Piezo-electronics: A paradigm for self-powered bioelectronics

Recent breakthroughs in electroactive piezo-biomaterials have driven significant progress towards the development of both, diagnostic and therapeutic purposes, enabling vital sign monitoring, such as heart rate, etc. while also supporting tissue regeneration. Bioelectronic medicine provides a promising method for controlling tissue and organ functions, with 'piezo-electronics' emphasizing the lasting role of electro-active piezo-biomaterials in self-powered devices. This article critically analyses a range of self-powered bioelectronic technologies, including wearable, implantable, regenerative, and cancer therapy applications. Piezoelectric nanogenerators (PENGs) are essential in wearable and implantable systems such as pressure and strain measurements, sensor for human-machine interface, self-powered pacemakers, deep brain stimulation, cochlear implant, tissue restoration and sustained drug delivery, controlled by electrical stimuli from PENGs etc. Regenerative bioelectronics play a key role in healing tissues, such as bone, neural, cardiac, tendon, ligament, skeletal muscle etc. using self-powered implants, which have ability to restore tissue functionality. Additionally, piezoelectric biomaterials are being utilized in cancer treatment, offering more targeted therapies with minimal side effects. Various cancerous tumors can be destroyed by reactive oxygen species (ROS), generated by piezo-biomaterials. Data science is also emerging as a crucial tool in optimizing self-powered bioelectronics, enhancing patient outcomes through data-driven strategies, and broadening the role of bioelectronic technologies in modern healthcare.

Advanced Piezoelectric Materials, Devices, and Systems for Orthopedic Medicine

Harnessing the robust electromechanical couplings, piezoelectric materials not only enable efficient bio-energy harvesting, physiological sensing and actuating but also open enormous opportunities for therapeutic treatments through surface polarization directly interacting with electroactive cells, tissues, and organs. Known for its highly oriented and hierarchical structure, collagen in natural bones produces local electrical signals to stimulate osteoblasts and promote bone formation, inspiring the application of piezoelectric materials in orthopedic medicine. Recent studies showed that piezoelectricity can impact microenvironments by regulating molecular sensors including ion channels, cytoskeletal elements, cell adhesion proteins, and other signaling pathways. This review thus focuses on discussing the pioneering applications of piezoelectricity in the diagnosis and treatment of orthopedic diseases, aiming to offer valuable insights for advancing next-generation medical technologies. Beginning with an introduction to the principles of piezoelectricity and various piezoelectric materials, this review paper delves into the mechanisms through which piezoelectric materials accelerated osteogenesis. A comprehensive overview of piezoelectric materials, devices, and systems enhancing bone tissue repair, alleviating inflammation at infection sites, and monitoring bone health is then provided, respectively. Finally, the major challenges faced by applications of piezoelectricity in orthopedic conditions are thoroughly discussed, along with a critical outlook on future development trends.

Piezoelectric Scaffolds as Smart Materials for Bone Tissue Engineering

Bone repair and regeneration require physiological cues, including mechanical, electrical, and biochemical activity. Many biomaterials have been investigated as bioactive scaffolds with excellent electrical properties. Amongst biomaterials, piezoelectric materials (PMs) are gaining attention in biomedicine, power harvesting, biomedical devices, and structural health monitoring. PMs have unique properties, such as the ability to affect physiological movements and deliver electrical stimuli to damaged bone or cells without an external power source. The crucial bone property is its piezoelectricity. Bones can generate electrical charges and potential in response to mechanical stimuli, as they influence bone growth and regeneration. Piezoelectric materials respond to human microenvironment stimuli and are an important factor in bone regeneration and repair. This manuscript is an overview of the fundamentals of the materials generating the piezoelectric effect and their influence on bone repair and regeneration. This paper focuses on the state of the art of piezoelectric materials, such as polymers, ceramics, and composites, and their application in bone tissue engineering. We present important information from the point of view of bone tissue engineering. We highlight promising upcoming approaches and new generations of piezoelectric materials.

Piezoelectrically and Topographically Engineered Scaffolds for Accelerating Bone Regeneration

Bone regeneration remains a critical concern across diverse medical disciplines, because it is a complex process that requires a combinatorial approach involving the integration of mechanical, electrical, and biological stimuli to emulate the native cellular microenvironment. In this context, piezoelectric scaffolds have attracted considerable interest owing to their remarkable ability to generate electric fields in response to dynamic forces. Nonetheless, the application of such scaffolds in bone tissue engineering has been limited by the lack of a scaffold that can simultaneously provide both the intricate electromechanical environment and the biocompatibility of the native bone tissue. Here, we present a pioneering biomimetic scaffold that combines the unique properties of piezoelectric and topographical enhancement with the inherent osteogenic abilities of hydroxyapatite (HAp). Notably, the novelty of this work lies in the incorporation of HAp into polyvinylidene fluoride-co-trifluoro ethylene in a freestanding form, leveraging its natural osteogenic potential within a piezoelectric framework. Through comprehensive in vitro and in vivo investigations, we demonstrate the remarkable potential of these scaffolds to accelerate bone regeneration. Moreover, we demonstrate and propose three pivotal mechanisms─(i) electrical, (ii) topographical, and (iii) paracrine─that collectively contribute to the facilitated bone healing process. Our findings present a synergistically derived biomimetic scaffold design with wide-ranging prospects for bone regeneration as well as various regenerative medicine applications.

