ABSTRACT
A previous investigation by our laboratory linked cellulose acetate degradation with adverse health effects in hemodialysis patients. To establish the accumulation of degradation products with time, a Monte Carlo model of degradation kinetics was developed. The model tracks changes in a population of molecules representative of the dialyzer membrane during the degradation process. The degradation calculation is a two step process: First, the model uses a random number to select an individual polymer molecule out of the population, and then a second random number is used to identify a site on the selected molecule for the degradation reaction to occur. After the reaction calculation, the resulting degraded molecules are redistributed into the population. The course of the reaction is determined by recalculating the molecular weight averages in the changing population as the calculations proceed. The model was validated using gel permeation chromatography molecular weight results and total acetyl content measurements on dialyzers stored up to 13.3 years after manufacture. It was found that the degradation reactions can be accurately modeled as random events and that the chain scissions and deacetylation events occur at constant rates. The shelf life of these devices was estimated using the model predictions and animal test results.
Subject(s)
Cellulose , Cellulose/analogs & derivatives , Kidneys, Artificial , Membranes, Artificial , Cellulose/adverse effects , Cellulose/chemistry , Drug Stability , Humans , In Vitro Techniques , Kidneys, Artificial/adverse effects , Models, Chemical , Molecular Weight , Monte Carlo Method , Safety , Time FactorsABSTRACT
Distributions of Ca-to Ca distances have been obtained from the crystal structure of hydroxyapatite for all biologically significant planes. Most frequently, calcium ions are separated by about 4, 6.3, 7.9, and 9.0 to 9.6 A. Frequent occurrence of distances at 10.4, 11.8, and 12.6 A result from a Ca ion in one repeating unit being paired with a Ca in another unit cell.
Subject(s)
Calcium , Dental Bonding , Dental Materials , Hydroxyapatites , Tooth/anatomy & histology , Adhesiveness , Calcium Phosphates , Crystallography , Humans , Molecular ConformationABSTRACT
A procedure for generating and quantitatively comparing possible cases of epitaxy and twinning has been devised and applied to the study of epitaxy between Ca5(PO4)3OH and other calcium orthophosphates. For any two given lattices, pairs of nets which match dimensionally within prescribed limits are found and sorted in order of increasing mismatch. The crystal structural parameters are used to generate and match atomic patterns corresponding to each pair of nets. Pattern matching is done by comparing magnitudes of vectors describing the immediate environment of each atom in turn, and does not require orienting the two patterns relative to one another. Atomic charges related by each vector are also considered. Use of the vector sets introduces the limitation that twinning involving reorientation in a contact plane cannot be distinguished from no reorientation (identity match). An additional method which uses these results to match complete patterns is suggested. The procedure is general in nature and has been applied here to the study of possible epitaxies between Ca5(PO4)3OH and Ca8H2(PO4)6·5H2O, Ca4O(PO4)2, CaHPO4, CaHPO4·2H2O, Ca(H2PO4)2·H2O, ß-Ca3(PO4)2 and Ca5(PO4)2SiO4. Of these cases, only the epitaxies Ca5(PO4)3OH/Ca8H2(PO4)6·5H2O and Ca5(PO4)3OH/Ca4O(PO4)2 appear to have sufficient structural similarity to occur in practice.