Superhero Arm




About: I build, I teach, I learn. Happiest when covered in saw dust, sweat and machine grease. Visit for more projects and info.

I met nine-year-old Aidan Robinson at a summer camp he attended called Superhero Cyborgs. This camp, run by KIDmob, is a place where kids with a variety of limb differences design their own prosthetic and orthotic devices. Each kid's design was impressive, but Aidan's playful ideas really resonated with me. To me, Aidan's ideas were a model for how prosthetic devices could and should be more appropriate, useful, and fun than those typically used for both children and adults.The typical prosthetic arm has a robotic hand or a hook that does not adapt well to a variety uses.

Aidan was born without a left arm below the elbow and has had many different types of prosthetics, all leaving him feeling almost better off without them. Despite these hurdles, and like the resilient and creative kid he is, Aidan adapted extremely well to life with a limb difference and is more active than most kids his age. Because the prosthetic is a part of him, we both felt that he needed a device that accurately reflected his personality and interests.

Aidan and his mom had two main goals for his new device. Aidan wanted access to multiple task specific attachments on one device, like a swiss army knife. Aidan had specific-enough interests to know exactly the tools he wanted on his arm, mostly consisting of things that helped him play favorite games or do everyday tasks. Aidan's mom, Jill, wanted something that would grow with him. Prosthetics are often expensive and custom fit. To be worth the cost, Aidan had to get long-term use and be more satisfied with the device overall.

I took the ideas Aidan and his mom came up with, and together we designed the following prosthetic device.


Step 1: Aidan's Ideas

The Superhero Cyborgs workshop was structured so that Aidan could work like a real designer, allowing him to be a creative Superhero and realize some of his ideas. He started off with some brainstorming sketches and moved on to do some really great prototyping work to test out some of his ideas. Aidan's sketch shows he has some really big ideas for a super adaptive, multifaceted device. But to get started, he kept it simple but cool and made himself a yellow LEGO character hand and a custom molded socket.

Step 2: The Socket Part 1: Thermoforming

Prosthetic sockets are traditionally expensive and fixed-size, even when they're designed for use by rapidly growing and changing kids. The expense of keeping a kid in clothes and shoes that fit is challenging enough, but adding on many thousands for a new prosthetic device can be financially overwhelming for some families. In addition, constantly having to use a device that doesn't quite fit is frustrating and can be painful.

The goal for the socket portion of the prosthetic device was to have something that could be reshaped and reformed to grow with Aidan.

My solution to this problem was to build the socket out of PLA plastic. PLA is printable on a Makerbot or similar 3D printer and can be reshaped with hot water. I named the parts of the socket the barnacle and the starfish. The barnacle is the part of the socket that has direct skin contact and extends to the outside and accepts the attachments. The starfish is the part that wraps around the end of Aiden's arm and gives the assembly most of it's structure.

The bottom part of the socket, the barnacle, works best if it is printed in two materials: PLA and Ninjaflex. To print in both materials, I switched out the filament about a third of the way through the print.

The file for both of the plastic socket parts are attached below.

Plastic parts can be formed with 180 degree Fahrenheit water on a plaster cast of the residual limb or directly on the skin. Let the part cool slightly after submerging it in water and before forming it onto the arm. During the fitting process, try not to deform details such as the receptacle for the ratchet wheel or the holes on the ends of the starfish's arms.

Step 3: The Socket Part 2: Sewing the Sock

In order to achieve good suspension (keeping the prosthetic on) a well-fit sock is very important. The sock is the fabric portion of the prosthetic socket that extends past the elbow.

I have done a fair amount of sewing in the past but I am not exactly a professional, so this part was hard for me.

I was able to get samples of a fabric from the company that used to make a product called Fabrifoam. I chose this material because it has a high coefficient of friction against skin on one side and it accepts Velcro very well on the other. Fabrifoam can be ordered here.

The sock is made from two pieces of fabric and a piece of Velcro. The first piece is a circle, about 3 inches in diameter, with about 10 slits cut around the edge towards the center to form a more dome-like shape. This circular piece has a 3/4 inch hole cut in the center to allow for the end of the barnacle to pass through it. The second piece of Fabrifoam is a rectangular piece that is sewn around the circular piece to create a tube. The seam between the two edges of the tube is connected by the Velcro strip.

Just like people's arms, not all socks will look the same, and they shouldn't necessarily. The point is to create something that will fit comfortably on the body, so feel free to make size and cut modifications as needed.

Step 4: The Socket Part 3: Assembly

Assembly of the socket is fairly simple.

First, take apart the ratchet knob. Take all the internal parts, along with the knob portion, out of the bottom housing and screw it into the top portion of the socket. Thread the cable through the holes in the ends of each of the starfish arms. If the cable is not long enough, longer lengths are available on McMaster Carr.

Second, cut small pieces of sticky-back Velcro to the ends of the starfish's arms.

Next, place the barnacle on the end of the residual limb. Pull the fabric sock over the limb so that the end of the barnacle extends out of the hole.

