Abstract:

Objective Ureteral stent tubes provide necessary support and drainage within the body, so the mechanical properties of the stent tube are crucial to ensurine its effectiveness and safety. For biodegradable ureteral stent tubes, their mechanical properties will gradually decrease over the degradation period. A study on the mechanical properties of ureteral stent undergoing in vitro degradation and on the performance of multi-objective optimization is purposed.

Method A mathematical model for the random hydrolysis of polymers was established and embeded into finite element simulation for degradation simulation. The writing of the VUMAT subroutine was adopted to control the simulation of cell failure in the degradation process of the stent, and a combination of finite element simulation, theoretical prediction, and experimental test results was adopted to study the mechanical properties of the ureteral stent under different degradation periods.

Results A corresponding three-dimensional geometric model was constructed based on ureteral stent prepared from a fiber-membrane. The accuracy of the geometric model was verified by comparing the finite element results with the actual experimental results. For the ureteral stent prepared from the fiber-membrane, the regular weaving structure of PGA and PGLA yarns evenly distributed on the stent after being combined demonstrated the best mechanical enhancement performance after high-temperature heat treatment. Therefore, stent tube C was selected for the study of its mechanical properties during the degradation cycle. The degradation process was expressed in the form of unit damage failure, and the numerical method of ABAQUS was used in combination with the user material subroutine (VUMAT) to automatically update material parameters based on degradation time. At the same time, a strength based failure criterion was applied to simulate the mechanical properties of fiber-membrane degradable ureteral stents under different degradation times. Based on this result, 20 initial sample points were generated using the optimal Latin hypercube sampling method, and a Kriging surrogate model was constructed using these sample points to predict the mechanical properties of the fiber-membrane ureteral stent before degradation at 0 weeks and 3 weeks of degradation. NSGA-II was adopted to optimize the structure of the fiber-membrane ureteral stent. After optimization using this algorithm, a set of Pareto solutions was obtained. The algorithm specified the optimal solution as the optimization result, and the stent structure parameters with the best mechanical properties during degradation were obtained. The optimized stent showed a 17.89% increase in radial compression performance and a 27.89% increase in axial tensile performance before degradation. After 3 weeks of degradation, the radial compression performance increased by 25.14% and the axial tensile performance increased by 33.62%. The optimized fiber-membrane degradable ureteral stent was found to possess improved mechanical properties before degradation and to maintain a high level of performance during a certain degradation period, thereby extending its support and drainage period in vivo.

Conclusion This study investigated the mechanical properties of degradable ureteral stent tubes with fiber-membrane structure before degradation and simulated degradation in vitro, and the following conclusions can be drawn. (1) Numerical simulation was conducted on the degradation process of fiber-membrane degradable ureteral stent, and the mechanical properties during the degradation cycle were studied. The effectiveness of the degradation model was verified through comparison with physical experiments, effectively solving the problems of long cycles and high costs in experimental and clinical testing. (2) Multi-objective optimization of the mechanical properties of stents was carried out based on a kriging surrogate model, and the evolution relationship of mechanical properties before and during degradation was obtained, providing reference for the design of biodegradable stent structures.

Key words: biodegradable ureteral stent tube, polymer degradation, mechanical property, polyglycolic acid, poly (lactide-co-glycolide), medical texile