Development and preclinical evaluation of a biodegradable ventricular septal defect occluder

Progress of biodegradable polymer application in cardiac occluders

Cardiac septal defect is the most prevalent congenital heart disease and is typically treated with open-heart surgery under cardiopulmonary bypass. Since the 1990s, with the advancement of interventional techniques and minimally invasive transthoracic closure techniques, cardiac occluder implantation represented by the Amplazter products has been the preferred treatment option. Currently, most occlusion devices used in clinical settings are primarily composed of Nitinol as the skeleton. Nevertheless, long-term follow-up studies have revealed various complications related to metal skeletons, including hemolysis, thrombus, metal allergy, cardiac erosion, and even severe atrioventricular block. Thus, occlusion devices made of biodegradable materials have become the focus of research. Over the past two decades, several bioabsorbable cardiac occluders for ventricular septal defect and atrial septal defect have been designed and trialed on animals or humans. This review summarizes the research progress of bioabsorbable cardiac occluders, the advantages and disadvantages of different biodegradable polymers used to fabricate occluders, and discusses future research directions concerning the structures and materials of bioabsorbable cardiac occluders.

4D Printing of Overall Radiopaque Customized Bionic Occlusion Devices

Percutaneous closure of ventricular septal defect (VSD) can effectively occlude abnormal blood flow between ventricles. However, commonly used Nitinol occlusion devices have non-negligible limitations, such as nondegradability leading to life-threatening embolization; limited device size predisposing to displacement and wear; only a few radiopaque markers resulting in inaccurate positioning. Nevertheless, the exploration of customized, biodegradable, and overall radiopaque occluders is still vacant. Here, overall radiopaque, biodegradable, and dynamic reconfigurable 4D printed VSD occluders are developed. Based on wavy bionic structures, various VSD occluders are designed and manufactured to adapt to the position diversity of VSD. The customized configuration, biocompatibility, and biodegradability of the developed 4D printed bionic occluders can eliminate the series of complications caused by traditional occluders. The overall radiopacity of 4D printed VSD occluders is validated ex vivo and in vivo, whereby accurate positioning can be assured. Notably, the preparation strategies for 4D printed occluders are scalable, eliminating the barriers to mass production, and marking a meaningful step in bridging the gap between modeling and clinical application of 4D printed occlusion devices. This work opens attractive perspectives for the rapid manufacturing of customized intelligent medical devices for which overall radiopacity, dynamic reconfigurability, biocompatibility, and biodegradability are sought.

Favorable mid-term performance of fully biodegradable implantable device for ventricular septal defect closure

Objectives

To assess the mid-term safety and efficacy of transthoracic perimembranous ventricular septal defect (Pm-VSD) closure using a new biodegradable device. Implantation entailed right subaxillary minithoracotomy under transesophageal echocardiography guidance.

Methods

Between October 2019 and January 2020, 13 patients (males, 5; mean age, 3.6 ± 2.5 years) with Pm-VSDs underwent transthoracic device closures at Zhengzhou University Central China Fuwai Hospital as described previously. Delivery pathways were established by manipulating a hollow probe from right atrium through tricuspid valve to right ventricle and then through VSDs to left ventricle, whereupon installation took place.

Results

All occluder implantations were successfully executed. Mean defect size was 4.1 ± 1.0 mm, and mean device waist size was 5.2 ± 1.1 mm. One patient (7.7%) with 1.5-mm residual shunt showed complete closure at discharge. There was 1 instance of postoperative incomplete right bundle branch block, which converted to complete right bundle branch block at month 1. During patient follow-up (mean, 24.6 ± 0.8 months), no device dislocations, new residual shunts, new valvular regurgitation, or detectable atrioventricular block ensued.

Conclusions

Closure of Pm-VSDs using a novel, fully biodegradable occluder in the manner described has proven safe and effective at mid-term follow-up. Long-term safety and efficacy of this device must be further corroborated in a large patient cohort going forward.

