Chronic Outward Force Research Articles

Efficacy of the Stripped AFX Aortic Cuff as a Scaffolding Bare Stent to Facilitate the Expansion of the Thoracoabdominal and Visceral Aorta during Thoracic Endovascular Aortic Repair for Complicated Stanford Type B Aortic Dissection.

During thoracic endovascular aortic repair for complicated Stanford type B aortic dissection, large bare stent placement for the abdominal aorta is sometimes necessary. In smaller abdominal aortic diameter cases, we used the stripped AFX aortic cuff as a scaffolding bare stent rather than the Zenith Dissection Endovascular Stent, which is a commercially available, large bare stent. In this study, we evaluated the feasibility of the stripped AFX aortic cuff and experiments were conducted to compare the stripped AFX and the Zenith Dissection Endovascular Stent. The type B aortic dissection patients treated with thoracic endovascular aortic repair using stripped AFX at three institutions between January 2014 and December 2017 were retrospectively reviewed. Clinical data, including technical success, perioperative complication, and overall survival, were evaluated. The experiment assessed the chronic outward force that reflected the load acting on the artery wall from the stent. Eight cases (seven males) were reviewed. The median (interquartile range, IQR) age of the patients was 60 years (46.3-70.3). The technical success rate was 100%, and no perioperative complications were observed. The median (IQR) follow-up period was 28.9 months (17.5-31.5). During the follow-up, one patient died of septic shock unrelated to aortic events. The median (IQR) diameter of the stripped AFX on the last follow-up CT was 23.5 mm (21.9-25.0). The chronic outward force of the Zenith Dissection Endovascular Stent was two to three times that of the stripped AFX. The stripped AFX aortic cuff is feasible and safe as a scaffolding stent during thoracic endovascular aortic repair for Stanford Type B aortic dissection.

Evaluation of the stress relaxation behavior of poly (L‐lactic acid) self‐expanding braided stents

AbstractStress relaxation of poly (L‐lactic acid) (PLLA) braided stents generated after crimping will significantly influence the radial properties of the stents, due to the viscoelasticity of PLLA. In particular, the environmental temperature and duration in the relaxation process are two vital effects on these radial performances. Thereby, a serious of crimping temperatures and holding durations were adopted in this study to investigate their influence on the radial properties of stents. The radial properties of the stent, including peak compression force, chronic outward force and energy loss, were characterized by radial mechanical tests. Results showed that the stress relaxation rates of PLLA braided stents increased with the increase of the crimp‐holding temperature and time. Specifically, the stress relaxation rates at 25, 37 and 42°C were 22.81%, 28.41% and 36.10% at hold time of 300 s. Moreover, the increase of stress relaxation can reduce chronic outward force of stents, which will furtherly cause complications such as insufficient supporting and the shift of stent and so on. Therefore, decreasing the crimp‐holding temperature and duration of stents is an effective method to reduce the stent stress relaxation. This work provides experimental references for the comprehensive evaluation of mechanical properties of PLLA braided stent in clinical application.

Computational modeling of braided venous stents — Effect of design features and device-tissue interaction on stent performance

Designing venous stents with desired properties is challenging due to the partly conflicting performance criteria, e.g., enhancing flexibility may be at odds with increasing patency. To evaluate the effect of design parameters on the mechanical performance of braided stents, computational simulations are performed using finite element analysis. Model validation is performed through comparison with measurements. Considered design features are stent length, wire diameter, pick rate, number of wires, and stent end-type, being either open-ended or closed looped. Based on the requirements of venous stents, tests are defined to study the effect of design variations with respect to the following key performance criteria: chronic outward force, crush resistance, conformability, and foreshortening. Computational modeling is demonstrated to be a valuable tool in the design process through its ability of assessing sensitivities of various performance metrics to the design parameters. Additionally, it is shown, using computational modeling, that the interaction between a braided stent and its surrounding anatomy has a significant impact on its performance. Therefore, taking into account device-tissue interaction is crucial for the proper assessment of stent performance.

How Controlled is the Expansion of VIATORR CX?

