## 2 Material and Methods  
### Micro Piezoelectric Actuator in the Endoscopic biopsy Channel  
To ensure actuator operation in the endoscopic biopsy channel, a light-weight surface milli-walker based on piezoelectric actuation was designed and reported in our recent research [30]. Although its overall size met the size limit of an endoscope, the actuator needs a pre-pressure track, which was provided by an electromagnet and corresponding power supply wire in actual installation, and the resultant overall size turned out to be too large to be integrated into the biopsy channel of an endoscope. Therefore, a permanent magnet orbit instead of an electromagnet is used to provide the magnetic field for the actuator in this paper. Despite the permanent magnet size, whose thickness is far smaller than that of an electromagnet, it still has a high magnetic induction intensity to provide enough pre-pressure, making it suitable for application in the endoscopic biopsy channel, as shown in Figure 1a. In addition, the overall size of the actuator is miniaturized to 4.5 mm \(\times\) 1.5 mm \(\times\) 1.5 mm (length \(\times\) width \(\times\) height) and its driving frequency is 10.4 kHz in this paper. The actuator weight is 0.063 g, in which the size of the thickness is 0.2 mm and the size of the piezoelectric ceramic is 1.5 mm \(\times\) 0.2 mm \(\times\) 3 mm, as seen in Figure 1b. Moreover, to enable the linear motion of the actuator in the endoscopic biopsy channel, the permanent magnet orbit is polished by sand paper to achieve a thickness of 0.35 mm and an effective working distance of 10 mm, and the object is shown in Figure 1c. Generally, the biopsy forceps, needles, and other medical consumables of an endoscope that can move in the endoscopic biopsy channel during surgery are installed with the help of a cleaning brush. In this work, we also employed the cleaning brush to install the actuator and permanent magnets in the endoscopy and realize their combination. The schematic diagram is shown in Figure 1a. The cleaning brush was modified to remove the hair brush of its head and use it as a guide line. Then, the permanent magnet orbit was fixed at the end of the cleaning brush using adhesive tape and the actuator was absorbed on the orbit by the magnetic field. After that, two wires were also fixed on the  
Figure 1: Method of installing the piezoelectric actuator in the endoscopic biopsy channel: **(a)** the schematic diagram of the piezoelectric actuator; **(b)** the dimensions of the actuator; **(c)** the actuator with an endoscope cleaning brush; and **(d)** the actuator in the biopsy channel. cleaning brush to ensure that the driving voltage could be applied outside. As described above, the actuator system could be installed in the endoscope biopsy channel as a whole, as shown in Figure 1c. The installation steps of the actuator and permanent magnet orbit in the endoscopic biopsy channel are as follows:  
1. Put the actuator on the permanent magnet orbit and fix the actuator through the magnetic field, making it unable to fall off during the installation process;
2. Insert the guide line from the entrance of the biopsy channel. During the installation process, the cleaning brush and the actuator must be inserted under a smooth condition;
3. Ensure the bending angle of the endoscope is not too large to avoid damage to the actuator and permanent magnet orbit when the actuator passes through the bending part of the endoscope;
4. When removing the actuator after use, it should be pulled out from the entrance of the biopsy channel slowly and lightly to prevent the actuator from falling off the track and leaving the endoscope during the pulling process. It should be noted here that due to the thickness of the permanent magnet orbit being only 0.35 mm, it is easy to break under collision, and thus, special attention should be paid to the installation process. Using the aforementioned steps, the actuator system and its supporting cleaning brush were successfully installed in the endoscopic biopsy channel, as shown in Figure 1d.