Interplay of Substrate Conductivity, Cellular Microenvironment, and Pulsatile Electrical Stimulation toward Osteogenesis of Human Mesenchymal Stem Cells in Vitro

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.

Synergistic effect of ultrasound and reinforced electrical environment by bioinspired periosteum for enhanced osteogenesis via immunomodulation of macrophage polarization through Piezo1

The periosteum plays a vital role in repairing bone defects. Researchers have demonstrated the existence of electrical potential in the periosteum and native bone, indicating that electrical signals are essential for functional bone regeneration. However, the clinical use of external electrical treatments has been limited due to their inconvenience and inefficacy. As an alternative, low-intensity pulsed ultrasound (LIPUS) is a noninvasive form of physical therapy that enhances bone regeneration. Furthermore, the wireless activation of piezoelectric biomaterials through ultrasound stimulation would generate electric charges precisely at the defect area, compensating for the insufficiency of external electrical stimulation and potentially promoting bone regeneration through the synergistic effect of mechanical and electrical stimulation. However, the optimal integration of LIPUS with an appropriate piezoelectric periosteum is yet to be explored. Herein, the BaTiO3/multiwalled-carbon nanotubes/collagen (BMC) membranes have been fabricated, possessing physicochemical properties including improved surface hydrophilicity, enhanced mechanical performance, ideal piezoelectricity, and outstanding biocompatibility, all of which are conducive to bone regeneration. When combined with LIPUS, the endogenous electrical microenvironment of native bone was recreated. After that, the wireless-generated electrical signals, along with the mechanical signals induced by LIPUS, were transferred to macrophages and activated Ca2+ influx through Piezo1. Ultimately, the regenerative effect of the BMC membrane with LIPUS stimulation (BMC + L) was confirmed in a mouse cranial defect model. Together, this research presents a co-engineering strategy that involves fabricating a novel biomimetic periosteum and utilizing the synergistic effect of ultrasound to enhance bone regeneration, which is achieved through the reinforcement of the electrical environment and the immunomodulation of macrophage polarization.

Decoupling the Influence of Poly(3,4-Ethylenedioxythiophene)-Collagen Composite Characteristics on Cell Stemness

Conductive polymers (CPs) are widely studied for their ability to influence a myriad of tissue systems. While their mixed ionic/electronic conductivity is commonly considered the primary driver of these benefits, the mechanisms by which CPs influence cell fate remain unclear. In this study, CP-biomaterial interactions are investigated using collagen, due to its widespread prevalence throughout the body and in tissue engineering constructs. Collagen is functionalized with both electrostatically and covalently bound derivatives of the CP poly(3,4-ethylenedioxythiophene) (PEDOT) doped via backbone-tethered sulfonate groups, which enable high solubility and loading to the collagen biomatrix. Intrinsically doped scaffolds are compared to those incorporated with a commercially available PEDOT formulation, which is complexed with polyanionic polystyrene sulfonate (PSS). Low loadings of intrinsically doped PEDOT do not increase substrate conductivity compared to collagen alone, enabling separate investigation into CP loading and conductivity. Interestingly, higher PEDOT loading bolsters human mesenchymal stromal (hMSC) cell gene expression of Oct-4 and NANOG, which are key transcription factors regulating cell stemness. Conductive collagen composites with commercial PEDOT:PSS do not significantly affect the expression of these transcription factors in hMSCs. Furthermore, it is demonstrated that PEDOT regulates cellular fate independently from physical changes to the material but directly to the loading of the polymer.

Advances in natural and synthetic macromolecules with stem cells and extracellular vesicles for orthopedic disease treatment

Serious orthopedic disorders resulting from myriad diseases and impairments continue to pose a considerable challenge to contemporary clinical care. Owing to its limited regenerative capacity, achieving complete bone tissue regeneration and complete functional restoration has proven challenging with existing treatments. By virtue of cellular regenerative and paracrine pathways, stem cells are extensively utilized in the restoration and regeneration of bone tissue; however, low survival and retention after transplantation severely limit their therapeutic effect. Meanwhile, biomolecule materials provide a delivery platform that improves stem cell survival, increases retention, and enhances therapeutic efficacy. In this review, we present the basic concepts of stem cells and extracellular vesicles from different sources, emphasizing the importance of using appropriate expansion methods and modification strategies. We then review different types of biomolecule materials, focusing on their design strategies. Moreover, we summarize several forms of biomaterial preparation and application strategies as well as current research on biomacromolecule materials loaded with stem cells and extracellular vesicles. Finally, we present the challenges currently impeding their clinical application for the treatment of orthopedic diseases. The article aims to provide researchers with new insights for subsequent investigations.

Smart Hydrogels for Tissue Regeneration

The rapid growth in the portion of the aging population has led to a consequent increase in demand for biomedical hydrogels, together with an assortment of challenges that need to be overcome in this field. Smart hydrogels can autonomously sense and respond to the physiological/pathological changes of the tissue microenvironment and continuously adapt the response according to the dynamic spatiotemporal shifts in conditions. This along with other favorable properties, make smart hydrogels excellent materials for employing toward improving the precision of treatment for age-related diseases. The key factor during the smart hydrogel design is on accurately identifying the characteristics of natural tissues and faithfully replicating the composition, structure, and biological functions of these tissues at the molecular level. Such hydrogels can accurately sense distinct physiological and external factors such as temperature and biologically active molecules, so they may in turn actively and promptly adjust their response, by regulating their own biological effects, thereby promoting damaged tissue repair. This review summarizes the design strategies employed in the creation of smart hydrogels, their response mechanisms, as well as their applications in field of tissue engineering; and concludes by briefly discussing the relevant challenges and future prospects.

Harmonizing Thickness and Permeability in Bone Tissue Engineering: A Novel Silk Fibroin Membrane Inspired by Spider Silk Dynamics

Guided bone regeneration gathers significant interest in the realm of bone tissue engineering; however, the interplay between membrane thickness and permeability continues to pose a challenge that can be addressed by the water-collecting mechanism of spider silk, where water droplets efficiently move from smooth filaments to rough conical nodules. Inspired by the natural design of spider silk, an innovative silk fibroin membrane is developed featuring directional fluid transportation via harmoniously integrating a smooth, dense layer with a rough, loose layer; conical microchannels are engineered in the smooth and compact layer. Consequently, double-layered membranes with cone-shaped microporous passageways (CSMP-DSF membrane) are designed for in situ bone repair. Through extensive in vitro testing, it is noted that the CSMP-DSF membrane guides liquid flow from the compact layer's surface to the loose layer, enabling rapid diffusion. Remarkably, the CSMP-DSF membrane demonstrates superior mechanical properties and resistance to bacterial adhesion. When applied in vivo, the CSMP-DSF membrane achieves results on par with the commercial Bio-Gide collagen membranes. This innovative integration of a cross-thickness wetting gradient structure offers a novel solution, harmonizing the often-conflicting requirements of material transport, mechanical strength, and barrier effectiveness, while also addressing issues related to tissue engineering scaffold perfusion.

