Measurement of mechanical properties of nitinol stent

Abstract: The mechanical performance parameters of the stent are very important for the use of the stent. The supporting force is the most important factor in determining the performance of the vascular stent. Only with good supporting performance can the stent fully support the blood vessels and smooth the blood flow. The near-equal atomic ratio nitinol alloy was selected for use as the scaffold material. DSC measured the phase transition temperature of the alloy wire, and the mechanical properties of the scaffold material were measured by tensile test. Through the determination of the support force of the stent and the comparison with well-known foreign stents, it was found that the radial force and elastic properties of Chinese-made vascular stents are good, and the end structure can be modified to achieve a better effect than that of foreign stents.

Key words: Nitinol shape memory alloy; vascular stent; radial support force; elasticity

In recent years, the incidence of aortic aneurysms has shown an upward trend. Intraoral treatment of aortic artery disease has become the main treatment method. Stent implantation is becoming more and more widely used in human cavity stenosis. The materials used in stents include stainless steel, magnesium alloy, shape memory alloy and other materials. Nitinol vascular stents have attracted more and more attention due to their good biocompatibility, shape memory properties and less thrombogenic properties. The radial support force is the resistance of the stent to radial external pressure or the strain of the stent on the external force acting on it. Radial support force is a very important performance index of vascular stents. It is related to whether the stent can be firmly attached to the blood vessel wall after it is unfolded, and whether the blood can be kept unblocked. The purpose of this paper is to explore the advantages of the mechanical properties of nitinol alloy as a vascular stent through the determination of the properties of nitinol alloy wire and the determination of the radial force of Chinese-made stents, and to guide the actual production.

1. Stent material

2. Experimental equipment

Differential scanning calorimetry (DSC, differentialscanningcalorimetry) determines the phase transition temperature of nitinol alloy wire and selects the appropriate experimental temperature; the universal mechanical stretching machine performs tensile tests on the stent wire to obtain the performance parameters of the nitinol wire at the operating temperature of the stent; DL-10 microcomputer-controlled electronic ultrafine testing machine compresses the abdominal aorta at various positions of Chinese-made stents and foreign stents, and measures the radial support force of one foot and performs comparative analysis.

3. Experimental process

3.1 Determination of mechanical properties

3.1.1 DSC determination

Take samples of the selected nitinol alloy wire and perform DSC measurements as shown in Figure 1. The phase transition temperature is 13.5℃. 6～26． 8℃, since the alloy stent is applied to the human body, the normal temperature of the human body’s body temperature is 36~37. Between 6℃, the alloy wire is at 13.5%. 6～26． It is in an austenitic state at 8℃, and the working state at body temperature is very stable. Therefore, the body temperature of 37℃ is selected as the experimental temperature for the following tests, which also meets the working requirements.

3.1.2 Determination of mechanical properties

3.2 Comparison of stent support force measurement

Using the DL-1O microcomputer-controlled electronic ultrafine testing machine of the Mechanics Research Laboratory of Tsinghua University, the DL-1O microcomputer-controlled electronic ultrafine testing machine was used to compare the supporting force performance of well-known foreign vascular stents (stents 1), the blood vessel stents (stents 2) made of nickel-titanium alloy wire selected in this article, and the blood vessel stents (stents 3, the same material as stents 2, the structure is slightly different) with fine-tuned structures woven from the same nickel-titanium alloy wire. Figure 3 shows the location map of each test area of the stent. Point A represents the position of the crown end of the stent, Point B represents the reinforcing ring of the stent, point C represents the middle position of the stent, and Point D represents the position of the tail end of the stent. During the test, rotate the points at each position separately, so that the appropriate points can be taken and measured by point-to-point pressure.

3.2.1 Radial force comparison of the three stents

The point-pressure method determines the radial force at different positions of each stent. As shown in Figure 4, the radial force at the crown end of stent 1 is lower than that of stent 2; and the radial force in the middle of stent 1 is significantly higher than that of stent 2 and stent 3. Clinically, it has been shown that the radial support force of stent 2 can fully meet the requirements of use. The excessive radial support force makes stent 1’s ability to deform with the body after implantation in the human body is poor, that is, the compliance is worse than that of stent 2 and 3.

3.2.2 Comparison of radial forces at the crown end

Due to the different dimensions of the crown end of the stent, the overall shrinkage of the stent to 25mm is equivalent to the compression capacity of the stent 1 with a single foot of 2.5mm, the compression capacity of the stent 2 with a single foot of 7.5mm, and the compression capacity of the stent 3 with a single foot of 2.5mm. As can be seen from the compression-radial force curve in Figure 5, the slope of the curve of stents 2 and 3 is close to 1, and the curve is approximately a straight line of Y=x, indicating that the elasticity of stents 2 and 3 is very good, while the slope of the curve of stent 1 is greater than 1, indicating that the rigidity of the stent is strong, Stent 2 Compared with stent 1, the radial force of the crown end of stent 2 is slightly greater than stent 1, and the elasticity and compliance of stent 2 are better than stent 1.

 4. conclusion

1) The experimental alloy wire is at 13. 6～26． It is in an austenitic state at 8℃, which is very soft and can be deformed into a shape that is easy to place a human catheter at will, and sent to a predetermined narrow position in the human body through the catheter; and it is in a very stable martensitic state within the body temperature range, and the stent quickly returns to its original state, supporting the blood vessels, and the working state at body temperature is very stable；

2) The mechanical properties of the selected nickel-titanium alloy wire at 37℃ are 1327.5MPa tensile strength, 7.69% elongation at break, and 24.7Gpa elastic modulus (the elastic modulus of the commonly used nickel-titanium alloy is 25~30gpa). The support elasticity is very good, and the flexibility and deformation with the body are also very good；

3) The stent has independent intellectual property rights, and the radial support force can.