Re-Differentiation Capacity of Human Chondrocytes in Vitro Following Electrical Stimulation with Capacitively Coupled Fields

Polypyrrole-based structures for activation of cellular functions under electrical stimulation

Polypyrrole (Ppy) is an electroconductive polymer used in various applications, including in vitro experiments with cell cultures under electrical stimulation (ES). Ppy can be applied in various forms and most importantly, it is biocompatible with cells. Ppy specifically directs ES to cells, which makes Ppy a potential polymer for the development of novel technologies for targeted tissue regeneration. The high potential of ES in combination with different Ppy-based systems, such as hydrogels, scaffolds, or Ppy-layers is advantageous to stimulate cellular differentiation towards neurogenic, cardiac, muscle, and osteogenic lineages. Different in-house devices and the principles of ES application used to stimulate cellular functions are reviewed and summarized. The focus of this review is to observe the most relevant studies and their in-house techniques regarding the application of Ppy-based materials for the use of bone, neural, cardiac, and muscle tissue regeneration under ES. Different types of Ppy materials, such as Ppy particles, layers/films, membranes, and 3D-shaped synthetic and natural scaffolds, as well as combining Ppy with different active molecules are reviewed.

Regenerative bioelectronics: A strategic roadmap for precision medicine

In the past few decades, stem cell-based regenerative engineering has demonstrated its significant potential to repair damaged tissues and to restore their functionalities. Despite such advancement in regenerative engineering, the clinical translation remains a major challenge. In the stance of personalized treatment, the recent progress in bioelectronic medicine likewise evolved as another important research domain of larger significance for human healthcare. Over the last several years, our research group has adopted biomaterials-based regenerative engineering strategies using innovative bioelectronic stimulation protocols based on either electric or magnetic stimuli to direct cellular differentiation on engineered biomaterials with a range of elastic stiffness or functional properties (electroactivity/magnetoactivity). In this article, the role of bioelectronics in stem cell-based regenerative engineering has been critically analyzed to stimulate futuristic research in the treatment of degenerative diseases as well as to address some fundamental questions in stem cell biology. Built on the concepts from two independent biomedical research domains (regenerative engineering and bioelectronic medicine), we propose a converging research theme, 'Regenerative Bioelectronics'. Further, a series of recommendations have been put forward to address the current challenges in bridging the gap in stem cell therapy and bioelectronic medicine. Enacting the strategic blueprint of bioelectronic-based regenerative engineering can potentially deliver the unmet clinical needs for treating incurable degenerative diseases.

Experimental and numerical methods to ensure comprehensible and replicable alternating current electrical stimulation experiments

Electrical stimulation has received increasing attention for decades for its application in regenerative medicine. Applications range from bone growth stimulation over cartilage regeneration to deep brain stimulation. Despite all research efforts, translation into clinical use has not yet been achieved in all fields. Recent critical assessments have identified limited documentation and monitoring of preclinical in vitro and in vivo experiments as possible reasons hampering clinical translation. In this work, we present experimental and numerical methods to determine the crucial quantities of electrical stimulation such as the electric field or current density. Knowing the stimulation quantities contributes to comprehending the biological response to electrical stimulation and to finally developing a reliable dose-response curve. To demonstrate the methods, we consider a direct contact electrical stimulation experiment that stands representative for a broad class of stimulation experiments. Electrochemical effects are addressed and methods to integrate them into numerical simulations are evaluated. A focus is laid on affordable lab equipment and reproducible open-source software solutions. Finally, clear guidelines to ensure replicability of electrical stimulation experiments are formulated.

Electrical Stimulation in Cartilage Tissue Engineering

Electrical stimulation (ES) has been frequently used in different biomedical applications both in vitro and in vivo. Numerous studies have demonstrated positive effects of ES on cellular functions, including metabolism, proliferation, and differentiation. The application of ES to cartilage tissue for increasing extracellular matrix formation is of interest, as cartilage is not able to restore its lesions owing to its avascular nature and lack of cells. Various ES approaches have been used to stimulate chondrogenic differentiation in chondrocytes and stem cells; however, there is a huge gap in systematizing ES protocols used for chondrogenic differentiation of cells. This review focuses on the application of ES for chondrocyte and mesenchymal stem cell chondrogenesis for cartilage tissue regeneration. The effects of different types of ES on cellular functions and chondrogenic differentiation are reviewed, systematically providing ES protocols and their advantageous effects. Moreover, cartilage 3D modeling using cells in scaffolds/hydrogels under ES are observed, and recommendations on reporting about the use of ES in different studies are provided to ensure adequate consolidation of knowledge in the area of ES. This review brings novel insights into the further application of ES in in vitro studies, which are promising for further cartilage repair techniques.

