Back-end Processing in Lab-on-a-Chip Fabrication (2011)

This thesis presents back-end processing steps for polymer labs-on-a-chip which permit the realization of complex applications like biological assays on chip.<br>Pre-treatment for surface cleaning, hydrophilization to promote capillary action, selective hydrophobization for flow control, dry reagent pre-storage to enable fully integrated chips as well as biocompatible sealing to ensure operation and biological activity after processing are covered. Most of these processing steps are developed on a sample lab-on-a-chip which aims for the on-chip amplification of mRNA by nucleic acid sequence based amplification (NASBA).<br>In detail, pre-treatment of chips with hydrogen peroxide is introduced which effectively sterilizes polymer surfaces. As hydrophilic coating, PEtOx-BP, PDMAA-BP and PEG are evaluated in respect to hydrophilicity. All coatings render polymer surfaces strongly hydrophilic (water contact angle < 40°). It is further determined that only PEG exhibits full NASBA compatibility.<br>A new method for the selective surface patterning of microfluidic chips with hydrophobic fluoropolymers is developed and demonstrated by the fabrication of hydrophobic valves via dispensing. It enables efficient optical quality control for the surface patterning due to incorporated dyes thus permitting low-cost production of highly reliable hydrophobic valves. Specifically, different dyes for fluoropolymers are investigated and two fluoropolymer-solvent- dye solutions based on fluorescent quantum dots (QD) and carbon black (CB) are presented in detail. The latter creates superhydrophobic surfaces (typical layer thickness: 2 µm) on arbitrary substrates, e.g. chips made from cyclic olefin copolymer (COC, ~ 157.9°), provides very good visibility for the visual quality control in polymer labs-on-a-chip and increases burst pressures of hydrophobic valves by up to 35 %. On the sample lab-on-a-chip, the quality control in combination with Teflon-CB as coating reduces the burst pressure variability from 14.5 % down to 6.1 % compared with Teflon-coated valves.<br>Two approaches for the pre-storage of bioreagents, i.e. spotting and drying as well as spotting and freeze-drying are presented and evaluated. Multiple 25 nL droplets of reagents are dispensed into sample reaction chambers while preventing any contact with the adjacent walls for the spotting and drying. The concept is validated for primers and probes by successfully conducting NASBA for mRNA of three different human papilloma virus (HPV) types, even after 2.5 months of storage. For spotting and freeze-drying, reagents are dispensed on frozen substrates whereas multiple droplets form small reagent pillars. These pillars are successfully freeze-dried on-chip using a custom-developed protocol. Rehydration and positive NASBA amplification of susceptible enzymes is demonstrated after three weeks.<br>Biocompatible sealing is realized via temperature diffusion bonding and adhesive bonding. The former process is optimized by the introduction of a transparent compound foil which allows for optical read-out techniques. With the adapted process, strongly bonded test chips are produced (delamination pressure > 3 bar). A potentially negative temperature impact of the sealing process on pre-stored reagents is further excluded for a minimum distance of 180 µm between interface and reagents. Finally, it is demonstrated that PEG<br>as hydrophilic coating can interfere with temperature diffusion bonding as residual PEG on the chip surface promotes delamination. For adhesive bonding as versatile approach for the sealing of polymer microfluidic chips, the influence of process parameters is investigated. Specifically, a process chain comprising pre-processing, adhesive transfer as well as post-processing is presented and parameter recommendations are provided. As device for adhesive transfer, a modified laminator is utilized which transfers thin layers of adhesive onto only the chip surface via a silicone roll. Using this device and a high temperature compatible (Tg > 100°C) epoxy adhesive, adhesive layers in the range of 2 - 4 µm can be reproducibly transferred (CV < 4 %). For best bonding results, it is recommended to provide 2.5 µm thin layers of adhesive in combination with a subsequent evacuation step at 10 mbar for 3 hours.<br>Further, it is proposed to integrate capture channels near large, featureless areas to compensate for variations in processing and thus prevent clogging of channels. With these recommendations, strongly bonded test chips (delamination pressure > 4 bar) can be reliably produced (yield > 80 %).<br>Additionally, the process is inherently compatible with Vistex as hydrophilic coating due to similar coupling chemistry.