The curvature of the small intestine imposes additional geometric constraints on the design of these devices. The walking sequence for forward movement is as follows (in the diagram, forward movement means right movement). Unfortunately, none of these simplifying assumptions hold for the modeling of the small intestine performed here.
The circumferential strain is calculated from the measured diameter (or radius) of the deformed specimen. In this analysis, the measure of the element in the azimuthal direction, AH, is dropped (this problem is asymmetric and the forces of interest are normal to this coordinate direction).
Criteria for Excessive Loading of the Intestines
Additionally, 2 = sin a where cu is the angle that the tangent to the meridian at s makes with the longitudinal axis of the problem. With the exception of the stress term, om, and its derivative ~ ( E , + v E , ) a"m this equation is purely kinematic, and om and a ( E ~ ~ v E c ) are determined for any deformation state by the experimental generated constitutive law This form of the shear-pull distribution equation is the basis for the numerical simulation of a robot segment's influence on the gut that follows in Section 4.6.
The compressive load on the inner wall of the intestine where such damage was evident was often less than a third of that which caused the specimen to rupture. This criterion was developed to predict the failure of linearly modeled materials as a limit on the allowable strain energy. Another ad hoc limitation on the acceptable levels of strain could be the magnitude of the normal traction force applied to the inner surface of the intestine.
So while this measurement of the robot's impact on the gut neglects the internal wall stresses in the membrane, be aware. This suggests that a measure that predicts the collapse of small intestinal blood vessels may be a well-reasoned criterion for limiting the maximal loading of the small intestine. Since these materials exhibit no dilatation, stresses that would otherwise tend to cause such volume changes can grow without affecting the deformation of the body.
In general, this term is removed from the stress tensor as it plays no role in the prediction of deformations in rubber. By analogy, an external load can "ring blood" from the membranes like water from a towel. For this gut modeling, the pressure is only one third of the sum of the meridional and circumferential stresses.
Elasticity Experiments of the Small Intestine
Experimental Goals and Issues
This is evident as pressure builds and the natural color of the bowel disappears. "Color" returns as soon as the load is removed, but it is not a desirable color. The isotropic stress tensor is a scalar multiple of the identity tensor, and this scalar is calculated to be one-third the trace of the Cauchy stress tensor. This is of course made less daunting if one can predict in advance the nature of the anisotropy by perceptive consideration of the internal structure of the material, thus reducing the number and complexity of the experiments required.
As designed, the grasping segments of the robot expand uniformly to push against the bowel walls, and the extensor segments are laterally compliant to minimize local loading asymmetries. So the robot can be assumed to produce approximately symmetric-ric radial expansion and axial extension in the gut. According to this assumption, intestinal segments can be considered as membranes subjected to internal axisymmetric loads.
So, the significant stresses to be considered in this gut study are the so-called "circular stress" and "meridional stress". This simplification also allows experimentation in the intestines of living pigs; this is a critical advantage and an original contribution of this research. Because the intestines are loosely supported in the abdomen by a thin, vascularized membrane, the mesentery, you can excise a portion of the small intestine that is still connected to its blood supply. This is true only if the intestinal portion is left in a ". tubular state.
Any attempt to produce strip-like test specimens for uniaxial tensile testing will cut its blood supply as its blood vessels surround the intestine. Unfortunately, this technique simultaneously stresses the intestines in both radial and longitudinal (axial) directions, which in turn produces a biaxial stress state in the membranes. Of course, the proposed robot provides a similar influence on the intestinal wall, but ideally one would prefer to measure the peripheral and axial material properties individually to thereby minimize the number of potentially invalid, simplifying assumptions made during data reduction and analysis.
Experimental Apparatus
If the ends of the specimen were prevented from moving, there would be no method of determining the magnitude of the longitudinal load on the membrane. This ensures that there will be longitudinal/axial stress (stress) in the membrane solely as a result of the internal pressure (see Figure 4.10). The clamping boundary condition determines that one plug must move freely in the longitudinal direction in order for the axial tensile forces to be calculated; this force is simply the internal pressure acting on the projected area of the specimen's cross-section (this was discussed in Section 4.4).
