Introduction: 3D Printing Prosthetic Hand - Make It Real Challenge: Please Vote
First Prize in the
Make It Real Challenge
Hello and welcome to my second instructable, the creation of a 3D scanned and printed prosthetic hand, named 'Scand'. I decided to submit this project for the Make it Real 3D competition so I would be very grateful if you could please vote for me! This was a real mechanical prosthetic hand prototype, designed for a real amputee as part of MY university final major project, it was not a group project. In this instructable, I hope to show you briefly how I made it and I hope that you like what you see... :) please vote
Scand, designed by Scott Allen, is a piece of functional body art for consumers with single upper-body amputations. Scand aims not only to enhance the user's quality of life through added functionality but also to blow disproportionate, semi-fake prostheses out of the water with dimensionally accurate yet honest styling that allows users to express their personalities and increase social acceptance by completing their body profiles (Marr's computational Theory of perception, 1988) and by reducing the occurrence of the 'Uncanny Valley Phenomena'; the sense of 'repulsion' caused by one discovering that a limb, you thought was real, being in fact a fake (Mori, 1970).
Existing designs of prostheses can be ugly, disproportionate, heavy, expensive and have low levels of social acceptance, particularly prosthetic hooks. Despite this, prosthetic hooks are favoured by many due to their higher levels of functionality. In an effort to retain this vital functionality, Scand incorporates this very mechanism, albeit a modified version.
First, however, a cast is made of the user's existing hand using a environmentally friendly, seaweed based Alginate formula and Cystacal-D Dental stone. Next a 3D scan of the cast is taken to produce a 3D CAD model. The CAD model is mirrored, shelled and then further manipulated, by the designer, to encompass the internal mechanism, fixing points, rubber sleeve and a cavity in the top for interchangeable push-fit objects such as a clock, compass or electronic device. The CAD data is then sent to a 3D printer where the new prosthesis is printed very slowly, layer by layer using a fine powder. The thumb and finger grip are printed in a different, rubberised material for maximised grip. An Aluminium sand cast mechanism with elasticated tension, enables the rubberised thumb to open and close with variable gripping force, permitting the user to grasp objects. The thumb mechanism is actuated by a conventional body worn harness and cable system; moving the opposite shoulder increases/releases the cable's tension. Retaining operational familiarity will help many users in the transition from older designs to the new Scand design.
This particular prototype was designed specifically for a Client, a Transradial amputee who lost his left hand to Cancer, aged 21. Scand's design has significantly improved this user's quality of life allowing him to grip fine objects such as food sachets, snooker cues and spice jars, objects that he was unable to pick up with his existing prosthesis. Additionally, Scand is lightweight and more balanced to the user's natural hand weight than his existing heavy prosthesis, easing day to day use and fatigue. The client felt that the individuality and proportional accuracy of Scand, coupled with its ability to reinstate functionality, would benefit many new amputees during rehabilitation. Consumer's will be able to commission their prosthesis styling, including colours, materials, inlays and a choice of insertable push-fit objects enabling them to proudly express their personalities, not mask their disabilities.
View the designer's website: www.scottallendesign.co.uk
Step 1: Creating the Mould for a Hand Cast
To create the CAD model for the 3D printer we will need to use a hand-held 3D scanner as trying to accurately reverse engineer an organically shaped individual's hand would be virtually impossible, however the scanner does not scan actual body parts as well as it does models, objects and casts. For this reason, as well as not having to always have the client/person's hand available to re-scan (should you need to), it makes more sense to scan a stone cast.
It took me three attempts to get an ideal cast, so learn from my mistakes
- first I used a lemonade bottle as the casting container, however this dictated the position of my clients hand so much that it was in a less than ideal position to house the internal mechanism I intended to build, so I redid it using a wider container (a 5L water bottle), although I ended up using more alginate.
This step shows how to make the mould for the stone cast.
To create this mould you need several things, some of which you could improvise on, but ideally:
Alginate impression material - such as that made by Hydrogum (available online)
A casting container - such as a 5L water bottle with the top cut off.
A mixing stick of some kind - wooden spoon, scrap wood etc
Luke warm water
Your own hand or client's hand
A mixing bowl/scrap container
1. Take the top off you casting container using scissors, you want it deep enough to pass the wrist by a could of inches.
