Fabrication of mixed-scale PMMA (Polymethyl methacrylate) fluidic device via thermal nanoimprint using a convex carbon mold

Abstract

Recently micro-/nanofluidic devices are widely used for various research areas including biological, chemical, and biomedical applications. Such mixed-scale micro-/nanofluidic devices are generally fabricated using photolithography and direct writing methods (e. g., e-beam lithography or focused ion beam milling) in series. However, the direct writing methods require high cost and long process time thus resulting in low throughput issue. PDMS (Polydimethylsiloxane) replication can overcome the low throughput issues. The PDMS replication method consists of a PDMS casting process on a pre-patterned mold and a subsequent curing processes. By this method, PDMS mixed-scale channel patterns can be replicated repeatedly, thus, total throughput of fabricated mixed-scale PDMS fluidic device is enhanced. However, the channel size is smaller, the more PDMS channels are collapsed due to the low Young’s modulus and hardness of PDMS. In this study, I developed the fabrication method of mixed-scale PMMA (Poly methyl methacrylate) fluidic device via simple thermal nanoimprint using a monolithic mixed-scale convex carbon mold (microchannel mold: width = ~ 50 m, height = ~ 5 m; nanochannel mold: width = ~ 600 nm, height = ~ 60 nm). The monolithic carbon mold was fabricated using carbon-MEMS consisting of two step photolithography processes and one step pyrolysis. In pyrolysis, polymer structures shrank dramatically and thus microscale photoresist structures were converted into sub-micro- or nanoscale carbon structures. In nanoimprint process, the shape of the monolithic mixed-scale convex carbon mold was transferred into a PMMA sheet while the polymer sheet was heated. After demolding the carbon mold from the patterned PMMA sheet, the patterns were accurately transferred on the PMMA sheet (microchannel: width = ~ 50 m, height = ~ 5 m; nanochannel: width = ~ 600 nm, height = ~ 60 nm). The pyrolyzed carbon mold could be easily demolded because of its curved side walls resulting from anisotropic volume reduction in pyrolysis. This special side wall geometry and good robustness of the carbon mold ensured reproducibility in nanoimprint. The mixed-scale channels were sealed by another thin PMMA sheet with low pressure and heat after oxygen plasma treatment. PMMA has higher Young’s modulus compared to PDMS (polydimethylsiloxane) so that the PMMA channels ensured consistent nanochannel fabrication and operation without channel collapse. The PMMA mixed-scale fluidic device was used to entrap single particles via diffusiophoresis. In the fluidic device, microchannels and nanochannels were smoothly connected via Kingfisher-beak-shaped 3D microfunnels that were converted from polymer triangular prims via pyrolysis. By filling two microchannels that are connected via multiple nanochannels with high concentration solution and low concentration solution respectively and controlling pressure difference between two microchannels, local concentration gradients can occur near the 3D microfunnels at the microchannel with low concentration. The localized concentration gradients generate local electric fields resulting in diffusiophoresis; the motion of charged particles along the localized electric fields. In this experiment, 1 μm-diameter charged single particles dispersed in the low concentrate solution were dragged from the microchannel into the 3D microfunnels via diffusiophoresis. Consequently, the unique 3D microfunnel worked as a chamber where single particle was entrapped; thus, single particles could be entrapped without external electric force in 3D microfunnels. The diffusiophoresis-based single particle entrapment experiment showed the potential of the mixed-scale channel networks as a single cell research tool.