A biomimetic approach for designing stent-graft structures: Caterpillar cuticle as design model

Small-Diameter Blood Vessel Substitutes: Biomimetic Approaches to Improve Patency

Small-dimeter blood vessels (<6 mm) are required in coronary bypass and peripheral bypass surgery to circumvent blocked arteries. However, they have poor patency rates due to thrombus formation, intimal hyperplasia at the distal anastomosis, and compliance mismatch between the native artery and the graft. This review covers the state-of-the-art technologies for improving graft patency with a focus on reducing compliance mismatch between the prosthesis and the native artery. The focus of this article is on biomimetic design strategies to match the compliance over a wide pressure range.

Design and computational optimization of compliance-matching aortic grafts

Introduction: Synthetic vascular grafts have been widely used in clinical practice for aortic replacement surgery. Despite their high rates of surgical success, they remain significantly less compliant than the native aorta, resulting in a phenomenon called compliance mismatch. This incompatibility of elastic properties may cause serious post-operative complications, including hypertension and myocardial hypertrophy. Methods: To mitigate the risk for these complications, we designed a multi-layer compliance-matching stent-graft, that we optimized computationally using finite element analysis, and subsequently evaluated in vitro. Results: We found that our compliance-matching grafts attained the distensibility of healthy human aortas, including those of young adults, thereby significantly exceeding the distensibility of gold-standard grafts. The compliant grafts maintained their properties in a wide range of conditions that are expected after the implantation. Furthermore, the computational model predicted the graft radius with enough accuracy to allow computational optimization to be performed effectively. Conclusion: Compliance-matching grafts may offer a valuable improvement over existing prostheses and they could potentially mitigate the risk for post-operative complications attributed to excessive graft stiffness.

Potential of auxetic designs in endovascular aortic repair: A computational study of their mechanical performance

With the rising popularity of endovascular aortic repair (EVAR) for aortic aneurysms and dissections, there is a crucial need for investigating the delayed appearance of post-EVAR complications such as stent-graft kinking, fracture and migration respectively. These complications have been noted to be influenced by the radial stiffness and bending flexibility attributes of stent-grafts. Auxetic designs with negative Poisson's ratio offer interesting advantages such as enhanced fracture toughness, superior indentation resistance and adaptive stiffness in response to intricate morphology for stenting applications over conventional stent designs. The objective of this study is to propose different auxetic stent candidates and to compare their mechanical performance with two conventional stent candidates for endovascular applications using numerical simulation through crimp/crushing tests for their radial stiffness and three-point bending/kinking tests for their flexibility, respectively. The results demonstrate that the novel hybrid auxetic designs (CRE and CSTAR) possess the best trade-off between radial stiffness and bending flexibility characteristics among all candidates for stent-graft applications.

The Technological Advancement to Engineer Next-Generation Stent-Grafts: Design, Material, and Fabrication Techniques

Endovascular treatment of aortic disorders has gained wide acceptance due to reduced physiological burden to the patient compared to open surgery, and ongoing stent-graft evolution has made aortic repair an option for patients with more complex anatomies. To date, commercial stent-grafts are typically developed from established production techniques with simple design structures and limited material ranges. Despite the numerous updated versions of stent-grafts by manufacturers, the reoccurrence of device-related complications raises questions about whether the current manfacturing methods are technically able to eliminate these problems. The technology trend to produce efficient medical devices, including stent-grafts and all similar implants, should eventually change direction to advanced manufacturing techniques. It is expected that through recent advancements, especially the emergence of 4D-printing and smart materials, unprecedented features can be defined for cardiovascular medical implants, like shape change and remote battery-free self-monitoring. 4D-printing technology promises adaptive functionality, a highly desirable feature enabling printed cardiovascular implants to physically transform with time to perform a programmed task. This review provides a thorough assessment of the established technologies for existing stent-grafts and provides technical commentaries on known failure modes. They then discuss the future of advanced technologies and the efforts needed to produce next-generation endovascular implants.

Thoracic aortic surgery: status and upcoming novelties

Several novel technological developments and surgical approaches have characterized the field of aortic surgery in the recent decade. The progressive introduction of endovascular procedures, minimally invasive surgical techniques and hybrid approaches have changed the practice in aortic surgery and generated new trends and questions. Also, the advancements in the manufacturing of tissue engineered vascular grafts as substitutes for aortic replacements are enlightening new avenues in the treatment of aortic disease. This review will provide an overview of the current novel perspectives, debates and trends in major thoracic aortic surgery.

A Comparative Study of the Efficacy by using Different Stent Grafts in Bell-Bottom Technique for the Treatment of Abdominal Aortic Aneurysm Concomitant with Iliac Artery Aneurysm

BACKGROUND

Bell-bottom technique (BBT) is one method to preserve the internal iliac artery during endovascular aneurysm repair (EVAR) for abdominal aortic aneurysm (AAA) that extends to iliac artery. The data on the efficacy of this technique are still limited. We sought to evaluate the midterm efficacy of BBT by using different stent grafts in the treatment of AAA combined with iliac artery aneurysm (IAA).

METHODS

From January 2011 to December 2016, AAA patients with IAA using BBT to preserve the internal iliac artery were retrospectively analyzed in our institution. Patients were followed up at 3, 6, and then every 12 months after surgery. The outcomes among 3 types of stent grafts (Zenith, Excluder, and Endurant) were compared. BBT-related end points including type Ib endoleak, IAA sac expansion, distal neck expansion, and rupture during follow-up were compared. Other events including perioperative death, any other types of endoleak, and corresponding management were also documented.