If independent measurements of these orthogonal properties are required for intact intestinal lumen, separate experimental systems must be created: one to expand radially without inducing axial loading, and one to perform a uniaxial tensile test of the tubular specimen. All sharp edges of these instruments were blunted for obvious reasons, but repeated grinding of the calipers' jaws at the measuring locations can abrade the membrane. Also, at low internal pressures, the experimental uncertainty of these measurements increases due to the extreme compliance of the sample.
This is because one must visually measure the distance between two ink dots on the surface of the intestinal segment. A very shallow burn mark was made on the outer membrane of the intestine, which could be marked with a felt-tip pen. Careful adjustment of the device's precision regulator could establish desired pressure loads in 0.1 psi increments with uncertainties less than 0.01 psi (less than 0.005 psi was typical).
Finally, to determine the stresses in the membrane, one must measure its wall thickness. One possible experimental technique measures the delays between the echoes of ultrasound pulses hitting the walls of the membrane. To this end, several piezoelectric ultrasound transducers were evaluated for measuring the thickness of excised intestinal tissue.
In Vivo Biomechanics Experiments
The apparatus is lowered so that a segment of the bowel can be attached as shown in Figure 4.11. The ability of the blood vessels supplying the segment to move with it prevented tension from building up in the mesentery. An estimate of the experimental uncertainty in the diameter measurement is 0.010 inch and for the length, about 0.020 inch.
This is likely to be reflected as a stiffening of the peripheral behavior compared to the longitudinal behavior. The membrane was squeezed between the jaws of the instrument as little as possible during these measurements. Furthermore, it provides a comparison between the elastic behavior of the living intestines with that of dissected specimens.
However, in vitro experiments can never be a valid measure of excessive strain on the intestinal tissue. In the initial configuration of the grasper segment, the bowel is unloaded and assumed to be tension-free. The longitudinal elements are progressively modeled until the right side of the expanded gripper segment is reached.
The shape of the gripper can now be changed and the modeling of the surrounding intestine begins again. In this figure, the risk of injury is reflected in the color of the membrane surface, with white representing excessive load/deformation. The societal benefits of the increased use of minimally invasive medicines were discussed (both for improved healthcare delivery and for potentially lower fiscal expenditure on healthcare).
These efforts highlighted unexpected issues that significantly affected the design of the fifth generation prototype. The proposed movement concept appears to be viable for safe and reliable endoscopic traversal of the small intestine.
In Vzvo Experiment a1 Observations
The Robotic Endoscope System Design
In an effort to provide a means of minimally invasive access to the entire small intestine, a self-propelled robotic endoscope has been conceived and patented. Initial motion theories were developed in an attempt to increase the reliability of such a device, i.e., the ability to crawl safely within a lubricated, steady, curved environment (including, for example, fail-safe redundancy mechanical). As an example, the extreme compliance of the intestinal membrane and its bending forced a reevaluation of the basic concepts of bowel movement.
And then a numerical model was developed to predict the interaction between a generic robot component and the intestinal membrane. Modeling the mechanical traction of the robot against the inner surface of the small intestine is beyond the domain of conventional robot grasping theory and concepts that tend to study fairly solid objects in contact. Furthermore, these studies provided a biologically inspired constraint on the acceptable stresses generated in the intestinal membrane.
Because these efforts represent a first attempt at robotic endoscopy of the small intestine, numerous areas for technical and scientific research remain unexplored. A detailed investigation into the use of the magnitude of membrane isotropic tension as a viable predictor of injury would be very valuable to properly model these living tissues. It would be useful to collect additional data that would allow an improved correlation between the meridional and circumferential elastic behavior of the small intestine for the animals studied.
0 All biomechanical analyzes and experiments relied on an assumption of "locally volume-preserving" deformations, i.e. incompressible behavior. This simplification should be verified by experiments that measure the wall thickness of the intestinal sample simultaneously with its length and diameter. This involves, in addition to testing advanced robot prototypes, the quasi-static computer simulation to be extended to model the intestinal response of several robot segments, i.e. a complete mechanism.
