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Biomedical subjects

D Cohn

Publications and source records attributed to D Cohn.

At least 37 records · Page 2Linked to original sources
PubMed1995

Derivatization of a new poly(ether urethane amide) containing chemically active sites.

A novel poly(ether urethane amide) was synthesized by chain-extending the prepolymer with fumaric acid, resulting in a polymer exhibiting superior mechanical properties and containing chemically active derivatization sites, stemming from the fumaramide double bond. The derivatization of the active sites was performed via a Michael-like addition of amine-terminated poly(ethylene oxide) chains, which when performed in bulk, greatly affected the mechanical properties of the polymer, except in the case in which a relatively high molecular weight was grafted. Surface grafting of the polymer led to a significantly lower effect on its mechanical properties, which was minimal when a high molecular weight molecule was used.

Absorption↗
PubMed1994

Elastic response of filament wound arterial prostheses under internal pressure.

Filament wound synthetic prostheses have anisotropic material properties and are therefore able to match closely the elastic properties of the replaced host vessels. Highly porous prosthesis walls are required to allow ingrowth of capillar cells from the outer surface of the graft in order to increase endothelium coverage of the luminal surface. The coating of highly porous grafts with biodegradable polymers has been shown to result in a sealed structure at the time of implantation followed by controlled porosity during the healing process. Accordingly, a new manufacturing process for a coated filament wound vascular graft is proposed in this work, which combines high potential porosity with high mechanical compliance. In vitro testing of its mechanical properties shows that the compliance can be controlled by changing the reinforcement angle and the coating material. Whereas the initial compliance of the coated structure expresses a composite material response, the post-degradation compliance reflects the highly compliant response of the filament wound non-woven scaffold. Hence, higher compliance values can be achieved by the proposed technique, compared with those of the commonly used synthetic grafts.

Anisotropy↗
PubMed1994

Modelling the anisotropic behaviour of filament wound vascular grafts.

The model according to the Law of Laplace, describing the mechanical behaviour of blood vessels and vascular grafts, was applied to filament wound arterial prostheses, which have been manufactured with different winding angles. By varying the winding angle, the anisotropic behaviour of the grafts could be changed and fitted to the anisotropic properties of natural blood vessels. Thus, the Laplace model had to be modified, and answers now to the requirement of responding to the anisotropic behaviour in hoop versus axial direction of the grafts. The experimental data of hoop and axial compliances obtained by biaxial inflation tests could be then correlated to the material properties of the vascular grafts measured by uniaxial tensile loading. It is shown that with the modified Laplace model the changes in the anisotropic behaviour due to different winding angles can be described and predicted. The calculated compliance values derived from the uniaxial tensile tests fitted the experimental data obtained by the biaxial inflation tests, although the calculated hoop compliance values tended to be higher than the experimental data.

Anisotropy↗
PubMed1994

Foscarnet-resistant herpes simplex virus infection in patients with AIDS.

Six human immunodeficiency virus-infected patients had clinical lesions of herpes simplex virus (HSV) type 2 that showed in vitro resistance to foscarnet. In each patient, lesions were unresponsive to foscarnet therapy or developed during daily suppressive foscarnet. Five patients had a history of intermittent or chronic foscarnet use for the management of acyclovir-resistant HSV infection, and 1 was receiving daily foscarnet for suppression of cytomegalovirus retinitis. Seven of 10 foscarnet-resistant isolates from 6 patients were susceptible to acyclovir in vitro, and 1 was of borderline susceptibility. In 3 patients, the administration of acyclovir, either alone or in combination with foscarnet, resulted in healing. Clinically significant resistance to foscarnet may occur in immunosuppressed patients with prior foscarnet exposure. Addition or substitution of acyclovir to foscarnet therapy may be a useful strategy for patients in whom foscarnet resistance is suspected, pending the results of in vitro susceptibility testing.

AIDS-Related Opportunistic Infections↗
PubMed1994

A study on the in vitro degradation of poly(lactic acid).

