The role of smart polymeric biomaterials in bone regeneration: a review

Addressing critical bone defects necessitates innovative solutions beyond traditional methods, which are constrained by issues such as immune rejection and donor scarcity. Smart polymeric biomaterials that respond to external stimuli have emerged as a promising alternative, fostering endogenous bone regeneration. Light-responsive polymers, employed in 3D-printed scaffolds and photothermal therapies, enhance antibacterial efficiency and bone repair. Thermo-responsive biomaterials show promise in controlled bioactive agent release, stimulating osteocyte differentiation and bone regeneration. Further, the integration of conductive elements into polymers improves electrical signal transmission, influencing cellular behavior positively. Innovations include advanced 3D-printed poly (l-lactic acid) scaffolds, polyurethane foam scaffolds promoting cell differentiation, and responsive polymeric biomaterials for osteogenic and antibacterial drug delivery. Other developments focus on enzyme-responsive and redox-responsive polymers, which offer potential for bone regeneration and combat infection. Biomaterials responsive to mechanical, magnetic, and acoustic stimuli also show potential in bone regeneration, including mechanically-responsive polymers, magnetic-responsive biomaterials with superparamagnetic iron oxide nanoparticles, and acoustic-responsive biomaterials. In conclusion, smart biopolymers are reshaping scaffold design and bone regeneration strategies. However, understanding their advantages and limitations is vital, indicating the need for continued exploratory research.

softer tissues.Such properties are also influenced by the cellular requirements for porosity (Kohane and Langer, 2008).
Biomaterial factors such as biocompatibility, mechanical properties, and surface properties affect cell attachment, osteointegration, and osteogenesis (Ogle, 2015).Regeneration efficacy can be enhanced by delivering bioactive agents that regulate bone metabolic signaling pathways and new bone formation (Wei et al., 2022).Bone Tissue Engineering (BTE), a multidisciplinary field with decades of accumulated data, holds promise for addressing bone defects.BTE employs cells, growth factors, dynamic stresses, and biomaterials to fabricate bespoke bioactive scaffolds -including metals, ceramics, or polymers -to enhance bone repair (Wang et al., 2021).
Smart polymeric biomaterials are instrumental in controlled drug delivery systems, detecting stimuli and releasing bioactive agents accordingly (Wei et al., 2017;Montoya et al., 2021).Their function relies on stimuli-sensitive moieties that, when exposed to stimuli, undergo changes triggering drug release (Bustamante-Torres et al., 2021;Zhang et al., 2021).Stimuli-responsive biopolymers have recently gained attention as valuable graft materials.External physical triggers or certain pathological microenvironments can alter these materials' configuration, influencing cell destiny and bolstering bone tissue therapy and regeneration (Sobol et al., 2011;Lavanya et al., 2020;Cerqueni et al., 2021;Sivakumar et al., 2022;Heng et al., 2023).This minireview will delve into the major types of smart polymers used in bone regeneration, outlining their functions, advantages, and limitations.

