General Design Specifications
Our client specified a number of general design specifications that must be met for the design of the model to be successful. These requirements are summarized to the left, and include cost, portability, safety, and use-specific requirements.
The client has requested that the model constructed be able to withstand at least 24 tube placement tests, which would give our system an advantage over some previously tested models. In addition, the client has several effective methods for simulating the esophagus, and thus our model would not need to reconstruct this capacity, but must be able to connect to these products.
Mechanical Design Specifications
We considered several different mechanical parameters of stomach and intestinal tissue that we thought would be important to simulate in our physical model. The parameters that we determined were most important were the maximal stress and strain in the longitudinal and transverse directions, the bulk modulus, the shear modulus, and the coefficient of friction. We found these values for both the stomach and the small intestines.
These values were found through literature searches, and were determined using techniques such as the Kolsky bar technique for the bulk and shear modulus, and rheometry for the coefficient of friction. We may use these same techniques to confirm the properties of any materials we decide to test for our final design.
Geometric Design Specifications
There is significant variation in the geometry of the human digestive tract both between individuals and in the same individual in different postures and feeding conditions. Due to this variation, we have selected dimensions for the average human population and will base our model on these values. This will capture a large bucket of the patient population.
Values for the duodenum and jejunum are reasonably established in the literature, and are given with a range of several centimeters. The values for stomach dimensions are less reliable, as anatomy texts will not give “typical” values for an organ of such high distensibility and variation. The geometry of the stomach will have great variation in the population from factors such as the time since the last meal of a patient. However, from the literature, ranges of values have been determined for the stomach dimensions.
Project Scope
The process of designing any engineering solution requires an understanding of what you are designing the product to do and what you are not. The general scope for our project is to develop a model to mimick the average adult patient stomach, duodenum, and jejunum. The model will only need to test nasojejunal feeding tubes. More detailed design specifications for our client can be found below.
Lubricity Specifications
The lubricity of the stomach and small intestine is extremely important to the funciton of the model that we construct, as the movement of the feeding tube dramatically changes due to these properties.
Although the lubricity of the stomach, duodenum, and jejunum is a critical part of the model, the method by which the mucus is secreted into these areas is not of concern. Therefore, the outer layer of mucus which prevents bacteria from entering the epithelium, and the means of cell proliferation within the stomach will not be addressed. Instead, the mechanical properties of the mucus, such as viscosity, are important in order to develop a suitable model to test the feeding tube placement. We have determined values for these properties through literature searches.
