Nils Paust

High-resolution permanent photoresist laminate TMMF for sealed microfluidic structures in biological applications

We demonstrate the use of photosensitive epoxy laminate TMMF S2045 for the fabrication and sealing of tapered microfluidic channels. The 45 μm thick resist enables the fabrication of shallow sealed cavities featuring extreme aspect ratios of less than 1:40 (h = 45 μm, w = 2000 μm). It also provides high resolution and enables minimum feature sizes of 10 μm. For the fabrication of free-standing structures, an aspect ratio of up to 7:1 was achieved. The dry-film photoresist can be applied easily by lamination onto structured substrates. The total thickness variation of the resist across a 100 mm wafer was determined to be less than ±0.6 μm. Process parameters for the fabrication and sealing of various micro-channels are discussed and optimized in this paper. The main focus was to minimize thermal impact during lamination, soft-bake, exposure and post–exposure bake, which could lead to lid sagging or channel clogging due to liquefaction of uncured resist. We tested TMMF according to ISO 10995-5 and found it to be non-cytotoxic, enabling its use for biological applications. Swelling of less than 5% for incubation of the dry-film resist in several biologically relevant solvents, buffers and cleaning solutions was observed. (Some figures in this article are in colour only in the electronic version)

Tomography based screening of flow field / current collector combinations for PEM water electrolysis

Current collectors in PEM water electrolysis perform several functions that influence overall performance. These include conducting heat and electrons, as well as transporting water and gas. X-ray tomography, standardized reconstruction, and calculation methods are used to compare the morphological and transport parameters of eight different current collectors functioning as flow fields for PEM water electrolysis. We find simple exponential relations between through-plane thermal conductivity λ and porosity p (λ20°C(p) = 1749p−1.306 − 4.420), as well as between through-plane electrical conductivity σ and water permeability K (σ(K) = 10−6 × K−0.6376). In addition, we use both local and global concepts to investigate the pore space of current collectors. We hereby investigate homogeneity and characteristic sizes, like mean pore diameter or mean distance between solid parts at the catalytic interface. Moreover, we find that the local concept of mean chord lengths can be used to explain electrical and thermal conductivity anisotropies. These chords can be used to predict the direction of the largest conductivity for fibrous current collectors.

Passive water management for µfuel-cells using capillary microstructures

In this work we present a novel system for the passive water management in polymer electrolyte fuel cells (PEMFC) based on capillary effects in microstructures. The system removes abundant water that occurs at low temperatures at a fuel cell cathode and secures the humidity of the electrolyte membrane on higher temperatures. Liquid water is removed by hydrophilic gas supply channels with a tapered cross section as presented previously, and further transported by a system of capillary channels and a layer of nonwoven material. To prevent the membrane from running dry, a storage area in the nonwoven layer is introduced, controlled by a novel passive capillary overflow valve. The valve controls whether water is stored or finally disposed by gravity and evaporation. Experiments in a model system show that the nonwoven material is capable of removing all liquid water that can be produced by the fuel cell. A miniaturized fuel cell utilizing the novel water removal system was fabricated and experiments show that the system can stabilize the performance during changes of electrical load. Clearing the drowned miniaturized fuel cell flow field was proven and required 2 min. To make the capillary effects available for the originally hydrophobic graphite composite materials that were used to fabricate the flow fields, hydrophilic grafting based on photochemistry was applied to the material and contact angles of about 40° could be achieved and preserved for at least three months.

Centrifugo-thermopneumatic fluid control for valving and aliquoting applied to multiplex real-time PCR on off-the-shelf centrifugal thermocycler

We introduce microfluidic automation of geometrically multiplexed real-time PCR to off-the-shelf Rotor-Gene Q thermocyclers (RGQ, QIAGEN GmbH, Hilden, Germany). For centrifugal fluid control the RGQ provides low and constant rotation of 400 rpm, only. Compatibility to this very limited flexibility of centrifugal actuation is achieved by using thermal gas compression and expansion for valving and aliquoting. In contrast to existing thermo-pneumatic actuation, centrifugo-thermopneumatic (CTP) fluid control employs the induced change of partial vapor pressure by global temperature control as actuation parameter for two new unit operations: CTP siphon valving and CTP two-stage aliquoting. CTP siphon valving was demonstrated to reliably transfer sample liquid in all cases (n = 35) and CTP two-step aliquoting transfers metered aliquots of 18.2 ± 1.2 μl (CV 6.7%, n = 8) into reaction cavities within 5 s (n = 24). Thermal characteristics of CTP two-stage aliquoting were found to be in good agreement with an introduced analytical model (R2 = 0.9876, n = 3). A microfluidic disk segment comprising both new unit operations was used for automation of real-time PCR amplification of Escherichia coli DNA. Required primers and probes were pre-stored in the reaction cavities and a comparison with reference reactions in conventional PCR tubes yielded the same PCR efficiency, repeatability, and reproducibility.