Engineered hydrogels for peripheral nerve repair

Peripheral nerve injury (PNI) is a complex disease that often appears in young adults. It is characterized by a high incidence, limited treatment options, and poor clinical outcomes. This disease not only causes dysfunction and psychological disorders in patients but also brings a heavy burden to the society. Currently, autologous nerve grafting is the gold standard in clinical treatment, but complications, such as the limited source of donor tissue and scar tissue formation, often further limit the therapeutic effect. Recently, a growing number of studies have used tissue-engineered materials to create a natural microenvironment similar to the nervous system and thus promote the regeneration of neural tissue and the recovery of impaired neural function with promising results. Hydrogels are often used as materials for the culture and differentiation of neurogenic cells due to their unique physical and chemical properties. Hydrogels can provide three-dimensional hydration networks that can be integrated into a variety of sizes and shapes to suit the morphology of neural tissues. In this review, we discuss the recent advances of engineered hydrogels for peripheral nerve repair and analyze the role of several different therapeutic strategies of hydrogels in PNI through the application characteristics of hydrogels in nerve tissue engineering (NTE). Furthermore, the prospects and challenges of the application of hydrogels in the treatment of PNI are also discussed.

Silk-based conductive materials for smart biointerfaces

Silk-based conductive materials are widely used in biointerface applications, such as artificial epidermal sensors, soft and implantable bioelectronics, and tissue/cell scaffolds. Such biointerface materials require coordinated physicochemical, biological, and mechanical properties to meet current practical needs and future sophisticated demands. However, it remains a challenge to formulate silk-based advanced materials with high electrical conductivity, good biocompatibility, mechanical robustness, and in some cases, tissue adhesion ability without compromising other physicochemical properties. In this review, we highlight recent progress in the development of functional conductive silk-based advanced materials with different morphologies. Then, we reviewed the advanced paradigms of these silk materials applied as wearable flexible sensors, implantable electronics, and tissue/cell engineering with perspectives on the application challenges. Silk-based conductive materials can serve as promising building blocks for biomedical devices in personalized healthcare and other fields of bioengineering.

Sonopiezoelectric Nanomedicine and Materdicine

Endogenous electric field is ubiquitous in a multitude of important living activities such as bone repair, cell signal transduction, and nerve regeneration, signifying that regulating the electric field in organisms is highly beneficial to maintain organism health. As an emerging and promising research direction, piezoelectric nanomedicine and materdicine precisely activated by ultrasound with synergetic advantages of deep tissue penetration, remote spatiotemporal selectivity, and mechanical-electrical energy interconversion, have been progressively utilized for disease treatment and tissue repair by participating in the modulation of endogenous electric field. This specific nanomedicine utilizing piezoelectric effect activated by ultrasound is typically regarded as "sonopiezoelectric nanomedicine". This comprehensive review summarizes and discusses the substantially employed sonopiezoelectric nanomaterials and nanotherapies to provide an insight into the internal mechanism of the corresponding biological behavior/effect of sonopiezoelectric biomaterials in versatile disease treatments. This review primarily focuses on the sonopiezoelectric biomaterials for biosensing, drug delivery, tumor therapy, tissue regeneration, antimicrobia, and further illuminates the underlying sonopiezoelectric mechanism. In addition, the challenges and developments/prospects of sonopiezoelectric nanomedicine are analyzed for promoting the further clinical translation. It is earnestly expected that this kind of nanomedicine/biomaterials-enabled sonopiezoelectric technology will provoke the comprehensive investigation and promote the clinical development of the next-generation multifunctional materdicine.

Piezoresistive MXene/Silk fibroin nanocomposite hydrogel for accelerating bone regeneration by Re-establishing electrical microenvironment

The electrical microenvironment plays an important role in bone repair. However, the underlying mechanism by which electrical stimulation (ES) promotes bone regeneration remains unclear, limiting the design of bone microenvironment–specific electroactive materials. Herein, by simple co-incubation in aqueous suspensions at physiological temperatures, biocompatible regenerated silk fibroin (RSF) is found to assemble into nanofibrils with a β-sheet structure on MXene nanosheets, which has been reported to inhibit the restacking and oxidation of MXene. An electroactive hydrogel based on RSF and bioencapsulated MXene is thus prepared to promote efficient bone regeneration. This MXene/RSF hydrogel also acts as a piezoresistive pressure transducer, which can potentially be utilized to monitor the electrophysiological microenvironment. RNA sequencing is performed to explore the underlying mechanisms, which can activate Ca²⁺/CALM signaling in favor of the direct osteogenesis process. ES is found to facilitate indirect osteogenesis by promoting the polarization of M2 macrophages, as well as stimulating the neogenesis and migration of endotheliocytes. Consistent improvements in bone regeneration and angiogenesis are observed with MXene/RSF hydrogels under ES in vivo. Collectively, the MXene/RSF hydrogel provides a distinctive and promising strategy for promoting direct osteogenesis, regulating immune microenvironment and neovascularization under ES, leading to re-establish electrical microenvironment for bone regeneration.

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MXene nanosheets could direct the selective growth of silk nanofibrils.

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Prepared MXene/RSF hydrogel exhibited good conductivity and sensing ability.

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The electroactive hydrogel could promote osteogenic differentiation of BMSCs by activating the Ca²⁺/CALM signaling pathway.

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The conductive system created an osteoblast–macrophage–endotheliocyte virtuous circle for bone microenvironment.

Synergetic Effect of Electrical and Topographical Cues in Aniline Trimer-Based Polyurethane Fibrous Scaffolds on Tissue Regeneration

Processibility and biodegradability of conductive polymers are major concerns when they are applied to tissue regeneration. This study synthesizes dissolvable and conductive aniline trimer-based polyurethane copolymers (DCPU) and processes them into scaffolds by using electrospinning with different patterns (random, oriented, and latticed). The effects of topographic cue changes on electrical signal transmission and further regulation of cell behaviors concerning bone tissue are researched. Results show that DCPU fibrous scaffolds possessed good hydrophilicity, swelling capacity, elasticity, and fast biodegradability in enzymatic liquid. In addition, the conductivity and efficiency of electrical signal transmission can be tuned by changing the surface’s topological structure. Among them, oriented DCPU scaffolds (DCPU-O) showed the best conductivity with the lowest ionic resistance value. Furthermore, the viability and proliferation results of bone mesenchymal stem cells (BMSCs) demonstrate a significant increase on three DCPU scaffolds compared to AT-free scaffolds (DPU-R). Especially, DCPU-O scaffolds exhibit superior abilities to promote cell proliferation because of their unique surface topography and excellent electroactivity. Concurrently, the DCPU-O scaffolds can synergistically promote osteogenic differentiation in terms of osteogenic differentiation and gene expression levels when combined with electrical stimulation. Together, these results suggest a promising use of DCPU-O fibrous scaffolds in the application of tissue regeneration.

Smart Hydrogels for Bone Reconstruction via Modulating the Microenvironment

Rapid and effective repair of injured or diseased bone defects remains a major challenge due to shortages of implants. Smart hydrogels that respond to internal and external stimuli to achieve therapeutic actions in a spatially and temporally controlled manner have recently attracted much attention for bone therapy and regeneration. These hydrogels can be modified by introducing responsive moieties or embedding nanoparticles to increase their capacity for bone repair. Under specific stimuli, smart hydrogels can achieve variable, programmable, and controllable changes on demand to modulate the microenvironment for promoting bone healing. In this review, we highlight the advantages of smart hydrogels and summarize their materials, gelation methods, and properties. Then, we overview the recent advances in developing hydrogels that respond to biochemical signals, electromagnetic energy, and physical stimuli, including single, dual, and multiple types of stimuli, to enable physiological and pathological bone repair by modulating the microenvironment. Then, we discuss the current challenges and future perspectives regarding the clinical translation of smart hydrogels.

