Design Alternatives and Analysis
To begin research into various design alternatives for the Teaching Heart, the team decided to divide the model into its main discrete structures. These structures were the Medium, Left Ventricle, Arteries and Veins, Resistance Vessels, and Resistance Mechanism. In the sections below, a brief overview of each structure's analysis is given (i.e. Pugh charts) to display how a final design was chosen. Following this analysis, a detailed diagram of the chosen design is displayed.
In creating the Pugh charts, the team identified 10 characteristics representative of the design specifications and weighted them based on their relative importance in the Teaching Heart design. Each characteristic was weighted between 1 and 10, with 1 being least significant and 10 being most significant. Additionally, each component was weighted between 1 and 10 for each characteristic based on the extent to which it fulfilled that characteristic. Note that some characteristics did not apply to every component.
The team weighted four characteristics as being most important (weight 10): sustainability, safety, the ability to model the circulatory system, and quantitative assessment of venous return. Sustainability is the primary problem associated with Dr. Wilkinson’s model; therefore it is important that the Teaching Heart model have a long lifespan. Components received a score of 10 if the longevity of the component was on the order of several years. Scores below that reflect a shorter longevity. Intended for use in the classroom setting, it is of utmost importance that the device be safe for its users. Components were ranked on safety based on the relative likelihood and severity of potential hazards; a score of 10 indicates less likely, negligible hazards, whereas scores of 1 indicate likely, severe hazards. Additionally, the device must appropriately model the circulatory system in order to succeed as a useful teaching aid. Components were ranked in this category based on the extent to which they fulfill the design specifications under this characteristic. Finally, students operate the device by changing the cardiac output to maintain a constant venous return; therefore, it is important that the student can quantitatively assess this return while using the device. Design alternatives were ranked based on the relative feasibility of this assessment.
Next, the team weighted four characteristics as being of next importance (score 9): size, weight, cost, and simplicity & ease of use. Because the device will likely be transported to and from different classrooms, it is important that the device be portable with a size and weight near those specified. Additionally, students come from a variety of backgrounds; thus the device must be accessible to those with and without prior knowledge of the subject or technical expertise. Components were ranked on the extent to which they exceeded, met, or fell short of these four specifications.
Finally, the team weighted resemblance to the circulatory system with a score of 7; while it is not pertinent to the functionality of the device, such similarity aids in comprehension. By having a device that physically resembles the circulatory system, the student is not distracted by differences in appearance and so can focus on the underlying concepts. Additionally, the team weighted the additional features with a score of 5. The client expressed that these features should be explored, but not take priority over the other requirements. Components were then ranked on the extent to which they exceeded, met, or fell short of these two specifications.
In creating the Pugh charts, the team identified 10 characteristics representative of the design specifications and weighted them based on their relative importance in the Teaching Heart design. Each characteristic was weighted between 1 and 10, with 1 being least significant and 10 being most significant. Additionally, each component was weighted between 1 and 10 for each characteristic based on the extent to which it fulfilled that characteristic. Note that some characteristics did not apply to every component.
The team weighted four characteristics as being most important (weight 10): sustainability, safety, the ability to model the circulatory system, and quantitative assessment of venous return. Sustainability is the primary problem associated with Dr. Wilkinson’s model; therefore it is important that the Teaching Heart model have a long lifespan. Components received a score of 10 if the longevity of the component was on the order of several years. Scores below that reflect a shorter longevity. Intended for use in the classroom setting, it is of utmost importance that the device be safe for its users. Components were ranked on safety based on the relative likelihood and severity of potential hazards; a score of 10 indicates less likely, negligible hazards, whereas scores of 1 indicate likely, severe hazards. Additionally, the device must appropriately model the circulatory system in order to succeed as a useful teaching aid. Components were ranked in this category based on the extent to which they fulfill the design specifications under this characteristic. Finally, students operate the device by changing the cardiac output to maintain a constant venous return; therefore, it is important that the student can quantitatively assess this return while using the device. Design alternatives were ranked based on the relative feasibility of this assessment.
Next, the team weighted four characteristics as being of next importance (score 9): size, weight, cost, and simplicity & ease of use. Because the device will likely be transported to and from different classrooms, it is important that the device be portable with a size and weight near those specified. Additionally, students come from a variety of backgrounds; thus the device must be accessible to those with and without prior knowledge of the subject or technical expertise. Components were ranked on the extent to which they exceeded, met, or fell short of these four specifications.
