Design and fabrication of a locomotive mechanism for capsule-type endoscopes using shape memory alloys (SMAs)

Endoscopes are medical devices to diagnose various kinds of diseases throughout the whole gastrointestinal tracks. Generally, they are divided into conventional push-type endoscopes and more recently developed wireless capsule-type endoscopes. The conventional endoscopes cannot reach the small intestines and generate pain and discomfort to patients due to the stiffness of their body. Such disadvantages do not exist in wireless capsule-type endoscopes. However, commercialized capsule-type endoscopes move passively by peristaltic waves (and the gravity), which makes it impossible for doctors to diagnose the areas of his or her interest more thoroughly and actively. To address this problem of passivity, a locomotive mechanism is proposed for wireless capsule-type endoscopes. Prototypes with micro brushless dc motors, ionic polymer metal composite actuator, and shape memory alloy (SMA) wires are designed and fabricated for preliminary tests. Based on the tests, spring-type SMA actuators are selected to be microactuators for capsule endoscopes. Thus, two-way linear actuators using a pair of SMA springs are developed based on a static analysis on them. Moreover, a simple and effective clamping device is developed based on biomimetic approach. A prototype endoscope with four pairs of SMA springs and four clampers was developed. It has 13 mm in diameter and 33 mm in total length, with a hollow space of 7.6 mm in diameter to house other parts that are needed for endoscopy such as a camera, an RF module, sensors, e.g., for endoscopic ultrasound, and a battery. A sequential control of the four actuators improves the efficiency of locomotion up to four times. To validate the performance of the proposed locomotive mechanism, a series of experiments were carried out including in-vitro tests. The results of the experiments indicate that the proposed locomotion mechanism is effective to be used for micro capsule-type endoscopes.