunctional delivery properties. Comprehensive assessment is required for material purity, residual monomers, in vivo stability, pharmacokinetics, biodistribution, and long-term biocompatibility. For bone-targeted nanogels, accurate evaluation of bone accumulation, metabolic pathways, and long-term retention remains particularly important for clinical translation.^127,128

In addition, commercialization is influenced by production costs, storage stability, and clinical acceptance. Compared with conventional bone therapeutics or biomaterials, multifunctional nanogel systems usually involve more complex preparation processes and higher manufacturing costs. Therefore, improving fabrication efficiency and demonstrating clear clinical advantages will be important for enhancing the economic feasibility and clinical adoption of nanogel-based therapies.^129,130

Future Perspectives and Research Directions

To address these translational challenges, future research on bone-targeted nanogels should focus on the integration of material engineering, advanced preclinical evaluation, and computational approaches.

First, manufacturing strategies should progress toward standardized and automated production. Microfluidic-assisted assembly and continuous manufacturing technologies may improve the reproducibility of nanogel size, network architecture, and surface functionalization while meeting GMP requirements for quality consistency.

Second, preclinical evaluation requires models with greater clinical relevance. Although rodent models provide valuable information on initial efficacy and safety, differences in bone structure and remodeling processes limit their ability to fully predict clinical outcomes. Therefore, large animal models of osteoporosis should be considered before clinical trials to evaluate long-term biodistribution, degradation behavior, immune responses, and chronic toxicity.

Finally, artificial intelligence and digital technologies may provide new approaches for personalized nanogel design. By integrating multi-omics data, material parameters, and nano–biointerface information, digital twin models may help predict nanogel behaviors under different physiological conditions, including protein corona formation, cellular uptake, and tissue distribution. Combined with interpretable AI approaches and experimental validation, these strategies may facilitate a transition from empirical material optimization toward data-driven design and provide new possibilities for personalized treatment of bone diseases.

Safety Evaluation and Potential Risks

The potential risks of nanogels in osteoporosis treatment require careful evaluation. First, degradation kinetics directly determine the therapeutic window. The degradation rate of nanogels should match the rate of new bone formation.^131,132 Rapid degradation may lead to loss of mechanical support and sustained release function. In contrast, slow degradation may occupy the lacuno-canalicular space and hinder new bone growth.¹³³ The potential toxicity of degradation products, such as lactic acid from PLGA or residual monomers from gelatin–siloxane systems, to bone marrow hematopoietic stem cells (HSCs) also requires attention.

Second, repeated administration may induce immune memory or anti-drug antibody (ADA) formation. PEGylation or other surface modifications can improve circulation time. However, they may also trigger adaptive immune responses, reducing therapeutic efficacy or causing systemic inflammation. Protein corona formation at the nano-bio interface may further activate the complement system or promote macrophage inflammatory cytokine release, increasing immune-related risks.^134–136

Mechanical homeostasis disruption should also be considered. Highly crosslinked or poorly compressible nanogels may cause local physical blockage in confined spaces such as the lacuno-canalicular system. This can impair nutrient exchange and mechanotransduction in bone cells, thereby interfering with bone remodeling.¹³⁷

In addition, pharmacokinetics and long-term accumulation risks are important. In elderly patients with osteoporosis, reduced liver and kidney function may lead to retention of nanomaterials and their degradation products in metabolic organs. This increases the risk of chronic toxicity or inflammation.^138,139

In summary, clinical translation of nanogels requires systematic evaluation of degradation kinetics, immune responses, mechanical interference, and in vivo accumulation.^140,141 The establishment of a bone-specific toxicological assessment system is essential to ensure safety and clinical feasibility.

Conclusion

In summary, this review has established a micro–nano–bio interfacial framework for understanding the design strategies and biological mechanisms of bone-targeted nanogels. This framework highlights the distinct features of nanogels compared with conventional macroscale hydrogel scaffolds. A major focus of this review is the relationship between nanogel viscoelasticity and adaptive transport within the LCS. Unlike rigid nanoparticles that mainly rely on size-dependent transport for tissue penetration, nanogels can respond to mechanical constraints in confined spaces through network rearrangement and stress relaxation. These properties enable nanogels to access deeper bone microenvironments. Therefore, tuning the dynamic mechanical characteristics of crosslinked networks represents a promising strategy for improving nanogel transport across complex bone barriers.

This review also emphasizes the role of protein corona formation in regulating the in vivo behavior of bone-targeted nanogels. Traditional nanomaterial design often focuses on reducing nonspecific protein adsorption through surface modification. However, increasing evidence suggests that the protein corona is not merely a biological barrier but an important interfacial layer that influences nanomaterial fate. Modulating surface chemistry and network architecture offers a rational approach to regulate protein corona composition and function, thereby improving nanogel stability, biodistribution, and interactions with the bone immune microenvironment.

Overall, the development of bone-targeted nanogels is shifting from a simple focus on delivery efficiency toward integrated regulation of mechanical adaptation, biointerface interactions, and microenvironment responsiveness. Future advances in precise manufacturing, AI-assisted design, and clinically relevant validation models may further facilitate the translation of nanogels for precision bone regeneration and osteoporosis therapy.

Funding Statement

This study was supported by the Young Scientists Fund (Class C) of the National Natural Science Foundation of China (No. 82608918), National Natural Science Foundation project (NO.82174260), Liaoning Province Science and Technology Department joint fund doctoral research project (NO:2023-BSBA-224).

Abbreviations

LCS, lacuno-canalicular system; MMP, matrix metalloproteinase; LDHs, layered double hydroxides; PMSNs, pollen-like mesoporous silica nanoparticles; ALP, alkaline phosphatase; BGP, Gla protein; OPN, osteopontin; PMOP, postmenopausal osteoporosis; GIOP, glucocorticoid-induced osteoporosis; AI, artificial intelligence; RF, random forest; SVR, support vector regression; DNNs, Deep neural networks; GMP, good manufacturing practice; CQAs, critical quality attributes; HSCs, hematopoietic stem cells; ADA, anti-drug antibody.

Data Sharing Statement

No datasets were generated or analyzed during the current study.

Disclosure

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.