Feasibility of biodegradable PLGA common bile duct stents: An in vitro and in vivo study

The current study investigates the feasibility of using a biodegradable polymeric stent in common bile duct (CBD) repair and reconstruction. Here, poly(l-lactide-co-glycolide) (PLGA, molar ratio LA/GA = 80/20) was processed into a circular tube- and dumbbell-shaped specimens to determine the in vitro degradation behavior in bile. The morphology, weight loss, and molecular weight changes were then investigated in conjunction with evaluations of the mechanical properties of the specimen. Circular tube-shaped PLGA stents with X-ray opacity were subsequently used in common bile duct exploration (CBDE) and primary suturing in canine models. Next, X-ray images of CBD stents in vivo were compared and levels of serum liver enzymes and a histological analysis were conducted after stent transplantation. The results showed that the PLGA stents exhibited the required biomedical properties and spontaneously disappeared from CBDs in 4–5 weeks. The degradation period and function match the requirements in repair and reconstruction of CBDs to support the duct, guide bile drainage, and reduce T-tube-related complications.     

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.     

Development and characterization of a coronary polylactic acid stent prototype generated by selective laser melting

In-stent restenosis is still an important issue and stent thrombosis is an unresolved risk after coronary intervention. Biodegradable stents would provide initial scaffolding of the stenosed segment and disappear subsequently. The additive manufacturing technology Selective Laser Melting (SLM) enables rapid, parallel, and raw material saving generation of complex 3- dimensional structures with extensive geometric freedom and is currently in use in orthopedic or dental applications. Here, SLM process parameters were adapted for poly-l-lactid acid (PLLA) and PLLA-co-poly-ε-caprolactone (PCL) powders to generate degradable coronary stent prototypes. Biocompatibility of both polymers was evidenced by assessment of cell morphology and of metabolic and adhesive activity at direct and indirect contact with human coronary artery smooth muscle cells, umbilical vein endothelial cells, and endothelial progenitor cells. γ-sterilization was demonstrated to guarantee safety of SLM-processed parts. From PLLA and PCL, stent prototypes were successfully generated and post-processing by spray- and dip-coating proved to thoroughly smoothen stent surfaces. In conclusion, for the first time, biodegradable polymers and the SLM technique were combined for the manufacturing of customized biodegradable coronary artery stent prototypes. SLM is advocated for the development of biodegradable coronary PLLA and PCL stents, potentially optimized for future bifurcation applications.     

A shape memory stent of poly(ε-caprolactone-co-dl-lactide) copolymer for potential treatment of esophageal stenosis

Biodegradable polymer stent with shape memory effect is expected to be developed in the treatment of esophageal stenosis, most likely due to traditional stents having such shortages as considerable rigidity and nondegradation. A tubular stent with the inner and outer diameters of 28 and 30 mm was manufactured from biodegradable poly(ε-caprolactone-co-dl-lactide) (PCLA) copolymer consisting of ε-caprolactone and dl-lactide at a weight ratio of 10/90. A series of tests were accomplished to investigate its properties including shape memory effects (SMEs), compression property and influence of in vitro degradation of polymer matrix on its shape recovery and dilation force. Significantly, an implantation of the stent into a dog model was performed to evaluate its function for the treatment of esophageal stenosis. The deformed stent needs about 36 s to recover its initial shape in vitro in 37°C warm water. The primary animal experiment in vivo has revealed that the implanted deformed stent could be triggered by body temperature and expectedly returned to a nearly-round shape to support esophageal wall. Therefore, the biodegradable intelligent polymer stent may be great potential to displace the conventional metallic stents for the esophageal stenosis therapy.     

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.

