One type of implantable device is a synthetic vascular graft such as is commonly used to replace damaged or dysfunctional arterial or venous pathways, for example at the site of an aneurysm or occlusion. Bypass grafts are often used to divert blood flow around damaged regions to restore blood flow. Another use of vascular prostheses is for creating a bypass shunt between an artery and vein, specifically for multiple needle access, such as is required for hemodialysis treatments. Following multiple percutaneous invasions into a vein, the vein may either collapse along the puncture track or become aneurysmal, leaky or fill with clot, causing significant risk of pulmonary embolization. Vascular prostheses have been used for many years as an alternative to patients' own veins for vascular access during hemodialysis.
Materials reach has led to the development of some synthetic materials for use in vascular prostheses. One example is polytetrafluoroethylene (PTFE), a porous organic material which can be expanded or stretched to a specific length and thickness. When thus expanded, the material consists of a network of interrelated nodes and fibrils. The diameters of the fibrils vary depending upon the conditions and rate at which the PTFE is stretched. An advantage of PTFE is that the diameters of the fibrils can be made much smaller than the diameters of fibers of knitted or woven fabrics which have been used for vascular prostheses. Moreover, the pore diameter and porosity of PTFE tubing can be adjusted to reduce the occurrence of thrombosis associated with vascular prostheses.
One drawback of PTFE tubing, however is that because of the relative elasticity of porous PTFE tubing, it is difficult to elicit natural occlusion of suture holes in vascular prostheses made from expanded PTFE tubing. As a result, blood cannot be withdrawn from a PTFE vascular graft until the graft has become surrounded by fibrotic tissue. This generally occurs within a minimum of 14 days after surgery.
Other materials which have been used to form prosthetic vascular grafts include autologous saphenous vein, Dacron.RTM. brand polyester fiber or other synthetic polyester fiber, modified bovine or carotid xenograft and human umbilical vein. None of these materials, however, overcomes the problems associated with early puncture of the graft.
Schanzer in U.S. Pat. No. 4,619,641 describes a two-piece coaxial double lumen arteriovenous graft. The Schanzer graft consists of an outer tube positioned over an inner tube, the space between being filled with a self-sealing adhesive. The configuration of this coaxial tube greatly increases the girth of the graft, and limits the flexibility of the lumen which conducts blood flow.
It is, therefore, an object of the invention to provide a vascular graft having a self-sealing capability. It is another object of the invention to provide such a graft that is easily manufactured.