Light responsive polymeric biomaterials for bone regeneration
Significant research underscores the utility of light-responsive polymers in precision drug delivery, boasting excellent control over spatial and temporal parameters and intensity, relevant in various medical conditions (Municoy et al., 2020;Pokharel and Park, 2022).In bone regeneration research, such materials can function as multifunctional scaffolds supporting bone repair or as drug delivery systems targeting antibacterial and osteogenic needs (Tomatsu et al., 2011).
One study reported a 3D-printed scaffold made of shapememory polyurethane (SMPU) and magnesium (Mg) for bone repair.The implanted scaffold can form a tight contact within bone structure by changing its shape between original conformation and compressed conformation, which was controlled by near-infrared (NIR) irradiation-induced photothermal effects.The scaffold demonstrated significant osteopromotive functions with in vitro and in vivo studies as shown in Figure 1A (Zhang et al., 2022).Photothermal therapy can also be used directly to bone defects, because heat at around 40 °C-43 °C can enhance proliferation and osteoblastic differentiation of mesenchymal stem cells (MSCs) (Liao et al., 2021).Tong et al. designed a biodegradable bone implant with black phosphorous (BP) nanosheets incorporated in poly (lactic-co-glycolic acid) (PLGA).After exposed to low intensity and periodic NIR, the implant significantly enhanced expressions of heat shock proteins, and increased osteogenesis in both cell and animal models (Tong et al., 2019).Furthermore, Zeng et al. developed a novel polydopamine-IR820-daptomycin coating for titanium bone implant.Under NIR irradiation, the composite had anti-bacterial efficiency.Additionally, the coating changed surface properties of implants, resulting in better contact with bones.The coating also significantly increased proliferation and osteogenic differentiation of bone marrow stem cells (Zeng et al., 2020).
Inflammatory response and low osteogenesis are two major issues that hinder bone regeneration.Kuang et al. developed a photo-responsive multicomponent hydrogel drug delivery system, which combined continuous drug release and NIR-controlled pulsatile drug release mechanisms together.In an osteoporosis animal model, the system can maintain parathyroid hormone (PTH) concentration in bone structure in a relatively stable manner, and thus promoted bone regeneration by achieving optimized osteoblast to osteoclast ratios (Kuang et al., 2021).In another study, osteo-inductive bone morphogenetic protein 2 (BMP-2) was attached to polydopamine-coated Mg-Ca carbonate microspheres which was incorporated into aspirin-containing hydroxybutyl chitosan (CS) hydrogel.The composite materials can release aspirin at early stage for anti-inflammatory effects, and then release BMP-2 to promote osteogenesis.Animal studies showed the presented composite promoted new bone formation (Wan et al., 2022).Furthermore, Wang et al. designed a NIRtriggered system with SrCl 2 -black phosphorous (BP)@PLGA microspheres, which can achieve on-demand release of Sr ions.After implanted into rat bone defects, the microspheres exhibit great biocompatibility and bone regeneration potential (Wang et al., 2018).

Thermo-responsive polymeric biomaterials for bone regeneration
Thermo-responsive biomaterials have piqued extensive interest, driven by their capacity for temperature-modulated bioactive agent release.Injectable systems enhance safety by transitioning phases without reliance on cross-linking agents, thereby avoiding denaturation.Moreover, the dynamic encapsulation process ensures therapeutic agents within the biomaterial.The rapid shift from a sol to a gel state at physiologic temperature eliminates the risk of premature burst release, optimizing the control of release kinetics (Duan et al., 2020).
Hydrogels' therapeutic applications can be hampered by mechanical inadequacies and shrinkage during cell culture.To overcome this, Zhu et al. (2023) devised a thermo/photo dualsensitive hydrogel, through physical and chemical cross-linking techniques, a thermo/photo dual-sensitive hydrogel was synthesized from methacrylated hydroxybutyl chitosan (MHBC) and chitin whisker (CHW).This M/C hydrogel exhibits a distinctive lamellar internal structure, and its mechanical properties and cellular compatibility can be tailored by modulating the M/C ratio.

Electrically-responsive polymeric biomaterials for bone regeneration
The impact of electrical currents on bone formation is welldocumented (Bassett et al., 1964), fostering the incorporation of conductive elements such as carbon nanofibers (Stout et al., 2012) and gold nanowires (Dvir et al., 2011) into conducting polymers.These polymers, soluble in organic solvents, can be blended with other polymers and processed into porous scaffolds, for instance, via electrospinning.Such uniformity enhances electrical signal transmission across the composite, influencing the behavior of all included cells.Substantial strides have been made in scaffold designs for bone tissue engineering.For instance, a 3D-printed poly (l-lactic acid) (PLLA) scaffold was designed, featuring a fiber diameter of 150 μm and an osteogenic pore size of 450 μm, both crucial for bone growth.The design parameters were set to achieve the desired scaffold size.With impressive cytocompatibility, elasticity resembling that of trabecular bone, and inherent piezoelectric properties encouraging the adhesion of fibrinogen-coated osteoblast-like cells, these scaffolds exhibit significant promise (Karanth et al., 2023).Other notable developments include a 3D polyurethane foam (PUF) scaffold coated with piezoelectric PVDF-HFP and mineralized calcium phosphate (CaP), which stimulated osteogenic cell differentiation and in vivo ectopic bone formation due to its components' synergistic effects (Ma et al., 2023a).BaTiO 3 nanofibers (BTNF) integrated into a poly (vinylidene fluoridetrifluoroethylene) (P(VDF-TrFE)) matrix created an anisotropic surface potential, bolstering mechanotransduction, in vitro osteogenesis, and in vivo bone regeneration as shown in Figure 1C (Bai et al., 2023).Wang et al. (2023) suggested a composite scaffold consisting of piezoelectric Whitlockite (WH) and polycaprolactone (PCL) that fostered neurovascularized bone tissue regeneration through sustained Mg 2+ release.A bifunctional composite formed by incorporating activated carbon nanotubes (ACNTs) into a polymethyl methacrylate (PMMA) matrix improved cell survival under electrical and magnetic stimuli (Li et al., 2022).A SiO 2 /PDMS composite electroactive membrane with embedded silicon dioxide electrets enhanced osteogenic differentiation and bone regrowth (Qiao et al., 2022).A pioneering approach introduced porous polymeric Fe 3 O 4 /GO scaffolds developed using cellulose and a co-dispersed nanosystem, exhibiting enhanced mechanical strength and antibacterial activities, as well as increased viability and proliferation of pre-osteoblast cell lines (Khan et al., 2022c).Lastly, a poly (l-lactic acid)-block-aniline pentamer (PLA-AP) and poly (lactic-co-glycolic acid)/hydroxyapatite (PLGA/HA-based) electroactive tissue engineering scaffold, loaded with the pSTAR-hBMP-4 plasmid, improved osteogenesis differentiation and bone healing, underlining the potential applications of multi-functional materials in bone tissue engineering (Cui et al., 2020).

