A novel surface treatment method using poly(ethyleneimine) (PEI), an amine-bearing polymer, was developed to enhance antibody binding on the poly(methyl methacrylate) (PMMA) microfluidic immunoassay device. By treating the PMMA surface of the microchannel on the microfluidic device with PEI, 10 times more active antibodies can be bound to the microchannel surface as compared to those without treatment or treated with the small amine-bearing molecule, hexamethylenediamine (HMD). Consequently, PEI surface modification greatly improved the immunoassay performance of the microfluidic device, making it more sensitive and reliable in the detection of IgG. The improvement can be attributed to the spacer effect as well as the functional amine groups provided by the polymeric PEI molecules. Due to the smaller dimensions (140x125 microm) of the microchannel, the time required for antibody diffusion and adsorption onto the microchannel surface was reduced to only several minutes, which was 10 times faster than the similar process carried out in 96-well plates. The microchip also had a wider detection dynamic range, from 5 to 1000 ng/mL, as compared to that of the microtiter plate (from 2 to 100 ng/mL). With the PEI surface modification, PMMA-based microchips can be effectively used for enzyme linked immunosorbent assays (ELISA) with a similar detection limit, but much less reagent consumption and shorter assay time as compared to the conventional 96-well plate.

The dynamic shear viscosity and the transient extensional viscosity of polycarbonate (PC), polymethyl methacrylate (PMMA), and polyvinyl blutyral (PVB) were measured at temperatures near and far above their glass transition temperatures. The temperature sensitivity of rheological properties was used to explain the displacement curves during embossing. Numerical simulation of the embossing process was also canied out to compare with the observed polymer flow patterns. It was found that the simulated flow pattern during isothermal embossing agrees fairly well with the experimental observation. The deviation between the simulated and experimental results at the late stage of embossing may be due to air entrapment between the mold feature and the polymer substrate. For non-isothermal embossing, the observed flow pattern can also be reasonably simulated. i.e. the polymer flows upward along the wall of the heated mold f a t u r e , and then compresses downward and squeezes outward. Temperature sensitivity of the dynamic shear viscosity and the transient extensional viscosity is similar for all three polymers. This correlates well with the initial displacement curves in isothermal embossing. Over a longer time, the strain hardening effect of the transient extensional viscoisity seems to play a major role in the displacement curves.

The relationship among processing conditions, material properties, and part quality in hot embossing was investigated for three optical polymers: pdycarbonate (PC), polymethyl methacrylate (PMMA), and polyvhyl butyral EVE%). A series of systematic embossing experiments was conducted using mold inserts having either smgle or multiple feature depths. The feature dimensions varied fi-om 90 to 3000 km. The processing conditions studied include embossing pressure, thermal cycles, and heating methods. The displacement profile, replication accuracy and molded-in stresses were measured experimentally. It was found that for isothermal embossing. both replication accuracy and birefringence pattern depend strongly on the processing conditions. For non-isothermal embossing, the molded parts showed excellent replication as long as the feature transfer was completed. The flow pattern under isothermal embossing resembles a b i d extensional flow. Under non-isothermal embossing, the polymer deformation involves an upward flow along the wall of mold features, followed by downward compression and outward squr-. Rheological characterization and hot embossing analysis are presented in Part II.

An electrokinetics-induced stagnation flow was created inside a microscale cross-channel. Compared to hydrodynamic-induced microfluidics, this flow system can be readily assembled and the operation is very simple due to a low pressure drop. Through image analysis, a fairly homogeneous, two-dimensional elongational flow was observed. The initial conformation of DNA molecules and residence time inside the flow field play important roles in determining the extent of DNA stretching. A coarse-grain molecular simulation agrees reasonably well with experimental observations.

In this study, a continuous flow dielectrophoresis (DEP) microfluidic chip was fabricated and utilized to sort out the microalgae (C. vulgaris) with different lipid contents. The proposed separation scheme is to allow that the microalgae with different lipid contents experience different negative or no DEP force at the separation electrode pair under the pressure-driven flow. The microalgae that experience stronger negative DEP will be directed to the side channel while those experience less negative or no DEP force will pass through the separation electrode pair to remain in the main channel. It was found that the higher the lipid content inside the microalgae, the higher the crossover frequency. Separation of the microalgae with 13% and 21% lipid contents, and 24% and 30%-35% lipid contents was achieved at the operating frequency 7 MHz, and 10 MHz, respectively. Moreover, separation can be further verified by measurement of the fluorescence intensity of the neutral lipid inside the sorted algal cells. V C 2014 AIP Publishing LLC.