These wheels use an airless tire design. The central 3d-printed core is surrounded by a helical coil of cable. I have successfully used these on a mars rover inspired robot, and they deal reasonably well with rocks, sticks, and loose dirt. 

The center has a 16mm square of 3mm holes, for use with a motor hub. 

The model source is a python script, which uses SolidPython to generate OpenSCAD code for the 3d model.

More information can be found at https://github.com/gwfellows/rover

Make sure to print with 4 or 5 solid layers and perimeters, as well as >30% infill, to ensure the wheels are strong enough.