Prashant N. Kumta

In vitro analysis of biodegradable polymer blend/hydroxyapatite composites for bone tissue engineering

Blends of biodegradable polymers, poly(caprolactone) and poly(D, L-lactic-co-glycolic acid), have been examined as scaffolds for applications in bone tissue engineering. Hydroxyapatite granules have been incorporated into the blends and porous discs were prepared. Mechanical properties and degradation rates in vitro of the composites were determined. The discs were seeded with rabbit bone marrow or cultured bone marrow stromal cells and incubated under physiological conditions. Polymer/ceramic scaffolds supported cell growth throughout the scaffold for 8 weeks. Scanning and transmission electron microscopy, and histological analyses were used to characterize the seeded composites. This study suggests the feasibility of using novel polymer/ceramic composites as scaffold in bone tissue engineering applications.

Si / TiN Nanocomposites Novel Anode Materials for Li ‐ Ion Batteries

Nanocomposites containing silicon and titanium nitride were synthesized by high‐energy mechanical milling. The process results in very fine particles distributed homogeneously inside the matrix. nanocomposites synthesized using different experimental conditions were evaluated for their electrochemical properties. Results indicate that in the composite alloys and dealloys with lithium during cycling, while remains inactive providing structural stability. A composite containing obtained after milling exhibited a stable capacity of , suggesting its promising nature. Preliminary cycling data show good capacity retention reflective of good phase and microstructural stability as verified by X‐ray diffraction and scanning electron microscopy analyses. ©2000 The Electrochemical Society

Interfacial Properties of the a-Si ∕ Cu :Active–Inactive Thin-Film Anode System for Lithium-Ion Batteries

Amorphous silicon thin films deposited on copper foil have been observed to exhibit near theoretical capacity for a limited number of cycles. The films, however, eventually delaminate, leading to failure of the anode. In order to better understand the mechanism of capacity retention and the ultimate failure mode of a model brittle active:elastic/plastic inactive anode system, the films were subjected to in situ adhesion tests while observing the film surface using scanning electron microscopy. Atomic force and transmission electron microscopy, and electrochemical cycling were conducted to analyze the emerging morphology of the films during cycling. The adhesion of the as-deposited Si film to the Cu substrate was measured to ∼7.7 J/m 2 , reflecting a weak interface adhesion strength. Plastic deformation of the underlying Cu substrate combined with a ratcheting mechanism is proposed to occur in the Si:Cu system, with delamination failure mode occurring only after the formation of an interface imperfection. From the analysis of slow rate cycling experiments, nucleation of a lithium compound based on the interdiffusion of Si and Cu is identified as the most probable cause of the ultimate delamination failure of the deposited film.

Nanostructured Si / TiB2 Composite Anodes for Li-Ion Batteries

Silicon and titanium boride nanocomposites were synthesized using high-energy mechanical milling. The nanocomposites obtained after mechanical milling consist of amorphous silicon and nanocrystalline titanium boride. The nanocomposite containing 40 mol % silicon obtained after milling for 20 h exhibits a stable capacity of ∼400 mAh/g. X-ray diffraction and scanning electron microscopy analyses indicated that the nanocomposite retains its initial phase and microstructure during electrochemical cycling. Premilling of the inactive TiB 2 component appears to increase the stability of the capacity of the nanocomposite electrodes due to a probable homogeneous distribution of the induced stress during cycling.

Rare-earth chalcogenides : an emerging class of optical materials

Sulphide compounds belong to the family of chalcogenides and are well known for their optical and electronic properties. They possess good optical properties because of their ability to transmit into the infrared (IR) region. Several sulphide glasses are known to exist which exhibit far infrared transmission and are also useful semiconductors. In recent years, there has been an increasing interest in IR materials to be used on surveillance equipment. This led to the identification of several new crystalline sulphide materials which can transmit very far into the IR region (up to a wavelength of 14 Μm). Crystalline and amorphous rare-earth sulphides emerged as a new class of materials, which possess several unique optical and electronic properties. This paper reviews the status of these rare-earth sulphide amorphous and polycrystalline materials, the techniques used to process these materials and discusses their structure, thermal, mechanical and optical properties. Conventional and emergent novel chemical processing techniques that are used for synthesizing these materials are reviewed in detail. The use of metallorganic precursors and the modification of their chemistry to tailor the composition of the final ceramic are illustrated. The potential of these chemical techniques and their advantages over the conventional solid state techniques used for processing sulphide ceramics is discussed, particularly in light of their successful applications in processing novel electronic and optical oxide ceramics.

