Ahalapitiya H. Jayatissa

Nano and micro mechanical properties of uncross-linked and cross-linked chitosan films

The aim of this study is to determine the nano and micro mechanical properties for uncross-linked and cross-linked chitosan films. Specifically, we looked at nanoindentation hardness, microhardness, and elastic modulus. It is important to study the nano and microscale mechanical properties of chitosan since chitosan has been widely used for biomedical applications. Using the solvent-cast method, the chitosan films were prepared at room temperature on the cleaned glass plates. The chitosan solution was prepared by dissolving chitosan in acetic acid 1% (v/v). Tripolyphosphate (TPP) was used to create the cross-links between amine groups in chitosan and phosphate groups in TPP. In this study, atomic force microscopy was used to measure the nanoindentation hardness and surface topography of the uncross-linked and cross-linked chitosan films. Elastic modulus was then calculated from the nanoindentation results. The effective elastic modulus was determined by microhardness with some modifications to previous theories. The microhardness of the chitosan films were measured using Vickers hardness meter under three different loads. Our results show that the microhardness and elastic modulus for cross-linked chitosan films are higher than the uncross-linked films. However, the cross-linked chitosan films show increased brittleness when compared to uncross-linked films. By increasing the load magnitude, the microhardness increases for both uncross-linked and cross-linked chitosan films.

Acceleration of biomimetic mineralization to apply in bone regeneration

The delivery of growth factors and therapeutic drugs into bone defects is a major clinical challenge. Biomimetically prepared bone-like mineral (BLM) containing a carbonated apatite layer can be used to deliver growth factors and drugs in a controlled manner. In the conventional biomimetic process, BLM can be deposited on the biodegradable polymer surfaces by soaking them in simulated body fluid (SBF) for 16 days or more. The aim of this study was to accelerate the biomimetic process of depositing BML in the polymer surfaces. We accelerated the deposition of mineral on 3D poly(lactic-co-glycolic acid) (PLGA) porous scaffolds to 36-48 h by modifying the biomimetic process parameters and applying surface treatments to PLGA scaffolds. The BLM was coated on scaffolds after surface treatments followed by incubation at 37 degrees C in 15 ml of 5x SBF. We characterized the BLM created using the accelerated biomineralization process with wide angle x-ray diffraction (XRD), Fourier transform infrared (FTIR) microscopy, and scanning electron microscopy (SEM). The FTIR and XRD analyses of mineralized scaffolds show similarities between biomimetically prepared BLM, and bone bioapatite and carbonated apatite. We also found that the BLM layer on the surface of scaffolds was stable even after 21 days immersed in Tris buffered saline and cell culture media. This study suggests that BLM was stable for at least 3 weeks in both media, and therefore, BLM has a potential for use as a carrier for biological molecules for localized release applications as well as bone tissue engineering applications.

The effect of graphene substrate on osteoblast cell adhesion and proliferation

Understanding the effect of graphene substrate on graphene-cell interaction is important for considering graphene as a potential candidate for biomedical applications. In this article, biocompatibility of few layers of graphene film transferred to different substrates was evaluated using osteoblasts. The substrates were oxidized silicon wafer (SiO2/Si stack), soda lime glass, and stainless steel. Chemical vapor deposition method was employed to synthesize graphene on copper substrate using methane and hydrogen as precursors. The quality and the thickness of graphene films on different substrates were estimated by Raman spectra, whereas the thickness of graphene film was confirmed by reflectance and transmittance spectroscopy. The study was also focused on cell attachment and morphology at two time points. The results show that graphene does not have any toxic effect on osteoblasts. The cell adhesion improves with graphene coated substrate than the substrate alone. It seems that graphene substrate properties play a dominant role in cell adhesion. The result of this study suggests that a layer of graphene on bone implants will be beneficial for osteoblast attachment and proliferation.

ZnO nanoparticles induced effects on nanomechanical behavior and cell viability of chitosan films

The aim of this paper is to develop novel chitosan-zinc oxide nanocomposite films for biomedical applications. The films were fabricated with 1, 5, 10 and 15% w/w of zinc oxide (ZnO) nanoparticles (NPs) incorporated with chitosan (CS) using a simple method. The prepared nanocomposite films were characterized using atomic force microscopy, Raman and X-ray diffraction studies. In addition, nano and micro mechanical properties were measured. It was found that the microhardness, nanohardness and its corresponding elastic modulus increased with the increase of ZnO NP percentage in the CS films. However, the ductility of films decreased as the percentage of ZnO NPs increased. Cell attachment and cytotoxicity of the prepared films at days two and five were evaluated in vitro using osteoblasts (OBs). It was observed that OB viability decreased in films with higher than 5% ZnO NPs. This result suggests that although ZnO NPs can improve the mechanical properties of pure CS films, only a low percentage of ZnO NPs can be applied for biomedical and bioengineering applications because of the cytotoxicity effects of these particles.

Mechanical and biological properties of chitosan/carbon nanotube nanocomposite films

In this article, different concentrations of multiwalled carbon nanotube (MWCNT) were homogeneously dispersed throughout the chitosan (CS) matrix. A simple solvent-cast method was used to fabricate chitosan films with 0.1, 0.5, and 1% of MWCNT with the average diameter around 30 nm. The CS/MWCNT films were characterized for structural, viscous and mechanical properties with optical microscopy, wide-angle X-ray diffraction, Raman spectroscopy, tensile test machine, and microindentation testing machine. Murine osteoblasts were used to examine the cell viability and attachment of the nanocomposite films at two time points. In comparison to the pure chitosan film, the mechanical properties, including the tensile modulus and strength of the films, were greatly improved by increasing the percentage of MWCNT. Furthermore, adding MWCNT up to 1% increased the viscosity of the chitosan solution by 15%. However, adding MWCNT decreased the samples ductility and transparency. In biological point of view, no toxic effect on osteoblasts was observed in the presence of different percentages of MWCNT at day 3 and day 7. This investigation suggested MWCNT could be a promising candidate for improving chitosan mechanical properties without inducing remarkable cytotoxicity on bone cells.

