Concept:
The Lyretail Prosthesis is a below-knee (BK) design for scuba diving. The name “Lyretail” references a family of fish known for their bright coloring and slightly asymmetrical tails, and it’s also a fun play on words (given that a scuba fin is a fake tail of sorts). The overall shape of the fin mimics the fin of a lyretail hogfish, the materials imitate the coloration of a tang fish, and the shape of the leg and some of the surface details are inspired by the eagle ray.
lyretail hogfish
tang
eagle ray
Research and Process for Lyretail:
My challenge to myself for my first prosthetic accessory project this semester began with some information-gathering on existing BK (below-knee) accessories for specific uses — mostly sports-related — due to my dedicated interest in functionality as well as form. A quick Google search showed me that there’s a multitude of designs for runners and ball players, far outshining any other athletic sports. In short, variety in below-knee special-use prostheses is mostly limited to land activities. Wouldn’t it be more interesting and more of a learning experience to address the need for a variety of water sports prostheses? I finally decided on creating an original concept and model for a scuba prosthetic accessory. This decision was clinched after finding out that my parents’ dive instructor and close friend Victor was a recent amputee and actually owned a diving prosthesis. I reached out to Victor around week two, and we’ve been in contact throughout most of my design process for the Lyretail.
Research began with the very simple need for better vocabulary about the world of prosthetics, so that when I needed to ask a question, I knew what terminology to use to get the right answers and to understand them. I found the Amputee Coalition, which is a nonprofit organization dedicated to advocacy and education about limb-loss. The Limb Loss Definition page on their site is incredibly helpful, and guided my questions for design success and better communication about limb-loss in general (http://www.amputee-coalition.org/resources/limb-loss-definitions/).
After getting my concept for a scuba accessory drawn out and semi-finalized (as well as after reading up on some limb-loss information), it occurred to me that this design actually needed to be a true prosthesis, not just an accessory. A conversation with Victor confirmed that suspicion. It needed to have a pylon that was water-proof or water-resistant, along with a shell with the same qualities. It needed to be nearly indestructible, so that there was almost no need to ever take it apart into component pieces to be cleaned or repaired. Victor gives his a “bath” after diving just to wash off the salt water and detritus, but that’s the extent of its maintenance. It would also be far more convenient for the leg design to be a whole piece that would replace the prosthesis the person used for walking and other day-to-day activities. On the other end, it would better for the flipper to be separated from the leg entirely, so that it could be easily adjusted to be tighter or looser. That would additionally mean that the diver could take off the flipper when leaving the water and replace it with a water shoe. What I found myself designing was two individual parts meant to work together functionally and aesthetically, but that were not married to each other. I would model a water-proof BK prosthesis with a purpose-driven ankle design, and I would model an easily adjustable scuba flipper.
An organization that provided a great amount of information about fabricating prostheses was the Digital Resource Foundation for the Orthotics & Prosthetics Community. Their website offered an excerpt article written by Michael J. Quigley from a book titled Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles, on prosthetic fabrication and management, and specifically a section titled “Computer-aided Design/Computer-aided Manufacturing” that answered a lot of unasked questions I had about the exact process of building a comfortable socket for the patient’s unique needs. After reading Quigley’s article on prosthetic management and talking to Victor about it, I adjusted my model to have a lower shell near the back of the knee. For my conceptual design, I am assuming the person has an BK amputation with a few inches of clearance below the knee. In order to make the fit comfortable when the knee is bent, there needed to be less material interfering with the flexed calf muscle. Victor also made the point that the pylon provided to me was much too high for the design. It needed to be half that height for the residual limb, or “stump,” to be able to fit in the top half of the shell comfortably within a socket. Another very useful site, though it’s not official, was this Amputation Help diary blog written by a recent BK amputee (http://www.amputationhelp.com/diary.html). It gave me valuable insight into an amputee’s experiences post-surgery, and all the work and collaboration that goes into making a comfortable, functional prosthesis. I learned about how prosthetists mold a socket and refine the socket as time goes by and the stump shrinks due to water loss and healing. It’s a great resource for learning the detail that goes into creating a prosthesis from the perspective of the patient.
