Electrical stimulation: Effective cue to direct osteogenic differentiation of mesenchymal stem cells?

Construction of Multicellular Neural Tissue Using Three-Dimensional Printing Technology: Cell Interaction

The study of the human nervous system remains challenging due to its inherent complexity and difficulty in obtaining original samples. Three-dimensional (3D) bioprinting is a rapidly evolving technology in the field of tissue engineering that has made significant contributions to several disciplines, including neuroscience. In order to more accurately reflect the intricate multicellular milieu of the in vivo environment, an increasing number of studies have commenced experimentation with the coprinting of diverse cell types. This article provides an overview of technical details and the application of 3D bioprinting with multiple cell types in the field of neuroscience, focusing on the challenges of coprinting and the research conducted based on multicellular printing. This review discusses cell interactions in coprinting systems, stem cell applications, the construction of brain-like organoids, the establishment of disease models, and the potential for integrating 3D bioprinting with other 3D culture techniques.

Inflammatory Microenvironment-Modulated Conductive Hydrogel Promotes Vascularized Bone Regeneration in Infected Bone Defects

Infected bone defects show a significant reduction in neovascularization during the healing process, primarily due to persistent bacterial infection and immune microenvironmental disorders. Existing treatments are difficult to simultaneously meet the requirements of antibacterial and anti-inflammatory treatments for infected bone defects, which is a key clinical therapeutic challenge that needs to be addressed. In this study, a conductive hydrogel based on copper nanoparticles was developed for controlling bacterial infection and remodeling the immune microenvironment. The hydrogel not only effectively eliminates bacteria that exist in the infected bone defect region but also transmits electrical signals to restore the disordered immune microenvironment. In vitro studies have shown that the hydrogel has excellent biocompatibility and can modulate macrophage polarization by transmitting electrical signals to reduce inflammation and promote neovascularization. In vivo studies further confirmed that the hydrogel scaffold not only rapidly cleared clinical bacterial infections but also significantly induced the formation of vascularized new bone tissue within 4 weeks. This work provides a simple and innovative strategy to fabricate copper-containing conductive hydrogels that show great potential for application in the field of therapeutics for infected bone regeneration.

Engineered MXene Biomaterials for Regenerative Medicine

MXene-based materials have attracted significant interest due to their distinct physical and chemical properties, which are relevant to fields such as energy storage, environmental science, and biomedicine. MXene has shown potential in the area of tissue regenerative medicine. However, research on its applications in tissue regeneration is still in its early stages, with a notable absence of comprehensive reviews. This review begins with a detailed description of the intrinsic properties of MXene, followed by a discussion of the various nanostructures that MXene can form, spanning from 0 to 3 dimensions. The focus then shifts to the applications of MXene-based biomaterials in tissue engineering, particularly in immunomodulation, wound healing, bone regeneration, and nerve regeneration. MXene's physicochemical properties, including conductivity, photothermal characteristics, and antibacterial properties, facilitate interactions with different cell types, influencing biological processes. These interactions highlight its potential in modulating cellular functions essential for tissue regeneration. Although the research on MXene in tissue regeneration is still developing, its versatile structural and physicochemical attributes suggest its potential role in advancing regenerative medicine.

From innovation to clinic: Emerging strategies harnessing electrically conductive polymers to enhance electrically stimulated peripheral nerve repair

Peripheral nerve repair (PNR) is a major healthcare challenge due to the limited regenerative capacity of the nervous system, often leading to severe functional impairments. While nerve autografts are the gold standard, their implications are constrained by issues such as donor site morbidity and limited availability, necessitating innovative alternatives like nerve guidance conduits (NGCs). However, the inherently slow nerve growth rate (∼1 mm/day) and prolonged neuroinflammation, delay recovery even with the use of passive (no-conductive) NGCs, resulting in muscle atrophy and loss of locomotor function. Electrical stimulation (ES) has the ability to enhance nerve regeneration rate by modulating the innate bioelectrical microenvironment of nerve tissue while simultaneously fostering a reparative environment through immunoregulation. In this context, electrically conductive polymer (ECP)-based biomaterials offer unique advantages for nerve repair combining their flexibility, akin to traditional plastics, and mixed ionic-electronic conductivity, similar to ionically conductive nerve tissue, as well as their biocompatibility and ease of fabrication. This review focuses on the progress, challenges, and emerging techniques for integrating ECP based NGCs with ES for functional nerve regeneration. It critically evaluates the various approaches using ECP based scaffolds, identifying gaps that have hindered clinical translation. Key challenges discussed include designing effective 3D NGCs with high electroactivity, optimizing ES modules, and better understanding of immunoregulation during nerve repair. The review also explores innovative strategies in material development and wireless, self-powered ES methods. Furthermore, it emphasizes the need for non-invasive ES delivery methods combined with hybrid ECP based neural scaffolds, highlighting future directions for advancing preclinical and clinical translation. Together, ECP based NGCs combined with ES represent a promising avenue for advancing PNR and improving patient outcomes.