The bioelectrical properties of bone tissue

Understanding the bioelectrical properties of bone tissue is key to developing new treatment strategies for bone diseases and injuries, as well as improving the design and fabrication of scaffold implants for bone tissue engineering. The bioelectrical properties of bone tissue can be attributed to the interaction of its various cell lineages (osteocyte, osteoblast and osteoclast) with the surrounding extracellular matrix, in the presence of various biomechanical stimuli arising from routine physical activities; and is best described as a combination and overlap of dielectric, piezoelectric, pyroelectric and ferroelectric properties, together with streaming potential and electro-osmosis. There is close interdependence and interaction of the various electroactive and electrosensitive components of bone tissue, including cell membrane potential, voltage-gated ion channels, intracellular signaling pathways, and cell surface receptors, together with various matrix components such as collagen, hydroxyapatite, proteoglycans and glycosaminoglycans. It is the remarkably complex web of interactive cross-talk between the organic and non-organic components of bone that define its electrophysiological properties, which in turn exerts a profound influence on its metabolism, homeostasis and regeneration in health and disease. This has spurred increasing interest in application of electroactive scaffolds in bone tissue engineering, to recapitulate the natural electrophysiological microenvironment of healthy bone tissue to facilitate bone defect repair.

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.

Nanostructure Mediated Piezoelectric Effect of Tetragonal BaTiO3 Coatings on Bone Mesenchymal Stem Cell Shape and Osteogenic Differentiation

In recent years, porous titanium (Ti) scaffolds with BaTiO3 coatings have been designed to promote bone regeneration. However, the phase transitions of BaTiO3 have been understudied, and their coatings have yielded low effective piezoelectric coefficients (EPCs < 1 pm/V). In addition, piezoelectric nanomaterials bring many advantages in eliciting cell-specific responses. However, no study has attempted to design a nanostructured BaTiO3 coating with high EPCs. Herein, nanoparticulate tetragonal phase BaTiO3 coatings with cube-like nanoparticles but different effective piezoelectric coefficients were fabricated via anodization combining two hydrothermal processes. The effects of nanostructure-mediated piezoelectricity on the spreading, proliferation, and osteogenic differentiation of human jaw bone marrow mesenchymal stem cells (hJBMSCs) were explored. We found that the nanostructured tetragonal BaTiO3 coatings exhibited good biocompatibility and an EPC-dependent inhibitory effect on hJBMSC proliferation. The nanostructured tetragonal BaTiO3 coatings of relatively smaller EPCs (<10 pm/V) exhibited hJBMSC elongation and reorientation, broad lamellipodia extension, strong intercellular connection and osteogenic differentiation enhancement. Overall, the improved hJBMSC characteristics make the nanostructured tetragonal BaTiO3 coatings promising for application on implant surfaces to promote osseointegration.

The interplay of collagen/bioactive glass nanoparticle coatings and electrical stimulation regimes distinctly enhanced osteogenic differentiation of human mesenchymal stem cells

Increasing research has incorporated bioactive glass nanoparticles (BGN) and electric field (EF) stimulation for bone tissue engineering and regeneration applications. However, their interplay and the effects of different EF stimulation regimes on osteogenic differentiation of human mesenchymal stem cells (hMSC) are less investigated. In this study, we introduced EF with negligible magnetic field strength through a well-characterized transformer-like coupling (TLC) system, and applied EF disrupted (4/4) or consecutive (12/12) regime on type I collagen (Col) coatings with/without BGN over 28 days. Additionally, dexamethasone was excluded to enable an accurate interpretation of BGN and EF in supporting osteogenic differentiation. Here, we demonstrated the influences of BGN and EF on collagen topography and maintaining coating stability. Coupled with the release profile of Si ions from the BGN, cell proliferation and calcium deposition were enhanced in the Col-BGN samples after 28 days. Further, osteogenic differentiation was initiated as early as d 7, and each EF regime was shown to activate distinct pathways. The disrupted (4/4) regime was associated with the BMP/Smad4 pathways that up-regulate Runx2/OCN gene expression on d 7, with a lesser effect on ALP activity. In contrast, the canonical Wnt/β-Catenin signaling pathway activated through mechanotransduction cues is associated with the consecutive (12/12) regime, with significantly elevated ALP activity and Sp7 gene expression reported on d 7. In summary, our results illustrated the synergistic effects of BGN and EF in different stimulation regimes on osteogenic differentiation that can be further exploited to enhance current bone tissue engineering and regeneration approaches. STATEMENT OF SIGNIFICANCE: The unique release mechanisms of silica from bioactive glass nanoparticles (BGN) were coupled with pulsatile electric field (EF) stimulation to support hMSC osteogenic differentiation, in the absence of dexamethasone. Furthermore, the interplay with consecutive (12/12) and disrupted (4/4) stimulation regimes was investigated. The reported physical, mechanical and topographical effects of BGN and EF on the collagen coating, hMSC and the distinct progression of osteogenic differentiation (canonical Wnt/β-Catenin and BMP/Smad) triggered by respective stimulation regime were not explicitly reported previously. These results provide the fundamentals for further exploitations on BGN composites with metal ions and rotation of EF regimes to enhance osteogenic differentiation. The goal is sustaining continual osteogenic differentiation and achieving a more physiologically-relevant state and bone constructs in vitro.

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

Mesenchymal stem cells (MSCs) play a major role in bone tissue engineering (BTE) thanks to their capacity for osteogenic differentiation and being easily available. In vivo, MSCs are exposed to an electroactive microenvironment in the bone niche, which has piezoelectric properties. The correlation between the electrically active milieu and bone's ability to adapt to mechanical stress and self-regenerate has led to using electrical stimulation (ES) as physical cue to direct MSCs differentiation towards the osteogenic lineage in BTE. This review summarizes the different techniques to electrically stimulate MSCs to induce their osteoblastogenesis in vitro, including general electrical stimulation and substrate mediated stimulation by means of conductive or piezoelectric cell culture supports. Several aspects are covered, including stimulation parameters, treatment times and cell culture media to summarize the best conditions for inducing MSCs osteogenic commitment by electrical stimulation, from a critical point of view. Electrical stimulation activates different signaling pathways, including bone morphogenetic protein (BMP) Smad-dependent or independent, regulated by mitogen activated protein kinases (MAPK), extracellular signal-regulated kinases (ERK) and p38. The roles of voltage gate calcium channels (VGCC) and integrins are also highlighted according to their application technique and parameters, mainly converging in the expression of RUNX2, the master regulator of the osteogenic differentiation pathway. Despite the evident lack of homogeneity in the approaches used, the ever-increasing scientific evidence confirms ES potential as an osteoinductive cue, mimicking aspects of the in vivo microenvironment and moving one step forward to the translation of this approach into clinic.

