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Biomedical subjects

H S Shim

Publications and source records attributed to H S Shim.

13 recordsLinked to original sources
PubMed1997

Grafting of PEO to glass, nitinol, and pyrolytic carbon surfaces by gamma irradiation.

Glass, nitinol, and pyrolytic carbon surfaces were grafted with poly (ethylene oxide) (PEO) and PEO-containing Pluronic surfactants by gamma irradiation. These substrates were coated with a primer layer of trichlorovinylsilane (TCVS), which allows grafting of organic polymers. The TCVS-coated substrates were adsorbed with PEO or Pluronics and exposed to 0.3 Mrad of gamma radiation to graft the polymer to the surface. PEO-grafted substrates were characterized by contact angle measurement, X-ray photoelectron spectroscopy, fibrinogen adsorption, and platelet adhesion and activation. Surface modification with PEO reduced fibrinogen adsorption by as much as 99%. Platelet adhesion was significnatly reduced or prevented on the modified surfaces. Protein- and platelet-resistance effects were independent of hydrophilicity of the PEO-grafted surfaces. Polymer grafting by gamma radiation to TCVS-coated substrates provides a facile process to improve thromboresistance of inorganic biomaterials.

Alloys↗
PubMed1986

Canine model for long-term evaluation of prosthetic mitral valves.

The evaluation of mechanical prosthetic heart valves would be aided by a more satisfactory animal model. For acute assessment, a variety of animals have been used, but for chronic studies, only larger animals (pigs, calves, baboons) have been employed, creating an expensive model with laboratory management difficulties. Previously, the use of dogs for chronic evaluation has been unsatisfactory because of the frequent occurrence of early sepsis and valve-related thrombotic deaths. We have modified our existing acute dog protocol to produce a successful chronic model. Our model employs perioperative systemic antibiotics, short cardiopulmonary bypass period (range 35-60 min), a minimum of perioperative intravenous lines, postoperative anticoagulation therapy, and strict postoperative antiseptic technique for blood sampling. To evaluate this model, 11 consecutive mongrel dogs underwent mitral valve replacement with either a standard Dacron sewing skirt or a newly devised carbon-coated Teflon sewing skirt No. 23 mm Bjork-Shiley Convexo Concave (CC) prosthetic valve. Nine animals (82%) survived and were evaluated after a predetermined observation interval of either 3 or 6 months for valve function, pannus formation, and possible carbon particle migration. At sacrifice, all animals had good hemodynamics and valve function, minimal pannus formation and no carbon washout. Consequently, this model provides a relatively inexpensive, reproducible method of chronic in vivo evaluation of prosthetic valve modifications.

Animals↗
PubMed1980

Gaseous flow through thin carbon films.

Thin carbon films, when used as coatings on prosthetic devices, must be a barrier to gases and physiological fluids. Using CO2 at room temperature, the gas permeability of carbon films ranging in thickness from about 200 to 500A was measured. The average permeability constant of 21 carbon films was determined to be 1.91 (+/- 1.02) x 10(-12) cm3-cm/cm2-sec-mmHg. This value is quite comparable to or smaller than that of nuclear graphites, which are considered to be impermeable to gases.

Biocompatible Materials↗
PubMed1980

The fatigue behavior of vapor-deposited carbon films.

Vapor-deposited carbon films (about 4000 to 5000 A thick) on stainless steel substrate were cyclically loaded to 10(6) cycles. The carbon films did not fail in fatigue at strain levels up to 13.12 x 10(-3). Rather, the failure in the carbon film occurred as a result of plastic deformation in the substrate; i.E., the failure was directly related to the endurance limit of the substrate material, which, when expressed as strain, was measured in this study to be about 8.0 - 10.88 x 10(-3). The endurance limit was also found to be very close to the elastic strain limit of the substrate. The implications of the findings for the use of carbon coated components in prosthetic devices are also discussed.

Carbon↗
PubMed1979

Developments in carbon prosthetics.