After adjusting the position of the barnacle and sock so that it fits comfortably, place the starfish over top of the limb. Tighten the ratchet knob so that it is comfortable but secure.

Finally, you can place a quick release clamp over the end of the barnacle and tighten it with the lever. The quick release knob holds everything together and creates an easy way to change out the attachments.

Step 5: Attachments Intro

This device has a flexible and simple attachment system and user-need driven attachments. We wanted Aidan to be able to change out the attachment by himself without help, and to realize anything he could dream up as an attachment in the future. For this reason, I kept the interface between the socket and the attachments simple.

The end of the socket has a 1/2 inch hole in it and each attachment mounts on a 1/2 inch rod. The attachments clamp on with a seat post clamp from a bicycle. Anything that mounts to a 1/2 inch rod can become an attachment for Aidan's prosthetic arm. So, when you're doing this yourself, have fun imagining the possibilities! The following attachment instructions are ideas inspired by and for Aidan, but feel free to change or adapt them to better suit your purposes.

Step 6: Attachment 1: Wii Nunchuck

One of Aidan's priorites was to get a Wii nunchuck as his prosthetic. Wii is a video game platform that often requires two-handed game play. By mounting the nunchuck controller to the end of Aidan's prosthetic, he would be able to experience and play Wii games more fully. Creating an interface for all buttons on the Wii nunchuck was outside the scope of this project, but engaging the accelerometer functions of the nunchuck made important progress towards increased access to what the game platform offers.

Translation: Aidan can now have more fun playing Wii.

In order to securely attach the Wii remote to the 1/2 inch rod, I cut the cord of the Wii remote, threaded it though a tube and re-soldered it together. I used epoxy putty to connect the tube to the remote.

Step 7: Attachment 2: Fork and Spoon

Aidan's a smart kid, and has already adapted well to eating with one arm. But, he wanted to be able to hold food in place more easily for cutting. I designed a fork attachment to make the process easier.

The fork attachment was made on a manual mill and drill press.

First, I milled down the end of an 8-inch piece of aluminum rod so that it had a flat section to bolt a fork. Next, I drilled a hole in middle of a fork handle and in the flat section of the aluminum rod. Finally, I tapped the hole in the aluminum rod and bolted the fork to it with the appropriately sized socket head cap screw.

The bolted joint gives Aidan the flexibility to adjust the fork's angle to best suit his needs in the moment.

Step 8: Attachments 3: Violin Bow

In Aidan's 4th grade class everyone learns to play the violin. Before this attachment, Aidan couldn't hold both the bow and violin at the same time.

Aidan and I chose to make a bow attachment because he would need more dexterity to use the fingerboard of the instrument than to use the bow and play successfully. The 3-D printed parts of this attachment are held together with 8-32 bolts. These bolts clamp onto the end of the violin bow. The spring in the base of the parts help to keep a relatively constant pressure on the violin's strings.

The file for the 3-D printed parts is attached below. As with the fork attachment, the aluminum rod for this part needs to be milled to a semicircle on one end.

Step 9: Attachments 4: Bike Riding Arm

Aidan's mom requested an attachment to help him ride a bike. All he needed was a second point of contact on the handle bars for steering and balance. This attachment provides him with that stability. It is intentionally reminiscent of Aidan's original LEGO character prototype that he built at the Superhero Cyborgs workshop.

The bike riding arm was made out of wood and painted with yellow spray paint. I drilled a hole in the back of the hand and inserted a 1/2 inch tube to interface with the socket.

Step 10: Attachment 5: LEGO Hand

The LEGO hand is my favorite attachment for a number of reasons. First of all, LEGO are a great building toy and this hand allows for a childhood dream of mine: the ability to build with LEGO on your own body! Secondly, this was one of Aidan's original ideas. Lastly, and most importantly, this attachment acts as a platform for Aidan to prototype future improvements or attachments to his prosthetic device with an already familiar medium.

Step 11: The Future

Before working on this project I had never thought much about prosthetics. When I saw how easy it was for the kids in the Superhero Cyborgs workshop to dream-up and realize ideas that fit their lives, it highlighted how unsatisfying commercially available designs could be. Looking at Aidan's sketches in comparison to the devices he had been using, it was clear that the "one-size fits all" approach was almost more limiting than his limb difference for him. I couldn't help thinking that those kids should have devices as awesome and unique as they are; that whenever possible, prosthetic devices can and should be a platform for showing-off the user's identity and interests as well as being a useful tool. Aidan had clearly done plenty of creative thinking on his own and was brimming with ideas for newer and better devices that fit his needs. I was lucky to work directly with him during the design process so we could put his thoughts and feedback straight to work.

I plan to refine the socket portion of Aidan's device and to work with him to create new attachments he can now prototype on his LEGO hand. Some in-progress ideas for future attachments include a water gun arm and a set of vice grips to hold onto anything that fits in their jaws...and have the firmest handshake of all time. I am excited to see more people try out this system so it can continue to develop and the list of awesome attachments can grow. Be sure to leave lots of comments when you try it!