     

In vitro study of drug-eluting stent coatings based on poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactide) incorporating cyclosporine A – drug release,polymer degradation and mechanical integrity

In this study, absorbable polymer stent coatings for localized drug delivery based on poly(l-lactide) (PLLA) and cyclosporine A (CsA) were developed and tested in vitro. Metallic stents were coated with different compositions of PLLA/CsA (70/30, 60/40, 50/50% w/w) and β-sterilized. The specimens were used to assess the drug release kinetics with HPLC. Sterilization influenced polymer degradation was measured with GPC. Mechanical integrity of the stent coatings was studied with SEM. The interconnection of the coated stents with a balloon-catheter was characterized by the measurement of stent dislodgment force. A migration assay was used to determine the inhibitory effect of the model drug CsA on smooth muscle cell (SMC) migration. The release of CsA was established over time periods up to 24 days in sodium chloride solution and in porcine blood plasma. An inhibition of SMC migration (max. 26–33%) was found for CsA concentrations of 4 × 10⁻⁵ to 4 × 10⁻⁷ mol/l. Marked molecular weight reduction (70–80%) of the PLLA matrix occurred after β-sterilization. We also observed a substantial decrease of in vitro degradation time. The maintenance of the mechanical integrity of the polymer coating during crimping and dilation of the specimens could be verified, and a sufficient stent dislodgment force of 0.8–0.9 N was measured.     

Improvement of stenting therapy with a silicon carbide coated tantalum stent

State of the art cardiovascular stent materials are a compromise between bulk properties and surface related properties. As a consequence, deficiencies in both characteristics lead to serious limitations of stenting therapy. Beside a dissatisfying X-ray visibility of current stent materials, which hinders precise angiographic control of the stent during implantation, insufficient hemocompatibility causes subacute vessel occlusions despite stringent anticoagulant medication. Additionally, bleeding complications result which further limit the therapeutical success. Therefore it is essential to develop a new coronary stent with improved material properties for the bulk of the stent and its surface. This is realized by a hybrid concept. The stent is manufactured from tantalum, having a high inherent radio-opacity. The stent is coated with amorphous silicon carbide, optimized for hemocompatibility. An appropriate deposition technology to maximize coating adhesion was developed. Amorphous silicon carbide was investigated in vitro and in vivo to assess its suitability for coronary stents.     

In vitro predegradation at elevated temperatures of poly(lactide)

In this study in vitro predegradation at elevated temperatures, used to obtain an increased degradation rate, was investigated. The in vitro degradation was followed by mass loss, molecular weight loss and changes in thermal properties. Two biodegradable polymers, the homopolymer PLLA and a copolymer PLA96 (96% L4%D lactide), were hydrolytically degraded at 90°C in a phosphate buffered solution. Both polymers, PLLA and PLA96, showed an initial linear degradation rate, but with longer implantation periods the degradation rate decreased and total degradation was best described as an asymptotic. Mass loss of the copolymer PLA96 was twice that of PLLA. The chemical analysis of the in vitro predegraded polymers coincided for both the decrease in molecular weight and the thermal properties with physiologically degraded poly(lactide). The results of this study show that although the degradation temperature is well above the glass transition temperature and not comparable to physiological temperatures, there seems to be good correlation between the in vitro degraded material and physiologically degraded material. In vitro predegradation enables investigation of the entire degradation process of a polymer in a short-term study. Moreover, in vitro predegradation allows direct comparison of the degradation rate of various polymers.     

In vitro and in vivo assessment of the biocompatibility of an Mg–6Zn alloy in the bile

