Surgeons at a hospital in Japan recently faced a dilemma before transplanting a parent's liver into a child: How exactly to trim the organ to fit the space in the child's smaller cavity while preserving its functions.

So they took a knife to a three-dimensional replica of the donor's liver built by a machine that resembles an office printer. The model helped the doctors figure out where to carve it, leading to a successful transplant last month.

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