Study of biodegradable and self-expandable PLLA helical biliary stent in vivo and in vitro

Biodegradable stents have advantages for the treatment of benign and malignant biliary stricture, especially eliminating the need for stent removal. In our present work, helical poly-l-lactic acids (PLLA) stent was fabricated and evaluated in vivo and in vitro. For in vivo study, bile duct injury canine models were made by transection of common bile ducts. Duct to duct anastomosis was done with helical PLLA biodegradable stents. Scanning electron microscopy (SEM) and histopathology were performed after three months. For In vitro study, sludge attachment assessment was performed. Polyethylene (PE) and PLLA membranes were immersed in human bile for two months. The samples were taken out and characterized by SEM. Self-expanding property of the helical stent was tested in 37°C water. The results demonstrate that the biodegradable stent had not only good biocompatibility, but also self-clearing effect to clear the attached sludge away. The self-expanding property facilitated stent implantation and also suggested possibility to be implanted endoscopically.     

The effects of types of degradable polymers on porcine chondrocyte adhesion, proliferation and gene expression

Understanding how a specific biomaterial may influence chondrocyte adhesion, proliferation and gene expression is important in cartilage tissue engineering. In this study several biodegradable polymers that are commonly used in tissue engineering were evaluated with respect to their influence on chondrocyte attachment, proliferation and gene expression. Primary cultures of porcine chondrocytes were performed in films made of poly-L-lactic acid (PLLA), poly-D,L-lactic acid (PDLLA), poly-(lactide-co-glycolide) (PLGA), or polycaprolactone (PCL). Chondrocytes adhered to PDLLA or PLGA after 1-day incubation better than to PLLA or PCL. After 7 or 14 day culture, the cell numbers on PDLLA or PLGA was still higher than PLLA or PCL. The results suggested that cell attachment and growth might depend on degradation rate of biodegradable polymers. Along with the fact that PDLLA or PLGA supported expression of chondrocyte specific genes more than PLLA or PCL, the former two materials seemed to be more suitable for cartilage tissue engineering than the latter ones. Besides, we found that chondrocyte phenotype prior to seeding was important in the expression of ECM proteins.     

基于PEG-PCL-PLLA杂臂星形高分子胶束的合成与性能

聚己内酯(PCL)和聚L-乳酸(PLLA)是胶束中常用的生物可降解高分子材料,将PCL和PLLA共聚或共混能改善PCL降解缓慢的缺陷,但PCL和PLLA同时作为杂臂星形高分子胶束的疏水部分却鲜少研究。文中合成了基于PEGPCL-PLLA的ABC杂臂星形高分子胶束与对应的线型三嵌段高分子胶束,研究了聚合物的不同结构对高分子胶束的结晶和粒径的影响。二者的化学结构、相对分子质量及其分布、结晶行为和粒径由核磁共振、差示扫描量热和动态光散射等测试来表征,其结果表明杂臂星形高分子胶束具有比线型嵌段高分子胶束有更强的结晶能力和更小的粒径。     

Tailoring impact toughness of poly(L-lactide)/poly(ε-caprolactone) (PLLA/PCL) blends by controlling crystallization of PLLA matrix

Melt blending poly(L-lactide) (PLLA) with various biodegradable polymers has been thought to be the most economic and effective route to toughen PLLA without compromising its biodegradability. Unfortunately, only very limited improvement in notched impact toughness can be achieved, although most of these blends show significant enhancement in tensile toughness. In this work, biodegradable poly(ε-caprolactone) (PCL) was used as an impact modifier to toughen PLLA and a nucleating agent was utilized to tailor the crystallization of PLLA matrix. Depending on the nucleating agent concentrations in the matrix and mold temperatures in injection molding, PLLA/PCL blends with a wide range of matrix crystallinity (10-50%) were prepared by practical injection molding. The results show that there is a linear relationship between PLLA matrix crystallinity and impact toughness. With the increase in PLLA crystalline content, toughening becomes much easier to achieve. PLLA crystals are believed to provide a path for the propagation of shear yielding needed for effective impact energy absorption, and then, excellent toughening effect can be obtained when these crystals percolate through the whole matrix. This investigation provides not only a new route to prepare sustainable PLLA products with good impact toughness but also a fresh insight into the importance of matrix crystallization in the toughening of semicrystalline polymers with a flexible polymer.     

