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This project extends the previous work on creating a prosthetic adaptation for percussion. The focus of this modification is to improve the original design. The primary objective is to develop a prosthetic attachment that can be easily attached and detached independently, while ensuring that the prosthesis is safe, durable, and user-friendly.
We have been assigned to improve a prosthetic arm attachment for a middle school girl who is a bilateral amputee. She currently uses the device to play percussion, but the current attachment point is weak and tends to move. We aim to modify and improve the original design so that it can be easily attached and detached independently and is user–friendly while still providing a firm hold.
We were requested not to meet with the family initially and proceed with the modification first. After the modification was completed, we would then contact the family. We are going off the base line to make an improved adaptation prosthesis that is easier to switch.
Initially, we contacted the previous group that developed the initial prosthesis to understand the design process and background research. We were able to obtain their original design for the drumstick holder and evaluated what strengths their design had. Our focus was to enhance the design by improving the grip, rotational control, and variability of the drumstick, as well as enabling independent attachment and detachment of the prosthesis. With our goals established, we independently researched different methods that would achieve them.
Our research led us to “activity-specific prosthetics,” which are prosthetic attachments designed for a singular purpose, as opposed to more typical and general-purpose prosthetic hands. These are made by a variety of companies, and there seem to be no industry standards for attaching these to users’ arms. We quickly realized that this type of prosthetic design depended strongly on the user’s needs and preferences, so we brainstormed concept designs based on what we knew about the client.
This design concept centers around using a commercial-off-the-shelf quick-release seatpost clamp, as seen on bicycles. On a typical bike, the seatpost shaft sits inside of a slightly larger shaft with a slit cut down its length. When the clamp is tightened, the outer shaft is squeezed onto the inner shaft, providing a frictional clamping force. As the attachment can be freely positioned before the clamp is engaged, this design allows complete rotational control for the user while still providing a strong hold (enough to support the weight of an adult man when used on bikes). Because the clamp is engaged and disengaged with only a single lever, this design should be easy for the client to manipulate without assistance. If the force required to clamp and unclamp proves too strong for the client, it can be easily reduced by extending the length of the lever arm. Pictured below is a typical commercially available seatpost clamp. Note the eccentric cam on the lever arm, which produces the clamping force. Also pictured is a typical outer seatpost shaft. The slit which allows the tube to flex can be seen.
Our next concept is a push-and-click device designed to offer partial rotational control, allowing the client to adjust positions based on her preferences. This device prioritizes safety, minimizing the risk of serious injury during use. Our primary goal is to ensure ease of use for the student, enabling her to effortlessly turn and adjust the prosthesis to suit her needs. Additionally, this design is highly manufacturable, as it can be efficiently produced using TPU at a minimal cost. Requires minimal digital input for the client.
This concept consists of a matching set of magnets embedded in the forward-facing surface of the arm and the base of the attachment. Rotational control is supported by the magnets (the user can choose the rough orientation of the attachment when connecting it to the arm), as well as by flex tubing similar to that seen on TIG welding heads. The tubing would extend from the attachment base to the working end, allowing a few degrees of twist and flex for the user to fine-tune the position beyond what the magnets allow. If the forces of drumming prove too strong for the flex tubing, alternate methods of rotational control might be considered. The hold force provided by the magnets needs to be fine-tuned to provide a strong hold while remaining viable for the client to remove without assistance.
We faced the challenge of determining which design should be further developed, as everyone had invested significant time into each concept.
We initially found the magnet design intriguing. However, in practical use, the magnet would likely encounter more issues. The only clear advantage we saw was its durability, as the magnet would last a long time without needing replacement.
Next was the Bike Collar design. Our goal for this design was to reduce cost and simplify the process. After running it through the decision matrix, we found that it scored highest in safety and prototyping. However, it fell short in critical areas like manufacturability and ease of use, which were essential for our needs.
Upon reviewing the decision matrix, we clearly established our values and goals for the ideal prosthesis: cost, manufacturability, fabrication, safety, durability, and the level of prototyping required. Ultimately, we chose the Push-and-Click prosthesis. This design scored highly in the areas of cost, manufacturability, fabrication, and ease of use, winning by a margin of 3 points in our decision matrix.
The attachment point has two primary components – a cylinder with grooves bored into the walls, and a cylindrical shaft with pins that fit inside of the grooves. The insert shaft will be part of the attachment. To use the device, the user will hold the attachment between their legs with the shaft sticking upward. They will then line up the locking grooves with the pins on the shaft, then push down and twist before letting go to allow the pins to slide into their locked positions. The attachment is then held securely in place by the spring providing a constant force against the pins.
This design earned an edge on the others thanks to providing a means of attachment which was secure while not requiring complex manipulation.
The Push-and-Click prosthesis is a 3D-designed prosthesis created for rotational use and comfort. This device was designed with a bilateral amputee in mind. The prosthesis consists of two parts, with the main cylindrical structure holding everything together, while the smaller device allows for adjustments. To use the device, you place the smaller component into the main body and twist it. This provides a secure yet easy method of control.
The main cylindrical structure serves as the primary body of the prosthesis. It is made out of TPU, which is flexible and durable. Additionally, it does not affect the client’s skin while she uses the prosthesis.