Investigation and optimization of In-Vitro behaviour of Perovskite barium titanate as a scaffold and protective coatings

The piezoelectric effect is widely known to have a significant physiological function in bone development, remodeling, and fracture repair. As a well-known piezoelectric material, barium titanate is particularly appealing as a scaffold layer to improve bone tissue engineering applications. Currently, the chemical bath deposition method is used to prepare green synthesized barium titanate coatings to improve mechanical and biological characteristics. Molarity of the solutions, an essential parameter in chemical synthesis, is changed at room temperature (0.1-1.2 Molar) to prepare coatings. The XRD spectra for as deposited coatings indicate amorphous behavior, while polycrystalline nature of coatings is observed after annealing (300 °C). Coatings prepared with solutions of relatively low molarities, i.e. from 0.1 to 0.8 M, exhibit mixed tetragonal - cubic phases. However, the tetragonal phase of Perovskite barium titanate is observed using solution molarities of 1.0 M and 1.2 M. Relatively high value of transmission, i.e. ∼80%, is observed for the coatings prepared with high molarities. Band gap of annealed coatings varies between 3.47 and 3.70 eV. For 1.2 M sample, the maximum spontaneous polarization (Ps) is 0.327x10-3 (μC/cm2) and the residual polarization (Pr) is 0.072x10-3 (μC/cm2). For 1.2M solution, a high hardness value (1510 HV) is recorded, with a fracture toughness of 28.80 MPam-1/2. Low values of weight loss, after dipping the coatings in simulated body fluid, is observed. The antibacterial activity of BaTiO3 is tested against E. coli and Bacillus subtilis. Drug encapsulation capability is also tested for different time intervals. As a result, CBD-based coatings are a promising nominee for use as scaffold and protective coatings.

Biological properties of polycaprolactone and barium titanate composite in biomedical applications

The ceramic-polymer composite materials are widely known for their exceptional mechanical and biological properties. Polycaprolactone (PCL) is a biodegradable polymer material extensively used in various biomedical applications. At the same time, barium titanate (BT), a ceramic material, exhibits piezoelectric properties similar to bone, which is essential for osseointegration. Furthermore, a composite material that combines the benefits of PCL and BT results in an innovative composite material with enhanced properties for biomedical applications. Thus, this review is organised into three sections. Firstly, it aims to provide an overview of the current research on evaluating biological properties, including antibacterial activity, cytotoxicity and osseointegration, of PCL polymeric matrices in its pure form and reinforced structures with ceramics, polymers and natural extracts. The second section investigates the biological properties of BT, both in its pure form and in combination with other supporting materials. Finally, the third section provides a summary of the biological properties of the PCLBT composite material. Furthermore, the existing challenges of PCL, BT and their composites, along with future research directions, have been presented. Therefore, this review will provide a state-of-the-art understanding of the biological properties of PCL and BT composites as potential futuristic materials in biomedical applications.

Mesenchymal Stem Cells Combined with a P(VDF-TrFE)/BaTiO3 Scaffold and Photobiomodulation Therapy Enhance Bone Repair in Rat Calvarial Defects

BACKGROUND

Tissue engineering and cell therapy have been the focus of investigations on how to treat challenging bone defects. This study aimed to produce and characterize a P(VDF-TrFE)/BaTiO₃ scaffold and evaluate the effect of mesenchymal stem cells (MSCs) combined with this scaffold and photobiomodulation (PBM) on bone repair.

METHODS AND RESULTS

P(VDF-TrFE)/BaTiO₃ was synthesized using an electrospinning technique and presented physical and chemical properties suitable for bone tissue engineering. This scaffold was implanted in rat calvarial defects (unilateral, 5 mm in diameter) and, 2 weeks post-implantation, MSCs were locally injected into these defects (n = 12/group). Photobiomodulation was then applied immediately, and again 48 and 96 h post-injection. The μCT and histological analyses showed an increment in bone formation, which exhibited a positive correlation with the treatments combined with the scaffold, with MSCs and PBM inducing more bone repair, followed by the scaffold combined with PBM, the scaffold combined with MSCs, and finally the scaffold alone (ANOVA, p ≤ 0.05).

CONCLUSIONS

The P(VDF-TrFE)/BaTiO₃ scaffold acted synergistically with MSCs and PBM to induce bone repair in rat calvarial defects. These findings emphasize the need to combine a range of techniques to regenerate large bone defects and provide avenues for further investigations on innovative tissue engineering approaches.

Bioinspired Piezoelectric Periosteum to Augment Bone Regeneration via Synergistic Immunomodulation and Osteogenesis

Ideal periosteum materials are required to participate in a sequence of bone repair-related physiological events, including the initial immune response, endogenous stem cell recruitment, angiogenesis, and osteogenesis. However, conventional tissue-engineered periosteal materials have difficulty achieving these functions by simply mimicking the periosteum via structural design or by loading exogenous stem cells, cytokines, or growth factors. Herein, we present a novel biomimetic periosteum preparation strategy to comprehensively enhance the bone regeneration effect using functionalized piezoelectric materials. The resulting biomimetic periosteum possessing an excellent piezoelectric effect and improved physicochemical properties was prepared using a biocompatible and biodegradable poly(3-hydroxybutyric acid-co-3-hydrovaleric acid) (PHBV) polymer matrix, antioxidized polydopamine-modified hydroxyapatite (PHA), and barium titanate (PBT), which were further incorporated into the polymer matrix to fabricate a multifunctional piezoelectric periosteum by a simple one-step spin-coating method. The addition of PHA and PBT dramatically enhanced the physicochemical properties and biological functions of the piezoelectric periosteum, resulting in improved surface hydrophilicity and roughness, enhanced mechanical performance, tunable degradation behavior, and stable and desired endogenous electrical stimulations, which is conducive to accelerating bone regeneration. Benefiting from endogenous piezoelectric stimulation and bioactive components, the as-fabricated biomimetic periosteum demonstrated favorable biocompatibility, osteogenic activity, and immunomodulatory functions in vitro, which not only promoted adhesion, proliferation, and spreading as well as osteogenesis of mesenchymal stem cells (MSCs) but also effectively induced M2 macrophage polarization, thereby suppressing reactive oxygen species (ROS)-induced inflammatory reactions. Through in vivo experiments, the biomimetic periosteum with endogenous piezoelectric stimulation synergistically accelerated the formation of new bone in a rat critical-sized cranial defect model. The whole defect was almost completely covered by new bone at 8 weeks post treatment, with a thickness close to that of the host bone. Collectively, with its favorable immunomodulatory and osteogenic properties, the biomimetic periosteum developed here represents a novel method to rapidly regenerate bone tissue using piezoelectric stimulation.