Micropatterned Polypyrrole/Hydroxyapatite Composite Coatings Promoting Osteoinductive Activity by Electrical Stimulation

Conductive polypyrrole (PPy) has excellent biocompatibility and structural stability. It is an ideal electroactive biomaterial that can apply exogenous electrical stimulation to promote osteoblast differentiation. However, PPy is a kind of bio-inert material, which does not have osteoinductive capacity. Therefore, we have introduced a kind of bioactive material, hydroxyapatite (HA), to construct PPy/HA composite to enhance bioactivity and osteoinduction. In addition, micron-topological morphology of scattered grid pattern has been designed and introduced to the PPy/HA coatings, which can further enhance the regulation ability of the coatings to the adhesion, proliferation and differentiation of MC3T3-E1 cells. In vitro simulated body fluids (SBFs) immersion test results have demonstrated that the fabricated micropatterned PPy/HA composite coatings perform bioactivity well and can promote the mineral deposition of HA on the surface. Moreover, it can also benefit the proliferation and osteognetic differentiation of MC3T3-E1 cells, when accompanied by external electrical stimulation (ES). In this study, we have successfully constructed electroactive and bioactive coatings, the method of which can potentially be applied to the surface functional modification of traditional bone repair metals.

Biomaterialomics: Data science-driven pathways to develop fourth-generation biomaterials

Conventional approaches to developing biomaterials and implants require intuitive tailoring of manufacturing protocols and biocompatibility assessment. This leads to longer development cycles, and high costs. To meet existing and unmet clinical needs, it is critical to accelerate the production of implantable biomaterials, implants and biomedical devices. Building on the Materials Genome Initiative, we define the concept 'biomaterialomics' as the integration of multi-omics data and high-dimensional analysis with artificial intelligence (AI) tools throughout the entire pipeline of biomaterials development. The Data Science-driven approach is envisioned to bring together on a single platform, the computational tools, databases, experimental methods, machine learning, and advanced manufacturing (e.g., 3D printing) to develop the fourth-generation biomaterials and implants, whose clinical performance will be predicted using 'digital twins'. While analysing the key elements of the concept of 'biomaterialomics', significant emphasis has been put forward to effectively utilize high-throughput biocompatibility data together with multiscale physics-based models, E-platform/online databases of clinical studies, data science approaches, including metadata management, AI/ Machine Learning (ML) algorithms and uncertainty predictions. Such integrated formulation will allow one to adopt cross-disciplinary approaches to establish processing-structure-property (PSP) linkages. A few published studies from the lead author's research group serve as representative examples to illustrate the formulation and relevance of the 'Biomaterialomics' approaches for three emerging research themes, i.e. patient-specific implants, additive manufacturing, and bioelectronic medicine. The increased adaptability of AI/ML tools in biomaterials science along with the training of the next generation researchers in data science are strongly recommended. STATEMENT OF SIGNIFICANCE: This leading opinion review paper emphasizes the need to integrate the concepts and algorithms of the data science with biomaterials science. Also, this paper emphasizes the need to establish a mathematically rigorous cross-disciplinary framework that will allow a systematic quantitative exploration and curation of critical biomaterials knowledge needed to drive objectively the innovation efforts within a suitable uncertainty quantification framework, as embodied in 'biomaterialomics' concept, which integrates multi-omics data and high-dimensional analysis with artificial intelligence (AI) tools, like machine learning. The formulation of this approach has been demonstrated for patient-specific implants, additive manufacturing, and bioelectronic medicine.

Electrical stimulation of titanium to promote stem cell orientation, elongation and osteogenesis

Electrical stimulation of cells allows exogenous electric signals as stimuli to manipulate cell growth, preferential orientation and bone remodelling. In this study, commercially pure titanium discs were utilised in combination with a custom-built bioreactor to investigate the cellular responses of human mesenchymal stem cells via in-vitro functional assays. Finite element analysis revealed the homogeneous delivery of electric field in the bioreactor chamber with no detection of current density fluctuation in the proposed model. The custom-built bioreactor with capacitive stimulation delivery system features long-term stimulation with homogeneous electric field, biocompatible, sterilisable, scalable design and cost-effective in the manufacturing process. Using a continuous stimulation regime of 100 and 200 mV/mm on cp Ti discs, viability tests revealed up to an approximately 5-fold increase of cell proliferation rate as compared to non-stimulated controls. The human mesenchymal stem cells showed more elongated and differentiated morphology under this regime, with evidence of nuclear elongation and cytoskeletal orientation perpendicular to the direction of electric field. The continuous stimulation did not cause pH fluctuations and hydrogen peroxide production caused by Faradic reactions, signifying the suitability for long-term toxic free stimulation as opposed to the commonly used direct stimulation regime. An approximate of 4-fold increase in alkaline phosphatase production and approximately 9-fold increase of calcium deposition were observed on 200 mV/mm exposed samples relative to non-stimulated controls. It is worth noting that early stem cell differentiation and matrix production were observed under the said electric field even without the presence of chemical inductive growth factors. STATEMENT OF SIGNIFICANCE: This manuscript presents a study on combining pure titanium (primarily preferred as medical implant materials) and electrical stimulation in a purpose-built bioreactor with capacitive stimulation delivery system. A continuous capacitive stimulation regime on titanium disc has resulted in enhanced stem cell orientation, nuclei elongation, proliferation and differentiation as compared to non-stimulated controls. We believe that this manuscript creates a paradigm for future studies on the evolution of healthcare treatments in the area of targeted therapy on implantable and wearable medical devices through tailored innovative electrical stimulation approach, thereby influencing therapeutic conductive and electroactive biomaterials research prospects and development.

Electroresponsive Hydrogels for Therapeutic Applications in the Brain

Electroresponsive hydrogels possess a conducting material component and respond to electric stimulation through reversible absorption and expulsion of water. The high level of hydration, soft elastomeric compliance, biocompatibility, and enhanced electrochemical properties render these hydrogels suitable for implantation in the brain to enhance the transmission of neural electric signals and ion transport. This review provides an overview of critical electroresponsive hydrogel properties for augmenting electric stimulation in the brain. A background on electric stimulation in the brain through electroresponsive hydrogels is provided. Common conducting materials and general techniques to integrate them into hydrogels are briefly discussed. This review focuses on and summarizes advances in electric stimulation of electroconductive hydrogels for therapeutic applications in the brain, such as for controlling delivery of drugs, directing neural stem cell differentiation and neurogenesis, improving neural biosensor capabilities, and enhancing neural electrode-tissue interfaces. The key challenges in each of these applications are discussed and recommendations for future research are also provided.

Regulation of stem cell fate using nanostructure-mediated physical signals

One of the major issues in tissue engineering is regulation of stem cell differentiation toward specific lineages. Unlike biological and chemical signals, physical signals with adjustable properties can be applied to stem cells in a timely and localized manner, thus making them a hot topic for research in the fields of biomaterials, tissue engineering, and cell biology. According to the signals sensed by cells, physical signals used for regulating stem cell fate can be classified into six categories: mechanical, light, thermal, electrical, acoustic, and magnetic. In most cases, external macroscopic physical fields cannot be used to modulate stem cell fate, as only the localized physical signals accepted by the surface receptors can regulate stem cell differentiation via nanoscale fibrin polysaccharide fibers. However, surface receptors related to certain kinds of physical signals are still unknown. Recently, significant progress has been made in the development of functional materials for energy conversion. Consequently, localized physical fields can be produced by absorbing energy from an external physical field and subsequently releasing another type of localized energy through functional nanostructures. Based on the above concepts, we propose a methodology that can be utilized for stem cell engineering and for the regulation of stem cell fate via nanostructure-mediated physical signals. In this review, the combined effect of various approaches and mechanisms of physical signals provides a perspective on stem cell fate promotion by nanostructure-mediated physical signals. We expect that this review will aid the development of remote-controlled and wireless platforms to physically guide stem cell differentiation both in vitro and in vivo, using optimized stimulation parameters and mechanistic investigations while driving the progress of research in the fields of materials science, cell biology, and clinical research.

