Finger-powered Digital Microfluidics for Micro Droplet Manipulation
Microfluidic devices that do not require bulky peripheral hardware, such as pumps and external battery/power supplies, are a suitable technology for portable applications in resource-constrained settings, such as point-of-care (POC) diagnosis in developed countries, environmental monitoring, and on-site forensic analysis, etc. The existing portable microfluidic devices are mostly based on microchannel structures, in which the pre-defined channels limit their functional flexibility, rendering them difficult to scale up. Digital microfluidics, on the other hand, can tackle this problem since they deal with discrete droplets individually and can therefore provide more on-demand flexibility and versatility. Most digital microfluidic devices, however, require external electric power sources.
We first propose finger-powered digital microfluidic (F-DMF) based on electrowetting on dielectric (EWOD). Instead of requiring an external power supply, our F-DMF uses piezoelectric elements to convert the mechanical energy produced by human fingers into electric voltage pulses for droplet manipulation. The voltage outputs of piezoelectric element mounted in cantilever beam configuration are studied theoretically and experimentally. Using this energy conversion scheme, the basic modes of droplet operations, such as droplet transport, splitting, and merging on EWOD devices are confirmed. The key assay steps involved in glucose detection and immunoassay are also successfully performed using F-DMF-EWOD.
Exploiting the same energy conversion scheme, F-DMF based on the electrophoretic transport of discrete droplets (EPD), which has the potential to overcome pinning and surface contamination often encountered in EWOD, is then presented. Successful EPD actuation, however, requires the piezoelectric elements to provide both sufficient charge and voltage pulse duration. These requirements are quantified using numerical models to predict the electrical charges induced on the droplets and the subsequent electrophoretic forces. The transport and merging of aqueous droplets as well as direct manipulation of body fluids is experimentally demonstrated using F-EPD-DMF. Further, a mechanical system and an efficient pin-assignment scheme are explored to facilitate the practical implementation of pre-programmed and functional actuation of droplets in the EPD-based system.
For the second part of this thesis, one practical issue in digital microfluidics biochip (DMFB) design is discussed: the droplet routing problem, which largely decides the performance and correctness of the system. The problem is formulated to a multi-agent path finding problem (MAPF) and an approximate algorithm based on Independent Detection (ID) is applied to solve the problem. The modified ID algorithm shows promising performance on selected benchmark problems with medium number of droplets (≤12). Overall, it achieves better timing result (~15% reduction) and total routing length (~50% reduction) with no compromise in fault tolerance (indicated by the total number of used cells), when compared with the previous best known results.