pH-responsive polymeric biomaterials for bone regeneration
Osteoporosis, a metabolic bone disorder, arises from excessive osteoclast activity which breaks down bone structure via the secretion of acid and proteinases.Consequently, the pH of osteoporotic bones is lower than that of healthy ones (Blair, 1998).Bacterial infection, another major impediment to bone regeneration, can also lead to an acidic microenvironment around infection sites.Accordingly, pH-responsive polymeric biomaterials are predominantly used for the delivery of osteogenic and antibacterial drugs.
Deng et al. ( 2020) designed a dual-layer polydopamine coating for bone implants, incorporating silver nanoparticles (NPs) in the first layer and apatite in the second (Figure 1D).In response to bacterial infection, the coating releases Ag + , Ca 2+ , and PO 4 3-ions.3D-printed polyetheretherketone scaffolds modified with this coating demonstrated superior antibacterial and osteogenic properties in vitro, and promoted bone ingrowth and osseointegration in vivo in an infected bone defect.Another study introduced a drug release system composed of Poly [2-(dimethylamino) ethyl methacrylate] (PDMAEMA), chitosan, and a minocycline drug reservoir.As bacteria induce a pH reduction around the system, the pH-responsive PDMAEMA hydrogel propels the drug from the reservoir on-demand for bacterial inhibition with remarkable efficacy (Chen et al., 2023).
Synergistic effects of BMP-2 and dexamethasone (Dex) are critical for osteoblastic differentiation and bone regeneration.Gan et al. engineered a pH-sensitive, chitosan-functionalized mesoporous silica nanoparticle (chi-MSN).The design involves covalently attaching BMP-2 to chitosan and encapsulating smaller Dex molecules within the mesopores.Once delivered into cells, a lower pH triggers the release of Dex following the initial release of BMP-2.This system resulted in a substantial increase in osteoblastic differentiation and new bone formation in vivo over a period of 4 weeks (Gan et al., 2015).Finally, George et al. developed an injectable Oligo [poly (ethylene glycol) fumarate]-dopamine (OPF-DOPA) hydrogel that forms crosslinks under low pH conditions, subsequently increasing its stiffness and slowing its degradation rate.Notably, the hydrogel adheres to bone structures, preventing displacement of bone implants (George et al., 2022).