Electrical field induce mBMSCs differentiation to osteoblast via protein adsorption enhancement

Electrical stimulation as a useful and simple method attracts a lot of attention due to its potential to influence cell behaviors. Reports on the change of cell interior structures and membrane under electrical field would be the possible mechanisms. However, changes in cell behavior caused by protein adsorption under different electric field has not been noticed and discussed yet. In this study, a composite hydrogel PDA-GO-PAAM with conductivity of 8.23 × 10^-4 S/cm and has similar elastic modulus with pure PAAM was fabricated. It was found that BSA adsorption was higher on composite hydrogel，while electrical stimulation would further enhance BSA adsorption. Cell experiments revealed that electrical stimulation of mBMSCs insignificantly affect cell proliferation, and strongly promoted the expression of cell adhesion factors compared to the unstimulated control. Meanwhile, mBMSCs showed a spreading morphology on composite hydrogel and such spreading became even wider under the electrical stimulation. Under the effect of electrical stimulation, the larger the cell adhesion area was found on the hydrogel, the more the osteoblasts genotype and phenotype expression were, especially under the parameter of 1 V/cm and 1 h. Our results hence illustrate that electrical stimulation regulates osteogenic differentiation of mBMSCs via tuning cell adhesion and cell spreading mediated by protein adsorption.

Electroactive Biomaterials and Systems for Cell Fate Determination and Tissue Regeneration: Design and Applications

During natural tissue regeneration, tissue microenvironment and stem cell niche including cell-cell interaction, soluble factors, and extracellular matrix (ECM) provide a train of biochemical and biophysical cues for modulation of cell behaviors and tissue functions. Design of functional biomaterials to mimic the tissue/cell microenvironment have great potentials for tissue regeneration applications. Recently, electroactive biomaterials have drawn increasing attentions not only as scaffolds for cell adhesion and structural support, but also as modulators to regulate cell/tissue behaviors and function, especially for electrically excitable cells and tissues. More importantly, electrostimulation can further modulate a myriad of biological processes, from cell cycle, migration, proliferation and differentiation to neural conduction, muscle contraction, embryogenesis, and tissue regeneration. In this review, endogenous bioelectricity and piezoelectricity are introduced. Then, design rationale of electroactive biomaterials is discussed for imitating dynamic cell microenvironment, as well as their mediated electrostimulation and the applying pathways. Recent advances in electroactive biomaterials are systematically overviewed for modulation of stem cell fate and tissue regeneration, mainly including nerve regeneration, bone tissue engineering, and cardiac tissue engineering. Finally, the significance for simulating the native tissue microenvironment is emphasized and the open challenges and future perspectives of electroactive biomaterials are concluded.

Electrical Stimulation and Conductive Polymers as a Powerful Toolbox for Tailoring Cell Behaviour in vitro

Electrical stimulation (ES) is a well-known method for guiding the behaviour of nerve cells in in vitro systems based on the response of these cells to an electric field. From this perspective, understanding how the electrochemical stimulus can be tuned for the design of a desired cell response is of great importance. Most biomedical studies propose the application of an electrical potential to cell culture arrays while examining the cell response regarding viability, morphology, and gene expression. Conversely, various studies failed to evaluate how the fine physicochemical properties of the materials used for cell culture influence the observed behaviours. Among the various materials used for culturing cells under ES, conductive polymers (CPs) are widely used either in pristine form or in addition to other polymers. CPs themselves do not possess the optimal surface for cell compatibility because of their hydrophobic nature, which leads to poor protein adhesion and, hence, poor bioactivity. Therefore, understanding how to tailor the chemical properties on the material surface will determine the obtention of improved ES platforms. Moreover, the structure of the material, either in a thin film or in porous electrospun scaffolds, also affects the biochemical response and needs to be considered. In this review, we examine how materials based on CPs influence cell behaviour under ES, and we compile the various ES setups and physicochemical properties that affect cell behaviour. This review concerns the culture of various cell types, such as neurons, fibroblasts, osteoblasts, and Schwann cells, and it also covers studies on stem cells prone to ES. To understand the mechanistic behaviour of these devices, we also examine studies presenting a more detailed biomolecular level of interaction. This review aims to guide the design of future ES setups regarding the influence of material properties and electrochemical conditions on the behaviour of in vitro cell studies.

Porous Electroactive and Biodegradable Polyurethane Membrane through Self-Doping Organogel

In order to improve the processability of conductive polyurethane (CPU) containing aniline oligomers, a new CPU containing aniline trimer (AT) and l-lysine (PUAT) are designed and synthesized. Further, the 3D porous PUAT membranes have been prepared by a simple gel cooperated with freeze-drying method. Chemical testings and conductive properties testify a self- doping model of PUAT based on the rich electronic l-lysine and electroaffinity AT moities. The self-doping behavior further endows the PUAT copolymers specific characteristics such as high electrical conductivity and the formation of the polaron lattice like-structure in good solvent dimethyl sulfoxide. The combination of organogel and freeze-drying could prevent the collapse of pore structure when the copolymers are molded as membranes. The synergistic effect of l-lysine and AT components has a strong influence on the dissolution, degradation, thermal stability, and mechanical properties of PUAT. The excellent properties of PUAT would broad the application of conductive polymers in biomedicine field.

Composites made of polyorganophosphazene and carbon nanotube up-regulating osteogenic activity of BMSCs under electrical stimulation

Bone is an electrically responsive tissue, so electroactive materials that can deliver electrical cues to bone are helpful for enhancing regeneration under electrical stimulation (ES), and conductive materials are crucial in ES transmission to determine osteogenesis. Compared with polyesters, biodegradable polyorganophosphazenes (POPPs) show superiority in the field of bone tissue engineering thanks to their rich phosphorus/nitrogen contents, suggesting that the combination of POPPs-based conductive substrates with ES may achieve synergistic enhancements on osteogenesis. Herein, conductive composite films were fabricated by blending poly[(alanine ethyl ester)-(glycine ethyl ester)]phosphazene (PAGP) with carbon nanotubes (CNTs). After surface modification with polydopamine (PDA), bone marrow mesenchymal stromal cells (BMSCs) were cultured on the films under ES, using the cells cultured on conductive films composed of poly(L-lactide) (PLLA) and CNTs as controls. The BMSCs on PAGP/CNT films demonstrated significantly faster proliferation rates and stronger osteogenic differentiation potentials than those on PLLA/CNT films, while cell attachments on the two PDA-coated substrates were similar. Under appropriate ES, further increases in the expressions of osteogenic markers as alkaline phosphatase, collagen I and calcium deposition were identified in comparison with the cases without ES. The contributions of the osteocompatible POPPs, the substrate conductivity and the ES treatment to enhanced osteogenesis suggested new strategies for the design of bone repair materials.