Alkaline earth metals for osteogenic scaffolds: From mechanisms to applications

Regeneration of bone defects is a significant challenge today. As alternative approaches to the autologous bone, scaffold materials have remarkable features in treating bone defects; however, the various properties of current scaffold materials still fall short of expectations. Due to the osteogenic capability of alkaline earth metals, their application in scaffold materials has become an effective approach to improving their properties. Furthermore, numerous studies have shown that combining alkaline earth metals leads to better osteogenic properties than applying them alone. In this review, the physicochemical and physiological characteristics of alkaline earth metals are introduced, mainly focusing on their mechanisms and applications in osteogenesis, especially magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Furthermore, this review highlights the possible cross-talk between pathways when alkaline earth metals are combined. Finally, some of the current drawbacks of scaffold materials are enumerated, such as the high corrosion rate of Mg scaffolds and defects in the mechanical properties of Ca scaffolds. Moreover, a brief perspective is also provided regarding future directions in this field. It is worth exploring that whether the levels of alkaline earth metals in newly regenerated bone differs from those in normal bone. The ideal ratio of each element in the bone tissue engineering scaffolds or the optimal concentration of each elemental ion in the created osteogenic environment still needs further exploration. The review not only summarizes the research developments in osteogenesis but also offers a direction for developing new scaffold materials.

A Comprehensive Review on Barium Titanate Nanoparticles as a Persuasive Piezoelectric Material for Biomedical Applications: Prospects and Challenges

Stimulation of cells with electrical cues is an imperative approach to interact with biological systems and has been exploited in clinical practices over a wide range of pathological ailments. This bioelectric interface has been extensively explored with the help of piezoelectric materials, leading to remarkable advancement in the past two decades. Among other members of this fraternity, colloidal perovskite barium titanate (BaTiO₃ ) has gained substantial interest due to its noteworthy properties which includes high dielectric constant and excellent ferroelectric properties along with acceptable biocompatibility. Significant progression is witnessed for BaTiO₃ nanoparticles (BaTiO₃ NPs) as potent candidates for biomedical applications and in wearable bioelectronics, making them a promising personal healthcare platform. The current review highlights the nanostructured piezoelectric bio interface of BaTiO₃ NPs in applications comprising drug delivery, tissue engineering, bioimaging, bioelectronics, and wearable devices. Particular attention has been dedicated toward the fabrication routes of BaTiO₃ NPs along with different approaches for its surface modifications. This review offers a comprehensive discussion on the utility of BaTiO₃ NPs as active devices rather than passive structural unit behaving as carriers for biomolecules. The employment of BaTiO₃ NPs presents new scenarios and opportunity in the vast field of nanomedicines for biomedical applications.

Multi-Parametric Exploration of a Selection of Piezoceramic Materials for Bone Graft Substitute Applications

This work was devoted to the first multi-parametric unitary comparative analysis of a selection of sintered piezoceramic materials synthesised by solid-state reactions, aiming to delineate the most promising biocompatible piezoelectric material, to be further implemented into macro-porous ceramic scaffolds fabricated by 3D printing technologies. The piezoceramics under scrutiny were: KNbO₃, LiNbO₃, LiTaO₃, BaTiO₃, Zr-doped BaTiO₃, and the (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃ solid solution (BCTZ). The XRD analysis revealed the high crystallinity of all sintered ceramics, while the best densification was achieved for the BaTiO₃-based materials via conventional sintering. Conjunctively, BCTZ yielded the best combination of functional properties-piezoelectric response (in terms of longitudinal piezoelectric constant and planar electromechanical coupling factor) and mechanical and in vitro osteoblast cell compatibility. The selected piezoceramic was further used as a base material for the robocasting fabrication of 3D macro-porous scaffolds (porosity of ~50%), which yielded a promising compressive strength of ~20 MPa (higher than that of trabecular bone), excellent cell colonization capability, and noteworthy cytocompatibility in osteoblast cell cultures, analogous to the biological control. Thereby, good prospects for the possible development of a new generation of synthetic bone graft substitutes endowed with the piezoelectric effect as a stimulus for the enhancement of osteogenic capacity were settled.

