Bioactive surface modifications through thermally sprayed hydroxyapatite composite coatings: a review of selective reinforcements.

Hydroxyapatite (HA) has been an excellent replacement for the natural bone in orthopedic applications owing to its close resemblance to the bone properties; however, it is brittle and has low strength. Surface modification techniques have been able to allay such mineral issues by depositing on substrate. These methods, being economical, impart mechanical strength without compromising biocompatibility. In this review article, the discussion is confined to plasma spray (high temperature) and other low temperature surface modification techniques: high-velocity oxy-fuel (HVOF) and cold spray. The processing temperature seems to significantly affect the performance of implants deposited with HA. Monolithic HA may not add enough strength to the bioimplants. Hence, this review discusses selective reinforcements to HA and their roles in enhancing the properties. Herein, a variety of selective reinforcements are discussed, such as carbon allotropes: graphene, carbon nanotubes, and nano diamond; metallic materials: Ag, Sr, Mg, and Ti; ceramic materials: Al2O3, SiO2, ZrO2, and TiO2; multi-materials: Al2O3-CNT/HA, Al2O3-TiO2/HA and others; and functionally graded composites: HA, 20 and 50 wt% Ti-6Al-4V/HA layered coating. Most of these reinforcements could not trade-off between biocompatibility and strength. The detailed in vitro and in vivo studies are still lacking. The literature on the relative effectiveness of these reinforcements is scanty, while the interface between HA coating and reinforcements is seldom explored. This review presents the suitability of thermal spray techniques based on the microstructure, mechanical, and biological properties. Therefore, it is envisaged that the present review can intrigue future researchers to understand the scope of surface coatings in achieving the better performance of implants at clinical trials.

High-Strength, Strongly Bonded Nanocomposite Hydrogels for Cartilage Repair.

Polyacrylamide-based hydrogels are widely used as potential candidates for cartilage replacement. However, their bioapplicability is sternly hampered due to their limited mechanical strength and puncture resistance. In the present work, the strength of polyacrylamide (PAM) hydrogels was increased using titanium oxide (TiO2) and carbon nanotubes (CNTs) separately and a combination of TiO2 with CNTs in a PAM matrix, which was interlinked by the bonding between nanoparticles and polymers with the deployment of density functional theory (DFT) approach. The synergistic effect and strong interfacial bonding of TiO2 and CNT nanoparticles with PAM are attributed to high compressive strength, elastic modulus (>0.43 and 2.340 MPa, respectively), and puncture resistance (estimated using the needle insertion test) for the PAM-TiO2-CNT hydrogel. The PAM-TiO2-CNT composite hydrogel revealed a significant self-healing phenomenon along with a sign toward the bioactivity and cytocompatibility by forming the apatite crystals in simulated body fluid as well as showing a cell viability of ∼99%, respectively. Furthermore, for new insights on interfacial bonding and structural and electronic features involved in the hydrogels, DFT was used. The PAM-TiO2-CNT composite model, constructed by two interfaces (PAM-TiO2 and PAM-CNT), was stabilized by H-bonding and van der Waals-type interactions. Employing the NCI plot, HOMO-LUMO gap, and natural population analysis tools, the PAM-TiO2-CNT composite has been found to be most stable. Therefore, the prepared polyacrylamide hydrogels in combination with the TiO2 and CNT can be a remarkable nanocomposite hydrogel for cartilage repair applications.

High-entropy alloys for water oxidation: a new class of electrocatalysts to look out for.

High-entropy alloys (HEAs) with five or more elements can provide near-continuous adsorption energies and can be optimized for superior persistent catalytic activity. This report presents electrochemical water oxidation facilitated by employing graphene and FeCoNiCuCr HEA nanoparticle based composites prepared via the mechanical milling of graphite-metal powders. The composite efficiently facilitates water oxidation with a low overpotential of 330 mV at 10 mA cm-2, and high specific and mass activities (∼143 mA cm-2 and 380 mA mg-1, respectively, at 1.75 V). Importantly, the composites exhibit excellent accelerated cycling stability with ∼99% current retention (after 3250 cycles). The HEA-based composites are anticipated to replace noble/precious metal based traditional electrocatalysts in the future, the use of which is a major obstacle in the technological scalability of electrochemical energy conversion and storage devices.

