Introduction: Adding Sense of Touch to a Prosthesis

If your hand has ever fallen asleep or gone numb for some reason, you know how how difficult it is to grab anything without a good sense of touch. Now imagine how amputees might feel! Scientists are currently working on adding a sense of touch to prostheses through several different, very high-tech means. I decided to sit down and see if I could figure out a way to do the same thing, but in a cheap, simple way. After all, not everyone can afford top-of-the-line prostheses with the latest technology. The solution I came up with was to use pneumatic or hydraulic lines that attach to the fingers at one end and to a brace at the other. The brace ends are arranged in a way so that when the brace is attached to a functional part of the arm and the prosthesis grips an object, the pressure from the grip will also create a feeling of being griped by the arm, wherever the brace is located. This Instructable is mainly to create a proof-of-concept model, which can be used to demonstrate to other people how this system would work.

Step 1: Materials

All I used for this was this kit I bought from Hobby Lobby for about $30, and a glue gun. I bought the kit mainly for the syringes and the tubing, but ended up using the wooden parts to make a semi-functional prosthetic arm, in order to better illustrate my concept.

Step 2: Making the Fingers

To make the "fingers of the hand, I took four of these long pieces and three of the smaller ones, and ran one öf the wooden dowels through all of them. I also glued them together so they would all lay flat and not move independently.

Step 3: Making the Thumb Joint

To make the thumb, I took two of these slightly shorter pieces and attached them to the outside of the fingers, using the excess remainder of the wooden dowel running through the fingers. I then ran a wooden dowel through the tips of both thumb pieces. This will be an attachment point for the thumb syringe and thumb tip.

Step 4: Making the Thumb Tip and Attaching Thumb Syringe

I used this flat piece as the thumb "tip." that part will do the actual gripping. I also glued the first syringe to the hand. In order to allow for pressure detection, the outside tube of the syringe, a.k.a. the barrel, needs to be glued to the wooden dowel between the two "bones" of the thumb joint. The flat part and the end of the syringe's plunger needs to be glued to the back of the flat piece that makes up the thumb tip.

Step 5: Adding the Finger Syringes

I pressed the thumb against the fingers of the hand. Looking at the back of the hand, I used a pen to roughly mark the halfway point of the thumb tip, so I know where to glue the syringes so they meet the thumb when the hand is closed. For the fingers, I decided not to glue anything to the flat parts of the plungers, since I figured that would give them a better, more independent sense of touch. The barrels of the syringes still need to be glued to the fingers of the hand, however. Take care not glue the plungers in place!

Step 6: Optional: Adding a Forearm

I originally wanted to add a forearm to the prosthesis, so I could attach it to my own arm. Unfortunately, the tubing that came with the kit was a bit short, which made attaching the forearm to my arm too difficult. I decided to do it anyway, for cosmetic purposes. I added some of these long wooden pieces and connected them to the hand by running a wooden dowel through all of them, creating a wrist joint. At the other end, I added a couple of small wooden pieces in between the longer ones and glued them all together, to give the forearm more support.

Step 7: Attaching the Tubing to the Syringes

The tubing was fairly easy to attach. I made sure all the lengths were the same for the finger syringes. For the thumb syringe, though, I used a slightly wider piece of tubing to connect two of the thinner ones like the ones in the fingers. Because the thumb syringe was pointing the opposite way of the finger syringes, it needed more length in order to curve back reach the brace.

Step 8: Making the Arm Brace

The arm brace was very simple. I just attached three wooden pieces together and made a U shape. One thing to note about the brace design is that it needs to be rigid. Because the flat part of the syringes attached to the brace will be pushing in against the arm whenever the prosthesis grips something, the brace needs to be rigid so that it doesn't stretch in reaction to the pushing by the syringes, which would cancel out the force being applied and therefore, the user would not experience the sense of touch as intended by the system. For that same reason, the brace needs to have a snug fit.

Step 9: Attaching the Syringes to the Brace

The syringes need to be attached to the brace in the same configuration they are attached to the hand of the prosthesis. That way, when the user grips something, they will similarly experience a gripping sensation in the area where the brace is installed. Because the tubing was so short and inflexible, I had to add a bit of reinforcement to the attachment points of the syringes, since the hot glue wasn't holding too well.

Step 10: Calibration

In order for the system to work properly, all the syringes on one end need to be fully closed (plungers all the way in) and all the syringes at the other end need to be partially open. The best way to do this was to remove all the tubing at one end, adjust ALL the syringes to where you want them, then attach all the tubing back. As far as how open you want the syringes, experience different levels. I noticed that about an inch of space inside the syringes was enough to provide enough movement at the brace end for the plungers to press on my arm, yet enough room at the fingers of the prosthesis to allow the hand to open and close without any problems.

Step 11: Conclusion and Afterthoughts

As you can see in the images, once the hand is closed and the fingers make contact, the plungers at the brace end close in and apply pressure to the user's arm. However, as I mentioned before, this model was more of a proof of concept than an actual working prosthesis. The good thing is that we truly live in an age of wonders. With the advent of 3D printing, this could be improved significantly. For example, instead of having tubes running all over it, a prosthesis could be engineered and printed with internal channels that run along its length. Not only that, but the channels could be made smaller so that more could be placed at the fingers and palm and at the brace end, giving the user a "higher-resolution" sense of touch. So, if anyone out there has the resources to improve on this, go for it. But remember that the goal of this is not a way to make money, but an economical way to give back to people who are missing limbs a little bit of the humanity they have lost. I sincerely hope that with this Instructable, that can be accomplished.

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