Vesselin Shanov

Biodegradable, elastomeric coatings with controlled anti-proliferative agent release for magnesium-based cardiovascular stents.

Vascular stent design continues to evolve to further improve the efficacy and minimize the risks associated with these devices. Drug-eluting coatings have been widely adopted and, more recently, biodegradable stents have been the focus of extensive evaluation. In this report, biodegradable elastomeric polyurethanes were synthesized and applied as drug-eluting coatings for a relatively new class of degradable vascular stents based on Mg. The dynamic degradation behavior, hemocompatibility and drug release were investigated for poly(carbonate urethane) urea (PCUU) and poly(ester urethane) urea (PEUU) coated magnesium alloy (AZ31) stents. Poly(lactic-co-glycolic acid) (PLGA) coated and bare stents were employed as control groups. The PCUU coating effectively slowed the Mg alloy corrosion in dynamic degradation testing compared to PEUU-coated, PLGA-coated and bare Mg alloy stents. This was confirmed by electron microscopy, energy-dispersive x-ray spectroscopy and magnesium ion release experiments. PCUU-coating of AZ31 was also associated with significantly reduced platelet adhesion in acute blood contact testing. Rat vascular smooth muscle cell (rSMC) proliferation was successfully inhibited when paclitaxel was released from pre-loaded PCUU coatings. The corrosion retardation, low thrombogenicity, drug loading capacity, and high elasticity make PCUU an attractive option for drug eluting coating on biodegradable metallic cardiovascular stents.

A Corrugated Graphene–Carbon Nanotube Composite as Electrode Material

A graphene and carbon nanotube (CNT) array composite was synthesized by chemical vapor deposition (CVD) and chemically treated after synthesis, yielding a novel corrugated structure, visually similar to a mushroom gill. This binder-free hybrid material was used to make an electrode that may find application in energy storage devices, such as supercapacitors. The electrode performance of the corrugated graphene/CNT array composite (CGCC) was compared to that of commercial glassy carbon. The results of the comparison are presented here, along with suggestions for further development of the CGCC electrode.

Kinetics of Growing Centimeter Long Carbon Nanotube Arrays

Carbon nanotubes (CNTs) are fascinating materials with outstanding mechanical, optical, thermal, and electrical properties [1-4]. CNTs also have a huge aspect ratio and a large sur‐ face area to volume ratio. Because of their unique properties, vertically aligned centimeter long CNT arrays have generated great interest for environmental sensors, biosensors, spin‐ ning CNT into yarn, super-capacitors, and super-hydrophobic materials for self-cleaning surfaces [5-11]. Yun et al. studied a needle-type biosensor based on CNTs to detect dopa‐ mine. Their results showed advantages of using CNT biosensors for detecting neurotrans‐ mitters [11]. Most of the envisioned applications require CNTs with high quality, a long length, and well aligned vertical orientation. Although many researchers have studied the synthesis of vertically aligned CNT arrays, the CNT growth mechanism still needs to be bet‐ ter understood. In addition, CNT lengths are typically limited to a few millimeters because the catalyst lifetime is usually less than one hour [1216]. Many groups have studied the ki‐ netics of CNT growth trying to improve CNT properties. Different observation methods [17-22] were used to determine the effect of the catalyst, buffer layers, carbon precursor, and deposition conditions on nanotube growth. One of the suggested growth mechanisms pos‐ tulates several steps [23]. First, the carbon source dissociates on the surface of the substrate. Next, the carbon atoms diffuse to the molten catalyst islands and dissolve. The metal-carbon solution formed reaches a supersaturated state. Finally, the carbon nanotubes start to grow from the carboncatalyst solution. In situ observation of CNTs during their nucleation and growth is a useful method to understand the growth mechanism, which might help to over‐ come the limitation of the short length of nanotubes, and to control array growth and quali‐ ty. Various remarkable approaches of in situ observation have been performed to affirm the growth mechanism of vertically aligned CNTs and also to obtain kinetics data such as

Flexible Low-Voltage Carbon Nanotube Heaters and their Applications

Carbon nanotube heaters recently gained more attention due to their efficiency and relative ease of fabrication. In this chapter, we report on the design and fabrication of low-voltage carbon nanotube (CNT) heaters and their potential applications. CNT sheets drawn from CNT arrays have been used to make the heaters. The sheet resistance of the CNT sheet is dependent on the number of layers accumulated during their formation, and it ranges from 3.57 kΩ/sq. for a 1-layer sheet to 6.03 Ω/sq. for a 300-layer sheet. The fabricated and studied CNT heaters revealed fast heating and cooling rate. Potential applications of these heating devices have been illustrated by manufactur‐ ing and testing heatable gloves and via deicing experiments using low-voltage CNT heaters.