Enzyme-responsive polymeric biomaterials for bone regeneration
Enzymes are integral to bone growth and remodeling, modulating various signaling pathways within bone tissue such as cell proliferation, adhesion, and osteogenesis.Within the context of bone regeneration, native enzymes present in bone tissue, like matrix metalloproteinases (MMPs), can initiate specific reactions crucial for drug delivery, diagnostics, and tissue repair.Enzymeresponsive polymers have great biocompatibility, selectivity, and efficiency, and have excellent potential for bone regeneration.
Materials responsive to enzymes have also been employed to counteract bone infections.Polyglutamic acid (PG) is a homogeneous polymer featuring amide crosslinkers cleavable by the V8 enzyme, which is secreted by Staphylococcus aureus.Ding et al. (2020) encapsulated AgNPs into Mesoporous silica nanoparticles (MSNs), which were then enveloped by PG and polyallylamine hydrochloride (PAH) layers using a layer-by-layer technique.These nanoparticles were ultimately placed onto a polydopamine-coated surface as a titanium bone implant coating.The modified implants exhibited exceptional antimicrobial effects and significantly enhanced new bone formation in a bacteria-infected rat model.In a similar study for periodontal treatment, Alkaline Phosphatase (ALP) -responsive polyphosphoester and minocycline hydrochloride (PPEM) was incorporated into a chitosan membrane, and the effects were evaluated in cell and animal models.The results confirmed release of antibiotic and osteogenic drugs from PPEM membrane and their effects (Li et al., 2019).
In a recent study, researchers engineered Matrix metalloproteinases (MMP)-responsive nanocapsules to deliver bone BMP-2 for bone fracture healing.These nanocapsules, formed via in situ 2-(methacryloyloxy) ethyl phosphorylcholine (MPC) polymerization, incorporated the isacryloylated VPLGVRTK peptide as MMP cleavable crosslinkers on the BMP-2 surface, maintaining the functionality of BMP-2 throughout the process.The nanocapsules were delivered to fracture site via malformed blood vessels and accumulated there.Once MMPs disrupted the capsule, BMP-2 was released, facilitating bone regeneration as demonstrated with in vivo studies as shown in Figure 1E (Qi et al., 2019).Various other enzymes, including tyrosinase, lysozyme, horseradish peroxidase, transglutaminase (TG), and alkaline phosphatase (AP), have been examined for their potential to induce beneficial reactions for bone regeneration (Yuan et al., 2018;Sood et al., 2022).

Redox-responsive polymeric biomaterials for bone regeneration
Redox signaling pathways, predicated on electron transfer and free radicals, underpin mammalian bone formation and regeneration, especially balancing reactive oxygen species (ROS) (Zhang et al., 2023).Recent discoveries underline the post-fracture influence of redox on cellular responses.
Well-known for their role in redox modulation, ceria nanoparticles have been incorporated into a polyurethane matrix alongside graphene, creating a multifunctional biomaterial.Ceria-graphene oxide hybrid nanoparticles were synthesized through a hydrothermal process that started with the sonication of graphene oxide in distilled water.Following this, cerium nitrate hexahydrate was incorporated into the mixture and stirred magnetically.The pH was elevated to 10 through the addition of an ammonia hydroxide solution, and stirring was continued.The composite was then placed into a Teflon-lined stainless steel hydrothermal reactor and kept in an oven.Particles were dried in a hot air oven.Ceria nanoparticles and reduced graphene oxide sheets were similarly synthesized, albeit without adding graphene oxide and the cerium precursor, respectively.The end products displayed enhanced properties, specifically in terms of radical scavenging and osteogenesis (Nilawar et al., 2023).Ceria-graphene oxide hybrid nanoparticles were synthesized via a hydrothermal process and demonstrated heightened radical-scavenging and osteogenic properties.The bioactivity of 3D-printed, porous PLA scaffolds can be augmented by ceria, fostering osteogenesis enhancement and antimicrobial properties as shown in Figure 1F (Nilawar and Chatterjee, 2022).Further, nanoceria-cellulose-gelatin scaffolds (CG-NCs) have been crafted to combat ROS-induced oxidative stress inhibiting bone repair, boasting superior mechanical properties, biomineralization capabilities, and promoting cell proliferation and differentiation (Singh et al., 2023).
Gelatin methacrylate (GelMA) hydrogels, enhanced with magnesium-seamed C-propylpyrogallol[4]arene (PgC (3) Mg), offering dual-release of bioactive Mg 2+ and antioxidants, boosting bioactivity and resilience to oxidative stress.The modified hydrogels decreased intracellular ROS levels and improved bone repair in severe cranial defects (Tan et al., 2022).Moreover, scaffold combining radially aligned mineralized collagen (RA-MC) fibers and nanosilicon (nSi) exhibited osteoconductivity and osteoinductivity, guiding reparative cells and reducing inflammation, thus showing promise for major bone defect repair (Mac et al., 2022).Additionally, sodium alginate hydrogel, embedded with calcium peroxide nanoparticles and vitamin C, has shown promising results in alleviating bone defect hypoxia and promoting bone healing under hypoxic conditions (Zhao et al., 2021).YQ Chen's research focuses on enhancing the biocompatibility, biosafety, and biodegradability of polysaccharide-based hydrogels, used as 3D scaffolds for bone healing.A photocrosslinked composite hydrogel was synthesized under UV irradiation, merging a novel, water-soluble phosphatefunctionalized chitosan (CSMAP), prepared with methacrylic anhydride (MA) and phosphonopropionic acid (P), and strontium phosphosilicate (SPS) bioceramic nanoparticles.The CSMAP-SPS hydrogel's porous network amplified mechanical strength and bioactive ion release.This hydrogel demonstrated superior biomineralization, cytocompatibility with preosteoblast MC3T3-E1 cells, and encouraged osteogenic differentiation and endothelial tube formation, suggesting potential utility in bone regeneration (Chen et al., 2021).