PurposeTo investigate and compare the physical properties of the new generation Gore VIATORR-Controlled Expansion Endoprosthesis (VCX) to those of the predecessor VIATORR stent in an in vitro experimental setup.Materials and MethodsA total of 12 stents (8 VCX; 4 VIATORR; GORE, USA) were examined. Radial resistive force (RRF) and chronic outward force (COF) were assessed using a radial force testing machine (RX-650, Machine Solutions Inc., USA). To assess the radial forces of the VCX above 8 mm, balloon expansion was performed between cycles.ResultsAll VCX stents show an abrupt decrease in COF at an external diameter of 8.3 mm; RRF decreases likewise at an external diameter of 8.5 mm. The predecessor VIATORR stent without the “controlled expansion” feature shows linear radial force reduction until full expansion at a diameter of 10 mm.The physical properties of the VCX can be altered by balloon modulation. Point of COF (RRF) reduction shifts to 8.5 mm (8.6 mm), 8.6 mm (8.8 mm) and 9.3 mm (9.6 mm) following modulation with a 8 mm, 9 mm and 10 mm balloon.ConclusionsThe VCX shows an abrupt and disproportionate decrease in COF and RRF at an external diameter of 8.3 mm, thus passive expansion to its nominal diameter of 10 mm is not to be expected. By means of balloon dilatation the physical properties of the stent can be altered, enabling customized TIPS creation. The previous VIATORR stent shows continuous COF and RRF until total expansion.

Analysis of mechanical properties of the stent's outflow segment during thoracic endovascular aortic repair

AIMS AND OBJECTIVES: To discuss the mechanical properties of stents and optimize the stents' selection scheme of overlapping segment through in vitro mechanical simulation of stent grafts. MATERIALS AND METHODS: The radial force tester was used to measure the radial force of different aortic covered stents, bare stents and the combination of the two. Linear relationship between the radial force at the distal end of the stent and oversize rate was plotted. The stent-induced aortic wall shear stress was evaluated intuitively by mechanical simulation. RESULTS: The radial force curve of the distal end of the thoracic aortic covered stent reflected the superelastic characteristics of aortic stent, and the characteristics was more obvious in stents composed of nickel-titanium than stainless; When different size of Hercules stents were combined with the same sized restrictive bare stent, the oversize rate of overlapping segment was the same, and the radial force was similar; At the initial compression diameter, the use of restrictive bare stent can improve the chronic outward force, but at high compression level, the chronic outward force of the overlapping segment was higher than that of the individual aortic covered stent. CONCLUSIONS: Based on in vitro mechanical simulation, the stent-vessel wall interaction after implantation can be more intuitively understood. The application of restrictive bare stents can change the radial force of the overlapping segment of the stents by reducing the oversize rate of the distal end of the stent-graft, but it can be harmful at a higher compression level.

Fabrication and Mechanical Property Evaluation of Peripheral Stent Made from Shape Memory Alloys

This study proposes to design and fabricate the peripheral stent using NiTi shape memory alloy wires by braiding technique. The 0.2 mm diameter NiTi wire was attached to the automatic braiding machine and fabricated the braided stent under the different height of circular rod holder. Heat-treatment was carried out at 450°C, 500°C, 550°C and 600°C for 10 and 20 minutes, respectively. Then transformation temperature, the shape of cell area, and mechanical properties were evaluated. The results show that complete superelasticity can be confirmed under body temperature in stent prototypes with heat-treatment temperature over 500°C irrespective of heat-treatment time. The prototype with the height of circular mold of 70 mm shown a good braiding pattern with high kink resistance. The radial force shown no significant difference at any section along the length profile. It is noted that stent with the height of circular mold at 70 mm, carrier speed of 30 rad/sec and take-up speed of 4.86 mm/s which shown radial resistive force (ROF) of 2.44 ± 0.05 N/mm, and chronic outward force (COF) of 0.533 ± 0.03 N/mm which is sufficient for vessel expansion.

