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

Fabrication of mixed-scale PMMA (polymethyl methacrylate) liquid device via thermal nanoimprinting using a convex carbon mold. In the nanoimprinting process, the shape of the monolithic mixed-scale convex carbon shape was transferred into a PMMA sheet while the polymer sheet was heated.

Necessity of mixed-scale fluidic device

As the demand for mixed-scale fluidic device increases, easy and cost-effective mixed-scale channel fabrication methods are required. Next, we suggest the necessity of new fabrication methods to overcome the limitations observed in conventional fabrication methods.

Conventional fabrication methods of mixed-scale fluidic device

Direct channel engraving

In the direct channel engraving method, the micro- and nanochannel patterns are sequentially etched on the substrate. Then, the uncovered areas of the substrate are etched by subsequent etching process, such as wet etching or dry etching, finally microchannel patterns are carved on the substrate.

Figure 1.3 Mixed-scale channel were fabricated on the glass substrate via direct engraving channel fabrication method

Replication

Compared to PDMS replication, nanoimprinting uses relatively hard materials such as UV-curable polymer or thermoplastic for liquid devices. Then a convex monolithic mold is placed on the thermoplastic surfaces and pressed with high pressure to print concave mold structures. During the impression, quasi-liquid state is thermoplastically deformed while filling the mold cavities - concave parts of the mold, then the opposite morphology of the mold is duplicated on the thermoplastic.

Whereas, in UV nanoimprint, a liq. condition UV-curing polymer is spin-coated on the substrate. The mold is then placed on the polymer layer and pressed down to be filled with UV-curable polymer in mold cavities. Because the UV curable polymer has low viscosity, it needs relatively low pressure compared to thermal nanoimprint process.

To escape thermal and pressure damage, UV-curable polymer is used, because it is a material in liquid state, therefore, the imprinting pressure and heating temperature are lower than the thermal process of nanoprinting.

Novel fabrication method of mixed-scale fluidic device

Carbon-MEMS

After fabricating mixed-scale channels through the direct etching method, several replica molds can be replicated from the mixed-scale channel structure, which is called a master mold. In addition, to make a monolithic mixed-scale convex carbon mold, additional photoresist patterning process was added for microchannel mold structures. After patterning the small photoresist (width ~ 1.1 μm, height ~ 210 nm), the large photoresist pattern (width ~ 100 μm, height ~ 25 μm) was added, thus, the photoresist structure with two different formed a precursor monolithic polymer and after pyrolysis, a monolithic composite mold fabrication was completed.

Specific point of the mixed scale carbon. mold took advantage of anisotropic volume reduction of polymers in pyrolysis. As described in this chapter, it can be expected that the most critical problems of the conventional channel fabrication method can be overcome by the mixed-scale monolithic convex mold composed of mechanically and chemically strong glassy carbon fabricated using the carbon-MEMS technique. This mold meets critical requirements for cost-effective mixed-scale mold manufacturing via exclusively microfabrication methods and alleviates the difficult precise alignment of two channel patterns at different scales.

Therefore, in this paper, carbon shape is used as a mixed channel shape in thermal nanoimprint process, and the superior accessibility is demonstrated in the next chapter.

Figure 1.5 SEM images of carbon structure via pyrolysis. (a) Suspended carbon nanowire and carbon mesh structures [42]

Thermal nanoimprint

It is essential that the UV curable polymer cures by exposure to UV light. Therefore, clear glass or quartz is used for the mold and base, but these are expensive materials and require a difficult manufacturing process. Thermoplastic, on the other hand, is commercialized in sheet form with various thicknesses from hundreds of micrometers to a number of millimeters, so it is more feasible to use it directly in the printing process.

If a mixed-scale convex mold is robust enough in the thermal nanoimprinting process, thermoplastic material is a good alternative to PDMS. During the nano-printing process, this mold transferred channel patterns onto the PMMA plates more than 40 times without mold damage. From these results, it is demonstrated that the limitations of fungi in thermal nanoprinting can be overcome and this fabrication method can replace the common PDMA replication method for liquid devices.

Figure 1.6 Via thermal nanoimprint process, mixed-scale channel was fabricated on the PMMA sheet

Thesis outline

Overview of mixed-scale fluidic device fabrication

This mixed-scale carbon shape consists of a nanochannel shape with a width of ~600 nm and a height of ~60 nm and a microchannel shape with a width of ~50 μm and a height of 5 ~ 6 μm with a kingfisher beak-shaped 3D structure. Using the mixed-shell convex carbon mold, channel patterns are transferred to the PMMA sheet. In Figure 3.3, the mixed shell carbon mold used was not damaged, but there was some contamination (Fig.

The mixed-scale convex carbon shape with 3D Kingfisher beak-shaped 3D carbon structure is shown in Figure 3.5 (a), and transferred concave PMMA channel network is shown in Figure 3.5 (b). As shown in the figure, the mixed-scale patterns were accurately transferred onto the PMMA sheet. Fluorescein sodium salt (Sigma Aldrich, Korea) dissolved in DI water) was filled into the mixed-scale PMMA channels (Fig.