The in vitro degradation of samples of L- and D,L-lactic acid polymers, P(L)LA and P(DL)LA respectively, having different molecular weights, morphology and/or geometry, has been studied through the determination of viscometric molecular weight, mass and mechanical properties as function of the immersion time in Ringer solution at 37 degrees C. In particular have been compared the degradation kinetics of P(L)LA, amorphous and crystalline, and of P(L)LA and P(DL)LA having different molecular weight and sample geometry. From the molecular weight versus the degradation time data, a degradation rate has been defined, as the derivative of the function best fitting the data, normalized to the molecular weight of the polymer at each time. The behavior of the degradation rate curves, plotted against the degradation time, has been interpreted and compared with relation to the initial physical and geometrical characteristics of the PLA samples.

Biodegradation, Environmental↗
PubMed1993

Stiffness variability and stress-dependent elastic response of synthetic fibre-reinforced composites for biomedical applications.

A major design requirement of biomaterial prostheses is to match their elastic properties with those of the natural host tissue. Composite materials address this requirement because their elastic properties can be altered accurately through composition and directionality parameters, and they can be designed to match closely the elastic properties of the biological tissues, in isocompliance, modulus gradient and anisotropy. This adds to a range of advantages of synthetic composite materials with respect to potential biomedical applications, which draw on their heterogeneity and anisotropy. This paper focuses on the elastic properties of synthetic fibre-reinforced composite materials that pertain to biomedical applications, and demonstrates the range of stiffnesses obtainable through selection of constituents and by choice of angle of reinforcement.

Anisotropy↗
PubMed1992

Introducing a selectively biodegradable filament wound arterial prosthesis: a short-term implantation study.

This article introduces a new compliant and selectively biodegradable filament wound vascular graft and reports the findings of a short-term implantation study. A basic feature of filament winding is its ability to tailor and better control the mechanical properties of the prosthesis, so that a closer match with the anisotropic properties of native arteries is achieved. The elastomeric vascular grafts comprise poly(ether urethane urea) fibers (Lycra) embedded in a two-component matrix consisting of poly(ether urethane) (Pellethane) and a highly flexible poly(ethylene glycol)/poly(lactic acid) biodegradable segmented copolymer (PELA). Typical tensile modulus values fall in the few megapascals (MPa) range, this being comparable to that of natural arteries. The wound graft exhibits excellent handling and suturability characteristics as well as enhanced burst strength. Furthermore, due to its biodegradable constituent, the prosthesis combines minimal intraoperative blood loss and high healing porosity. The graft displays initially negligible in vitro water permeation, which increases gradually with time. In this short-term study, the prostheses were implanted in the canine carotid, and their biological performance was compared to that of expanded Gore-Tex. The luminal surface of the wound grafts was coated with a thin layer of pseudointima, strongly adhered to the prosthesis surface. Contrasting with the very stiff Gore-Tex grafts, the filament wound prostheses retained their high compliance, being highly pulsatile upon explanation. Histological studies fully corroborated these findings, underscoring the healing properties of these new filament wound vascular prostheses.

Animals↗
PubMed1992

Compliance and ultimate strength of composite arterial prostheses.

A further stage is reported in a comprehensive project aimed at developing new composites for soft tissue implants. The composite prostheses are made by a filament-winding technique comprising Lycra elastomeric fibres embedded in an elastomeric matrix of mostly Pellethane. New experimental results of compliance of filament-wound tubes and of the ultimate strength are presented and compared with theoretical predictions. It is shown that the functions of these properties in the winding angle are given by common composite material models. The compliance is predicted accurately by the expression based on a transformation of the principal elastic constants, whereby a maximum is expected at a winding angle of around 45 degrees. The ultimate strength is compared with two failure criteria, of which the maximum work criterion gives an excellent fit.

Animals↗
PubMed1992

Grafting reactions and heparin adsorption of poly(amidoamine)-grafted poly(urethane amide)s.