There is a great clinical need for biodegradable bile duct stents. Biodegradable stents made of an Mg–6Zn alloy were investigated in both vivo animal experiment and in vitro cell experiments. During the in vivo experiments, blood biochemical tests were performed to determine serum magnesium, serum creatinine (CREA), blood urea nitro-gen (BUN), serum lipase (LPS), total bilirubin (TB) and glutamic-pyruvic transaminase (GPT) levels. Moreover, tissue samples of common bile duct (CBD), liver and kidney were taken for histological evaluation. In the in vitro experiments, primary mouse extrahepatic bile duct epithelial cells (MEBDECs) were isolated and cultured. Cytotoxicity testing was carried out using the MTT method. Flow cytometry analyses with propidium iodide staining were performed to evaluate the effect of Mg–6Zn alloy extracts on cell cycle. The in vivo experiments revealed no significant differences (P > 0.05) in serum magnesium, CREA, BUN, LPS, TB or GPT before and after the operation. Based on the HE results, hepatocytes, bile duct epithelial cells, renal glomerulus and renal tubule tissues did not present significant necrosis. In the in vitro experiments, the cell relative growth rate curve did not change significantly from 20 to 40 % extracts. In vitro experiments showed that 20–40 % Mg–6Zn extracts are bio-safe for MEBDECs. In vivo experiments showed that Mg–6Zn stents did not affect several important bio-chemical parameters or, harm the function or morphology of the CBD, kidney, pancreas and liver. Our data suggested that this Mg–6Zn alloy is a safe biocompatible material for CBD.     

Biodegradation of porous versus non-porous poly(L-lactic acid) films

The influence of porosity on the degradation rate of poly(L-lactic acid) (PLLA) films was investigated in vitro and in vivo. Non-porous, porous and combi (porous with a non-porous layer) PLLA films were used. Changes in Mw, Mn, polydispersity (Mw/Mn) ratio, melting temperature (T _m), heat of fusion, tensile strength, E-modulus, mass and the remaining surface area of cross-sections of the PLLA films were measured. In general, during the degradation process, the porous film has the highest Mw, Mn, Mw/Mn ratio and T _m, while the non-porous film has the lowest. In contrast, the highest heat of fusion values were observed for the non-porous film, indicating the presence of relatively smaller molecules forming crystalline domains more easily. The tensile strength and E-modulus of the non-porous film decrease faster than those of the porous and the combi film. None of the three types of films showed massive mass loss in vitro nor a significant decrease in remaining polymer surface area in light microscopical sections in vitro and in vivo. Heavy surface erosion of the non-porous layer of the combi film was observed after 180 days, turning the combi film into a porous film. This is also indicated by the changes in tensile strength, Mw, Mw/Mn, T _m and heat of fusion as a function of time. It is concluded that non-porous PLLA degrades faster than porous PLLA. Thus, in our model, porosity is an important determinant of the degradation rate of PLLA films.     

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.     

Tissue biocompatibility of new biodegradable drug-eluting stent materials

Drug-eluting stents are a recent innovation for endovascular and endourethral purposes. The aim of this study was to assess the biocompatibility of new biodegradable drug-eluting stent materials in vivo. Rods made of SR-PLDLA (self-reinforced poly-96l,4d-lactic acid) covered with P(50l/50d)LA and rods made of 96l/4D SR-PLA and covered with P(50l/50d)LA including indomethacin 3.3 μg/mm² or dexamethasone 1.5 μg/mm², were inserted into the dorsal muscles of 20 rabbits serving as test animals. Rods made of silicone and organotin-positive polyvinylchloride were used as negative and positive controls. The animals were sacrificed after 1 week, 1 month, 2 months or 4 months. Histological changes attributable to the operative trauma were seen in all specimens at 1 week and 1 month. At 2 months both dexamethasone and indomethacin induced less fibrosis than the plain SR-PLDLA covered with P(50l/50d)LA without drug. At 4 months dexamethasone induced both chronic inflammatory changes and foreign body reaction, whereas the reactions in the indomethacin and drug-free plain SR-PLDLA groups were insignificant. The new biodegradable drug-eluting stent materials are highly biocompatible. Drug-eluting biodegradable stents may offer a promising new treatment modality for vascular and urethral diseases. However, further studies are needed to demonstrate their feasibility and efficacy.     