Effect of Piezoelectric BaTiO3 Filler on Mechanical and Magnetoelectric Properties of Zn0.25Co0.75Fe2O4/PVDF-TrFE Composites

Polymer-based multiferroics, combining magnetic and piezoelectric properties, are studied experimentally-from synthesis to multi-parameter characterization-in view of their prospects for fabricating biocompatible scaffolds. The main advantage of these systems is facile generation of mechanical deformations and electric signals in response to external magnetic fields. Herein, we address the composites based on PVDF-TrFE polymer matrices filled with a combination of piezoelectric (BaTiO_3, BTO) and/or ferrimagnetic (Zn_0.25Co_0.75Fe₂O₄, ZCFO) particles. It is shown that the presence of BTO micron-size particles favors stripe-type structuring of the ZCFO filler and enhances the magnetoelectric response of the sample up to 18.6 mV/(cm∙Oe). Besides that, the admixing of BTO particles is crucial because the mechanical properties of the composite filled with only ZCFO is much less efficient in transforming magnetic excitations into the mechanical and electric responses. Attention is focused on the local surfacial mechanical properties since those, to a great extent, determine the fate of stem cells cultivated on these surfaces. The nano-indentation tests are accomplished with the aid of scanning probe microscopy technique. With their proven suitable mechanical properties, a high level of magnetoelectric conversion and also biocompatibility, the composites of the considered type are enticing as the materials for multiferroic-based polymer scaffolds.

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.

Polydopamine Codoped BaTiO3-Functionalized Polyvinylidene Fluoride Coating as a Piezo-Biomaterial Platform for an Enhanced Cellular Response and Bioactivity

For a number of clinical applications, Ti6Al4V implants with bioactive coatings are used. However, the deposition of a functional polymeric coating with desired physical properties, biocompatibility, and long-term stability remains largely unexplored. Among widely investigated synthetic biomaterials, polyvinylidene fluoride (PVDF) with β-polymorph and barium titanate (BaTiO₃, BT) are considered as good examples of piezo-biopolymers and bioceramics, respectively. In this work, an adherent PVDF-based nanocomposite coating is deposited onto a Ti6Al4V substrate to explore the impact of its functional characteristics (piezoactivity) on cellular behavior and bioactivity (apatite growth and mineralized matrix formation). The precursor solution was prepared by physically grafting PVDF with polydopamine (pDOPA), forming mPVDF. Subsequently, mPVDF was reinforced with BaTiO₃ nanoparticles in dimethylformamide/acetone solution, and the resulting nanocomposite (mPVDF-BT) was then spray-coated onto a roughened Ti6Al4V substrate using an airbrush at 140 °C under a pressure of 2 bar. The reproducibility of this simple yet effective processing approach to deposit chemically stable and adherent coatings was established. Remarkably, the modification with pDOPA and reinforcement with BaTiO₃ nanoparticles resulted in an enhanced β-fraction of PVDF up to 96%. This nanocomposite encouraged cellular viability of preosteoblasts (∼158% at day 5) and characteristic spreading, in vitro. Our findings indicate that the mPVDF-BT coating facilitated faster nucleation and growth of the biomineralized apatite layer with ∼70% coverage within 3 days of incubation in the simulated body fluid. In addition, the coupling among surface polar energy (5.5 mN/m), fractional polarity (∼117%), roughness (8.7 μm), and fibrous morphology also endorsed better cellular behavior. Taken together, this coating deposition strategy will pave the pathway toward designing cell-instructive surface-modified Ti6Al4V biomaterials with tailored biomineralization and bioactivity properties for musculoskeletal reconstruction and regeneration applications.

Enhancement of Bone Regeneration Through the Converse Piezoelectric Effect, A Novel Approach for Applying Mechanical Stimulation

Serious bone injuries have devastating effects on the lives of patients including limiting working ability and high cost. Orthopedic implants can aid in healing injuries to an extent that exceeds the natural regenerative capabilities of bone to repair fractures or large bone defects. Autografts and allografts are the standard implants used, but disadvantages such as donor site complications, a limited quantity of transplantable bone, and high costs have led to an increased demand for synthetic bone graft substitutes. However, replicating the complex physiological properties of biological bone, much less recapitulating its complex tissue functions, is challenging. Extensive efforts to design biocompatible implants that mimic the natural healing processes in bone have led to the investigation of piezoelectric smart materials because the bone has natural piezoelectric properties. Piezoelectric materials facilitate bone regeneration either by accumulating electric charge in response to mechanical stress, which mimics bioelectric signals through the direct piezoelectric effect or by providing mechanical stimulation in response to electrical stimulation through the converse piezoelectric effect. Although both effects are beneficial, the converse piezoelectric effect can address bone atrophy from stress shielding and immobility by improving the mechanical response of a healing defect. Mechanical stimulation has a positive impact on bone regeneration by activating cellular pathways that increase bone formation and decrease bone resorption. This review will highlight the potential of the converse piezoelectric effect to enhance bone regeneration by discussing the activation of beneficial cellular pathways, the properties of piezoelectric biomaterials, and the potential for the more effective administration of the converse piezoelectric effect using wireless control.

Texturized P(VDF-TrFE)/BT membrane enhances bone neoformation in calvaria defects regardless of the association with photobiomodulation therapy in ovariectomized rats

OBJECTIVES

The purpose of this investigation was to evaluate in vivo the response of bone tissue to photobiomodulation when associated with texturized P(VDF-TrFE)/BT in calvaria defects of ovariectomized rats.

MATERIALS AND METHODS

Wistar Hannover rats were submitted to ovariectomy/control surgery. Calvaria bone defects of 5-mm diameter were performed after 90 days of ovariectomy. The animals were divided into OVX (without laser (L) and membrane), OVX + P(VDF-TrFE)/BT, OVX + P(VDF-TrFE)/BT + L, and OVX + PTFE + L. It was utilized a low-intensity gallium-aluminum-arsenide laser (GaAlAs) with 780-nm wavelength and 30-J/cm² energy density in 12 sessions (120 s). Thirty days after the bone defect the animals were euthanized for histological, microtomographic, and molecular evaluation. Quantitative analysis was analyzed by statistical software for p < 0.05.