As described in figure 4.3, we can localize solute gradient region in Kingfisher beak-shaped 3D microfunnels clearly fabricated by mixed-scale carbon molding and nanoimprinting process. This PMMA mixed-scale fluidic device enabled single particle capture by diffusiophoresis at Kingfisher beak-shaped 3D microfunnels due to their characteristic 3D architecture. 17] Yunjeong Lee, Yeongjin Lim and Heungjoo Shin, “Mixed-scale channel networks influence kingfisher-beak-shaped 3D microfunnels for efficient single-particle trapping”, Nanoscale.

Figure 2.1 Schematic of a mixed-scale monolithic convex carbon mold fabrication.

A monolithic mixed-scale convex mold fabrication – Carbon-MEMS

One step mixed-scale channel replication and sealing -

Finally, a mixed-scale fluidic device is complete with no-flow, no-collapsed, hydrophilic surfaces. As shown in Figure 3.4, the Young's modulus and hardness of SU-8 increased dramatically by about 5 times and 10 times each. From figure 3.9(b), it was demonstrated that the nanochannels were clearly closed and there was no channel collapse or channel leakage compared to the corresponding SEM image of the nanochannel (Fig.

In general, the study of biomolecules is necessary for biomedical research, for which micro/nanofluidics or mixed-scale fluidic devices are commonly used as a bioanalysis tool, which are called lab-on-a-chip. Similarly, in the case of Figure 4.6 (c), 2 and half micropipe structures were placed between microchannels and nanochannels. In the same case in Figure 4.4, several additional particles were dragged into the 3D micropipe after 30 minutes of single particle trapping.

In this research, the fabrication of mixed-scale PMMA fluidic device was developed via thermal nanoimprint using a monolithic mixed-scale convex carbon mold. Using carbon MEMS technique, a mixed-scale monolithic convex carbon structure was fabricated by two-step photolithography and a pyrolysis process. This result demonstrated the applicability of mixed-scale PMMA fluidic device with 3D microfunnel for a wide range of research areas such as single cell study and nanoelectroporation.

Figure 2.2 (a) Schematic of a mixed-scale channel replication via thermal nanoimprint using mixed- mixed-scale carbon mold

Characterization of carbon mold

Pyrolyzed carbon morphology

Durability and reusability of carbon mold

Characterization of PMMA (polymethyl methacrylate) channels

Transferred micro-/nanochannel morphology

This 3D carbon structure was transferred onto the PMMA, resulting in Kingfisher beak-shaped 3D microfunnel (Fig. Through the 3D microfunnel, the deep and large horizontal microchannel is vertically and smoothly connected to nanochannel because the cross section of 3D microfunnel is gradually reduced to nanochannel size.

Figure 3.5 (a) SEM image of mixed-scale carbon mold and (b) magnified nanochannel carbon mold image

Characterization of nanochannel fabrication

1 μm SU-8 small-scale precursor width was reduced to ~600 nm in carbon, and the corresponding nanochannel width is nearly 600 nm. 420 nm-height SU-8 was reduced to ~60 nm-height carbon structure and which were etched 60 nm depth channels on PMMA.

Sealing test of mixed-scale channel networks

Those images were taken with inverted confocal microscopy at UOBC (Multiphoton Confocal Microscope, LSM 780 16 NLO setup, Carl Zeiss).

Figure 3.9 (a) Completed mixed-scale fluidic device after oxygen plasma assisted thermal bonding comparing with a 25 cents coin

Necessity

In previous research, Kingfisher beak-shaped 3D microfunnel was successfully used as a single particle chamber and the particles were dragged one by one by electric power. Even though the effectiveness of the unique 3D microfunnel structure has been demonstrated, too much strong electric field and result of randomly trapped particles need to be supplemented. The strong electric field tightly trapped particles that can lead to cell damage in real bioengineering research and result of random trapping means low efficiency of the manipulation method.

Here, we attempt to use Kingfisher beak-shaped 3D microfunnel structure for single particle capture via diffusiophoresis method. From this research, the utility of the Kingfisher beak-shaped 3D microfunnel in terms of diffusiophoresis and single particle chamber can be demonstrated.

Diffusiophoresis

Here, k is the Boltzmann constant, T is the temperature, e is the proton charge, Z is the valence of the symmetric Z:Z electrolyte, n is the local salt concentration, and 𝐷+ and 𝐷− are the diffusion coefficients of the cation and anion, respectively. The size of this electric field depends on the difference in diffusion coefficients of the ions and the length on which the gradient is placed. Due to the pressure gradient within the surface of the particles, a counter-drive occurs so that the particles can move.

However, for the sake of completeness, chemiphoresis effect can be ignored since chemiphoresis has less influence on transport rates [52]. As one of the advanced applications of diffusiophoresis, we used this phenomenon as a driving force for the capture of single particles. In this paper, we use the unique structure of Kingfisher beak-shaped 3D microfunnel structure for solute gradient localization.

This is the first time to show the possibility of diffusiophoresis as a single source of driving particle trapping.

Figure 4.1 Essential mechanism of electrolyte diffusiophoresis. The mechanism consists of two parallel additive phenomena: electrophoresis and chemiphoresis [53]

Experiment

Results

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