Moving research direction in the field of metallic bioresorbable stents-A mini-review

A review on pancreatic duct stents: materials and emerging trends

Pancreatic duct strictures, which can arise from trauma, inflammation, or malignancy, often result in complications such as duct obstruction, pancreatic parenchymal hypertension, and ischemia. Endoscopic stenting is an effective therapeutic approach for managing these strictures. However, traditional plastic pancreatic duct stents fail to conform to the physiological curvature of the pancreas, while metal pancreatic duct stents with flared ends reduce migration but are associated with complications such as de novo strictures. Additionally, plastic and metal pancreatic duct stents require surgical removal. Whereas biodegradable pancreatic duct stents present a promising alternative due to their superior biocompatibility and ability to decompose into non-toxic materials, potentially eliminating the need for additional surgeries. Despite these advantages, biodegradable pancreatic duct stents remain in the experiment stage. This review assesses current materials of pancreatic duct stents, and emphasizes recent advancements in biodegradable options and emerging trends in clinical applications.

Evaluation of the interface of metallic-coated biodegradable polymeric stents with prokaryotic and eukaryotic cells

Polymeric coronary stents, like the ABSORB™, are commonly used to treat atherosclerosis due to their bioresorbable and cell-compatible polymer structure. However, they face challenges such as high strut thickness, high elastic recoil, and lack of radiopacity. This study aims to address these limitations by modifying degradable stents produced by additive manufacturing with poly(lactic acid) (PLA) and poly(ε-caprolactone) (PCL) with degradable metallic coatings, specifically zinc (Zn) and magnesium (Mg), deposited via radiofrequency (rf) magnetron sputtering. The characterisation included the evaluation of the degradation of the coatings, antibacterial, anti-thrombogenicity, radiopacity, and mechanical properties. The results showed that the metallic coatings inhibited bacterial growth, though Mg exhibited a high degradation rate. Thrombogenicity studies showed that Zn-coated stents had anticoagulant properties, while Mg-coated and controls were thrombogenic. Zn coatings significantly improved radiopacity, enhancing contrast by 43 %. Mechanical testing revealed that metallic coatings reduced yield strength and, thus, diminished elastic recoil after stent expansion. Zn-coated stents improved cyclic compression resistance by 270 % for PCL stents, with PLA-based stents showing smaller improvements. The coatings also enhanced crush resistance, particularly for Zn-coated PCL stents. Overall, Zn-coated polymers have emerged as the premier prototype due to their superior biological and mechanical performance, appropriate degradation during the stent life, and ability to provide the appropriate radiopacity to medical devices.

A comparative in vitro study of distinct and novel stent geometries on mechanical performances of poly-L-lactic acid cardiovascular stents

BACKGROUND

Poly-L-lactic acid (PLLA) is one of the representative polymeric materials serving as bioresorbable stents (BRS) for cardiovascular disease due to its proper biodegradation, high biocompatibility, and adequate mechanical properties among polymer candidates for BRS. However, PLLA BRS as cardiovascular stents also have limitations because their mechanical properties including low radial strength and high elastic recoil are inferior to those of metallic-based BRS stents.

METHODS

In the study, we developed and manufactured distinct and novel types of stent geometries for investigating mechanical properties of thin-walled PLLA BRS (110 μm) for cardiovascular applications. Five key mechanical tests, including radial strength, crimping profile, flexibility, elastic recoil, and foreshortening were performed through a comprehensive analysis. In addition, we applied the finite element method for further validation and insight of mechanical behaviors of the PLLA BRS.

RESULTS

Results revealed that Model 2 had advantages in high flexibility as well as radial strengths, which would be a proper option for complex and acutely curved lesions. Model 3 would be an optimum selection for stent placement in mild target site due to its strength in minimum elastic recoil. Even though Model 4 showed the highest radial strength, finite element simulation showed that the geometry caused higher maximum stress than that of Model 2 and Model 3 during the crimping process. Model 1 showed the most vulnerable geometry among the tested models in both in vitro and finite element analysis.

CONCLUSION

Such data may suggest potential guidance in regard to understanding the mechanical behaviors of PLLA BRS as not only applicable cardiovascular but also peripheral and intracranial stents.

Polydiolcitrate-MoS2 Composite for 3D Printing Radio-Opaque, Bioresorbable Vascular Scaffolds

Implantable polymeric biodegradable devices, such as biodegradable vascular scaffolds, cannot be fully visualized using standard X-ray-based techniques, compromising their performance due to malposition after deployment. To address this challenge, we describe a new radiopaque and photocurable liquid polymer-ceramic composite (mPDC-MoS2) consisting of methacrylated poly(1,12 dodecamethylene citrate) (mPDC) and molybdenum disulfide (MoS2) nanosheets. The composite was used as an ink with microcontinuous liquid interface production (μCLIP) to fabricate bioresorbable vascular scaffolds (BVS). Prints exhibited excellent crimping and expansion mechanics without strut failures and, importantly, with X-ray visibility in air and muscle tissue. Notably, MoS2 nanosheets displayed physical degradation over time in phosphate-buffered saline solution, suggesting the potential for producing radiopaque, fully bioresorbable devices. mPDC-MoS2 is a promising bioresorbable X-ray-visible composite material suitable for 3D printing medical devices, such as vascular scaffolds, that require noninvasive X-ray-based monitoring techniques for implantation and evaluation. This innovative biomaterial composite system holds significant promise for the development of biocompatible, fluoroscopically visible medical implants, potentially enhancing patient outcomes and reducing medical complications.