A Finite Element Investigation on Material and Design Parameters of Ventricular Septal Defect Occluder Devices

Background and Objective: Ventricular septal defects (VSDs) are the most common form of congenital heart defects. The incidence of VSD accounts for 40% of all congenital heart defects (CHDs). With the development of interventional therapy technology, transcatheter VSD closure was introduced as an alternative to open heart surgery. Clinical trials of VSD occluders have yielded promising results, and with the development of new material technologies, biodegradable materials have been introduced into the application of occluders. At present, the research on the mechanical properties of occluders is focused on experimental and clinical trials, and numerical simulation is still a considerable challenge due to the braided nature of the VSD occluder. Finite element analysis (FEA) has proven to be a valid and efficient method to virtually investigate and optimize the mechanical behavior of minimally invasive devices. The objective of this study is to explore the axial resistive performance through experimental and computational testing, and to present the systematic evaluation of the effect of various material and braid parameters by FEA. Methods: In this study, an experimental test was used to investigate the axial resistive force (ARF) of VSD Nitinol occluders under axial displacement loading (ADL), then the corresponding numerical simulation was developed and compared with the experimental results to verify the effectiveness. Based on the above validation, numerical simulations of VSD occluders with different materials (polydioxanone (PDO) and Nitinol with different austenite moduli) and braid parameters (wire density, wire diameter, and angle between left and right discs) provided a clear presentation of mechanical behaviors that included the maximal axial resistive force (MARF), maximal axial displacement (MAD) and initial axial stiffness (IAS), the stress distribution and the maximum principal strain distribution of the device under ADL. Results: The results showed that: (1) In the experimental testing, the axial resistive force (ARF) of the tested occluder, caused by axial displacement loading (ADL), was recorded and it increased linearly from 0 to 4.91 N before reducing. Subsequent computational testing showed that a similar performance in the ARF was experienced, albeit that the peak value of ARF was smaller. (2) The investigated design parameters of wire density, wire diameter and the angle between the left and right discs demonstrated an effective improvement (7.59%, 9.48%, 1.28%, respectively, for MARF, and 1.28%, 1.80%, 3.07%, respectively, for IAS) for the mechanical performance for Nitinol occluders. (3) The most influencing factor was the material; the performance rose by 30% as the Nitinol austenite modulus (EA) increased by 10,000 MPa. The performance of Nitinol was better than that of PDO for certain wire diameters, and the performance improved more obviously (1.80% for Nitinol and 0.64% for PDO in IAS, 9.48% for Nitinol and 2.00% for PDO in MARF) with the increase in wire diameter. (4) For all of the models, the maximum stresses under ADL were distributed at the edge of the disc on the loaded side of the occluders. Conclusions: The experimental testing presented in the study showed that the mechanical performance of the Nitinol occluder and the MARF prove that it has sufficient ability to resist falling out from its intended placement. This study also represents the first experimentally validated computational model of braided occluders, and provides a perception of the influence of geometrical and material parameters in these systems. The results could further provide meaningful suggestions for the design of biodegradable VSD closure devices and to realize a series of applications for biodegradable materials in VSD.

Finite Element Analysis and Bench Testing of Ventricular Septal Defect Occluder

Complications after transcatheter closure of the ventricular septal defect (VSD) is significantly associated with the mechanical behaviour of the VSD occluder. This study aims to investigate the effect of structural parameters of the VSD occluder. A mechanical model of the VSD occluder was constructed by theoretical modelling. The mechanical properties of the VSD occluders with different braiding angles (30°, 45°, 60°), materials (nitinol (NiTi), polydioxanone (PDO)) and waist-heights (3 mm, 4 mm) were analysed and validated by bench tests. For the 30°NiTi, 45°NiTi, 60°NiTi and 45°PDO occluders, the bending angles at the waist under 1 mm radial shrinkage were 112°, 121°, 155° and 155°, respectively. And the maximum principal strains at the waist were 16.62%, 8.19%, 1.20%, and 0.66%, respectively. The maximum radial deformations with 0.5 rad axial bending at the waist were 1.73, 1.44, 0.41 and 1.68 mm, respectively. When the occluders were implanted into VSD with the mean thickness of 3.5 mm, high stress appeared at the margin and the contact area, and the area with the 3-mm-occluder was much larger. In conclusion, the 60°NiTi occluder showed better ability to fit the deformation of the defect than the other NiTi occluders, and the 45°PDO occluder performed better under compression conditions but poorly under bending conditions than the 45°NiTi occluder. The choice of the appropriate waist-height is beneficial to eliminate associative complication by reducing the contact stress.

