Biological systems can do a lot of things that machines can't.
Biological life also has to solve a lot of problems that machines don't. For example, no doctor or mechanic will come along and fix every bruise or scrape; the organism must do that itself. But a car that gets dented is taken to a special building where specially trained people work to fix it. Likewise, a car does not have to digest hay and oats, or run away from wolves, or find its own way to water, the way horses do.
We humans are willing to make many tradeoffs in pursuit of efficiency. In particular, we have learned to value a machine that can do one thing well--such as refining oil or turning a shaft--rather than asking a single machine to gather, process, and consume its own fuel, much less repair itself. And it is only recently that we are asking a few of our machines to even pretend to nuzzle us with affection.
So when I say "improvement," let it be understood that I'm talking about improvements in effectiveness for narrow tasks, not in overall functionality.
Machines are becoming useful for more and more things. Some of those things are inspired by what we see other organisms doing, but the design usually turns out rather different. We see birds; we make airplanes; the airplanes have neither feathers, muscles, nor feet.
Biology uses molecular machines to make more molecular machines, as well as structures at all scales up to whales and dinosaurs. The first inspiration is to make a machine that will perform the same function. But what biological functionality can be discarded, if all we want is a machine that does a narrow task?
A mechanical approach to molecular fabrication might be about as similar to a cell as an airplane--or a helicopter!--is to a bird. Even so, there is no bright-line division between biological and engineered approaches. For the past few decades, researchers have been making modest changes to natural bacteria in order to fabricate chemicals like insulin. Today, a few researchers are talking about engineering and building a bacteria-like life form from scratch. Our cells contain machines--mitochondria--that were once independent bacteria, but have become mere chemical factories, supplied with most of their building blocks pre-made by the enclosing cell.
So let's think about the options for an engineered biologically-inspired system. What functionality can we dispense with to make the engineering task easier and the machine more efficient?
- Biological molecular machines rely on slow thermodynamic relaxation and diffusion processes for efficient operation. An engineered machine, especially if built with strong materials, could transfer forces between machines, balancing loads faster and in a smaller volume.
- Biological molecular machines work in the drag of water. We already know that some enzymes do not need water. Perhaps we could engineer chemical reaction mediators that work in vacuum.
- All biological systems are limited to design spaces that are accessible to incremental blind improvement. Obviously, we don't need to stick with that limitation.
- Biological organisms must metabolize complex and varying chemical inputs. We can provide refined chemicals.
- Organisms maintain internal state in the face of external perturbations via a series of complex feedback loops. Machines may maintain their state by being designed to be insensitive to perturbations. This may waste some matter or some energy, but historically that has been OK.
- Organisms must self-repair. Machines don't have to. If they're cheap enough to rebuild, they can just be replaced when they break. (This does not preclude recycling.)
- Organisms must resist predators and parasites. Some machine-structure materials are susceptible to attack by oxygen, water, or organisms, but in general machines can be protected by simple barriers.
- Organisms must grow from smaller to larger instances, from the inside out. Machines can be built externally in their "adult" form.
- Organisms must, with few exceptions, maintain all the processes of life at all times. Machines, being simpler, can usually be frozen in place and restarted.
Here's a key thing to notice: These limitations are mostly independent of each other. Bypassing any one of them, or any combination, may point to a useful spot in the design space. Thus, asking which biological limitations could be relaxed, and what a system without some of these constraints might look like, could provide very fruitful grounds for development of research targets.
Manufacturing systems based on diamondoid mechanosynthesis aim to bypass all the above limitations at once, and may represent a “sweet spot” in the molecular design space. However, most of the biological limitations appear to be mutually independent, implying the existence of a number of potentially useful and accessible systems distributed among numerous identifiable dimensions of the design space.