You see, a biological computing device is like a banana.
Let's say that I want something to eat, so I go into my kitchen and find a banana. Is my banana "functional" as food? Well, it's very important for food to be fresh and healthy, so I'd better make sure my banana is fresh. But just how fresh is "fresh enough"? Let us consider some different ways that my banana might look:
|A spectrum of banana freshness (credits: yellow, spotted, brown, rotted)|
The second banana is getting on in age and starting to develop spots. It might be a bit mushy inside, and my daughter will definitely not eat it, but I'd be happy enough to chow down.
The third banana probably won't taste good to eat on its own, but it's just perfect for making banana bread or other recipes that transform a banana from centerpiece to simply tasty flavoring.
And as for the fourth... I don't think I'd even want to feed that melted mess to livestock.
As you can probably see, the notion of a "fresh banana" is not a fixed concept, but a spectrum, and "fresh enough" depends entirely on what exactly we want to do with that banana.
Biological computing devices are the same way: a device has to be very high-performance in multiple dimensions (uniform yellow) to be safe and useful for complex circuits or applications like precision medical therapy, while simpler and less safety-critical circuits can tolerate some problems (spotted yellow), some applications just need a nudge or two in the right direction (brown), and some devices probably aren't good for anything at all (rotten).
Right now, the vast majority of our available biological devices are metaphorical brown bananas, with just a few spotted bananas available. Understanding that fact is the first step, and getting on with building some fresher bananas is the second.