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Biomedical subjects

D J Lyman

Publications and source records attributed to D J Lyman.

At least 19 recordsLinked to original sources
PubMed1992

Effects of a vascular graft/natural artery compliance mismatch on pulsatile flow.

Attempts have been made to correlate small-diameter vascular graft patency with compliance matching between the graft and the host artery. Without knowledge about the mechanisms of failure by compliance mismatch, however, such correlations remain empirical. We have developed a flow system which mimics the flow in peripheral arteries and techniques to model a compliance mismatch in a straight elastic tube, as might occur with vascular repair. Our goal was to investigate one proposed mechanism of graft failure by compliance mismatch, that of a blood flow disturbance. Flow visualization experiments showed that, under pulsatile flow, a compliance mismatch caused trapping of 40 microns microspheres at the wall near the distal or downstream anastomosis. This suggests that the presence of a microscopic flow separation or stagnant zone in vivo may contribute to the intimal hyperplasia and thrombosis seen in failed grafts.

Acceleration↗
PubMed1991

Investigation of early surface delamination observed in retrieved heat-pressed tibial inserts.

The objective was to examine possible reasons for delamination observed in tibial inserts of the porous-coated anatomic (PCA) knee replacement. To date, 33 PCA inserts have been forwarded to the authors' labs. Of these 33, 52% showed severe delamination within four years of implantation. Visual, structural, and mechanical analyses were conducted and data compared on the heat-pressed PCA type and the common machined inserts. Twenty inserts of the two different types were examined. Visual data using polarized light microscopy showed the presence of a surface layer separated from the middle region of the heat-pressed inserts by a line of demarcation 250-580 microns beneath the articulating surface. This anomaly was not observed in machined inserts. Structural analysis showed the new heat-pressed inserts had increased crystallinity in the surface layer when compared to new machined inserts. The retrieved heat-pressed inserts showed increased crystallinity in the surface and middle regions. There was a slight increase in surface crystallinity in the retrieved machined inserts. Microhardness data showed that there was an increased hardness associated with the crystallinity seen on the heat-pressed inserts. Orthopedic surgeons should be aware of early delamination and surface failure in heat-pressed inserts.

Hardness↗
PubMed1990

Predicting the compliance of small diameter vascular grafts from uniaxial tensile tests.

The compliance hypothesis states that the compliance of vascular grafts should match that of the host artery for optimal patency. Although this has not been proven, the literature shows that much effort has gone into measuring compliance. Uniaxial circumferential tensile tests are simpler than compliance tests, but do not give the compliance (a multiaxial property) directly. Therefore, we have used mechanical models to correlate the two. Simple models suffer from inappropriate simplifying assumptions. In a clinically useful range, a Laplace law model and an incremental elasticity model do not predict the compliance from the rigidity as well as does a model derived from finite elasticity. This latter model has helped locate sources of errors. Variations in graft thickness, diameter, and anisotropy may be responsible for scatter in the experimental correlations between compliance and uniaxial rigidity.

Animals↗
PubMed1989

Solid state line scan cameras for measuring strain in soft tissues and implants.

The line scan camera, or LSC, is an inexpensive and easily applied technique for optical strain measurement of soft biomaterials. The LSC is based on a linear array of photodiodes; in gauging applications, where measurements between dark/light interfaces are important, the digital nature of the array can be exploited. Advantages of the LSC include low cost, high frequency response, applicability to front- or back-lighted samples, insensitivity to stray and nonuniform lighting as well as to accidental overexposure, ease and linearity of calibration, and lack of temperature sensitivity. With the 1024 element arrays used herein, the relative resolution is theoretically limited to 1 part in 1024, or 0.1%; in practice, the relative resolution is somewhat poorer. LSCs have been successfully used in mechanical tests to measure the diameter of arteries and compliant vascular grafts, the longitudinal strain of vascular grafts, and the dynamic diameter of elastic tube models of graft/artery systems in pulsatile flow visualization experiments.

Animals↗
PubMed1988

Finite elasticity modeling of the biaxial and uniaxial properties of compliant vascular grafts.

Compliant vascular grafts were modeled by finite elasticity theory. A linear, biaxial model satisfactorily described the stress-strain behavior in inflation tests, where the sample length was fixed longitudinally and inflated. The model was then used to predict the behavior in a longitudinal test (where the longitudinal stretch was varied while keeping the pressure zero), and in a uniaxial test of a circumferential strip. The model satisfactorily predicted the longitudinal Young's modulus measured in the longitudinal test. The model was less successful in predicting the circumferential Young's modulus measured in the uniaxial test, possibly because the state of stress was not purely uniaxial.

Blood Vessel Prosthesis↗
PubMed1987

In vitro and in vivo characterization of synthetic polymer/biopolymer composites.

Collagen, extracted from rat tail tendons using dilute acetic acid, was fabricated into films for subsequent characterization and biocompatibility testing. The reconstituted collagen was characterized with infrared spectroscopy, solution viscosity, contact angle, and tensile testing techniques and was found to be pure with molecular and physical properties consistent with findings of previous researchers. Composites composed of collagen coated on urethane and Silastic Rubber films were fabricated to give improved tear resistance. The biocompatibility of the composites and individual polymers was evaluated by discs implanted in the paravertebral muscle of rabbits. After four weeks none of the materials induced any gross changes in the muscle. Histopathological evaluation revealed a fibrous capsule around all of the materials. Collagen and collagen composites exhibited a stronger reaction as evidenced by a larger fibroblast layer and a variety of inflammatory cells, lymphocytes, eosinophils, and macrophages. The urethane was rated with a response index of 1.5 versus 3.25 for the urethane/collagen composite; Silastic Rubber rated a response index of 1.67 versus 3.12 for the Silastic Rubber/collagen composite; collagen rated a response index of 3.3. The polyester sutures also induced a reaction with a larger fibrous capsule but fewer inflammatory cells as compared to collagen and collagen composites.

