Alexander started designing the Springfree Trampoline way back in 1987 when his wife didn't want to get a trampoline due to the danger of snapping her kids' limbs.
Being a good father, and a curious engineer, he saw this as an engineering issue rather than a safety issue, and he set about improving the tramp. It turned out to be a harder challenge than expected, and he enlisted the help of his graduate students to solve some of the tricky engineering issues. In all, it took around 15 years for the idea to become a reality. The final design uses fibre-glass reinforced plastic rods to help bouncing instead of using those nasty old springs that pinch and injure people so often.
The rods are placed diagonally from the metal frame and support the mat from beneath so that anyone bouncing on it can't land between the springs and hit the frame - no more chipped teeth. It's unclear whether this also solves the old 'static shock when dismounting on a hot day' problem but it's certainly made the backyard nutcracker substantially safer. Very safe and still just as bouncy - test trampolines are subjected to three million bounces as part of the rigorous testing, simulating 10 years of usage.