Compressive mechanical properties and deformation behavior of porous polymer blends of poly(ε-caprolactone) and poly(l-lactic acid)

Porous biodegradable polymeric scaffolds are developed by physically blending two different kinds of biodegradable polymers, PCL, and PLLA, for application in tissue engineering. The main objective of the development of this material is to control the mechanical properties, such as, elastic modulus and strength. The results from mechanical testing showed that the compressive mechanical properties of PCL/PLLA scaffold can be varied by changing the blend ratio. It also showed that these properties can be well predicted by the rule of mixture. The primary deformation mechanism of the scaffolds was found to be localized buckling of struts surrounding the pores. Localized ductile failure caused by PCL phase tends to be suppressed with increasing PLLA content. The immiscibility of PCL and PLLA caused the phase-separation morphology that strongly affected the macroscopic mechanical properties and the microscopic deformation behavior.     

Assessment of structure integrity,corrosion behavior and microstructure change of AZ31B stent in porcine coronary arteries

Magnesium alloy coronary stent becomes a hot research topic due to its biodegradable character for avoiding late thrombosis and late restenosis. However, fracture of Mg alloy stent was a common issue after implantation. In this study, 18 drug-eluting biodegradable AZ31 B stents were implanted into porcine coronary arteries to assess its structural integrity, corrosion behavior and microstructure change in vivo.The coronary artery tissue responses to AZ31 B stent implantation were detected by quantitative coronary angiography and optical coherence tomography at the set time periods. In addition, further analyses were focused on the stent structure integrity, corrosion behaviors and the microstructure change of Mg alloy stents after implantation. A large number of fractures on stent struts were observed by high-resolution transmission X-ray tomography clearly. Moreover, degradation products, twins and grain refinement that appeared in Mg alloy stent matrix after implantation were also observed during the study. Inferred from this study, it is shown that the loss of AZ31 B stent structural integrity may be the result of stress concentration, degradation and microstructure change.     

Preparation and characterization of electrospun poly(ε-caprolactone)-poly(l-lactic acid) nanofiber tubes

Recently, attempts have been made to develop nanofiber tubes suitable for nerve regeneration made of biodegradable nanofibers. Among all polymeric nanofibers, poly(ε-caprolactone) (PCL) is distinctively known for better mechanical stability and poly(l-lactic acid) (PLLA) for relatively faster biodegradability. Our purpose of study is to investigate their blending compatibility and the ability to form nanofiber tubes via electrospinning. We electrospun the PCL–PLLA nanofiber tubular using different blend ratios of PCL–PLLA. The electrospun nanofibers were continuously deposited over high speed rotating mandrel to fabricate nanofiber tubes having inner diameter of 2 mm and the wall thickness of 55–65 μm. The diameters of nanofibers were between 715 and 860 nm. The morphologies of PCL–PLLA nanofiber tubes were examined under scanning electron microscope, and showed better structural stability and formability than the neat PLLA nanofibers. Fourier transform infrared spectroscopy study revealed that the PCL–PLLA blend nanofiber exhibited characteristic peaks of both PCL and PLLA and was composition-dependent. Raman and X-ray diffraction studies showed that the increasing PCL ratio in the PCL–PLLA blend increased crystallinity of PCL–PLLA blends. Differential scanning calorimetry revealed recrystallization peaks in PCL–PLLA blends ratios of 1:2 and 1:1. Based on characterization, the electrospun PCL–PLLA nanofiber tubes is considered to be a better candidate for further in vivo or in vitro investigation, and resolve biocompatibility issues in tissue engineering.     

Biomineralization of five polymers in human bile

Insertion of polymeric biliary endoprostheses is widely used as a method of palliation of strictures of the biliary tree. Study of biomineralization process of the polymers in bile is important for clogging of the stent in a few months and is still an existing problem. Bile sludge accumulation on five types of polymers in vitro in 1 and 3 weeks was primarily investigated in this paper. The polymers PE, PTFE, PLLA, PHBV, PTSG (poly(butylene terephthalate)-co-poly(butylenes succinate)-b-poly(ethylene glycol) were used in experiment. All five types of polymers induce biliary sludge that may cause the clog. However, PLLA, a biodegradable polymer, may resist sludge accumulation by polymer pieces peeled from surface during biodegradation process in bile.     

Poly(l-lactic acid) monofilaments for biodegradable braided self-expanding stent

Although poly(l-lactic acid) (PLLA) is the widely used material for bioresorbable stents, publications on PLLA braided stents are still lacking. In this paper, the PLLA monofilaments were prepared via melt spinning and solid-state drawing. The total draw ratios are from 2.8 to 30.8. The properties of the monofilaments, such as crystallinity, elastic modulus, elongation at break, were characterized. Due to the relatively good mechanical properties, PLLA monofilaments with draw ratio of 16.8 were used to braid carotid stents. The curve of the radial force of the braided PLLA stent almost overlaps with that of a Carotid Wallstent with similar parameters, which indicates that the braided PLLA stent can provide adequate support to the carotid lesion.