Comparison between a dedicated venous stent and standard composite Wallstent–Z stent approach to iliofemoral venous stenting: Intermediate-term outcomes

ObjectiveDedicated venous stents have not been used in the management of symptomatic chronic iliofemoral venous obstruction (CIVO) until recently. The Bard Venovo stent (Becton, Dickinson, and Co, Franklin Lakes, NJ) is one such stent noted to have an increased chronic outward force and radial resistive force compared with the Wallstent (Boston Scientific, Marlborough, MA). In the present study, we evaluated the outcomes following the use of the Bard Venovo stent vs a matched cohort of limbs that had undergone stenting with the Wallstent–Zenith (Z) stent (Cook Medical Inc, Bloomington, IN) composite configuration. MethodsA review of contemporaneously entered electronic medical record data for 167 patients (167 limbs) with initial iliofemoral stents placed from 2019 to 2020 for quality of life (QOL)-impairing CIVO that had failed conservative therapy was performed. The visual analog scale for pain score (score, 0-10), grade of swelling (score, 0-4), venous clinical severity score (score, 0-27), and the 20-item chronic venous insufficiency quality of life questionnaire instrument for QOL were evaluated before and after intervention to assess the effects of stenting. A Kaplan-Meier analysis was used to examine primary, primary-assisted and secondary stent patency, and analysis of variance with repeated measures was used to compare clinical outcomes. ResultsA total of 167 limbs had undergone Bard Venovo stenting (56 men and 111 women). Their median age was 61 years. The laterality was right and left in 70 and 97 limbs, respectively. Post-thrombotic syndrome was seen in 84 limbs and nonthrombotic iliac vein lesions/May-Thurner syndrome in 83 limbs. At 6 months, the venous clinical severity score had improved from 7 to 4 in the limbs with a unilateral Venovo (UV) stent and from 5 to 4 in the composite Wallstent–Z stent group (P = .9). The grade of swelling had improved from 3 to 1 in the UV group and from 3 to 1 in the composite group (P = .6), and the visual analog scale for pain score had improved from 7 to 2 in the UV group and from 5 to 0 in the composite group (P = .007). At 12 months, ulcers had healed in 53% (8 of 15) of the UV group and 56% (5 of 9) of the composite group (P = .7). The global 20-item chronic venous insufficiency quality of life questionnaire scores had improved from 58 to 28 in the UV group and from 59 to 40 in the composite group (P = .6). The cumulative primary, primary-assisted, and secondary patency at 18 months was 81%, 97%, and 98% in the UV group and 87%, 98%, and 100% in the composite group, respectively (P > .4). No difference in the reintervention rates was noted between the two groups (P = .5). ConclusionsFor patients who had undergone stenting for QOL-impairing CIVO, the results with the Bard Venovo venous stent were comparable to those with the composite Wallstent–Z stent configuration for clinical outcomes, QOL improvement, and stent patency. Further study is, however, required to confirm this improvement in the long term.

Different properties of poly(L-lactic acid) monofilaments and its corresponding braided springs after constrained and unconstrained annealing.

Thermal annealing is widely applied to enhance the mechanical performance of PLLA monofilaments, which brings in a variety of expected strengths through different constrained methods. In this work, samples with constrained and unconstrained annealing process were both prepared and characterized, including mechanical performance, surface morphology, radial supporting performance and axial flexibility. Experimental results revealed that the monofilaments under constrained annealing showed higher elastic modulus with 6.4GPa, which were higher than those without any constraint. While the maximal elongation at break with 51.11% were observed in unconstrained annealed monofilaments. Few changes were presented in the molecular weight between the two types of samples. Moreover, the springs under constrained annealing inhibited the most reliable radial supporting performance with higher radial compression force and chronic outward force, 0.665N/mm and 0.14N respectively. However, unconstrained annealing springs showed better flexibility with 0.178N bending stiffness and 1.58N maximum bending force. These results suggested that filaments and springs with various properties can be obtained under different annealing conditions.