Mimicking the electrophysiological microenvironment of bone tissue using electroactive materials to promote its regeneration

The process of bone tissue repair and regeneration is complex and requires a variety of physiological signals, including biochemical, electrical and mechanical signals, which collaborate to ensure functional recovery. The inherent piezoelectric properties of bone tissues can convert mechanical stimulation into electrical effects, which play significant roles in bone maturation, remodeling and reconstruction. Electroactive materials, including conductive materials, piezoelectric materials and electret materials, can simulate the physiological and electrical microenvironment of bone tissue, thereby promoting bone regeneration and reconstruction. In this paper, the structures and performances of different types of electroactive materials and their applications in the field of bone repair and regeneration are reviewed, particularly by providing the results from in vivo evaluations using various animal models. Their advantages and disadvantages as bone repair materials are discussed, and the methods for tuning their performances are also described, with the aim of providing an up-to-date account of the proposed topics.

Applications of Graphene and Its Derivatives in Bone Repair: Advantages for Promoting Bone Formation and Providing Real-Time Detection, Challenges and Future Prospects

During continuous innovation in the preparation, characterization and application of various bone repair materials for several decades, nanomaterials have exhibited many unique advantages. As a kind of representative two-dimensional nanomaterials, graphene and its derivatives (GDs) such as graphene oxide and reduced graphene oxide have shown promising potential for the application in bone repair based on their excellent mechanical properties, electrical conductivity, large specific surface area (SSA) and atomic structure stability. Herein, we reviewed the updated application of them in bone repair in order to present, as comprehensively, as possible, their specific advantages, challenges and current solutions. Firstly, how their advantages have been utilized in bone repair materials with improved bone formation ability was discussed. Especially, the effects of further functionalization or modification were emphasized. Then, the signaling pathways involved in GDs-induced osteogenic differentiation of stem cells and immunomodulatory mechanism of GDs-induced bone regeneration were discussed. On the other hand, their applications as contrast agents in the field of bone repair were summarized. In addition, we also reviewed the progress and related principles of the effects of GDs parameters on cytotoxicity and residues. At last, the future research was prospected.

Graphene Hybrid Materials for Controlling Cellular Microenvironments

Cellular microenvironments are known as key factors controlling various cell functions, including adhesion, growth, migration, differentiation, and apoptosis. Many materials, including proteins, polymers, and metal hybrid composites, are reportedly effective in regulating cellular microenvironments, mostly via reshaping and manipulating cell morphologies, which ultimately affect cytoskeletal dynamics and related genetic behaviors. Recently, graphene and its derivatives have emerged as promising materials in biomedical research owing to their biocompatible properties as well as unique physicochemical characteristics. In this review, we will highlight and discuss recent studies reporting the regulation of the cellular microenvironment, with particular focus on the use of graphene derivatives or graphene hybrid materials to effectively control stem cell differentiation and cancer cell functions and behaviors. We hope that this review will accelerate research on the use of graphene derivatives to regulate various cellular microenvironments, which will ultimately be useful for both cancer therapy and stem cell-based regenerative medicine.

Biomimetic titanium implant coated with extracellular matrix enhances and accelerates osteogenesis

Aim: To evaluate the biological function of titanium implants coated with cell-derived mineralized extracellular matrix, which mimics a bony microenvironment. Materials & methods: A biomimetic titanium implant was fabricated primarily by modifying the titanium surface with TiO₂ nanotubes or sand-blasted, acid-etched topography, then was coated with mineralized extracellular matrix constructed by culturing bone marrow mesenchymal stromal cells. The osteogenic ability of biomimetic titanium surface in vitro and in vivo were evaluated. Results: In vitro and in vivo studies revealed that the biomimetic titanium implant enhanced and accelerated osteogenesis of bone marrow stromal cells by increasing cell proliferation and calcium deposition. Conclusion: By combining surface topography modification with biological coating, the results provided a valuable method to produce biomimetic titanium implants with excellent osteogenic ability.

A bone matrix-simulating scaffold to alleviate replicative senescence of mesenchymal stem cells during long-term expansion

Replicative senescence during in vitro augmentation, which is mostly induced by the loss of physiological microenvironment, hinders the application of mesenchymal stem cells (MSCs) in the clinic. Here, we investigated whether MSCs senescence could be prevented by bio-scaffold mimicking the natural tissue matrix. Human umbilical cord mesenchymal stem cells (hUCMSCs) exhibited a senescent phenotype during a long-term passage in the conventional culture dish. To fabricate the bone matrix, a naturally based matrix composed of nano-hydroxyapatite/chitosan/poly lactide-co-glycolide (nHA/CS/PLGA) was produced. Long-term passage resulted in an obvious increase in the expression of senescence markers and a reduction in the expression of master genes involved in tissue regeneration. Functional assay confirmed that nHA/CS/PLGA scaffold preserved the proliferation and differentiation of hUCMSCs even after being passaged 27 times. Moreover, in vivo ectopic bone formation assay revealed that the bone formation of hUCMSCs cultured on the nano-scaffolds for the long term was as robust as the cells in the early passage. In summary, our results demonstrate that nHA/CS/PLGA scaffold effectively preserves the stemness and youth of hUCMSCs in the long-term passage. Taken advantage of its compatibility and bioactivity, nHA/CS/PLGA scaffold is of great potential in large-scale expansion of MSCs for stem cell therapy and tissue engineering.

Synthesis of Conductive Carbon Aerogels Decorated with β-Tricalcium Phosphate Nanocrystallites

There has been substantial interest in research aimed at conductive carbon-based supports since the discovery that the electrical stimulus can have dramatic effect on cell behavior. Among these carbon-aerogels decorated with biocompatible polymers were suggested as future materials for tissue engineering. However, high reaction temperatures required for the synthesis of the aerogels tend to impair the stability of the polymeric networks. Herein, we report a synthetic route towards carbon-aerogel scaffolds decorated with biocompatible ceramic nanoparticles of tricalcium phosphate. The composites can be prepared at temperature as high as 1100 °C without significant effect on the morphology of the composite which is comparable with the original aerogel framework. Although the conductivity of the composites tends to decrease with the increasing ceramic content the measured conductivity values are similar to those previously reported on polymer-functionalized carbon-aerogels. The cell culture study revealed that the developed constructs support cell proliferation and provide good cell attachment suggesting them as potentially good candidates for tissue-engineering applications.

Electrical stimulation in bone tissue engineering treatments

Electrical stimulation (EStim) has been shown to promote bone healing and regeneration both in animal experiments and clinical treatments. Therefore, incorporating EStim into promising new bone tissue engineering (BTE) therapies is a logical next step. The goal of current BTE research is to develop combinations of cells, scaffolds, and chemical and physical stimuli that optimize treatment outcomes. Recent studies demonstrating EStim's positive osteogenic effects at the cellular and molecular level provide intriguing clues to the underlying mechanisms by which it promotes bone healing. In this review, we discuss results of recent in vitro and in vivo research focused on using EStim to promote bone healing and regeneration and consider possible strategies for its application to improve outcomes in BTE treatments. Technical aspects of exposing cells and tissues to EStim in in vitro and in vivo model systems are also discussed.

A microelectrode array chip for osteogenic differentiation of mesenchymal stem cells under electrical stimulation

Electrical stimulation (ES) as an easy and effective inducing method has been widely used in induction differentiation of stem cells, e.g. osteogenic differentiation of mesenchymal stem cells (MSCs) for bone healing and bone tissue therapies. However, the micro-effect of an inhomogeneous electric field has rarely been investigated for ES in induction differentiation, and conventionally used ex situ assays may preclude accurate assessment due to variation from cell inoculation and treatments. Here, a novel electrical stimulation method with a microelectrode array chip is proposed for osteogenic differentiation of MSCs. The electric field applied onto the MSCs by the microelectrode array is designed similarly with a natural aggregation distribution of differentiated MSCs. The proposed ES method accelerates osteoblast proliferation and differentiation in the electrode array region and generates a larger amount of mineralized deposits, which are assayed via in situ alizarin red staining and morphology observation as well as immunocytochemistry. In addition, this method allows a direct in situ assessment to compare the osteogenic differentiation of MSCs with and without ES on a single chip to avoid culture environment difference. The method provides a fundamental platform for investigating induced differentiation of stem cells and allows integration with multifunctional cell assays to achieve in situ tracking for the differentiation process of stem cells.