Differently terminated poly(amidoamine) (PAA) oligomers were grafted on the surface of poly(ether urethane amide)s (PEUAm), with fumaric or maleic acid moieties. The grafting reaction was Michael-type addition of amino groups to activated double bonds in the PEUAm backbone. PAAs having primary amino, or secondary amino end-groups were directly grafted on the surface of PEUAm sheets. For vinyl-terminated chains an alpha, omega amino-polyether spacer was introduced initially, following the same addition mechanism. Ungrafted and grafted materials were characterized, besides other analytical techniques, by ATR FT-IR spectroscopy. The heparin adsorption on PEAUm films was analysed after its elution from heparinized samples, quantified by coagulation tests (aPTT), and related to the presence of the PAAs chains grafted on to the surface. Results indicate that PAA-grafted PEUAm elastomeric biomaterials, display enhanced heparin adsorption abilities.

Adsorption↗
PubMed1991

Histological and biochemical distinctiveness of atypical aldosterone-producing adenomas responsive to upright posture and angiotensin.

Fifteen patients with primary aldosteronism were classified as angiotensin II-unresponsive aldosterone-producing adenoma (AII-U APA, n = 9), or angiotensin II-responsive aldosterone-producing adenoma (AII-R APA, n = 6), based on the responsiveness of aldosterone to upright posture and to angiotensin II infusion. Lack of aldosterone response to angiotensin II infusion immediately postoperatively in the AII-R APA subtype was consistent with previous responsiveness residing solely within the adenoma. Cortisol levels in five of the six patients with AII-R APA failed to suppress normally with dexamethasone consistent with some autonomous production of cortisol by the adenoma. In contrast, cortisol levels suppressed normally during dexamethasone administration in all patients with AII-U APA. This biochemical distinction can be added to the previously described overproduction of 18-oxo cortisol in AII-U APA but not in AII-R APA. Histological examination of adenoma sections revealed predominantly (greater than or equal to 50%) zona fasciculata type cells in AII-U APA. In contrast, AII-R APA contained less than 20% zona fasciculata type. Thus, biochemical differences between AII-U APA and AII-R APA subtypes of primary aldosteronism may be due to underlying differences in cellular composition of the aldosterone-producing adenomas.

Adenoma↗
PubMed1991

Hamartoma of the breast.

A case of hamartoma of the breast is reported and the literature reviewed. The clinical, radiological and pathological features of this rare condition are discussed, together with the difficulties in diagnosis. Familiarity with this lesion should enable it to be recognized more frequently in future.

Breast Neoplasms↗
PubMed1990

Utilization of composite laminate theory in the design of synthetic soft tissues for biomedical prostheses.

There are several advantages of using composite design considerations for the preparation of biomedical soft tissues. Using a composite laminate design, a wide range for compliance results, proving that the prosthesis compliance can be altered without a concomitant variation of other properties. The trend of compliance as a function of the reinforcement angle is discussed for an angle-ply composite of low compliance constituents, as well as the implications for stress-strain behaviour. Experimental examples pertinent to prosthetic arterial design are presented.

Biocompatible Materials↗
PubMed1990

New arterial prostheses by filament winding.

A new manufacturing method for vascular grafts, based on a filament winding technique, is unveiled. The concept that pilots this method is presented and analysed in detail alongside the experimental results. A basic feature of filament winding is its ability to produce a two-phase structure built of a continuous fibre-reinforced polymeric matrix, shaped according to the shape of a mandrel. This structure offers a number of advantages over common vascular graft designs, e.g. better control of the mechanical properties and closer match with anisotropic properties of native arteries, and more degrees of design freedom with respect to pore size, biodegradability and biocompatibility. The experimental section offers a range of potential constituent materials, and presents an example of a Lycra fibre/Pellethane matrix prototypical prosthesis.

Blood Vessel Prosthesis↗
PubMed1990

Long-term evaluation of a new selectively biodegradable vascular graft coated with polyethylene oxide-polylactic acid for right ventricular conduit. An experimental study.