Centrifugal Step Emulsification can Produce Water in Oil Emulsions with Extremely High Internal Volume Fractions

The high throughput preparation of emulsions with high internal volume fractions is important for many different applications, e.g., drug delivery. However, most emulsification techniques reach only low internal volume fractions and need stable flow rates that are often difficult to control. Here, we present a centrifugal high throughput step emulsification disk for the fast and easy production of emulsions with high internal volume fractions above 95%. The disk produces droplets at generation rates of up to 3700 droplets/s and, for the first time, enables the generation of emulsions with internal volume fractions of >97%. The coefficient of variation between droplet sizes is very good (4%). We apply our system to show the in situ generation of gel emulsion. In the future, the recently introduced unit operation of centrifugal step emulsification may be used for the high throughput production of droplets as reaction compartments for clinical diagnostics or as starting material for micromaterial synthesis.

Design of a Passive and Portable DMFC Operating in All Orientations

A microfluidic layout concept for passive and portable Direct Methanol fuel Cells (DMFCs) is presented. We proofed this concept by developing a DMFC that continuously runs for 40 hours in all orientations without the need for any active components such as pumps or valves. In contrast to our previous work [1], the system now is truly portable. In any orientation of the DMFC, a bubble driven self regulating supply mechanism safely removes carbon dioxide and transports at least 3.5 times more methanol to the anode than critically needed to sustain DMFC operation. On the cathode, diffusive oxygen supply and the transport of the reaction product water along a capillary gradient out of the DMFC ensures a stable performance. Compared to our previous work [1], the power output was increased by a factor of 2.5 and reached p = 5.5 mW cm-2. A stable power output for 40 hours of p = 4 mW cm-2 was achieved for the preferred vertical position with bubbles moving against buoyancy forces. In the most challenging horizontal position with the anode facing downwards, a power output of at least p = 3.1 mW cm-2 was reached for the same period of time.

Digital droplet LAMP as a microfluidic app on standard laboratory devices

Digital nucleic acid amplification methods are a growing research field that allows for absolute quantification of DNA making the need of standard curves redundant. However, most of the existing digital amplification systems require specialized laboratory devices and costly investments. The required disposable cartridges are device specific and not interchangeable. Here, we present digital droplet loop-mediated isothermal amplification (ddLAMP) as a microfluidic app on standard laboratory devices. ddLAMP is implemented on a disposable polymer chip (DropChip) in the format of a standard microscope slide. After off-chip DNA denaturation, the reaction mix is emulsified in the DropChip in a mini centrifuge for 6 minutes. The DropChip is transferred to an in situ thermal cycler for 1 hour of incubation. Afterwards, a fluorescence scan in a microarray scanner is performed. The DropChip allows for absolute quantification with a dynamic range of 15–1500 DNA copies per μl. Assay conditions were optimized for ddLAMP and comparison of ddPCR and ddLAMP for genomic E. coli DNA reveals very good concordance.

Fully passive degassing and fuel supply in direct methanol fuel cells

In this paper a micro direct methanol fuel cell (muDMFC) is presented, which is operated in a completely passive way, i.e. the cell does not require an external pump for fuel supply. The surface energy of deformed CO2 bubbles, generated as a reaction product during DMFC operation, is employed to supply methanol to the anode. In contrast to a digital valve based approach presented earlier by Meng et. al. [1], a tapered channel is applied to achieve a pumping mechanism. This way the pump rates can be adapted to the requirements of a specific cell. The presented study reveals that this concept is able to maintain the supply for all typical DMFC operation conditions. Experimental results are presented that demonstrate the continuous operation of a passive muDMFC for more than 15 hours.

Micro structured flow field for passive water management in miniaturized PEM fuel cells

In this work a novel flow field for the passive water management in proton exchange membrane fuel cells (PEMFC) is described. A triangular micro channel forces condensing water droplets to detach from the gas diffusion layer (GDL) in order to ensure proper oxygen supply. Water droplets are lifted into a secondary channel, and transported out of the fuel cell by capillary forces. Different droplet shapes inside the channels are identified. Preferred shapes cover the GDL only slightly and can be attained for contact angles typical for fuel cell materials. The new channel design was compared in a test fuel cell to standard square channels in particular in the starting phase of the cell at low working temperatures (22degC). The new channel design keeps the cell at 95% of its initial performance compared to 60% when using the standard design.

Numerical investigations on electric field characteristics with respect to capacitive detection of free-flying droplets.

In this paper a multi-disciplinary simulation of a capacitive droplet sensor based on an open plate capacitor as transducing element is presented. The numerical simulations are based on the finite volume method (FVM), including calculations of an electric field which changes according to the presence of a liquid droplet. The volume of fluid (VOF) method is applied for the simulation of the ejection process of a liquid droplet out of a dispenser nozzle. The simulations were realised using the computational fluid dynamic (CFD) software CFD ACE+. The investigated capacitive sensing principle enables to determine the volume of a micro droplet passing the sensor capacitor due to the induced change in capacity. It could be found that single droplets in the considered volume range of 5 nL < Vdrop < 100 nL lead to a linear change of the capacity up to ΔQ < 30 fC. The sensitivity of the focused capacitor geometry was evaluated to be Si = 0.3 fC/nL. The simulation results are validated by experiments which exhibit good agreement.