A review on hydroxyapatite coatings for the biomedical applications: experimental and theoretical perspectives.

The production of hydroxyapatite (HAP) composite coatings has continuously been investigated for bone tissue applications during the last few decades due to their significant bioactivity and osteoconductivity. Herein, we highlight the recent experimental and theoretical progresses on HAP coatings, which may bridge the existing gap between theory and practice. The experimental studies mainly deal with electrochemical (EC) and electrophoretic (EP) deposition for the synthesis of nano-HAP in the form of coatings. Additionally, the biocompatible coating method for the fabrication of HAP composite coatings, the plasma spraying (PS) technique, and its mechanism are discussed in this review. Furthermore, the adhesion strength, mechanical, tribological and electrochemical phenomena of HAP composite coatings are critically analyzed. Their ameliorated bactericidal activity is also discussed to recognize the possibility of substituted HAP coatings from a clinical perspective. In addition, computational studies on the HAP system are explored in this report, including the first-principles density functional theory, ab initio modeling and molecular dynamics simulations. Thus, the main significance of this review is to present a collective discussion on the structural features, interfacial mechanics and binding aspects by experimental and theoretical investigations for HAP-based biomaterials, which can provide clear insights for the future research related to orthopedic applications.

Influence of Oxidation Degree of Graphene Oxide on Its Nuclear Relaxivity and Contrast in MRI.

Graphene oxide (GO) serves as a versatile platform for various applications, with the oxygen content of GO playing an important role in governing its properties. In the present study, different GO types covering a wide range of oxidation degree were prepared using our newly developed two-step method involving ball milling of graphite followed by its oxidation to GO. In addition to the variations in their physicochemical properties, the different GO types exhibited differences in proton relaxivity due to their paramagnetic nature. Nuclear magnetic resonance spectroscopy studies showed that the degree of oxidation of GO perturbs its nuclear relaxation properties and, together with intercalated Mn2+ ions, provides large contrast variation in magnetic resonance imaging (MRI). The study for the first time reveals that the surface chemistry of GO affects its relaxivity and opens up new avenues for developing tunable GO-based contrast agents in magnetic resonance imaging for diagnostics and therapies.

Inner Sphere Electron Transfer Promotion on Homogeneously Dispersed Fe-Nx Centers for Energy-Efficient Oxygen Reduction Reaction.

The study reports the optimized incorporation of pyridinic nitrogen in nitrogen-doped carbon nanotubes (CNTs) to realize effective Fe-Nx centers throughout the framework. The study unveils nitrogen as a valuable asset to promote the homogeneous dispersion of Fe moieties throughout the CNT framework, which is a necessary component to institute uniform Fe-Nx centers. In addition, pyridinic nitrogen causes disruption in strongly delocalized π-electrons, which impart electron-withdrawing nature in the carbon matrix, resulting in an anodic shift in oxygen reduction reaction (ORR) onset potential (Eonset). The direct interaction of Fe-Nx with O2, as evidenced by poisoning and computational studies, ensures the preferential inner sphere electron transfer mechanism. Despite the alkaline medium, the outer sphere electron transfer mechanism was muted, with suppressed HO2- generation, preferential 4e- reduction pathways, and excellent cyclic stability. The study indicates the dependency of ORR half-wave potential on the electron transfer mechanism. The poisoning study unveils the direct involvement of Fe-Nx electroactive centers in facilitating ORR in alkaline medium. It further indicates a noticable increase (up to ∼25%) in peroxide generation-an unwanted ORR intermediate-and concomitant reduction in average electron transfer no. per oxygen molecule.

