Biomedical subjects

A C Duncan

Publications and source records attributed to A C Duncan.

10 recordsLinked to original sources

PubMed2006

Effect of laser modified surface microtopochemistry on endothelial cell growth.

The introduction of microelectronics technology in the area of biological sciences has brought forth previously unforeseeable applications such as DNA or protein biochips, miniaturized, multiparametric biosensors for high performance multianalyte assays, DNA sequencing, biocomputers, and substrates for controlled cell growth (i.e. tissue engineering). We developed and investigated a new method using "cold" excimer laser beam technology combined with microlithographical techniques to create surfaces with well defined 3D microdomains in order to delineate critical microscopic surface features governing cell-material interactions. Microfabricated surfaces with microgrooves 30-3 microm deep, 10 - 1 microm wide spaced 30 microm apart were obtained with micron resolution, by "microsculpturing" polymer model surfaces using a computer controlled laser KrF excimer beam coupled with a microlithographic projection technique. The laser beam after exiting a mask was focused onto the polymer target surface via an optical setup allowing for a 10-fold reduction of the mask pattern. Various 3D micropatterned features were obtained at the micron level. Reproducible submicron features could also be obtained using this method. Subsequently, model human umbilical endothelial cells (HUVEC) were cultured on the laser microfabricated surfaces in order to study the effects of specific microscopic surface features on cell deposition and orientation. Cell deposition patterns were found to be microstructure dependant, and showed cell orientation dependency for features in the cell range dimension, a behaviour significantly different from that of a previously studied cell model (osteoprogenitor cell). This model may be a promising in so far as it is very rapid (a time frame less than a second per square centimeter of micropatterned surface) and provides further insights into the effects of surface microtopography on cell response with possible applications in the field of biosensors, biomedical and/or pharmaceutical engineering sciences.

Cell Culture Techniques↗

PubMed2002

Laser microfabricated model surfaces for controlled cell growth.

The relatively recent applications of microelectronics technology into the biological sciences arena has drastically revolutionized the field. New foreseeable applications include miniaturized, multiparametric biosensors for high performance multianalyte assays or DNA sequencing, biocomputers, and substrates for controlled cell growth (i.e. tissue engineering). The objectives of this work were to investigate a new method combining microphotolithographical techniques with laser excimer beam technology to create surfaces with well defined 3-D microdomains in order to delineate critical microscopic surface features governing material-cell interaction. Another obvious application of this study pertains to the fabrication of cell-based biosensors. Microfabricated surfaces were obtained with micron resolution, by "microsculpturing" polymer model surfaces using a laser excimer KrF beam coupled with a microlithographic projection technique. The laser beam after exiting a mask was focused onto the polymer target surface via an optical setup allowing for a 10-fold reduction of the mask pattern. Various 3-D micropatterned features were obtained at the micron level. Reproducible submicron features could also be obtained using this method. Subsequently, model osteoblast-like cells were plated onto the laser microfabricated surfaces in order to study the effects of particular surface microtopography on preferential cell deposition and orientation. Preferential cell deposition was observed on surfaces presenting "smooth" microtopographical transitions. This system may provide an interesting model for further insights into correlations between 3-D surface microtopography and cell response with new applications in the field biosensor, biomaterial and pharmaceutical engineering sciences (e.g. new cell based biosensors, controlled synthesis of immobilized cell derived active ingredients).

Biocompatible Materials↗

PubMed2002

Is estradiol cardioprotection a nitric oxide-mediated effect?