Electrical Stimulation Therapy - Dedicated to the Perfect Plastic Repair

Tissue repair and reconstruction are a clinical difficulty. Bioelectricity has been identified as a critical factor in supporting tissue and cell viability during the repair process, presenting substantial potential for clinical application. This review delves into various sources of electrical stimulation and identifies appropriate electrode materials for clinical use. It also highlights the biological mechanisms of electrical stimulation at both the subcellular and cellular levels, elucidating how these interactions facilitate the repair and regeneration processes across different organs. Moreover, specific electrode materials and stimulation sources are outlined, detailing their impact on cellular activity. The future development trends are projected from two perspectives: the optimization of equipment performance and the fulfillment of clinical demands, focusing on the feasibility, safety, and cost-effectiveness of technologies.

Piezoelectric Stimulation Induces Osteogenesis in Mesenchymal Stem Cells Cultured on Electroactive Two-Dimensional Substrates

Physical cues have been shown to be effective in inducing osteogenic differentiation of mesenchymal stem cells (MSCs). Here, we propose piezoelectric stimulation as a potential osteogenic cue mimicking the electroactive properties of bone's extracellular matrix. When combined with a magnetostrictive component, piezoelectric polymers can be used for MSC stimulation by applying an external magnetic field. The deformation of the magnetostrictive component produces a deformation in the polymer matrix, generating a change in the surface charge that induces an electric field that can be transmitted to the cells. Cell adhesion, cytoskeleton changes, and metabolomics are the first evidence of MSC osteoblastogenesis and can be used to study initial MSC response to this kind of stimulation. In the current study, poly(vinylidene) fluoride (PVDF) piezoelectric films with and without cobalt ferrite oxide (CFO) crystallized from the melt in the presence of the ionic liquid 1-butyl-3-methyl-imidazolium chloride ([Bmim][Cl]) were produced. [Bmim][Cl] allowed the production of the β-phase, the most electroactive phase, even without CFO. After ionic liquid removal, PVDF and PVDF-CFO films presented high percentages of the β-phase and similar crystalline content. Incorporating CFO nanoparticles was effective, allowing the electromechanical stimulation of MSCs by applying a magnetic field with a bioreactor. Before stimulation, the initial response of MSCs was characterized in static conditions, showing that the produced films were biocompatible and noncytotoxic, allowing MSC adhesion and proliferation in the short term. Stimulation experiments revealed that MSCs electromechanically stimulated for 3 days in PVDF-CFO supports showed longer focal adhesions and decreased vimentin cytoskeletal density, both signals of early osteogenic differentiation. Furthermore, they rearranged their energy metabolism toward an osteogenic phenotype after 7 days of culture under the same stimulation. The results prove that MSCs respond to electromechanical stimulation by osteogenic differentiation.

Bone morphogenetic protein-2 and pulsed electrical stimulation synergistically promoted osteogenic differentiation on MC-3T3-E1 cells

Electrical stimulation (ES) plays an important role in regulating cell osteoblast differentiation. As a noninvasive rehabilitation therapy method, Es has a unique role in postoperative recovery. Bone morphogenetic protein-2 (BMP-2) is the most commonly used bioactive molecule in in situ tissue engineering scaffolds, and it plays an important regulatory role in the whole process of bone injury repair. In this study, the osteogenic regulation of MC-3T3-E1 cells was studied by combining pulsed electrical stimulation (PES) and different concentrations of BMP-2. The results showed that PES and BMP-2 could synergically promote the proliferation of MC-3T3-E1 cells. The qPCR results of osteoblast-related genes showed that PES was synergistic with BMP-2 to promote osteoblast differentiation mainly through the regulation of the Smad/BMP and insulin like growth factor 1 (IGF1) signaling pathways. The expression level of alkaline phosphatase (ALP) and alizarin red staining further demonstrated the synergistic effect of PES and BMP-2 on promoting osteogenic differentiation and mineralization of cells. PES and BMP-2 could also synergically promote cell proliferation, expression of collagen I (COL-I) and ALP, and cell mineralization on the 3D-printed polylactic acid scaffold. These results suggest that the use of PES can enhance the osteogenic effect of in situ bone repair scaffolds containing BMP-2, reduce the dose of BMP-2 alone, and reduce the possible side effects of high-dose BMP-2 in vivo.