Single Atom Nucleation of Cobalt over Graphene Oxide: Theory and Experimental Study.

Nucleation and growth mechanism of Co electrodeposition over graphene oxide (GO) was studied using cyclic voltammetry (CV) and chronoamperometry techniques. The studies were performed over an aqueous electrolyte containing 10 mM CoSO4·7H2O maintained at an initial pH of 2.5. CV studies established that the deposition mechanism was diffusion-controlled and irreversible. Chronoamperometry studies revealed the presence of three concurrent processes: an initial adsorption current, which indicated adatom layer formation, 3D nucleation and growth of Co islands over GO, and hydrogen evolution over the deposited Co nanoclusters. It was observed that the nucleation rate increased with increasing the overpotential (η) for deposition (from 2.71 × 104 cm-2 s-1 at η = 0.35 V to 3.62 × 106 cm-2 s-1 at η = 0.90 V). Application of the classical theory of nucleation over the chronoamperometry results suggested that the free energy of formation of the critical nucleus was lower than room temperature thermal energy. This indicated that the nucleation and growth process was not activation-controlled but rather a kinetically controlled process. Application of Milchev's atomistic theory revealed that every single atom of Co deposited over the GO sheet was a supercritical nucleus that could grow into a cluster irreversibly.

Modulation of protein-graphene oxide interactions with varying degrees of oxidation.

The degree of oxidation of graphene oxide (GO) has been shown to be important for its toxicity and drug-loading efficiency. However, the effect of its variations on GO-protein interaction remains unclear. Here, we evaluate the effect of the different oxidation degrees of GO on its interaction with human ubiquitin (8.6 kDa) using solution state nuclear magnetic resonance (NMR) spectroscopy in combination with other biophysical techniques. Our findings show that the interaction between the protein and the different GO samples is weak and electrostatic in nature. It involves fast dynamic exchange of the protein molecules from the surface of the GO. As the oxidation degree of the GO increases, the extent of the interaction with the protein changes. The interaction of the protein with GO can thus be modulated by tuning the degree of oxidation. This study opens up new avenues to design appropriate graphenic materials for use in various biomedical fields such as drug delivery, biomedical devices and imaging.

Synthesis of Graphene Oxide-Fe3O4 Based Nanocomposites Using the Mechanochemical Method and in Vitro Magnetic Hyperthermia.

The study presented in this work consists of two parts: The first part is the synthesis of Graphene oxide-Fe3O4 nanocomposites by a mechanochemical method which, is a mechanical process that is likely to yield extremely heterogeneous particles. The second part includes a study on the efficacy of these Graphene oxide-Fe3O4 nanocomposites to kill cancerous cells. Iron powder, ball milled along with graphene oxide in a toluene medium, underwent a controlled oxidation process. Different phases of GO-Fe3O4 nanocomposites were obtained based on the composition used for milling. As synthesized nanocomposites were characterized by x-ray diffraction (XRD), alternating magnetic field (AFM), Raman spectroscopy, and a vibrating sample magnetometer (VSM). Additionally, the magnetic properties required to obtain high SAR values (Specific Absorption Rate-Power absorbed per unit mass of the magnetic nanocomposite in the presence of an applied magnetic field) for the composite were optimized by varying the milling time. Nanocomposites milled for different extents of time have shown differential behavior for magneto thermic heating. The magnetic composites synthesized by the ball milled method were able to retain the functional groups of graphene oxide. The efficacy of the magnetic nanocomposites for killing of cancerous cells is studied in vitro using HeLa cells in the presence of an AC (Alternating Current) magnetic field. The morphology of the HeLa cells subjected to 10 min of AC magnetic field changed considerably, indicating the death of the cells.

Correlation between defect density in mechanically milled graphite and total oxygen content of graphene oxide produced from oxidizing the milled graphite.