Others smart polymeric biomaterials for bone regeneration
Several stimuli-responsive biomaterials warrant further exploration, particularly in mechanical, magnetic, and acoustic domains.In the mechanical field, Wolff's 1892 hypothesis proposed bone's responsiveness to biophysical stimuli, shedding light on bone and tissue healing as well as the impact of workouts and machine-induced stress on bone and mesenchymal tissue development (Ma et al., 2023b).On the other hand, specific polymers are capable of responding to compression, shear and other mechanical stimuli with network structural change or polymeric degradation, which affect bone implant design and drug delivery.PLA scaffold reinforced with 20% magnesium  (Chu et al., 2017;Chu et al., 2019).The results suggest that carefully chosen stress-responsive polymers may play important roles in bone regeneration.Magneticresponsive biomaterials also show potential, particularly when pristine superparamagnetic iron oxide nanoparticles (pSPIONs) are incorporated into additively manufactured scaffolds.Such scaffolds, composed of chitosan (CS), poly (vinyl alcohol) (PVA), and hydroxyapatite (HA), exhibit enhanced magnetic properties useful for magnetic hyperthermia and bone regeneration.Notably, the presence of pSPIONs increases cell adherence, proliferation, and ALP expression in human osteosarcoma Saos-2 cells, making these scaffolds a promising choice for bone regenerative applications (Tavares et al., 2023).The synergy of low-intensity pulsed ultrasound and lipid microbubbles with 3D-printed PLGA/TCP scaffolds has also been demonstrated to enhance bone marrow stem cell growth and differentiation, representing a potential strategy for bone regeneration (Jin et al., 2023).Furthermore, titanium-hydroxyapatite and titanium-wollastonite composites exhibit physicochemical and biocompatible properties conducive to future bone implants, underscoring the potential of metalceramic composites in bone implant advancements (Shanmuganantha et al., 2022).

Discussion and conclusion
Smart biopolymers exhibit potential in bone regeneration via innovative scaffold construction, material enhancement, and tailored drug delivery, thereby providing diverse therapeutic avenues.As reviewed before, light-responsive polymers enhance antibacterial effectiveness and bone repair in 3D-printed scaffolds and photothermal treatments.Thermo-responsive materials, conductive polymers, and pH-responsive biomaterials have demonstrated potential in controlled drug release, improved cellular behavior, and combating osteoporosis and infections, respectively.Also, enzymeresponsive polymers and redox signaling pathways targeting materials have shown promise in bone regeneration and infection mitigation.Stimuli-responsive materials have made advances in the mechanical, magnetic, and acoustic domains.Notably, magnetic-responsive biomaterials enhance cell adherence and proliferation, while acousticresponsive materials stimulate stem cell growth and bone differentiation.The advantages and limitations of smart biopolymers were summarized in Table 1.
Despite the considerable potential of smart biopolymers in bone regeneration, obstacles persist.Interactions of biomaterials depend on factors like size, charge, and shape, and the application of responsive polymers remains challenging due to deep tissue penetration limits, mechanical properties variability, and potential induction of oxidative stress.In conclusion, remarkable advances in bone regeneration have been made, but the ideal polymeric materials for this purpose remain to be developed.Their biophysical and biochemical properties should be further exploited to guide material design and fabrication.Additionally, more theoretical and experimental studies are needed to facilitate controllable manipulation to explore their potentials.Furthermore, interdisciplinary collaborations with artificial intelligence (AI) may also foster designs of polymeric biomaterials for bone repair.AI-assisted techniques such as multi stimuli-responsive methodologies and robocasting may enable biomaterial customization with potential breakthroughs in pore shape control and deep tissue penetration to unlock their full potential in bone regeneration.

TABLE 1
Smart polymeric biomaterials: representative references, advantages and limitations.