Nature has evolved various forms of biological nanomachines - small-scale devices composed of chemically reacting biological molecules. Simply referred to as bionanomachines in this book, they consist of molecules that are abundantly found in living organisms, such as carbohydrates, lipids, proteins, and nucleic acids. They are in the nanometer to micrometer range and thus not visible to the human eye. They are machines capable of biochemical interaction with molecules. Figure 2.1 shows some examples of bio-nanomachines, including protein molecules that catalyze chemical reactions (i.e., enzymes), regulate flow of molecules (transport channels), or produce motion using chemical energy (motor proteins); deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) molecules that store genetic information; vesicles that mediate the transport of protein molecules within cells; and viruses that infect biological cells to replicate. Bio-nanomachines in this book also extend to cellular organelles that provide specific functions within cells and even whole cells that are built from billions of bio-nanomachines capable of interacting with a wide variety of molecules in the environment.

A single bio-nanomachine can be viewed as a functional unit that interacts with molecular signals [1,2]. A bio-nanomachine may respond to input signals by transmitting output signals, changing its internal state, or modifying its functionality. For instance, an enzyme, a catalyst of chemical reactions, responds to specific substrate molecules by producing product molecules. A DNA molecule, a storage of genetic information, responds to molecular signals in the cell by changing its state by switching on and off particular genes.