Application and Development Prospect of Nanoscale Iron Based Metal-Organic Frameworks in Biomedicine

Metal-organic frameworks (MOFs) are coordination polymers that comprise metal ions/clusters and organic ligands. MOFs have been extensively employed in different fields (eg, gas adsorption, energy storage, chemical separation, catalysis, and sensing) for their versatility, high porosity, and adjustable geometry. To be specific, Fe2+/Fe3+ exhibits unique redox chemistry, photochemical and electrical properties, as well as catalytic activity. Fe-based MOFs have been widely investigated in numerous biomedical fields over the past few years. In this study, the key index requirements of Fe-MOF materials in the biomedical field are summarized, and a conclusion is drawn in terms of the latest application progress, development prospects, and future challenges of Fe-based MOFs as drug delivery systems, antibacterial therapeutics, biocatalysts, imaging agents, and biosensors in the biomedical field.

The enhancement of mechanical properties and uniform degradation of electrodeposited Fe-Zn alloys by multilayered design for biodegradable stent applications

Pure Fe is a potential biodegradable stent material due to its better biocompatibility and mechanical properties, but its degradation rate needs to be improved. Alloying with Zn to form Fe-Zn alloy is anticipated to meet the degradation rate requirements while retaining the iron's inherent properties. Therefore, Fe-Zn alloys with monolayered and multilayered structures were prepared by electrodeposition. The alloys' composition, microstructure, mechanical properties, in vitro degradation and biocompatibility were assessed. Results showed that the Zn content ranged from 2.1 wt% to 11.6 wt%. After annealing at 450°C, all the alloys consisted of α(Fe) solid solution and Zn-rich B2 ordered coherent phase, except for the alloy with 11.6 wt% Zn content, in which a Fe₃Zn₁₀ phase appeared. The layered structure consisted of alternating columnar-grain and nano-grain layers, which compensated for the intrinsic brittleness of electrodeposited metals and improved the galvanic effect of the alloy, thus increasing the strength and plasticity and changing the corrosion from localized to uniform while augmenting the corrosion rate. The yield strength of the multilayered alloy exceeded 350 MPa, its elongation was more than 20%, and its corrosion rate obtained by immersion test in Hank's solution reached 0.367 mm·y^-1. Fe-Zn alloys with lower Zn content had good cytocompatibility with the human umbilical vein endothelial cells and good blood compatibility. The above results verified that the multilayered Fe-Zn alloy prepared by electrodeposition presented enhanced mechanical properties, higher degradation rate, uniform degradation mechanism and good biocompatibility. It should be qualified for the application of biodegradable stents. STATEMENT OF SIGNIFICANCE: A potential biodegradable Fe-Zn alloy, which is difficult to be obtained by the metallurgical method, was prepared by electrodeposition to solve the low degradation rate of iron-based biomaterials. A multilayered microstructure design composed of alternating columnar-grain and nano-grain layers was achieved by changing the electrical parameters. The layered design compensated for the intrinsic poor plasticity of electrodeposited metals. It increased the galvanic effect of the alloy, thus augmenting the corrosion rate and changing the corrosion mode of the alloy from localized to uniform corrosion. The yield strength of multilayered alloy exceeded 350 MPa; its elongation was more than 20%. Moreover, the layered alloy had good cytocompatibility and blood compatibility. It indicates that the alloy is qualified for biodegradable stent application.

Structure Optimization of a Fe-Mn-Pd Alloy by Equal-Channel Angular Pressing for Biomedical Use

In this work, a Fe-Mn-Pd alloy was produced by methods of equal channel angular pressing (ECAP) in order to obtain an alloy with a high rate of degradation for the development of biodegradable devices. Special efforts were made to the obtaining of an ultrafine-grained structure of alloys in a fully austenitic state at temperatures of 300 °C and 450 °C. Further investigation of its effect on the corrosion rate and mechanical properties was carried out. The formation of an austenitic structure with structural element sizes of 100-250 nm after deformation was confirmed by X-ray diffraction analysis. ECAP proved to be the reason for a significant increase in strength with maximum σUTS = 1669 MPa and σYS = 1577 MPa while maintaining satisfactory plasticity. The alloy degradation rate was investigated using the potentiodynamic polarization analysis. The corrosion rate of the alloy after ECAP (~1 mm/y) is higher than that of the coarse-grained state and significantly higher than that of annealed iron (~0.2 mm/y). ECAP in both modes did not impair the biocompatibility of the Fe-Mn-Pd alloy and the colonization of the sample surface by cells.