Browsing by Subject "Droplet"

Microfluidic biochips are now being used for biochemical applications such as high-throughput DNA sequencing, point-of-care clinical diagnostics, and immunoassays. In particular, digital microfluidic biochips (DMFBs) are especially promising. They manipulate liquid as discrete droplets of nanoliter or picoliter volumes based on the principle of electrowetting-on-dielectric under voltage-based electrode actuation. DMFBs have been commercially adopted for sample preparation and clinical diagnostics. Techniques have also been developed for high-level synthesis, module placement, and droplet routing.

However, reliability is a major concern in the use of DMFBs for laboratory protocols. In addition to manufacturing defects and imperfections, faults can also arise during a bioassay. For example, excessive or prolonged actuation voltage may lead to electrode breakdown and charge trapping, and DNA fouling may lead to the malfunction of electrodes. Faults may eventually result in errors in droplet operations. If an unexpected error appears during an experiment, the outcome of the experiment will be incorrect. The repetition of an experiment leads to wastage of valuable reagents and time.

Therefore, it is necessary to ensure the correctness of the hardware and bioassay execution on the biochip. In this thesis, we focus on three types of reliability: biochip testing, error/fault recovery, and fault-tolerant synthesis. First, when a biochip is fabricated, defects might occur in parts of the biochip. Therefore, our objective is to develop biochip testing methods to detect and locate faults. Second, to faults that appear during droplet operation or in the hardware, we develop error-recovery procedures and redundancy solutions. Finally, we develop fault-tolerant synthesis techniques so that even if faults occur during droplet operations (e.g., unbalance splitting), the bioassay can proceed unimpeded. The proposed solutions are applied to two new types of biochip platforms, namely micro-electrode-dot-array (MEDA) and digital acoustofluidics.

This thesis addresses mass transfer in multi-phase single particle systems. By using a novel technique based upon the micropipette, the dissolution of liquid and gas droplets in a liquid medium can be observed. Three classes of experimental systems are observed: pure liquid droplet dissolution in a pure liquid environment, miscible mixture liquid droplet dissolution in a pure liquid environment, and solute-containing liquid droplet dissolution in a pure liquid environment. Experiments on the dissolution of pure droplets of water in n-alcohols and n-alkanes showed that water droplets dissolved ten times faster in the alcohols as compared to in the alkanes. When solubility was taken into account, however, and diffusion coefficients calculated using the Epstein-Plesset equation, diffusion constants for alkanes were twenty five times higher in alkanes than for the corresponding alcohol (for example 12.5 vs 0.5 x 10-8 cm2/s for pentane and pentanol). This difference in rates of diffusion of the single molecules reflects the effect of hydrogen bonding on small solute diffusion, which is expounded upon in Chapter 2.

A model for the dissolution of a droplet containing a mixture, each component of which is soluble in the surrounding liquid medium is presented in Chapter 3. The model is based upon a reduced surface area approximation and the assumption of ideal homogenous mixing : Mass flux (dm_i)/dt=〖Afrac〗_i D_i (c_i-c_s){1/R+1/√(πD_i t)}, where Afraci is the area fraction of component i, ci and cs are the initial and saturation concentrations of the droplet material in the surrounding medium, respectively, R is the radius of the droplet, t is time, and Di is the coefficient of diffusion of component i in the surrounding medium. This model was tested for the dissolution of ethyl acetate and butyl acetate in water and the dissolution of butyl acetate and amyl acetate in water, and was found to provide a good fit. In Chapter 4, a partial differential equation, R^2/D ├ ∂c/∂t┤|_η=(∝η)/D ∂c/∂η+(∂^2 c)/〖∂η〗^2 +2/η ∂c/∂η, is presented for the dissolution of a solute containing droplet in a liquid medium, and shell or bead formation is predicted. In Chapter 5, an application of the solute containing droplet dissolution is presented in which suspensions of glassified protein microspehres are used to improve the injectability of protein based pharmaceuticals. Injectability is related to viscosity, and the viscosity of a suspension may be predicted to follow the Krieger Dougherty equation: (η(Φ))/η_0 =(1-Φ/Φ_m )^(-2.5Φ_m ) , where Φ is the volume fraction of the suspensate, η is the viscosity of the overall suspension, η0 is the viscosity of the suspending fluid, and Φm is the maximum possible volume fraction. Finally, in Chapter 6, various experimental methods used to generate droplets are addressed.

Electrowetting-on-dielectric (EWD) digital microfluidics is a droplet-based fluid handling technology capable of radically accelerating the pace of genome engineering research. EWD-based laboratory-on-chip (LoC) platforms demonstrate excellent performance in automating labor-intensive laboratory protocols at ever smaller scales. Until now, there has not been an effective means of gene transfer demonstrated in EWD microfluidic platforms. This thesis describes the theoretical and experimental approaches developed in the demonstration of an EWD-enabled electrotransfer device. Standard microfabrication methods were employed in the integration of electroporation (EP) and EWD device architectures. These devices enabled the droplet-based bulk transformation of E. coli with plasmid and oligo DNA. Peak on-chip transformation efficiencies for the EP/EWD device rivaled that of comparable benchtop protocols. Additionally, ultrasound induced in-droplet microstreaming was developed as a means of improving on-chip electroporation. The advent of electroporation in an EWD platform offers synthetic biologists a reconfigurable, programmable, and scalable fluid handling platform capable of automating next-generation genome engineering methods. This capability will drive the discovery and production of exotic biomaterials by providing the instrumentation necessary for rapidly generating ultra-rich genomic diversity at arbitrary volumetric scales.