Electric field stimulation-responsive hydrogels for bone regeneration: from mechanisms to applications

Abstract

The continuous extension of human life expectancy and the global trend of population aging have contributed to a marked increase in the incidence of musculoskeletal diseases, with fractures and osteoporosis being prominent examples. Consequently, promoting bone regeneration is a crucial medical challenge that demands immediate attention. As early as the mid-20th century, researchers revealed that electrical stimulation could effectively promote the healing and regeneration of bone tissue. This is achieved by mimicking the endogenous electric field within bone tissue, which influences cellular behavior and molecular mechanisms. In recent years, electroactive hydrogels responsive to electric field stimulation have been developed and applied to regulate cell functions at different stages of bone regeneration. This paper elaborates on the regulatory effects of electrical stimulation on MSCs, macrophages, and vascular endothelial cells during the process of bone regeneration. It also involves the activation of relevant ion channels and signaling pathways. Subsequently, it comprehensively reviews various electric-field-responsive hydrogels developed in recent years, covering aspects such as material selection, preparation methods, characteristics, and their applications in bone regeneration. Ultimately, it provides an objective summary of the existing deficiencies in hydrogel materials and research, and looks ahead to future development directions.

Fig. 1

Graphic abstract of this article. This article mainly discusses the following three parts: (1) Cellular and molecular mechanisms related to bone regeneration; (2) Several electrical hydrogels responsive to electric field stimulation; (3) Biomedical applications of electrical hydrogels responsive to electric field stimulation

Fig. 2

Bone regeneration is a complex and interconnected process that occurs in distinct phases. In the first phase (week 1), hematoma formation occurs, where the coagulation cascade triggers both pro-inflammatory and anti-inflammatory events, coordinated by IL-1, IL-6, TNF-α, VEGF, and RANKL, involving M1 and M2 macrophages, Th1 and Th2 cells, and fibroblasts. During week 2 to 3 soft callus formation and angiogenesis occur, involving endothelial cells, hypertrophic chondrocytes, and osteoblasts. During week 4−17 complex callus formation is characterized by matrix mineralization and woven bone development, with the participation of endothelial cells, osteocytes, osteoblasts, and osteoclasts. The bone remodeling phase (week 18–52) involves remodeling using TGF-β and MMPs, which affects endothelial cells, osteocytes, osteoblasts, and osteoclasts, thereby improving the healed fracture and restoring the functional bone structure. This complex temporal and cellular orchestration is the foundation for the successful regeneration of bone tissue. The figure here is based on the work of S. Park and Y. Niu et al. VEGF vascular endothelial growth factor, RANKL nuclear factor κB ligand, MMP matrix metalloproteinases.^, Copyright © 2024 Elsevier Ltd

Fig. 3

Electrical stimulation activates signaling pathways in BMSCs. a G-protein-coupled receptors bind PLC, releasing Ca²⁺ from the endoplasmic reticulum. The increase in intracellular calcium concentration activates protein kinase C, which in turn activates the MAPK pathway. b VGCC allows calcium to enter the cytoplasm, bind to CaM, interact with CaMK, and promote OSX expression. c Bone morphogenetic protein receptors can be activated by a combination of BMP ligands and electrical stimulation, activating the Smad-dependent pathway, leading to RUNX2 expression, or by activating the MAPK extracellular signal-regulated kinase (ERK) and p38 in the activation-independent pathway, which in turn can induce RUNX2 and OSX expression. d Piezoelectric substrates with associated surface potentials can enhance protein adsorption and change their conformation, exposing adhesion domains recognized by integrins. Integrins mediate the response by activating FAK. e Notch signaling pathway may be activated by electrical stimulation. When the NICD is cleaved and enters the nucleus, it promotes the expression of Hey1, Hes5, and Hes1. f Cell-cell junctions and piezoelectric stimulation can activate the Wnt/β-catenin signaling pathway. Due to the decrease in intracellular calcium concentration, β-catenin is released from E-cadherin, accumulating in the cytoplasm and translocating to the nucleus, promoting TCF/LEF expression. PLC phospholipase C, PKC protein kinase C, MAPK mitogen-activated protein kinase, VGCC voltage-gated calcium channels, CaM calmodulin, CaMK calmodulin-dependent protein kinase, OSX osterix, BMP bone morphogenetic protein, RUNX2 runt-related transcription factor, FAK focal adhesion kinase, NICD notch intracellular domain, TCF/LEF T cell factor/lymphoid enhancer factor. Copyright © 2024 Elsevier Ltd