Unleashing the Potential of Electroactive Hybrid Biomaterials and Self-Powered Systems for Bone Therapeutics

The incidence of large bone defects caused by traumatic injury is increasing worldwide, and the tissue regeneration process requires a long recovery time due to limited self-healing capability. Endogenous bioelectrical phenomena have been well recognized as critical biophysical factors in bone remodeling and regeneration. Inspired by bioelectricity, electrical stimulation has been widely considered an external intervention to induce the osteogenic lineage of cells and enhance the synthesis of the extracellular matrix, thereby accelerating bone regeneration. With ongoing advances in biomaterials and energy-harvesting techniques, electroactive biomaterials and self-powered systems have been considered biomimetic approaches to ensure functional recovery by recapitulating the natural electrophysiological microenvironment of healthy bone tissue. In this review, we first introduce the role of bioelectricity and the endogenous electric field in bone tissue and summarize different techniques to electrically stimulate cells and tissue. Next, we highlight the latest progress in exploring electroactive hybrid biomaterials as well as self-powered systems such as triboelectric and piezoelectric-based nanogenerators and photovoltaic cell-based devices and their implementation in bone tissue engineering. Finally, we emphasize the significance of simulating the target tissue's electrophysiological microenvironment and propose the opportunities and challenges faced by electroactive hybrid biomaterials and self-powered bioelectronics for bone repair strategies.

Assemblable 3D biomimetic microenvironment for hMSC osteogenic differentiation

Adequate simulation mimicking a tissue's native environment is one of the elemental premises in tissue engineering. Although various attempts have been made to induce human mesenchymal stem cells (hMSC) into an osteogenic pathway, they are still far from widespread clinical application. Most strategies focus primarily on providing a specific type of cue, inadequately replicating the complexity of the bone microenvironment. An alternative multifunctional platform for hMSC osteogenic differentiation has been produced. It is based on poly(vinylidene fluoride) (PVDF) and cobalt ferrites magnetoelectric microspheres, functionalized with collagen and gelatin, and packed in a 3D arrangement. This platform is capable of performing mechanical stimulation of piezoelectric PVDF, mimicking the bones electromechanical biophysical cues. Surface functionalization with extracellular matrix biomolecules and osteogenic medium complete this all-round approach. hMSC were cultured in osteogenic inducing conditions and tested for proliferation, surface biomarkers, and gene expression to evaluate their osteogenic commitment.

Bioelectronics for electrical stimulation: materials, devices and biomedical applications

Bioelectronics is a hot research topic, yet an important tool, as it facilitates the creation of advanced medical devices that interact with biological systems to effectively diagnose, monitor and treat a broad spectrum of health conditions. Electrical stimulation (ES) is a pivotal technique in bioelectronics, offering a precise, non-pharmacological means to modulate and control biological processes across molecular, cellular, tissue, and organ levels. This method holds the potential to restore or enhance physiological functions compromised by diseases or injuries by integrating sophisticated electrical signals, device interfaces, and designs tailored to specific biological mechanisms. This review explains the mechanisms by which ES influences cellular behaviors, introduces the essential stimulation principles, discusses the performance requirements for optimal ES systems, and highlights the representative applications. From this review, we can realize the potential of ES based bioelectronics in therapy, regenerative medicine and rehabilitation engineering technologies, ranging from tissue engineering to neurological technologies, and the modulation of cardiovascular and cognitive functions. This review underscores the versatility of ES in various biomedical contexts and emphasizes the need to adapt to complex biological and clinical landscapes it addresses.