Finally, the team weighted resemblance to the circulatory system with a score of 7; while it is not pertinent to the functionality of the device, such similarity aids in comprehension. By having a device that physically resembles the circulatory system, the student is not distracted by differences in appearance and so can focus on the underlying concepts. Additionally, the team weighted the additional features with a score of 5. The client expressed that these features should be explored, but not take priority over the other requirements. Components were then ranked on the extent to which they exceeded, met, or fell short of these two specifications.
Medium
Below is the updated Pugh chart which ultimately led to the team's decision to use water as the model's medium. In the progress report, the team originally had chosen aqueous glycol. But after consultation with the professor and analyzing the options, a new Pugh chart was created. Through this analysis water was chosen.
Three alternatives were considered for use as the medium within the Teaching Heart model: air, water, and aqueous glycol.
Three alternatives were considered for use as the medium within the Teaching Heart model: air, water, and aqueous glycol.
- Compressed air is the current medium used in Dr. Wilkinson’s model. It is a lightweight option, lending to the model’s portability. However, it is not readily available in the classroom, and compressed air canisters must be purchased to refill the system.
- Water is a readily available at zero cost, making it the most affordable option. However, unlike air, water would add weight to the model. Though, if the device is emptied after each use, this would not pose a problem for its portability.
- Aqueous glycol was identified during research as a medium commonly used to model blood because of their similar properties. Therefore, this medium would most closely resemble the actual circulatory system. However, aqueous glycol is not readily available in the classroom and would be an ongoing cost for using the device.
Medium Pugh Chart
Left Ventricle
Below are the Pugh Charts which ultimately lead to the team's decision to use a Fuel Line Primer Bulb for the model's left ventricle.
Four alternatives were considered for the left ventricle: a reusable resuscitation bag, an elastomer core, a PVC balloon, and a fuel line primer bulb.
Four alternatives were considered for the left ventricle: a reusable resuscitation bag, an elastomer core, a PVC balloon, and a fuel line primer bulb.
- Dr. Wilkinson’s current model utilizes a resuscitation bag to model the left ventricle. The resuscitation bag is cost-effective and allows the user to easily manipulate both the rate and intensity of pumping. However, the bags are disposable and so may not prove sustainable over the course of several academic years. The bags also are used solely with air and so may be difficult to integrate a liquid medium if the team decides to choose that route.
- An elastomer core is an out-of-the-box option the Teaching Heart team considered. Commonly used in animal toys, the material is compliant yet highly durable which are both advantageous attributes to the design of the left ventricle. It should be noted that holes would need to be drilled on either end of the chamber to allow for flow through the core while pumping. This could easily be achieved but would brings about another issue, lack of unidirectional flow.
- The PVC blowing balloon option is highly similar to the elastomer core. It is durable and easily compressed. Furthermore, it would need to be drilled to obtain a second hole allowing flow through the balloon. Unidirectional flow would be a concern here as well. Additionally, the PVC blowing balloon is generally less than two inches in diameter at maximum and would limit the stroke volume of the system greatly.\
- The last option heavily considered was a fuel-line primer bulb. The idea to use this for the left ventricle came about when the Teaching Heart team was at the local hardware store searching for ideas. Besides being durable and compliant, an added advantage to the primer bulb is its everyday use in pumping a liquid medium (i.e. fuel) in marine sports products. Since the team is highly considering using water in the system this would be an added bonus. While not as drastic as the PVC blowing balloon, one slight drawback to the bulb is its size. The bulb could administer a decent stroke volume just nothing as ideal as that of the resuscitation bag. However, the main reason this option is highly considered to be used as the left ventricle is the integration of check valves in the inlet and outlet. This feature allows for unidirectional flow, something paramount in modeling left ventricular circulation.
Left Ventricle Pugh Chart
Arteries and Veins
Below is the Pugh chart which ultimately lead to the team's decision to use a Rigid Chamber for the model's arteries and veins.
Three alternatives were considered for modeling the arteries and veins: balloons, tubing, and chambers.
Three alternatives were considered for modeling the arteries and veins: balloons, tubing, and chambers.
- The current prototype utilizes latex balloons to model the compliance of the arteries and veins. While these effectively model the difference in compliance between the arteries and veins, latex balloons are not sustainable. The balloons rot and must be replaced every year. Additionally, the balloons are subject to tears causing the medium, air, to leak. Therefore, the team investigated other, more sustainable, balloon options. Polyurethane balloons, commonly used for advertisement purposes, are compliant but are far more sustainable than latex balloons. With a balloon design, tubing would also be used to connect the balloons to the left ventricle. This tubing would be consistent with that chosen to model the capillary bed.