Moving forward, my biggest hope for this project is that the attitude towards people with limb differences shifts from thinking they need a "replacement" for something that's "missing" from their body, to thinking their difference reveals a blank canvas to be filled with devices that are personalized, useful, and fun for the user.

For more information see this article in the Atlantic or my portfolio website.



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    162 Discussions


    4 years ago on Introduction

    Hi Coby,

    I am Paul from Ontario, Canada my son Ethan who born without a left hand after elbow. Me and my wife was trying prosthesis for him since he was 4 months, but completely failed due to weight of the prosthesis, he never wanted to use, always complained its so heavy. I was searching online to find other prosthesis manufacturer who can make better, while searching I found your work. That is a eye opener for me. Details of the work inspired me to make one. I already looked for makerbot 3D replicator printer and ninja flex, I am hoping I will able to make one. Thanks for all the design files. You are a savior. Our prayers are always with you.

    2 replies

    Reply 2 years ago


    Your son has roughly what I was born with. Please take my comments in that light.

    While I am hesitant to tell a parent how to parent, please read my comments about how my parents chose to deal with it on the link below. Simply put, they explicitly decided to ignore it. Now, I am almost 60 and am very happy with my parent's choice. And yes, I am a parent of a now almost 25 yo son. If he had been born without a hand, I would have done it *exactly* as they did.

    And, no, having one hand does *not* make me handicapped. I went to high school wit another guy with one hand whose parents had made a different choice. *He* was handicapped.


    Reply 3 years ago

    I can't believe I missed this comment. Please do tell me how it worked out for you. I am happy to help in any way I can if you need assistance. Wishing you and Ethan the best.


    3 years ago

    This is really touching... You're a great guy! I wish all the best you deseve it

    1 reply

    4 years ago on Introduction

    I followed your profile here from your multi-colored sweater. I think this instructable may be the coolest thing I've seen all week.

    1 reply

    Reply 3 years ago

    Thanks. I'm glad you like it. Sorry it took me so long to reply to this comment. It must have slipped though my radar.


    4 years ago on Introduction

    Hello Coby,

    I'm Davids from Ghana and have a son Caleb, 4 years old who was born without a right hand below the elbow quite similar to Aidan's but a bit closer to the wrist. It's been really difficult for us - Caleb, me and his mum. In that, our society is that which looks at disabilities as a form of curse or a misfortune. It has not been easy for him integrating into society as he doesn't get the friendly relationship he and us anticipate.

    But when I look into Caleb, he has got much more than we could even imagine. He's very active, happy, playful and above all talented. A great football talent that he is, I've always been thinking of a way to help see my boy through his dreams.

    He's been curious lately as my wife tells me, about what happened to his arms....and that his schoolmates have been staring overly at him especially when he's playing soccer. Though I'm currently not staying with my family in Ghana, but communication with them is very swift and I do feel feel pity for Caleb when he always asks me on the phone: "Dad, how can I play football without my hand? I can't throw the ball when it goes to throw over". The least I can always do is to motivate and reassure him that it's gonna be ok.

    Please Coby, is there any advice or help that you and other like minded individuals or organizations do to help Caleb?

    You can reach me out through or my cell #: +974 77531847.... Any help extended to poor Caleb and my family will be highly appreciated.


    4 years ago on Step 10

    I'm not quite sure why the lego hand is hollow. I think that a solid piece (so that the person printing it could choose the infill) would lend itself to a much more solid and usable product (while still being lite)

    1 reply

    Reply 4 years ago on Step 10

    The LEGO hand is hollow mostly because support material is less expensive than model material. If it was made on a MakerBot or similar printer I would have used a lattice structure for the internal supports as you suggest.


    4 years ago on Introduction

    Excellent idea you have my respect sir, many posts here are cool but useless, this idea is awesome, I hope you continues this work.


    1 reply

    4 years ago on Introduction

    Aidan is going to be a very positive part of humanity's future if his mind continues to develop at this level of creativity!

    1 reply
    Matt Makes

    4 years ago on Introduction

    Really impressive! I've seen a few other prosthetic related 3D print tutorials but none of them have been as elegant, intuitive or accessible as this one is. This seems like a real solution that someone with access to a desktop style 3D printer could potentially utilize to help someone with a disability. I have to say that I love the quick change attachments, the Lego one is especially impressive and is genius in it's simplicity. I'm glad you're associated with Instructables because no one would stand a chance against the sheer thought, heart, and design of this project in the Dremel contest otherwise. Very well done, you should be proud of what you created and the joy it has brought.

    Best Regards!

    2 replies
    CobyUngerMatt Makes

    Reply 4 years ago on Introduction

    Thank you. I am glad you like the quick change attachments. Aiden really had a great visions that steered the direction of the design and fabrication.

    I entered some contests just for fun, but I can't win any of them.

    Matt MakesMatt Makes

    Reply 4 years ago on Introduction

    And kudos to Aiden for coming up with such a great concept. The uninhibited creativity of kids is always impressive!