Biodegradable ureteral stents: influence of critical point carbon dioxide drying for success in surgical procedure

Temporary ureteral stents are used to ensure the patency of a ureter, which may be compromised, for example, by a kidney stone or a surgical procedure. The forgotten stent is a complication feared by urologists, as it has been associated with increased morbidity. A biodegradable ureteral stent would avoid these risks, as well as the need for the second surgery. In this work, with the objective to make this biodegradable stents a in vivo success, four formulations with different concentrations of gelatin and alginate and varying the concentrations of crosslink agent in order to obtained a higher mechanical properties were studied. A new method of injection moulding and critical point CO2 drying was developed to fabricate the stents. The use of supercritical carbon dioxide for the drying process has proven to be of major importance to develop the proposed materials. When the stents are dried at room temperature, even though the lumen is kept open, upon immersion in water the swelling of the polymeric stent takes place both to the exterior and to the inner core of the tubing.  Decreasing, as a consequence the inner diameter of the stent. As a result, it was not possible to pass the guidewire to conduct the stent to the animal ureter. On the other hand, the stents dried using supercritical fluid carbon dioxide at 40ºC and 100 bar present a memory-shape effect. In this case, the swelling if preferably towards the exterior, not compromising the inner diameter of the stent. The morphology of the samples was analyzed by scanning electron microscopy. X-ray scan demonstrated the radiopacicty of the stents. All formulations shown a good mechanical performance comparing with the commercial stent (Biosof duo, Porges). The degradation of the biodegradable ureteral stents was assessed in artificial urine solution. Complete degradation of the materials occurred between 3 and 15 days. Additionally, the preliminary in vivo validation of the ureteral stents has proven to be successful. Two ureteral stent were placed in the ureters of a female pig, following the normal surgical procedure. The animal remained asymptomatic, with normal urine flow and after 15 days de ureteral stents were totally degraded.