Biomedical subjects

P S Ayyaswamy

Publications and source records attributed to P S Ayyaswamy.

10 recordsLinked to original sources

PubMed2006

Escherichia coli biofilms formed under low-shear modeled microgravity in a ground-based system.

Bacterial biofilms cause chronic diseases that are difficult to control. Since biofilm formation in space is well documented and planktonic cells become more resistant and virulent under modeled microgravity, it is important to determine the effect of this gravity condition on biofilms. Inclusion of glass microcarrier beads of appropriate dimensions and density with medium and inoculum, in vessels specially designed to permit ground-based investigations into aspects of low-shear modeled microgravity (LSMMG), facilitated these studies. Mathematical modeling of microcarrier behavior based on experimental conditions demonstrated that they satisfied the criteria for LSMMG conditions. Experimental observations confirmed that the microcarrier trajectory in the LSMMG vessel concurred with the predicted model. At 24 h, the LSMMG Escherichia coli biofilms were thicker than their normal-gravity counterparts and exhibited increased resistance to the general stressors salt and ethanol and to two antibiotics (penicillin and chloramphenicol). Biofilms of a mutant of E. coli, deficient in sigma(s), were impaired in developing LSMMG-conferred resistance to the general stressors but not to the antibiotics, indicating two separate pathways of LSMMG-conferred resistance.

Anti-Bacterial Agents↗

PubMed2001

Surface transformation of bioactive glass in bioreactors simulating microgravity conditions. Part I: experimental study.

Surface modified bioactive glass with surface properties akin to those of the bone mineral phase is an attractive candidate for use as a microcarrier material for 3-D growth of bone-like tissue in rotating wall vessel bioreactors (RWVs). The critical surface properties of this material are the result of reaction in solution. Because an RWV environment is completely different from conditions previously employed for bioactive glass testing, a detailed study of the surface reactions is warranted. Under properly chosen conditions, RWVs can also provide a simulated microgravity environment for the bioactive glass (BG) particles. In this sense, this study is also a report on the behavior of a bioactive material under microgravity conditions simulated on earth. A high aspect ratio vessel (HARV) and carefully selected experimental conditions enabled the simulation of microgravity in our laboratory. A complimentary numerical study was simultaneously conducted to ascertain the appropriateness of the experimental parameters (particle size, particle density, medium density, medium viscosity, and rotational speed) that ensure simulated microgravity conditions for the glass particles in the HARV. Physiological solutions (pH 7.4) with and without electrolytes, and also with serum proteins, were used to study the change in surface character resulting from simulated microgravity. Control tests at normal gravity, both static and dynamic, were also conducted. Solution and surface analyses revealed major effects of simulated microgravity. The rates of leaching of constituent ions (Si-, Ca-, and P-ions) were greatly increased in all solutions tested. The enhanced dissolution was followed by the enhanced formation of bone-like minerals at the BG surface. This enhancement is expected to affect adsorption of serum proteins and attachment molecules, which, in turn, may favorably affect bone cell adhesion and function. The findings of the study are important for the use of bioactive materials as microcarriers to generate and analyze 3-D bone-like tissue structures in bioreactors under microgravity conditions or otherwise.

Bioreactors↗

PubMed2001

Surface transformation of bioactive glass in bioreactors simulating microgravity conditions. Part II: numerical simulations.

The effects of simulated microgravity on the surface modification of bioactive glass (BG) in solution were studied using a numerical method. Models were developed for estimating the mass transfers of different chemical species from the surface of bioactive glass particles (microcarriers) suspended in the rotating liquid medium of a NASA-designed high aspect ratio vessel (HARV) bioreactor and on the bottom surface of a static vial. The concentration profiles resulting from chemical reactions and ionic transports were ascertained. Numerical results for the transport under simulated microgravity in the HARV and at normal gravity in the static vial were compared. These results were also compared with those of experiments to verify the enhancement of the reaction kinetics under simulated microgravity conditions. The experimental and numerical studies confirm that simulated microgravity conditions lead to the quick achievement of bioactive glass surface modification.

Bioreactors↗

PubMed2001

3D bone tissue engineered with bioactive microspheres in simulated microgravity.

