Growth kinetics, phase transitions, and cracking in cholesterol gallstones
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T. N. Blanton Analytical Technology Division, Eastman Kodak Company, Rochester, New York 14652-3712 (Received 20 May 1994; accepted 3 October 1994)
The growth kinetics of cholesterol gallstones have been studied by growing crystals from melted gallstones. The resulting microstructures are spherulitic which is essentially the same as the structures seen in natural gallstones prior to melting. The cholesterol crystals when observed in hot stage microscopy emerge from a unique nucleation center growing radially in the [001] direction with constant rate. The DSC thermograph of a natural gallstone is initially similar to that of cholesterol monohydrate. Upon melting, cholesterol monohydrate changes to anhydrous cholesterol; both forms are crystalline and exhibit polymorphic transformations. Synthetic stones grown from cholesterol were anhydrous and have a phase change at temperatures close to human body temperature. Optical microscopy established that this phase transformation cracks the spherulitic crystals perpendicular to the fast growth direction. Thermal expansion measurements demonstrate that upon heating, the low density, low temperature phase is transformed to a high density phase. This phase transformation and repeated cracking may prove to be useful in destroying natural gallstones, while suppressing this transformation and its associated cracking might aid in securing other solid cholesterol deposits within the human body.
I. INTRODUCTION Cholesterol is a lipid compound made from hydrocarbons. It plays an important role in human body functions. Biomaterials can be synthesized, processed, and have microstructures just as conventional materials.1'2 Biomaterials obey established materials science principles. The structure and chemistry of cholesterol are widely known.3"5 Gallstones may be classified according to their composition as cholesterol-rich, mixed, or pigment stones. Cholesterol is the major constituent of almost all gallstones found in the western hemisphere.6 Some concretions can be pulverized inside a patient's body using ultrasonic extracorporeal shock wave lithotripsy (ESWL). Gallstone lithotripsy is successful in only about 20% of the patients treated using this procedure, whereas kidney stone lithotripsy is successful in over 95% of the patients.7 The procedure of using ESWL for treating gallstone patients and an attempt to optimize this technique has necessitated the study of material properties of gallstones, especially the mechanical properties. Cholesterol can occur with two chemistries either as anhydrous or as monohydrate cholesterol. Both forms are crystalline and exhibit polymorphic transitions. 811 Anhydrous cholesterol has a polymorphic transition a few degrees below body temperature. Cholesterol mono216
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J. Mater. Res., Vol. 10, No. 1, Jan 1995
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hydrate exhibits a separate polymorphic transition at around 86 °C and a smectic liquid crystalline phase transition at about 124 °C. n It is widely believed that gallstones are made up
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