Protein adsorption on titanium dioxide: Effects on double layer and semiconductor space charge region studied by EIS

Semiconducting electrodes for neural interfacing: a review

In the past 50 years, the advent of electronic technology to directly interface with neural tissue has transformed the fields of medicine and biology. Devices that restore or even replace impaired bodily functions, such as deep brain stimulators and cochlear implants, have ushered in a new treatment era for previously intractable conditions. Meanwhile, electrodes for recording and stimulating neural activity have allowed researchers to unravel the vast complexities of the human nervous system. Recent advances in semiconducting materials have allowed effective interfaces between electrodes and neuronal tissue through novel devices and structures. Often these are unattainable using conventional metallic electrodes. These have translated into advances in research and treatment. The development of semiconducting materials opens new avenues in neural interfacing. This review considers this emerging class of electrodes and how it can facilitate electrical, optical, and chemical sensing and modulation with high spatial and temporal precision. Semiconducting electrodes have advanced electrically based neural interfacing technologies owing to their unique electrochemical and photo-electrochemical attributes. Key operation modalities, namely sensing and stimulation in electrical, biochemical, and optical domains, are discussed, highlighting their contrast to metallic electrodes from the application and characterization perspective.

Titanium-protein nanocomposites as new biomaterials produced by high-pressure torsion

The development of new biomaterials with outstanding mechanical properties and high biocompatibility has been a significant challenge in the last decades. Nanocrystalline metals have provided new opportunities in producing high-strength biomaterials, but the biocompatibility of these nanometals needs to be improved. In this study, we introduce metal-protein nanocomposites as high-strength biomaterials with superior biocompatibility. Small proportions of bovine serum albumin (2 and 5 vol%), an abundant protein in the mammalian body, are added to titanium, and two nanocomposites are synthesized using a severe plastic deformation process of high-pressure torsion. These new biomaterials show not only a high hardness similar to nanocrystalline pure titanium but also exhibit better biocompatibility (including cellular metabolic activity, cell cycle parameters and DNA fragmentation profile) compared to nano-titanium. These results introduce a pathway to design new biocompatible composites by employing compounds from the human body.

Effect of Cationic/Anionic Diffusion Dominated Passive Film Growth on Tribocorrosion

Tribocorrosion behaviours of nickel(Ni) and (niobium)Nb in sodium sulfate(Na2SO4) solution under potentiodynamic and potentiostatic conditions were studied. Under the potentiodynamic condition, the passivation was early broken, accompanied by a sharp increase in frictional coefficient on Nb. The current was more fluctuant, and larger material loss appeared at the higher potential in the potentiostatic condition. However, these phenomena did not occur for Ni, and it even showed lower material loss at the higher potential in the potentiostatic tribocorrosion test. The differences in tribocorrosion behaviour had a close relationship to the passive film growth mechanism, which decided the passive film/metal interface structure. Nb with anionic diffusion dominated mechanism in passive growth would cause the accumulation of oxygen vacancies at the passive film/metal interface. This may weaken the adhesion between the metal and the passive film. However, with the cationic diffusion dominated passive film growth on Ni, cation vacancies concentrated at the passive film/tribo-film interface, and this did not affect the adhesion between metal and passive film. Ni or other passive elements with the cationic diffusion-dominated mechanism in passive film growth were recommended as the alloying element for improving the tribocorrosion resistance of alloys.

