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For our final design solution, the only components that we will be utilizing from the old existing prototype is the top and bottom plate. The components that make up our chamber are down below. The final prototype with the implemented components is shown in Figure 1.
5x 24-Inch Steel 304L Rings
1x Top plate of the chamber
1x Bottom plate of the chamber
1x Neoprene air duct chamber with thin wire inside
1x Waist Seal connected to top plate of the chamber
44x 6-32 1 inch Phillips starhead screws w/ nuts
2x Slotted flat metal strips
3x Rolls of duct tape
2x Bicycle foot straps
2x Rolls of Garage Sealer
Figure 1: Final Design Prototype
There are some components we designed to meet our requirements. To solve the issue of collapsibility, steel rings were implemented to withstand the negative pressure along with neoprene fabrics. To ensure that the chamber is able to sustain the negative pressure for periods of time, the waistband seal also plays a significant role in restricting air movement across the chamber. Lastly, foot straps are installed to help with user comfortability and greater control over the moving chamber.
Final Design in further detail are sectioned as follows:
Reinforcement Rings
Waist Seal
Foot Straps
Top/Bottom Plate
Testing Results
Reinforcement Rings
Chamber collapse was a prominent issue with the prior prototype and was demonstrated to cause significant volume decrease and pressure loss. To solve this problem, we decided to install reinforcement rings that would provide additional structural support to the chamber to thus increase its ability to withstand higher uniform pressure loads without collapsing. The first decision that was made was the selection of materials suitable for the rings. Figure 2 shows the different materials considered.
Figure 2: Reinforcement Ring Material Considerations
After careful review, we chose stainless steel grade 304L as the ideal material for our chamber. Stainless steel is commonly used in the industry which makes it easy for purchase; it's high young's modulus also meant that it would be able to resist plastic deformation.
The rings were zip tied along the grooves (in black) on the inside of the neoprene chamber body and secured with vinyl cement and seam sealer. While the chamber didn't collapse on itself, the rings were prone to tilt slightly within the neoprene chamber as shown in figure 3. Through further testings, it was decided that the ring tilting was not a major concern as it did not affect how the chamber was generating negative pressure. Figure 4 shows the testing chamber with and without the reinforcement rings.
Figure 3: Slight Ring Tilt
Figure 4: Chamber with (right) & without (left) Rings
2. Waist Seal
The waist seal is another critical component that encloses the lower body of the user inside the device and prevents air leakage when negative pressure is generated inside the chamber. Thus, a proper waist seal must meet the conditions to one - comfortably wrap around any user’s waist, and two - restrict air movement across the seal.
The first iteration of the waist seal consisted of a two layered neoprene membrane. The initial thought was the overall air leakage could be minimized by adding layers to the waist seal. While the idea holds true, testing revealed that the connection between the user's waist and the seal itself played a larger part in restricting air movement.
With that in mind, the second and current iteration of the waist seal was derived from a previous seated LBNP device currently not in use. This neoprene seal has a waist belt that can be fastened onto the user's waist and worked similarly to a corset. Immediately, improvements were noticed that this seal worked better in preventing pressure loss from the waist area of the user. Figure 5 shows the differences between the old waist seal to the new waist seal.
Figure 5: Old (left) vs New (right) Waist Seal
3. Foot Straps
Foot straps are another key component we wanted to add to the chamber. Although testing procedures look rather simple, in actuality there is a lot of pressure and force going into expanding the chamber. Involving both passive and active testing, the user lies within a chamber for approximately 1 hour and 30 minutes. From our testing experiences, it was uncomfortable without having any support on our feet. To help with this, we decided to install foot straps (figure 6) at the bottom plate so the user would have greater control over the movement of the chamber.
The foot straps are sandwiched between the metal slot holder and a garage sealer through the bottom plate. The garage sealer helps restrict air from leaking through where the drill was made and the protruding screws were filed off so that its flush with the metal slot holder.
Figure 6: Foot Strap on the Bottom Plate
4. Top / Bottom Plate
Top and bottom plates shown in figure 7 are the components used in the previous design. They consist of steel and they are directly connected to the neoprene chamber. The main point we focused was making sure there were no leaks. Since we had to take plates on and off oftentimes to add components such as reinforcement rings and foot straps, we were worried leaks might occur due to loose screws. To test if there are leaks in those screws, we did a soapy water leak test whenever we assembled back our device. The test result indicated that there were no leaks in top/bottom plates.
Figure 7: Top (left) & Bottom (right) Plate
5. Testing Results
In the first part of the test procedure, test subject will be seated upright for 15 mins while their jugular vein diameter is measured in order to get a baseline under regular conditions. Next, measurements are taken while the subject is in supine position for the microgravity condition. A final measurement is taken while the subject actively expanding the chamber and generating around -25 mmHg.
Ideally, an effective LBNP device would redistribute blood pressure from the upper body to the lower body, decreasing intracranial pressure thus helping prevent SANS. From the ultrasound images taken by Ryan Kassel using the LBNP chamber (Figure 8), internal jugular vein cross section can be observed under different scenarios. The upright condition represents normal blood gradients within the user in regular gravity conditions and the supine condition represents blood gradients in microgravity conditions. In the passive LBNP condition, the jugular vein CSA decreases to around 43.3mm2, indicating significant blood redistribution to the lower body. In the active squat LBNP condition, the jugular vein CSA also decreases to around 53.6mm2. Comparing the jugular vein size between the supine and the LBNP conditions, it can be concluded that the device is indeed effective at decreasing the jugular vein size around 28.9 ~ 42%.
Figure 8: Internal Jugular Vein Cross Section under different scenarios
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