     

Polymer architecture as key to unprecedented high-resolution 3D-printing performance: The case of biodegradable hexa-functional telechelic urethane-based poly-ε-caprolactone

Two-photon polymerization (2PP) is a high-resolution 3D-printing technology with a very rapidly expanding field of applications, including tissue engineering (TE). In this field, 2PP offers unprecedented possibilities for systematic studies of both cell–cell and cell–material interactions in 3D. For TE applications, the reliable production of biodegradable micro-scaffolds in porous, complex architectures is essential. However, the number of biodegradable materials that support the required level of spatial resolution is very limited, being a major bottleneck for the use of 2PP in the TE field.Herein, we introduce a hexa-functional urethane-based biodegradable precursor that overcomes the limitations associated with the high-resolution printing of current biodegradable precursors. The precursor is a telechelic urethane-based poly-ε-caprolactone (PCL) possessing three acrylate functionalities at each polymer end group which enables the reliable production of complex architectures owing to its superior physical properties as compared to the traditional di-acrylate terminated analogs. The newly developed hexa-functional telechelic urethane-based PCL reveals enhanced crosslinking kinetics and one order of magnitude higher Young’s modulus compared to the di-functional precursor (57.8 versus 6.3 MPa), providing an efficient and solvent-free 2PP processing at fast scanning speeds of up to 100 mm s⁻¹ with unprecedented feature resolutions (143 ± 18 nm at 100 mm s⁻¹ scanning speed). The crosslinked hexa-functional polymer combines strength and flexibility owing to the segregation between its hard polyacrylate and soft PCL segments, which makes it suitable for biological systems in contrast to the highly crosslinked and rigid structures typically manufactured by 2PP. Furthermore, it revealed lower degradation rate compared to its di-functional analog, which can be considered as an advantage in terms of biocompatibility due to the slower formation of acidic degradation products. Extracts of the developed polymers did not show a cytotoxic effect on the L929 fibroblasts as confirmed via ISO 10993-5 standard protocol. The presented precursor design constitutes a simple and effective approach that can be easily translated towards other biodegradable polymers for the manufacturing of biodegradable constructs with nano-scale precision, offering for the first time to use the true capabilities of 2PP for TE applications with the use of synthetic biodegradable polymers.     

Expression of basal lamina components by Schwann cells cultured on poly(lactic acid) (PLLA) and poly(caprolactone) (PCL) membranes

The present in vitro study investigated the expression of basal lamina components by Schwann cells (SCs) cultivated on PCL and PLLA membranes prepared by solvent evaporation. Cultures of SCs were obtained from sciatic nerves from neonatal Sprague Dawley rats and seeded on 24 well culture plates containing the polymer membranes. The purity of the cultures was evaluated with a Schwann cell marker antibody (anti-S-100). After one week, the cultures were fixed and processed for immunocytochemistry by using antibodies against type IV collagen, laminin I and II. Positive labeling against the studied molecules was observed, indicating that such biomaterials positively stimulate Schwann cell adhesion and proliferation. Overall, the present results provide evidence that membrane-derived biodegradable polymers, particularly those derived from PLLA, are able to provide adequate substrate and stimulate SCs to produce ECM molecules, what may have in turn positive effects in vivo, influencing the peripheral nerve regeneration process.     

Accelerated endothelialization with a polymer-free sirolimus-eluting antibody-coated stent

Drug-eluting stents have shown an impressive reduction of in-stent restenosis for many years. However, stent thrombosis due to incomplete/late endothelialization has raised major safety concerns. To overcome these problems, we developed for the first time a polymer-free sirolimus-eluting antibody-coated stent (PFSEACS) by combining polymer free and endothelial progenitor cell-capture pro-healing approaches. In the first phase, the stents were prepared by loading sirolimus on the porous outer stent surface and directly fixing the anti-CD34 antibodies without any medium carriers on the blood contacting surface. The dose and elution of sirolimus, the amount and stability of anti-CD34 antibody immobilization, and the rate of CD34+ cell capture were evaluated. In the second phase, the stents were validated in an animal model of coronary arteries in pigs. The stent was observed to start collecting endothelial progenitor cells ~2 h after stent implantation and exhibited greatly enhanced endothelialization while maintaining an excellent anti-restenosis activity comparable to the polymer-free sirolimus-eluting stents. Overall, both in vitro and in vivo evaluations indicated that novel PFSEACSs exhibited facilitated endothelialization with excellent anti-restenosis activity and thus should merit further clinical studies.     

