Think of an ultra-precise glue gun: this is how 3D printers make objects, caking layers upon layers of polymers until the whole model is complete. Right now, the printed objects themselves are often rough and not body-safe, with crevices that collect gunk easily. People usually recast the models in silicon before use.
A mini cottage industry is already springing up around the concept, with the New York Toy Collective scanning people's appendages and printing them into custom gifts, and MakerLove offering free models for anyone with access to the right printers.
Below is a gallery of what you can download for free from MakerLove.