PubMed Health⌕ Search

Biomedical subjects

H D Keith

Publications and source records attributed to H D Keith.

7 recordsLinked to original sources
PubMed1987

Hierarchical structure in polymeric materials.

The diversity of monomers available for synthesis of high polymers makes it possible to prepare a wide variety of long-chain macromolecular compounds. It is instructive to consider a hierarchical organization of structure in polymers at four successive levels--the molecular, nano-, micro-, and macrolevels--and to examine how interactions at and between these various levels of structure have important and often quite specific influences. Examples are drawn from semicrystalline polymers with flexible chains, liquid-crystalline polymers composed of rigid macromolecules, and amorphous polymers. Structural hierarchies in biocomposite systems are also discussed, particularly in soft connective tissues such as tendon and intervertebral disk.

Animals↗
PubMed1983

Testing the hydrocephalus shunt valve.

Unwanted results of shunting for hydrocephalus include slit and asymmetrical ventricles and the conversion of communicating hydrocephalus to noncommunicating hydrocephalus. Evidence suggests this may be due to a mismatch between the pressure/flow performance characteristics of the shunt valve and the pathologic hydrodynamics of the ventricular cerebrospinal fluid. Commercially available shunts are currently tested using a steady-state pressure/flow method. We have utilized a bench testing method which incorporates pulsed flow and varying compliance. Results indicate that with minimal compliance in the system, opening and closing pressures differ markedly in value with pulsed flow as compared to steady-state testing. The greater the compliance the less deviation was noted. It is recommended more effort be made to understand the value of matching pressure/flow characteristics of valve to the individual hydrocephalic state.

Biomechanical Phenomena↗
PubMed1969

Crystallization of DNA from dilute solution.

Hexagonal platelet crystals about 1 mu across and about 150 A thick have been precipitated by the addition of ethanol to solutions of ultrasonically degraded, salmon sperm DNA in sodium cacodylate buffer. X-ray powder diffraction patterns from concentrated slurries of these crystals in mother liquor show intense rings at 23.9 A and at about 3.4 A; the former arise from lateral packing and the latter from an axial periodicity of the DNA helix. Evidence is presented to indicate that native DNA helices crystallize by chain folding and that no irreversible denaturation results from the crystallization process.

Animals↗
PubMedDate not supplied

Testing of cerebrospinal fluid shunt systems under dynamic flow conditions.

There exists considerable disagreement regarding the experimental methods that are used to evaluate cerebrospinal fluid (CSF) shunts. Nearly everyone who has studied CSF shunts has attempted to define shunt characteristics by either steady pressure or steady flow-rate tests of only the valve component of the shunt. However, a valved shunt assembly must actually function in a very dynamic environment. A series of bench tests have been designed to determine the dynamic characteristics of complete shunt systems. Thirty CSF shunt assemblies, consisting of six assemblies from five different manufacturers, have been tested. The tests show that the response times for CSF shunts vary from 0.17 min to 0.46 min, and that the mean pressure maintained in a dynamic flow environment may be very different from the pressure maintained under steady flow conditions.

Cerebrospinal Fluid Shunts↗