On Producing Linear Dilution Gradient of a Sample with a Digital Microfluidic Biochip

Digital micro fluidic (DMF) biochips are now being extensively used to automate several biochemical laboratory protocols such as clinical analysis, point-of-care diagnostics, and polymerase chain reaction (PCR). In many biological assays, e.g., in bacterial susceptibility tests, samples and reagents are required in multiple concentration (or dilution) factors, satisfying certain "gradient" patterns such as linear, exponential, or parabolic. Dilution gradients are usually prepared with continuous-flowmicrofluidic devices, however, they suffer from inflexibility, non-programmability, and from large requirement of costly stock solutions. DMF biochips, on the other hand, are shown to produce, more efficiently, a set of random dilution factors. However, all existing algorithms fail to optimize cost or performance when a certain gradient pattern is required. In this work, we present an algorithm to generate any arbitrary linear gradient, on-chip, with minimum wastage, while satisfying a required accuracy in concentration factors. We present new theoretical results on the number of mix-split operations and waste computation, and prove an upper bound on storage requirement. The corresponding layout design of the biochip is also proposed. Simulation results on different linear gradients show a significant improvement in sample cost over three earlier algorithms used for the generation of multiple concentrations.