Biomedical subjects

M K Chaudhury

Publications and source records attributed to M K Chaudhury.

8 recordsLinked to original sources

PubMed2001

Fast drop movements resulting from the phase change on a gradient surface.

The movement of liquid drops on a surface with a radial surface tension gradient is described here. When saturated steam passes over a colder hydrophobic substrate, numerous water droplets nucleate and grow by coalescence with the surrounding drops. The merging droplets exhibit two-dimensional random motion somewhat like the Brownian movements of colloidal particles. When a surface tension gradient is designed into the substrate surface, the random movements of droplets are biased toward the more wettable side of the surface. Powered by the energies of coalescence and collimated by the forces of the chemical gradient, small drops (0.1 to 0.3 millimeter) display speeds that are hundreds to thousands of times faster than those of typical Marangoni flows. This effect has implications for passively enhancing heat transfer in heat exchangers and heat pipes.

Journal Article↗

PubMed1999

Surface properties and hemocompatibility of alkyl-siloxane monolayers supported on silicone rubber: effect of alkyl chain length and ionic functionality.

Self-assembled monolayers of alkylsiloxanes supported on poly(dimethylsiloxane) (PDMS) rubber were used as model systems to study the relation between blood compatibility and surface composition. The inner lumen of PDMS tubes were first treated with an oxygen plasma. The resultant oxidized surfaces were post-derivatized by reaction with alkyltrichlorosilanes to form the monolayer films. The alkyl chain lengths used were slightly longer than in a previous study, and this may alter the phase-state of the monolayer from liquid-like to crystalline. The chemical properties of the monolayer were controlled by varying the chemical composition of the alkyltrichlorosilanes used. Terminal functionalities included -CH3, -CF3, -COOH, -SO3H and -(CH2CH2O)4OH. Surface derivatization was verified with static contact angle measurements and X-ray photoelectron spectroscopy. Blood compatibility was evaluated using a canine ex vivo arterio-venous series shunt model. Surfaces grafted with hydrophobic head groups such as -CH3 and -CF3 were significantly less thrombogenic than the surfaces composed of ionic head groups such as -COOH and -SO3H. Surfaces enriched in -(CH2CH2O)4OH had an intermediate thrombogenicity. Silastic pump grade tubing and polyethylene tubing, used as controls, were found to be the least thrombogenic of all the surfaces tested.

Animals↗

PubMed1997

Effect of saline exposure on the surface and bulk properties of medical grade silicone elastomers.

Medical-grade silicone elastomers were subjected to accelerated aging in saline to verify the hydrolytic stability of the elastomer. Tensile strength, elongation at break, and the elastomer stress measured at 100% or 200% elongation did not change significantly for peroxide-cured sheeting aged in 37 degrees C or 100 degrees C saline for 45 h. Under similar condition, hydrosilylation cured sheeting behaved similarly; however, increases in stresses measured at 100% and 200% elongation were observed after the first hour of treatment. After the first hour, the physical properties remained relatively constant. On either elastomer, initial liquid drop advancing contact angles for water ranged from 110 degrees to 115 degrees, and in no case was a change of > 6 degrees observed as a result of aging in saline for 45 h at temperatures up to 97 degrees C. The high advancing angles indicated that the surface remained largely hydrophobic. Initial liquid drop receding contact angles ranged from 48 degrees to 64 degrees, with receding contact angles being more sensitive to accelerated aging, in one case decreasing to 14 degrees. Similar decreases in receding contact angle were observed on polyethylene subjected to the same accelerated aging conditions. Decreases in receding contact angle were not considered to be indicative of extensive hydrolysis. The observed contact angle phenomena are consistent with current views of contact angle hysteresis being caused by surface heterogeneity. There was no evidence of significant surface or bulk siloxane hydrolysis under these accelerated aging conditions.

Hydrolysis↗

PubMed1995

Surface and blood-contacting properties of alkylsiloxane monolayers supported on silicone rubber.

