A Route to Three‐Dimensional Structures in a Microfluidic Device: Stop‐Flow Interference Lithography

Abstract

Polymeric structures with repeating 2D and 3D motifs at the micrometer scale and below have a variety of uses. Patterned 2D structures have been shown to have myriad applications in biosensors, [1] tissue engineering, [2] and diagnostic assay systems.[3] The availability of more sophisticated 3D structures would enable important advances in photonics, [4] information storage, [5] and tissue engineering. [6,7] Many techniques have been developed to fabricate complex 3D structures at the micrometer scale and below using either top-down or bottomup approaches. Bottom-up approaches such as polymer phase separation, [8] molecular self-assembly, [9] or colloidal assembly [10] are cheap and can cover large areas but face defects and limitations in the type and the geometry of structures that can be formed. While top-down methods offer precise size and shape control, the necessary point-by-point or layer-by-layer processing makes methods such as gray-scale photolithography, [11] direct 3D writing, [12] or two-photon lithography [13] very time-consuming.Recently, interference lithography (IL) [14][15][16][17][18][19][20] has emerged as an attractive alternative technique that allows the rational design of complex and defect-free 1D, 2D, and 3D patterns over large areas. Besides being fast and efficient, IL also affords control over geometrical parameters, such as symmetry and volume fractions, of the structures formed. IL performed using an elastomeric poly(dimethylsiloxane) (PDMS) phase mask [17] has the further advantage of providing simple and cheap processing, since only a single collimated beam is needed to form 3D interference patterns. However, phase-mask interference lithography (PMIL) [15,17] is typically performed by flood exposing a spin-coated layer of photoresist film through a phase mask, thus imposing some restrictions on the shape and material properties of the structures formed as well as limiting the throughput because of the serial nature of processing. Furthermore, such processes are not easily amenable to the fabrication of chemically anisotropic structures. Chemical anisotropy refers to the presence of multiple segregated chemical functionalities or gradients of one or more chemical functionality in a structure.

Porous polymer particles—A comprehensive guide to synthesis, characterization, functionalization and applications

“…Nevertheless, few parameters such as viscosity and tube distance to the lower phase are of importance. Finally, flow lithography techniques pioneered by Doyle [256][257][258][259] have drawn attention as a potential technique for porous particle production. Although generally being considered as a microfluidic technique, there are distinct differences.…”

Multi-Beam Interference Advances and Applications: Nano-Electronics, Photonic Crystals, Metamaterials, Subwavelength Structures, Optical Trapping, and Biomedical Structures

“…Two decades later, multiple MBIL exposures were proposed to generate more complex 2D patterns in a photoresist [138]. Since then, a wide range of structures have been recorded via MBIL using near-infrared [129,[139][140][141][142], visible light [18,31,32,62,88,[143][144][145][146][147][148][149][150][151][152][153], ultraviolet (UV) [62,77,78,99,115,[154][155][156][157][158][159][160], deep-UV [92,101,142,[161][162][163], and extreme-UV sources [164][165][166][167].…”

“…A wide array of photosensitive materials have been used to record the interference patterns formed by MBIL and include positive resists [115,157,184], negative resists [99,185], hybrid organic-inorganic materials [165,186,187], extreme-UV photoresists [188], silsesquioxane-based photoresists [129], 20 μm holographic polymer-dispersed liquid crystals [189], amorphous-chalcogenide-semiconductor thin films [190], titanium-containing monomer films [191], red-sensitive photopolymers [145], polyimide foils [51], biocompatible polymers [50], oligomer films [153], and even silica [192] and chalcogenide [193] glasses. In the most general terms, given sufficient optical intensity, an interference pattern may be recorded in or on any material that responds to laser illumination at a given wavelength [160], to include direct writing on metallic surfaces via laser interference metallurgy [194] and direct laser interference patterning of -conjugated polymers [195].…”

Stop flow lithography in perfluoropolyether (PFPE) microfluidic channels

“…(1) The channel material should be compatible 40 with soft lithography to allow rapid prototyping of channels with various geometries. SFL devices have been fabricated with multiple inlet pathways or layered channels to create structured micro-flows for the synthesis of chemically patterned particles [1][2][3][4][5][6][7][8] .…”

“…(3) Oxygen permeable flow channels are necessary because the process requires local inhibition of polymerization near channel interfaces via oxygen permeation. By virtue of this localized 50 inhibition layer, particles can be advected through flow without sticking to the device walls.…”