Effect of Heat-Treatment Temperature on Radial Force of Atrial Depressurized Device Made from Shape Memory Alloys for Heart Failure Treatment

Nowadays, NiTi shape memory alloys have been extensively used for medical applications especially as transcatheter [6]. This research aims to investigate the effect of heat-treatment temperature on radial force of NiTi self-expanding atrial depressurized device. The function of this ADD is to expand and sustain the hole in the atrial septal and release the pressure in the heart chamber to an acceptable level, and subsequently, prevent it from closing again, without a repeating surgery. The device was made by braiding 72 NiTi wires with diameter 0.1 mm and formed to double-disc shape with 8 mm fenestration size. Heat-treatments were carried out at 560°C, 580°C, and 600°C for 10 min, respectively to obtain suitable mechanical properties and shape setting. In order to evaluate the basic performance of the device, phase transformation temperatures and radial force were investigated. The results show that ADD device can be formed perfectly and excellent superelasticity can be confirmed in all specimens at room temperature, irrespective of heat-treatment temperature. It is confirmed that both radial resistive force and chronic outward force greatly increase while the fenestration size decreases with increasing of compression load. This ensures that the device is able to resist compression of the heart wall. It can be concluded that the optimum heat-treatment temperature is 560°C since the low heat-treatment temperature produces less oxidation and the greatest recovery force.

Enhanced mechanical properties of poly(L‐lactide) braided stent with six‐arm poly(L‐lactide‐co‐ε‐caprolactone) coating cross‐linked by hexamethylene diisocyanate

AbstractCompared with traditional metal stents, biodegradable polymer stents have better biocompatibility but poorer mechanical properties in the treatment of vascular stenosis. It is necessary to improve the mechanical properties of biodegradable polymer stents for further application. For this purpose, we prepared a poly(L‐lactide) braided stent covered with six‐arm poly(L‐lactide‐co‐ε‐caprolactone) (6S‐PLCL) coating cross‐linked by hexamethylene diisocyanate (HDI) in different proportions. The coating causes an effective constraint on the movement of the monofilaments in the axial direction at the intersection. Due to this constraint, the coating enhanced mechanical properties of the stent and effectively reduced the axial elongation after crimping. Furthermore, the coated stent with the mass ratio of 5:1 (6S‐PLCL:HDI) showed the highest chronic outward force and the lowest axial elongation compared to the other mass ratios. The results can provide suggestions for improving the mechanical properties of polymer‐braided stents as compared to traditional metal stents.

Evaluation of mechanical properties of poly(L‐lactic acid) braided stents with axial stiffeners

AbstractPoly(l‐lactic acid) (PLLA) has been widely used in constructing biodegradable braided stents due to high biocompatibility and biodegradability. However, poor mechanical properties of this stent are main weaknesses. In this paper, the mechanical properties of PLLA braided stents containing axial stiffeners were evaluated. The stents with stiffener diameters of 0.11, 0.19 and 0.28 mm are named SS11, SS19 and SS28, respectively, and the stent without stiffener is named SS00 as control. The results show that compared with SS00, the bending stiffness of SS11, SS19 and SS28 is increased by 0.77, 1.76, and 2.70 times, respectively. Meanwhile, chronic outward force at an indicated use diameter of 5 mm is increased by 0.83, 0.50, and 0.21 times, but elastic recovery rate is decreased by 5.6%, 14.6%, 22.5%. In addition, radial stiffness of SS11, SS19, and SS28 are increased by 2.81, 2.08 and 1.62 times, respectively. Therefore, the radial support performance is improved and the bending flexibility is weakened by adding stiffeners. Taken together, SS11 can provide sufficient radial support force and low axial force in clinical applications. This study may provide a reference for optimizing the mechanical properties of the biodegradable braided stents.

Mechanical characteristics of venous stents to overcome challenges of venous outflow obstruction.

Many challenges posed by the venous outflow obstruction (VOO) are unique to the venous system which require specific mechanical characteristics of stents to overcome them. Therefore, this article aims to review and discuss these challenges and the stent mechanical characteristics required to overcome them. A literature search was performed with Pubmed with the terms "mechanical characteristics of venous stents" and "mechanical properties of venous stents". The venous system poses unique anatomical, physiological and pathological challenges when compared to the arterial counterpart. Several mechanical characteristics specific to venous stents which include the size, inherent forces, and flexibility are important to overcome these unique challenges when treating VOO. The most important stent inherent forces for venous stents include the chronic outward force, radial resistive force, and crush resistance. Various stent materials and designs, particularly laser-cut versus braided-structure stents, and open-cell versus closed-cell stents, determine the mechanical characteristics including the inherent forces of the venous stents. The desired mechanical characteristics of stents needed to overcome the venous system challenges often in conflict or with opposing effects. Therefore, it is important to fine adjust and optimize these characteristics. There are many anatomical, physiological, and pathological challenges unique to the venous system which need to be overcome by the stent system mechanically when treating VOO. Therefore, dedicated venous stents with optimal mechanical characteristics are being developed with different designs and properties to treat VOO. Further research and innovations are needed to improve the stent technology that targets the venous system specifically.