It has been reported that defect density in ball-milled graphite lattice increases with the milling time. Guided by this, we hypothesized that the oxygen content of graphene oxide can be substantially enhanced by oxidizing ball-milled graphite and also, the oxygen content would monotonically increase with the milling time as the defect sites would be preferred sites for oxidation. Interestingly, we observed that this correlation was not directly proportional for all milling hours. Even though, the defect density of graphite monotonically increased with milling time, the oxygen content of graphene oxide initially increased and then decreased. This was due to milling time dependent change in the size of the graphite plates and consequent relative abundance of the different oxygen containing functional groups on graphene oxide (GO) produced from the milled graphite.

First Report on High Entropy Alloy Nanoparticle Decorated Graphene.

This is the first report on synthesis of multimetal high entropy alloy (HEA) nanoparticle-few layer graphene composite. A two-step methodology for synthesizing multi-component HEA nanoparticle-graphene composite is provided. In the first step, high purity graphite powder was mechanically milled with metal powders (Ni, Cr, Co, Cu, Fe) to produce multimetal-graphite composite. This composite was then sonicated with sodium lauryl sulphate (SLS) for 2 hours to produce a dispersion of graphene decorated with multi-component nanoparticles with face centred cubic structure. Potentiodynamic polarization and electrochemical impedance spectroscopy methods revealed that the HEA nanoparticle graphene composite possess excellent corrosion resistance properties which was better than the corrosion resistance exhibited by milled and exfoliated graphene. The HEA nanoparticle-graphene composite can be used for corrosion resistant coating applications.

Phase formation and stability of Ag-60 at%Cu alloy nanoparticles synthesized by chemical routes in aqueous media.

The present work reports the nature of the evolution of an array of nanoparticles during the synthesis of alloy nanoparticles of Ag-60 at%Cu by the co-reduction of metal salt precursors using NaBH4 in an aqueous medium. This was achieved by studying samples extracted at different intervals of time from the reaction bath. The microstructural characterization reveals that at the initial stage of synthesis, a single-phase solid solution of alloy nanoparticles of very small sizes was formed; however, as the reaction time increases, a network of chains of nanoparticles evolves containing particles rich in either Ag or Cu. Keeping the particles in the reaction bath for a longer time, the chemistry of the network changes further with the chain containing an Ag-rich core and Cu2O as the shell. In the present study, we tried to rationalize the evolution of the phases from the observed results.

A novel algorithm for reducing false arrhythmia alarms in intensive care units.

Alarm fatigue in intensive care units (ICU) is one of the top healthcare issues in the US. False alarms in ICU will decrease the quality of care and staff response time over the alarms. Normally, false alarm will cause desensitization of the clinical staff which leads to warnings and misleading, if the triggered alarm is true. In this study, we have proposed a multi-model ensemble approach to reduce the false alarm rate in monitoring systems. We have used 750 patient records from PhysioNet database. At First arrhythmia based features from electrocardiogram (ECG), arterial blood pressure (ABP) and photoplethysmogram (PPG) features were extracted from the records. Next, the dataset has been separated into two subsets on the basis of available features information. The first dataset (DS1) is the combination of ECG physiological, ABP and PPG features. Their correlation coefficient and p-values criteria have been applied for relevant alarm-wise feature-set selection, and random forest classifier was used for model development and validation. The threshold based approach was used on second dataset (DS2) which is the combination of arrhythmia, ABP and PPG features. The developed ensemble model is able to achieve sensitivity 83.33-100 % (average 95.56 %) being true alarms and suppress false alarms rate 66.67-89% (average 77.25%). The predictability of classifier shows the advantage to deal with unbalanced set of information, therefore overall model performance has reached to 83.96% accuracy.

Nonequilibrium microstructures for Ag-Ni nanowires.