Recent developments in next-generation occlusion devices

Transcatheter closure has been widely accepted as a highly effective way to treat abnormal blood flows and/or embolization of thrombus in the heart. It allows the closure of four types of congenital heart defects (CHDs) and stroke-associated left atrial appendage (LAA). The four types of CHDs include atrial septal defect (ASD), patent foramen ovale (PFO), patent ductus arteriosus (PDA), and ventricular septal defect (VSD). Advancements in the materials and configurations of occlusion devices have spurred the transition from open-heart surgery with high complexity and morbidity, or lifelong medication with a high risk of bleeding, to minimally invasive deployment. A variety of occlusion devices have been developed over the past few decades, particularly novel ones represented by biodegradable and 3D-printed occlusion devices, which are considered as next-generation alternatives to conventional Nitinol-based occlusion devices due to biodegradability, customization, and improved biocompatibility. The aim here is to comprehensively review the next-generation occlusion devices in terms of materials, configurations, manufacturing methods, deployment strategies, and (if available) experimental results or clinical data. The current challenges and the direction of future work are also proposed. STATEMENT OF SIGNIFICANCE: Implantation of occlusion devices has become a widely accepted and highly effective treatment for occluding abnormal blood/thrombus flow within the heart. Due to the serious complications such as erosion and displacement of conventional Nitinol-based occluders, next-generation occluders with reduced risk of complications and improved biocompatibility has emerged. Here, we comprehensively review the next-generation occluders developed for atrial septal defect (ASD), patent foramen ovale (PFO), patent ductus arteriosus (PDA), ventricular septal defect (VSD), and left atrial appendage (LAA), with special emphasis on biodegradable occluders. Besides, intelligent materials (e.g., automatically deployable shape memory polymers) and rapid customized manufacturing methods (3D/4D printing) for the fabrication of occluders are also introduced. Lastly, the directions of future work are highlighted.

Key Regulatory Differentially Expressed Genes in the Blood of Atrial Septal Defect Children Treated With Occlusion Devices

Atrial septal defects (ASDs) are the most common types of cardiac septal defects in congenital heart defects. In addition to traditional therapy, interventional closure has become the main treatment method. However, the molecular events and mechanisms underlying the repair progress by occlusion device remain unknown. In this study, we aimed to characterize differentially expressed genes (DEGs) in the blood of patients treated with occlusion devices (metal or poly-L-lactic acid devices) using RNA-sequencing, and further validated them by qRT-PCR analysis to finally determine the expression of key mediating genes after closure of ASD treatment. The result showed that total 1,045 genes and 1,523 genes were expressed differently with significance in metal and poly-L-lactic acid devices treatment, respectively. The 115 overlap genes from the different sub-analyses are illustrated. The similarities and differences in gene expression reflect that the body response process involved after interventional therapy for ASDs has both different parts that do not overlap and the same part that crosses. The same portion of body response regulatory genes are key regulatory genes expressed in the blood of patients with ASDs treated with closure devices. The gene ontology enrichment analysis showed that biological processes affected in metal device therapy are immune response with CXCR4 genes and poly-L-lactic acid device treatment, and the key pathways are nuclear-transcribed mRNA catabolic process and proteins targeting endoplasmic reticulum process with ribosomal proteins (such as RPS26). We confirmed that CXCR4, TOB1, and DDIT4 gene expression are significantly downregulated toward the pre-therapy level after the post-treatment in both therapy groups by qRT-PCR. Our study suggests that the potential role of CXCR4, DDIT4, and TOB1 may be key regulatory genes in the process of endothelialization in the repair progress of ASDs, providing molecular insights into this progress for future studies.

An innovative occluder for cardiac defect: 3D printing and a biocompatibility research based on self-developed bioabsorbable material-LA-GA-TMC