Electronic properties and surface potential evaluations at the protein nano-biofilm/oxide interface: Impact on corrosion and biodegradation

The formation of a protein nano-biofilm, which exhibits a special electronic behavior, on the surface of metals or oxide biomaterials considerably influences the crucial subsequent interactions, particularly the corrosion and biodegradation processes. This study discusses the impact of electrical surface potential (ESP) of a single or nano-biofilm of albumin protein on the electrochemical interactions and electronic property evolutions (e.g., charge carriers, space charge capacitance (SCC), and band bending) occurring on the surface oxide of CoCrMo implants. Scanning Kelvin probe force microscopy (SKPFM) results indicated that ESP or surface charge distribution on a single or nano-biofilm of the albumin protein is lower than that of a CoCrMo complex oxide layer, which hinders the charge transfer at the protein/electrolyte interface. Using a complementary approach, which involved performing Mott-Schottky analysis at the electrolyte/protein/oxide interface, it was revealed that the albumin protein significantly increases the SCC magnitude and number of n-type charge carrier owing to increased band bending at the SCC/protein interface; this facilitated the acceleration of metal ion release and metal-protein complex formation. The nanoscale SKPFM and electrochemical analyses performed in this study provide a better understanding of the role of protein molecules in corrosion/biodegradation of metallic biomaterials at the protein nano-biofilm/oxide interface.

Titanium and Protein Adsorption: An Overview of Mechanisms and Effects of Surface Features

Titanium and its alloys, specially Ti6Al4V, are among the most employed materials in orthopedic and dental implants. Cells response and osseointegration of implant devices are strongly dependent on the body-biomaterial interface zone. This interface is mainly defined by proteins: They adsorb immediately after implantation from blood and biological fluids, forming a layer on implant surfaces. Therefore, it is of utmost importance to understand which features of biomaterials surfaces influence formation of the protein layer and how to guide it. In this paper, relevant literature of the last 15 years about protein adsorption on titanium-based materials is reviewed. How the surface characteristics affect protein adsorption is investigated, aiming to provide an as comprehensive a picture as possible of adsorption mechanisms and type of chemical bonding with the surface, as well as of the characterization techniques effectively applied to model and real implant surfaces. Surface free energy, charge, microroughness, and hydroxylation degree have been found to be the main surface parameters to affect the amount of adsorbed proteins. On the other hand, the conformation of adsorbed proteins is mainly dictated by the protein structure, surface topography at the nano-scale, and exposed functional groups. Protein adsorption on titanium surfaces still needs further clarification, in particular concerning adsorption from complex protein solutions. In addition, characterization techniques to investigate and compare the different aspects of protein adsorption on different surfaces (in terms of roughness and chemistry) shall be developed.

A semi-GCMC simulation study of electrolytic capacitors with adsorbed titrating peptides

We use semi-grand canonical Monte Carlo simulations to study an electrolytic capacitor with an adsorbed peptide on the electrode surfaces. Only homogeneous peptides are considered, consisting of only a single residue type. We find that the classical double-hump camel-shaped differential capacitance in such systems is augmented by the addition of a third peak, due to the capacitance contribution of the peptide, essentially superimposed on the salt contribution. This mechanistic picture is justified using a simple mean-field analysis. We find that the position of this third peak can be tuned to various surface potential values by adjusting the ambient pH of the electrolyte solution. We investigate the effect of changing the residue type and the concentration of the adsorbed peptide and of the supporting electrolyte. Varying the residue species and pH allows one to modify the capacitance profile as a function of surface potential, facilitating the design of varying discharging patterns for the capacitor.

Conduction Band of Hematite Can Mediate Cytochrome Reduction by Fe(II) under Dark and Anoxic Conditions

While it was recently reported that the conduction band of iron minerals can mediate electron transfer between Fe(II) and different Fe(III) lattice sites during Fe(II)-catalyzed mineral transformation, it is unclear whether such a conduction band mediation pathway occurs in the microbial Fe(II) oxidation system under dark and anoxic subsurface conditions. Here, using cytochrome c (c-Cyts) as a model protein of microbial Fe(II) oxidation, the in vitro kinetics and thermodynamics of c-Cyts reduction by Fe(II) were studied. The results showed that the rates of c-Cyts reduction were greatly enhanced in the presence of the semiconducting mineral hematite (Hem, α-Fe₂O₃). The electrochemical experiments separating Fe(II) and c-Cyts demonstrated that electrons from Fe(II) to the electrode or from the electrode to c-Cyts could directly penetrate hematite, resulting in enhanced current. Independent photochemical and photoluminescence experiments indicated that c-Cyts could be directly reduced by the conduction band electrons of hematite which were generated under light illumination. In c-Cyts+Fe(II)+Hem, the redox potential of Fe(II)-Hem was shifted from -0.15 to -0.18 V and that of c-Cyts+Hem changed slightly from -0.05 to -0.04 V. For the bulk hematite, Mott-Schottky plots illustrated that the flat band was shifted negatively and positively in the presence of Fe(II) and oxidized c-Cyts, respectively, and the surface electron/charge density was higher in the presence of Fe(II)/c-Cyts. As a consequence, the redox gradients from adsorbed Fe(II) to adsorbed c-Cyts allow electron transfer across the conduction band of hematite and facilitate c-Cyts reduction. This mechanistic study on conduction band-mediating electron transfer could help interpret the role of semiconducting minerals in the microbial Fe(II) oxidation process under dark anoxic conditions.

The Influence of Microstructure on the Passive Layer Chemistry and Corrosion Resistance for Some Titanium-Based Alloys

The effect of microstructure and chemistry on the kinetics of passive layer growth and passivity breakdown of some Ti-based alloys, namely Ti-6Al-4V, Ti-6Al-7Nb and TC21 alloys, was studied. The rate of pitting corrosion was evaluated using cyclic polarization measurements. Chronoamperometry was applied to assess the passive layer growth kinetics and breakdown. Microstructure influence on the uniform corrosion rate of these alloys was also investigated employing dynamic electrochemical impedance spectroscopy (DEIS). Corrosion studies were performed in 0.9% NaCl solution at 37 °C, and the obtained results were compared with ultrapure Ti (99.99%). The different phases of the microstructure were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Chemical composition and chemistry of the corroded surfaces were studied using X-ray photoelectron spectroscopy (XPS) analysis. For all studied alloys, the microstructure consisted of α matrix, which was strengthened by β phase. The highest and the lowest values of the β phase's volume fraction were recorded for TC21 and Ti-Al-Nb alloys, respectively. The susceptibility of the investigated alloys toward pitting corrosion was enhanced following the sequence: Ti-6Al-7Nb < Ti-6Al-4V << TC21. Ti-6Al-7Nb alloy recorded the lowest pitting corrosion resistance (Rpit) among studied alloys, approaching that of pure Ti. The obvious changes in the microstructure of these alloys, together with XPS findings, were adopted to interpret the pronounced variation in the corrosion behavior of these materials.

Characterization of double layer alterations induced by charged particles and protein-membrane interactions using contactless impedance spectroscopy

Double layer interactions between charged particles and surfaces play a vital role in a variety of technical and biological systems because they determine the stability of, e.g., protein-membrane biointerfaces. The underlying theoretical principle is based on the overlap of two different double layers that induce surface charges to be shifted to a new equilibrium distribution, which can be approximated by the Poisson-Boltzmann equation. In the present work we show theoretical and experimental results involving double layer capacitance of surfaces that exhibit charge regulation behavior. Charge regulation is an important parameter to consider when investigating protein-membrane interactions because it defines surface properties between ideal constant charge and constant potential behavior. In this work we introduce a novel theoretical model that also includes charge regulation behavior and can assess changes of double layer disruptions at TiO(2) and supported lipid-bilayers (SLB). The selected surfaces represent important biointerfaces that can be found on implants or cell membranes. We also demonstrate that contactless impedance spectroscopy is well suited to measure double layer capacitance interactions using differently charged silica beads. The combination of a theoretical model with experimental data allowed us further to identify charge regulation effects during protein adsorption (BSA and Annexin V) events at supported lipid-bilayers (SLB) used as a simple cell membrane model. Finally, the first indications of changed charge regulation behavior during protein surface crystallization events were also documented.