Autologous urothelial cells transplantation onto a prefabricated capsular stent for tissue engineered ureteral reconstruction

In this study, we have fabricated an artificial ureter by transplantation of in vitro-expanded urothelial cells onto an in vivo-prefabricated capsular stent using tissue engineering methods. Spiral poly (l-lactic acid) (PLLA) stents were transplanted into the subcutaneous of Wistar rats for a period of 1, 2 or 3 weeks to induce the formation of connective tissue capsules on their surfaces. The capsular PLLA stents were then decellularized and further recellularized with bladder epithelial cells to fabricate artificial ureters. The results showed that the entrapped cells in all capsules remained continuously proliferation and lined up in continuous layers. In addition, the urothelial cells on the capsular stents with an embedding period of 2 or 3 weeks showed higher proliferative viability compared with the cells on the stents with an embedding time of 1 week (P < 0.05). The results of the study indicated that the prefabricated capsular stents could serve as alternative cell carriers for tissue engineered ureters, especially with embedding time from 2 to 3 weeks.     

Characterization and in vivo evaluation of a bio-corrodible nitrided iron stent

A bio-corrodible nitrided iron stent was developed using a vacuum plasma nitriding technique. In the nitrided iron stents, the tensile strength, radial strength, stiffness and in vitro electrochemical corrosion rate were significantly increased compared with those of the control pure iron stent. To evaluate its performance in vivo, the deployment of the nitrided iron stents in juvenile pig iliac arteries was performed. At 3 or 6 months postoperatively, the stented vessels remained patent well; however, slight luminal loss resulting from intimal hyperplasia and relative stenosis of the stented vessel segment with piglets growth were observed by 12 months; no thrombosis or local tissue necrosis was found. At 1 month postoperatively, a nearly intact layer of endothelial cells formed on the stented vessel wall. Additionally, a decreased inflammation scoring, considerably corroded struts and corrosion products accumulation were seen. These findings indicate the potential of this nitrided iron stent as an attractive biodegradable stent.     

可生物降解镁合金血管支架管坯的材料制备及成形研究进展

对可生物降解镁合金血管支架的研究现状进行了综述。镁合金作为新型可降解物材料成为了研究热点,其中血管支架是其最有前景的应用方向之一。镁合金微细管材成形困难及镁合金血管支架腐蚀速率过快,是制约其大规模临床应用的2个主要因素。作者介绍了近期国内外的相关研究,包括改善镁合金力学性能,以及为提高成形极限采取的新成形方法,为提高镁合金耐腐蚀性而采取的各种处理方法等。     

可降解铁基心血管支架材料的研究进展

概述了可降解铁基心血管支架材料的研究历史及现状,介绍了可降解铁基心血管支架的优势及其早期动物实验结果。结果表明,铁基合金是一种非常适宜制作可降解心血管支架的材料,但其存在降解速度过慢的问题。在分析可降解铁基合金的最新研究进展基础上,认为在提高铁基合金的降解速度方面虽然目前已经进行了很多有益的尝试,但是尚无极具意义的结果。同时提出,应在对人体环境下铁基合金的降解行为研究基础上发展新的提高铁基合金降解速度的方法。     

Nickel-free stainless steel avoids neointima formation following coronary stent implantation

Abstract

SUS316L stainless steel and cobalt–chromium and platinum–chromium alloys are widely used platforms for coronary stents. These alloys also contain nickel (Ni), which reportedly induces allergic reactions in some subjects and is known to have various cellular effects. The effects of Ni on neointima formation after stent implantation remain unknown, however. We developed coronary stents made of Ni-free high-nitrogen austenitic stainless steel prepared using a N₂-gas pressurized electroslag remelting (P-ESR) process. Neointima formation and inflammatory responses following stent implantation in porcine coronary arteries were then compared between the Ni-free and SUS316L stainless steel stents. We found significantly less neointima formation and inflammation in arteries implanted with Ni-free stents, as compared to SUS316L stents. Notably, Ni²⁺ was eluted into the medium from SUS316L but not from Ni-free stainless steel. Mechanistically, Ni²⁺ increased levels of hypoxia inducible factor protein-1α (HIF-1α) and its target genes in cultured smooth muscle cells. HIF-1α and their target gene levels were also increased in the vascular wall at SUS316L stent sites but not at Ni-free stent sites. The Ni-free stainless steel coronary stent reduces neointima formation, in part by avoiding activation of inflammatory processes via the Ni-HIF pathway. The Ni-free-stainless steel stent is a promising new coronary stent platform.     