Cross-linked chitosan improves the mechanical properties of calcium phosphate-chitosan cement

Calcium phosphate (CaP) cements are highly applicable and valuable materials for filling bone defects by minimally invasive procedures. The chitosan (CS) biopolymer is also considered as one of the promising biomaterial candidates in bone tissue engineering. In the present study, some key features of CaP-CS were significantly improved by developing a novel CaP-CS composite. For this purpose, CS was the first cross-linked with tripolyphosphate (TPP) and then mixed with CaP matrix. A group of CaP-CS samples without cross-linking was also prepared. Samples were fabricated and tested based on the known standards. Additionally, the effect of different powder (P) to liquid (L) ratios was also investigated. Both cross-linked and uncross-linked CaP-CS samples showed excellent washout resistance. The most significant effects were observed on Youngs modulus and compressive strength in wet condition as well as surface hardness. In dry conditions, the Youngs modulus of cross-linked samples was slightly improved. Based on the presented results, cross-linking does not have a significant effect on porosity. As expected, by increasing the P/L ratio of a sample, ductility and injectability were decreased. However, in the most cases, mechanical properties were enhanced. The results have shown that cross-linking can improve the mechanical properties of CaP-CS and hence it can be used for bone tissue engineering applications.

Comparison study of graphene based conductive nanocomposites using poly(methyl methacrylate) and polypyrrole as matrix materials

Graphene was used as the filler to mix with two kinds of polymer materials, poly(methyl methacrylate) (PMMA) and polypyrrole (PPy). PMMA is an insulator and PPy is an intrinsic semiconducting/conducting polymer. Graphene/PMMA nanocomposite (GrPMMA) was produced by solution blending and spin-coating deposition, and graphene/PPy nanocomposite (GrPPy) was produced by in situ polymerization and doctor-blade coating. The X-ray diffraction and Raman spectroscopy were used to characterize the structures of GrPMMA and GrPPy. The electrical conductivity was studied as a function of graphene concentration for both GrPMMA and GrPPy. The electrical conductivity of PMMA was improved drastically after adding graphene, and the electrical conductivity of GrPMMA increases as the graphene concentration increases. However, the electrical conductivity of PPy decreases after adding graphene, and the electrical conductivity of GrPPy decreases as the graphene concentration increases. The difference in change in electrical conductivity of PMMA and PPy after addition of graphene may be due to the different electron transport mechanisms of those two nanocomposite materials.

Effect of factors on RFID tag readability-statistical analysis

Radio frequency identification, or RFID, is a generic term for technologies that use radio waves to automatically identify individual items. This paper examines the effect of different factors such as distance to the tag from antenna, height and position of antenna, effect of metals, and interference from other sources on readability of the tag. Experiments were conducted and readings were taken to consider the effect of the factors to identify the critical ones that affects the readability of the tag. Design of Experiments (DOE) was used; specifically 2k factorial design and 2(k−1) fractional factorial designs were considered to determine which factor/s contributes to the variation of readability. In addition, combined effects of factors were also analyzed. SASTM software package was used to validate the data statistically. The two designs were compared and the best out of the two was chosen to develop a model. The developed model can be used to read the intensity of the tag (output) with 99.08% accuracy. Based on the analysis of the data we concluded that half fractional factorial design was the best fit for the experiment.

Effect of UV irradiation on gas sensing behavior of nanocrystalline ZnO thin films

The effect of UV irradiation on ZnO thin film based gas sensor was investigated. Zinc oxide thin films were deposited on an alkali free glass substrate by magnetron sputtering system using zinc target. The UV irradiation of the ZnO thin films was measured to understand the change of microstructure, electrical properties, optical properties and gas sensing characteristics. The X-ray diffraction patterns and SEM images revealed that the films have a nanocrystalline structure. The optical properties of ZnO films were not affected by the UV irradiation significantly. The gas sensing behavior of zinc oxide thin films were enhanced by UV irradiation for a shorter period whereas sensing characteristics were degraded for a longer irradiation period. It was also observed that the dependence of gas sensing characteristics was correlated with the change of electrical properties and crystallinity of films.

Synthesis and characterization of transferable graphene by CVD method

Synthesis of large-area graphene has been realized on the top of copper substrates by chemical vapor deposition (CVD) at ambient pressure with the flow of a mixture of hydrogen and methane gases. The graphene layers were transferred to the oxidized silicon wafers employing a undercutting method. The intense G (1570 cm−1) and 2D (2700 cm−1) bands observed from Raman spectroscopy reveals the presence of graphene on top of oxidized silicon wafer. Reflectance spectroscopy was measured and compared with the theoretical calculation based on Fresnels approach estimating the thickness of the graphene layer to be 0.32 Å. The optical spectra of graphene layers could be reproduced using the complex refractive index of graphene indicating that the optical properties of graphene is not significantly different from that of in-plane graphite. As part of the device fabrication, graphene layers were tested for oxygen sensitivity in hydrogen as a function of gas composition at different temperatures.