Maximilian Focke

Microthermoforming of microfluidic substrates by soft lithography (µTSL): optimization using design of experiments

We present a detailed analysis of microthermoforming by soft lithography (μTSL) for replication of foil-based microfluidic substrates. The process was systematically optimized by design of experiments (DOE) enabling fabrication of defect-free lab-on-a-chip devices. After the assessment of typical error patterns we optimized the process toward the minimum deviation between mold and thermoformed foil substrates. The following process parameters have most significant impact on the dimensional responses (p 40% relative impact. The DOE results in an empirical process model with a maximum deviation between the prediction and experimental proof of 2% for the optimum parameter set. Finally, process optimization is validated by the fabrication and testing of a microfluidic structure for blood plasma separation from human whole blood. The optimized process enabled metering of a nominal volume of 4.0 μl of blood plasma with an accuracy deviation of 3% and a metering precision of ±7.0%. The μTSL process takes about 30 min and easily enables the replication of 300 μm wide microchannels having vertical sidewalls without any draft angles in a well-controllable way. It proves to be suitable for multiple applications in the field of microfluidic devices. S Online supplementary data available from stacks.iop.org/JMM/21/115002/mmedia (Some figures in this article are in colour only in the electronic version)

Microfluidic cartridges for DNA purification and genotyping processed in standard laboratory instruments

Two microfluidic cartridges intended for upgrading standard laboratory instruments with automated liquid handling capability by use of centrifugal forces are presented. The first microfluidic cartridge enables purification of DNA from human whole blood and is operated in a standard laboratory centrifuge. The second microfluidic catridge enables genotyping of pathogens by geometrically multiplexed real-time PCR. It is operated in a slightly modified off-the-shelf thermal cycler. Both solutions aim at smart and cost-efficient ways to automate work flows in laboratories. The DNA purification cartridge automates all liquid handling steps starting from a lysed blood sample to PCR ready DNA. The cartridge contains two manually crushable glass ampoules with liquid reagents. The DNA yield extracted from a 32 μl blood sample is 192 ± 30 ng which corresponds to 53 ± 8% of a reference extraction. The genotyping cartridge is applied to analyse isolates of the multi-resistant Staphyloccus aureus (MRSA) by real-time PCR. The wells contain pre-stored dry reagents such as primers and probes. Evaluation of the system with 44 genotyping assays showed a 100% specificity and agreement with the reference assays in standard tubes. The lower limit of detection was well below 10 copies of DNA per reaction.

Microthermoforming and Sealing of COP Films to Form Thin Walled Lab-On-A-Chip Cartridges

This paper describes a process chain to form and seal thermoplastic films to obtain enclosed microfluidic cartridges with thin walls. Especially applications requiring fast thermocycling such as polymerase chain reaction (PCR) highly benefit from this lab-on-a-foil approach, due to an improved heat transfer. The cartridge fabrication is a two-step process comprising microthermoforming by soft lithography followed by a novel gas pressure assisted thermal sealing. As film material a 188 μm thick cyclic olefin polymer (COP) is used, which is microthermoformed over a male Polydimethylsiloxane (PDMS) mold to shape the intended thin walled microstructures. The process parameters have been optimized and feature sizes in the range of a few micrometers to millimeters with aspect ratios up to three are replicated. The forming of the thermoplastic film is caused by heating the film above glass transition temperature (Tg) and applying a gas pressure of 310 kPa. After cooling and demolding the COP structures are thermally sealed with a special coextruded COC film. The coextruded film consists of a temperature stable TOPAS COC 6013 (Tg ~ 135 °C) layer and a thin layer of TOPAS COC 8007 (Tg ~ 79 °C) acting like hot melt. An additional mold for bond support, which is commonly needed for positive thermoformed structures, has been eliminated by a gas pressure assisted bonding approach. The processes enable fast and flexible prototyping of thin walled microfluidic cartridges within 8 – 10 hours.