When it came to designing the ankle joint, I had a bit of a leg-up (no pun intended) on that topic. Victor’s dive prosthesis had an original locking mechanism on the ankle (actually designed and fabricated by him) that meant he would lock the ankle at a 90 degree or 0 degree angle for walking or swimming. He told me it was preferable to be able to lock it in the 0 degree position while swimming, and let the flipper move up and down on its own while the joint stayed stationary. I decided to use the general flavor of that idea without taking his design. After talking to Victor, I read up on the available types of prosthetic foot/ankle combinations on the Hanger Clinic’s website (http://www.hangerclinic.com/limb-loss/adult-lower-extremity/Pages/Feet-and-Ankles.aspx). It was pretty clear I needed to use a simple single-axis joint: it would be much less prone to water damage, given that the range of motion would be very basic and it didn’t need much engineering to achieve. It would also mean I could design my own locking mechanism. The foot/ankle joint I modeled utilizes a dial that moves the ankle into one of three possible positions between 0 and 90 degrees, and locks into place using simple notches built into the rotating joint mechanism. While I was at it, I made the pylon have adjustable height, so that while it was being fitted to a new user, it could be easily slid up or down and then bolted in the right height position for the person.
The MIT Media Lab is doing incredible work merging biology with mechanics to fabricate realistic motion and dynamics in prostheses. Their website (https://www.media.mit.edu/projects/powered-ankle-foot-prosthesis/overview/) is absolutely fascinating, and demonstrated to me (after reading a few articles on biomechanics) that my strengths lie in design and modeling, not necessarily engineering. However, I am strongly inspired by their joining of beauty with realistic functionality, and I kept some of their key goals in mind while modeling the Lyretail. To use a quote from the Biomechatronics section of their site, I seek “to accelerate the merging of body and machine” and create something that mimics the form and function of a human leg as well as perform beautifully at its prescribed athletic task.
The rest of my research for Lyretail was gained through conversations with Victor, who brings both areas of knowledge to the table: diving and prosthetics. Since he is actually a dive master, not just an instructor, I felt free to ask him technical questions about whether certain elements of the design would impact its performance underwater. One of those topics of conversation included the shape of the fin. The cutouts in the top of the flipper should not cause a noticeable difference in drag while swimming. He did note, however, that the slightly bulged shape of the tips of the fin may affect propulsion a little. If I were going to redesign, I would make the flipper flatter across the front to improve performance. The buckle and adjustable back of the fin were a suggestion from Victor also. Full coverage on the back of the ankle is much less snug than you would think, and you also can’t change the fit. So I cut off the heel of the design and created a ratcheting and releasing button that would allow the user to easily adjust the tightness of the fin using one hand on the outside of the ankle. For the leg portion of the design, Victor anticipates no issue with drag regarding the open design of the shell. He suggested later on in the process that the shell should be attached to the socket in several places (not just the ankle) to improve stability. I have added carbon fiber attachments from the socket to the shell in both the front and the back of the design.
no naked edges
Modeling:
For the final stretch of modeling the Lyretail, my goals were making the pylon adjustable to individuals, and finalizing the lockable ankle joint. This was my process for modifying the pylon:
- DupEdge on the top pylon’s end to get the beginning of a new pipe
- ExtrudeCrv on that to bring it down into the lower pylon, then BooleanDifference to remove the areas of the top and botton pylons where the middle pylon sits
- Chamfer the edges of the top and bottom pylons
- Using interpolate point curves, draw the outline of the protrusions behind the pylons that will hold the screws used to adjust fit and height
- Using a combination of NetworkSrf and Sweep2, make the protrusions solid, then Cap the end, and BooleanUnion with the pylon
- Create a hole for the screw casing using a cylinder and BooleanDifference, then FilletEdge
- Using cylinders, helix curves, a circle, Sweep1, and BooleanUnion to create the screw thread, then a bigger cylinder and BooleanDifference to create the hole the screw goes into
- Create a closed, filletCornered rectangular curve to Split the end of the pylon, then delete the unnecessary pieces of the surface - this creates the notch in the pylon. Use PlanarSrf to close off the openings, and fillet the edges.
