(Invited) Development of Stress Measurable Vessel Model using Photoelastic Materials
Authors: Sakurai Junpei , Abe Shuhei , Kumeno Yuki , Hori Shimon , Hata Seiichi , Ikeda Seiichi , Saito Azusa , Kawakami Masaru , Furukawa Hidemitsu 
Published: February 2020

Today, intravascular catheter surgeries, which are kind of minimally invasive procedures, are widely used surgical techniques. As compared to conventional open surgery, incision size is small, and burden on patients is reduce. Therefore, the wounds associated with pain is small, and time to heal is also short. On the other hand, since operative field was restricted during surgery, operators need to have sufficient skill for intravascular catheter surgeries. To obtain it, operators must practice using artificial surgical simulators. Materials for surgical simulators are requires having physical properties as like human organs. Although lab animals have similar physical properties, they are not appropriate from an ethical perspective. Moreover, recently, silicone-based materials are used for surgical simulators, but their physical properties do not mimic those of human organs. Gel materials are expected for materials of surgical simulators, because their mechanical properties are similar to human organs and they contain large amounts of moisture. Besides, gel material can apply for fabrication of the complicate shape of human organs using 3D printer.

The issue of practice using conventional surgical simulators is quantitative evaluation of operator skill. In the case of intravascular catheter surgeries, measurement of applied force to vessel wall by catheter and guide wire is an important function. Since vessel walls of patients are delicate, if devices applied excess force, and unexpected accidents such as vascular occlusion are attracted.

In this study, we focused on the photoelasticity of gel materials. If vessel models are made of these gel materials, it is possible them to evaluate quantitative score of operator skill. To realize these surgical simulators, we developed the stress measurement system for photoelasticity of gel materials. Besides, we evaluated the physical properties such as mechanical properties and photoelastic coefficient of several gel materials using our developed system.

We constructed new stress measurement system for photoelasticity of gel materials Our system consisted of several polarizers and wave plates, and it could measure the value of stress and direction of stress in X-Y plane from RGB values on color map. In order to minimize the overlap ratio of the same color in the color map, two 1 wave plates were insert to conventional observation system of circularly polarized wave and the insertion angle of two 1 wave plates is 30 degrees. We suggested analysis algorithm which enables to eliminate the effects of incident light and ambient light. As the results, we succeed to measure the tensile stress by our system during the tensile test of PVA hydrogel. Measurement error was less than 6.3% at the stress range from 0 to 80 kPa, and it is sufficient measurement resolution for evaluate stress during practice of intravascular catheter surgeries.

Generally, Young’s moduli of human vessels depend on the organ and/or symptoms, and they are ranged from 0.02 to 3 MPa. Although mechanical properties of gel material can be adjusted by changing kinds of formulas and their concentrations, it is difficult that only one type gel coverall the range of Young’s moduli of human vessels. Therefore, the materials of surgical simulators are selected on demand from patient of case history. Finally, we investigated the Young’s moduli and photoelastic coefficients of two types of designable gels such as inter cross-linking network gels and double networks gels. As the results, two type gels exhibited photoelasticity. Young’s moduli of inter cross-linking network gels are approximately 1 MPa and the effect of formula concentration was slightly. On the other hand, the range of Young’s modulus of double networks gels was 0.12 to 0.77 MPa for strain range from 0 to 15%, and it was within the range of Young’s modulus of human vessels. Their photoelastic coefficients were 0.012 to 0.022 ×10-9 Pa-1. The measurement error of our system was 7.3%, and was acceptable.