Research to Clinics: Clinical Translation Considerations for Anodized Nano-Engineered Titanium Implants

Titania nanotubes (TNTs) fabricated on titanium orthopedic and dental implants have shown significant potential in "proof of concept" in vitro, ex vivo, and short-term in vivo studies. However, most studies do not focus on a clear direction for future research towards clinical translation, and there exists a knowledge gap in identifying key research challenges that must be addressed to progress to the clinical setting. This review focuses on such challenges with respect to anodized titanium implants modified with TNTs, including optimized fabrication on clinically utilized microrough surfaces, clinically relevant bioactivity assessments, and controlled/tailored local release of therapeutics. Further, long-term in vivo investigations in compromised animal models under loading conditions are needed. We also discuss and detail challenges and progress related to the mechanical stability of TNT-based implants, corrosion resistance/electrochemical stability, optimized cleaning/sterilization, packaging/aging, and nanotoxicity concerns. This extensive, clinical translation focused review of TNTs modified Ti implants aims to foster improved understanding of key research gaps and advances, informing future research in this domain.

Engineered titanium implants for localized drug delivery: recent advances and perspectives of Titania nanotubes arrays

INTRODUCTION

Therapeutics delivery to bones to treat skeletal diseases or prevent postsurgical infections is challenging due to complex and solid bone structure that limits blood supply and diffusion of therapeutics administered by systemic routes to reach effective concentration. Titanium (Ti) and their alloys are employed as mainstream implant materials in orthopedics and dentistry; having superior mechanical/biocompatibility properties which could provide an alternative solution to address this problem.

AREAS COVERED

This review presents an overview of recent development of Ti drug-releasing implants, with emphasis on nanoengineered Titania nanotubes (TNTs) structures, for solving key problems to improve implants osseointegration, overcome inflammation and infection together with providing localized drug delivery (LDD) for bone diseases including cancer. Critical analysis of the advantages/disadvantages of developed concepts is discussed, their drug loading/releasing performances and specific applications.

EXPERT OPINION

LDD to bones can address many disorders and postsurgical conditions such as inflammation, implants rejection and infection. To this end, TNTs-Ti implants represent a potential promise for the development of new generation of multifunctional implants with drug release functions. Even this concept is extensively explored recently, there is a strong need for more preclinical studies using animal models to confirm the long-term safety and stability of TNTs-Ti implants for real-life medical applications.

Titania nanopores with dual micro-/nano-topography for selective cellular bioactivity

This letter describes a simple surface modification strategy based on a single-step electrochemical anodization towards generating dual micro- and nano-rough horizontally-aligned TiO₂ nanopores on the surface of clinically utilized micro-grooved titanium implants. Primary macrophages, osteoblasts and fibroblasts were cultured on the nano-engineered implants, and it was demonstrated that the modified surfaces selectively reduced the proliferation of macrophages (immunomodulation), while augmenting the activity of osteoblasts (osseo-integration) and fibroblasts (soft-tissue integration). Additionally, the mechanically robust nanopores also stimulated osteoblast and fibroblast adhesion, attachment and alignment along the direction of the pores/grooves, while macrophages remained oval-shaped and sparsely distributed. This study for the first time reports the use of cost-effectively prepared nano-engineered titanium surface via anodization, with aligned multi-scale micro/nano features for selective cellular bioactivity, without the use of any therapeutics.

Tailoring the immuno-responsiveness of anodized nano-engineered titanium implants

Owing to its biocompatibility and corrosion resistance, titanium is one of the most commonly used implantable biomaterials. Numerous in vitro and in vivo investigations have established that titanium surfaces with a nanoscale topography outperform conventional smooth or micro-rough surfaces in terms of achieving desirable bonding with bone (i.e. enhanced bioactivity). Among these nanoscale topographical modifications, ordered nanostructures fabricated via electrochemical anodization, especially titania nanotubes (TNTs), are particularly attractive. This is due to their ability to augment bioactivity, deliver drugs and the potential for easy/cost-effective translation into the current implant market. However, the potential of TNT-modified implants to modulate the host immune-inflammatory response, which is critical for achieving timely osseointegration, remains relatively unexplored. Such immunomodulatory effects may be achieved by modifying the physical and chemical properties of the TNTs. Furthermore, therapeutic/bioactive enhancements performed on these nano-engineered implants (such as antibacterial or osteogenic functions) are likely to illicit an immune response which needs to be appropriately controlled. The lack of sufficient in-depth studies with respect to immune cell responses to TNTs has created research gaps that must be addressed in order to facilitate the design of the next generation of immuno-modulatory titanium implants. This review article focuses on the chemical, topographical and mechanical features of TNT-modified implants that can be manipulated in order to achieve immuno-modulation, as well as providing an insight into how modulating the immune response can augment implant performance.

