A detailed design analysis of a lumenally delivered, flexible, balloon-assisted, sterile endoscopic overtube.

Abstract

The goal of this project was to develop a sterile surgical device for gaining peritoneal cavity access without external incisions via the body's major natural orifices. Such Natural Orifice Translumenal Endoscopic Surgery (NOTES) interventions have recently received innovative attention as the next step in minimally-invasive surgery, however, the safety and efficacy depends greatly on sterility. The research for this thesis focused primarily on gastric interventions is divided into four main foci: tensile testing and determination of the most accurate material model for human gastric tissue, FEA analysis of the balloon dilation, experimentally validating these results using a silicone tissue phantom, and the paper design of a theoretical prototype. Tensile testing the human gastric tissue provided the only material properties for the entire stress-strain curve known to the literature to be accurate. A series of tests were conducted on several different freshly donated organs. Statistical analyses were performed comparing the inner and outer layers, and the 0° and 90° orientation. These results showed that while visually different, the elastic portion of the stress-strain curve showed no statistical differences between layers or orientation. A FE model was created in 2D axisymmetric and 3D to determine the minimum size incision needed to dilate large enough to allow passage for the device and endoscope without inducing irreversible damage to the tissue. Conclusions from tensile testing led to the material model being hyperelastic, homogeneous, and isotropic. ABAQUS Explicit was used to model the quasi-steady state problem and to more effectively manage contact definitions in the 3D simulation. The models were also simulated with a silicone material for experimental validation. Using the same assumptions from the ABAQUS model, a physical experiment was performed with the silicone tissue phantom. From a circular incision of 10mm, several final diameters were tested. Rigid objects were used to dilate rather than balloons for ease of visualization. The surface had nodal coordinates drawn on the material and digital images taken before and after dilation. Coordinates were extracted using xyExtract.exe, and strains calculated with a user defined MATLAB program. Taking the above work into consideration, three different prototype designs were proposed. All three incorporate dual-donut balloons as the primary means of dilating the initial incision, holding the device in place, and providing a mechanical means for ensuring sterility while maintaining insufflation. The first embodiment simply incorporates dual balloons on the end of a PTFE sheath. The second utilizes a corkscrew-shaped singular balloon crimped in the middle to form the dual expansion zones. Finally, the last prototype design uses a second outer sheath to encapsulate both balloons which provides the dilatory force to the tissue, and acts as a long cylindrical balloon to stabilize the length of the sheath.