Recent Advances in Nanomaterials for Modulation of Stem Cell Differentiation and Its Therapeutic Applications

Challenges in directed differentiation and survival limit the clinical use of stem cells despite their promising therapeutic potential in regenerative medicine. Nanotechnology has emerged as a powerful tool to address these challenges and enable precise control over stem cell fate. In particular, nanomaterials can mimic an extracellular matrix and provide specific cues to guide stem cell differentiation and proliferation in the field of nanotechnology. For instance, recent studies have demonstrated that nanostructured surfaces and scaffolds can enhance stem cell lineage commitment modulated by intracellular regulation and external stimulation, such as reactive oxygen species (ROS) scavenging, autophagy, or electrical stimulation. Furthermore, nanoframework-based and upconversion nanoparticles can be used to deliver bioactive molecules, growth factors, and genetic materials to facilitate stem cell differentiation and tissue regeneration. The increasing use of nanostructures in stem cell research has led to the development of new therapeutic approaches. Therefore, this review provides an overview of recent advances in nanomaterials for modulating stem cell differentiation, including metal-, carbon-, and peptide-based strategies. In addition, we highlight the potential of these nano-enabled technologies for clinical applications of stem cell therapy by focusing on improving the differentiation efficiency and therapeutics. We believe that this review will inspire researchers to intensify their efforts and deepen their understanding, thereby accelerating the development of stem cell differentiation modulation, therapeutic applications in the pharmaceutical industry, and stem cell therapeutics.

Advances in electroactive biomaterials: Through the lens of electrical stimulation promoting bone regeneration strategy

The regenerative capacity of bone is indispensable for growth, given that accidental injury is almost inevitable. Bone regenerative capacity is relevant for the aging population globally and for the repair of large bone defects after osteotomy (e.g., following removal of malignant bone tumours). Among the many therapeutic modalities proposed to bone regeneration, electrical stimulation has attracted significant attention owing to its economic convenience and exceptional curative effects, and various electroactive biomaterials have emerged. This review summarizes the current knowledge and progress regarding electrical stimulation strategies for improving bone repair. Such strategies range from traditional methods of delivering electrical stimulation via electroconductive materials using external power sources to self-powered biomaterials, such as piezoelectric materials and nanogenerators. Electrical stimulation and osteogenesis are related via bone piezoelectricity. This review examines cell behaviour and the potential mechanisms of electrostimulation via electroactive biomaterials in bone healing, aiming to provide new insights regarding the mechanisms of bone regeneration using electroactive biomaterials.

The translational potential of this article

This review examines the roles of electroactive biomaterials in rehabilitating the electrical microenvironment to facilitate bone regeneration, addressing current progress in electrical biomaterials and the mechanisms whereby electrical cues mediate bone regeneration. Interactions between osteogenesis-related cells and electroactive biomaterials are summarized, leading to proposals regarding the use of electrical stimulation-based therapies to accelerate bone healing.

Osteogenic differentiation of human mesenchymal stem cells on electroactive substrates

This study investigates the effect of electroactivity and electrical charge distribution on the biological response of human bone marrow stem cells (hBMSCs) cultured in monolayer on flat poly(vinylidene fluoride), PVDF, substrates. Differences in cell behaviour, including proliferation, expression of multipotency markers CD90, CD105 and CD73, and expression of genes characteristic of different mesenchymal lineages, were observed both during expansion in basal medium before reaching confluence and in confluent cultures in osteogenic induction medium. The crystallisation of PVDF in the electrically neutral α-phase or in the electroactive phase β, both unpoled and poled, has been found to have an important influence on the biological response. In addition, the presence of a permanent positive or negative surface electrical charge distribution in phase β substrates has also shown a significant effect on cell behaviour.

Synergy between 3D-extruded electroconductive scaffolds and electrical stimulation to improve bone tissue engineering strategies

In this work, we propose a simple, reliable, and versatile strategy to create 3D electroconductive scaffolds suitable for bone tissue engineering (TE) applications with electrical stimulation (ES). The proposed scaffolds are made of 3D-extruded poly(ε-caprolactone) (PCL), subjected to alkaline treatment, and of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), anchored to PCL with one of two different crosslinkers: (3-glycidyloxypropyl)trimethoxysilane (GOPS) and divinyl sulfone (DVS). Both cross-linkers allowed the formation of a homogenous and continuous coating of PEDOT:PSS to PCL. We show that these PEDOT:PSS coatings are electroconductive (11.3-20.1 S cm-1), stable (up to 21 days in saline solution), and allow the immobilization of gelatin (Gel) to further improve bioactivity. In vitro mineralization of the corresponding 3D conductive scaffolds was greatly enhanced (GOPS(NaOH)-Gel - 3.1 fold, DVS(NaOH)-Gel - 2.0 fold) and cell colonization and proliferation were the highest for the DVS(NaOH)-Gel scaffold. In silico modelling of ES application in DVS(NaOH)-Gel scaffolds indicates that the electrical field distribution is homogeneous, which reduces the probability of formation of faradaic products. Osteogenic differentiation of human bone marrow derived mesenchymal stem/stromal cells (hBM-MSCs) was performed under ES. Importantly, our results clearly demonstrated a synergistic effect of scaffold electroconductivity and ES on the enhancement of MSC osteogenic differentiation, particularly on cell-secreted calcium deposition and the upregulation of osteogenic gene markers such as COL I, OC and CACNA1C. These scaffolds hold promise for future clinical applications, including manufacturing of personalized bone TE grafts for transplantation with enhanced maturation/functionality or bioelectronic devices.