How to correctly estimate the electric field in capacitively coupled systems for tissue engineering: a comparative study

Capacitively Coupled (CCoupled) electric fields are used to stimulate cell cultures in Tissue Engineering. Knowing the electric field (E-Field) magnitude in the culture medium is fundamental to establish a relationship between stimulus strength and cellular effects. We analysed eight CCoupled studies and sought to corroborate the reported estimates of the E-Field in the culture medium. First, we reviewed the basic physics underlying CCoupled stimulation and delineated three approaches to estimate the E-field. Using these approaches, we found that the reported values were overestimated in five studies, four of which were based on incorrect assumptions. In all studies, insufficient information was provided to reproduce the setup exactly. Creating electrical models of the experimental setup should improve the accuracy of the E-field estimates and enhance reproducibility. For this purpose, we developed a free open-source tool, the E-field Calculator for CCoupled systems, which is available for download from an internet hosting platform.

Numerical study on the effect of capacitively coupled electrical stimulation on biological cells considering model uncertainties

Electrical stimulation of biological samples such as tissues and cell cultures attracts growing attention due to its capability of enhancing cell activity, proliferation, and differentiation. Eventually, a profound knowledge of the underlying mechanisms paves the way for innovative therapeutic devices. Capacitive coupling is one option of delivering electric fields to biological samples that has advantages regarding biocompatibility. However, its biological mechanism of interaction is not well understood. Experimental findings could be related to voltage-gated channels, which are triggered by changes of the transmembrane potential. Numerical simulations by the finite element method provide a possibility to estimate the transmembrane potential. Since a full resolution of the cell membrane within a macroscopic model would lead to prohibitively expensive models, we suggest the adaptation of an approximate finite element method. Starting from a basic 2.5D model, the chosen method is validated and applied to realistic experimental situations. To understand the influence of the dielectric properties on the modelling outcome, uncertainty quantification techniques are employed. A frequency-dependent influence of the uncertain dielectric properties of the cell membrane on the modelling outcome is revealed. This may have practical implications for future experimental studies. Our methodology can be easily adapted for computational studies relying on experimental data.

Integration of clinical perspective into biomimetic bioreactor design for orthopedics

The challenges to accommodate multiple tissue formation metrics in conventional bioreactors have resulted in an increased interest to explore novel bioreactor designs. Bioreactors allow researchers to isolate variables in controlled environments to quantify cell response. While current bioreactor designs can effectively provide either mechanical, electrical, or chemical stimuli to the controlled environment, these systems lack the ability to combine all these stimuli simultaneously to better recapitulate the physiological environment. Introducing a dynamic and systematic combination of biomimetic stimuli bioreactor systems could tremendously enhance its clinical relevance in research. Thus, cues from different tissue responses should be studied collectively and included in the design of a biomimetic bioreactor platform. This review begins by providing a summary on the progression of bioreactors from simple to complex designs, focusing on the major advances in bioreactor technology and the approaches employed to better simulate in vivo conditions. The current state of bioreactors in terms of their clinical relevance is also analyzed. Finally, this review provides a comprehensive overview of individual biophysical stimuli and their role in establishing a biomimetic microenvironment for tissue engineering. To date, the most advanced bioreactor designs only incorporate one or two stimuli. Thus, the cell response measured is likely unrelated to the actual clinical performance. Integrating clinically relevant stimuli in bioreactor designs to study cell response can further advance the understanding of physical phenomenon naturally occurring in the body. In the future, the clinically informed biomimetic bioreactor could yield more efficiently translatable results for improved patient care.

Phenolated alginate-collagen hydrogel induced chondrogenic capacity of human amniotic mesenchymal stem cells

Horseradish peroxidase (HRP)-catalyzed hydrogels are considered to be an important platform for tissue engineering applications. In this study, we investigated the chondrogenic capacity of phenolated (1.2%) alginate-(0.5%) collagen hydrogel on human amniotic mesenchymal stem cells after 21 days. Using NMR, FTIR analyses, and SEM imaging, we studied the phenolation and structure of alginate-collagen hydrogel. For physicochemical evaluations, gelation time, mechanical properties, swelling, and degradation rate were assessed. The survival rate was monitored using the MTT assay and DAPI staining. Western blotting was performed to measure the chondrogenic differentiation of cells. NMR showed successful phenolation of the alginate-collagen hydrogel. FTIR exhibited the interaction between the functional groups of collagen with phenolated alginate. SEM showed the existence of collagen microfibrils in the alginate-collagen hydrogel. Compared to phenolated alginate, the addition of collagen increased hydrogel elasticity by 10%. Both swelling rate and biodegradability were reduced in the presence of collagen. We noted an increased survival rate in phenolated alginate-collagen compared to the control cells (p < 0.05). Western blotting revealed the increase of chondrocyte-associated proteins such as SOX9 and COL2A1 in phenolated-alginate-collagen hydrogels after 21 days. These data showed that phenolated alginate-collagen hydrogel is an appropriate 3 D substrate to induce chondrogenic capacity of human mesenchymal stem cells.