This study adopted the latest self-developed bioabsorbable material lactide-glycolide-1,3-trimethylene carbonate (LA-GA-TMC) and applied the three-dimensional (3D) printing technique to manufacture the occluder for cardiac septal defects, so as to realize the individualized treatment of cardiac septal defects. At the same time, its biosafety was evaluated, with an aim to establish foundation for futural large-scale animal experiment and clinical trial. The traditional "one-pot synthesis" was modified, and the "two-step synthesis method" was utilized to synthesize the LA-GA-TMC terpolymer at the lactide: glycolide: trimethylene carbonate ratio of 6:1:1.7. Afterward, the synthesized terpolymer was used as the raw material to fabricate the occluder model via using 3D printing technique. Then, its biocompatibility was comprehensively evaluated through cytocompatibility, blood compatibility, and histocompatibility. The occluder made from LA-GA-TMC 3D printing had favorable ductility and recoverability; besides, it possessed the temperature-control feature, and the relative cell proliferation rates in extract liquids at various concentrations were all >70%, suggesting that it had favorable cytocompatibility. Moreover, hemolytic experiment revealed that its hemolytic rate was <5%, dynamic blood coagulation experiment demonstrated that the sample material moderately activated the blood coagulation, and the above findings suggested that it had good blood compatibility. In addition, implanting experiment in vivo revealed that its histocompatibility was superior to the traditional nitinol and the emerging poly-l-lactic acid. It is completely feasible to manufacture the cardiac septal defects occluder based on the novel absorbable material LA-GA-TMC, which has favorable biocompatibility, through 3D printing technique and it possesses broad prospects in large-scale animal experiment and clinical trial.

A novel biodegradable occluder for the closure of ventricular septal defects: immediate and medium-term results in a canine model

OBJECTIVES

The feasibility of an Amplatzer septal occluder closure of ventricular septal defects (VSDs) under echocardiographic guidance has already been proven. However, the technique is not used routinely owing to high-rate atrioventricular blocks and the non-absorbability of the occluder. Here, we aimed to evaluate the safety, biocompatibility and effectiveness of a new biodegradable occluder.

METHODS

A total of 18 adult beagle dogs [mean body weight 8.85 kg (range 8.10-10.40 kg)] were enrolled in our study from March 2015 to July 2018. VSD animal models were prepared by thoracic intervention, and the new bioabsorbable occluder was implanted under the guidance of transthoracic echocardiography. Laboratory examinations, transthoracic echocardiography and electrocardiography were performed after surgery, and pathological samples from dogs were obtained after euthanasia and examined in the 1st, 3rd, 6th, 9th, 12th and 24th months to evaluate the safety, biocompatibility and effectiveness of the biodegradable occluder.

RESULTS

All the dogs received successful implantations of the biodegradable occluder and survived, showing no follow-up-related complications (such as dislocation, arrhythmia and recanalization). Biochemical tests showed no significant abnormalities or differences, except for an increase in white blood cells and C-reactive proteins on the day after the operation. Histopathological examinations revealed that the bioabsorbable occluders were biocompatible, while immunohistochemical evaluations and electron microscopy showed endothelial cells growing on the occluder surface.

CONCLUSIONS

The effective occlusion of VSD, good biocompatibility, rapid endothelialization and lack of complications shown by the biodegradable occluder in this study suggest that the device meets acceptable clinical safety and has potential application prospects.

Biodegradable atrial septal defect occluders: A current review

Atrial septal defect (ASD) is a common structural congenital heart disease. With the development of interventional closure devices and transcatheter techniques, interventional closure therapy has become the most well-accepted therapeutic alternative worldwide, as it offers a number of advantages over conventional therapies such as improved safety, easier operation, lower complication rates and invasiveness, and shorter anesthetic time and hospitalizations. During the past decades, various types of occluders based on nondegradable shape memory alloys have been used in clinical applications. Considering that the permanent existence of foreign nondegradable materials in vivo can cause many potential complications in the long term, the research and development of biodegradable occluders has emerged as a crucial issue for interventional treatment of ASD. This review aims to summarize partially or fully biodegradable occlusion devices currently reported in the literature from the aspects of design, construction, and evaluation of animal experiments. Furthermore, a comparison is made on the advantages and disadvantages of the materials used in biodegradable ASD occlusion devices, followed by an analysis of the problems and limitations of the occlusion devices. Finally, several strategies are proposed for future development of biodegradable cardiac septal defect occlusion devices. STATEMENT OF SIGNIFICANCE: Although occlusion devices based on nondegradable alloys have been widely used in clinical applications and saved numerouspatients, biodegradable occlusion devices may offer some advantages such as fewer complications, acceptable biocompatibility, and particularly temporary existence, thereby leaving "native" tissue behind, which will certainly become the development trend in the long term. This review summarizes almost all partially or fully biodegradable occlusion devices currently reported in the literature from the aspects of design, construction, and evaluation of animal experiments. Furthermore, a comparison is made on the advantages and disadvantages of the materials used in biodegradable ASD occlusion devices, followed by an analysis of the problems and limitations of the occlusion devices. Finally, several strategies are proposed for future development of biodegradable cardiac septal defect occlusion devices.