The majority of carbon-coated prosthetic devices in use today are coated with a unique form of carbon, low-temperature isotropic (LTI) carbon. The wide acceptance of this special form of carbon is a direct result of LTI carbon's demonstrated biocompatibility, its mechanical properties, and its inertness. The LTI carbon deposition process, however, places severe constraints on the size and type of substrate that can be coated. The substrates must be small so that they may be supported in a fluidized bed and further must be able to withstand temperatures in excess of 1200 degrees C. Recent technological advancements have removed the requirement that an object to be coated must be suspended in a fluidized bed and have also made possible the deposition of isotropic carbon at near room temperature. These developments expand the application of carbon-surfaced components into areas of prosthetics not previously possible. This paper describes some of the new applications and results.

Animals↗
PubMed1977

The strength of LTI carbon dental implants.

In vitro mechanical tests have been performed on a variety of LTI pyrolytic carbon blade-type dental implants, and the test results have been analyzed using an analytical model. Tensile stresses at fracture were calculated to be about 5 X 10(4) psi and 8 X 10(3) psi in the LTI carbon coating and the graphite substrate, respectively. These values are close to their respective fracture strengths. The fracture loads predicted by the model are in good agreement with data obtained from the mechanical tests and are higher than forces expected in mastication.

Carbon↗
PubMed1977

The microstructure of isotropic vapor-deposited carbon films.

The structure of thin, vapor-deposited carbon films was characterized by transmission electron microscopy and electron diffraction. Selected area electron diffraction showed very weak and broad peaks, indicating that these carbons contain extremely small crystallites whose dimension in the crystallographic c-direction is about 8 to 10 a. The observed diffraction bands are (h, k, 1 = 0) type reflections, which suggests that individual crystallites consist of graphitic layer planes stacked in parallel groups but with no order between atoms in adjacent planes (turbostratic). The carbon films exhibit no preferred orientation, indicating that the small crystallites are randomly oriented in the film and that the films are therefore isotropic. The measured density (1.8 g/cm3) and the structure of the vapor-deposited carbons are accordingly similar to those of low-temperature isotropic (LTI) pyrolytic carbons.

Carbon↗
PubMed1977

The wear of titanium, titanium alloy, and UHMW polyethylene caused by LTI carbon and Stellite 21.

The comparative wear resistance of a commercially pure titanium (A-70), a titanium alloy (Beta III), and a UHMW polyethylene (Lennite) has been evaluated by employing a test procedure described previously. Either an LTI carbon or a Stellite 21 was the disk material. All material combinations exhibited a low volume wear rate ranging from about 1.2 x 10(-6) to 1.6 x 10(-6) mm3/km. The wear behavior of pure titanium seems to be related not only to its mechanical properties but also to its chemical reactivity with the test environment. A comparison of the current results with earlier data for LTI carbons suggests that LTI carbons may be used as a component material for many artificial joints.

Alloys↗
PubMed1976

The mechanical behavior of LTI carbon dental implants.

LTI pyrolytic carbon blade-type dental implants consisting of a graphite substrate and an LTI pyrolytic carbon coating have a strength that increases with the coating thickness. For implants having a coating thickness of about 0.03 in., average fracture loads of about 1500 lb and 230 lb were obtained in axial compressive loading and eccentric loading (e.g., axial compressive loading plus a bending moment), respectively. Depending on the type of loading, the maximum stresses in the graphite substrate were calculated to be very close to its compressive or tensile fracture strength. Also studied was the effect of a variety of defects on the overall strength of the implants.

Animals↗
PubMed1976

The adhesion of thin carbon films to metallic substrates.

As part of the development of carbon-coated prosthetic devices, the adhesion of thin carbon films to metallic substrates has been studied. The bond strength of carbon films about 5000 A thick on Ti-6A1-4V and stainless steel was measured in a pull test and found to be greater than 4700 psi. Auger electron spectroscopy showed a reactive film/substrate interface. The ultimate bond strength was found to be dependent on the substrate and the deposition parameters.

Adhesiveness↗