Ternary hydrogels with tunable mechanical and self-healing properties based on the synergistic effects of multiple dynamic bonds

Ternary hydrogels with tunable mechanical properties were prepared based on the synergistic effects of hydrogen bonds and imine bonds.

Mussel-Inspired Conducting Copolymer with Aniline Tetramer as Intelligent Biological Adhesive for Bone Tissue Engineering

Electrically conducting polymers have been emerging as intelligent bioactive materials for regulating cell behaviors and bone tissue regeneration. Additionally, poor adhesion between conventional implants and native bone tissue may lead to displacement, local inflammation, and unnecessary secondary surgery. Thus, a conductive bioadhesive with strong adhesion performance provides an effective approach to fulfill fixation and regeneration of comminuted bone fracture. Inspired by mussel chemistry, we designed the conductive copolymers poly{[aniline tetramer methacrylamide]-co-[dopamine methacrylamide]-co-[poly(ethylene glycol) methyl ether methacrylate]} [poly(ATMA-co-DOPAMA-co-PEGMA); AT:conductive aniline tetramer; DOPA:dopamine; PEG:poly(ethylene glycol))] with AT content 3.0, 6.0, and 9.0 mol %, respectively. The adhesive strength of this copolymer was enhanced during tensile process perhaps due to the synergistic effects of H-bonding, π-π interactions, and polymer long-chain entanglement, reaching up to 1.28 MPa with 6 mol % AT. Biological characterizations of preosteoblasts indicated that the bioadhesives exhibited desirable biocompatibility. In addition, the osteogenic differentiation was synergistically enhanced by the conductive substrate and electrical stimulation with a square wave, frequency of 100 Hz, 50% duty cycle, and electrical potential of 500 mV, as indicated by ALP activity, calcium deposition, and expression of osteogenic genes. The ALP activity at 14 days and calcium deposition at 28 days on the 9 mol % AT group were significantly higher than that on PLGA under electrical stimulation. The expression value of OPN for 9 mol % AT group was notably upregulated by 5.9-fold compared with PLGA at 7 days under electrical stimulation. Overall, the conductive polymers with strong adhesion can synergistically upregulate the cellular activity combining with electrical stimulation and might be a promising bioadhesive for orthopedic and dental applications.

Hierarchically designed bone scaffolds: From internal cues to external stimuli

Bone tissue engineering utilizes three critical elements - cells, scaffolds, and bioactive factors - to recapitulate the bone tissue microenvironment, inducing the formation of new bone. Recent advances in materials development have enabled the production of scaffolds that more effectively mimic the hierarchical features of bone matrix, ranging from molecular composition to nano/micro-scale biochemical and physical features. This review summarizes recent advances within the field in utilizing these features of native bone to guide the hierarchical design of materials and scaffolds. Biomimetic strategies discussed in this review cover several levels of hierarchical design, including the development of element-doped compositions of bioceramics, the usage of molecular templates for in vitro biomineralization at the nanoscale, the fabrication of biomimetic scaffold architecture at the micro- and nanoscale, and the application of external physical stimuli at the macroscale to regulate bone growth. Developments at each level are discussed with an emphasis on their in vitro and in vivo outcomes in promoting osteogenic tissue development. Ultimately, these hierarchically designed scaffolds can complement or even replace the usage of cells and biological elements, which present clinical and regulatory barriers to translation. As the field progresses ever closer to clinical translation, the creation of viable therapies will thus benefit from further development of hierarchically designed materials and scaffolds.

Bone Repair and Regenerative Biomaterials: Towards Recapitulating the Microenvironment

Biomaterials and tissue engineering scaffolds play a central role to repair bone defects. Although ceramic derivatives have been historically used to repair bone, hybrid materials have emerged as viable alternatives. The rationale for hybrid bone biomaterials is to recapitulate the native bone composition to which these materials are intended to replace. In addition to the mechanical and dimensional stability, bone repair scaffolds are needed to provide suitable microenvironments for cells. Therefore, scaffolds serve more than a mere structural template suggesting a need for better and interactive biomaterials. In this review article, we aim to provide a summary of the current materials used in bone tissue engineering. Due to the ever-increasing scientific publications on this topic, this review cannot be exhaustive; however, we attempted to provide readers with the latest advance without being redundant. Furthermore, every attempt is made to ensure that seminal works and significant research findings are included, with minimal bias. After a concise review of crystalline calcium phosphates and non-crystalline bioactive glasses, the remaining sections of the manuscript are focused on organic-inorganic hybrid materials.

Measuring Changes in Electrical Impedance During Cell-Mediated Mineralization

Background: The fundamental electrical properties of bone have been attributed to the organic collagen and the inorganic mineral component; however, contributions of individual components within bone tissue toward the measured electrical properties are not known. In our study, we investigated the electrical properties of cell-mediated mineral deposition process and compared our results with cell-free mineralization. Materials and Methods: Saos-2 cells encapsulated within gelatin methacrylate (GelMA) hydrogels were chemically stimulated in osteogenic medium for a period of 4 weeks. The morphology, composition, and mechanical properties of the mineralized constructs were characterized using bright-field imaging, scanning electron microscopy (SEM) energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy (FITR), nuclear magnetic resonance spectroscopy (NMR), micro-CT, immunostaining, and mechanical compression tests. In parallel, a custom-made device was used to measure the electrical impedance of mineralized constructs. All results were compared with cell-free GelMA hydrogels mineralized through the simulated body fluid approach. Results: Results demonstrate a decrease in the electrical impedance of deposited mineral in both cell-mineralized and cell-free mineralized samples. Conclusions: This study establishes a model system to investigate in vivo and in vitro mineralization processes.

A Mussel-Inspired Persistent ROS-Scavenging, Electroactive, and Osteoinductive Scaffold Based on Electrochemical-Driven In Situ Nanoassembly

Conductive polymers are promising for bone regeneration because they can regulate cell behavior through electrical stimulation; moreover, they are antioxidative agents that can be used to protect cells and tissues from damage originating from reactive oxygen species (ROS). However, conductive polymers lack affinity to cells and osteoinductivity, which limits their application in tissue engineering. Herein, an electroactive, cell affinitive, persistent ROS-scavenging, and osteoinductive porous Ti scaffold is prepared by the on-surface in situ assembly of a polypyrrole-polydopamine-hydroxyapatite (PPy-PDA-HA) film through a layer-by-layer pulse electrodeposition (LBL-PED) method. During LBL-PED, the PPy-PDA nanoparticles (NPs) and HA NPs are in situ synthesized and uniformly coated on a porous scaffold from inside to outside. PDA is entangled with and doped into PPy to enhance the ROS scavenging rate of the scaffold and realize repeatable, efficient ROS scavenging over a long period of time. HA and electrical stimulation synergistically promote osteogenic cell differentiation on PPy-PDA-HA films. Ultimately, the PPy-PDA-HA porous scaffold provides excellent bone regeneration through the synergistic effects of electroactivity, cell affinity, and antioxidative activity of the PPy-PDA NPs and the osteoinductivity of HA NPs. This study provides a new strategy for functionalizing porous scaffolds that show great promise as implants for tissue regeneration.

