Basic Study on Photoelastic Stress Measurement for Surgery Simulator using Gel Materials

Abstract

Catheterization procedure is a typical example of minimally invasive medical treatment. This has the advantage that the physical burden on the patient can be remarkably reduced as compared with the conventional medical treatment method involving craniotomy or hemorrhage etc. However, on the other hand, since visual and tactile restrictions are limited at the time of surgery, physicians are required to have expert skill and many experiences. To raise the success rate of surgery irrespective of the amount of surgical experience, we have conducted a simple model using a glass tube and a simulation operation using an experimental animal, but they have low bio-reproducibility and ethical problems was there. In recent years, due to the development of new materials and the appearance of 3D printers, shape reproducibility has improved. A more desirable function of the surgical simulator is to visualize the force applied to the vessel wall surface by the catheter. This is because there is a danger of attracting vascular occlusion etc. when a large force is applied to the wall surface of the blood vessel. Therefore, it is considered important to quantitatively evaluate the operational feeling for simulated surgery. Therefore, we aim to manufacture a vascular surgical simulator with high bio-reproducibility that enables real-time stress measurement. In this research, stress visualization method using photoelastic effect is applied to surgical simulator. By using the photoelastic effect, it becomes possible to check the stress state of the entire field of view, and it is possible to obtain a stress state when a medical instrument such as a coil or a stent is inserted. Therefore, it is useful not only for training but also for the development of medical instruments. Heretofore, examples using PDMS and polyurethane elastomer as a surgical simulator material have been reported, but the longitudinal modulus of elasticity is high and it is unsuitable for a blood vessel model. Therefore, we focused on PVA hydrogel as surgical simulator material. PVA (polyvinyl alcohol) hydrogel is a material that can be expected to reproduce vascular tissue because its physical properties can be easily changed by changing the compounding ratio and degree of polymerization of powdered PVA. By changing the compounding ratio of powdered PVA in the range of 4-15 wt.%, We realized PVA hydrogel's longitudinal elastic modulus of 60-450 kPa. Since the longitudinal elastic modulus of human vascular tissue is 20-3000 kPa, PVA hydrogel is useful for reproducing soft vascular tissue. Further, the average photoelastic coefficient of PVA hydrogel is 2.30 × 10-9 Pa-1, which has a photoelastic coefficient greater than that of polyurethane elastomer which is a photoelastic material useful for photoelastic stress measurement. (photoelastic coefficient of polyurethane elastomer is 1.45 × 10-9 Pa-1.) Therefore, it was confirmed that it is a useful material for stress measurement using photoelastic effect. In this study, a simple blood vessel model was prepared using PVA hydrogel and simulated surgery using a catheter was performed. In addition, a photoelastic observation system was constructed for stress measurement, and a method for correcting the influence due to the presence of incident light, ambient light, and initial stress was examined. As a result, we successfully extracted and visualized the stress applied to the simple blood vessel model.