Several years ago I bought this absolutely terrific “HYDRODYNAMIC Building Set” by Bridge Street Toys:
It has who know how many parts, and lets you build an endless variety of complex toy chemical plants.
Leo and I have played with it quite a few times but, as with his various programmable toys, only recently has he got to the point where he is expert enough at both the small manipulations required, and at understanding the overall of what’s going on, to envision and then create his own inventions (admittedly, with a lot of help from me in practical realization details!)
Yesterday he decided be wanted to build an artificial heart. It’s a little hard to build an artificial heart with just one pump and a bunch of tubing and inflexible reservoirs , but … well, okay, so it doesn’t have to literally be an artificial heart; we’ll leave that for first grade! 🙂
Anyway, the kit is well thought out, works very well, and is a lot of fun for all ages.
Two minor nits: First, the green connectors don’t stay on very well under pressure, or any amount of stress. I’m not sure what to do about that other than plan carefully to avoid overpressure and stress. The other nit is that the nib at the pump head that adapts the pump to the tubing size breaks off easily. We broke two. The good news is that the good folks at Bridge Street Toys are extremely helpful, and sent is replacement nibs right away.
You’ll want to buy several meters of extra tubing (available at any hardware store), as you’ll end up cutting it up into random-length pieces and then end up not having the length piece you want. You’ll also want some food coloring. (I bought several quart-sized food coloring containers as an Amazon add on, which is enough food coloring to color Lake Ontario, I think. You can probably make due with those tiny squeeze bottles!)
 In high school, I think it was, I actually built a sort of artificial heart for a science fair, using an aquarium pump and really real artificial heart valves that my dad,the cardiologist, got me. I don’t think I won anything, but it was pretty cool, and not very far off, in concept, to the Jarvis 7, which hadn’t yet been invented, I think, or at least not widely used. Several years later I realized that the artificial heart valves were made of titanium, or something really expensive like that, and asked my dad how he managed to get two of them for his son’s stupid science fair project. As I recall the answer was that they were used, and that the patient didn’t need them any longer. Yuk!