- The team also considered using compliant tubing to model the arteries and veins. Specifically, latex-rubber and polyethylene tubing were considered because of their elastic properties. This elasticity is necessary to model the distensibility of the actual arteries and veins. Difference in compliance between the arteries and veins would be achieved by manipulation of the tube dimensions or use of an external sleeve. However, use of tubing would require a change in the way students utilize the device. Currently, users manipulate cardiac output in order to maintain a constant venous return; therefore, the model would need a mechanism to gauge the volume of medium within the arteries and veins.
- Chambers were also considered to model the compliance of the arteries and veins. The closed chamber has a variable amount of air trapped above the fluid. By changing the relative amount of trapped air, and therefore the fluid height within each chamber, one can achieve the compliance differential between the arteries and veins enumerated in the design specifications. Additionally, the chamber design is durable and sustainable. However, this added durability is at the cost of portability. The rigid chamber would be bulkier and therefore less portable than the other options described above. Similar to the balloons, tubing would be used to connect the arteries and veins to the left ventricle and capillary bed.
Arteries and Veins Pugh Chart
Capillary Bed
Below is the Pugh Chart which ultimately lead to the team's decision. upon determination of the Resistance Mechanism, to use Vinyl Tubing for the model's capillary bed.
Five alternatives were considered for the capillary bed: nylon, latex-rubber, and PVC tubing, and piping.
Five alternatives were considered for the capillary bed: nylon, latex-rubber, and PVC tubing, and piping.
- Nylon tubing is durable and affordable. It can accommodate a wide variety of fittings which is beneficial for creating a network of capillaries. While the rigidity of nylon tubing lends to its durability, it may prove difficult to utilize external clamps to vary the resistance if that method is ultimately chosen. Therefore, there is dependence on the resistance mechanism when choosing the appropriate tubing material.
- Latex-rubber tubing is affordable and can accommodate a wide variety of fittings. However, it is not very durable. The tubing is prone to cracking or tearing at fitting interfaces. Additionally, it is susceptible to certain fluids, which causes the tubing to weaken over time. This susceptibility would pose a problem if a fluid is chosen as the medium.
- PVC tubing, similar to nylon tubing, is durable and can accommodate a wide variety of fittings. Additionally, PVC tubing is more affordable than nylon tubing. Slightly less rigid than nylon tubing, it is compatible with all types of resistance mechanisms.
- PVC or metal pipes are both durable and can accommodate a wide variety of fittings. However, pipes also pose a few obstacles relative to the design specifications. Piping and its associated fittings are more expensive than flexible tubing. Additionally, piping can be heavy, thus decreasing the portability of the device and potentially surpassing the upper weight limit dictated by the specifications.
Capillary Bed Pugh Chart
Resistance Mechanism
Below is the Pugh Chart which ultimately lead to the team's decision. upon determination of the Resistance Vessels, to use Clamp-Style Needle Pinch Valves for the model's resistance mechanism.
Three alternatives were considered to vary the resistance within the capillary bed: external clamps, needle pinch valves, and exchangeable tubes.
Three alternatives were considered to vary the resistance within the capillary bed: external clamps, needle pinch valves, and exchangeable tubes.
- The current model utilizes external clamps to vary resistance in the capillary bed. This mechanism is affordable and allows for varied resistance from zero to infinity (flow blocked completely); however, the resistances allowed within that range are not finely controlled.
- Needle pinch valves are metered, external valves that allow for fine control of flow; however, these valves are slightly more expensive than the external clamps.
- Unlike resisting flow through a channel by gating a certain point (like with the clamps and pinch valves) exchangeable tubes with varied diameters can also be used to vary resistance through the capillary bed. Just like vessels constrict and dilate under different conditions in the body, tubing of different diameter can be inserted into the model. While this more closely resembles the actual peripheral resistance experienced during circulation, this method poses several potential problems. The exchange of parts during use would decrease the ease and simplicity of use. Additionally, the security of the connections between the arteries and veins and the capillary bed may be compromised by the constant switching of resistance vessels.
Resistance Mechanism Final Pugh Chart
Chosen Design
Below is a detailed diagram of the chosen design. Further information on specific parts and dimensions can be found in subsequent pages on the website.