The rotational capabilities of the prosthesis allow the smaller device to rotate the joint at the top, providing rotational control in 45-degree increments. This enables the client to position her arm in various ways. The goal of the design was to ensure that the rotational device remained secure while giving the client control over her desired arm positions.
The Push-and-Click mechanism gives the prosthesis its name. The design features L-shaped ridges inside the main cylindrical structure, ensuring that the smaller component locks into place securely, offering stability while the client holds something.
The ease of manufacturing, thanks to the use of TPU and 3D design, allows for reduced costs in creating prototypes of this device. This also gives us complete control over the design and customization.
Given how small the pins on the insert are, we were concerned about their resistance to bending. As such, we performed a stress analysis on the pins to determine if they would stay safely above the yield strength of our 3D-Printed 316 Stainless Steel.
The picture below illustrates our analysis. The pin is treated as a cantilever with one fixed end, with the force distributed along its full length. As there are four pins, each pin will experience ¼ of the applied load of 10 lbf. The pin is square. We calculate the moment of area (I), the moment about the area of interest (M), and the maximum resulting stress (σ), which was ultimately determined to be significantly less than the yield strength of the material (σ_y).
The reaction force of the spring was an important part of the analysis to ensure that the the child wouldn’t have a problem with the prosthesis coming apart or being to hard to attach the two pieces. It was determined that a reasonable amount of force to fulfill these two requirements was about 10 lbf. Given the space limitations of the prosthesis attachment, the spring would need to have a outer diameter smaller than 0.80 in.
A spring with the following specs was found: Length : 1.5″ OD: 0.66″ spring constant (k): 34 lbs/in. With this spring, the distance of compression, given our max length within the prosthesis, would be a 0.50″ for the variable x. Using the mentioned values and the equation F = -kx the resultant force would be 15.98 lb. This is the value determined to best fit both requirements of sturdiness and ability to use.
Creep resistance was one thing that we wanted to look at to ensure that the insert would not deform overtime. Since TPU was a relatively soft material, we wanted to make sure it would withstand the resistance of a spring overtime, particularly the notches. TPU is a viscoelastic material, it will behave more like a viscous fluid when under stress or strain; this gives it unique creep and cyclic fatigue properties. Both tests show that TPU has unique properties that may not be favorable for a part that receives continuous cyclic stress.
Figures A and B show that TPU during creep testing will continue to exponentially increase until failure. This is due to the unique properties of the viscoelastic material. Once the load is removed from the TPU, the material does not ‘spring’ back to its original shape or remain deformed; it returns to its original shape slowly overtime as shown in figure C, but never back to its original shape. Cyclic fatigue also showed cyclic hysteresis, which is unfavorable since it is associated with plastic deformation.
To develop the prosthetic for this project, we utilized the previous CAD designs, ensuring consistency with the original Spring 2024 design. Using a 3D printer and a TPU filament similar to what the final design would be printed in, we fabricated multiple iterations of the design. TPU was selected for its flexibility, durability, and skin-safe properties, ensuring it met the client’s needs. Despite not having direct access to the client, we relied on detailed design, old documentation, existing measurements, and collaboration with previous team members. The iterative process allowed us to refine the design efficiently, focusing on a push and click attachment system that is easy to use and adaptable for future expansions. Each iteration underwent rigorous testing to confirm fitment and functionality, ultimately resulting in a design that aligns with the client’s expectations and lays a solid foundation for future enhancements.
The testing phase for the prosthetic attachment was highly successful. The design was specifically tailored to be compatible with the prosthetic arm while incorporating the ability to accommodate future attachments.
We were initially concerned with the creep deformation of the pins that protrude from the insert to interface with the outer shaft, as they would be subject to a constant force due to the embedded spring. To determine how significant this issue would be, we constructed a rudimentary test rig to hold the pins under a constant five pounds of force. The rig was left in place for 72 hours, and upon investigation, we discovered significant deformation of the pins.
While we found that the pins rebounded somewhat after several days of rest, we concluded that this would be unacceptable in the final design. After some debate, we settled on using a 1/8th inch aluminum sheet insert cut to size and then embedded within the insert, to take the place of the TPU pins. Testing after this addition showed complete resistance to creep, eliminating the problem.
This prosthetic device presents minimal risk to the client. The only potential concern is a minor pinch point in the push-down mechanism, but this has been designed to minimize any significant hazard. Additionally, the TPU material ensures skin safety while providing the durability required for daily playing. Following basic precautions, the device can be used confidently and safely by the client.
Our primary objective was to enhance the design created by the Spring 2024 prosthetic team, ensuring it catered to the specific needs of a middle school student passionate for music. The prosthetic enables the client to participate fully in music class and offers an ability for future expansions. The twist-and-remove attachment system is user-friendly and adaptable, allowing for seamless integration with other potential attachments. The TPU filament chosen for the project ensured a balance of comfort, durability, and safety for the client.
This project was a valuable learning experience. Building upon the previous team’s work provided insights into the challenges they faced and allowed us to improve our approach. Throughout this process, we refined our skills in communication, design, and problem-solving, all of which are essential traits for successful engineering. Beyond the technical achievements, this project reinforced the importance of creating innovative solutions to empower individuals with physical impairments, enabling them to pursue their passions and overcome challenges.