BACKGROUND: Estradiol exerts a number of biological effects that support extensive observational data suggesting a protective role for estrogen in cardiovascular disease prevention. These include effects on lipid and carbohydrate metabolism, coagulation/fibrinolysis as well as a possible effect on vascular reactivity. It has been proposed that this might be mediated by vascular endothelial nitric oxide (NO) production. Accordingly, we designed complementary in-vivo and in-vitro studies to investigate this hypothesis further. METHODS: Firstly, in a group of 10 healthy post-menopausal women, bilateral venous occlusion plethysmography was used to examine forearm vasoconstrictor responses to intrabrachial N(G)-monomethyl-l-arginine (l-NMMA; a substrate inhibitor of nitric oxide synthase) both before and after 4 weeks of treatment with transdermal 17beta-estradiol (E(2)) (80 microg/day). Secondly, we examined the direct effects of acute (24 h) and chronic (7 days) treatment with E(2) (10 pmol/l and 10 nmol/l) on endothelial nitric oxide synthase (eNOS) gene expression in cultured human aortic endothelial cells. RESULTS: No significant differences were observed between the vasoconstrictor responses to l-NMMA (2, 4, 8 micromol/min) before and after E(2) treatment. Comparison of E(2)-treated endothelial cells with control cells showed no significant increase in eNOS mRNA expression following either acute or chronic estradiol treatment. CONCLUSIONS: The present studies do not provide evidence for an eNOS-mediated cardioprotective response to estrogen and therefore suggest that additional mechanisms other than the endothelial NO system may have an important role in the cardiovascular effects of estrogen.

Administration, Cutaneous↗

PubMed1999

Screening of Zulu medicinal plants for angiotensin converting enzyme (ACE) inhibitors.

Twenty plants used by traditional healers in South Africa for the treatment of high blood pressure were investigated for their anti-hypertensive properties, utilizing the angiotensin converting enzyme assay. A hit rate of 65% was achieved, with the highest inhibition (97%) obtained by Adenopodia spicata leaves. A further seven plants exhibited an inhibition greater than 70% and five more over 50%. The leaves of the plants showed the greatest levels of inhibition. There was little difference in the overall hit rate between ethanolic and aqueous extracts, although in most cases there was a marked difference in activity between aqueous and ethanolic extracts from the same species. Plants exhibiting inhibition levels greater than 50% were further tested for the presence of tannins in order to eliminate possible false positives. Active plants that did not contain tannins were Agapanthus africanus, Agave americana, Clausena anisata, Dietes iridioides, Mesembruanthemum spp., Stangeria eriopus and Tulbaghia violacea.

Angiotensin-Converting Enzyme Inhibitors↗

PubMed1999

The effect of estradiol and a combined estradiol/progestagen preparation on insulin sensitivity in healthy postmenopausal women.

Abnormalities of carbohydrate metabolism and insulin sensitivity have been reported in estrogen deficiency. Estrogen replacement appears to result in an improvement in these parameters, although progestagens may antagonize these effects. We have examined the effects of transdermal estradiol and oral norethisterone on insulin sensitivity using the hyperinsulinemic euglycemic clamp method by performing a randomized, double blind, placebo-controlled study in 22 healthy women after a surgically induced menopause. After baseline measurements, subjects were randomized to receive either transdermal 17beta-estradiol (50 microg) or matching placebo patches for 6 weeks. The subjects were then further randomized to receive either estradiol in combination with oral norethisterone (1 mg) or a matching oral placebo preparation, crossing over after 6 weeks, with assessment of insulin sensitivity at the end of each treatment. No significant increase in insulin sensitivity was observed after 6 weeks of transdermal 17beta-estradiol treatment (95% confidence interval, -0.54, 1.86; P = 0.27). Addition of norethisterone for a further 6 weeks had no detectable effect on insulin sensitivity (95% confidence interval, -1.65, 1.10; P = 0.65). The results of this study using transdermal estradiol do not support previous reports that unopposed estrogens exert potentially beneficial effects on insulin sensitivity and suggest that the addition of an oral progestagen confers no clinically important risk or benefit. It is therefore unlikely that effects on insulin sensitivity contribute appreciably to the cardioprotective benefits attributed to hormone replacement therapy.

Administration, Cutaneous↗

PubMed1997

Viscoelasticity of dynamically fixed bioprosthetic valves. II. Effect of glutaraldehyde concentration.