Vibrational and thermal study on the in vitro and in vivo degradation of a poly(lactic acid)-based bioabsorbable periodontal membrane

Fourier transform Raman (FT-Raman), attenuated total reflection/Fourier transform infrared (ATR/FT-IR) spectra and differential scanning calorimetry (DSC) measurements were performed on a poly(lactic acid)-based biodegradable periodontal membrane in order to study its in vitro and in vivo degradation mechanism and kinetics. For this purpose, the hydrolitic in vitro degradation of the membrane was investigated in two aqueous media: saline phosphate buffer (SPB, pH=7.4) and 0.01 M NaOH solution. Moreover, a membrane implanted in vivo for four weeks for treatment of contiguous vertical bony defects, was examined. Vibrational and thermal measurements show that the membrane has a prevalently amorphous structure and is composed of low molecular weight polymeric chains. The degradation is faster in NaOH solution than in SPB and occurs heterogeneously without any significative increase in crystallinity. The DSC and spectroscopic measurements are discussed in comparison with the trend of % weight loss and show a progressive decrease in molecular weight. Regarding the Raman analysis, the I₈₇₅/I₁₄₅₂ intensity ratio was identified as a marker of the degree of degradation. Regarding the in vivo degradation, the presence, spectroscopically revealed, of a biological component entrapped in the membrane proves the good integration of the membrane with the surrounding tissues. The membrane seems to degrade faster in vivo than in vitro. A comparison with the degradation mechanism and kinetics of a periodontal membrane previously studied, Vicryl® periodontal mesh, is made.     

Formation of bone-like apatite on poly(L-lactide) to improve osteoblast-like compatibility in vitro and in vivo

The biomimetic apatite coating process was adopted to modify poly(L-lactide) (PLLA) surfaces with osteoblasts-like cell compatibility. The apatite coating was formed on the pre-hydrolyzed PLLA film and scaffold surfaces by incubating in simulated body fluid (SBF). Scanning electron microscopy and energy dispersive X-ray analyzer were utilized to characterize the composition and the structure of the apatite coating. The cytocompatibility of the modified PLLA films was investigated by testing osteoblast-like attachment, proliferation, alkaline phosphatase (ALP) activity, and cell cycle. Subsequently, the modified PLLA scaffolds were co-cultured with the osteoblasts-like in vitro and subcutaneously implanted into nude mice. The experimental results showed that the formed apatite had a nano-sized particle and matrix configuration. The surface modification of PLLA with apatite coating significantly promoted osteoblast-like compatibility. After a four-week culture in vivo, no significant inflammatory signs were observed in the implanted regions and osteoblast-like congeries with bone-like structure began to form in the scaffolds. The positive results of this study suggest a good way to produce desirable PLLA biomaterials for bone tissue engineering.     

In vitro blood compatibility of poly (hydroxybutyrate-co-hydroxyhexanoate) and the influence of surface modification by alkali treatment

In vitro blood compatibility of poly (hydroxybutyrate-co-hydroxyhexanoate) (PHBHHx) was evaluated in comparison with poly (L-lactic acid) (PLLA) by a haemolysis assay, in vitro platelet adhesion test and coagulation measurements including plasma recalcification time (PRT), plasma prothrombin time (PT) and kinetic clotting time. The results showed that PHBHHx exhibited better blood compatibility than PLLA. Furthermore, PHBHHx film was modified by NaOH treatment to improve the surface hydrophilic property and the influence of the surface modification on the blood compatibility was investigated. Surface properties including hydrophilic property, surface appearance and functional groups were characterized by water contact angle measurement, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results showed that the hydrophilic property of PHBHHx film was obviously improved by the NaOH treatment. It was also shown that the NaOH treatment could significantly enhance the blood compatibility of PHBHHx by prolonging PRT, PT, and kinetic clotting time and decreasing platelet activation. It is thought that the improvement in the hydrophilic property mainly contributes to the enhancement of blood compatibility.     

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.     

