This is the main body of my hidden blade. Check the other four in my shop: https://www.shapeways.com/shops/sicarius-lair
CAUTION: watch the size of the hidden blade. It's for a 23cm (9,055in) or bigger arm. It measures about 44,4cm (17,48in) when fully extended. Which means you get a 21,4cm (8,42in) blade.
Please note that this is a BETA product, due to money issues, I didn't got one for myself yet. However, I did some simulations in SolidWorks, it everything looks good.
WARNING: I think I'll be doing the right thing to warn one liiiiiittle detail. Due to the eagle drawing on the top of body, Shapeways says there could be a little problem with it. IT'S NOT A STRUCTURE PROBLEM. But the eagle might look like a jackdaw :D (just kidding). Check in the image section the only problem with "White Strong...".
My hidden blade has two working methods. You can keep it inside until you pull a string attached to your finger, so the blade will go out, until you manually put it back in. You can also keep it in with the force of a spring, until you wave your forearm, and the blade will be locked by a metal plate, until you pull it (manually or with a ring attached to it).
-Strong line (multifilament line, superline, any strong line that you can make knots with)
-Extension spring or elastic band (nothing to strong, or it could damage the plastic structure)
-Bendable metal plate (to build the lock for the second method, you can use a saw, and get a small piece of it)
-Small "pen-spring" (that little spring inside those "click-pens")
-Velcro straps/belts, to place it in your arm.