OBJECTIVE: We have previously shown the benefits of dynamic fixation over conventional static fixation of bioprosthetic valves. In an attempt to increase the durability of bioprosthetic heart valves, we explored the benefit of low-concentration glutaraldehyde dynamic fixation. METHODS: Pig aortic valves obtained fresh from the abattoir and excised with the entire root were dynamically fixed in glutaraldehyde phosphate buffer solutions varying in concentration from 0.05% to 2.5%. Denaturation temperatures were measured and mechanical testing was performed at low (3 mm/sec) to high physiologic rates (30 mm/sec) at 37 degrees C in isotonic modified Hanks solution. RESULTS: When fixed dynamically in 0.05% glutaraldehyde solution for 24 hours, the tissue reached a degree of cross-linking (denaturation temperature = 82.8 degrees +/- 0.6 degree C) significantly higher than that obtained for 0.05% static fixation (denaturation temperature = 79.3 degrees +/- 0.9 degree C) (p < 0.05) but similar to that for conventional static fixation in 0.5% glutaraldehyde solution (denaturation temperature = 83.5 degrees +/- 0.3 degree C). After fixation in low-concentration glutaraldehyde (0.05%), final relaxation slopes and moduli in the circumferential direction were significantly higher than those for the statically fixed tissue but similar to those for the fresh tissue. However, both dynamic and static fixation had the effect of increasing tissue extensibility to similar extents in both directions, irrespective of glutaraldehyde concentration. CONCLUSIONS: Dynamic glutaraldehyde fixation of a porcine aortic valve at lower concentrations resulted in a better degree of cross-linking and a material with biomechanical properties that more closely mimic those of natural heart valve tissue.

Animals↗

PubMed1997

Effect of C4-, C8- and C18-alkylation of poly(vinyl alcohol) hydrogels on the adsorption of albumin and fibrinogen from buffer and plasma: limited correlation with platelet interactions.

Polyvinyl alcohol (PVA) hydrogel was partially alkylated with short (C4), intermediate (C8) and long (C18) alkyl chains to test the hypothesis that an alkylated surface might promote enhanced interaction with albumin and thus exhibit low platelet thrombogenicity. PVA hydrogel was reacted with alkyl halides (C4, C8 or C18) and coated onto polyethylene. The effect of surface alkylation (extent of alkylation and alkyl chain length) on the adsorption of human serum albumin and fibrinogen to these surfaces was investigated in both buffer and plasma. Platelet interactions were investigated in vitro using flow cytometry methods. The maximum surface concentrations of albumin and fibrinogen adsorbed from buffer onto PVA and alkylated PVA were characteristic of monolayers. At low concentrations differences in adsorption among the surfaces appeared to be related to hydrophobicity as determined by dynamic advancing water contact angle, and to degree of alkylation as determined by angle dependent XPS analysis. Alkyl chain length dependence was not observed. Adsorption from plasma was considerably lower than from buffer, except for albumin on C8-PVA where monolayer adsorption was observed. Fibrinogen adsorption from plasma was similar on PVA, C8-PVA and C18-PVA, but was higher on C4-PVA. For albumin adsorption from plasma, the initial slope of the adsorption-concentration curve was highest for C18-PVA, suggesting higher albumin affinity despite the low degree of substitution of the C18-PVA material. These data suggest possible selectivity of the C18 alkylated PVA for albumin. Platelet studies showed that C4-PVA was the least platelet reactive (microparticle generation and P-selectin expression) of the alkyl derivatized materials.

Adsorption↗

PubMed1996

Dynamic glutaraldehyde fixation of a porcine aortic valve xenograft. I. Effect of fixation conditions on the final tissue viscoelastic properties.

Sixty porcine aortic valves were fixed under dynamic conditions at specific durations, pressures and vibration rates in a 0.5% glutaraldehyde phosphate buffer (pH 7.4, 0.2 M). Tensile relaxation tests were performed at low through high extension rates (0.3, 3 and 30 mm s-1) and tissue denaturation temperatures were determined by the hydrothermal isometric tension method. Conventional statically fixed valves and fresh valves were used as controls. No differences between dynamic and static treatment were observed at pulsation rates above those expected in the physiological range (i.e. above 1.2 Hz) or at higher pressures such as 30 mmHg. However, differences in both stress relaxation rates and denaturation temperatures were delineated in milder fixation conditions, i.e. at low pressures (< 4 mmHg) and low vibration rates similar to that of the normal heart beat (approximately 1.2 Hz). In these conditions the relaxation rate of the dynamically fixed tissue (-7.4 +/- 0.7% of stress remaining per log(s)) was similar to that of the fresh tissue (-6.7 +/- 1.2% log(s-1)) and significantly higher than the statically treated tissue (-3.9 +/- 1.7% log(s-1)). The rates of stress relaxation appeared to be strain rate dependent in both radial and circumferential directions when the tissues were strained at physiological rates during testing (> approximately 15000% min-1). Dynamically treated valves showed higher denaturation temperatures (mean +/- SD) (89.4 +/- 0.5 degree C) compared with the statically fixed (82.7 +/- 1.4 degrees C) or untreated (fresh) valves (65.5 +/- 0.8 degree C). The results suggest a higher degree of internal cross-linking owing possibly to enhanced penetration of the glutaraldehyde reagent and a greater accessability of reactive cross-linking sites on the collagen molecules. Better stress relaxation rates are likely associated with an increase in potential shearing between adjacent collagen fibres thus preserving the natural stress-reducing mechanism of the fresh, untreated valves. The dynamically treated valves therefore possess characteristics that may enable them to better resist long-term mechanical fatigue and in vivo degradation.