Three-dimensional (3D) osteoblast cell cultures were obtained in rotating-wall vessels (RWV), simulating microgravity. Three types of bioactive microcarriers, specifically modified bioactive glass particles, bioceramic hollow microspheres, and biodegradable bioactive glass-polymer composite microspheres, were developed and used with osteoblasts. The surfaces of composite microspheres fully transformed into bone apatite after 2-wk immersion in simulated physiological fluid, which demonstrated their bone-bonding ability. The motion of microcarriers in RWVs was photographically recorded and numerically analyzed. The trajectories of hollow microspheres showed that they migrated and eventually stayed around at the central region of the RWV. At their surfaces, shear stresses were low. In contrast, solid glass or polymer particles moved toward and finally bounced off the outer wall of the RWVs. Cell culture studies in the RWV using bone marrow stromal cells showed that the cells attached to and formed 3D aggregates with the hollow microspheres. Extracellular matrix and mineralization were observed in the aggregates. Cell culture studies also confirmed the ability of the composite microspheres to support 3D bone-like tissue formation. These data suggest that the new hollow bioceramic microspheres and degradable composite microspheres can be used as microcarriers for 3D bone tissue engineering in microgravity. They also have potential applications as drug delivery systems.

Animals↗

PubMed2000

New bioactive, degradable composite microspheres as tissue engineering substrates.

Novel bioactive, degradable polymer/glass/ceramic composite microspheres were developed using a solid-in-oil-in-water (s/o/w) emulsion solvent removal method. Modified bioactive glass (MBG) powders were encapsulated into the polylactic acid (PLA) matrix. Scanning electron microscopy and energy-dispersive X-ray analyses revealed that the MBG powders were mostly embedded in the polymer matrix, and submicron-size pores were present at the surface. Immersion in simulated physiological fluid (SPF) was used to evaluate the surface reactivity of the microspheres. The polymeric surface was fully transformed into carbonated calcium hydroxyapatite (c-HA) after 3 weeks of immersion. In contrast, PLA microspheres showed no evidence of any calcium phosphate deposition. Ion concentration analyses revealed a decrease in Ca and P concentrations and an increase in Si concentration in the SPF immersed with composite microspheres during the 3-week period. The Ca and P uptake rates decreased after 2 days of incubation. This coincided with the decrease of the Si release rate. These data lend support to the suggestion that the Si released from the MBG powders present in the polymer matrix is involved in the formation of the Ca-P layer. Our results support the concept that these new bioactive, degradable composite microspheres may serve as microcarriers for synthesis of bone and other tissues in vitro and in vivo.

Biocompatible Materials↗

PubMed1999

Fabrication, characterization and evaluation of bioceramic hollow microspheres used as microcarriers for 3-D bone tissue formation in rotating bioreactors.

Novel bioactive ceramic hollow microspheres with an apparent density in the range 0.8-1.0 g cm(-3) have been developed as microcarriers for 3-D bone tissue formation in rotating-wall vessels (RWV). Hollow ceramic microspheres with a composition of 58-72% SiO2, 28-42% Al2O3 (wt%) and an apparent density 0.8-1.0 g cm(-3) were pretreated in 1.0 N NaOH for 2 h before being coated with synthesized calcium hydroxyapatite (HA) particulate sol. The HA-coated hollow microspheres were sintered for 1 h at 600, 800 and 1000 degrees C. SEM analysis revealed that the grain size and pore size of the calcium phosphate coating increased with the sintering temperature. FTIR analysis showed that crystalline calcium hydroxyapatite was present in the coatings sintered at 600 and 800 degrees C. When sintered at 1000 degrees C, the coating consisted of alpha-tricalcium phosphate. All the coatings adhered well, independent of sintering temperature. The trajectory analysis revealed that the hollow microsphere remained suspended in a rotating-wall vessel (RWV), and experienced a low shear stress (approximately 0.6 dyn cm(-2)). Cell culture studies using rat bone marrow stromal cells and osteosarcoma cells (ROS 17/2.8) showed that the cells attached to and formed 3-D aggregates with the hollow microspheres in a RWV. Extracellular matrix was observed in the aggregates. These data suggest that these hollow bioactive ceramic microspheres can be used as microcarriers for 3-D bone tissue formation in vitro, as well as for the study of the effects of microgravity on bone cell functions.