Use of the Cascade expandable net to treat cerebral vasospasm \u2013 initial clinical experience from a single centre with in vitro benchside tests

BackgroundThe use of self-expanding stents to treat post-hemorrhagic cerebral vasospasm was recently described. We sought to determine the clinical efficacy of the Cascade device to treat delayed cerebral vasospasm (DCV). We performed benchside tests to determine the chronic outward force exerted by the Cascade in comparison to the Solitaire.MethodsThe chronic outward force (COF) of the Cascade M agile and Cascade L Agile was tested with equivalent tests of the Solitaire 4x20mm. Further tests to determine the forces generated in pre-formed tubes of 1.5–6 mm were performed using both fully and partially unsheathed Cascades.A retrospective review to identify all patients with aSAH and DCV treated with a Cascade device between January 2020 and July 2021. We recorded the treatment arterial vessel diameters and hemorrhagic or ischemic complications.ResultsIn vitro the Cascade generated greater radial force than the Solitaire. The force generated by the Cascade M Agile at 1.5 mm was approximately 64% higher than the Solitaire 6x40mm and approximately 350% higher than the Solitaire 4x20mm.4 patients with DCV were identified all of whom were treated with a cascade device. In all cases there was a significant improvement in the diameter of the vasospastic vessels treated with an average diameter increase of approximately 300%. There were no complications from the Cascade. Delayed CT angiography showed persistent dilatation of the segments treated with the Cascade at 24 h.ConclusionThe Cascade is a safe and effective device when used to treat DCV secondary to aSAH. Larger studies are required to validate our initial results.

A computational study of effects of material properties, strain level, and friction coefficient on smart stent behavior and peripheral artery performance during the interaction process

Peripheral artery stenting (PAS) is an effective alternative for peripheral endarterectomy. Smart stents can be used to minimize the problems of interaction between peripheral arteries and smart stent. To evaluate the biomechanical properties of smart stents and their interactions with the peripheral artery, a 3D nonlinear finite element method (FEM) model that was composed of a peripheral artery and a smart stent was built. The present simulation modeled smart superelasticity material based on thermodynamics of the Helmholtz free energy (Auricchio theory) and Peripheral artery based on hyperelastic models (Mooney-Rivlin and Ogden). Additionally, the present study used FEM to assess the influences of the material properties, strain level, and friction coefficient (with attention to smart stent’s materials properties) of the newly designed smart stent during crimping and its interaction with the peripheral artery. The results showed that the smart stent with high Af: Austenite finish temperature, 90% crimping, and friction coefficient of 0.1 between smart stent and peripheral artery performed better mechanically and clinically, which can be attributed to suitable Chronic Outward Force (COF), high Radial Resistive Force (RRF), whole mechanical hysteresis regarding superelastic performance, the high martensite formation, the low stress on the peripheral artery, and the high strain on the internal curvature of the smart stent and peripheral artery. Moreover, it was found that the Mooney-Rivlin model showed a better mechanical performance that the Ogden model given the distribution of stress and strain level on the peripheral artery. These models were appropriate for the description of the peripheral artery performance and smart stent behavior with considering material properties, strain level, and friction coefficient during the interaction process.