Animals↗

PubMed1996

Preparation and characterization of alkylated poly(vinyl alcohol) hydrogels using alkyl halides.

A poly(vinyl alcohol) hydrogel coated onto polyethylene was partially alkylated by reaction with an alkylhalide (C4, C8, or C18) in the presence of a deprotonating agent (sodium ethoxide or potassium tert-butoxiDATE Surface coverage determined by X-ray photoelectron spectroscopy (XPS) was respectively approximately 34, 25, and approximately 8% for the C4, C8, and C18 modified surfaces. Statistically significant differences were observed in the fraction of C8 and C18 grafted alkyl groups as a function of depth (i.e. take-off angle) indicating the presence of a verticle composition gradient. All three surfaces showed maximal surface coverage of alkyl groups after 1 h reaction. At this reaction time, no further coverage was observed beyond a base/PVA ratio twenty times greater than the stoichiometric ratio. The advancing contact angle data exhibited an increase in hydrophobicity that correlated with the degree of coverage obtained by XPS: 90 +/- 1, 83 +/- 0.5, and 71 +/- 1 deg for C4, C8 and C18 alkylated PVA, and 55 +/- 2 deg for PVA respectively. Large contact angle hysteresis was observed on all three surfaces consistent with surface heterogeneity.

Alkylation↗

PubMedDate not supplied

Effect of dynamic glutaraldehyde fixation on the viscoelastic properties of bovine pericardial tissue.

We have previously proposed dynamic fixation as an alternative method to fix a porcine aortic heart valve xenograft with better tissue fixation and better preservation of its natural biomechanical properties. Bovine pericardium was fixed under dynamic conditions, low pressures (< 4 mmHg) and low vibration rate (1.2 Hz) in a 0.5% glutaraldehyde phosphate buffer (pH 7.4, 0.2 M). After fixation, tensile testing (i.e. relaxation and stress-strain curves) was performed at low and high extension rates (3 and 30 mm s(-1)) and tissue denaturation temperatures were determined by the hydrothermal isometric tension method. Conventional fresh and statically fixed pericardium were used as controls. In this instance, we found no significant biomechanical differences between the dynamically and statically fixed pericardial tissue (e.g. moduli and stress relaxation). However, differences in tissue extensibility were delineated, since the extensibility of the dynamically fixed tissue was closer to that of the fresh tissue compared to that of the statically fixed tissue. The final relaxation rate of the dynamically fixed tissue (-3.5 +/- 1.0% of stress remaining per log(second)) was similar to that of the statically fixed tissue (-3.2 +/- 0.60% log(s(-1))) and significantly lower than the fresh tissue(-9.5 +/- 1.2% log(s(-1))). The denaturation temperatures of the dynamically fixed pericardial tissue (mean +/- SD) (86.0 +/- 1.2 degrees C) and the statically fixed (85.2 +/- 1.6 degrees C) were similar but significantly higher than that of the untreated (fresh) valves (69.3 +/- 0.4 degrees C). The results suggest a similar degree of internal cross-linking for both statically and dynamically fixed pericardium. Although fundamental structural differences exist between both porcine and bovine xenograft tissue, how these differences contribute to biomechanical differences in the effects of dynamic versus static fixation remain to be explained.

Animals↗