Thumbs Up!

Introduction: Thumbs Up!

Well, this project was done a while ago during as an seminar work at one of our classes at university together with my colleague and friend marko. The assistive tech contest encouraged me to finally write down our steps and considerations.

Our goal for this project was to provide a first draft of a thumb prothesis to restore elemental grasping functionality after a traumatic thumb amputation or otherwise absent thumb. The main objective and most present design principle was to utilize affordable and almost everywhere accessible materials, to facilitate access to prosthesis even in developing countries. Well, we used a 3D printer, but compared to professionalists level equipment in prosthetics this can be considered as low cost.

Step 1: The Amputation Pattern

This artificial thumb is used to gain basic grasping functions normally provided by the thumb, if the thumb is entirely amputated without participation of other structures of the hand and wrist. An intact palm, backhand and functional index finger on the affected side is required. As a next design step this artificial thumb could be adopted to be also used by patients with only a partially amputation of the thumb. This artificial thumb is suitable for both sides (simply mirrored); the depicted one in this instructable is the right hand side.

Step 2: Materials

  • one glove
  • bowden cable: three different lengths are needed, in sum they are about 0,15m in length
  • cable: normally wire cable is used but a tearproof thread will also work, it should provide at least a little bit of stiffness
  • joints: we made them out of cable ties, alternatively you could use any semi-elastic pieces, like littler rubber pieces (e.g. bicycle tube)
  • again some cable ties were used to mount the bowden cable onto the glove, alternatively you could use any kind of thread
  • phalanges and the basis: were 3d printed (see: base, finger, thimble)

for the whole build process no elaborated tools were used

Step 3: Design

The phalanges and the basis are 3d printed and for rapid prototyping purposes we used another awesome project called "Flexy-Hand 2" by Gyrobot (CC-licenced, see the project on thingiverse). We used the upper two parts of the thumb as well as the base and adopted them for our use.

You can find all of our designs on tinkercad: base + finger Additionally we added a little thimble for the index finder to better transmit the force applied from the index finger to the bowden cable: thimbleFor the base/harness, we also thought about exchanging the 3d printed version to some other material, e.g. carbon fiber epoxy after making plaster cast to get the exact shape of the hand.

Step 4: Build

  1. First we printed the 3d printed parts out of PLA and then without smoothing the parts we connected them with our cable ties. Those were fixed in the slots with some superglue after we tuned the right size of the cable ties. As these will open the finger via elastic forces, the right amount of elasticity is crucial.
  2. After this, we put in the thread, through the base and all finger parts until the fingertip. A simple knot was used to secure the thread there and some access thread was left there to allow for some adjustments later.
  3. The base was fixed to the glove, although we removed it later again (because our real thumb got in our way)
  4. Mount the bowden cable on the glove and leave gaps between the joints of the index finger. We mounted this bowden cable to the glove with cable ties, but i guess sewing would be prettier.
  5. Pull the thread all the way through until the tip of the index finger and mount it on the little thimble.
  6. At this point we adjusted the right length of thread by undoing the knot on the tip of the thumb.

Step 5: Functionality

A tensing of the index finger causes an extension of the length of the cable on the back of the index finger. This pull on the rope is transformed into a respective movement on the artificial thumb. The rope is guided through the prosthesis on the base of the thumb in order to subsequently guide the cable on the inside of the thumb. This control cannot be decoupled and is not affected by an extension of the index finger or other fingers. This direct power transmission also acts as a feedback mechanism, as under appropriate load a pressure on the index finger tip is propagated through the bowden cable.

Step 6: Lesions Learned

Our experiences from the first prototype revealed the following difficulties und positive sides of our design

  • Comfort is compromised by the unpadded attachment of the bowden cable to the glove, due to the unfinished nature of our first prototype.
  • Deflection by flexible bowden cables seems to be more difficult than initially thought, a good interplay of flexibility and stiffness must be found to keep the stability as high as possible while keeping the space required as low as possible.
  • The ratio of finger movement vs. movement of the prosthesis needs to be further adjusted. To achieve this a better form of the phalanges would be necessary. With this prototype a complete contraction of the index finger leads to a relatively small contraction of the artificial thumb.
  • Our joints prove to be too stiff to build up adequate finger pressure. The material used (plastic cable ties) seems to be suitable, especially in terms of availability and cost. We used two cable ties per joint and this turned out to be too much.
  • A proper testing of the thumb prosthesis has proved to be difficult for us (double thumb owner).
  • The natural roughness of the 3d printed surfaces turned out to be very good in order to grab objects (fast printing is nice here).
  • All parts we used were easily available. Even the bowden cable was sourced in a bike store in our vicinity, from their trash bin, for free.

Thank you for reading this little instructable. I would love to hear that someone makes use out of this.

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    Reply 1 year ago

    Thank you very much, i appreciate it