RESULTS

Histological parameters showed bone tissue formation at the borders of all group defects. The association of photobiomodulation and texturized P(VDF-TrFE)/BT was not synergistic and did not show significant changes in morphometric analysis and biomarkers gene expression. Nevertheless, texturized P(VDF-TrFE)/BT membrane enhanced bone repair regardless of the association with photobiomodulation therapy, with an increase of connectivity density when compared to the OVX + PTFE + L group. The association of photobiomodulation therapy and PTFE was synergistic, increasing the expression of Runx2, Alp, Bsp, Bglap, Sp7, and Rankl, even though not enough to reflect significance in the morphometric parameters.

CONCLUSIONS

The utilization of texturized P (VDF-TrFE)/BT, regardless of the association with photobiomodulation therapy, enhanced bone repair in an experimental model of osteoporosis.

Recent advances of polymer-based piezoelectric composites for biomedical applications

Over the past decades, electronics have become central to many aspects of biomedicine and wearable device technologies as a promising personalized healthcare platform. Lead-free piezoelectric materials for converting mechanical into electrical energy through piezoelectric transduction are of significant value in a diverse range of technological applications. Organic piezoelectric biomaterials have attracted widespread attention as the functional materials in the biomedical devices due to their advantages of excellent biocompatibility. They include synthetic and biological polymers. Many biopolymers have been discovered to possess piezoelectricity in an appreciable amount, however their investigation is still preliminary. Due to their piezoelectric properties, better known synthetic fluorinated polymers have been intensively investigated and applied in biomedical applications including controlled drug delivery systems, tissue engineering, microfluidic and artificial muscle actuators, among others. Piezoelectric polymers, especially poly (vinylidene fluoride) (PVDF) and its copolymers are increasingly receiving interest as smart biomaterials due to their ability to convert physiological movements to electrical signals when in a controllable and reproducible manner. Despite possessing the greatest piezoelectric coefficients among all piezoelectric polymers, it is often desirable to increase the electrical outputs. The most promising routes toward significant improvements in the piezoelectric response and energy-harvesting performance of such materials is loading them with various inorganic nanofillers and/or applying some modification during the fabrication process. This paper offers a comprehensive review of the principles, properties, and applications of organic piezoelectric biomaterials (polymers and polymer/ceramic composites) with special attention on PVDF-based polymers and their composites in sensors, drug delivery and tissue engineering. Subsequently focuses on the most common fabrication routes to produce piezoelectric scaffolds, tissue and sensors which is electrospinning process. Promising upcoming strategies and new piezoelectric materials and fabrication techniques for these applications are presented to enable a future integration among these applications.

Enhanced osteogenic differentiation of mesenchymal stem cells on P(VDF-TrFE) layer coated microelectrodes

Electrical stimulation has been proved to be critical to regulate cell behavior. But, cell behavior is also susceptible to multiple parameters of the adverse interferences such as surface current, electrochemical reaction products, and non-uniform compositions, which often occur during direct electric stimulation. To effectively prevent the adverse interferences, a novel piezoelectric poly(vinylidene fluoride-trfluoroethylene)(P(VDF-TrFE)) layer was designed to coat onto the indium tin oxide (ITO) planar microelectrode. We found the electrical stimulation was able to regulate the osteogenic differentiation of mesenchymal stem cells (MSCs) through calcium-mediated PKC signaling pathway. Meanwhile, the surface charge of the designed P(VDF-TrFE) coating layer could be easily controlled by the pre-polarization process, which was demonstrated to trigger integrin-mediated FAK signaling pathway, finally up-regulating the osteogenic differentiation of MSCs. Strikingly, the crosstalk in the downstream of the two signaling cascades further strengthened the ERK pathway activation for osteogenic differentiation of MSCs. This P(VDF-TrFE) layer coated electrical stimulation microelectrodes therefore provide a distinct strategy to manipulate multiple-elements of biomaterial surface to regulate stem cell fate commitment.

Enhanced cellular osteogenic differentiation on CoFe2O4/P(VDF-TrFE) nanocomposite coatings under static magnetic field

Cellular responses can be regulated and manipulated through combining stimuli-responsive biomaterial with external stimulus. In this present, the magneto-responsive CoFe₂O₄/P(VDF-TrFE) nanocomposite coatings were designed to understand cell behaviors of preosteoblasts, as well as get insight into the underlying mechanism of osteogenic differentiation under static magnetic field (SMF). CoFe₂O₄/P(VDF-TrFE) nanocomposite coatings with differential magnetic property (low, medium and high magnetization) were prepared by incorporation of different mass fraction of CoFe₂O₄ nanoparticles (6%, 13 %, 20 %) into P(VDF-TrFE) matrix. Cell experiments indicated that all nanocomposite coatings with the assistance of SMF could promote the cell attachment, proliferation and osteogenic differentiation of MC3T3-E1 cells. Among different nanocomposite coatings, low magnetization coating (6%) showed a higher ALP activity and gene expression of Runx2, Col-I, OCN. Molecular biology assays demonstrated that the combination of nanocomposite coatings and SMF could significantly up-regulate the expression level of α2β1 integrin and p-ERK. Whereas, the addition of inhibitor U0126 down-regulated sharply the expression level of p-ERK, which indicated that cellular osteogenic differentiation of MC3T3-E1 cells was governed through α2β1 integrin-mediated MEK/ERK signaling pathways during CoFe₂O₄/P(VDF-TrFE) nanocomposite coatings were combined with SMF. This work provided a promising strategy to enhance cellular osteogenic differentiation through a remote-control manner, which exhibited great potential in the application of bone tissue repair and regeneration.