Surface modified electrospun nanofibrous scaffolds for nerve tissue engineering

The development of biodegradable polymeric scaffolds with surface properties that dominate interactions between the material and biological environment is of great interest in biomedical applications. In this regard, poly-ε-caprolactone (PCL) nanofibrous scaffolds were fabricated by an electrospinning process and surface modified by a simple plasma treatment process for enhancing the Schwann cell adhesion, proliferation and interactions with nanofibers necessary for nerve tissue formation. The hydrophilicity of surface modified PCL nanofibrous scaffolds (p-PCL) was evaluated by contact angle and x-ray photoelectron spectroscopy studies. Naturally derived polymers such as collagen are frequently used for the fabrication of biocomposite PCL/collagen scaffolds, though the feasibility of procuring large amounts of natural materials for clinical applications remains a concern, along with their cost and mechanical stability. The proliferation of Schwann cells on p-PCL nanofibrous scaffolds showed a 17% increase in cell proliferation compared to those on PCL/collagen nanofibrous scaffolds after 8 days of cell culture. Schwann cells were found to attach and proliferate on surface modified PCL nanofibrous scaffolds expressing bipolar elongations, retaining their normal morphology. The results of our study showed that plasma treated PCL nanofibrous scaffolds are a cost-effective material compared to PCL/collagen scaffolds, and can potentially serve as an ideal tissue engineered scaffold, especially for peripheral nerve regeneration.     

Characterization of anterior cruciate ligament cells and bone marrow stromal cells on various biodegradable polymeric films

In this study, the adhesion, proliferation and morphology of rabbit anterior cruciate ligament (ACL) cells and bone marrow stromal cells (bMSCs) on synthetic biodegradable polymeric films were investigated. Tissue culture polystyrene (TCP) was used as control. Seven biodegradable polymers were used; they are as follows: poly(-caprolactone) (PCL), poly( -lactide) ( -PLA), poly( -lactide) ( -PLA), PLA/PCL (50:50), PLA/PCL (75:25), high molecular weight (HMW) poly( -lactide–co-glycolide (PLGA50:50) and HMW PLGA75:25. Polymeric film substrates were manufactured using solvent spin-casting technique. After 8 h of cell culture, a high percentage of ACL cells was found attached to PLGA50:50 (38.6±8.4%) and TCP (39.3±6.1%) as compared to the other six polymeric films (p≤0.001). As for bMSCs, 76.4±10%, 76.3±16% and 76.1±19% of seeded bMSCs were adhered to TCP, PLGA50:50 and PLGA75:25, respectively. These were significantly more than those of the other five polymeric films (p<0.001). At Day 5, bMSCs were found to proliferate faster on TCP (by 7±0.8-fold of initial cell seeding number), -PLA (by 5.6±1.6-fold), PLGA50:50 (by 9.3±1.3-fold) and PLGA75:25 (by 5.8±1.3-fold) than on PCL, PLLA and PCL/PLA (50:50, 25:75) (p<0.001). ACL cells had a greater fold expansion on TCP (by 3.5±0.2-fold), PLGA50:50 (by 3.1±0.4-fold) and PLGA75:25 (by 3.9±0.4-fold) than on the other five polymer substrates (p<0.001). From these results, HMW PLGA (50:50, 75:25) was shown more likely to allow bMSCs and ACL cells to attach and proliferate, and bMSCs attached and proliferated faster than ACL cells.     

Mechanical properties and in vivo performance of a novel sliding-lock bioabsorbable poly-<Emphasis Type="Italic">p</Emphasis>-dioxanone stent

A bioabsorbable poly-p-dioxanone (PPDO) stent with a novel sliding-lock structure was fabricated to treat stenotic peripheral vessels. The sliding-lock PPDO stents have greater radial strength (107 kPa) than PPDO stents with conventional net-tube structure (32 kPa). The sliding-lock PPDO stents were implanted into the iliac arteries of pigs, and implantation success rate was 90% indicating the feasibility of this design. Additionally, we found that sliding-lock PPDO stents kept vessels patent, although by 3 and 6 months post implantation, luminal diameter decreased slightly due to intimal hyperplasia. At 1 month post implantation, the stents were sparsely covered with endothelial cells, and by 6 months, the stents were mostly absorbed and inflammatory reaction gradually decreased as the stents were absorbed. This study shows favorable mechanical strength, degradability and efficacy for the sliding-lock PPDO stents, and supports further research and development of this unique design of polymer stents for applications in vascular devices.