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Our goal is to build a prosthetic that will allow a student to play in their high school band without using their primary Hero Arm. The prosthetic will have to be versatile and allow them to play a multitude of percussion instruments. Likewise, ease of maintenance, durability, and comfort will be paramount. The field of prosthetics is ever-changing with unique needs for each individual. Having a custom-designed mechanism to help this individual will not only take away the stress of potentially damaging their main prosthesis but also allow them to seamlessly live their life like everyone else.
There are two main challenges with this design project. The first is how to attach the prosthetic. The second is how to change out the different instrument sticks that will be used. There are additional problems branching off from these two main issues including comfort, safety, durability, and aesthetics.
Design Specifications:
The holders shouldn’t be painful or awkward for use.
The holders for the mallets/drumsticks must be secure.
The holders must attach to her arms without slipping.
Each holder shouldn’t weigh more than her Hero arm at 1-2lbs.
The holders need to be used daily in her band class for 35 min each day.
The band director got back to us and gave us a more detailed list of how the instruments are used. Drumsticks and mallets are both about 16″ and are held 4-5″ from the end opposite the striking end. The angle between the surface of the instrument and the stick/mallet when striking should be close to zero. For the mallets, her reach would need to be anywhere from 12-20+” away, and most of the music she would be playing is on the outside of that range.
We talked to medical personnel who deal with prosthetics and said ensuring she wasn’t allergic, that the attachment didn’t chafe or blister her, and that there was support for the prosthetics to be held to her torso in order to help with fatigue were all very important.
We spoke with the family, and her mother informed us that she has no allergies to materials, and the sizes for her arms. Neither the family nor the band director had a fixed idea of what they wanted from us, but the mother said that having an object to grip in her Hero arm wouldn’t work as well since both of her arms are used, and she only has the one Hero arm.
Quick Connect (Fixed)
The thought behind this design was a basic arm that had a quick connect system to grip the end of the drumsticks or mallets. The prosthetic had a suction attachment to her arm, shoulder straps, and a clamp on the hand to hold the instruments.
Pros: Easy to interchange sticks and to operate
Cons: May need modification to sticks to use properly.
Drill Chuck Clamp (Adjustable)
The thought behind this design was a basic arm that had a drill chuck clamp to grip on the end of the drumsticks or mallets. The prosthetic had a compression strap attachment to her arm and a clamp on the hand to hold the instruments.
Pros: This would allow for more variable sizes in what it could hold, and would firmly hold them.
Cons: This would be hard to clean and might be harder to change out the sticks.
Compression Clamp (Collapsable)
The thought behind this design was a basic arm that had a bicycle seat-style clamp to grip on the end of the drumsticks or mallets. The prosthetic had a suction attachment to her arm, shoulder straps, and a clamp on the hand to hold the instruments.
Pro: These would be very firm to hold the instruments, and would be easy to use/maintain.
Cons: They wouldn’t allow for various-size instruments to be used.
We selected the quick release design. We think that this has the simplest design, easiest use for the child, is safe to use, and is the cheapest design to make. A compression release would be an easy way for them to change out the instruments that they are using in a quick manner.
The design for this project is of 3 parts: a hand to hold the instruments, an arm link to mount the end pieces, and the mesh to attach the prosthetic to the arm. We went through with the button operated quick release system for the hand assembly. The mesh that was for her arm was adapted from a design that was found online, and scans from the Hero Arm.
The arm assembly is 5 inches from the open end of the instrument mount at the front to the end where the arm is attached. At the widest, the prosthetic is just over 7 inches in circumference, matching the circumference of the arm. The mesh is tightened down with wire and has a quick release for safety. The hand has a button that acts to compress against springs that push the bottom “teeth” up, pinning the instrument in place. The distance the button moves is 0.2in, which is more than enough distance to release the instruments.
This is the hand assembly, which is what grips the instruments. It works by springs in tension on the interior of the design to clamp on the instruments. This is offset by compressing the button on top and creates enough room for the instrument to fall out. The housing and pieces are a minimum of 0.1in thick for the sake of resilience for impacts. The material for the hand is ABS plastic, which has a modulus of elasticity of 290kpsi. Using analysis from ME4010, the estimated deflection of this hand piece is around 1/hundred thousandths of an inch from a 5lbf impact, so it should be exceptionally sturdy compared to the impacts of the drumming. This assembly totals 0.15lb, and is made of 6 independent parts.
