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Biomedical subjects

M V Sefton

Publications and source records attributed to M V Sefton.

At least 91 records · Page 5Linked to original sources
PubMed1983

Microencapsulation of erythrocytes in Eudragit-RL-coated calcium alginate.

Human erythrocytes have been encapsulated in calcium alginate gel beads (approx. 2 mm diameter) which were subsequently coated with a water-insoluble cationic polyacrylate, Eudragit RL. Stained histological sections as well as scanning and transmission electron micrographs clearly showed the presence of a dense capsule wall, approx. 60 micron thick after 60 min of exposure to an aqueous emulsion of Eudragit, surrounding a sponge-like alginate core. Eudragit beads could withstand greater compressive forces before mechanical failure than uncoated beads. The effect of alginate concentration, coating time and ionic strength on the compressive load at failure, indicated the likelihood that coating involved ionic interactions between the polycationic Eudragit and the polyanionic alginate. Encapsulated erythrocytes continued to consume glucose but at a rate one-third higher than normal which attributed to an under estimate in the number of immobilized cells or to an actual increase in cell metabolism due to immobilization. Alginate/Eudragit encapsulated erythrocytes also continued to reversibly bind oxygen even after 21 days storage at 4 degrees C. Although in vivo stability and biocompatibility remain at question, this system is a model of that required to encapsulate pancreatic islets for the treatment of insulin-dependent diabetes.

Alginates↗
PubMed1983

Properties of a heparin-poly(vinyl alcohol) hydrogel coating.

Some physical, chemical, and biological characteristics of a heparin-PVA hydrogel potentially useful as a thromboresistant coating for the preparation of small-diameter vascular prostheses and blood-sampling catheters have been determined. The molecular weight between crosslinks in the acetal crosslinked gel was approximately 8000, permitting proteins the size of albumin to enter the interior of the gel. The release rate of heparin from a gel containing 7 mg/g gel was 10(-2) micrograms/g min which was significantly lower than the reported minimum required for thromboresistance of ionically heparinized materials. Nevertheless, in vitro biological activity was observed in both thrombin time and plasma recalcification time assays, which could not be attributed to the release of heparin into the incubated plasma. Correlation of final heparin contents with the amount of terminal amino acid residues in three samples of heparin suggests that the heparin is bound to the PVA in the gel through the amino acid terminus; this provides a plausible explanation for the retention of biological activity by the immobilized heparin.

Animals↗
PubMed1983

Relationship between release rate and surface concentration for heparinized materials.

Mathematical models are used to predict surface concentrations that result from the release of heparin into flowing blood and stagnant or well-mixed plasma. Two release rates--4 X 10(-2) and 3 X 10(-5) micrograms/cm2 min--are considered, which describe elution from an ionically heparinized material and from an immobilized heparin-PVA hydrogel, respectively. When heparin is released at the higher rate into blood flowing in cylindrical tubes with dimensions characteristic of the vasculature, or annular tubes representative of catheter experiments, a minimum surface concentration of 0.5 micrograms/mL is attained virtually at the tube inlet. Release at the lower rate requires tube lengths of several thousand meters to attain the same critical value. Similarly, heparin released from a suspension of beads at the higher rate leads to critical surface concentrations of 0.2 micrograms/mL within a fraction of a second in stagnant plasma, or ca. 5 s in a well-mixed environment. At the lower release rate, 45 or 100 min must elapse before the same level is achieved. These results support the validity of 4 X 10(-2) micrograms/cm2 min as a reasonable minimum release rate to produce a heparin microenvironment sufficient to prevent thrombosis. The lower rate is shown to be insufficient to generate a critical concentration, thus supporting the argument that heparin-PVA does not owe its biological activity to a heparin microenvironment. The model equations can be applied to the release of any material to determine surface concentrations.

Blood Coagulation↗
PubMed1983

Stain length passive dosimeter for monitoring of carbon monoxide.

A new passive dosimeter for the personal monitoring of carbon monoxide exposure in the workplace has been developed. This new type of sampling device does not require follow-up analysis of the collected sample to determine the exposure. Rather, the time-weighted average concentration is determined directly from the length of a colored stain which is produced instantaneously during the exposure period in a specially prepared indicator tube. The colored stain length is a function of both contaminant concentration and exposure time. The effects of carbon monoxide concentration in the range of 0.5 T TLV to 2.0 X TLV on the response of the dosimeter were evaluated in a dynamic exposure chamber. In this range of concentration, plots of stain length against exposure collapsed onto a single curve. At the TLV exposure the stain length was 2.1 cm and the 95% confidence interval about this point was +/- 17%. Relative humidity in the range of 26 to 92% and face velocity in the range of 0.01 to 1.0 m/s (2 to 200 fpm) did not significantly affect the dosimeter response. These data were fitted to the appropriate model equation with a correlation coefficient of 0.968. By eliminating the need for follow-up analysis, this stain length dosimeter significantly reduces the cost of monitoring. The instantaneous readout eliminates the delay between exposure and quantification, enabling more careful control of the workplace environment.

Carbon Monoxide↗
PubMed1982

Microencapsulation of erythrocytes.

Human erythrocytes have been encapsulated in a polyacrylate membrane by a simple precipitation process. The encapsulated cells appeared to remain functional after encapsulation: the consumption of glucose and the ability to reversibly bind oxygen was unimpaired. Furthermore, storage at 4 degrees C for almost 6 months had no effect on the P50 and n50 values. This is the first time to our knowledge that live mammalian cells have been encapsulated in a polymer other than alginate.

Acrylic Resins↗
PubMed1981

Patency of heparinized SBS shunts at high shear rates.

The patency of 50 cm long, 1.7 mm ID heparin-polyvinyl alcohol (PVA) coated SBS arterio-venous shunts in pigs at shear rates greater than 1000 s-1 was found to be not different from that of identical shunts coated with PVA but without heparin. This was attributed to the absence of any measureable effect of surface bound heparin on platelet related thrombus formation at high shear rates. On the other hand, platelet adhesion values determined in the absence of flow by the open static method decreased with increasing heparin content in heparin-PVA films. The low overall patency (average life of 170 minutes) of the PVA coated SBS shunts (with and without heparin) was presumed to be related to the absence of circulating heparin during surgery and the consequent presence of tissue thromboplastin or cellular debris during the immediate postoperative period. Alternative protocols are needed to test heparinized materials at low shear rates in the absence of systemic heparin to properly assess the potential thrombo-resistance of such materials.

Animals↗
PubMed1980

Insulin permeability of hydrophilic polyacrylate membranes.

The diffusive permeability of insulin in polyhydroxyethyl methacrylate (37.1% water), polyhydroxyethyl acrylate (51.8% water), polymethacrylic acid (67.5% water), and cuprophane PT-150 membranes was determined and correlated with the weight fraction of water in the membrane. Insulin diffusivity in the membranes was determined to be 1.0 x 10(-6) cm2/sec, which is in reasonable agreement with literature values for insulin diffusivity in water.

Acrylates↗
PubMed1979

Heparinized styrene-butadiene-styrene elastomers.

A heparinized high-strength elastomer has been developed which is potentially useful as a nonthrombogenic vascular prosthesis. A surface hydroxylated styrene-butadiene-styrene (SBS) block copolymer with at least 40% extent of reaction after glow-discharge cleaning was coated with a 20% acetylated polyvinyl alcohol/heparin mixture containing glutaraldehyde and magnesium chloride. After curing at 80 degrees C for 100 min, the polyvinyl alcohol, heparin, and hydroxylated SBS were covalently bound to each other by acetal bridges. The effects of the various substrate and coating parameters were optimized to achieve very strong adhesion between the coating layer and the surface hydroxylated SBS. Heparin was not leached from the surface of the new material using 3M saline at pH 7.4 despite a detection limit of 10(-5) micrograms heparin/cm2 min. Prolonged partial thromboplastin times of greater than 1200 sec were observed (control: PTT = 120 sec). Preliminary ex vivo testing using a simple arteriovenous shunt in the leg of a rabbit showed good thromboresistance. The heparinized SBS shunt chamber remained patent for more than two hours without desorption of heparin. It was concluded that surface hydroxylated SBS heparinized by acetal coupling owed its thromboresistance to the heparin covalently bound to the surface and not to a microenvironment of heparin in solution at the blood/material interface.

Biocompatible Materials↗
PubMed1976

Surface hydroxylation of styrene-butadiene-styrene block copolymers for biomaterials.

This work pertains to the development of high strength elastomers potentially useful as nonthrombogenic cardiovascular prostheses. Triblock copolymers of the styrene-butadiene-styrene type have been subjected to surface hydroxylation which provide reactive sites at the surface for the subsequent coupling of heparin while retaining the unique mechanical properties of the SBS copolymers. Curves of hydroxyl content versus the copolymer film thickness demonstrate the effect of swelling in the surface region on the product distribution and on the time dependence of the hydroxylation process. In addition, the effect of time, temperature, and the composition of the reaction bath on the diffusion/reaction process is shown. Finally, the general applicability of this surface modification scheme to the development of biomaterials is discussed.

Adsorption↗
PubMedDate not supplied

A model of thrombin inactivation in heparinized and nonheparinized tubes with consequences for thrombus formation.

The role of flow and mass transport in determining procoagulant concentration at the wall of synthetic and natural cylindrical blood vessels is analyzed theoretically. The model assumes steady laminar flow and considers, in addition to the fluid dynamic parameters, three rate-determining steps: production of procoagulant (thrombin) and its inactivation at the wall, as well as inactivation in the fluid bulk. The ratio of thrombin wall concentration to production rate Cw/N emerges as a critical parameter in characterizing the behavior of the tube wall. With a wall-inactivation rate typical of heparinized materials, Cw/N = 11.1 s/cm, independent of flow (shear rate) and axial position. This is significantly less than the range of Cw/N (50-500 s/cm) for which the thrombin concentration is high enough to result in significant fibrin formation and thrombosis. Hence little fibrin formation and a high degree of thromboresistance is expected for heparinized materials. Nonheparinized materials have Cw/N values above this range, which are only weakly dependent on shear rate and diameter, suggesting that flow-induced dispersion of thrombin (or other procoagulants) has limited impact on the thrombin wall concentration. These latter results appear to refute the conventional wisdom that attributes the relative patency of large-diameter vessels and differences between venous and arterial thrombi to such flow effects. It is likely that additional factors such as flow pulsatility and wall geometry must be considered to account for these observations.

Blood Coagulation↗
PubMedDate not supplied

Characterization of canine platelet P-selectin (CD 62) and its utility in flow cytometry platelet studies.

1. P-selectin (PADGEM, GMP140, or CD62) a member of lectin-like adhesive proteins is expressed on the surface of activated degranulated canine platelets and is the calcium-dependent receptor for leukocyte adhesion. 2. The electrophoretic mobility of P-selectin, by Western blot analysis and immunoprecipitation from radiolabeled membranes of canine and human platelets, was similar or identical and immunocytochemical studies localized P-selectin in internal vesicles similar to the alpha granule localization in human platelets. 3. Two antibodies to human P-selectin KC4.1 and AC1.2 crossreacted with canine platelets whose surface binding, in response to agonists thrombin, calcium ionophore (A23187), phorbol esters and ADP, was similar. 4. Anti-P-selectin antibodies in conjunction with crossreacting anti-GPIIb/IIIa antibodies (A2A9, 7E3, RUU-PL7F12) enables the analysis of activated platelets, platelet-derived microparticles and platelet-leukocyte interactions in canine models by flow cytometry.

Adenosine Diphosphate↗
PubMedDate not supplied

HEMA/MMMA microcapsule implants in hemiparkinsonian rat brain: biocompatibility assessment using [3H]PK11195 as a marker for gliosis.