Nanocrystalline TiN Derived by a Two-Step Halide Approach for Electrochemical Capacitors

Titanium nitride (TiN) nanocrystallites exhibiting high specific surface area (∼128 m 2 /g) for electrochemical capacitor application were obtained by a two-step halide approach comprised of a room-temperature TiCl 4 (1)-NH 3 (g) reaction followed by heat-treatment under NH 3 atmosphere. The synthesized nitride powders were agglomerates containing spherical crystallites ∼ 10 nm in diam and exhibit a specific surface area ranging from 128 to 23 m 2 /g depending on the heat-treatment temperature. The specific capacitance evaluated by cyclic voltammetry in 1 M KOH electrolyte ranged from 238 to 24 F/g depending on the heat-treatment temperature and the scan rates employed. Structural characterization was performed using X-ray diffraction, helium pycnometry, N 2 adsorption for specific surface area measurements using the Brunauer-Emmett-Teller method, Fourier transform infrared spectroscopy, and high-resolution transmission electron microscopy. Results of these studies are presented and discussed.

Characterization of osteoblast-like behavior of cultured bone marrow stromal cells on various polymer surfaces

The creation of novel bone substitutes requires a detailed understanding of the interaction between cells and materials. This study was designed to test certain polymers, specifically poly(caprolactone) (PCL), poly(D,L-lactic-CO-glycolic acid) (PLGA), and combinations of these polymers for their ability to support bone marrow stromal cell proliferation and differentiation. Bone marrow stromal cells were cultured from New Zealand White rabbits and were seeded onto glass slides coated with a thin layer of PCL, PLGA, and combinations of these two polymers in both a 40:60 and a 10:90 ratio. Growth curves were compared. At the end of 2 weeks, the cells were stained for both matrix mineralization and alkaline phosphatase activity. There was no statistically significant difference in growth rate of the cells on any polymer or polymer combination. However, there was a striking difference in Von Kossa staining and alkaline phosphatase staining. Cells on PCL did not show Von Kossa staining or alkaline phosphatase staining. However, in the 40:60 and 10:90 blends, there was both positive Von Kossa and alkaline phosphatase staining. These data indicate that PCL alone may not be a satisfactory material for the creation of a bone substitute. However, it may be used in combination with PLGA for the creation of a bone substitute material.

In vivo study of magnesium plate and screw degradation and bone fracture healing.

Each year, millions of Americans suffer bone fractures, often requiring internal fixation. Current devices, like plates and screws, are made with permanent metals or resorbable polymers. Permanent metals provide strength and biocompatibility, but cause long-term complications and may require removal. Resorbable polymers reduce long-term complications, but are unsuitable for many load-bearing applications. To mitigate complications, degradable magnesium (Mg) alloys are being developed for craniofacial and orthopedic applications. Their combination of strength and degradation make them ideal for bone fixation. Previously, we conducted a pilot study comparing Mg and titanium devices with a rabbit ulna fracture model. We observed Mg device degradation, with uninhibited healing. Interestingly, we observed bone formation around degrading Mg, but not titanium, devices. These results highlighted the potential for these fixation devices. To better assess their efficacy, we conducted a more thorough study assessing 99.9% Mg devices in a similar rabbit ulna fracture model. Device degradation, fracture healing, and bone formation were evaluated using microcomputed tomography, histology and biomechanical tests. We observed device degradation throughout, and calculated a corrosion rate of 0.40±0.04mm/year after 8 weeks. In addition, we observed fracture healing by 8 weeks, and maturation after 16 weeks. In accordance with our pilot study, we observed bone formation surrounding Mg devices, with complete overgrowth by 16 weeks. Bend tests revealed no difference in flexural load of healed ulnae with Mg devices compared to intact ulnae. These data suggest that Mg devices provide stabilization to facilitate healing, while degrading and stimulating new bone formation.

Sn/C Composite Anodes for Li-Ion Batteries

Composites of Sn/C were synthesized by infiltrating tetraethyltin into mechanically milled polystyrene resin powder. Heat-treatment of the precursor in Ar atmosphere at 600°C results in crystalline tin and amorphous carbon. The resultant composite exhibits a capacity of 480 mAh/g with good capacity retention (0.15% loss/cycle). Transmission electron microscopy analysis indicates nanocrystalline (∼200 nm) spherical tin particles embedded in large carbon particles (=2 to 10 μm). The embedment of nanocrystalline tin within carbon appears to be the main reason for the good cyclability suggesting the excellent potential of these Sn/C based composites for use as alternative anodes for lithium-ion batteries.

Synthesis and electrochemical characterization of LiMO2 (M=Ni, Ni0.75Co0.25) for rechargeable lithium ion batteries

Abstract A particulate sol-gel (PSG) process has been developed for synthesizing LiNiO2 and LiNixCo1−xO2 (x=0.75) powders, utilizing the reaction of lithium hydroxide with nickel acetate in a water–ethanol solvent system. Thermal analyses of the as-prepared xerogels show a one step weight loss with a single exothermic peak indicating its chemical homogeneity. Infrared and chemical analyses results of the as-prepared xerogels suggest the possible formation of Ni(OH)(OAc). Generation of lithium carbonate as an intermediate product during the initial heat treatment of the precursors and its decomposition during subsequent heat treatment is the main rate determining step responsible for the formation of the desired lithiated metal oxide. The presence of residual lithium carbonate in the synthesized materials may cause loss in the discharge capacity of the materials. The morphology of the residual lithium carbonate has been investigated using electron microscopy, while its effect on the electrochemical properties has been exemplified.