Animals↗

PubMed1997

Dynamics of a microcarrier particle in the simulated microgravity environment of a rotating-wall vessel.

Rotating-wall vessel (RWV), a low-shear, low turbulence microcarrier culture system provides a simulated microgravity environment suitable for 3-dimensional tissue culture. In this paper, the motion of a microcarrier particle in the rotating fluid has been analytically/numerically studied. If the microcarrier is less dense than the surrounding liquid medium, it eventually migrates towards an equilibrium state in the fluid. This state corresponds to a stationary location in the inertial frame of reference or equivalently, a circular orbit about the rotational axis in a rotating frame. If the particle is denser, it may move away indefinitely to reach or collide with the outer wall of the rotating vessel (outer boundary of the rotating fluid). Such a collision may damage the cells and could be undesirable for tissue culture. We have calculated migration times for a denser microcarrier to reach the outer wall of the vessel. Several factors--rotational speed, fluid viscosity, density difference between that of the microcarrier and the fluid, microcarrier radius, and the initial position of the microcarrier--were found to affect this migration time. We have also evaluated the variation of the fluid shear stress on the microcarrier surface. Decreasing the density difference between the microcarrier and the fluid, and decreasing the size of the microcarrier, can both decrease the maximum shear stress. The results for a solid, a hollow, and a hollow-porous microcarrier show that with a denser microcarrier material, the hollow or hollow-porous spherical microcarriers are preferable in order to increase the suspension time and decrease the maximum shear stress. The results of this study are thought to be useful for the development of optimal conditions for cell growth and metabolism in RWVs.

Cell Culture Techniques↗

PubMed1986

Perfused phantom models of microwave irradiated tissue.

The theoretical basis, practical design considerations, and prototype testing of a perfused model suitable for simulation studies of microwave heated tissue are presented. A parallel tube heat exchanger configuration is used to simulate the internal convection effects of blood flow. The global thermal response of the phantom, on a scale of several tube spacings, is shown theoretically to be nearly identical to that predicted by Pennes' bioheat equation, which is known to give a reasonable representation of tissue under many conditions. A parametric study is provided for the relationships between the tube size, spacing and material properties and the simulated perfusion rate. A prototype with a physiologically reasonable perfusion rate was tested using a typical hyperthermia applicator. The measured thermal response of the phantom compares favorably with the numerical solution of the bioheat equation under the same irradiation conditions. This similarity sheds light on the unexpected success of the bioheat equation for modeling the thermal response of real tissue.

Blood Circulation↗

PubMed1986

Small-scale temperature fluctuations in perfused tissue during local hyperthermia.

We develop analytical expressions (scaling laws) for the local temperature fluctuations near isolated and countercurrent blood vessels during hyperthermia. These scaling laws relate the magnitude of such fluctuations to the size of the heated region and to the thermal equilibration length of the vessels. A new equilibration length is identified for countercurrent vessels. Significant temperature differences are predicted between the vessels and the immediately adjacent tissue when the equilibration length is comparable to or longer than the size of the heated tissue region. Countercurrent vessels are shown to have shorter equilibration lengths and produce smaller temperature fluctuations than isolated vessels of the same size.

Blood Vessels↗

PubMed1986

Heat transport mechanisms in vascular tissues: a model comparison.

We have conducted a parametric comparison of three different vascular models for describing heat transport in tissue. Analytical and numerical methods were used to predict the gross temperature distribution throughout the tissue and the small-scale temperature gradients associated with thermally significant blood vessels. The models are: an array of unidirectional vessels, an array of countercurrent vessels, and a set of large vessels feeding small vessels which then drain into large vessels. We show that three continuum formulations of bioheat transfer (directed perfusion, effective conductivity, and a temperature-dependent heat sink) are limiting cases of the vascular models with respect to the thermal equilibration length of the vessels. When this length is comparable to the width of the heated region of tissue, the local temperature changes near the vessels can be comparable to the gross temperature elevation. These results are important to the use of thermal techniques used to measure the blood perfusion rate and in the treatment of cancer with local hyperthermia.

Biomechanical Phenomena↗