Direct coupled electrical stimulation towards improved osteogenic differentiation of human mesenchymal stem/stromal cells: a comparative study of different protocols

Electrical stimulation (ES) has been described as a promising tool for bone tissue engineering, being known to promote vital cellular processes such as cell proliferation, migration, and differentiation. Despite the high variability of applied protocol parameters, direct coupled electric fields have been successfully applied to promote osteogenic and osteoinductive processes in vitro and in vivo. Our work aims to study the viability, proliferation, and osteogenic differentiation of human bone marrow-derived mesenchymal stem/stromal cells when subjected to five different ES protocols. The protocols were specifically selected to understand the biological effects of different parts of the generated waveform for typical direct-coupled stimuli. In vitro culture studies evidenced variations in cell responses with different electric field magnitudes (numerically predicted) and exposure protocols, mainly regarding tissue mineralization (calcium contents) and osteogenic marker gene expression while maintaining high cell viability and regular morphology. Overall, our results highlight the importance of numerical guided experiments to optimize ES parameters towards improved in vitro osteogenesis protocols.

Electro-mechanical coupling directs endothelial activities through intracellular calcium ion deployment

Conversion between mechanical and electrical cues is usually considered unidirectional in cells with cardiomyocytes being an exception. Here, we discover a material-induced external electric field (Eex) triggers an electro-mechanical coupling feedback loop in cells other than cardiomyocytes, human umbilical vein endothelial cells (HUVECs), by opening their mechanosensitive Piezo1 channels. When HUVECs are cultured on patterned piezoelectric materials, the materials generate Eex (confined at the cellular scale) to polarize intracellular calcium ions ([Ca2+]i), forming a built-in electric field (Ein) opposing Eex. Furthermore, the [Ca2+]i polarization stimulates HUVECs to shrink their cytoskeletons, activating Piezo1 channels to induce influx of extracellular Ca2+ that gradually increases Ein to balance Eex. Such an electro-mechanical coupling feedback loop directs pre-angiogenic activities such as alignment, elongation, and migration of HUVECs. Activated calcium dynamics during the coupling further modulate the downstream angiogenesis-inducing eNOS/NO pathway. These findings lay a foundation for developing new ways of electrical stimulation-based disease treatment.

Hydroxyapatite-filled osteoinductive and piezoelectric nanofibers for bone tissue engineering

Osteoporotic-related fractures are among the leading causes of chronic disease morbidity in Europe and in the US. While a significant percentage of fractures can be repaired naturally, in delayed-union and non-union fractures surgical intervention is necessary for proper bone regeneration. Given the current lack of optimized clinical techniques to adequately address this issue, bone tissue engineering (BTE) strategies focusing on the development of scaffolds for temporarily replacing damaged bone and supporting its regeneration process have been gaining interest. The piezoelectric properties of bone, which have an important role in tissue homeostasis and regeneration, have been frequently neglected in the design of BTE scaffolds. Therefore, in this study, we developed novel hydroxyapatite (HAp)-filled osteoinductive and piezoelectric poly(vinylidene fluoride-co-tetrafluoroethylene) (PVDF-TrFE) nanofibers via electrospinning capable of replicating the tissue's fibrous extracellular matrix (ECM) composition and native piezoelectric properties. The developed PVDF-TrFE/HAp nanofibers had biomimetic collagen fibril-like diameters, as well as enhanced piezoelectric and surface properties, which translated into a better capacity to assist the mineralization process and cell proliferation. The biological cues provided by the HAp nanoparticles enhanced the osteogenic differentiation of seeded human mesenchymal stem/stromal cells (MSCs) as observed by the increased ALP activity, cell-secreted calcium deposition and osteogenic gene expression levels observed for the HAp-containing fibers. Overall, our findings describe the potential of combining PVDF-TrFE and HAp for developing electroactive and osteoinductive nanofibers capable of supporting bone tissue regeneration.

