Prostheses are impregnated with gelatin, Chitosan and collagen, which decrease the permeability . Preclotting of coated prostheses have been advocated for many years for their non-porous surface which minimizes blood loss of the patients. Together with the facts that such prostheses are readily available, soft, pliable and easy handle, have extended their use particularly for repair of complex con genital cardiac. (102) At least three layers of collagen ,mixed with a plasticizer, are applied to a tubular graft , so the graft become blood – tight without the need for preclotting , The collagen is preferably obtained from bovine skin which has been processed by acid digestion to give a fibril dispersion of high purity . An aqueous purified collagen slurry including a plasticizer is applied to the graft by massage to cover the entire surface area , this is claimed to provide a flexible graft with good handle .After the required number of coatings, the collagen is parallel – linked by exposure to formaldehyde vapor , porosity is reduced to less than about 1% before coating . The collagen which is produced by this process is in the form of a white homogenous smooth paste ,which is added to a plasticizer such as glycerin, or other biologically acceptable material .Collagen slurry contains between about 5 and 50% by weight of collagen, the plasticizer will be between 4 and 12% and from 10 to 25% ethanol which may be present to hasten the evaporation of water . To apply the coating the slurry is infected into the lumen of the graft and massaged manually to ensure that entire inner surface is covered , any excess of slurry is removed through one of the open ends ,the grafts are then left to dry for about half an hour at room temperature . The coating and drying processes are repeated at least three times.
The coating is parallel- linked by exposure to formaldehyde vapor for five minutes, the graft is air dried for 15 minutes and then vacuum dried for 24 hours to remove moisture and any excess formaldehyde .A test for blood tightness of a coated graft compared with one that had no collagen coating it was found that porosity of the coated prosthesis was ,22 ml/minutes . cm2 (considered as zero being within limits of experimental error), while for the uncoated fabric was 36 ml / minutes . cm2 , as well as a reduced porosity collagen coated vascular grafts prepared in accordance with this invention ,exhibit reduced thrombogenicity compared with coated grafts. (103) besides porosity, good handling and suturing characteristics satisfactory healing (rapid tissue growth) , mechanical and chemical stability, good tensile strength and resistance to deterioration are the major requirements of vascular grafts. Since vasculars are subjected to static pressure and repeated stress of pulsation in application, they should have good dilation and creep resistance. Crimp in grafts is for handling and length sizing, crimped geometry improves the grafts bending ability without kinking. All the grafts being the highest , composite grafts structures which are obtained by impregnation with biodegradable compounds, resulted in stiffer structure with less elasticity and externally supported grafts exhibited good dimensional stability . (4)