Low-porosity woven Dacron grafts have been used extensively as an extracardiac conduit in the surgical treatment of congenital and acquired heart diseases involving total heparinization. Caution is still warranted in their use, however, because of long-term complications, including calcification and development of obstructive fibrous peel. In contrast, high-porosity grafts offer much better tissue anchorage and healing but cannot be used under heparinization. We have developed a compound vascular prosthesis in which a knitted Dacron graft is coated with a polymeric biodegradable sealant. Polyethylene oxide-polylactic acid-segmented copolymers comprised the degradable component of the graft. In vitro studies showed that the coated prosthesis exhibited a highly flexible elastomer-like mechanical response. The prostheses were completely watertight, and significant degradation started after 1 week, with absorption completed after 3 weeks. Seven woven and six knitted polyethylene oxide-polylactic acid-coated Dacron grafts used as extracordiac conduits (16 mm), connecting the right ventricle and the pulmonary artery were implanted in dogs. The dogs were killed after 12 to 18 months, and the results are reported. Scanning electron microscopy examination showed peel detachment and nonhomogenous intimal surface with fenestrations in the woven graft group, but complete healing and incorporation of the pseudointima with homogenous, thin lining of the luminal surface in the polyethylene oxide-polylactic acid-coated group. Histologic studies indicated much superior healing and anchorage of the periprosthetic tissue and the pseudointima in the polyethylene oxide-polylactic acid-coated grafts. The biodegradable polymer was fully degraded and exhibited a complete incorporation of the compound vascular prosthesis. This study indicates the superior healing properties of these selectively biodegradable grafts, which might increase long-term patency and decrease complications of right ventricular conduits.

Animals↗
PubMed1989

Use of high-performance polyethylene fibres as a reinforcing phase in poly(methylmethacrylate) bone cement.

First results are presented concerning the elastic and ultimate mechanical behaviour of p(MMA) bone cement reinforced with as-received and surface-modified Spectra 900 polyethylene fibres. Even though the surface chemistry and reactivity of the fibres was modified, the surface oxidation and surface grafting treatments of the polyethylene fibres apparently did not significantly affect the mechanical properties of the polyethylene-reinforced p(MMA) bone cement or improve the interfacial bonding. This may be attributed to the rather unfavourable area-to-volume ratio of PE fibres for such treatments, as well as to the necessarily low content of PE fibres in the bone cement which does not allow a clear differentiation between the various samples.

Bone Cements↗
PubMed1989

Compositional and structural analysis of PELA biodegradable block copolymers degrading under in vitro conditions.

This paper describes an investigation of the in vitro degradation of some polyethylene oxide/polylactic acid block copolymers. It has been found that the faster the degradation, the more basic the incubation medium and, as expected, the higher the temperature. The addition of enzyme proved to have no effect. This study shows that some polyethylene oxide/polylactic acid copolymers exhibit a steady increase in polylactic acid content, as degradation proceeds. This behaviour was attributed to the solubilization effect of the hydrophilic polyethylene oxide chains on the extraction of polyethylene oxide/polylactic acid-degrading fragments. Our findings indicate that polylactic acid blocks are cleaved randomly, the lactoyl end unit playing no special role. In accordance with data published for other biodegradable polymers, the crystallinity of polyethylene oxide/polylactic acid increases as degradation proceeds.

Biodegradation, Environmental↗
PubMed1988

Biodegradable PEO/PLA block copolymers.

Poly(ether ester) block copolymers based on polyethylene oxide (PEO) and polylactic acid (PLA) segments were synthesized and characterized, with the aim of developing a new family of bioadsorbable polymers. The materials developed were tailored to meet various mechanical and degradation requirements, and overcome the limitations of the few existing biodegradable polymers. The copolymeric matrices were characterized by means of infrared spectroscopy, differential scanning calorimetry, and nuclear magnetic resonance spectroscopy. The composition of the copolymers synthesized varied between 20 and 84 mol% lactic acid, with PEO chains in the 600-6000 molecular weight range. The solubility properties of the copolymers in a series of organic solvents are described. The equilibrium water content and the water contact angle of various matrices were determined and related to their composition and structure. The incorporation of PEO into the chain yielded highly hydrophilic materials, with equilibrium water contents higher than 60%. Stress/strain curves are presented.

Biocompatible Materials↗