Cell Behavior Changes and Enzymatic Biodegradation of Hybrid Electrospun Poly(3-hydroxybutyrate)-Based Scaffolds with an Enhanced Piezoresponse after the Addition of Reduced Graphene Oxide

This is the first comprehensive study of the impact of biodegradation on the structure, surface potential, mechanical and piezoelectric properties of poly(3-hydroxybutyrate) (PHB) scaffolds supplemented with reduced graphene oxide (rGO) as well as cell behavior under static and dynamic mechanical conditions. There is no effect of the rGO addition up to 1.0 wt% on the rate of enzymatic biodegradation of PHB scaffolds for 30 d. The biodegradation of scaffolds leads to the depolymerization of the amorphous phase, resulting in an increase in the degree of crystallinity. Because of more regular dipole order in the crystalline phase, surface potential of all fibers increases after the biodegradation, with a maximum (361 ± 5 mV) after the addition of 1 wt% rGO into PHB as compared to pristine PHB fibers. By contrast, PHB-0.7rGO fibers manifest the strongest effective vertical (0.59 ± 0.03 pm V^-1 ) and lateral (1.06 ± 0.02 pm V^-1 ) piezoresponse owing to a greater presence of electroactive β-phase. In vitro assays involving primary human fibroblasts reveal equal biocompatibility and faster cell proliferation on PHB-0.7rGO scaffolds compared to pure PHB and nonpiezoelectric polycaprolactone scaffolds. Thus, the developed biodegradable PHB-rGO scaffolds with enhanced piezoresponse are promising for tissue-engineering applications.

Electrically stimulated 3D bioprinting of gelatin-polypyrrole hydrogel with dynamic semi-IPN network induces osteogenesis via collective signaling and immunopolarization

In recent years, three-dimensional (3D) bioprinting of conductive hydrogels has made significant progress in the fabrication of high-resolution biomimetic structures with gradual complexity. However, the lack of an effective cross-linking strategy, ideal shear-thinning, appropriate yield strength, and higher print fidelity with excellent biofunctionality remains a challenge for developing cell-laden constructs, hindering the progress of extrusion-based 3D printing of conductive polymers. In this study, a highly stable and conductive bioink was developed based on polypyrrole-grafted gelatin methacryloyl (GelMA-PPy) with a triple cross-linking (thermo-photo-ionically) strategy for direct ink writing-based 3D printing applications. The triple-cross-linked hydrogel with dynamic semi-inner penetrating polymer network (semi-IPN) displayed excellent shear-thinning properties, with improved shape fidelity and structural stability during 3D printing. The as-fabricated hydrogel ink also exhibited "plug-like non-Newtonian" flow behavior with minimal disturbance. The bioprinted GelMA-PPy-Fe hydrogel showed higher cytocompatibility (93%) of human bone mesenchymal stem cells (hBMSCs) under microcurrent stimulation (250 mV/20 min/day). Moreover, the self-supporting and tunable mechanical properties of the GelMA-PPy bioink allowed 3D printing of high-resolution biological architectures. As a proof of concept, we printed a full-thickness rat bone model to demonstrate the structural stability. Transcriptomic analysis revealed that the 3D bioprinted hBMSCs highly expressed gene hallmarks for NOTCH/mitogen-activated protein kinase (MAPK)/SMAD signaling while down-regulating the Wnt/β-Catenin and epigenetic signaling pathways during osteogenic differentiation for up to 7 days. These results suggest that the developed GelMA-PPy bioink is highly stable and non-toxic to hBMSCs and can serve as a promising platform for bone tissue engineering applications.