RESULTS

A total of 125 patients with 141 IAAs were identified. Ninety-eight patients (78.4%) with 113 lesions (80.4%) received a median follow-up time of 38 months. The incidence of type Ib endoleak was 22.9%, 8.3%, 11.9%, and 14.2% (P = 0.19) in Zenith, Excluder, Endurant group, and total patients, respectively. The incidence of IAA sac enlargement was 17.1%, 5.6%, 7.1%, and 9.7% (P = 0.20). The incidence of IAA rupture was 8.6%, 0.0%, 0.0%, and 2.7% (P = 0.03). The incidence of IAA neck enlargement was 34.3%, 13.9%, 16.7%, and 21.2% (P = 0.07). Totally, 14 cases (10.7%) received further treatment for BBT-related issues.

CONCLUSIONS

Although BBT remains a safe and effective treatment option to preserve internal iliac artery during standard EVAR with acceptable complication rates in Asians, different IAA rupture rates were found among 3 different stent grafts. Our data for the first time revealed that the type of stent grafts has influence on the final clinical outcome. Based on that, iliac extension should be selected appropriately while treating AAA-IAA.

Compliance Study of Endovascular Stent Grafts Incorporated with Polyester and Polyurethane Graft Materials in both Stented and Unstented Zones

Compliance mismatch between stent graft and host artery may induce complications and blood flow disorders. However, few studies have been reported on stent graft compliance. This study aims to explore the deformation and compliance of stent graft in stented and unstented zones under three pressure ranges. Compliance of two stent grafts incorporated with polyurethane graft (nitinol-PU) and polyester graft (nitinol-PET) materials respectively were tested; the stents used in the two stent grafts were identical. For the circumferential deformation of the stent grafts under each pressure range, the nitinol-PET stent graft was uniform in both zones. The nitinol-PU stent graft was circumferentially uniform in the stented zone, however, it was nonuniform in the unstented zone. The compliance of the PU graft material was 15 times higher than that of the PET graft. No significant difference in compliance was observed between stented and unstented zones of the nitinol-PET stent graft regardless of the applied pressure range. However, for the nitinol-PU stent graft, compliance of the unstented PU region was approximately twice that of the stented region; thus, compliance along the length of the nitinol-PU stent graft was not constant and different from that of the nitinol-PET stent graft.

Old Myths, New Concerns: the Long-Term Effects of Ascending Aorta Replacement with Dacron Grafts. Not All That Glitters Is Gold

Synthetic grafts are widely used in cardiac and vascular surgery since the mid-1970s. Despite their general good performance, inability of mimicking the elastomechanical characteristics of the native arterial tissue, and the consequent lack of adequate compliance, leads to a cascade of hemodynamic and biological alterations deeply affecting cardiovascular homeostasis. Those concerns have been reconsidered in more contemporaneous surgical and experimental reports which also triggered some research efforts in the tissue engineering field towards the realization of biomimetic arterial surrogates. The present review focuses on the significance of the “compliance mismatch” phenomenon occurring after aortic root or ascending aorta replacement with prosthetic grafts and discusses the clinical reflexes of this state of tissue incompatibility, as the loss of the native elastomechanical properties of the aorta can translate into detrimental effects on the normal efficiency of the aortic root complex with impact in the long-term results of patients undergoing aortic replacement.

Medical Textiles as Vascular Implants and Their Success to Mimic Natural Arteries

Vascular implants belong to a specialised class of medical textiles. The basic purpose of a vascular implant (graft and stent) is to act as an artificial conduit or substitute for a diseased artery. However, the long-term healing function depends on its ability to mimic the mechanical and biological behaviour of the artery. This requires a thorough understanding of the structure and function of an artery, which can then be translated into a synthetic structure based on the capabilities of the manufacturing method utilised. Common textile manufacturing techniques, such as weaving, knitting, braiding, and electrospinning, are frequently used to design vascular implants for research and commercial purposes for the past decades. However, the ability to match attributes of a vascular substitute to those of a native artery still remains a challenge. The synthetic implants have been found to cause disturbance in biological, biomechanical, and hemodynamic parameters at the implant site, which has been widely attributed to their structural design. In this work, we reviewed the design aspect of textile vascular implants and compared them to the structure of a natural artery as a basis for assessing the level of success as an implant. The outcome of this work is expected to encourage future design strategies for developing improved long lasting vascular implants.

A new design concept for knitted external vein-graft support mesh

Autologous vein-graft failure significantly limits the long-term efficacy of coronary artery bypass procedures. The major cause behind this complication is biomechanical mismatch between the vein and coronary artery. The implanted vein experiences a sudden increase (10-12 fold) in luminal pressures. The resulting vein over-distension or 'ballooning' initiates wall thickening phenomenon and ultimate occlusion. Therefore, a primary goal in improving the longevity of a coronary bypass procedure is to inhibit vein over-distension using mechanical constriction. The idea of using an external vein-graft support mesh has demonstrated sustained benefits and wide acceptance in experimental studies. Nitinol based knitted structures have offered more promising mechanical features than other mesh designs owing to their unique loosely looped construction. However, the conventional plain knit construction still exhibits limitations (radial compliance, deployment ease, flexibility, and bending stresses) which limit this design from proving its real clinical advantage. The new knitted mesh design presented in this study is based on the concept of composite knitting utilising high modulus (nitinol and polyester) and low modulus (polyurethane) material components. The experimental comparison of the new design with a plain knit design demonstrated significant improvement in biomechanical (compliance, flexibility, extensibility, viscoelasticity) and procedural (deployment limit) parameters. The results are indicative of the promising role of new mesh in restoring the lost compliance and pulsatility of vein-graft at high arterial pressures. This way it can assist in controlled vein-graft remodelling and stepwise restoration of vein mechanical homoeostasis. Also, improvement in deployment limit parameter offers more flexibility for a surgeon to use a wide range of vein diameters, which may otherwise be rendered unusable for a plain knit mesh.