https://phys.org/news/2018-11-bioreactor-device-frogs-regenerate-legs.html

The scientists 3-D printed the bioreactor out of silicon and filled it with hydrogel—a sticky glob of polymers. They laced the hydrogel with hydrating silk proteins that promote healing and regeneration, then added progesterone. Progesterone is best known for its role in preparing the uterus for pregnancy, but the hormone has also been shown to promote nerve, blood vessel, and bone tissue repair.

The researchers split the frogs into three groups: experimental, control, and sham. For the experimental and sham group, they sutured the device on the frogs immediately after limb amputation. In the experimental group, the bioreactor released progesterone onto the amputation site. In all cases, they removed the devices after 24 hours.