Utilisation of a wrinkled film-based structure for the sensing measurement of a vibro-impact capsule robot

Abstract

According to a pre-strain strategy, a wrinkled structure for measuring the contact pressure between the intestinal wall and the vibro-impact capsule robot is proposed for the potential application of early bowel cancer detection. An analytical model of this wrinkled structure is established by using an energy method. Based on Hertz theory, a theoretical expression for the elastic modulus of bowel tissue and the amplitude of this structure, so the current generated, is obtained. It is found that the sensitivity of the structure is dependent on its wrinkled amplitude. Our simulation results show that, for a static capsule, the greater the contact pressure between the wrinkled structure and the capsule is, the greater the current of this mechanism is generated, indicating the bowel tissue becomes stiffer. For a dynamic capsule, our simulation results reveal that the faster the average velocity of the capsule is, the greater the current is generated. These relationships are explained by modelling the hoop pressure of the intestine on the capsule robot validated via finite element analysis. The findings of this paper can provide design guidelines for fabricating the proposed mechanism integrated onto the vibro-impact capsule robot for diagnostic and locomotion tracking purposes.

Appendix

When the magnetic inner mass is excited by an external magnetic field, the capsule moves in the intestine, and its motion can be modelled via the following equations,

$$\begin{aligned} \begin{aligned} M_{\textrm{m}}{\ddot{Z}}_{\textrm{m}} = F_{\textrm{e}} - F_{\textrm{i}},\\ M_{\textrm{c}}{\ddot{Z}}_{\textrm{c}} = F_{\textrm{f}} + F_{\textrm{i}}, \end{aligned} \end{aligned}$$

(A.1)

where $F_{\textrm{e}}= P_{\textrm{d}}\cos (\omega t)$ is the excitation, $F_{\textrm{f}}$ denotes environmental resistance. According to the contact case between the magnet and the capsule, the mutual interactive force $F_{\textrm{i}}$ can be written as

$$\begin{aligned} F_{\textrm{i}} = {\left\{ \begin{array}{ll} k (Z_{\textrm{m}} - Z_{\textrm{c}}) + c (V_{\textrm{m}} - V_{\textrm{c}}), &\quad \text {if } Z_{\textrm{m}} - Z_{\textrm{c}} < G_1,\\ k (Z_{\textrm{m}} - Z_{\textrm{c}}) + c (V_{\textrm{m}} - V_{\textrm{c}}) + k_1 (Z_{\textrm{m}} - Z_{\textrm{c}} - G_1), &\quad \text {if } Z_{\textrm{m}} - Z_{\textrm{c}} \ge G_1,\\ \end{array}\right. } \end{aligned}$$

(A.2)

where $Z_{\textrm{m}}$ and $V_{\textrm{m}}$ are the displacement and velocity of the magnet, $Z_{\textrm{c}}$ and $V_{\textrm{c}}$ are the displacement and velocity of the capsule, and $G_1 = 1.6~{\textrm{mm}}$ is a gap between the magnet and the constraint. Because the friction models have no significant effect on the dynamics of the capsule, the frictional resistance acting on the capsule $F_{\textrm{f}}$ is modelled as the Coulomb friction and is expressed as

$$\begin{aligned} {\left\{ \begin{array}{ll} F_{\textrm{f}} \in [-P_{\textrm{f}}, P_{\textrm{f}}], &\quad \text {if } V_{\textrm{c}} = 0,\\ F_{\textrm{f}} = -{{\,\textrm{sign}\,}}(V_{\textrm{c}}) P_{\textrm{f}} &\quad\text {if } V_{\textrm{c}} \ne 0,\\ \end{array}\right. } \end{aligned}$$

(A.3)

where $P_{\textrm{f}} = 2.5~{\textrm{N}}$ denotes the static friction and is identified by experiment [38].