New Applications and Techniques for Nanotube Superfiber Development

Abstract Nanotubes are a unique class of materials because their properties depend not only on their composition but also on their geometry. The diameter, number of walls, length, chirality, van der Waals forces, and quality all affect the properties and performance of nanotubes. This dependence on geometry is what makes scaling-up nanotubes to form bulk material so challenging. Nanotubes are also unusual because they stick together to form bundles or strands. Nanotube superfiber materials are fibrous assemblages of nanotubes and strands. The hope and dream of researchers around the world is that nanotube superfiber materials will have broad applications and change engineering design. This chapter gives a perspective on nanotube superfiber development. This chapter discusses new applications—where we think we can go with the material properties and what applications will be enabled—and new techniques for developing superfiber material.

MODELING THE ELECTRICAL IMPEDANCE OF CARBON NANOTUBE RIBBON

Carbon Nanotube (CNT) ribbon is a thin layer of aligned, partially overlapping CNTs drawn from a forest of CNTs grown on a substrate. The electrical properties of the ribbon must be understood to put this material into multifunctional applications. Measurements show that CNT ribbon exhibits interesting characteristics including frequency-dependent electrical impedance. The impedance is mainly a combination of resistive and capacitive impedance. The magnitude of the impedance of ribbon increases moderately with increasing frequency then decreases significantly at higher frequency, MHz and above. An electrical model was developed to approximate the electrical impedance of the CNT ribbon. Based on this model, some important properties of the CNT ribbon can be understood. The ribbon capacitance, CNT–CNT contact resistance and resistivity can be approximated using the model. This information is useful in determining the suitability of ribbon for different applications. Methods to improve the electrical conduction of CNT ribbon are also discussed.

Hybrid scaffolds of Mg alloy mesh reinforced polymer/extracellular matrix composite for critical-sized calvarial defect reconstruction

The challenge of developing scaffolds to reconstruct critical‐sized calvarial defects without the addition of high levels of exogenous growth factor remains relevant. Both osteogenic regenerative efficacy and suitable mechanical properties for the temporary scaffold system are of importance. In this study, a Mg alloy mesh reinforced polymer/demineralized bone matrix (DBM) hybrid scaffold was designed where the hybrid scaffold was fabricated by a concurrent electrospinning/electrospraying of poly(lactic‐co‐glycolic acid) (PLGA) polymer and DBM suspended in hyaluronic acid (HA). The Mg alloy mesh significantly increased the flexural strength and modulus of PLGA/DBM hybrid scaffold. In vitro results demonstrated that the Mg alloy mesh reinforced PLGA/DBM hybrid scaffold (Mg‐PLGA@HA&DBM) exhibited a stronger ability to promote the proliferation of bone marrow stem cells (BMSCs) and induce BMSC osteogenic differentiation compared with control scaffolding materials lacking critical components. In vivo osteogenesis studies were performed in a rat critical‐sized calvarial defect model and incorporated a variety of histological stains and immunohistochemical staining of osteocalcin. At 12 weeks, the rat model data showed that the degree of bone repair for the Mg‐PLGA@HA&DBM scaffold was significantly greater than for those scaffolds lacking one or more of the principal components. Although complete defect filling was not achieved, the improved mechanical properties, promotion of BMSC proliferation and induction of BMSC osteogenic differentiation, and improved promotion of bone repair in the rat critical‐sized calvarial defect model make Mg alloy mesh reinforced PLGA/DBM hybrid scaffold an attractive option for the repair of critical‐sized bone defects where the addition of exogenous isolated growth factors is not employed.