Self-assembled monolayers of alkylsiloxanes supported on polydimethyl siloxane (PDMS) rubber were used as model systems to study the relation between blood compatibility and surface chemistry. The inner lumen of PDMS tubes was first treated with an oxygen plasma. The resultant oxidized surfaces were postderivatized by reacting them with alkyltrichlorosilanes to form the monolayer films. The chemical properties of the monolayers were controlled by varying the head-group chemical compositions. Surface derivatization was verified using variable-angle X-ray photoelectron spectroscopy (XPS or ESCA). Blood compatibility was evaluated using a canine ex vivo arteriovenous series shunt model. Surfaces grafted with hydrophobic head-groups as -CH3 and -CF3 had significantly lower platelet and fibrinogen deposition than the surfaces composed of hydrophilic groups such as -CO2CH3, -(CH2CH2O)3COCH3, and -(OCH2CH2)3OH.

Animals↗

PubMed1987

Mechanism of DNA (Southern) and protein (Western) blotting on cellulose nitrate and other membranes.

The transfer of DNA fractions from hydrophilic gels to nitrocellulose membranes (Southern blotting) which was soon followed by the description of an analogous procedure for RNA (Northern blotting), and somewhat later for proteins (Western blotting), has rapidly become an important separation and characterization method in molecular biology, genetic engineering, and immunological detection. Surface tension measurements have shown that the interfacial attraction between DNA and cellulose esters (-delta G132) in aqueous media can be considerable. The weaker binding energy of proteins to cellulose nitrate and to cellulose acetate may be compared to hydrophobic interaction chromatography, as on account of the somewhat lower [-delta G132] values, it often is necessary to "fix" them more tightly onto nitrocellulose by using high salt concentrations. The binding energy of RNA to both cellulose esters also is rather low. In addition to the effect of high ionic strength, the effect of adding methanol, and the effects of denaturation, heating and drying on the energy of attachment of the biopolymers to cellulose esters, have been studied. Cationized nylon membranes have been advocated recently, especially for electrophoretic transfer of nucleic acids (in which process high salt concentrations cannot easily be used). With positively charged nylon membranes, the attachment mainly occurs through the electrostatic attraction between the strongly negatively charged nucleic acids (or proteins) and the positively charged membrane. Also, more apolar membranes (of polyvinyl difluoride) have been proposed, which manifest a strong interfacial (hydrophobic) attraction to all the above biopolymers (regardless of their electrostatic charge). However, with these two novel membrane types it is no longer possible to exploit the large difference in binding energy between DNA and RNA, which makes cellulose nitrate membranes so uniquely suited for RNA-DNA hybridization assays.

Animals↗

PubMed1987

Monopolar surfaces.

Following the development of a methodology for determining the apolar components as well as the electron donor and the electron acceptor parameters of the surface tension of polar surfaces, surfaces of a number of quite common materials were found to manifest virtually only electron donor properties and no, or hardly, any electron acceptor properties. Such materials may be called monopolar; they can strongly interact with bipolar materials (e.g., with polar liquids such as water); but one single polar parameter of a monopolar material cannot contribute to its energy of cohesion. Monopolar materials manifesting only electron acceptor properties also may exist, but they do not appear to occur in as great an abundance. Among the electron donor monopolar materials are: polymethylmethacrylate, polyvinylalcohol, polyethyleneglycol, proteins, many polysaccharides, phospholipids, nonionic surfactants, cellulose esters, etc. Strongly monopolar materials of the same sign repel each other when immersed or dissolved in water or other polar liquids. The interfacial tension between strongly monopolar surfaces and water has a negative value. This leads to a tendency for water to penetrate between facing surfaces of a monopolar substance and hence, to repulsion between the molecules or particles of such a monopolar material, when immersed in water, and thus to pronounced solubility or dispersibility. Monopolar repulsion energies can far outweigh Lifshitz-van der Waals attractions as well as electrostatic and "steric" repulsions. In aqueous systems the commonly observed stabilization effects, which usually are ascribed to "steric" stabilization, may in many instances be attributed to monopolar repulsion between nonionic stabilizing molecules. The repulsion between monopolar molecules of the same sign can also lead to phase separation in aqueous solutions (or suspensions), where not only two, but multiple phases are possible. Negative interfacial tensions between monopolar surfactants and the brine phase can be the driving force for the formation of microemulsions; such negative interfacial tensions ultimately decay and stabilize at a value very close to zero. Strongly monopolar macromolecules or particles surrounded by oriented water molecules of hydration can still repel each other, albeit to an attenuated degree. This repulsion was earlier perceived as caused by "hydration pressure". A few of the relevant colloid and surface phenomena are reviewed and re-examined in the light of the influence of surface monopolarity on these phenomena.