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.

Piezoelectric Scaffolds as Smart Materials for Neural Tissue Engineering

Injury to the central or peripheral nervous systems leads to the loss of cognitive and/or sensorimotor capabilities, which still lacks an effective treatment. Tissue engineering in the post-injury brain represents a promising option for cellular replacement and rescue, providing a cell scaffold for either transplanted or resident cells. Tissue engineering relies on scaffolds for supporting cell differentiation and growth with recent emphasis on stimuli responsive scaffolds, sometimes called smart scaffolds. One of the representatives of this material group is piezoelectric scaffolds, being able to generate electrical charges under mechanical stimulation, which creates a real prospect for using such scaffolds in non-invasive therapy of neural tissue. This paper summarizes the recent knowledge on piezoelectric materials used for tissue engineering, especially neural tissue engineering. The most used materials for tissue engineering strategies are reported together with the main achievements, challenges, and future needs for research and actual therapies. This review provides thus a compilation of the most relevant results and strategies and serves as a starting point for novel research pathways in the most relevant and challenging open questions.

Cell Therapy: Effect of Locally Injected Mesenchymal Stromal Cells Derived from Bone Marrow or Adipose Tissue on Bone Regeneration of Rat Calvarial Defects

Treatment of large bone defects is a challenging clinical situation that may be benefited from cell therapies based on regenerative medicine. This study was conducted to evaluate the effect of local injection of bone marrow-derived mesenchymal stromal cells (BM-MSCs) or adipose tissue-derived MSCs (AT-MSCs) on the regeneration of rat calvarial defects. BM-MSCs and AT-MSCs were characterized based on their expression of specific surface markers; cell viability was evaluated after injection with a 21-G needle. Defects measuring 5 mm that were created in rat calvaria were injected with BM-MSCs, AT-MSCs, or vehicle-phosphate-buffered saline (Control) 2 weeks post-defect creation. Cells were tracked by bioluminescence, and 4 weeks post-injection, the newly formed bone was evaluated by µCT, histology, nanoindentation, and gene expression of bone markers. BM-MSCs and AT-MSCs exhibited the characteristics of MSCs and maintained their viability after passing through the 21-G needle. Injection of both BM-MSCs and AT-MSCs resulted in increased bone formation compared to that in Control and with similar mechanical properties as those of native bone. The expression of genes associated with bone formation was higher in the newly formed bone induced by BM-MSCs, whereas the expression of genes involved in bone resorption was higher in the AT-MSC group. Cell therapy based on local injection of BM-MSCs or AT-MSCs is effective in delivering cells that induced a significant improvement in bone healing. Despite differences observed in molecular cues between BM-MSCs and AT-MSCs, both cells had the ability to induce bone tissue formation at comparable amounts and properties. These results may drive new cell therapy approaches toward complete bone regeneration.

Effect of stem cells combined with a polymer/ceramic membrane on osteoporotic bone repair

Cell therapy associated with guided bone regeneration (GBR) can be used to treat bone defects under challenging conditions such as osteoporosis. This study aimed to evaluate the effect of mesenchymal stem cells (MSCs) in combination with a poly(vinylidene-trifluoroethylene)/barium titanate (PVDF-TrFE/BT) membrane on bone repair in osteoporotic rats. Osteoporosis was induced in female rats by bilateral removal of the ovaries (OVX) or sham surgery (SHAM), and the osteoporotic condition was characterized after 5 months by microtomographic and morphometric analyses. Calvarial defects were created in osteoporotic rats that immediately received the PVDF-TrFE/BT membrane. After 2 weeks, bone marrow-derived MSCs from healthy rats, characterized by the expression of surface markers using flow cytometry, or phosphate-buffered saline (PBS) (Control) were injected into the defects and bone formation was evaluated 4 weeks post-injection by microtomographic, morphometric, and histological analyses. A reduction in the amount of bone tissue in the femurs of OVX compared with SHAM rats confirmed the osteoporotic condition of the experimental model. More bone formation was observed when the defects were injected with MSCs compared to that with PBS. The modification that we are proposing in this study for the classical GBR approach where cells are locally injected after a membrane implantation may be a promising therapeutic strategy to increase bone formation under osteoporotic condition.

Piezoelectric materials as stimulatory biomedical materials and scaffolds for bone repair

The process of bone repair and regeneration requires multiple physiological cues including biochemical, electrical and mechanical - that act together to ensure functional recovery. Myriad materials have been explored as bioactive scaffolds to deliver these cues locally to the damage site, amongst these piezoelectric materials have demonstrated significant potential for tissue engineering and regeneration, especially for bone repair. Piezoelectric materials have been widely explored for power generation and harvesting, structural health monitoring, and use in biomedical devices. They have the ability to deform with physiological movements and consequently deliver electrical stimulation to cells or damaged tissue without the need of an external power source. Bone itself is piezoelectric and the charges/potentials it generates in response to mechanical activity are capable of enhancing bone growth. Piezoelectric materials are capable of stimulating the physiological electrical microenvironment, and can play a vital role to stimulate regeneration and repair. This review gives an overview of the association of piezoelectric effect with bone repair, and focuses on state-of-the-art piezoelectric materials (polymers, ceramics and their composites), the fabrication routes to produce piezoelectric scaffolds, and their application in bone repair. Important characteristics of these materials from the perspective of bone tissue engineering are highlighted. Promising upcoming strategies and new piezoelectric materials for this application are presented.