2. Mix one part alginate with two parts water (or as specified on your packet)
3. Stir well with your mixing stick and work into a thick paste, work quickly as it sets quickly too!
4. Now quickly pour the mixture into your casting container/ 5L bottle
5. Now slowly insert the hand you intend to cast into the casting container and wait for about 10minutes until completely set.
6. Try to peel a gap at the wrist to release the vacuum within, now heave and tug to loosen the hand from the mould, you'll most likely need two people, once loosened carefully pull the hand from the mould, do not wriggle fingers within the mould.
Step 2: Using the Mould to Create the Hand Cast
To create this cast you need several things, some of which you could improvise on, but ideally:
Block of Cystacal D dental stone
Powdered Cystacal D dental stone
Rotary sander with hose outlet
Milkshake mixer or equivalent food style mixer
A new mixing stick - scrap wood/spoon etc
My apologies but some of the photos did not all import in the correct orientation, despite them all being rotated correctly..hmm
If you do not mix the mixture well enough you will get lumps (see poor result in photos) and you will have to redo the whole process (steps 1&2).
For each cast:
MKI - The thumb was tucked too far underneath
MKII - The surface finish was poor
MKII - Perfect
1. Grind your block of Cystacal D dental stone using the rotary sander, mix with the out hose with water to create a milky liquid
2. Mix this liquid with the powdered Cystacal D dental stone using the food mixer
3. Place your mould onto the vibrating platform (machine) and pour your thick, gloopy mixture into the cavity.
4. Turn on the vibrating platform and let the vibrations cause the mixture to enter all the tiny gaps inside.
5. Turn off the vibrating platform and leave to set.
6. Cut out the cast from the mould.
7. Admire your model and all its amazing detail including finger prints!
Step 3: 3D Scanning
For this step you will need to use a 3D scanner such as a z-corp 800 hand held scanner.
There are two ways in which you can scan the cast, you may stick the reflective positioning dots onto the model directly or onto a board beneath, meaning you can reuse the relatively expensive dots to scan many objects, although you will need to conduct both a top and bottom scan.
1. Plug the scanner into your laptop
2. Calibrate the scanner
3. stick the dots to your model or board and begin to scan
4. Make sure that when scanning that your optimum proximity bar remains green such as in the photo (to the left of the laptop's screen).
5. Use the realtime model on screen to watch your progress and find any missed patches. Slow sweeps work best, you may of course recover previously scanned areas, providing you have not moved your model.
Step 4: CAD Modelling
The scan data, a polygonal mesh needs to be converted into a patch network and then a nurbs surface, from here you can export an iges part file, which can be solidified and remodelled within a CAD package such as Pro/Engineer, Solidworks or Rhino. Don't be caught out by scale, check whether you are working to mm or inches! It could be a costly mistake. Remember to mirror the data in order to give you the OTHER hand (the right hand mirrored into a left hand in this case).
For the CAD remodelling I did several things
1. Took the solid '.prt file' and created several new workplanes from which to sketch onto (a scanned model will not instantly have logically located work planes as it was not built within the CAD package).
2. Next I created a simple rectangular sketch and cut the wrist nice and flat with an extrude (remove material selected), the wrist was previously a bit lumpy from the liquid mixture poured into the mould.
3. Then I removed the thumb and used that as a separate part file named "thumb".
4. Then I shelled the hand to a wall thickness of 2mm
5. Then I created a circular cut in the wrist for inserting the internal mechanism (to come).
6. Then I created three screw holes to affix this mechanism, 120 degrees from one another, which I strengthened with thicker wall thicknesses around the holes.
7. To build the watch rim I then I sketched a circle on a projected plane above the top of the hand aka 'back of the hand'. The sketch was then projected onto the surface of the hand itself.
8. A smaller circle was then sketched on a projected plane and the two circles were merged or 'blended' using influences curves around this blend.
9. I filled this blend to create the solid rim walls (you cannot print surfaces using 3d printers, surfaces have no thickness!)
10. I created a simple circular extrude to remove the inner material, but not all the way through as I needed to retain a base.
11. Then I cut a smaller hole in the base allowing the clock (or other insertable object) to be popped out from the inside of the hand.