Progress in TiO2 nanotube coatings for biomedical applications: a review

Titanium dioxide nanotubes (TNTs) have drawn wide attention and been extensively applied in the field of biomedicine, due to their large specific surface area, good corrosion resistance, excellent biocompatibility, and enhanced bioactivity. This review describes the preparation of TNTs and the surface modification that entrust the nanotubes with better antibacterial property and enhanced osteoblast adhesion, proliferation, and differentiation. Considering the contact between TNTs' surface and surrounding tissues after implantation, the interactions between TNTs (with properties including their diameter, length, wettability, and crystalline phase) and proteins, platelets, bacteria, and cells are illustrated. The state of the art in the applications of TNTs in dentistry, orthopedic implants, and cardiovascular stents are introduced. In particular, the application of TNTs in biosensing has attracted much attention due to its ability for the rapid diagnosis of diseases. Finally, the difficulties and challenges in the practical application of TNTs are also discussed.

The effect of ordered and partially ordered surface topography on bone cell responses: a review

Current understanding of the role of ordered and partially ordered surface topography in bone cell responses for bone implant design.

Synthesis of human hair fiber-impregnated chitosan beads functionalized with citric acid for the adsorption of lysozyme

Herein, a novel CA-CS/HHF composite was firstly constructed by impregnating human hair fiber into a CS matrix and then functionalized with citric acid.

Protein interactions with layers of TiO2 nanotube and nanopore arrays: Morphology and surface charge influence

In the present work we investigate the key factors involved in the interaction of small-sized charged proteins with TiO₂ nanostructures, i.e. albumin (negatively charged), histone (positively charged). We examine anodic nanotubes with specific morphology (simultaneous control over diameter and length, e.g. diameter - 15, 50 or 100nm, length - 250nm up to 10μm) and nanopores. The nanostructures surface area has a direct influence on the amount of bound protein, nonetheless the protein physical properties as electric charge and size (in relation to nanotopography and biomaterial's electric charge) are crucial too. The highest quantity of adsorbed protein is registered for histone, for 100nm diameter nanotubes (10μm length) while higher values are registered for 15nm diameter nanotubes when normalizing protein adsorption to nanostructures' surface unit area (evaluated from dye desorption measurements) - consistent with theoretical considerations. The proteins presence on the nanostructures is evaluated by XPS and ToF-SIMS; additionally, we qualitatively assess their presence along the nanostructures length by ToF-SIMS depth profiles, with decreasing concentration towards the bottom.

STATEMENT OF SIGNIFICANCE

Surface nanostructuring of titanium biomedical devices with TiO₂ nanotubes was shown to significantly influence the adhesion, proliferation and differentiation of mesenchymal stem cells (and other cells too). A high level of control over the nanoscale topography and over the surface area of such 1D nanostructures enables a direct influence on protein adhesion. Herein, we investigate and show how the nanostructure morphology (nanotube diameter and length) influences the interactions with small-sized charged proteins, using as model proteins bovine serum albumin (negatively charged) and histone (positively charged). We show that the protein charge strongly influences their adhesion to the TiO₂ nanostructures. Protein adhesion is quantified by ELISA measurements and determination of the nanostructures' total surface area. We use a quantitative surface charge model to describe charge interactions and obtain an increased magnitude of the surface charge density at the top edges of the nanotubes. In addition, we track the proteins presence on and inside the nanostructures. We believe that these aspects are crucial for applications where the incorporation of active molecules such as proteins, drugs, growth factors, etc., into nanotubes is desired.

