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Two final designs have demonstrated success in minimizing the effect of external forces on blood pressure readings. Both physical shields can maintain systolic and diastolic blood pressure readings within +/- 5 mmHg of control readings, even when acted on by up to 25 lbs of force. Each design prioritized arm adjustability and ease of application, requiring minimal training for medical staff to use them.
Wall Shield
Wall Shield
The Wall Shield is a 3-component design involving a base plate, a wall, and a set screw. The wall design allows for size adjustability via a linear slider that comes from a rail connected to the wall that fits snugly into the base plate. The pieces fit in loose enough that they may easily slide through one another with ease, yet give minimal room for rattling. When the desired size is set between the wall and the base plate, the set screw may be applied so that it presses against the wall's rail, acting as a clamp to hold it in place inside of the base plate.
Set Screw Locking Mechanism
The wall design uses a set screw locking mechanism which allows for ease of linear sliding for size adjustments and a strong, rigid hold on the size once it is screwed into place. The set screw screws into the baseplate as the wall rail is inside, and as it tightens, the flat end of the screw presses against the rail, clamping it down. As this locking mechanism does not rely on any holes, grooves, or notches, the sizing is not confined to a finite amount of possible sizes, allowing for any arm that falls within the size range to comfortably fit within the device.
Testing/Results
Testing/Results
Protection Validation
3 sets of blood pressure measurements were first taken as a control. Then force was applied to the arm for 3 separate measurements to see the effects of the disturbances on the data. Finally, another 3 test were conducted with the shield present.
The data on the right shows the systolic and diastolic measurement variations for the three different testing conditions. What could be found is that the shield is able to mitigate the effects of the disturbance applied on the cuff.
Flexible Shield
Flexible Shield
The flexible shield design is made of a polycarbonate plastic that allows for flexibility as opposed to the other previous designs. Due to this flexibility, the flexible shield is able to change in shape and sizes radially so that it may fit a large range of patient's with varying arm sizes. When bent, the spring tension of the material provides rigidity against external forces such as a surgeon leaning on the device.
Polycarbonate Sheet after cut
Polycarbonate Sheet after Bending
Polycarbonate Plastic Sheet
A polycarbonate plastic sheet is cut using a CNC mill to make a 152.4mm x 400 mm rectangle with 27 5mm diameter holes that are evenly spaced out by 15 mm. This sheet is then bent into shape using a brake to create the cylindrical-like design that goes around the arm.
When the polycarbonate plastic is bent around the arm, there is potential energy stored in the spring-like characteristic of the material. When a force is applied to the outside of the shield, the spring-like characteristic of the material resists changes caused by the external disturbance.
Zip-tie/Velcro Strap
Zip ties will be used to fasten the shield by lining up the holes and weaving a zip-tie through them as shown in the figure to the right. Velcro straps can also be used by wrapping the shield and attaching the Velcro to itself once the shield is in the proper position.
Testing/Results
Testing/Results
Deformation Testing
To see how much load the shield can handle, weights of 2.5 lbs were added to the top of the cuff, and the vertical diameter was measured. Plotting this data on the right, we can see that there is a relatively linear plot. This is because the prediction that the outside shield would act as a spring was true. Hook's law states that there is a linear relationship between force and linear deformation when a load is applied to a spring. The spring constant of the material is approximately 0.56 lbs/mm.
Protection Validation
Force Sensing Resistors (FSRs) were applied to the outside of a blood pressure cuff during a measurement, and disturbances were added. The "Unprotected Cuff" plot shows the resulting BP cuff readings in blue and the added disturbances in red. What was found was that the disturbances caused random peaks and valleys in the normal BP measurement curve, causing a mis-reading.
When the shield was applied, the FSRs were moved to the outside of the shield to measure the external disturbances. The resulting data can be seen in the "Shielded Cuff" plots. The blue data shows a relatively normal measurement curve while the data in red shows that disturbances were present. This shows that the shield was effective in preventing the mis-readings in forces up to 20 lbs.
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