The effect of biphasic electrical stimulation on osteoblast function at anodized nanotubular titanium surfaces

Nanomaterials and synergistic low-intensity direct current (LIDC) stimulation technology for orthopedic implantable medical devices

Nanomaterials play a significant role in biomedical research and applications because of their unique biological, mechanical, and electrical properties. In recent years, they have been utilized to improve the functionality and reliability of a wide range of implantable medical devices ranging from well-established orthopedic residual hardware devices (e.g., hip implants) that can repair defects in skeletal systems to emerging tissue engineering scaffolds that can repair or replace organ functions. This review summarizes the applications and efficacies of these nanomaterials that include synthetic or naturally occurring metals, polymers, ceramics, and composites in orthopedic implants, the largest market segment of implantable medical devices. The importance of synergistic engineering techniques that can augment or enhance the performance of nanomaterial applications in orthopedic implants is also discussed, the focus being on a low-intensity direct electric current (LIDC) stimulation technology to promote the long-term antibacterial efficacy of oligodynamic metal-based surfaces by ionization, while potentially accelerating tissue growth and osseointegration. While many nanomaterials have clearly demonstrated their ability to provide more effective implantable medical surfaces, further decisive investigations are necessary before they can translate into medically safe and commercially viable clinical applications. The article concludes with a discussion about some of the critical impending issues with the application of nanomaterials-based technologies in implantable medical devices, and potential directions to address these.

The effect of sinusoidal AC electric stimulation of 3D PCL/CNT and PCL/β-TCP based bio-composites on cellular activities for bone tissue regeneration

Various physical stimulations have been widely applied to tissue regenerative applications. In particular, for bone tissue regeneration, several experimental studies have reported that electric stimulation can enhance the mineral formation in cultured osteoblasts and even alter the pattern of gene expression, promoting bone tissue formation. However, to date, for rapid-prototyped polycaprolactone (PCL)-based composites of pure PCL and dispersed materials including carbon nanotubes and β-tricalcium phosphate (TCP), the effect of electric stimulation on various cellular activities has not been analyzed. Here, a sinusoidal AC electric field (55 ± 8 mV cm^-1 and 60 Hz) between parallel electrodes was applied to three-dimensional scaffolds (pure PCL, PCL/CNT-0.2 wt%, and PCL/β-TCP-20 wt%) cultured with osteoblast-like cells (MG63) 30 min per day for 14 days. When exposed to electric stimulation, alkaline phosphatase and calcium mineralization were enhanced in all scaffolds, and the PCL/β-TCP scaffold in particular showed the highest improvement in bone mineralization compared with other scaffolds. In this work, we surmised that the improvement may have been due to chemical precipitation of the calcium ions from the PCL/β-TCP scaffolds. To evaluate the effect of the released calcium ions from the composite scaffold, we observed the cellular behavior (cellular contraction) of proliferated cells under electric stimulation. The results indicate that in addition to the applied electric field conditions, the scaffold materials are also an important parameter for successful electric stimulation.

Combined effects of direct current stimulation and immobilized BMP-2 for enhancement of osteogenesis

Direct current (DC) stimulation has been used to promote bone repair and osteogenesis, but problems associated with the implanted metal electrodes may limit its application and compromise the therapeutic results. The replacement of the metal electrodes with a biodegradable conductive polymer film can potentially overcome these problems. In our work, polypyrrole/chitosan films comprising polypyrrole nanoparticles dispersed in a chitosan matrix were prepared. The polypyrrole/chitosan film meets the requirements for DC delivery, as indicated by its electrical conductivity, biodegradability, and mechanical properties. The film supports osteoblast growth to the same degree as dentine discs (a bone-like mineralized substrate), confirming that it is non-cytotoxic. Our results showed that optimal DC stimulation was achieved with 200 µA for 4 h per day, and under this condition, osteoblast metabolic activity on Day 7 increased by 1.8-fold over that without DC stimulation. To further improve osteogenesis on the polypyrrole/chitosan film, bone morphogenetic protein-2 (BMP-2) was covalently immobilized on the film surface. Osteoblasts cultured on the BMP-2-functionalized polypyrrole/chitosan film and subjected to the optimal DC stimulation exhibited a significant increase in cellular metabolic activity (2.3-fold on Day 7), ALP activity (1.7-fold on Day 21) and mineralization (twofold on Day 21) over those cultured on polypyrrole/chitosan film without DC stimulation. Osteogenic gene expression results showed that BMP-2 and DC stimulation by itself enhanced osteoblast differentiation, and a combination of these two factors resulted in synergistic effects on osteoblast differentiation and maturation.