This is the arm assembly, which attaches the hand assembly to the mesh cuff. This part must have a large surface on either end to correctly and securely mount the hand piece and the mesh for attaching to the arm, both of which are different in size. Additionally, it has to extend her reach a few inches to reach the notes she needs to play. The hand assembly attaches at the small nose, and the mesh will be epoxied around the wide end to fit more securely on her arm. This part is the thickest and sturdiest part, and is the strongest individual piece. Care will need to be taken in order to ensure the end pieces are attached securely. The piece is a single part weighing 0.42lbs, and is made of ABS plastic with a Modulus of Elasticity of 290 kpsi.
This is the mesh cuff to attach the arm assembly to her arm.
MECHANICAL
Tensile modulus 2080 MPa
Tensile Strength Yield,47.1 MPa
Tensile Elongation Yield, 3.20 mm ASTM
Flexural Modulus 3.20 mm ASTM D790 2210 MPa
Flexural Strength 3.20 mm ASTM D790 75.5 MPa
Per the National Institute of Health “Body-powered prostheses require cable operation forces between 33 and 131 N.”
With a estimated 4lbs or 1.81 kg of weight the amount of force needed to be held up is 17.75 N.
The cuffs and attachments were 3D printed and joined using CA Glue. A gel padding was attached to the inside of the cuffs and cut to size. Through an open side, the spring and button were mounted inside the button case and a removable panel was inserted to hide the internal mechanism. A final tightening knob with nylon string was routed through the cuff and mounted to the side of the cuff for easy access.
Through bending and shaking it was found that weaknesses at the corners of the mechanism were prone to snapping and stress-relieving curvature was needed to keep this from happening. Likewise, the size needed to be increased to allow proper fitting.
Attaching an instrument: place the instrument in the front of the holder until it reaches the button. Depress the button until the hole aligns and the instrument can be pushed through the button assembly. Once the instrument is at the desired holding depth, release the button to hold the instrument in place.
Releasing an instrument: press the button until the instrument is loose, then let gravity slide the instrument out the front, or pull it out directly.
Attaching to arm: slide the cuff over the end of the arm. Press the tensioner down into the locked position, and twist clockwise until tight but not uncomfortable.
Releasing from an arm: pull up on the tensioner until it clicks in the release position, and the cord will now unwind. Once the cuff is loose enough, remove it from the arm.
First, there was brainstorming about the initial design type for the hand attachment. We came up with 3 ideas for different types of attachments, and chose one design to go forward with, a button design. The initial hand assembly was created and 3D printed initially in multiple pieces for ease of assembly, but refinements to the design were necessary for durability.
Subsequent designs accounted for a thicker button assembly, an integrated mounting location for the arm cuff, and a separate cuff sized for the end of the arm it was to be attached to. Getting the cuff to fit, and not be a chafing or blood constriction hazard for the client was the next major problem to solve. A wire tensioner with a quick release mechanism was chosen over 2 position locking mechanisms for fitting purposes, and gel padding was added to the interior lining of the cuff to prevent chafing on the arm. Multiple versions were printed for sizing and for durability reasons.
The design of the hand assembly went through multiple iterations as well. Some designs were attempted that tried to have an integrated cover and an exterior that had natural hand looking features. The aesthetic features as an idea were quickly replaced by an exterior shell that mounted to a cover over a lighter and more compact mounting. Since the button in the hand assembly was operated by spring compression, we had to try out multiple springs in their stiffness, but also their size in diameter and length. Some were too tall and had to be cut to fit, while others were too stiff or loose for us to consider as easy to operate. The button had to be reprinted in a couple of designs to account for installation and tolerances for the printing machines.
This project was interesting to work on, since it has biomechanical issues that we had never had to deal with before, in addition to the structural issues we are used to having from previous classwork. The design challenges of adapting a mechanism to a human body were difficult, and there’s definitely an increase of concerns that are present that are not present in other projects we have worked on. Chafing and constriction of healthy limbs from blood were safety concerns that had to be addressed before any other concerns, including functionality, could be solved. Help from Dr. Canfield about solving structural issues and help from Chris Mills concerning 3D printing was necessary and we are thankful for their help.
Video link to YouTube:
