A multi-material printer can include a housing that is configured to be handheld. The housing can be configured to receive first and second printable materials from respective first and second containers. A nozzle can define an outlet of the multi-material printer. At least one actuator can be configured to cause first and second printable materials from the first and second containers to flow at a respective constant rate. The multi-material printer can be configured to simultaneously extrude the first and second printable materials from the outlet. The multi-material printer can be used for fabricating a scaffold from filaments comprising the first and second materials directly within the body of a human or animal.