Microencapsulation of dopamine-secreting cells in biocompatible, semi-permeable polymer membranes has been proposed as an alternative strategy for dopamine replacement for Parkinson's disease. In order to assess the viability of this proposal, dopamine-secreting PC12 cells were immunoisolated via microencapsulation in a 75:25 2-hydroxyethyl methacrylate/methyl methacrylate (HEMA/MMA) copolymer. A submerged nozzle-liquid jet method was used to produce small diameter (400 microm) microcapsules, which were stereotaxically implanted in the denervated striatum of hemi-Parkinsonian rats. A 96% survival rate was associated with the implantation surgery and no deleterious side effects were apparent. Light microscopy revealed good biocompatibility between the HEMA/MMA copolymer and the host brain, as evidenced by the absence of gross tissue damage at the neuronal tissue/capsule interface. Autoradiographic analyses using [3H]PK11195 as marker for reactive astrocytes revealed a moderate inflammatory response, confined to the immediate vicinity of the injection tract. Quantitative analyses indicated that the local tissue response did not differ significantly between brains implanted with PC12-containing capsules and those implanted with vehicle-containing capsules. Taken together, these results support the biocompatibility of HEMA/MMA copolymer as well as the feasibility and safety of stereotaxic implantation of microcapsules.

Animals↗
PubMedDate not supplied

Immunoblot analysis of proteins associated with HEMA-MMA microcapsules: human serum proteins in vitro and rat proteins following implantation.

Human serum proteins and their fragments, associated with hydroxyethyl methacrylate-methyl methacrylate (HEMA-MMA) copolymer microcapsules, were characterized using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblot analysis. Capsules were incubated with serum for 1 week in vitro and then dissolved in ethanol to also precipitate the adsorbed protein. The precipitate was dissolved in 2% (w/v) SDS (the 'capsule eluate') to be assayed by electrophoresis. The majority of proteins probed for in the immunoblots were detected in the capsule eluates. These included fibronectin, plasminogen, IgG, vitronectin, Factor B, Factor H, Factor I, C3, but not beta-lipoprotein, fibrinogen, HMWK, or IgM. Complement activation fragments were detected in both the immunoblots of the capsule eluates and the medium containing serum without capsules. Thus, the adsorption of these fragments, formed independent of capsule presence, may be partially or completely responsible for the complement fragments associated with capsules. The prevention of complement activation by the addition of 5.8 mM EDTA, at the beginning of the week-long incubation, resulted in fewer low-molecular-weight C3 fragments associated with capsules. Rat proteins were also detected in immunoblots of the eluate of 'free-floating' capsules from the rat peritoneal cavity following implantation for 1 day using anti-human antibodies. Detected proteins included HMWK, fibrinogen, antithrombin III, transferrin, alpha1-antitrypsin, fibronectin, albumin, alpha2-macroglobulin, vitronectin, beta2-microglobulin, Factor B and Factor I. Rat fibrinogen, IgG, and complement C3 fragments were also detected in these immunoblots, but with monoclonal antibodies against the rat proteins.

Animals↗
PubMedDate not supplied

A piezoelectric driven controlled release micropump for insulin delivery.

A piezoelectric disk bender was used to augment the rate of insulin transport across a membrane. Repeated displacement of the disk resulted in a pressure driven flow across the membrane, in addition to the basal delivery obtained because of the concentration difference between the insulin and downstream reservoirs. Augmentation increased nonlinearly with voltage, higher augmentation being achieved at voltages greater than 50 V DC (50% duty cycle). Augmentation was also much greater at frequencies near resonance (e.g., near 200 Hz for a 1.2 microns cellulose acetate membrane) and with smaller insulin reservoirs. Greater augmentations with reasonable basal rates were obtained with membranes that had high diffusive permeabilities and a high ratio of hydraulic to diffusive permeability. Theoretic calculations, using an equivalent circuit model of piezoelectric action, suggested that although the pressure increase in the insulin reservoir was approximately 0.19% of the maximum possible, this pressure increase was sufficient to account for approximately 95% of the augmented flow. Although further improvements are necessary, this device has the potential of delivering insulin to diabetics to better match supply and demand.

Equipment Design↗