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B Narasimhan

Publications and source records attributed to B Narasimhan.

8 recordsLinked to original sources
PubMed2001

Mathematical models describing polymer dissolution: consequences for drug delivery.

Polymer dissolution is an important phenomenon in polymer science and engineering that has found applications in areas like microlithography, controlled drug delivery, and plastics recycling. This review focuses on the modeling efforts to understand the physics of the drug release process from dissolving polymers. A brief review of the experimentally observed dissolution behavior is presented, thus motivating the modeling of the mechanism of dissolution. The main modeling contributions have been classified into two broad approaches - phenomenological models and Fickian equations, and anomalous transport models and scaling law-based approaches. The underlying principles and the important features of each approach are discussed. Details of the important models and their corresponding predictions are provided. Experimental results seem to be qualitatively consistent with the present picture.

Drug Delivery Systems↗
PubMed2001

Microphase separation in bioerodible copolymers for drug delivery.

This research examines the microstructure of bioerodible polyanhydrides with an eye towards precise design of drug delivery devices. Our main hypothesis is that the bioerodible copolymer poly(1,6-bis-p-carboxyphenoxyhexane-co-sebacic anhydride) (CPH : SA) undergoes micro-phase separation at certain copolymer compositions due to differences in relative hydrophobicity of the co-monomers, resulting in thermodynamic partitioning of drugs incorporated into these copolymers. We investigate the thermal properties, degree of crystallinity, and surface microstructure of several compositions of CPH : SA using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and atomic force microscopy (AFM). We observe that the degree of crystallinity decreases, while the crystal lamellar thickness increases with CPH content. Phase-imaging using AFM indicates the presence of micro-domains in 20 : 80 and 80 : 20 CPH : SA, while poly(SA) and 50 : 50 CPH : SA show no micro-phase separation. Finally, drug-polymer interactions are studied by loading the polymers with different amounts of brilliant blue (hydrophilic) and p-nitroaniline (hydrophobic). DSC and WAXD analysis shows that loading hydrophobic drugs into relatively hydrophobic polymers (poly(SA)) lowers melting point that becomes more pronounced with increased drug loading.

Biodegradation, Environmental↗
PubMed2000

The inference of antigen selection on Ig genes.

Analysis of somatic mutations in V regions of Ig genes is important for understanding various biological processes. It is customary to estimate Ag selection on Ig genes by assessment of replacement (R) as opposed to silent (S) mutations in the complementary-determining regions and S as opposed to R mutations in the framework regions. In the past such an evaluation was performed using a binomial distribution model equation, which is inappropriate for Ig genes in which mutations have four different distribution possibilities (R and S mutations in the complementary-determining region and/or framework regions of the gene). In the present work, we propose a multinomial distribution model for assessment of Ag selection. Side-by-side application of multinomial and binomial models on 86 previously established Ig sequences disclosed 8 discrepancies, leading to opposite statistical conclusions about Ag selection. We suggest the use of the multinomial model for all future analysis of Ag selection.

Antigens↗
PubMed1999

Bone mineral acquisition in healthy Asian, Hispanic, black, and Caucasian youth: a longitudinal study.

Ethnic and gender differences in bone mineral acquisition were examined in a longitudinal study of 423 healthy Asian, black, Hispanic, and white males and females (aged 9-25 yr). Bone mass of the spine, femoral neck, total hip, and whole body was measured annually for up to 4 yr by dual energy x-ray absorptiometry. Age-adjusted mean bone mineral curves for areal (BMD) and volumetric (BMAD) bone mineral density were compared for the 4 ethnic groups. Consistent differences in areal and volumetric bone density were observed only between black and nonblack subjects. Among females, blacks had greater mean levels of BMD and BMAD at all skeletal sites. Differences among Asians, Hispanics, and white females were significant for femoral neck BMD, whole body BMD, and whole body bone mineral content/height ratio, for which Asians had significantly lower values; femoral neck BMAD in Asian and white females was lower than that in Hispanics. Like the females, black males had consistently greater mean values than nonblacks for all BMD and BMAD measurements. A few differences were also observed among nonblack male subjects. Whites had greater mean total hip BMD, whole body BMD, and whole body bone mineral content/height ratio than Asian and Hispanic males; Hispanics had lower spine BMD than white and Asian males. The tempo of gains in BMD varied by gender and skeletal site. In females, total hip, spine, and whole body BMD reached a plateau at 14.1, 15.7, and 16.4 yr, respectively. For males, gains in BMD leveled off at 15.7 yr for total hip and at age 17.6 yr for spine and whole body. Black and Asian females and Asian males tended to reach a plateau in BMD earlier than the other ethnic groups. The use of gender- and ethnic-specific standards is recommended when interpreting pediatric bone densitometry data.

Absorptiometry, Photon↗
PubMed1997

Molecular analysis of drug delivery systems controlled by dissolution of the polymer carrier.

Dissolution-controlled drug delivery systems are characterized by a phase erosion of the polymer carrier that is associated with fast or slow dissolution of the macromolecular chains. The molecular nature of the dissolution phenomenon was examined by analyzing the water transport process and the subsequent polymer chain disentanglement that is usually characterized by a snake-like motion of the chain (reptation). The results indicate that the polymer molecular weight, water, polymer and drug diffusion coefficients, equilibrium water concentration in the polymer, and water-polymer interaction parameter can control the mechanism and rate of drug release. A new model for this process was developed, and its predictions are compared with experimental studies of drug delivery from poly(vinyl alcohol)-based systems.

Cimetidine↗
PubMed1995

Characterization of scyllo-inositol-containing phosphatidylinositol in plant cells.

The structure of in vivo [3H]myo-inositol-labeled phosphatidylinositols in barley seeds were investigated by chemical degradation. In this report we present data that suggests the presence of scyllo-inositol-containing phosphatidylinositol in addition to the commonly occurring myo-inositol-containing phosphatidylinositol.

Chromatography, Paper↗
PubMedDate not supplied

Self-diffusion and molecular mobility in PVA-based dissolution-controlled systems for drug delivery.

Nuclear magnetic resonance (NMR) microscopy has been used to monitor the hydration of poly(vinyl alcohol) (PVA) samples of varying molecular weight. One-dimensional profiles weighted to predominantly show the variation of water concentration were acquired every 3 min during the first 30 min of hydration and subsequently at 1 and 2 h. Diffusion-weighted profiles obtained after 30 min and 1 and 2 h were used to calculate the spatial variation of the water self-diffusion coefficient. The resulting data provide supporting evidence for the hypothesis that phenomena such as reptation are important near the glassy/rubbery interface of polymers during dissolution, while the diffusion gradually changes to Zimm type near the rubbery/solvent interface.

Diffusion↗