A poly(l‐lactic acid) braided stent with high mechanical properties during in vitro degradation in bile

AbstractIn this article, the mechanical properties and physicochemical indexes of a poly(l‐lactic acid) (PLLA) braided stent during in vitro degradation in bile for 270 days are characterized to investigate its applications in treating biliary strictures. The PLLA braided stent always maintains structural integrity well macroscopically without cracks or pits microscopically. During degradation, radial compressive force and chronic outward force are increased by 25.1% and 14.3%, respectively, bending stiffness and axial force are increased by 3.0% and 4.9%, respectively. These results indicate that PLLA braided stent can keep sufficient radial force and good bending performance during degradation in bile for 270 days, showing promising application prospects in treating severe benign biliary strictures demanding support for over half a year. Furthermore, almost no mass loss and 42.4% attenuation in molecular weight indicate the degradation mechanism of bulk erosion without obvious autocatalysis, causing decrease in bile pH. Degradation rate constant is 0.002 day−1 and crystallinity is increased by 8%. These results explain that PLLA braided stent can keep high mechanical properties during degradation in bile for 270 days, which is in the early stage of degradation. This study may bring new ideas into the applications of PLLA braided stents in treating biliary strictures.

Impact of chronic outward force on arterial responses of proximal and distal of long superficial femoral artery stent

BackgroundSelf-expanding nitinol stent (SENS) implantation is commonly oversized in the superficial femoral artery (SFA), and leads to chronic outward force (COF) and in-stent restenosis (ISR). This study aimed to investigate the impact of COF of oversizing SENS on ISR of SFA.MethodsIn patients with implanted SENS in SFA, intimal hyperplasia especially between proximal segment and distal segment was evaluated by quantitative angiography, and the impact of COF on mid-term angiographic outcomes was investigated. In addition, porcine model with implanted SENS was used to evaluate the impact of COF on angiographic and histopathologic outcomes at 1 month. Excised stented arteries were evaluated by histopathologic analysis.ResultsWe analyzed 65 SENS in 61 patients with follow-up angiography at 6 months to 1 year. The baseline diameter was 6.8 ± 0.71 mm and length were 97.0 ± 33.8 mm for the SENS. The ratio of the diameter of the stent to the reference vessel was 1.3 ± 0.24 at the proximal portion and 1.53 ± 0.27 at the distal portion (P < 0.001). In the long SFA stent, stent-to-vessel ratio was significantly higher in the distal stent than in the proximal stent (1.3 ± 0.2 vs. 1.55 ± 0.25, P = 0.001). ISR incidence was higher at the distal stent (37.3% vs 52.6%, P = 0.029). All 11 pigs survived for 4 weeks after SENS implantation. The vessel diameter was 4.04 ± 0.40 mm (control group) vs 4.45 ± 0.63 mm (oversized group), and the implanted stent diameter was 5.27 ± 0.46 mm vs. 7.18 ± 0.4 mm (P = 0.001). The stent-to-vessel diameter ratio was 1.31 ± 0.12 versus 1.63 ± 0.20 (P < 0.001). After 4 weeks, restenosis % was 29.5 ± 12.9% versus 46.8 ± 21.5% (P = 0.016). The neointimal area was 5.37 ± 1.15 mm2 vs. 8.53 ± 5.18 mm2 (P = 0.05). The restenosis % was 39.34 ± 8.53% versus 63.97 ± 17.1% (P = 0.001).ConclusionsCOF is an important cause of restenosis in the distal portion of the SFA stent. Optimal sizing of the SFA stent is important to reduce the incidence of restenosis. Therefore, COF was an important factor of restenosis following distal SFA stenting.

Can a self-expanding pediatric stent expand with an artery? Relationship of stent design to vascular biology.

A large-diameter, intravascular, self-expanding stent system capable of continued expansion during somatic and vascular growth was modeled with finite element analysis (FEA), manufactured and tested in an animal model. Children can quickly outgrow intravascular stents. If a stent could expand after implantation in arteries this would be ideal for use in pediatric patients. Computer-aided design and FEA were used to design and manufacture large-diameter, self-expanding nitinol stents with both high and low chronic outward force (COF). Four distinct stents with similar designs but with variable lengths and strut thicknesses were manufactured. Fourteen of these stents were implanted in the abdominal aortas or iliac arteries of four juvenile swine. All animals survived without complication to their designated time points of harvest (90 or 180-days), and all stents expanded to greater diameters than the adjacent non-stented artery. Luminal diameter growth was 34-49% and 20-23% for stented and non-stented segments, respectively. Histologic examination revealed variable degrees of the internal elastic lamina and/or medial disruption with a mean injury score ranging from 0.70 ± 0.56 to 1.23 ± 0.21 and low COF stents implanted in smaller arteries having a larger injury score. Inflammatory responses and stenosis formation were minimal and ranged from 0.50 ± 0.71 to 3.00 ± 0.00 and 5.52 ± 1.05% to 14.68 ± 9.12%, respectively. The stent's COF did not correlate with vessel expansion or vascular injury. Self-expanding stents can mirror and even exceed somatic growth. Although longer-term testing is needed, it may be possible to custom tailor self-expanding stents to expand after arterial implantation in pediatric patients.

