Testing

Overview

Testing was conducted to evaluate both the reliability and durability of the prosthetic socket as well as patient comfort and safety, four of our main design goals.

Device reliability and durability was evaluated through five different tests: Force sensor calibration and validation, air tightness of air bladders (full inflation), airtightness force decay, cyclic air bladder testing, and height variation optimization test. Safety and comfort was evaluated through two different tests: finite element analysis (FEA) and force on residuum while in use. Details on the purpose and result of each test can be found below in the test specifics.

Overall this device satisfied many of our original design goals. We were able to accomplish a relatively airtight bladder that has a predictable air and force decay rate, seen in figure 5 and 7. Additionally all the bladders designed outperformed our original change in height desire of 6.5 mm, with the minimum change in height in the popliteal bladder with approximately 11.5mm including inflation and vacuum, seen in figure 3. This dramatic change in height means volume fluctuations of approximately 15% could easily be accounted for without the need for donning and doffing socks as required with traditional prosthetics. In addition, our force on the residuum test had promising results showing our design increased and decreased pressures as intended, proving it to be a viable solution to diurnal volumetric changes in the residual limb, results seen in figure 14.

Test Specifics

FSR Validation and Testing

This test was completed first to ensure the force readings were properly calibrated for the rest of the testing and for in-socket use.
Essential for understanding the reliability of force measurements.

Result: The FSR validation testing resulted in an R-value between the actual mass and fit predicted mass of 0.918, meaning the estimated force fit the validation tests.

Figure 1: Validation Curve for Large FSR. Blue points are fit-predicted mass vs actual mass per each trial. Ideal 1:1 fit represented as a red line. The R-squared value for fit data was 0.918.

Inflation Height

This test was completed to determine if the custom air bladders were able to deform in height at least 6.5mm.
Essential for ensuring the socket would be able to adapt for the average volume changes amputees face.

Result: The deformation ranged from 7.59mm to 26.08mm (155.91%-301.83%), both significantly greater than desired height change of 6.5mm. Height change for all air bladders can be seen below in Figure 3.

**It is important to note that this does not include vacuum data which we assume would add approximately 5 mm of overall height to the change in height.

Figure 2: Caliper measuring the height of the air bladder when in resting position.

Figure 3: Custom air bladders and their respective change in height and percent of original height calculated.

Figure 4: Set up of Airtightness of Bladders. Recording height changes over time on video.

Airtightness of Bladders

This test was used to determine how long the air bladders were able to retain air over time, providing a quantitative measurement for airtightness.
Essential for determining intervals of inflation while in use to provide reliable volume changes.

Result: Custom designed air bladders took approximately 170 seconds to deflate and deflated at a relatively constant and linear rate, seen below in Figure 5.

Figure 5: Graph of height change over time for curved test bladder for all trials.

Airtightness Force Decay

This test was used to determine the force decay over time due to air leakage.
Essential for determining intervals of inflation while in use and threshold force values to maintain reliable volume changes.

Result: Air bladders had a relatively consistent and linear decline in force for all trials, seen in Figure 4. Force decreased approximately 9N over 120 seconds.

Figure 6: Set up of Airtightness Force Decay test using custom test rig.

Figure 7: Force vs time graph for all trails.

Figure 8: Set up of Cyclic air bladder testing using a custom test rig.

Cyclic Airbladder Testing

An air bladder was repeatedly inflated with each cycle counted until the air bladder failed to inflate.
Essential for understanding the durability of air bladders while in use.

Result: Each cycle took approximately 11 seconds to run and ran over the course of 28 days. There were a total of 219927 cycles completed with no sign of degradation.

Height Variation Optimal Force

An air bladder was placed in a rig of varying heights, maximally inflated, and maximum force outputs were determined.
Essential for understanding optimal bladder height range for maximum force output. Pertains to reliability and durability of the device in use.

Result: Maximum force was achieved when the bladder was at its smallest height, then had a steady decline until approximately 55% of its height where it began to increase again slightly until reaching a plateau at approximately 100% of its height, graph can be seen in Figure 9.

Figure 9: Graph of Average force vs percent of resting height.

Finite Element Analysis

Finite element analysis was completed to evaluate stress, displacement, and strain of socket insert while in use.
Essential to understand if insert can safely withstand use by the average user.

Result: Stress, strain, and displacement on the socket insert was minimal, meaning our device would safely withstand the weight of the average user, results can be seen below in Figures 10-12.

Figure 10: FEA of stress on socket insert. Dark blue indicates 1.786e-03 von Mises while red indicates 2.91e01 von Mises. Note no red was seen in this analysis.

Figure 11: FEA of displacement on socket insert. Dark blue indicates 1.000e-30 von mm while red indicates 4.908e-01mm. Note location of red on knee bend.

Figure 12: FEA of strain on socket insert. Dark blue indicates 1.324e-06 estrn while red indicates 4.331e-03 von estrn. Note no red was seen in this analysis.

Figure 13: Artificial silicone residuum with FSR on anatomical locations of interest.

Force on Residuum in Use

A model residuum was created and FSRs were placed on boney or sensitive areas and force was recorded while an adaptable socket was in use.
Essential for understanding if the device works as intended and an approximation of patient comfort.
Note: This device was not fit on a patient as we believe our device needs more development to ensure maximal patient safety.

Result: Overall force decreased in sensitive areas and increases in tolerant areas, with exception of the proximal fibula, results can be seen below in Figure 14. This test shows the relative comfort of a patient would be increasing while in use compared to when the residual limb was just set inside the socket insert.

Figure 14: Anterior and posterior view of residual limb force sensor placement. Light grey area indicates pressure tolerant areas while grey hashed area indicates pressure sensitive areas.

Made by Savanna Turner

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