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Biomedical subjects

D Cohn

Publications and source records attributed to D Cohn.

At least 19 recordsLinked to original sources
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

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↗
PubMed1988

Biodegradable PELA block copolymers: in vitro degradation and tissue reaction.

Degradation of, and tissue reaction elicited by a series of polyethylene oxide (PEO)/polylactic acid (PLA) PELA block copolymers were studied in vitro and in vivo. In particular, the effect of pH, temperature and enzymatic activity was addressed. The mass loss was faster, the more basic the media, while, expectedly, PELA copolymers degraded faster with the higher temperature. The addition of an enzyme (carboxylic ester hydrolase) had no effect. The degradation process strongly affected the mechanical properties of the materials under investigation, the elongation at break dropping drastically after two days of degradation. After seven days, only gross observation of the extensively degraded samples was possible. The in vivo studies compared the tissue reaction elicited by various PELA copolymers to that evoked by PLA. Evaluation of tissue reaction observed with a PELA sample after sterilization with gamma radiation showed acute inflammation with considerable dispersion of the material, 12 days after implantation. The granulomatous reaction observed with PELA copolymers after ethylene oxide sterilization was identical to the reaction observed with PLA.

Animals↗
PubMed1987

Morphological study of biodegradable PEO/PLA block copolymers.

A series of PEO/PLA copolymers, covering a wide range of compositions and segmental lengths, was synthesized, and their morphology was investigated by means of DSC and IR studies. For matrices comprising PEO chains with molecular weights below 3400, no soft-segment crystallinity was detected. When long hard segments were present, essentially monophasic, semicrystalline polymers were obtained, with PLA blocks melting around 130 degrees C. Polymers containing longer soft segments (PEO 6000) exhibited a two-phase matrix, with both components being able to crystallize. The relative degree of crystallinity of PEO and PLA blocks was also determined. The thermal history of the sample strongly affected the morphology of the matrix, especially when both blocks were long enough to crystallize. To further explore these polymers, solvent cast films were prepared and their morphology assessed. Casting from acetone, which is an excellent solvent for PLA, resulted in hard blocks exhibiting lower degrees of crystallinity, while methanol had a similar effect on PEO soft segments.

Biocompatible Materials↗
PubMed1986

The effects of daily cyclophosphamide administration on the development and extent of primary experimental interstitial nephritis in rats.

We examined the effects of daily cyclophosphamide administration on the development and extent of tubulointerstitial nephritis produced in rats injected with tubular basement membranes in adjuvant. 15 mg/kg/day of cyclophosphamide completely blocked the development of interstitial lesions, while 2 mg/kg/day enhanced the degree of interstitial injury. When cyclophosphamide in the higher dose was started early in disease, 12 days after immunization, protection from progression was also observed as well as significant reductive improvement. If cyclophosphamide was administered late in disease, 21 days after immunization, no further progression was demonstrable, but substantial injury remained. In the latter two experiments, the beneficial effects of cyclophosphamide could not be explained by a reduction in anti-tubular basement membrane antibodies bound to the kidney. In groups of immunized rats that were tested, however, cyclophosphamide was able to non-specifically impair the delayed-type hypersensitivity response to tubular antigen and PPD. We conclude, therefore, that cyclophosphamide, in high but not low dosage, if given before damage is extensive and prolonged, may successfully inhibit the cellular immune response producing primary interstitial nephritis.

Animals↗
PubMed1985

Demodectic oviposition in the eyelid.

The normally torpid existence of demodectic mites of the eyelid changes with the onset of oviposition. There occurs a burst of activity characterized by flexion, extension and rotation.

Animals↗
PubMed1985

Inhibitory role of dietary protein restriction on the development and expression of immune-mediated antitubular basement membrane-induced tubulointerstitial nephritis in rats.

The protective effect of dietary protein restriction on the development and expression of immune-mediated interstitial nephritis was evaluated in Brown Norway rats with anti-tubular basement membrane disease. In the first series of experiments, pair-fed rats received low protein (LP) (3% casein) or normal protein (NP) (27% casein), normocaloric diets. After 6 wk, each group was immunized with renal tubular antigen in adjuvant to produce anti-tubular basement membrane antibody (alpha TBM-Ab) and tubulointerstitial nephritis. The kidneys harvested from NP rats after four more weeks on the diet had histologically more severe interstitial disease than the LP rats (histologic severity; NP = 3.1 +/- 0.2 vs. LP = 1.1 +/- 0.3; P less than 0.001), and serum creatinine values were concordantly different (NP = 1.34 +/- 0.02 vs. LP = 0.82 +/- 0.03). Titers of alpha TBM-Ab were similar in both groups, while the T cell-mediated immune response, as measured by delayed-type hypersensitivity (DTH), was nonspecifically impaired in LP rats when compared with the NP group. Admixture cotransfers of LP plus NP cells failed to demonstrate active suppression as an explanation for the depressed DTH in LP rats. The therapeutic role of dietary protein restriction was also examined in rats with established alpha TBM disease. In these experiments, rats were first immunized and fed NP diets for 4 wk (histologic severity = 3.0 +/- 0.2; creatinine = 1.78 +/- 0.02), and then were divided into two groups and followed for six more weeks on either LP or NP diets. LP rats, under these conditions, developed less disease than those fed NP diet (histologic severity; NP = 3.2 +/- 0.3 vs. LP = 1.4 +/- 0.2; P less than 0.001), and serum creatinine values were concordantly different (NP = 1.92 +/- 0.05 vs. LP = 0.97 +/- 0.02). Again, the titers of alpha TBM-Ab in both LP and NP groups were similar. These data collectively suggest that LP diet has a protective effect both on the development and extent of tubulointerstitial nephritis that is perhaps, in part, related to the selective abrogation of effector T cell immunity.

Animals↗