Electrically Conductive and 3D-Printable Oxidized Alginate-Gelatin Polypyrrole:PSS Hydrogels for Tissue Engineering

Electroactive hydrogels can be used to influence cell response and maturation by electrical stimulation. However, hydrogel formulations which are 3D printable, electroactive, cytocompatible, and allow cell adhesion, remain a challenge in the design of such stimuli-responsive biomaterials for tissue engineering. Here, a combination of pyrrole with a high gelatin-content oxidized alginate-gelatin (ADA-GEL) hydrogel is reported, offering 3D-printability of hydrogel precursors to prepare cytocompatible and electrically conductive hydrogel scaffolds. By oxidation of pyrrole, electroactive polypyrrole:polystyrenesulfonate (PPy:PSS) is synthesized inside the ADA-GEL matrix. The hydrogels are assessed regarding their electrical/mechanical properties, 3D-printability, and cytocompatibility. It is possible to prepare open-porous scaffolds via bioplotting which are electrically conductive and have a higher cell seeding efficiency in scaffold depth in comparison to flat 2D hydrogels, which is confirmed via multiphoton fluorescence microscopy. The formation of an interpenetrating polypyrrole matrix in the hydrogel matrix increases the conductivity and stiffness of the hydrogels, maintaining the capacity of the gels to promote cell adhesion and proliferation. The results demonstrate that a 3D-printable ADA-GEL can be rendered conductive (ADA-GEL-PPy:PSS), and that such hydrogel formulations have promise for cell therapies, in vitro cell culture, and electrical-stimulation assisted tissue engineering.

Proximity-induced electrodeformation and membrane capacitance coupling between cells

Membrane capacitance and transmembrane potential are sensitive to the proximity of neighboring biological cells which eventually induces anisotropic perturbation of the local electric field distribution in a cell assembly and/or a tissue. The development of robust and reliable multiphysics approaches is essential to solve the challenge of analyzing proximity-induced capacitance coupling (CC) between cells. In this study, we ask to what extent this CC is a minor perturbation on the individual cells or whether it can fundamentally affect bio-electromechanical cues. A key component of our continuum electromechanical analysis is the consideration of elastic models of cells under steady state electric field excitation to characterize electrodeformation (ED). Analyzing the difference between the ED force for a pair of cells and its counterpart for a single reference cell allows us to determine a separation distance-orientation angle diagram providing evidence of a separation distance beyond which the electrostatic interactions between a pair of biological cells become inconsequential for the ED. An attenuation-amplification transition of ED force in this diagram suggests that anisotropy induced by the orientation angle of the cell pair relative to the applied electric field direction has a significant influence on ED and CC. We furthermore observe that the shape of this diagram changes when extracellular conductivity is varied. The results obtained are then contrasted with the corresponding diagrams of similar cell configurations under an oscillating electric field excitation below and above the α-dispersion frequency. This investigation may provide new opportunities for further assessment of electromechanical properties of engineered tissues.

Feasibility of Human Platelet Lysate as an Alternative to Foetal Bovine Serum for In Vitro Expansion of Chondrocytes

Osteoarthritis (OA) is a degenerative joint disease that affects a lot of people worldwide. Current treatment for OA mainly focuses on halting or slowing down the disease progress and to improve the patient’s quality of life and functionality. Autologous chondrocyte implantation (ACI) is a new treatment modality with the potential to promote regeneration of worn cartilage. Traditionally, foetal bovine serum (FBS) is used to expand the chondrocytes. However, the use of FBS is not ideal for the expansion of cells mean for clinical applications as it possesses the risk of animal pathogen transmission and animal protein transfer to host. Human platelet lysate (HPL) appears to be a suitable alternative to FBS as it is rich in biological factors that enhance cell proliferation. Thus far, HPL has been found to be superior in promoting chondrocyte proliferation compared to FBS. However, both HPL and FBS cannot prevent chondrocyte dedifferentiation. Discrepant results have been reported for the maintenance of chondrocyte redifferentiation potential by HPL. These differences are likely due to the diversity in the HPL preparation methods. In the future, more studies on HPL need to be performed to develop a standardized technique which is capable of producing HPL that can maintain the chondrocyte redifferentiation potential reproducibly. This review discusses the in vitro expansion of chondrocytes with FBS and HPL, focusing on its capability to promote the proliferation and maintain the chondrogenic characteristics of chondrocytes.

