Cardiovascular diseases are the primary cause of death globally, accounting for approximately 32% of all deaths [
1,
2]. Among these, coronary artery disease (CAD) is one of the most prevalent cardiovascular diseases, resulting in substantially more deaths than cancer, respiratory diseases, and diabetes, thus obtruding a major health and economic burden on most developed nations. The deposition of fatty substances, cholesterol, cellular waste materials, calcium, and fibrin on the walls of blood arteries causes CAD. It subsequently advances towards plaque formation and blockage of blood vessels, which is termed as atherosclerosis. The blockage of coronary arteries due to the deposition of fatty tissue developed on the walls of arteries results in reduced blood flow, thus causing damage to heart muscles. This also results in hypertension further accompanied by angina in patients. Several risk factors such as age, obesity, smoking, sedentary lifestyle, and diet as well as existing disease conditions like diabetes, non-alcoholic fatty liver disease and hypertension contribute towards this pathological condition. Evidence also suggests that changes in blood parameters including increased levels of triglyceride (normal range <150 mg/dL) and low-density lipoprotein (LDL; (normal range <100 mg/dL)) with decreased levels of high-density lipoprotein (HDL; normal range >40 mg/ml) [
3] can also make a person more prone to developing CAD [
4].
The typical anatomy of blood vessel involves three layers: intima (innermost), media (middle layer) and adventitia (outermost) [
5] as described in
Figure 1A. With the onset of atherosclerosis, the intima gets damaged, due to the plaque formation, leading to complications in blood flow through the vessels [
6]. In a clinical setup, coronary artery blockages are predominantly treated by an invasive coronary artery bypass graft (CABG) surgery, in which an alternative artery or a vein from the body is used to bypass the blocked part to resume proper circulation [
7]. However, there are certain disadvantages associated with CABG that include harvesting of blood vessels from the body, requirement of healthy patent conduits, degeneration of these grafts over time and requirement of longer recovery time [
8]. The current gold standard treatment approach is percutaneous coronary intervention (PCI), which is a minimally invasive surgical procedure to treat stenosis (abnormal narrowing of blood vessels). The procedure relies on the use of a tiny catheter with a folded balloon on its tip which is threaded through the blocked artery. On reaching the blocked site, the balloon is inflated to compress the plaque against the wall of the artery (balloon angioplasty) [
9]. In recent years, there have been modifications made to the surgical procedure that involve the incorporation of stents. These stents have proven to be effective in reducing the occurrence of abrupt vessel closure, preventing the re-accumulation of plaque and restenosis when compared to balloon angioplasty. Consequently, this has resulted in a decrease in the rate of target lesion revascularization (TLR) [
10]. A stent is a mesh-type structure that is loaded over a guidewire, delivered via a balloon catheter to the site of the blocked artery and deployed at the required site, as shown in
Figure 1B. Different types of stents that are presently available in the market/clinics/research are categorized as 1) bare metal stent, 2) drug-eluting stent, 3) bioengineered stent, 4) bio-resorbable vascular scaffold (BVS), and 5) dual therapy stent. Some of the earliest stents were fabricated using metals (mostly stainless steel), however, they were associated with restenosis, i.e., re-narrowing of the blood vessels after a certain period [
11]. This was mainly due to the migration and over-proliferation of smooth muscle cells from the intima layer to the blood vessel lining. [
6] Further, they were also associated with an immunological response by the body and discomfort of having a metal implant placed inside the artery for lifetime. To combat the host immune response and reduce restenosis, new generation stents incorporated/coated with drug molecules alone or drug molecules encapsulated within a polymeric material were developed (known as drug-eluting stents; DES). Such a method resulted in controlling the neointimal proliferation and reducing restenosis [
12]. However, with time, the coating of biodegradable polymer and drug was seen to wear off from the metal surface, leaving behind bare metal stent (BMS) which had its own disadvantages as described above [
13]. BVS is a non-metallic mesh tube that looks like a stent but progressively dissolves once the blocked artery can function normally. It became popular to overcome the issues with metallic stents. Such stents/scaffolds are made of resorbable polymers or metals that will degrade within a definite time point and hence will not remain in the body for lifetime. On the other hand, bioengineered stents enabled the supply of such polymeric stents along with cells for a better healing process. Dual therapy stent is one of the advanced versions of stents in which, along with aspirin, an antiplatelet medicine like clopidogrel, prasugrel or ticagrelor is given to the patient to control thrombosis [
14]. While designing most of the advanced and new generation bioresorbable stents, there are certain critical requirements which must be fulfilled. These are radial strength, thinner struts, biocompatibility, radio-opacity for X-ray and magnetic resonance imaging (MRI) visualization, acute and chronic recoil resistance, deliverability, lower crossing profile and long-term integrity [
15]. Further, some design considerations that need to be examined include the degradation and drug elution profile (incorporated within the stents), biocompatibility of degradable/non-degradable by-products, shelf-life, deterioration in the mechanical properties and resorption time of the stent [
16]. In lieu of the recent advancement in stent manufacturing and designing techniques, use of advanced manufacturing/ 3D printing technology has emerged as a new direction that enables customized patient-specific implant fabrication.
This review gives a comprehensive aspect on the state-of-the-art of the techniques that have translational potential for implant fabrication in treating cardiovascular diseases. It also considers the advancement in material design which is crucial for the fabrication of bioresorbable polymeric stents. We have described the studies involving different types of automated manufacturing technologies encompassing 3D printing, which are currently being experimented on by researchers to develop better quality stents. These techniques could reduce the time from bench to bedside and become a feasible option in the clinical setup.
Figure 1.
A) Architecture of blood vessel. Endothelial cells positioned in a single layer on subendothelial extracellular matrix and internal elastic lamina make up the tunica intima. The external elastic lamina, extracellular matrix, and smooth muscle cells make up the tunica media. Fibroblasts and stem/progenitor cells are distributed throughout the connective tissue that makes up the tunica adventitia. B) Procedure for percutaneous coronary intervention. It employs a catheter (a thin flexible tube) to place a stent to open-up blood vessels. The balloon is initially inflated, followed by the expansion of the stent, which pushes the plaque against the artery wall. After the stent has been successfully implanted, the balloon is deflated and removed, leaving the stent to keep open the artery.