Materials for cardiovascular devices

Cardiovascular devices have several requirements for the materials used in their fabrication . In order to fabricate an economical, safe and effective device, chemical, physical and biological properties of the material must be carefully evaluated along with its processability characteristic. Materials utilized up to date in cardiovascular devices come from the broad range of commercially available materials, but they have to be categorically tested for being acceptable for use in contact with blood. Application for these materials include extracorporeal devices, catheter and tubing which are inserted into a blood vessel, or devices which can be permanently implanted. Specifically, these biomedical materials include polymers such as polyethyeneterephthalate and polytetrafluoroethylene used in vascular grafts, biodegradable polymers such as polylactic/glicolic acid copolymers used as experimental vascular grafts, biologically derived materials such as glutaraldehyde tanned porcine heart valves for heart valve prostheses, bioderived macromolecules such as aldehyde tanned collagen used as a biodegradable coating on textile vascular grafts, passive coating such as hydrogels used to increase lubricity of catheters, bioactive coating such as bound heparin to reduce the thrombosis on catheters, and carbons such as pyrolitic carbon used as a heart valve component. Another large family of materials that show interesting properties for biomedical applications is that of polyurethanes. These materials are biocompatibile and do not present any undesirable reaction with the biological fluids. Polyurethanes have been used in diverse biomedical applications such as endotracheal tubing, vascular prosthesis, cardiac assisting devices and aorto-coronary by-bass, artificial cardiac valve, pace-maker insulators, roller blood pump tubing for the artificial heart, breast implants and dialysis membranes. After an overview of the general properties of the materials for cardiovascular devices, this talk will be restricted to the properties of the materials for vascular grafts with particular emphasis on the small-diameter vascular grafts area. We will also illustrate our own approach to vascular grafts fabrication from polyurethanes which relies on a spray technique associated to a phase-inversion effect of a polymer solution. The principle, which is at the base of this process, is to use a thermodynamically unstable synthetic polymer solution to produce spongy tubular membranes through the deposition of polymer layers of controlled porosity onto a rotating mandrel. As a result of this unique material processing, membranes display a tridimensional, interconnected filamentous porous structure with a hydrophilic behavior. Structural properties of these membranes are such that display a very open luminal surface and a high wall porosity, while the entire membranes show low hydraulic permeability (HP). The HP measured collecting in the first minute the water passing through the membrane wall under a head pressure of 120 mmHg resulted 39 +/- 8 ml/min/cm2 afterwards HP reduced functioning of time. This is attributed to the membrane wall structure which is compressible and, therefore, adapts dynamically to varations of luminal pressure (LP). By varying some of the parameter of the "spraying, phase-inversion" technique the fine gel-like structure of the tubular membranes can be widely varied, that is membranes can be fabricated with wall structure features different from those of the luminal surface. In addition this techinque allows us to prepare materials incorporating bioactive peptides, for instance endothelial cell growth factors, which will be slowly released to stimulate cell proliferation.