Establishment of a New Device for Electrical Stimulation of Non-Degenerative Cartilage Cells In Vitro

In cell-based therapies for cartilage lesions, the main problem is still the formation of fibrous cartilage, caused by underlying de-differentiation processes ex vivo. Biophysical stimulation is a promising approach to optimize cell-based procedures and to adapt them more closely to physiological conditions. The occurrence of mechano-electrical transduction phenomena within cartilage tissue is physiological and based on streaming and diffusion potentials. The application of exogenous electric fields can be used to mimic endogenous fields and, thus, support the differentiation of chondrocytes in vitro. For this purpose, we have developed a new device for electrical stimulation of chondrocytes, which operates on the basis of capacitive coupling of alternating electric fields. The reusable and sterilizable stimulation device allows the simultaneous use of 12 cavities with independently applicable fields using only one main supply. The first parameter settings for the stimulation of human non-degenerative chondrocytes, seeded on collagen type I elastin-based scaffolds, were derived from numerical electric field simulations. Our first results suggest that applied alternating electric fields induce chondrogenic re-differentiation at the gene and especially at the protein level of human de-differentiated chondrocytes in a frequency-dependent manner. In future studies, further parameter optimizations will be performed to improve the differentiation capacity of human cartilage cells.

Computational study on electromechanics of electroactive hydrogels for cartilage-tissue repair

BACKGROUND AND OBJECTIVE

The self-repair capability of articular cartilage is limited because of non-vascularization and low turnover of its extracellular matrix. Regenerating hyaline cartilage remains a significant clinical challenge as most non-surgical and surgical treatments provide only mid-term relief. Eventually, further pain and mobility loss occur for many patients in the long run due to further joint deterioration. Repair of articular cartilage tissue using electroactive scaffolds and biophysical stimuli like electrical and osmotic stimulation may have the potential to heal cartilage defects occurring due to trauma, osteoarthritis, or sport-related injuries. Therefore, the focus of the current study is to present a computational model of electroactive hydrogels for the cartilage-tissue repair as a first step towards an optimized experimental design.

METHODS

The multiphysics transport model that mainly includes the Poisson-Nernst-Planck equations and the mechanical equation is used to find the electrical stimulation response of the polyelectrolyte hydrogels. Based upon this, a numerical model on electromechanics of electroactive hydrogels seeded with chondrocytes is presented employing the open-source software FEniCS, which is a Python library for finite-element analysis.

RESULTS

We analyzed the ionic concentrations and electric potential in a hydrogel sample and the cell culture medium, the osmotic pressure created due to ionic concentration variations and the resulting hydrogel displacement. The proposed mathematical model was validated with examples from literature.

CONCLUSIONS

The presented model for the electrical and osmotic stimulation of a hydrogel sample can serve as a useful tool for the development and analysis of a cartilaginous scaffold employing electrical stimulation. By analyzing various parameters, we pave the way for future research on a finer scale using open-source software.

Graphene Oxide Reinforcement Enhances the Piezoelectric and Mechanical Properties of Poly(3-hydroxybutyrate-co-3-hydroxy valerate)-Based Nanofibrous Scaffolds for Improved Proliferation of Chondrocytes and ECM Production

The discovery of piezoelectricity in natural cartilage has inspired the development of piezoelectric biomaterials for its repair and regeneration using tissue engineering approaches. In the present work, piezoelectric scaffolds composed of poly(3-hydroxybutyrate-co-3-hydroxy valerate) (PB) and graphene oxide (GO) have been successfully fabricated by the electrospinning technology. The fabricated scaffolds were examined for their morphological, physical, chemical, piezoelectric, and biological characterizations. The fiber diameter was found to be in the range of 600-800 nm appropriate for chondrogenic growth. Reinforcement of 1.5% GO enhanced the tensile strength of PB to 2.08 ± 0.33 MPa compared to PB alone (0.59 ± 0.12). Reinforcement of GO significantly enhances the piezoelectric coefficient (d₃₃), and for 0.5, 1, and 1.5% GO in PB, it was found to be 0.12 ± 0.015, 0.57 ± 0.19, and 0.94 ± 0.03 pC/N, respectively, and corresponding voltages of 11.84 ± 1.4, 54.69 ± 18.29, and 100.2 ± 3.2 mV, respectively, were generated. The biological activity of the smart piezo scaffolds was also evaluated on freshly isolated goat chondrocytes. The GO-reinforced scaffold showed higher cell proliferation and cell adhesion as confirmed by alamarBlue assay and field emission scanning electron microscopy imaging. The GO-reinforced scaffold has demonstrated significantly higher extracellular matrix production compared to PB as confirmed by histochemistry and real-time polymerase chain reaction. Hence, the GO-based piezoelectric PB electrospun scaffold can be a better alternative for cell-free and growth factor-free approach for cartilage tissue engineering.

