  We report the first realization of a biomolecular AND gate function with
double-sigmoid response (sigmoid in both inputs). Two enzyme biomarker inputs
activate the gate output signal which can then be used as indicating liver
injury, but only when both of these inputs have elevated pathophysiological
concentrations, effectively corresponding to logic-1 of the binary gate
functioning. At lower, normal physiological concentrations, defined as logic-0
inputs, the liver-injury output levels are not obtained. High-quality gate
functioning in handling of various sources of noise, on time scales of
relevance to potential applications is enabled by utilizing "filtering"
effected by a simple added biocatalytic process. The resulting gate response is
sigmoid in both inputs when proper system parameters are chosen, and the gate
properties are theoretically analyzed within a model devised to evaluate its
noise-handling properties.