Investigation of In Vitro Bone Cell Adhesion and Proliferation on Ti Using Direct Current Stimulation

Our objective was to establish an in vitro cell culture protocol to improve bone cell attachment and proliferation on Ti substrate using direct current stimulation. For this purpose, a custom made electrical stimulator was developed and a varying range of direct currents, from 5 to 25 µA, were used to study the current stimulation effect on bone cells cultured on conducting Ti samples in vitro. Cell-materials interaction was studied for a maximum of 5 days by culturing with human fetal osteoblast cells (hFOB). The direct current was applied in every 8 h time interval and the duration of electrical stimulation was kept constant at 15 min for all cases. In vitro results showed that direct current stimulation significantly favored bone cell attachment and proliferation in comparison to nonstimulated Ti surface. Immunochemistry and confocal microscopy results confirmed that the cell adhesion was most pronounced on 25 µA direct current stimulated Ti surfaces as hFOB cells expressed higher vinculin protein with increasing amount of direct current. Furthermore, MTT assay results established that cells grew 30% higher in number under 25 µA electrical stimulation as compared to nonstimulated Ti surface after 5 days of culture period. In this work we have successfully established a simple and cost effective in vitro protocol offering easy and rapid analysis of bone cell-materials interaction which can be used in promotion of bone cell attachment and growth on Ti substrate using direct current electrical stimulation in an in vitro model.

Electrical stimulation: a novel tool for tissue engineering

New advances in tissue engineering are being made through the application of different types of electrical stimuli to influence cell proliferation and differentiation. Developments made in the last decade have allowed us to improve the structure and functionality of tissue-engineered products through the use of growth factors, hormones, drugs, physical stimuli, bioreactor use, and two-dimensional (2-D) and three-dimensional (3-D) artificial extracellular matrices (with various material properties and topography). Another potential type of stimulus is electricity, which is important in the physiology and development of the majority of all human tissues. Despite its great potential, its role in tissue regeneration and its ability to influence cell migration, orientation, proliferation, and differentiation has rarely been considered in tissue engineering. This review highlights the importance of endogenous electrical stimulation, gathering the current knowledge on its natural occurrence and role in vivo, discussing the novel methods of delivering this stimulus and examining its cellular and tissue level effects, while evaluating how the technique could benefit the tissue engineering discipline in the future.

TiO2 nanotubes as drug nanoreservoirs for the regulation of mobility and differentiation of mesenchymal stem cells

The extracellular microenvironment plays a key role in the regulation of cellular behavior. To mimic the natural extracellular microenvironment, TiO(2) nanotube (TNT) arrays as drug nanoreservoirs for loading of bone morphogenetic protein 2 (BMP2) were constructed on titanium substrates and then covered with multilayered coatings of gelatin/chitosan (Gel/Chi) for controlled drug release. The multilayered coatings were constructed via a spin-assisted layer-by-layer assembly technique. The successful fabrication of this system was monitored by field emission scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy and contact angle measurements. Multilayered coating with Gel/Chi retained the drug bioactivity and release properties, which were revealed by superoxide dismutase activity measurement. In addition, cytoskeleton observation and wound healing assay confirmed that BMP2-loaded and multilayer-coated TNT arrays were able to stimulate motogenic responses of mesenchymal stem cells (MSCs). More importantly, the system demonstrated that it was capable of promoting the osteoblastic differentiation of MSCs. This study may have potential impact on the development of bone implants for enhanced bone osseointegration.