Animals↗
PubMed1985

Evaluation of the gas transfer characteristics of porous copolyurethane oxygenator membranes.

Porous copolyurethane membranes were prepared by solvent casting thin polymer films onto fibrous substrates. Pore size, water filtration rates, and gas transmission rates were measured to determine the influence of substrate, polymer type, and coating thickness on the membrane properties in the dry and wet state. Cellulosic substrates were not as satisfactory as were polyethylene, polyester, or nylon substrates. Porous membranes formed on these latter substrates had gas transfer rates similar to silicone rubber membranes. Although O2 transfer was similar to Gore-Tex and Celgard, CO2 transfer was less, apparently due to fewer pores in the copolyurethane membranes and possible wetting of the pores by water.

Biological Transport↗
PubMed1985

Evaluation of the thrombogenicity of selected microporous oxygenator membranes.

A comparative study of the thrombogenicity of several microporous membranes was performed using an ex vivo system simulating the conditions in an artificial lung. A study of platelet adhesion and adsorbed protein layer indicated that the membranes separated into three groups. Copolyurethane membranes adsorbed very few platelets and appeared to be most compatible; microporous polypropylene and polytetrafluoroethylene membranes were intermediate in their response; silicone rubber and silicone rubber-coated paper membranes showed numerous platelet clots, often involving neutrophils and fibrin, and were the least blood compatible.

Dimethylpolysiloxanes↗
PubMed1980

The interaction of plasma proteins with polymers. I. Relationship between polymer surface energy and protein adsorption/desorption.

Adsorption of bovine albumin, gamma-globulin and fibrinogen from phosphate buffer solution (pH = 7.5) onto several polymer films was studied using the radioiodinated proteins (125I). The kinetics of desorption of the proteinated polymer films in bovine plasma was determined. Contact angle measurements on these same polymers allowed the calculation of dispersive (WA d) and polar (Ip) components of the polymer-protein solution system. Results from these measurements show that the nondispersive-dispersive force balance at the polymer-protein solution interface, expressed by the Ip/WA d ratio, is an important factor for binding of proteins on polymer surfaces. The purity of fibrinogen and the cleaning procedures for the polymer surfaces influence the absolute values of proteins adsorbed on polymer surfaces.

Adsorption↗
PubMed1978

Compliance as a factor effecting the patency of a copolyurethane vascular graft.

A new solid-wall vascular graft which has a compliance approximating that of the natural artery has been prepared from a copolyether-urethane material. Six of nine of these compliant grafts implanted in dogs were patent on removal, the longest implant time being 77 days for a 4-mm I.D. femoral artery graft. This is in contrast to noncompliant grafts of the same copolyurethane in which failure usually occurred within 48 hr.

Animals↗
PubMed1977

Block copolyurethanes in the urinary system.

Free floating segments of block copolyurethane were implanted into rat bladders and compared with similar implants of silicone rubber. This urethane was far superior to silicone rubber in its ability to withstand incrustation in the rat bladder. Ten dogs had a segment of right ureter replaced by a tubular segment of smooth (or film) urethane and were followed with intravenous urography. At 1 month, eight of 10 dogs had right hydronephrosis. At the time of sacrifice (4 or 8 months) seven dogs had reduced hydronephrosis, and in three dogs intravenous urography was unsatisfactory for grading. Only two of the ten dogs had significant urinary infection. A smooth protein-like coasting appeared along the lumen of the graft, incrustation was minimal, and new mucosa and smooth muscle regeneration was seen. This material was well tolerated in the urinary tracts of dogs and rats.

Animals↗
PubMed1975

Polymers in contact with the body.

The clinical use of polymeric materials in the body to repair and restore damaged or diseased tissues and organs is substantially increasing on an annual basis. Concomitant with this use is an increase in materials related research on medically used polymers. Information on the historical and current clinical use of polymeric materials is provided in order to establish a basis for the philosophy and problems encountered in assessing the acceptability of various polymers in the biological system. The requisite properties which must be demonstrated by a polymer in contact with the body are discussed from two viewpoints, i.e., the effects of the material on the stability of the host and the effects of the host on the stability of the material. In addition, the effects of synthesis, processing, storage, sterilization, implantation, and possible degradation of polymers are discussed, poly(ethylene terephthalate) being used as an example.

Biomedical Engineering↗
PubMed1975

The effects of chemical structure and surface properties of synthetic polymers on the coagulation of blood. IV. The relation between polymer morphology and protein adsorption.

Although these studies are preliminary, they do not show that the chemical structure of the block copolymer is not the only determinant of its blood properties; one must be equally concerned with the effect of its domain-matrix size and purity. These features, which are affected by factors such as block size, mold surface, and general fabrication procedures, as well as chemical structure, must be controlled if one is to have reproducible results in the function of an implant.

Adsorption↗