Motorola Mobility is tapping into 3D printing as a way to make parts for user-customized smartphones, a concept being chased by the company under an initiative called Project Ara. Motorola has signed a deal with 3D Systems, which will use 3D printing to make smartphone enclosures and other modules for the highly modular smartphones. 3D Systems, which makes custom parts using 3D printing, did not respond to a request for comment on the other parts it will supply.