This study presents a straightforward two-step fabrication process of durable, completely superhydrophobic microchannels in PDMS. First, a composite material of PDMS/PTFE particles is prepared and used to replicate a master microstructure. Superhydrophobic surfaces are formed by subsequent plasma treatment, in which the PDMS is isotropically etched and PTFE particles are excavated. We compare the advancing and receding contact angles of intrinsic PDMS samples and composite PTFE/PDMS samples (1 wt %, 8 wt %, and 15 wt % PTFE particle concentration) and demonstrate that both the horizontal and vertical surfaces are indeed superhydrophobic. The best superhydrophobicity is observed for samples with a PTFE particle concentration of 15 wt %, which have advancing and receding contact angles of 159° ± 4° and 158° ± 3°, respectively.

Gel electrophoresis is one of the most applied and standardized tools for separation and analysis of macromolecules and their fragments in academic research and in industry. In this work we present a novel approach for conducting on-demand electrophoretic separations of DNA molecules in open microfluidic (OM) systems on planar polymer substrates. The approach combines advantages of slab gel, capillary- and chip-based methods offering low consumable costs (<0.1$) circumventing cost-intensive microfluidic chip fabrication, short process times (5 min per analysis) and high sensitivity (4 ng/μL dsDNA) combined with reasonable resolution (17 bases). The open microfluidic separation system comprises two opposing reservoirs of 2-4 μL in volume, a semi-contact written gel line acting as separation channel interconnecting the reservoirs and sample injected into the line via non-contact droplet dispensing and thus enabling the precise control of the injection plug and sample concentration. Evaporation is prevented by covering aqueous structures with PCR-grade mineral oil while maintaining surface temperature at 15°C. The liquid gel line exhibits a semi-circular cross section of adaptable width (∼200-600 μm) and height (∼30-80 μm) as well as a typical length of 15-55 mm. Layout of such liquid structures is adaptable on-demand not requiring time consuming and repetitive fabrication steps. The approach was successfully demonstrated by the separation of a standard label-free DNA ladder (100-1000 bp) at 100 V/cm via in-line staining and laser induced fluorescent end-point detection using an automated prototype.

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We present a novel measurement method based on the gravimetric principles adapted from the ASTM E542 and ISO 4787 standards for quantitative volume determination in the sub-microliter range. Such a method is particularly important for the calibration of non-contact micro dispensers as well as other microfluidic devices. The novel method is based on the linear regression analysis of continuously monitored gravimetric results and therefore is referred to as ‘gravimetric regression method (GRM)’. In this context, the regression analysis is necessary to compensate the mass loss due to evaporation that is significant for very small dispensing volumes. A full assessment of the measurement uncertainty of GRM is presented and results in a standard measurement uncertainty around 6 nl for dosage volumes in the range from 40 nl to 1 µl. The GRM has been experimentally benchmarked with a dual-dye ratiometric photometric method (Artel Inc., Westbrook, ME, USA), which can provide traceability of measurement to the International System of Units (SI) through reference standards maintained by NIST. Good precision (max. CV = 2.8%) and consistency (bias around 7 nl in the volume range from 40 to 400 nl) have been observed comparing the two methods. Based on the ASTM and ISO standards on the one hand and the benchmark with the photometric method on the other hand, two different approaches for establishing traceability for the GRM are discussed.

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Electrowetting is used to assist the delivery of droplets by a contact method. The electroassisted liquid dispensing technique enables to monitor the drop size via the voltage applied between the tool, i.e., silicon cantilevers and the deposition surface. Voltages ranging from 0to210V are used to deposit water-glycerol drops with diameters and volumes in the ranges of 5–40μm and 20fL–14pL. The presented results demonstrate that electrowetting-assisted deposition is of special interest for patterning applications requiring large features to be directly and quickly written using a minimum volume of reagents.

We present a disposable, normally closed, non-contact dispensing valve for the sub-µL range. The miniaturized solenoid valve (diameter: 8 mm, height: 27.25 mm) is compatible to standard Luer-Lock interfaces. A highly dynamic actuation principle enables opening times down to 1 ms. The dispensing performance was evaluated for water (η = 1.03 mPas) and a 66% (w/w) glycerol/water solution (η = 16.98 mPas), at pressures varying from 200 to 800 mbar. The experimentally determined minimal dispensing volume was 163 nL (CV 1.6%) for water and 123 nL (CV 4.5%) for 66% (w/w) glycerol/water. The low-cost polymer valve enables high precision dispensing of liquid volumes down to the lower end of the sub-µL range comparable to high-end non-disposable micro-dispensing valves.

Microfluidic systems fabricated in polydimethylsiloxane (PDMS) enable a broad variety of applications and are widespread in the field of Lab-on-a-Chip. Here we demonstrate semi-contact-writing, a novel method for fabrication of polymer based molds for casting microfluidic PDMS chips in a highly flexible, time and cost-efficient manner. The method is related to direct-writing of an aqueous polymer solution on a planar glass substrate and substitutes conventional, time- and cost-consuming UV-lithography. This technique facilitates on-demand prototyping in a low-cost manner and is therefore ideally suited for rapid chip layout iterations. No cleanroom facilities and less expertise are required. Fabrication time from scratch to ready-to-use PDMS-chip is less than 5 h. This polymer writing method enables structure widths down to 140 μm and controllable structure heights ranging from 5.5 μm for writing single layers up to 98 μm by stacking. As a unique property, freely selectable height variations across a substrate can be achieved by application of local stacking. Furthermore, the molds exhibit low surface roughness (Ra  =  24 nm, RRMS  =  28 nm) and high fidelity edge sharpness. We validated the method by fabrication of molds to cast PDMS chips for droplet based flow-through PCR with single-cell sensitivity.