Titania nanotubes for orchestrating osteogenesis at the bone-implant interface

Titanium implants can fail due to inappropriate biomechanics at the bone-implant interface that leads to suboptimal osseointegration. Titania nanotubes (TNTs) fabricated on Ti implants by the electrochemical process have emerged as a promising modification strategy to facilitate osseointegration. TNTs enable augmentation of bone cell functions at the bone-implant interface and can be tailored to incorporate multiple functionalities including the loading of active biomolecules into the nanotubes to target anabolic processes in bone conditions such as osteoporotic fractures. Advanced functions can be introduced, including biopolymers, nanoparticles and electrical stimulation to release growth factors in a desired manner. This review describes the application of TNTs for enhancing osteogenesis at the bone-implant interface, as an alternative approach to systemic delivery of therapeutic agents.

Effect of microgrooved surface topography on osteoblast maturation and protein adsorption

Microgrooved surfaces have been used extensively to influence cell contact guidance. Guiding cell growth, extracellular matrix deposition, and mineralization is important for bone implant longevity. In this study, we investigated the osteoblast response to microgrooved metallic surfaces in serum-supplemented medium. Groove spacing was comparable with the spread osteoblast size. Focal adhesions were observed to confine to the intervening ridge/groove boundaries. Osteoblasts bridged over the grooves and were unable to conform to the concave shape of the underlying grooves. Microgrooved surfaces induced higher osteoblast proliferation and metabolic activity after 14 days in osteogenic medium compared with as-received surfaces, resulting in higher mineralization and alignment of cell-secreted collagen after 28 days. To establish whether preferential cell attachment at the ridge/groove boundaries was influenced by the adhesion proteins contained in the serum-supplemented media, fluorescently labeled fibronectin was adsorbed onto the microgrooved substrates at low concentrations, mimicking the concentrations found in blood serum. Fibronectin was found to selectively adsorb onto the ridge/groove boundaries, the osteoblast focal adhesion sites, suggesting that protein adsorption may have influenced the cell attachment pattern.

Fabrication of different crystallographically oriented TiO2 nanotube arrays used in dye-sensitized solar cells

Different crystallographically oriented TiO₂ NTAs were fabricated via anodizing Ti film in an electrolyte with different water content.

Functionalization of monolithic and porous three-dimensional graphene by one-step chitosan electrodeposition for enzymatic biosensor

Biological modification of monolithic and porous 3D graphene is of great significance for extending its application in fabricating highly sensitive biosensors. The present work reports on the first biofunctionalization of monolithic and freestanding 3D graphene foam for one-step preparation of reagentless enzymatic biosensors by controllable chitosan (CS) electrodeposition technology. Using a homogeneous three-component electrodeposition solution containing a ferrocene (Fc) grafted CS hybrid (Fc-CS), glucose oxidase (GOD), and single-walled carbon nanotubes (SWNTs), a homogeneous biocomposite film of Fc-CS/SWNTs/GOD was immobilized on the surface of 3D graphene foam by one-step electrodeposition. The Fc groups grafted on chitosan can be stably immobilized on the 3D graphene surface and keep their original electrochemical activity. The SWNTs doped into the Fc-CS matrix act as a nanowire to facilitate electron transfer and improve the conductivity of the biocomposite film. Combined with the extraordinary properties of 3D graphene foam including large active surface area, high conductivity, and fast mass transport dynamics, the 3D graphene based enzymatic biosensor achieved a large linear range (5.0 μM to 19.8 mM), a low detection limit (1.2 μM), and rapid response (reaching the 95% steady-state response within 8 s) for reagentless detection of glucose in the phosphate buffer solution.

Facile method to enhance the adhesion of TiO₂ nanotube arrays to Ti substrate

The weak adhesion of anodic TiO2 nanotube arrays (TNTAs) to the underlying Ti substrate compromises many promising applications. In this work, a compact oxide layer between TNTAs and Ti substrate is introduced by employing an additional anodization in a fluoride-free electrolyte. The additional anodization results in an about 200 nm thick compact layer near the nanotube bottoms. Scratch test demonstrates that the critical load of TNTAs with the compact oxide layer is a more than threefold increase in comparison with those without the compact layer. Moreover, this facile method can also improve the photoactivity and supercapacitor performances of TNTAs markedly.

Local co-delivery and release of antimicrobial peptide and RGD using porous TiO2

The co-delivery system with AMP and RGD on porous titanium showed excellent biocompatibility and antimicrobial activity.