Summary of Current Coronary Artery Disease Procedures
Coronary artery disease (CAD), the leading cause of death in the US, is caused by cholesterol deposits accumulating in the arterial wall and limiting blood flow (Brown et al.). Currently, there are two types of procedures for CAD: angioplasty and bypass graft surgery. In angioplasty, the surgeon uses a reticular metal stent to support the arterial walls at the area of blockage. As a result, the stent can only protect the area of blockage; other parts of the coronary artery remain at risk. Furthermore, angioplasty is not suitable for many patients because “1) the coronary artery is too small or 2) there is a complete blockage that cannot be crossed with the balloon” (Michaels and Chatterjee). Coronary artery bypass surgery is an open-heart surgery in which the surgeon attaches the ends of a graft above and below the area of blockage. By using a healthy human blood vessel or an anti-thrombogenic material for the grafts, the surgery eliminates CAD’s chance of recurrence. It is also appropriate for most CAD cases. A heart-lung bypass machine is usually needed to stop the patient’s heart and conduct surgery safely; the off-pump method is suitable for some but more challenging (Lawton).
Diagrams for percutaneous coronary intervention (angioplasty) and coronary artery bypass grafting (Doenst et al.).
Traditional bypass grafts are either small-diameter human blood vessels or synthetic tubes. Surgeons prefer autologous human blood vessels—the saphenous vein, the internal thoracic artery, etc.—because they have nearly the same structure and properties as a coronary artery. However, cutting a blood vessel may cause pain and derived diseases. Vein grafts also have poor long-term patency because of potential clogging as it thickens Meanwhile, the use of allogenic grafts greatly depends on the availability and ABO match (Valentini et al.).
On the other hand, synthetic grafts have “unlimited availability and consistent quality and patency” (“Role of prosthetic conduits in coronary artery bypass grafting”). They also “[eliminate] the donor-site morbidity” (Jia et al.). Nevertheless, they are known for short longevity since blood flow is directly exposed to the graft’s inner surface; the synthetic material’s surface thrombogenicity can create another blockage (Desai et al.). Materials are shaped into vessels similar in thickness and diameter to the coronary artery. This report researches two typical synthetic materials, polytetrafluoroethylene (PTFE or Teflon) and polyethylene terephthalate (PET or Dacron). Both materials are biocompatible, strong, and flexible.
Biomedical engineers have recently developed several hybrid designs, or designs combining tissues with synthetic scaffolds; they introduced new materials for grafting or scaffolding, such as polyglycolic acid (PGA) and poly(l-lactide-co-ε-caprolactone) (PLCL). The material is printed into a porous vascular structure and later seeded with human cells. The cells grow on the scaffold and eventually form a surface mimicking that of the coronary artery. After the graft is implanted in vivo, the material will degrade as new tissues grow. Because the designs incorporate natural human cells, they are expected to promote tissue regeneration, improve the graft’s interaction with other body tissues, and offer long-term stability. Potential drawbacks include high production complexity and high costs. Hybrid designs are yet to be standardized, regulated, and clinically adopted.
“The process of producing a tissue engineered vascular graft through scaffold-based method” (Wee and Langer).
Evaluation Matrix
The evaluation matrix below displays the properties of the four synthetic materials—PTFE, PET, PGA, and commercial PLCL. The second column is the relative weight of each property, or how important the property is to the success of a coronary artery bypass graft, from 1 to 10. The ultimate tensile strength and modulus of elasticity have weights of 10 because they determine the graft’s ability to accommodate the blood pressure. Longevity and infection, which have a weight of 9, ensure that the graft maintains its mechanical integrity and does not expose the patient to more diseases. The melting point reflects the difficulty of printing and modifying the material and is not directly related to the graft’s performance in vivo, so it has a weight of 5. The material’s mechanical performance and the patient’s health are much more important than its cost; hence, the cost of the graft is assigned a weight of 4.
According to the evaluation matrix, commercial PLCL has the highest sum of weighted products; therefore, it is the best material for a coronary artery bypass graft. To create a scaffold as light as possible, engineers must select biomaterials with low density. PLCL has the lowest density among all materials. If multiplying the densities by the normal volume of a coronary artery, the resulting mass of a PLCL scaffold will be 6 to 9 times smaller than that of other grafts or scaffolds. It is noteworthy that the final hybrid-design graft may have a different density since the PLCL scaffold is seeded with human cells. PLCL has an ultimate tensile strength—16.1 MPa (Jeong)—most similar to that of a healthy human coronary artery—0.5-3MPa (Enis et al.). Its modulus of elasticity is 0.012 GPa (Jeong) compared to the healthy human coronary artery’s 0.0015 GPa (Wang et al.). As a highly elastic material, PLCL can accommodate a wide range of pressure well, yet it may cause instability because of deformation. PLCL has a melting point of 175°C, lower than other materials and thus easier to print and modify. It has an ideal resistance against infection. In an actual hybrid-design graft, building a “[PLCL] bilayer membrane composed of a porous layer and a compact layer” can effectively prevent bacterial penetration (Abe et al.). It degrades slowly, providing enough time for new tissues to grow and mature. It does not release toxins or carcinogens during degradation. Though both the material and the tissue engineering technology are very expensive, PLCL’s properties are very suitable for a strong and elastic graft safe for the human body.
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