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biodegradable polymers:
several biodegradable polymers such as polylactide (pla), polyglycolide (pga), poly(glycolide-co-lactide) (plga), poly(dioxanone), poly(trimethylene carbonate), poly(carbonate), and so on have been used extensively or tested on a wide range of medical applications due to their good biocompatibility, controllable biodegradability, and relatively good processability. pla, pga, and plga are bioresorbable polyesters belonging to the group of poly ?-hydroxy acids. these polymers degrade by nonspecific hydrolytic scission of their ester bonds.
-the hydrolysis of pla yields lactic acid, which is a normal byproduct of anaerobic metabolism in the human body and is incorporated in the tricarboxylic acid (tca) cycle to be finally excreted by the body as carbon dioxide and water.
-pga biodegrades by a combination of hydrolytic scission and enzymatic (esterase) action producing glycolic acid which either can enter the tca cycle or is excreted in urine and can be eliminated as carbon dioxide and water.
surface modifications for improving biocompatability:
prevention of thrombus formation is important in clinical applications where blood is in contact such as hemodialysis membranes and tubes, artificial heart and heart–lung machines, prosthetic valves, and artificial vascular grafts. in spite of the use of anticoagulants, considerable platelet deposition and thrombus formation take place on the artificial surfaces.
heparin, one of the complex carbohydrates known as mucopolysaccharides or glycosaminoglycan, is currently used to prevent formation of clots. in general, heparin is well tolerated and devoid of serious consequences. however, it allows platelet adhesion to foreign surfaces and may cause hemorrhagic complications such as subdural hematoma, retroperitoneal hematoma, gastrointestinal bleeding, hemorrhage into joints, ocular and retinal bleeding, and bleeding at surgical sites. these difficulties give rise to an interest in developing new methods of hemocompatible materials.
many different groups have studied immobilization of heparin on the polymeric surfaces, heparin analogs and heparin–prostaglandin or heparin–fibrinolytic enzyme conjugates. the major drawback of these surfaces is that they are not stable in the blood environment. it has not been firmly established that a slow leakage of heparin is needed for it to be effective as an immobilized antithrombogenic agent if not, its effectiveness could be hindered by being “coated over” with an adsorbed layer of more common proteins such as albumin and fibrinogen.
albumin-coated surfaces have been studied because surfaces that resisted platelet adhesion in vitro were noted to adsorb albumin preferentially .fibronectin coatings have been used in in vitro endothelial cell seeding to prepare a surface similar to the natural blood vessel lumen . also, algin-coated surfaces have been studied due to their good biocompatibility and biodegradability.
recently, plasma gas discharge and corona treatment with reactive groups introduced on the polymeric surfaces have emerged as other ways to modify biomaterial surfaces.
hydropinghobic coatings composed of silicon- and fluorine-containing polymeric materials as well as polyurethanes have been studied because of the relatively good clinical performances of silastic, teflon, and polyurethane polymers in cardiovascular implants and devices. polymeric fluorocarbon coatings deposited from a tetrafluoroethylene gas discharge have been found to greatly enhance resistance to both acute thrombotic occlusion and embolization in small diameter dacron grafts.
hydropinghilic coatings have also been popular because of their low interfacial tension in biological environments. hydrogels as well as various combinations of hydropinghilic and hydropinghobic monomers have been studied on the premise that there will be an optimum polar-dispersion force ratio which could be matched on the surfaces of the most passivating proteins. the passive surface may induce less clot formation. polyethylene oxide coated surfaces have been found to resist protein adsorption and cell adhesion and have therefore been proposed as potential “blood compatible” coatings.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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