Fig. 4

Ion channels play an important role in bone injury and repair. After the bone and joint injury, danger signals are triggered to alarm. These alarm signals bind to PRRs on immune cells, thereby promoting M1 macrophage polarization and fibroblast activation through ion channels, including TRPV4, TRPA1, and ASIC1a. The regulated release of pro-inflammatory cytokines (including TNF, IL-1β, IL-6, and IL-18) and matrix-degrading enzymes (such as matrix metalloproteinases, or MMPs, and a disintegrin and metalloproteinase, or ADAM) is controlled by ion channels (including Nav1.7, TRPV4, and Piezo) and amplifies the inflammatory response. This cascade reaction leads to cartilage degeneration, heterotopic ossification, synovial inflammation, and joint pain. PRR pattern recognition receptor, MMP matrix metalloproteinase, ADAM a disintegrin and metallo-proteinases. Copyright 2024, Springer Nature

Fig. 5

Microstructure and conductive properties of conductive hydrogels. a Microscopic molecular structures of graphene and conjugated π-polymers. b The conductive hydrogel exhibits superior conductivity and tensile properties, and can activate a light-emitting diode. Copyright 2024, American Association for the Advancement of Science. c, d SEM and TEM images of micron-sized fibers. e Conductivity of the conductive hydrogel. Copyright © 2024 Elsevier Ltd. f Conductivity of HAGN hydrogels containing different concentrations (0%, 10%, 50%) of graphite for 72 h. Copyright 2024, Wiley-VCH

Fig. 6

Structures and piezoelectric capabilities of piezoelectric hydrogels. a Dopamine-modified barium titanate participates in the crosslinking of hydrogels. Representative SEM images of different hydrogel samples, where the red arrows indicate piezoelectric nanoparticles. Copyright 2023, Springer Nature. b SEM images of GelMA, GelMA + c-BTO and GelMA + t-BTO. c–f Electromechanical responses of various hydrogels before and after ultrasonic stimulation under 200 kPa, 50–300 kPa, 0.5–3.5 Hz. Copyright © 2024 Elsevier Ltd

Fig. 7

Other electrical hydrogels (a–c) Nanotriboelectric generator combined with conductive hydrogel. a A schematic diagram of a fully implantable battery-free BD-ES system allows patients to perform active or passive functional exercises under guidance. b, c Output voltage signals of HTP-NG under different pressures and in response to knee bending. Copyright 2024, American Association for the Advancement of Science. d The combination of the drug and the hydrogel under electrostatic adsorption. Transmission electron microscope (TEM) image. Copyright 2024, American Chemical Society

Fig. 8

Conductive devices a–d Characterization of the PLA/KNN@PDA thin-film nanogenerator. a Schematic illustration of the structure of the PLA/KNN@PDA nanogenerator. b SEM image of the PLA/KNN@PDA nanofibers (scale bar: 2 µm). c EDS elemental mapping images of the PLA/KNN@PDA nanofibers. d Open-circuit voltage and short-circuit current generated by the PLA/KNN@PDA thin-film nanogenerator under ultrasonic stimulation. Copyright 2024, Wiley-VCH. e, f Schematic diagrams and photographs of the fabrication process of the self-made ES device. Copyright 2024, American Chemical Society

Fig. 9

The effect of electrical hydrogels on stimulating bone regeneration. a Repair chicken bone defects using bovine ECM hydrogel and biocompatible mPCL scaffolds in the chick embryo CAM model. c, c1, c2: New bone formation and bridging were observed in the Stro-4+ seeded ECM/mPCL scaffold group. Copyright 2020, Wiley-VCH. b, c Osteogenic activity of BMSCs in Z - CS/β - TCP/GO hydrogels. ALP strain staining of BMSCs cultured with different scaffolds for 7 and 14 days, scale bar, 200 μm (b). Alizarin red S staining of cells cultured with varying scaffolds for 14 and 21 days, scale bar, 200 μm (c). Copyright 2023, MDPI. ECM extracellular matrix, mPCL melt electro-written medical-grade polycaprolactone, CAM chorioallantoic membrane, ALP alkaline phosphatase

Fig. 10

Electrical hydrogels can be used to monitor bone regeneration. a–d In vivo NIR-FL/MR imaging studies of different scaffolds through a rat calvarial defect model. NIR-FL images (a) and FL intensity curves (c) of ALP expression in the bone defect area at different times (λex = 660 nm). MR images (b) and signal intensity curves (d) for monitoring scaffold degradation. Copyright © 2024 Elsevier Ltd. LCR meter Inductance capacitance resistance meter, NIR-FL/MR near-infrared fluorescence/magnetic resonance imaging