This is what got me into prop making. I always wanted to make the things I saw in films. Often, film props don't actually do all the cool things we see them do. This is different. It moves. A monster attached to my back with a touch activated 2 axis controlled tentacle! There are some necessary skills for this type of prosthetic creation. This project involves lots of sculpting, engineering, soldering, prototype circuit design, foam latex mixing and some luck. I'm completely self taught and with some passion, I know anyone can do this. I'm going to go over the how as much as I can!
Step 1: Armatures, Sculpting and Mold Making
First we'll discuss the body of the creature. You'll need a hollow object to sculpt on--this is later for mounting electronics to. There are a number of steps involved for sculpting a skin for a mechanical tentacle armature. We'll get to that!
The Monster Body
I used a metal bowl to sculpt on. Asymmetry is pretty fun to look at and unsettling, too. There are many techniques out there but just remember to keep reference photos in front of you the whole time you're sculpting.
This is a bit tricky. You see, if we just sculpted a tentacle out of clay and cast it, it would be solid. We need to fit the armature in it so it moves!
Prepping the Armature
You can see that the tentacle is made from transparent discs. What you need to do after making this armature (I know, I know, we'll get to that!) is fill in the spaces with cotton balls and cover the whole thing in plastic wrap that has been sprayed with a release agent (or pam). At the base of the armature, you'll want to make keys out of clay (these keep the tentacle armature in the center of the mold later on when we pour foam latex!)
You'll be making a 2 part mold of this solid wrapped armature using alginate and plaster (one time use!) or silicone and plaster (repeatable). Once you've made a plaster positive of the armature, you're ready to add clay to this positive and start designing your tentacle!
You'll want to make the skin about 1/4" to 1/3" thick. You could go thicker but it may make ugly folds. Trust foam latex to be forgiving.
Step 2: Casting Foam Latex (You're Going to Need a Bigger Boat.)
Foam Latex Mixer is a position in a film special effects crews. One person who is an expert at foam latex. Needless to say (but I'll say it) there is a lot to know about foam latex. She is a cruel beast. I have to assume you know how to mix foam latex and bake it. There could be multiple instructables on foam latex and the subtleties of recipe changes, ie flow enhancers, softeners.
There are plenty of online resources for foam latex. In fact, I made a foam latex oven using online resources!
The important thing to get right in this, besides the foam latex, is setting up your mold keys. If you don't have clean edges and strong keys (they keep everything lined up), you'll end up with crooked results. Or it won't turn out at all. The foam latex gods will come down and smite your work.
Step 3: THE TENTACLE ARMATURE
The Control Mechanism
I wanted to be able to actively control the tentacle while wearing it. To do this, I turned to servo testing circuits. They are simple circuits to make. I attached a diagram. Instead of using a potentiometer to sweep the servo arm, I replaced it with a force sensitive resistor! So, to move the tentacle all I have to do is apply pressure to my fingers. I made two circuits, one for each servo to control X/Y axis of tentacle.
The Mechanical Elements
The center of the spine is made from a brake spring intended for a motorcycle. The spines themselves are acrylic. There are 4 holes in each one for my control cables. I decided to use FIREWIRE which is a type of fishing line that's sturdy and slick (mm!). The set up is pretty elegant. You tie off the ends of the 4 lines and then run it through each disk. If you have any, I recommend wrapping the armature in a mesh to keep it from looking too bumpy from the space between spines. More spines is good too!
I cut mine out with a table scroll saw. I have a laser cutter now and wow would that have been safer and faster. But hey, that was a whole mustache ago.
Step 4: Painting
You're dealing with foam so you may want to use alcohol based paints that are used in air brushing. :] Remember that it is foam LATEX so you can use liquid taxes to adhere pieces, add texture or patch a tear. I realize there's a lot going on with this project so ask me questions and I'll do my best to answer! Want to see a video of it working on me? Go to my instagram at HardlyHumanFX :]