A Novel Gradient Degradable Ureteral Stent In A Porcine Model

Objective:This study was designed to investigate the microstructure, degradation time,mechanical property and biocompatibility of a poly (lactic acid-glycolicacid)/polycaprolactone degradable ureteral stent in vitro. We also describe thedegradation mode, and physiological and histological responses in a porcine model.Methods:Fabrication and tension test of the stents: PCL and PLGA (80:20) were mixed atspecific mass percentages (5%,10%,15%,20%,25%, and30%PCL with PLGA).The mixtures were dissolved in trichloromethane to prepare a5%solution. Theelectrospinning technique was used to fabricate the ureteral stent. The micro-structureof the stents were investigated by scanning electron microscopy. The tensile propertiesof the scaffolds were determined by tensile testing of scaffold specimens in a tensilefixture of a dynamic mechanical analyzer.In vitro degradation test: We chose the stents with mass percentages of5%,15%,and25%PCL/PLGA as the samples for testing degradation. The scaffolds was cutinto small pieces (10mm). After recording the weight, the samples were immersed inthe urine to evaluate its degradability at the temperature of37°C. The urine came fromthe healthy volunteers, and was replaced every day. At each observation points (7,14,21,28,42,49, and56days), three parallel samples were removed and the residualweight ratio were investigated.MTT: The culture medium with serum was used as the extraction vehicle. Theconcentration of the extraction vehicle and material was0.2g/ml. HeLa cells wereused in test. The highest concentration was100%extract, and the other solutions wereextract diluted at ratios of1:2,1:4,1:8, and1:16. Culture medium was used as negative control, and phenol as the positive control. The cell viability (%) wasrecorded.Tissue compatibility: The15%PCL/PLGA stents were cut into small pieces of1cm. Three samples were implanted into each side of the rabbit muscle separated by aninterval of1cm. The control group received an F6ureteral stent made of polyurethane.1,4, and12weeks after implantation, two rabbits were killed via air embolism at eachobservation points (7,28, and84days). The implanted stents were removed alongwith the surrounding tissues, and samples were fixed in formalin prior to paraffinembedding and hematoxylin and eosin staining.Animal experiment: The15%PCL/PLGA and25%PCL/PLGA were used tofabricate ureteral stent which can degrade from distal end to the proximal end bydouble-needle electrospinning. A total of10female Changbai pigs were used in thestudy. Six PCL/PLGA stents and4polyurethane stents were ureteroscopically insertedunilaterally in2groups of animals. Excretory urogram, and blood and urine tests wereperformed on different observation points (14,28,42,56and70days). On day70allpigs underwent necropsy by using overdose of anesthesia for microscopic andhistological evaluation.Results:Stents were white, with a length of15–20cm, an inner diameter of1.5mm, andan outer diameter of2.0mm. The stent was soft with some toughness and memory,and it did not break after bending. The scanning electron microscopy showed thestents were composed of nano-fibers with a multi-pore structure. The diameters of thefiber were60-1200nm, and the pore size were50-180um, different mass percentagesstents had the same structure. The tension test results showed that with increasingPCL concentration, the tensile strength of the stent gradually increased. ThePCL/PLGA stent with5%PCL exhibited the lowest breaking strength, but thisbreaking strength was still greater than2MPa.All three curves were almost linear, and the5%PCL/PLGA stent disintegratedinto many segments by day28. The15%PCL/PLGA stent disintegrated into segments by day42, but the25%PCL/PLGA stent did not disintegrate until day56. Theseresults provide strong evidence that the degradation time can be increased by the useof increasing PCL concentration.The absorbance of the negative control group and samples exposed to theleaching solutions of different concentration were all greater than0.5, whereas theOD492value of the positive phenol-exposed group was less than0.1. There were nosignificant differences between the absorbance values of the negative control groupand cells exposed to the leaching solutions of different concentration (p>0.05), but allof these values were significantly higher than that of positive control group (p<0.05).The results indicate that the scaffolds fabricated by electrospinning are not cytotoxic.The histological results showed that1week after the operation, manyinflammatory cells were present forming a layer1mm thick at the boundary andmany mononuclear giant cells also were seen. Four weeks later, the number ofinflammatory cells had decreased significantly, but some mononuclear giant cellswere still observed. The boundary became fibrotic. Twelve weeks after the operation,only a few inflammatory cells were located in the boundary; however, the fibrosispresented no apparent increase compared with that of the4th week. The histologicalappearance of the control group showed no differences with the experimental group atthe1st and4th weeks, but at the12th week, we observed significant fibrosis andthicker fibrous layers in the control group.All10animals were stented successfully. There were no differences in bodyweight and serum creatinean in either group at any time point(p>0.05). Urinalysisshowed that the urine PH had no differences between the two groups(p>0.05), but thewhite blood cells in the polyurethane group was apparently higher after8weeks(p<0.05). Two weeks after the implantation, the PCL/PLGA stents showed nochange in length. All PCL/PLGA stents began to degrade at4weeks, and4stentsdegraded nearly50%in length at six weeks. Four stents were completely degraded byweek8, and all stents were completely degraded by week10. There was no incidentof ureteral obstruction due to degraded stent fragments and no retained pieces in the kidneys of test animals. IVP was done to assess kidney function and hydronephrosis.In PCL/PLGA group, nephrosis was observed after2week, and the nephrosis wasdecreased at4weeks. However, only1case remained mild hydronephrosis at10weeks. On week six, no significant differences were observed in hydronephrosisseverity between PCL/PLGA and polyurethane stented pigs(p>0.05). As indwellingtime increased beyond8weeks, the level of hydronephrosis increased significantly inthe polyurethane group (p<0.05). It remained constant in the PCL/PLGA group.Contrast material entry into the kidney/ureters of polyurethane stented pigs wasdelayed by week8(which approached significance) vs. PCL/PLGA stented pigs,suggesting decreased kidney function in the former.On day70all pigs underwent necropsy by using overdose of anesthesia. The sizeof stented kidneys in each group was similar to nonstented kidneys. No stents pieceswas observed in the renal pelvic, ureter and bladder. In the polyurethane group, amassively calcified stent was observed. On day70histological findings indicatinginflammation or hydronephrosis were more frequent and severe in stented kidneysthan in nonstented kidneys. Interestingly, bullous edema, an irregular surface andincreased thickness of the urothelia were present only in the polyurethane group. Itappears that the PCL/PLGA stent produced less bladder irritation and inflammation.No difference in histological severity score was observed in the middle and distalureter sections and bladder of PCL/PLGA and polyurethane stented animals (p>0.05).In contrast, PCL/PLGA stented pigs had significantly lower mean severity scores inthe kidney and proximal ureter sections compared to polyurethane stentcontrols(p<0.05).Conclusions:The PCL/PLGA ureteral stent can be fabricated by electrospinning technique anddouble-needle electrospinning can fabricate the ureteral stent which can degrade fromdistal end to the proximal end. The PCL/PLGA ureteral stent had excellentmechanical properties, with the increasing of PCL concentration, the mechanicalproperties of the stent gradually increased. The degradation time of15%and25% PCL/PLGA sent was6and8weeks, respectively. The PCL/PLGA stent has nocytotoxicity and has a good histocompatibility. Animal study showed the stent candegrade from the distal end to the proximal end, and the stent had equivalent drainageeffects, less hydronephrosis and more biocompatibility compared to conventionalstents.