Preparation of a poly(L-lactic acid) braided stent with high radial force

In this paper, a poly(L-lactic acid) (PLLA) braided stent with high radial force is prepared by optimizing the processes of monofilament forming and stent annealing. Firstly, three kinds of PLLA monofilaments with different diameters are prepared by melt extrusion, then two kinds of thicker monofilaments are further solid-state drawn to have diameters similar to that of the third thinner monofilament. The monofilament that is solid-state drawn at the larger draw ratio shows sufficient tensile mechanical properties and can be used for braiding stents, which is owing to the promotion of oriented crystallization. Secondly, PLLA braided stents are annealed at different temperatures. Stent not annealed shows much lower chronic outward force (COF) and much higher radial shrinkage rate (RSR) than those of annealed stents. Moreover, the COF is increased by 115% and the RSR is decreased by 59% as the annealing temperature is gradually increased from 80℃ to 160℃. Therefore, PLLA braided stents can be annealed at higher temperatures below their melting temperature to enhance the radial force. These results are due to the improved crystallinity induced by the thermal motion of molecular chains. This study may provide helpful suggestions for the preparation of biodegradable braided stents with high radial force.

Thoracic aortic parallel stent-graft behaviour when subjected to radial loading

To manage complex aortic arch disease using minimally invasive techniques, interventionalists have reported the use of multiple stent-graft devices deployed in a parallel configuration. The structural device-device and device-artery interactions arising during aortic arch parallel endografting, also known as chimney thoracic endovascular aortic repair (ch-TEVAR), is not well understood. Through the use of a radial force testing system we sought to characterise both the loading and deformation behaviour of parallel endografts in representative ch-TEVAR configurations. Four commercially available devices (Bentley BeGraft, Gore TAG, Gore Viabahn, and Medtronic Valiant) were subjected to uniform radial load individually, and in six combinations, to quantify loading profiles. Image data collected during testing were analysed to evaluate mechanical deformations in terms of gutters, chimney and main endograft compression, as well as graft infolding. Parallel endografting was found to increase radial loads when compared to standard TEVAR. Chronic outward force during ch-TEVAR was dependent on main endograft manufacturer, with TAG combinations leading to consistently higher loads than Valiant, but independent of chimney graft type. Endograft deformations were dependent on chimney graft type, with Viabahn combinations presenting with lower gutter areas and increased lumen compression than BeGraft. Chimney graft deformations were also influenced by deployment arrangement in the case of double ch-TEVAR. This study emphasizes the significant variability in both radial loads and mechanical deformations between clinically relevant ch-TEVAR configurations.

A poly(L‐lactic acid) monofilament with high mechanical properties for application in biodegradable biliary stents

AbstractIn this article, a poly(L‐lactic acid) (PLLA) monofilament with high mechanical properties which can be used to construct biodegradable biliary stents (BBS) is prepared. Compared with a commercial polydioxanone (PDO) suture, the PLLA monofilament has much higher Young's modulus. As a result, chronic outward force (COF) of the stent braided by PLLA monofilament is more than three times higher than that of the stent braided by PDO suture. Furthermore, by physicochemical indexes characterizations, it is acceptable that the PLLA monofilament has stable Young's modulus and higher elongation at break without brittle fracture risk during degradation in bile. In contrast, although PDO suture can also maintain Young's modulus well during degradation, its elongation at break is attenuated rapidly, which shows a high brittle fracture risk. These results indicate that PLLA monofilament is more suitable for constructing BBS with high COF, which are promising in treating more complex biliary strictures that need longer support time.