IntroductionTeeth are one of the most important tissues of the human body. However, due to bacterial invasion and trauma, dental pulp necrosis and dentin defects can occur, resulting in the deterioration of tooth structures and the compromise of physiological functions.1 Dental caries is a pertinent example. Severe forms of this condition can lead to damage to the dentin matrix, thus exposing the dental pulp to the oral microenvironment. The dental pulp is susceptible to infection by microorganisms, which can result in inflammation and necrosis.2,3,4 In clinical, the treatment of pulp-related caries commonly involves the utilization of medical pastes or pulp capping, avoiding further inflammation and bacterial invasion.5 However, when the dental pulp was seriously infected or necrotic, the therapeutic approach entails root canal therapy (RCT), which uses inert materials to replace the necrotic tissue.6,7 It should be noted that the currently applicable filling materials such as calcium hydroxide, exhibit a limited ability to promote odontogenic differentiation of dental pulp cells and dentin formation.6,8 The long-term delayed regeneration and structure integration of pulp-dentin complex can increase the brittleness of teeth, loss of defense ability and teeth vitality.7 Generally, dentin caries is associated with simultaneous damage to the dentin and pulp. Moreover, due to the close structural connection and interdependent functions between dental pulp and dentin,9,10 the concept of integration regeneration for pulp-dentin complex has attracted increasing attention.2,11Dental pulp-dentin complex is densely innervated with multiple nerve fibers, that discriminate the functions of sense external stimuli such as pain and temperature.7,12 Besides, sensory nerves could secret numerous neurotransmitters and neurotrophic factors to regulate inflammation, angiogenesis and the odontogenic differentiation of dental pulp cells, thus actively participating in pulp-dentin complex repair.7,13,14,15 In the sensory nerve-deficient microenvironment, the supplementation of activin B has been confirmed to promote the proliferation and reduced the apoptosis of dental pulp stem cells (DPSCs).15 It is reported that Schwann cells (SCs) possess the capacity of secreting extracellular vesicles to induce angiogenesis and neurite outgrowth.6,16,17 Moreover, SCs-derived extracellular vesicles can also activate SDF-1/CXCR4 axis to recruit endogenous stem cells, and thus contribute to the formation of structures similar to the pulp-dentin complex.6,16 Hence, innervation should be fully considered when designing biomaterials for dental pulp-dentin complex regeneration. Unfortunately, to our knowledge, current strategies are mainly focused on antibacterial,18,19 immune regulation,20,21 and mineral deposition,22,23,24 which largely ignored the great significance of innervation, leading to unsatisfactory therapeutic outcomes. Therefore, it remains challenging to achieve innervated pulp-dentin complex regeneration.7,25Silicate bioceramics have gained increasing attention in regenerative medicine owing to their excellent biocompatibility and biodegradability,26,27 and have been successfully employed for tissue regeneration, including bone, cartilage, skin, muscles, and nerve tissues.28,29,30,31,32 The bioactive ions released from silicate bioceramics could create beneficial ionic microenvironments for regulating multiple cell behaviors, including proliferation, migration and differentiation.32,33,34,35 Lithium (Li), one of the essential trace elements, possesses notable neuroactive and neuroprotective effects.36,37,38 Studies have shown that Li ions enhance the migration, differentiation and myelination of SCs via β-catenin signals.39,40 Moreover, Li ions could promote the neuronal differentiation of neural stem cells by activating the PI3K-AKT signaling pathway.32 It is reported that Li-containing fillers could stimulate DPSCs migration and odontogenic differentiation through activating Wnt/β-catenin pathway, inducing dentin formation.41 Besides, as the main component of tooth, calcium (Ca) actively participates in mineral deposition and dentin formation.42 Ca ions also play crucial roles in neurogenesis.43,44 Moreover, silicon (Si) ions have been demonstrated to induce the osteogenic differentiation of bone mesenchymal stem cells, and promote the neurite outgrowth of dorsal root ganglion neurons as well as secreting neuropeptides.45,46 Hence, it is reasonable to speculate that Li-Ca-Si (LCS) bioceramics could act as bioactive factors to induce neurogenesis odontogenesis, which further promote innervated pulp-dentin complex regeneration.Herein, we successfully prepared an injectable bioceramics-containing composite hydrogel composed of LCS bioceramics particles and gelatin methacryloyl (GelMA) matrix for the treatment of pulp-dentin complex defects (Fig. 1). Firstly, GelMA was chosen as a filler for in situ injected into the defects in a minimally invasive manner, serving as a barrier to protect pulp tissues. Secondly, LCS bioceramic particles were incorporated into the hydrogel, functioning as bioactive factors for regulating cell behavior. Specifically, the released bioactive Li, Ca, Si ions have the capacity to enhance the migration, and neurogenic differentiation of SCs, as well as the odontogenic differentiation of DPSCs. Besides, the function of composite hydrogel in mediating neurogenesis-odontogenesis couples was investigated. Finally, the effectiveness of bioceramics-containing composite hydrogel on innervated pulp-dentin complex regeneration was explored in vivo. Overall, we expect that this study could offer a new concept for achieving functional pulp-dentin regeneration via inducing innervation by biomaterials.Fig. 1Schematic diagram of the preparation of an injectable bioceramics-containing composite hydrogel and its application in the regeneration of innervated dentin-pulp complex. The injectable composite hydrogel was prepared by incorporating lithium calcium silicate (LCS) bioceramics into GelMA hydrogel. The bioceramics-containing composite hydrogel was in situ injected and then crosslinked by blue light to fill the defects. Multiple bioactive Li, Ca and Si ions released by the composite hydrogel could promote odontogenesis and neurogenesis, thus contributing to the regeneration of dentin-pulp complex with innervationFull size imageResultsPreparation and characterization of LCS bioceramics and injectable composite hydrogelLCS bioceramics particles were synthesized by sol-gel method as our previous report.33,47 As shown in Fig. 2a, LCS particles exhibited an irregular morphology with micron-scale size. The XRD pattern showed that all the diffraction peaks of particles could be well indexed into Li2Ca2Si2O7 phase (Fig. 2b). Fig. S1 showed that the particle size of LCS bioceramics particles is below 50 μm. GelMA hydrogel was synthesized according to previous studies.45,48 As shown in Fig. 2c, the storage modulus (G’) of the hydrogel was immediately increased and significantly higher than loss modulus (G”) after exposed to blue, indicating its light-triggered gelation properties. Figure 2d showed the representative images of sol-gel transition process of composite hydrogel.Fig. 2Characterization of Li2Ca2Si2O7 (LCS) bioceramics particles and bioceramics-containing composite hydrogel. a SEM image of LCS particles. b XRD pattern of LCS particles. c Modulus-time curves of composite hydrogel with blue light crosslinking. d Digital photographs of hydrogel before and after crosslinked by blue light. e SEM images of these composite hydrogels. f The stress-strain curve of composite hydrogels. g The compression modulus analysis of composite hydrogels (n = 3). h Swelling ratio of the hydrogels (n = 6). i Equilibrium water content of the hydrogels (n = 6). *P