STATEMENT OF SIGNIFICANCE

Electrical stimulation/electrical microenvironment are known effect the process of bone regeneration by altering the cellular response and are crucial in maintaining tissue functionality. Piezoelectric materials, owing to their capability of generating charges/potentials in response to mechanical deformations, have displayed great potential for fabricating smart stimulatory scaffolds for bone tissue engineering. The growing interest of the scientific community and compelling results of the published research articles has been the motivation of this review article. This article summarizes the significant progress in the field with a focus on the fabrication aspects of piezoelectric materials. The review of both material and cellular aspects on this topic ensures that this paper appeals to both material scientists and tissue engineers.

Three-dimensional piezoelectric fibrous scaffolds selectively promote mesenchymal stem cell differentiation

The discovery of electric fields in biological tissues has led to efforts in developing technologies utilizing electrical stimulation for therapeutic applications. Native tissues, such as cartilage and bone, exhibit piezoelectric behavior, wherein electrical activity can be generated due to mechanical deformation. Yet, the use of piezoelectric materials have largely been unexplored as a potential strategy in tissue engineering, wherein a piezoelectric biomaterial acts as a scaffold to promote cell behavior and the formation of large tissues. Here we show, for the first time, that piezoelectric materials can be fabricated into flexible, three-dimensional fibrous scaffolds and can be used to stimulate human mesenchymal stem cell differentiation and corresponding extracellular matrix/tissue formation in physiological loading conditions. Piezoelectric scaffolds that exhibit low voltage output, or streaming potential, promoted chondrogenic differentiation and piezoelectric scaffolds with a high voltage output promoted osteogenic differentiation. Electromechanical stimulus promoted greater differentiation than mechanical loading alone. Results demonstrate the additive effect of electromechanical stimulus on stem cell differentiation, which is an important design consideration for tissue engineering scaffolds. Piezoelectric, smart materials are attractive as scaffolds for regenerative medicine strategies due to their inherent electrical properties without the need for external power sources for electrical stimulation.

Potential of Osteoblastic Cells Derived from Bone Marrow and Adipose Tissue Associated with a Polymer/Ceramic Composite to Repair Bone Tissue

One of the tissue engineering strategies to promote bone regeneration is the association of cells and biomaterials. In this context, the aim of this study was to evaluate if cell source, either from bone marrow or adipose tissue, affects bone repair induced by osteoblastic cells associated with a membrane of poly(vinylidene-trifluoroethylene)/barium titanate (PVDF-TrFE/BT). Mesenchymal stem cells (MSC) were isolated from rat bone marrow and adipose tissue and characterized by detection of several surface markers. Also, both cell populations were cultured under osteogenic conditions and it was observed that MSC from bone marrow were more osteogenic than MSC from adipose tissue. The bone repair was evaluated in rat calvarial defects implanted with PVDF-TrFE/BT membrane and locally injected with (1) osteoblastic cells differentiated from MSC from bone marrow, (2) osteoblastic cells differentiated from MSC from adipose tissue or (3) phosphate-buffered saline. Luciferase-expressing osteoblastic cells derived from bone marrow and adipose tissue were detected in bone defects after cell injection during 25 days without difference in luciferin signal between cells from both sources. Corroborating the in vitro findings, osteoblastic cells from bone marrow combined with the PVDF-TrFE/BT membrane increased the bone formation, whereas osteoblastic cells from adipose tissue did not enhance the bone repair induced by the membrane itself. Based on these findings, it is possible to conclude that, by combining a membrane with cells in this rat model, cell source matters and that bone marrow could be a more suitable source of cells for therapies to engineer bone.

Electroactive BaTiO3 nanoparticle-functionalized fibrous scaffolds enhance osteogenic differentiation of mesenchymal stem cells

It has been proven that the surface topographic cues of fiber arrangement can induce osteogenic differentiation of mesenchymal stem cells. However, this effect alone is weak and insufficient to meet the needs of regenerative medicine. In this work, electroactivity concept was introduced to enhance the osteoinductivity of fibrous scaffolds. The randomly oriented and aligned electroactive fibrous scaffolds of poly-(l-lactic acid) (PLLA) with incorporation of ferroelectric ceramic BaTiO₃ (BTO) nanoparticles (NPs) were fabricated by electrospinning. Physicochemical properties, including fiber morphology, microstructure, composition, thermal stability, surface roughness, and surface wettability, of these fibrous scaffolds were studied. The dielectric properties of the scaffolds were evaluated. The results showed that the randomly oriented BTO/PLLA composite fibrous scaffolds had the highest dielectric permittivity of 1.19, which is of the same order of magnitude as the natural bone. The combined effects of fiber orientation and electrical activity on the osteogenic responses of bone marrow mesenchymal stem cells (BM-MSCs) were specifically investigated. Randomly oriented composite fibrous scaffolds significantly promoted polygonal spreading and encouraged early osteogenic differentiation in BM-MSCs, whereas aligned composite fibrous scaffolds promoted cell elongation and discouraged osteogenic differentiation. These results evidenced that randomly fiber orientation and biomimetic electric activity have combining effects on osteogenic differentiation of BM-MSCs. Our findings indicate that coupling effects of multi-physical properties should be paid more attention to mimic the microenvironment for enhancing osteogenic differentiation of BM-MSCs.