Electrical implications of corrosion for osseointegration of titanium implants

The success rate of titanium implants for dental and orthopedic applications depends on the ability of surrounding bone tissue to integrate with the surface of the device, and it remains far from ideal in patients with bone compromised by physiological factors. The electrical properties and electrical stimulation of bone have been shown to control its growth and healing and can enhance osseointegration. Bone cells are also sensitive to the chemical products generated during corrosion events, but less is known about how the electrical signals associated with corrosion might affect osseointegration. The metallic nature of the materials used for implant applications and the corrosive environments found in the human body, in combination with the continuous and cyclic loads to which these implants are exposed, may lead to corrosion and its corresponding electrochemical products. The abnormal electrical currents produced during corrosion can convert any metallic implant into an electrode, and the negative impact on the surrounding tissue due to these extreme signals could be an additional cause of poor performance and rejection of implants. Here, we review basic aspects of the electrical properties and electrical stimulation of bone, as well as fundamental concepts of aqueous corrosion and its electrical and clinical implications.

Decreased Staphylococcus aureus biofilm growth on anodized nanotubular titanium and the effect of electrical stimulation

Infection is a major problem in orthopedics leading to implant failure. It is a challenging task to treat orthopedic implant infection, which may lead to implant replacement and, in severe cases, may result in amputation and mortality. Infection poses an even further risk as bacteria are beginning to develop resistance against commonly used antibiotics. Therefore, in this research a combination of various approaches was used to fight implant infection without resorting to the use of antibiotics. Specifically, conventional titanium was altered through a process of anodization and electrical stimulation to reduce Staphylococcus aureus growth. It was shown that when a 15-30 V electrical stimulation was coupled with anodized nanotubular titanium a significant decrease in S. aureus biofilm formation was observed, compared with non-anodized and non-electrically stimulated titanium after 2 days culture. The decrease in biofilm formation observed here was explained by the presence of fluorine on the surfaces of anodized nanotubular titanium. Thus, coupling the positive influences of anodization and electrical stimulation could be a promising way to fight titanium-based orthopedic device-related infections.

Osteoblast function on electrically conductive electrospun PLA/MWCNTs nanofibers

The electrospinning process was utilized successfully to fabricate the random oriented and aligned electrically conductive nanofibers of biodegradable poly-DL-lactide (PLA) in which multiwalled carbon nanotubes (MWCNTs) were embedded. The topographical features of the composite nanofibers were characterized by SEM. The dispersion and alignment of MWCNTs in nanofiber matrix were observed by TEM. The in vitro degradation was characterized in terms of the morphological change, the mass loss and the reduction of polymer molecular weight as well as the decrease of pH value of degradation media. In particular, these conductive nanofiber meshes offered a unique system to study the synergistic effect of topographic cues and electrical stimulation on osteoblasts outgrowth as a way of exploring their potential application in bone tissue engineering. The results of obsteoblasts assay unstimulated showed that the aligned nanofibers as topographic cues could enhance the extension and direct the outgrowth of obsteoblasts better than random fibers. In the presence of direct current (DC) of 100 μA, the obsteoblasts on all samples grew along the electrical current direction. The cellular elongation and proliferation were mainly dependent on the electrical stimulation whereas the topographical features played a minor role in them. Therefore, electrical stimulation with an appropriate DC value imparted on conductive substrate had great potential in application of bone tissue engineering.

Nanoscale surface modifications of medically relevant metals: state-of-the art and perspectives

Evidence that nanoscale surface properties stimulate and guide various molecular and biological processes at the implant/tissue interface is fostering a new trend in designing implantable metals. Cutting-edge expertise and techniques drawn from widely separated fields, such as nanotechnology, materials engineering and biology, have been advantageously exploited to nanoengineer surfaces in ways that control and direct these processes in predictable manners. In this review, we present and discuss the state-of-the-art of nanotechnology-based approaches currently adopted to modify the surface of metals used for orthopedic and dental applications, and also briefly consider their use in the cardiovascular field. The effects of nanoengineered surfaces on various in vitro molecular and cellular events are firstly discussed. This review also provides an overview of in vivo and clinical studies with nanostructured metallic implants, and addresses the potential influence of nanotopography on biomechanical events at interfaces. Ultimately, the objective of this work is to give the readership a comprehensive picture of the current advances, future developments and challenges in the application of the infinitesimally small to biomedical surface science. We believe that an integrated understanding of the in vitro and particularly of the in vivo behavior is mandatory for the proper exploitation of nanostructured implantable metals and, indeed, of all biomaterials.