Generation of Customizable Micro-wavy Pattern through Grayscale Direct Image Lithography

Abstract

With the increasing amount of research work in surface studies, a more effective method of producing patterned microstructures is highly desired due to the geometric limitations and complex fabricating process of current techniques. This paper presents an efficient and cost-effective method to generate customizable micro-wavy pattern using direct image lithography. This method utilizes a grayscale Gaussian distribution effect to model inaccuracies inherent in the polymerization process, which are normally regarded as trivial matters or errors. The measured surface profiles and the mathematical prediction show a good agreement, demonstrating the ability of this method to generate wavy patterns with precisely controlled features. An accurate pattern can be generated with customizable parameters (wavelength, amplitude, wave shape, pattern profile, and overall dimension). This mask-free photolithography approach provides a rapid fabrication method that is capable of generating complex and non-uniform 3D wavy patterns with the wavelength ranging from 12 μm to 2100 μm and an amplitude-to-wavelength ratio as large as 300%. Microfluidic devices with pure wavy and wavy-herringbone patterns suitable for capture of circulating tumor cells are made as a demonstrative application. A completely customized microfluidic device with wavy patterns can be created within a few hours without access to clean room or commercial photolithography equipment.

Figure 1. Analysis of the resin polymerization process.

(a) Theoretical curing model and actual curing result of the photocurable resin under the UV exposure of one pixel area. (b) Profile of photocurable resin cured under the exposure of grayscale gradient. (c) Curve fitting of the experimental result.

Figure 2. Mathematical model of curing shapes.

(a) Variation of single pixel curing models at increased grayscale levels. (b) The curing model under the exposure of a pattern with continuous pixels at the same grayscale values of 200. (c–e) The curing model under the exposure of a pattern with gray pixels (with grayscale value of 200) alternating with black pixels (with grayscale value of 0). The numbers of the black pixels between two nearby gray pixels are 1 in (c), 2 in (d) and 3 in (e,f) The curing model of a single wave containing five grayscale pixels. (g–i) Curing models of wave series containing multiple grayscale pixels in each wave. The numbers of grayscale pixels in each wave are 3 in (g), 9 in (h) and 11 in (i), respectively. (j) Single wave curing models according to different pixel numbers in each wave. (k) Parameters of grayscale values and number of pixels applied in (j). (In (a–i), Green curves represent the curing shape of a single pixel; Red curves represent the final curing shapes of the entire exposure pattern. In (j), blue curves represent curing shapes of each whole wave).

Figure 3. Pattern generating process.

(a) CAD drawings of uniformly distributed herringbone lines. (b) CAD drawings of a column of lines with the gradient determined by the gap distance. (c,d) The grayscale exposure image generated from drawing (a,b), respectively.

Figure 4. The schematic diagram of the wavy pattern projection system.

Figure 5. Comparison between experimental result and mathematical model.

(a–d) Mathematical curing models, profilometer measurement results, and the comparison between the exposure images in grayscale (e–h) and the digital microscope images of fabricated wavy patterns (i–l).

Figure 6

(a) The digital microscope image of a single wave. (b) The microscope image of the fabricated wavy-herringbone pattern. (c) The digital microscope image of the wavy pattern on PDMS after replication. (Side view) (d) Image of the wavy pattern with a wavelength of 2100 μm. (Comparing with a penny in thickness of 1.52 mm) (e) Microscope image of the wavy pattern with the increasing gradient of the wavelength. (Ratio of the gradient change is 120%) (f) A microfluidic device with wavy patterns. The digital microscope image of the fabricated wavy concentric circles patterns in (g) top view and (h) oblique view, respectively.

Figure 7

Microfluidic devices with (a) pure wavy pattern and (b) wavy-herringbone pattern, respectively. Microscope image of captured CTCs in microfluidic devices with (c) pure wavy pattern and (d) wavy-herringbone pattern, respectively.