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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
PubMed2002

The parallel helices of the intermediate filaments of alpha-keratin.

Recent Fourier transform infrared spectroscopy (FTIR) with attenuated total reflection technique (ATR) has been applied to alpha-keratin fibers (horse-hair) extended in water both at 21 and 95 degrees C. Infrared absorption bands in the Amide 1 region indicated that at extensions to 40-50% strain in water at 21 degrees C alpha-helices had completely disappeared and parallel beta-sheets were formed [Appl. Spectrosc. 55 (2001) 552]. However, when the hair fibers were extended to the same strain at 95 degrees C in water the result was the formation of anti-parallel beta-sheets. These results suggest that the relatively more stable anti-parallel beta-state [Polymer 10 (1969) 810] is only attained in extended alpha-keratin fibers at elevated temperatures and must result from major molecular rearrangement. It was concluded that the alpha-helices in the intermediate filaments (IFs) of alpha-keratin fibers must be parallel. This is in contrast to the previously accepted orientation of anti-parallel alpha-helices, based primarily on findings of X-ray diffraction studies of the structure of beta-keratin in highly extended fibers [Polymer 10 (1969) 810; Keratins, IL: Thomas Springfield (1972); Nature 316 (1985) 767].

Animals↗
PubMed2001

Vascular graft healing. II. FTIR analysis of polyester graft samples from implanted bi-grafts.

FTIR-ATR analysis has shown that the 4-step process for preclotting polyester vascular grafts results in a uniform and reproducible fibrin coating of the polyester fibers. Western blot analyses have shown that FN and VEGF are also present in this fibrin coating. FTIR-ATR analyses of explanted grafts indicate that, while the in vivo healing of these preclotted polyester grafts proceed through the inflammation, proliferation, and remodeling phases of normal wound healing, these phases are modified. Because the fibrin coating provides a nonporous barrier between peri-graft tissue and the flowing blood, these molecular changes are controlled by the interactions of blood-borne constituents with the lumenal surface of the preclotted graft. Also, a well prepared preclotted polyester graft shows a minimal inflammatory response. After implantation, the fibrin preclot is more than 90% gone by the fifth day. However, the proliferation phase, involving synthesis of new protein and polysaccharide materials to replace the fibrin, appears to have begun by the third day. Detection of collagen I in the 5-day explants suggests that the overlapping remodeling phase of healing has begun. Protein and saccharide materials continue to be synthesized and remodeled, and, by the tenth day, collagen IV is detected. By 14-days post-implantation, there is an increase in collagen IV and cellular membrane lipids. Because collagen IV is an indicator of the presence of endothelial cells, some of these cellular membranes must be of endothelial origin. Thus, it appears that FTIR-ATR can be a useful tool in the study of vascular healing.

Animals↗
PubMed1999

Vascular graft healing: I. FTIR analysis of an implant model for studying the healing of a vascular graft.

FTIR analysis of sequential biopsy samples of the primary and replacement segments obtained from 70 to 80-cm long carotid-femoral bypass grafts implanted bilaterally in the dog indicate that the healing pattern is similar over the entire length of these grafts. Preliminary analysis of the spectra also indicates that the major developments in the formation of the biological layer appear to occur during the first 4 weeks after implantation. Collagen IV could be detected by both FTIR and antibody staining in the 8-week samples. Thus, this study supports the application of FTIR attenuated total reflectance to determine the components of the biological tissue that forms on an implanted vascular graft surface.

Animals↗
PubMed1994

Morphology of block copolyurethanes: V. The effect of -CH2CH2- vs -CH2- spacers between aromatic rings.

Copolyether-urethane-ureas based on 1,2-ethylene bis(4-phenylisocyanate), polypropylene glycol and ethylene diamine were synthesized by both the standard two-step and a multi-step procedure and their properties compared with analogous copolyurethanes based on methylene bis(4-phenylisocyanate). The infrared and dynamic mechanical properties indicate differences in the packing structure of the hard domains between these two copolymer systems. The infrared data suggest that the introduction of a -CH2CH2- spacer between the two aromatic rings of the diisocyanate results in a coplanar packing in the hard domains for these copolyurethanes.

Biomechanical Phenomena↗
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↗