In vitro evaluation of electrospun polyvinylidene fluoride hybrid nanoparticles as direct piezoelectric membranes for guided bone regeneration

A polyvinylidene fluoride (PVDF) piezoelectric membrane containing carbon nanotubes (CNTs) and graphene oxide (GO) additives was prepared, with special emphasis on the piezoelectric activity of the aligned fibers. Fibroblast viability on membranes was measured to study cytotoxicity. Osteoprogenitor D1 cells were cultured, and mineralization of piezoelectric composite membranes was assessed by ultrasound stimulation. Results showed that the electrospun microstructures were anisotropically aligned fibers. As the GO content increased to 1.0 wt% (0.2 wt% interval), the β phase in PVDF slightly increased but showed the opposite trend with the increase in CNT. Excessive addition of GO and CNT hindered the growth of the β phase in PVDF. The direct piezoelectric activity and mechanical properties showed the same trend as the β phase in PVDF. Moreover, GO/PVDF with the same nanoparticle content showed better performance than CNT/PVDF composites. In this study, a comparison of the generated piezoelectric specific voltage (unit: 10^-3 Vg^-1 cm^-2, linear stretch, g₃₃) with control PVDF only (0.55 ± 0.16) revealed that the two composites containing 0.8 wt% GO- and 0.2 wt% CNT- with 15 wt% PVDF exhibited excellent piezoelectric voltages, which were 3.37 ± 1.05 and 1.45 ± 0.07 (10^-3 Vg^-1 cm^-2), respectively. In vitro cultures of these two groups in contact with D1 cells showed significantly higher alkaline phosphatase secretion than the PVDF only group within 1-10 days of cell culture. Further application of ultrasound stimulation showed that the piezoelectric membrane differentiated D1 cells earlier than without ultrasound and induced higher proliferation and mineralization. This developing piezoelectric effect is expected to generate voltage through activities to enhance microcurrent stimulation in vivo.

Magnetically Activated Piezoelectric 3D Platform Based on Poly(Vinylidene) Fluoride Microspheres for Osteogenic Differentiation of Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) osteogenic commitment before injection enhances bone regeneration therapy results. Piezoelectric stimulation may be an effective cue to promote MSCs pre-differentiation, and poly(vinylidene) fluoride (PVDF) cell culture supports, when combined with CoFe2O4 (CFO), offer a wireless in vitro stimulation strategy. Under an external magnetic field, CFO shift and magnetostriction deform the polymer matrix varying the polymer surface charge due to the piezoelectric effect. To test the effect of piezoelectric stimulation on MSCs, our approach is based on a gelatin hydrogel with embedded MSCs and PVDF-CFO electroactive microspheres. Microspheres were produced by electrospray technique, favouring CFO incorporation, crystallisation in β-phase (85%) and a crystallinity degree of around 55%. The absence of cytotoxicity of the 3D construct was confirmed 24 h after cell encapsulation. Cells were viable, evenly distributed in the hydrogel matrix and surrounded by microspheres, allowing local stimulation. Hydrogels were stimulated using a magnetic bioreactor, and no significant changes were observed in MSCs proliferation in the short or long term. Nevertheless, piezoelectric stimulation upregulated RUNX2 expression after 7 days, indicating the activation of the osteogenic differentiation pathway. These results open the door for optimising a stimulation protocol allowing the application of the magnetically activated 3D electroactive cell culture support for MSCs pre-differentiation before transplantation.

The Synergistic Effect of Cyclic Tensile Force and Periodontal Ligament Cell-Laden Calcium Silicate/Gelatin Methacrylate Auxetic Hydrogel Scaffolds for Bone Regeneration

The development of 3D printing technologies has allowed us to fabricate complex novel scaffolds for bone regeneration. In this study, we reported the incorporation of different concentrations of calcium silicate (CS) powder into fish gelatin methacrylate (FGelMa) for the fabrication of CS/FGelMa auxetic bio-scaffolds using 3D printing technology. Our results showed that CS could be successfully incorporated into FGelMa without influencing the original structural components of FGelMa. Furthermore, it conveyed that CS modifications both the mechanical properties and degradation rates of the scaffolds were improved in accordance with the concentrations of CS upon modifications of CS. In addition, the presence of CS enhanced the adhesion and proliferation of human periodontal ligament cells (hPDLs) cultured in the scaffold. Further osteogenic evaluation also confirmed that CS was able to enhance the osteogenic capabilities via activation of downstream intracellular factors such as pFAK/FAK and pERK/ERK. More interestingly, it was noted that the application of extrinsic biomechanical stimulation to the auxetic scaffolds further enhanced the proliferation and differentiation of hPDLs cells and secretion of osteogenic-related markers when compared to CS/FGelMa hydrogels without tensile stimulation. This prompted us to explore the related mechanism behind this interesting phenomenon. Subsequent studies showed that biomechanical stimulation works via YAP, which is a biomechanical cue. Taken together, our results showed that novel auxetic scaffolds could be fabricated by combining different aspects of science and technology, in order to improve the future chances of clinical applications for bone regeneration.