Activation functions hard-limiting

The feasibility and limitations of molecular machines can hardly be emphasized any better than by Feynman s mixed message [1], namely that An internal combustion engine of molecular scale is impossible. Other chemical reactions, liberating energy when cold, can be used instead. Nanoscale machines, like their macroscopic counterparts, require power supplies of appropriate kinds and magnitudes for their functions. While macroscopic machines enjoy the simplicity of distinct active (ON) and inactive (OFF) states in the presence and absence of power supplies, respectively, molecular machines are in perpetual Brow-... 

An example of such a situation was considered at the end of the preceding chapter the system with two oscillatory isozymes (fig. 3.23) contains two instability mechanisms coupled in parallel. Compared with the model based on a single product-activated enzyme, new behavioural modes may be observed, such as birhythmicity, hard excitation and multiple oscillatory domains as a function of a control parameter. The modes of dynamic behaviour in that model remain, however, limited, because it contains only two variables. For complex oscillations such as bursting or chaos to occur, it is necessary that the system contain at least three variables. 

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