Generation of precision microstructures based on reconfigurable photoresponsive hydrogels for high-resolution polymer replication and microoptics

Abstract

Microstructured molds are essential for fabricating various components ranging from precision optics and microstructured surfaces to microfluidics. However, conventional fabrication technology such as photolithography requires expensive equipment and a large number of processing steps. Here, we report a facile method to fabricate micromolds based on a reusable photoresponsive hydrogel: Uniform micropatterns are engraved into the hydrogel surface using photo masks under UV irradiation within a few minutes. Patterns are replicated using polydimethylsiloxane with minimum feature size of 40 μm and smoothness of R_q ~ 3.4 nm. After replication, the patterns can be fully erased by light thus allowing for reuse as a new mold without notable loss in performance. Utilizing greyscale lithography, patterns with different height levels can be produced within the same exposure step. We demonstrate the versatility of this method by fabricating diffractive optical elements devices and a microlens array and microfluidic device with 100 µm wide channels.

Fig. 1. Preparation of AM/AZO-CD hydrogel based micromold displays and their subsequent usage in a replication process.

a Structuring and replication process on the acrylamide-azobenzene/cyclodextrin (AM-AZO/CD) hydrogel based micromold display; (b) Mechanism of light responsive AM-AZO/CD hydrogel; (c) Surface profile of the micromold, i.e., the structured hydrogel after 1 min of structured UV irradiation, (d) PDMS component replicated from the micromold; (e) the PDMS surface characterization shows high smoothness with a R_q ~ 3.4 nm.

Fig. 2. Optimization and usage of the micromold display.

a Increasing crosslinker ratio results in AM/AZO-CD hydrogels with a higher elastic module; (b) Higher crosslinker ratio causes a decrease of the engraving speed on the hydrogel, but facilitate handing of the micromold displays; consequently, the hydrogel formulation with 1.6 mol% crosslinker was chosen for further experiments; (c) Engraving height increases with increasing exposure time, reaching 10.6 μm after 15 min UV irradiation; (d) Illustration of consecutive setting/resetting cycles of the micromold display with different topographies; (e) Optical pictures and WLI characterization of four PDMS substrates replicated from micromolds generated on the micromold display with a smiley (1st and 3rd) and a deer (2nd and 4th) structure. Data in a, b and c are presented as mean values ± SD. Error bars represent the standard deviation from three samples.

Fig. 3. Micromold display structures generated using digital light processing (DLP).

a Scheme of maskless projection lithography system based on a digital micromirror device (DMD); (b) WLI profile of replicated PDMS with a triangle array; (c) Engraving height increases over exposure time reaching 11 μm after 10 min of illumination; (d) The profile of replicated PDMS with a tree structure showing the ability of the micromold display to fabricate complex shapes; (e, f) Two consecutive lithography-replication process based on reversibility of the AM/AZO-CD hydrogel via digital masks using the custom-made DMD; (g) A square array greyscale digital mask used for grayscale lithography, and (h, i) WLI image and feature profile of obtained structure on the replicated PDMS with different height; (j) A grayscale flower mask used for grayscale lithography and (k, l) WLI image and feature profile of replicated flower on PDMS with different height based on the varied transparency of the mask. Data in c are presented as mean values ± SD. Error bars represent the standard deviation from three samples.

Fig. 4. Optical and microfluidic devices fabrication using the developed micromold display.

a Scanning electron micrograph and (b) 3D topography of PDMS microlens array and (c) corresponding diffraction pattern; (d) The binary hologram used as the digital mask for lithography using the Gerchberg-Saxton (GS) algorithm (the inset is original user-designed image that was used to generate the corresponding digital masks); (e) 3D profiles of the replicated PDMS DOE devices determined by WLI which show a feature resolution of about 30 μm and (f) corresponding diffraction patterns; (g) WLI characterization and (h) feature profile of the replicated microchannel in PDMS at different locations along the channel; (i) Optical picture of the microchannel filled with dyed water showing diffusive mixing.