Poly(Vinylidene Fluoride-Trifluorethylene)/barium titanate membrane promotes de novo bone formation and may modulate gene expression in osteoporotic rat model

Osteoporosis is a chronic disease that impairs proper bone remodeling. Guided bone regeneration is a surgical technique that improves bone defect in a particular region through new bone formation, using barrier materials (e.g. membranes) to protect the space adjacent to the bone defect. The polytetrafluorethylene membrane is widely used in guided bone regeneration, however, new membranes are being investigated. The purpose of this study was to evaluate the effect of P(VDFTrFE)/BT [poly(vinylidene fluoride-trifluoroethylene)/barium titanate] membrane on in vivo bone formation. Twenty-three Wistar rats were submitted to bilateral ovariectomy. Five animals were subjected to sham surgery. After 150 days, bone defects were created and filled with P(VDF-TrFE)/BT membrane or PTFE membrane (except for the sham and OVX groups). After 4 weeks, the animals were euthanized and calvaria samples were subjected to histomorphometric and computed microtomography analysis (microCT), besides real time polymerase chain reaction (real time PCR) to evaluate gene expression. The histomorphometric analysis showed that the animals that received the P(VDF-TrFE)/BT membrane presented morphometric parameters similar or even better compared to the animals that received the PTFE membrane. The comparison between groups showed that gene expression of RUNX2, BSP, OPN, OSX and RANKL were lower on P(VDF-TrFE)/BT membrane; the gene expression of ALP, OC, RANK and CTSK were similar and the gene expression of OPG, CALCR and MMP9 were higher when compared to PTFE. The results showed that the P(VDF-TrFE)/BT membrane favors bone formation, and therefore, may be considered a promising biomaterial to support bone repair in a situation of osteoporosis.

Capacitive Feedthroughs for Medical Implants

Important technological advances in the last decades paved the road to a great success story for electrically stimulating medical implants, including cochlear implants or implants for deep brain stimulation. However, there are still many challenges in reducing side effects and improving functionality and comfort for the patient. Two of the main challenges are the wish for smaller implants on one hand, and the demand for more stimulation channels on the other hand. But these two aims lead to a conflict of interests. This paper presents a novel design for an electrical feedthrough, the so called capacitive feedthrough, which allows both reducing the size, and increasing the number of included channels. Capacitive feedthroughs combine the functionality of a coupling capacitor and an electrical feedthrough within one and the same structure. The paper also discusses the progress and the challenges of the first produced demonstrators. The concept bears a high potential in improving current feedthrough technology, and could be applied on all kinds of electrical medical implants, even if its implementation might be challenging.

Electrically Polarized Biomaterials

Electrically polarized biomaterials and their interactions with the surrounding biological environment is important for understanding the host response, growth and inhibition of biological species as well as the long-term fate and performance of the implants. Polarized materials possess electrical charges at the surface due to polar or electret properties. As these surfaces are at the frontier of biological reactions understanding biological interactions at the interface with polarized biomaterials requires a convergence of understanding multiple disciplines. This article discusses progress that has taken place in the fields of surface and interface science, materials science and biomedical device engineering to obtain a better perspective of such interactions.

Participation of MicroRNA-34a and RANKL on bone repair induced by poly(vinylidene-trifluoroethylene)/barium titanate membrane

The poly(vinylidene-trifluoroethylene)/barium titanate (PVDF) membrane enhances in vitro osteoblast differentiation and in vivo bone repair. Here, we hypothesized that this higher bone repair could be also due to bone resorption inhibition mediated by a microRNA (miR)/RANKL circuit. To test our hypothesis, the large-scale miR expression of bone tissue grown on PVDF and polytetrafluoroethylene (PTFE) membranes was evaluated to identify potential RANKL-targeted miRs modulated by PVDF. The animal model used was rat calvarial defects implanted with either PVDF or PTFE. At 4 and 8 weeks, the bone tissue grown on membranes was submitted to a large-scale analysis of miRs by microarray. The expression of miR-34a and some of its targets, including RANKL, were evaluated by real-time polimerase chain reaction and osteoclast activity was detected by tartrate-resistant acid phosphatase (TRAP) staining. Among more than 250 miRs, twelve, including miR-34a, were simultaneously higher expressed (≥2 fold) at 4 and 8 weeks on PVDF. The higher expression of miR-34a was concomitant with a reduced expression of all its evaluated targets, including RANKL. Additionally, more TRAP-positive cells were observed in bone tissue grown on PTFE compared with PVDF in both time points. In conclusion, our results suggest that the higher bone formation induced by PVDF could be, at least in part, triggered by a miR-34a increase and RANKL decrease, which may inhibit osteoclast differentiation and activity, and bone resorption.

Association of mesenchymal stem cells and osteoblasts for bone repair

Aim: We tested the hypothesis that the association of bone marrow mesenchymal stem cells (MSCs) and osteoblasts (OBs) optimize bone repair. Materials & Methods: MSCs were cultured in growth or osteogenic medium and seeded into gelatin sponge prior to implantation. Defects were created into rat calvariae and implanted with gelatin sponge without cells, with MSCs, with OBs and with association of MSCs and OBs. Histological analysis and micro-CT-based histomorphometry were carried out after 4 weeks. Results: Increased bone formation was observed in defects treated with cells and bone volume was greater in defects treated with either OBs or MSCs/OBs. Conclusion: Association of MSCs and OBs did not increase the process of bone repair compared with cell-based therapy using either MSCs or OBs alone.