Development of a Biocompatible Material for Biomedical Graft Applications

Abstract

Cardiovascular diseases are the one of the leading causes of death among humans worldwide, with the most prominent being coronary artery disease. Typically affecting older patients, the most widely used surgical intervention is coronary artery bypass grafting. This invasive procedure involves repurposing some of the patient’s other blood vessels to redirect the flow of blood to the heart muscles. Despite a clear preference for this surgery, many patients are unable to receive it due to a lack of suitable donor vessels, and those that do undergo it must also recover from the simultaneous harvesting procedure. There is high demand for artificial blood vessels suitable for use in coronary bypass surgery and other small-diameter vascular graft applications, yet so far, no viable product is available. Grafting patches and conduits already exist and are widely used for larger vessels such as the aorta, but the unique challenges inherent in small-diameter vessels – especially compliance mismatch – have prevented the same technology from scaling down.

Researchers are approaching the problem from both bioengineering and mechanical engineering viewpoints, with many competing ideas being developed. It is vital for a successful vascular graft to closely match the mechanical properties of the native blood vessel, while also allowing the body to integrate the implanted graft with no rejection. This research project investigates using varying mixtures of the synthetic biocompatible materials thermoplastic polyurethanes (TPU) and polydimethylsiloxane (PDMS) and how the mechanical properties of the composition relate to vascular grafts. Sample batches of the blended material were produced via solution mixing and tensile tests were undertaken to characterise their stress-strain behaviour. The specimens that were tested exhibit far greater elongations than found in previous literature at similar stresses. When compared to the average physiological values for maximum stress and strain of the coronary artery, a blend composed of 70% TPU, 30% PDMS, and 1 phr (part per hundred rubber) EMA was found to have the closest stress at a similar elongation (1.976 MPa compared to 1.44 ± MPa, and a maximum elongation of 1,332%. These findings show a lot of promise and prove that these blends and how they are produced are worthy of continued research.

Additionally, a preliminary investigation into stepped thickness geometry was conducted based on prior research by the project supervisor, with the intent of discovering if and how the inclusion of wider sections changes the energy absorption characteristics compared to a standard tensile profile. In general, a noticeable increase in energy absorption by stepped specimens compared to those of a uniform thickness was observed suggesting a relationship that is worth further investigation.