My process for modeling the dial and ankle:
- ExtractIsocurve to pull two curves from the rotating horizontal pipe. From those, create a cylinder that hugs the horizontal pipe. FilletEdge the cylinder.
- Using InterpolatePointsCurve and FilletCorners, draw a dial in one plane. Offset that curve with rounded corners, then move it back to the cylinder, Loft the two curves, FilletEdge, and then BooleanUnion with the back cylinder. But wait! My BooleanUnion didn’t work cleanly, so use Dir to show the direction of the surface normals on both pieces. Reverse direction on the one that’s inside out, then BooleanUnion again. Steal one of the screws from the top pylons, Scale3D, and snap it to the center of the dial, then BooleanDifference.
- For the notches in the ankle joint, create three circles along the edge in one plane, then ExtrudeCurve and BooleanDifference. Fillet those edges!
material assignments by layer
Materials:
- For the pylons and ankle joint, I used Steel Ultra Scratched with the bump and scale turned down to be less dramatic. This is close to what Victor’s pylons looks like after significant wear and tear.
- For the socket, a Carbon Fiber Gloss Material with roughness turned down, scale turned down, black and grey colors, gaps turned down, edge width turned up, weave noise turned up, and contrast turned down. All this results in a pretty realistic copy of the carbon fiber material on Victor’s dive leg socket.
- For the leg shell and some detail pieces, I used a Metallic Paint, so that I could control the color distribution and clarity. I’m mimicking the coloration of a Tang Fish, so I’m looking for a very vibrant gradient from red to orange, contrasted with deep blues on the details and the fin.
- For the flipper and detail spots, I used a tire material with less roughness, and changed the color to get the gradient on the front of the leg that mimics the shape of an eagle ray.
- The SACH foot is a mold tech material with stipples. I didn’t think the material I used for previous renders did the aesthetic of the design any favors, so I changed it up. Hopefully it doesn’t look like a basketball (the MoldTech bump material definitely did).
- The buckle is a hard rough plastic in black.
- The screw casing and dial are a Tarnished Metal material, which gives it the look of being used in salt water, but not entirely eaten away at.
- The screws are a Metal Rust material, because screws would definitely experience wear over time due to the salt, and would have to be replaced periodically.
- Occlusion Material on all parts for occlusion passes on all renders.
Works Cited
Bowker, John H., and John W. Michael. Atlas of limb prosthetics: surgical, prosthetic, and rehabilitation principles. St. Louis: Mosby Year Book, 2002. Print.
Gauge, Robert. "DIARY." AmputationHelp.Com. N.p., 24 Nov. 2014. Web. 03 Mar. 2017. <http://www.amputationhelp.com/diary.html>.
"Amputee Coalition." Resources and news for amputees, amputation, limb loss, caregivers and healthcare providers. The Amputee Coalition, 2017. Web. 03 Mar. 2017. <http://www.amputee-coalition.org/resources/limb-loss-definitions/>.
"Project Overview ‹ Powered Ankle-Foot Prosthesis – MIT Media Lab." MIT Media Lab. Massachusetts Institute of Technology , n.d. Web. 03 Mar. 2017. <https://www.media.mit.edu/projects/powered-ankle-foot-prosthesis/overview/>.
"Prosthetic Feet." Prosthetic Feet – Hanger Clinic. Hanger Clinic, 2015. Web. 03 Mar. 2017.
<http://www.hangerclinic.com/limb-loss/adult-lower-extremity/Pages/Feet-and-Ankles.aspx>.
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