This work illustrates that a variety of nanowire microstructures can be obtained either by controlling the nanowire formation kinetics or by suitable thermal processing of as-deposited nanowires with nonequilibrium metastable microstructure. In the present work, 200-nm diameter Ag-Ni nanowires with similar compositions, but with significantly different microstructures, were electrodeposited. A 15 mA deposition current produced nanowires in which Ag-rich crystalline nanoparticles were embedded in a Ni-rich amorphous matrix. A 3 mA deposition current produced nanowires in which an Ag-rich crystalline phase formed a backbone-like configuration in the axial region of the nanowire, whereas the peripheral region contained Ni-rich nanocrystalline and amorphous phases. Isothermal annealing of the nanowires illustrated a phase evolution pathway that was extremely sensitive to the initial nanowire microstructure.

Poly(vinylidene fluoride)-based flexible and lightweight materials for attenuating microwave radiations.

Two unique materials were developed, like graphene oxide (GO) sheets covalently grafted on to barium titanate (BT) nanoparticles and cobalt nanowires (Co-NWs), to attenuate the electromagnetic (EM) radiations in poly(vinylidene fluoride) (PVDF)-based composites. The rationale behind using either a ferroelectric or a ferromagnetic material in combination with intrinsically conducting nanoparticles (multiwall carbon nanotubes, CNTs), is to induce both electrical and magnetic dipoles in the system. Two key properties, namely, enhanced dielectric constant and magnetic permeability, were determined. PVDF/BT-GO composites exhibited higher dielectric constant compared to PVDF/BT and PVDF/GO composites. Co-NWs, which were synthesized by electrodeposition, exhibited saturation magnetization (Ms) of 40 emu/g and coercivity (Hc) of 300 G. Three phase hybrid composites were prepared by mixing CNTs with either BT-GO or Co-NWs in PVDF by solution blending. These nanoparticles showed high electrical conductivity and significant attenuation of EM radiations both in the X-band and in the Ku-band frequency. In addition, BT-GO/CNT and Co-NWs/CNT particles also enhanced the thermal conductivity of PVDF by ca. 8.7- and 9.3-fold in striking contrast to neat PVDF. This study open new avenues to design flexible and lightweight electromagnetic interference shielding materials by careful selection of functional nanoparticles.

Synergetic effect of size and morphology of cobalt ferrite nanoparticles on proton relaxivity.

Cobalt ferrite nanoparticles with average sizes of 14, 9 and 6 nm were synthesised by the chemical co-precipitation technique. Average particle sizes were varied by changing the chitosan surfactant to precursor molar ratio in the reaction mixture. Transmission electron microscopy images revealed a faceted and irregular morphology for the as-synthesised nanoparticles. Magnetic measurements revealed a ferromagnetic nature for the 14 and 9 nm particles and a superparamagnetic nature for the 6 nm particles. An increase in saturation magnetisation with increasing particle size was noted. Relaxivity measurements were carried out to determine T2 value as a function of particle size using nuclear magnetic resonance measurements. The relaxivity coefficient increased with decrease in particle size and decrease in the saturation magnetisation value. The observed trend in the change of relaxivity value with particle size was attributed to the faceted nature of as-synthesised nanoparticles. Faceted morphology results in the creation of high gradient of magnetic field in the regions adjacent to the facet edges increasing the relaxivity value. The effect of edges in increasing the relaxivity value increases with decrease in the particle size because of an increase in the total number of edges per particle dispersion.

Effect of reflux time on nanoparticle shape.

In the present work, Pt nanoparticles were produced from a reaction mixture containing a trace amount of cobalt carbonyl salt acting as a shape inducer. Nanoparticle shape evolution during reaction mixture reflux was monitored by characterizing particles extracted from the reaction mixture at different times. It was observed that 5 min of reflux produced spherical nanoparticles, 30 min of reflux produced cube shaped nanoparticles, and 60 min of reflux produced truncated octahedron morphology nanoparticles. It is illustrated that during nanoparticle synthesis the reflux process can provide energy needed for shape transformation from a metastable cube morphology to a truncated octahedron morphology which is thermodynamically the most stable geometry for fcc crystals. An optimization of the reaction reflux is thus needed for isolating metastable shapes.