12. To clock itself is from habitat, I reverse engineered this clock to check it's fit within the hand, throughout the modelling process.
13. To create the rubber sleeve I copied the geometry of the organic wrist profile to build a new part which I then blended to a circle profile on a projected plane. I thickened these surfaces to give a wall thickness and I created a protrusion to fit into a recess within the wrist ensuring a smooth transition from the hand shell to the rubber sleeve component.
14. The thumb was just left solid for strength, however it could have been shelled to reduce cost.
15. Cut a slither from the index finger and use the cut part to form a new part (a rubber insert)
16. All parts were used to build an assembly file and check the overall model appearance.
Step 5: 3D Printing
Now you will want to save your individual part files as '.stl' files.
This is different depending on your modelling package, but generally speaking try and input the minimum step heights etc that your system will default to as this will give you the best surface finish, but may increase your file sizes.
Send them to a 3D printer or company who can offer this service. I printed the hand shell in the most rigid of the rubberised materials DM_9795, as I wanted to avoid brittle materials which would shatter on bashes or knocks. I printed the thumb and sleeve in a more flexible material DM_9770. If you have to pay for support material then you may which to consider which orientation you print the model in.
Once printed you will need to carefully remove the support material with a jet wash or by hand using water only. Caution; smaller or delicate parts are likely to break under high pressured water. Let the model completely dry out for 48 hours (it will sweat), then finish with fine grade wet and dry paper, make sure you remove all support material, sometimes you can feel it better with your fingers than you can see it with your eyes!
Step 6: Internal Mechanism
To make this mechanism I modified an existing prosthetic hook, made by Hosmer USA. You need to:
1. Disassemble and store internal ball bearings safely, then saw off the hooks themselves
2. Saw off the cable arm too, but not completely, just leave a little metal to weld onto.
3. Take some aluminium rod (mine was 8x120mm) and bend it into the desired curve then weld this to what was the cable lever arm. Now file and wet & dry.
4. Drill a hole into the new welded arm to form a loop hole for the cable to be crimped around.
5. Take your cabling and some brass tubing, now crimp them very tightly.
6. Drill the screw holes and another hole to create the channel for the cabling to fit through, keeping all cabling internal. I printed a drilling template from my CAD to do this.
6. Drill a hole in the thumb to slot the levering arm onto.
Step 7: Painting
Since I used a rubberised material I decided to apply a flexible paint to avoid cracking if the hand was to flex.
I used a spray gun for a nice thin, even coat. I tried to dye the rubberised parts with a leather dye (as recommended by others) however I found that it never seeped in enough and I was able to remove it all with water. I decided to leave these parts in their neutral colour as they looked pretty cool like this anyway.
You may want to knock up a quick jig to hold the hand and/or rotate it whilst spraying.
1. Smoothen your parts using wet and dry
2. Blast off any dust with compressed air
3. Sit the hand on a jig and spray whilst rotating it.
4. Allow coat to dry, then sand back again, particularly in between the fingers.
5. Apply another coat and repeat the process until you're happy :)
Step 8: Assembly
Assembly is pretty simple
Ensure that you fit the mechanism from the inside of the sleeve, screw the mechanism in and then mark out the slot for the cable to slot through. Then drop in the ball bearings and screw all together, pretty much as you disassembled it. Finally screw it into the fixing holes that were created on CAD. It would be much better to incorporate brass fixings than screw directly into the material as it doesn't bite well. I had to fill the holes with glue and rethread as this was more sturdy but I could not source brass fixings of the correct size.
Then glue the thin slither of printed rubber into the index finger cavity to create your enhanced grip. I used gorilla glue as its very strong!
Step 9: Final Piece - Please Vote for Me!
...and here it is, the final piece worn by my client. He requested a modern looking prosthesis with a james bond aesthetic. It functioned well and allowed him to complete tasks with fine grip that he was previously unable to complete such as being able to hold sachets to feed the cat and holding spice jars.
Thank you and I hope you've enjoyed this instructable, especially as it took two days to upload it all :P
I would be so grateful if you would vote for me in the make it real challenge, over 1000 hours went into this whole project (including research, reports, folios etc). It was exhausting, but welllll worth it! :P
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