Conductive nanostructured Si biomaterials enhance osteogeneration through electrical stimulation

It is well known that the differentiation of stem cells is affected by the cell culture medium, the scaffold surface and electrochemical signals. However, stimulation of patterned biomaterials seeded with stem cell cultures has not been explored. Herein the effect of electrical stimulation on osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells (rBMSCs) cultured on solid and nanoporous micropyramid patterned Si surfaces was evaluated. It was found that both stimulation and scaffold patterning significantly enhanced osteo-differentiation. The stimulated nanoporous micropyramid scaffolds were more promising compared to the stimulated solid micropyramid surfaces, as they significantly promoted the osteogenic differentiation of rBMSCs via BMP/Smad signaling pathway. Particularly, as compared to the unstimulated patterned biomaterials, the stimulated patterned scaffolds allowed for a significant increase in core binding factor alpha l, alkaline phosphatase, the alpha l chain of type I Col, osteocalcin, and osteonectin, all of which are characteristic for osteo-differentiation. The proposed combination of electrical stimulation with scaffold patterning may provide novel promising strategies for bone tissue engineering and regenerative medicine.

Dual functional approaches for osteogenesis coupled angiogenesis in bone tissue engineering

Bone fracture healing is a multistep and overlapping process of inflammation, angiogenesis and osteogenesis. It is initiated by inflammation, causing the release of various cytokines and growth factors. It leads to the recruitment of stem cells and formation of vasculature resulting in the functional bone formation. This combined phenomenon is used by bone tissue engineers from past few years to address the problem of vasculature and osteogenic differentiation during bone regeneration. In this review, we have discussed all major studies reporting the dual functioning approach to promote osteogenesis coupled angiogenesis using various scaffolds. These scaffolds are broadly classified into four types based on the nature of their structural and functional components. The functionality of the scaffold is either due to the structural components or the loaded cargo which conducts or induces the coupled functionality. Dual delivery system for osteoinductive and angioinductive factors ensures the co-delivery of two different types of molecules to induce osteogenesis and angiogenesis. Single delivery scaffold for angioinductive and osteoinductive molecule releases single type of molecules which could induce both angiogenesis and osteogenesis. Osteoconductive scaffold consisted of bone constituents releases angioinductive factors. Osteoconductive and angioconductive scaffold composed of components which provide the native substrate features for osteogenesis and angiogenesis. This review article also discusses the studies highlighting the synergism of physico-chemical stimuli as dual functioning feature to enhance angiogenesis and osteogenesis simultaneously. In addition, this article covers one of the least discussed area of the bone regeneration i.e. 'cartilage formation as a median between angiogenesis and osteogenesis'.

Recent Advances in Biomaterials Science and Engineering Research in India: A Minireview

Biomedical research in health innovation and product development encompasses convergent technologies that primarily integrate biomaterials science and engineering at its core. Particularly, research in this area is instrumental for the implementation of biomedical devices (BMDs) that offer innovative solutions to help maintain and improve quality of life of patients worldwide. Despite achieving extraordinary success, implantable BMDs are still confronted with complex engineering and biological challenges that need to addressed for augmenting device performance and prolonging lifetime in vivo. Biofabrication of tissue constructs, designing novel biomaterials and employing rational biomaterial design approaches, surface engineering of implants, point of care diagnostics and micro/nano-based biosensors, smart drug delivery systems, and noninvasive imaging methodologies are among strategies exploited for improving clinical performance of implantable BMDs. In India, advances in biomedical technologies have dramatically advanced health care over the last few decades and the country is well-positioned to identify opportunities and translate emerging solutions. In this article, we attempt to capture the recent advances in biomedical research and development progressing across the country and highlight the significant research work accomplished in the areas of biomaterials science and engineering.

Electric field-responsive nanoparticles and electric fields: physical, chemical, biological mechanisms and therapeutic prospects

Electric fields are among physical stimuli that have revolutionized therapy. Occurring endogenously or exogenously, the electric field can be used as a trigger for controlled drug release from electroresponsive drug delivery systems, can stimulate wound healing and cell proliferation, may enhance endocytosis or guide stem cell differentiation. Electric field pulses may be applied to induce cell fusion, can increase the penetration of therapeutic agents into cells, or can be applied as a standalone therapy to ablate tumors. This review describes the main therapeutic trends and overviews the main physical, chemical and biological mechanisms underlying the actions of electric fields. Overall, the electric field can be used in therapeutic approaches in several ways. The electric field can act on drug carriers, cells and tissues. Understanding the multiple effects of this powerful tool will help harnessing its full therapeutic potential in an efficient and safe way.

The Foreign Body Response Demystified

The human body is endowed with an uncanny ability to distinguish self from foreign. The implantation of a foreign object inside a mammalian host activates complex signaling cascades, which lead to biological encapsulation of the implant. This reaction by the host system to a foreign object is known as foreign body response (FBR). Over the last few decades, it has been increasingly important to have a deeper insight into the mechanisms of FBR is needed to develop biomaterials for better integration with living systems. In the light of recent advances in tissue engineering and regenerative medicine, particularly in the field of biosensors and biodegradable tissue engineering scaffolds, the classical concepts related to the FBR have acquired new dimensions. The aim of this review is to provide a holistic view of the FBR, while critically analyzing the challenges, which need to be addressed in the future to overcome this innate response. In particular, this review discusses the relevant experimental methodology to assess the host response. The role of erosion and degradation behavior on FBR with biodegradable polymers is largely explored. Apart from the discussion on temporal progression of FBR, an emphasis has been given to the design of next-generation biomaterials with favorable host response.

Modulating Surface Potential by Controlling the β Phase Content in Poly(vinylidene fluoridetrifluoroethylene) Membranes Enhances Bone Regeneration

Bioelectricity plays a vital role in living organisms. Although electrical stimulation is introduced in the field of bone regeneration, the concept of a dose-response relationship between surface potential and osteogenesis is not thoroughly studied. To optimize the osteogenic properties of different surface potentials, a flexible piezoelectric membrane, poly(vinylidene fluoridetrifluoroethylene) [P(VDF-TrFE)], is fabricated by annealing treatment to control its β phases. The surface potential and piezoelectric coefficients (d₃₃ ) of the membranes can be regulated by increasing β phase contents. Compared with d₃₃  = 20 pC N^-1 (surface potential = -78 mV) and unpolarized membranes, bone marrow mesenchymal stem cells (BM-MSCs) cultured on the d₃₃  = 10 pC N^-1 (surface potential = -53 mV) membranes have better osteogenic properties. In vivo, d₃₃  = 10 pC N^-1 membranes result in rapid bone regeneration and complete mature bone-structure formation. BM-MSCs on d₃₃  = 10 pC N^-1 membranes have the lowest reactive oxygen species level and the highest mitochondrial membrane electric potential, implying that these membranes provide the best electrical qunantity for BM-MSCs' proliferation and energy metabolism. This study establishes an effective method to control the surface potential of P(VDF-Trfe) membranes and highlights the importance of optimized electrical stimulation in bone regeneration.