2     Result
2.1 Experiment on the influence of needle insertion speed on puncture force
Combined with the experimental data obtained, a set of data at 150mm/min of  IPN was selected as a typical sample, and the needle insertion process can be divided into four stages, as shown in Fig.3. The first stage:the surface of the polyurethane film is in contact with the tip of the needle, and the tip is deformed under the action of the tip displacement. The tip does not puncture the surface, and the needle resistance gradually increases, which corresponds to the a stage in Fig.3 on the data. The second stage:the local pressure on the surface of the polyurethane film reaches the critical value of its tensile strength, the needle tip punctures the surface, the film stress is released to a certain extent, the needle resistance decreases significantly, and the first peak is formed, which corresponds to the b stage in Fig.3 on the data. The third stage:the contact surface between the tip side wall and the polyurethane film increases, and the tip contacts with the bottom surface of the polyurethane film, but the bottom surface is not pierced, and the needle resistance rises again, which corresponds to the c stage in Fig.3 on the data[20]. The fourth stage:the needle tip pierces the bottom surface, and the polyurethane film is penetrated to form a second peak, which corresponds to the d stage in Fig.3 on the data. 
There are differences in the tip process of different needles, which makes the relationship between the peak values different. For example, different oblique cutting angles of the section will cause the first peak to be much smaller than (or slightly larger than) the second peak. In this experiment, the maximum peak value was selected as the value of puncture force.

Fig.3. The force stage of the needle insertion process
Statistical analysis was performed on the experimental data of each group at different speeds, and it was found that only the difference between the value of the needle speed of 50mm/min and the other four groups was statistically significant (p＜0.05). It shows that although the puncture force increases with the increase of the puncture speed, the difference is not statistically significant when the puncture speed is above 100mm/min. The piercing force at five different speeds is shown in Fig.4. 
The average value of the puncture force at the puncture speed of 150mm/min was taken as the puncture force of each needle. The puncture force data of conventional needles are shown in Table 4. The puncture force data of the self-destructing needle is shown in Table 3, of which 30G indicates that the diameter of the needle tube is 0.3mm, 31G indicates that the diameter of the needle tube is 0.25mm, and the small diameter needle shows a small puncture force on the whole. However, the influence of the needle tip process on the puncture force can not be ignored, and there will be a case where the puncture force of the large diameter needle is smaller than that of the small diameter needle.

Table 3 Needle Piercing Force of Self-Destructing Insulin Pen

Fig.4. Piercing force at different injection speeds

Table 4 Experimental data of conventional insulin pen needle

Note:According to the numerical value of tilt resistance, the descending order is arranged.