Crystallinity assessment and in vitro cytotoxicity of polylactide scaffolds for biomedical applications

Bioresorbable polylactides are one of the most important materials for tissue engineering applications. In this work we have prepared scaffolds based on the two optically pure stereoisomers: poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA). The crystalline structure and morphology were evaluated by DSC, AFM and X-ray diffraction. PLLA and PDLA crystallized in the α form and the equimolar PLLA/PDLA blend, crystallized in the stereocomplex form, were analyzed by a proliferation assay in contact with mouse L-929 and human fibroblasts and neonatal keratinocytes for in vitro cytotoxicity evaluation. SEM analysis was conducted to determine the cell morphology, spreading and adhesion when in contact with the different polymer surfaces. The preserved proliferation rate showed in MTT tests and the high colonization on the surface of polylactides observed by SEM denote that PLLA, PDLA and the equimolar PLLA/PDLA are useful biodegradable materials in which the crystalline characteristics can be tuned for specific biomedical applications.     

Biocompatibility and degradation mechanisms of predegraded and non-predegraded poly(lactide) implants: an animal study

In all patients treated with as-polymerized poly(l-lactide) (PLLA), a welling at the site of implantation was observed after three years of implantation. These swellings seem to be related with degrading PLLA and the formation of particles of high crystallinity. To avoid these complications, poly(96%l-, 4%d-lactide) (PLA96) was developed that possesses lower crystallinity that probably results in a faster and more complete degradation. To study the cause of the swelling of PLLA implants and to study the degradation of PLA96 long-term implantation studies are required. Considering the very slow degradation rate of as--polymerized PLLA, in vitro predegradation was performed at elevated temperatures (90°C) to simulate long-term physiological degradation. In this study a comparison was made between the histopathological reaction to non-degraded and predegraded PLLA, PLA96 and polyethylene (PE) discs implanted subcutaneously in rats. Animals were sacrificed after a postoperative period varying from 4 to 52 weeks. Chemical, light-and electron microscopical analysis and semi-quantitative measurements were performed. Based on the chemical analysis, the degradation rate of PLA96 was higher compared with PLLA. The histological reaction to non-degraded PLLA and PLA96 discs was very mild. The histological reaction to the predegraded implants was qualitatively similar to the reaction to the non-degraded implants, however, quantitatively an increase was noted. A number of predegraded PLLA and PLA96 discs showed an increase of volume with implantation time caused by the formation of fields of polymer debris accompanied by a granulomatous inflammatory reaction. The debris zone was found to consist of both polylactide polymer fragments and small remnants of degenerated cells. From our results it can be concluded that, when compared to PLLA, the degradation of PLA96 is enhanced. Subcutaneously implanted predegraded PLLA and PLA96 discs can induce a swelling similar to that observed with PLLA implants in patients. So, in vitro predegradation followed by in vivo implantation might be used as a model to predict late complications during clinical use.     

Short-term <Emphasis Type="Italic">in vitro</Emphasis> and <Emphasis Type="Italic">in vivo</Emphasis> biocompatibility of a biodegradable polyurethane foam based on 1,4-butanediisocyanate

In this study short-term in vitro and in vivo biocompatibility apects of a biodegradable polyurethane (PU) foam were evaluated. The PU consists of hard urethane segments and amorphous soft segments based on a copolyester of dl-lactide and -caprolactone. The urethane segments are of uniform length and synthesized with 1,4-butanediisocyanate. The foam has good mechanical properties and will be used for tissue regeneration applications. Degradation tests were carried out in a buffer solution for twelve weeks. Cytotoxicity was determined using extract and direct contact test methods with incubation periods varying form 24 to 72 h. The foam was implanted subcutaneously for one, four and twelve weeks and the tissue response to the material was histologically evaluated.In vitro, the mass loss was 3.4% after twelve weeks. In the cytotoxicity tests the PU caused no abnormal growth behaviour, nor morphological changes or inhibition in metabolic activity. The in vivo studies showed no toxic tissue response to the PU. Connective tissue ingrowth, accompanied by vascular ingrowth was complete at twelve weeks. In vivo degradation had started within four to twelve weeks.In conclusion, the PU shows a good in vitro and in vivo biocompatibility in these short-term experiments.     

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。