Colloids↗

PubMed1986

Nature of the antigen-antibody interaction. Primary and secondary bonds: optimal conditions for association and dissociation.

All antigen-antibody (AG-AB) bonds are weak physical bonds; covalent bonds are not encountered. The main bonds involved are: (I) Coulombic bonds; (II) Ca2+-bridges; (III) hydrogen bonds; (IV) Lifshitz--van der Waals bonds. Combinations of III and IV occur as the "bonds" usually alluded to as hydrophobic (H phi) interactions. In primary bonds, mainly types I and IV occur; types II and III are quite rare. Secondary bonds, which evolve after a certain time-lapse (varying from minutes to days), mainly involve type IV bonds and H phi interactions, while hydrogen bonds sensu stricto have been known to play a role in rare instances. In affinity chromatography involving AG-AB interactions, complete elution with mild eluents usually is desirable. It would thus appear essential to let little time elapse between AG-AB complex formation and the elution step, to minimize strengthening of the AG-AB interaction by secondary bond formation. For expeditious elution, it also is important to avoid using dehydrating agents (which tend to decrease the bond distance between AG and AB and thus could strengthen the bond in a number of ways). The degree of involvement of types I and IV bonds as well as of H phi interactions varies considerably among different AG-AB systems. These components thus have to be measured separately in order to determine the optimal conditions for elution. The parameters of the liquid medium that may be modulated to influence dissociation (or association) are, for example, surface tension, pH, ionic strength, dielectric constant, temperature, admixture of dehydrating agents or of chaotropic salts. The influence of variations in each of these parameters on types I, III and IV bonds (and thus also on H phi interactions) will be delineated. Because of the misleading implications of the term "hydrophobic interactions", it seems preferable henceforth to allude to them as "interfacial forces".

Animals↗

PubMed1981

Correlation between membrane expansion and temperature-induced membrane fusion.

For each phospholipid membrane, there is a characteristic phase transition temperature, and for each phospholipid spherical membrane, there is a specific 'fusion' temperature. In order to examine the possible correlation between temperature-induced membrane fusion and membrane expansion, the relationship between the physical states of phospholipid membranes at both temperatures have been investigated by the use of the monolayer system. Monolayer expansion studies have indicated that the increase in area per lipid molecule, caused by increasing the temperature from the phase transition to the fusion temperature, is approximately the same for five different phospholipids used. With the same temperature increase, phospholipid monolayers containing cholesterol did not expand appreciably. This correlates qualitatively with the greater inhibition of membrane fusion seen in the spherical phospholipid membrane systems when cholesterol was incorporated in the membrane. The effect of pH on the expansion of phosphatidylserine monolayers was also studied in relation to membrane fusion phenomena. The shift in fusion temperature of the spherical phospholipid membranes due to the change of pH is explained by the shift in phase transition temperatures of lipid membranes. The expanded area per molecule in the monolayer caused by increasing the temperature from the phase transition to the fusion temperature was approximately the same irrespective of surface charge densities.

Cholesterol↗