Numerical Simulations as Means for Tailoring Electrically Conductive Hydrogels Towards Cartilage Tissue Engineering by Electrical Stimulation

Cartilage regeneration is a clinical challenge. In recent years, hydrogels have emerged as implantable scaffolds in cartilage tissue engineering. Similarly, electrical stimulation has been employed to improve matrix synthesis of cartilage cells, and thus to foster engineering and regeneration of cartilage tissue. The combination of hydrogels and electrical stimulation may pave the way for new clinical treatment of cartilage lesions. To find the optimal electric properties of hydrogels, theoretical considerations and corresponding numerical simulations are needed to identify well-suited initial parameters for experimental studies. We present the theoretical analysis of a hydrogel in a frequently used electrical stimulation device for cartilage regeneration and tissue engineering. By means of equivalent circuits, finite element analysis, and uncertainty quantification, we elucidate the influence of the geometric and dielectric properties of cell-seeded hydrogels on the capacitive-coupling electrical field stimulation. Moreover, we discuss the possibility of cellular organisation inside the hydrogel due to forces generated by the external electric field. The introduced methodology is easily reusable by other researchers and allows to directly develop novel electrical stimulation study designs. Thus, this study paves the way for the design of future experimental studies using electrically conductive hydrogels and electrical stimulation for tissue engineering.

Establishment and Evaluation of an In Vitro System for Biophysical Stimulation of Human Osteoblasts

While several studies investigated the effects of mechanical or electrical stimulation on osseointegration and bone fracture healing, little is known about the molecular and cellular impact of combined biophysical stimulation on peri-implant osseointegration. Therefore, we established an in vitro system, capable of applying shear stress and electric fields simultaneously. Capacitively coupled electric fields were used for electrical stimulation, while roughened Ti6Al4V bodies conducted harmonically oscillating micromotions on collagen scaffolds seeded with human osteoblasts. Different variations of single and combined stimulation were applied for three days, while samples loaded with Ti6Al4V bodies and untreated samples served as control. Metabolic activity, expression of osteogenic markers and bone remodeling markers were investigated. While combined stimulation showed no substantial benefit compared to sole mechanical stimulation, we observed that 25 µm micromotions applied by roughened Ti6Al4V bodies led to a significant increase in gene expression of osteocalcin and tissue inhibitor of metalloprotease 1. Additionally, we found an increase in metabolic activity and expression of bone remodeling markers with reduced procollagen type 1 synthesis after 100 mV_RMS electrical stimulation. We were able to trigger specific cellular behaviors using different biophysical stimuli. In future studies, different variations of electrical stimulation will be combined with interfacial micromotions.

Devitalizing Effect of High Hydrostatic Pressure on Human Cells-Influence on Cell Death in Osteoblasts and Chondrocytes

Chemical and physical processing of allografts is associated with a significant reduction in biomechanics. Therefore, treatment of tissue with high hydrostatic pressure (HHP) offers the possibility to devitalize tissue gently without changing biomechanical properties. To obtain an initial assessment of the effectiveness of HHP treatment, human osteoblasts and chondrocytes were treated with different HHPs (100–150 MPa, 250–300 MPa, 450–500 MPa). Devitalization efficiency was determined by analyzing the metabolic activity via WST-1(water-soluble tetrazolium salt) assay. The type of cell death was detected with an apoptosis/necrosis ELISA (enzyme-linked immune sorbent assay) and flow cytometry. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were carried out to detect the degree of cell destruction. After HHP treatment, the metabolic activities of both cell types decreased, whereas HHP of 250 MPa and higher resulted in metabolic inactivation. Further, the highest HHP range induced mostly necrosis while the lower HHP ranges induced apoptosis and necrosis equally. FESEM and TEM analyses of treated osteoblasts revealed pressure-dependent cell damage. In the present study, it could be proven that a pressure range of 250–300 MPa can be used for cell devitalization. However, in order to treat bone and cartilage tissue gently with HHP, the results of our cell experiments must be verified for tissue samples in future studies.