Invisible Material
An invisible material, also known as an invisible wall, refers to any solid object that characters and/or objects can collide with, but which is not visible to the player. These are typically used as barriers to block off areas that aren't supposed to be accessible, and can be separated into two distinct categories: thick-layer surfaces that obstruct lateral movement, and thin-layer barriers that prevent movement through layers.
While some games include invisible materials as part of their toolsets, others have glitches or other tools that enable players to create their own.
Official Invisible Materials
LittleBigPlanet PS Vita
The first instance of an official Invisible Material can be found in LittleBigPlanet PS Vita. In Create Mode it displays as a flat, unlit, transparent surface, similar to Sticker Panel at 0% opacity. In terms of physical attributes, it is lightweight, has no gravity, and cannot be grabbed. While the material itself turns turns invisible in Play Mode, any decorations placed onto it will remain visible; stickers placed onto a piece of Invisible Material will not be visible in either Play Mode or Create Mode, however.
LittleBigPlanet Karting
LittleBigPlanet Karting also has its own invisible material, known as Force Field. In Create Mode it has a light green tint, but like with other materials in this game, the player can change the colour in its tweak menu.
LittleBigPlanet 2
An Invisible Material was added to LittleBigPlanet 2 in the DC Comics Premium Level Kit, unlocked by completing Sackboy Vs The Mecha-Lex. This version has a similar appearance to the Clear Plastic material found elsewhere in the pack, and takes on the physical properties of rubber. Unlike its PS Vita counterpart, decorations placed onto this material will not render in Play Mode.
D.I.Y. Invisible Objects
In the original LittleBigPlanet, one method of create invisible walls took advantage of materials with rounded bevels; materials such as Basic Rubber or Dark Matter will disappear entirely if shrunken past a certain point, or cut into thin enough strips.
In LittleBigPlanet 2, the Button and both kinds of Lever received an "Electronics and Cable Visibility" tweak; these default to 'Switch Visible', but can be set to 'No' to make them invisible, allowing them to be used as invisible surfaces. Thin-layer barriers that prevent layer movement can also be created relatively easily by lethalising a thin piece of Hologram with Plasma, and tweaking it to match the player colour. This works because Sackthings will not attempt to change layers if there is a lethalised object between them and the layer they're trying to switch to, even if it is not considered solid.
Although LittleBigPlanet 3 does not include a dedicated Invisible Material in its base toolset, the Opacity Tweaker can adjust the opacity of any object it is attached to, and even turn it invisible; through this, any material in the game can be turned invisible without removing its physical properties. A less intentional method for creating invisible objects comes in the Camera Excluder; if the Material Changer is used to convert a piece of Camera Excluder into some other material, the new material will retain its behaviour of turning invisible in Play Mode.