Abstract
Rapid prototyping (RP) of microfluidic channels in liquid photopolymers using standard lithography (SL) involves multiple deposition steps and curing by ultraviolet (UV) light for the construction of a microstructure layer. In this work, the conflicting effect of oxygen diffusion and UV curing of liquid polyurethane methacrylate (PUMA) is investigated in microfabrication and utilized to reduce the deposition steps and to obtain a monolithic product. The conventional fabrication process is altered to control for the best use of the oxygen presence in polymerization. A novel and modified lithography technique is introduced in which a single step of PUMA coating and two steps of UV exposure are used to create a microchannel. The first exposure is maskless and incorporates oxygen diffusion into PUMA for inhibition of the polymerization of a thin layer from the top surface while the UV rays penetrate the photopolymer. The second exposure is for transferring the patterns of the microfluidic channels from the contact photomask onto the uncured material. The UV curing of PUMA as the main substrate in the presence of oxygen is characterized analytically and experimentally. A few typical elastomeric microstructures are manufactured. It is demonstrated that the obtained heights of the fabricated structures in PUMA are associated with the oxygen concentration and the UV dose. The proposed technique is promising for the RP of molds and microfluidic channels in terms of shorter processing time, fewer fabrication steps and creation of microstructure layers with higher integrity.
Highlights
Fabrication of microfluidic devices using elastomeric materials such as thermal-curing polydimethylsiloxane (PDMS) by soft lithography/molding is a common research approach[1]
The general fabrication of a microstructure in polyurethane methacrylate (PUMA) resin is performed by standard lithography (SL) in the following steps, as illustrated in Fig. 1: A) resin deposition for the base layer, B1) complete UV curing of the base layer, B2) resin deposition for the structure layer, and C) microstructure pattern transfer onto uncured polymer resin followed by a final step of washing the un-crosslinked PUMA
When most of the PUMA film is in a liquid state, the transmission at different thicknesses of material (1.34mm, 3.0mm, and 9.7mm) is almost constant, followed by slight decline at higher exposure doses when the resin has been mostly polymerized
Summary
Fabrication of microfluidic devices using elastomeric materials such as thermal-curing polydimethylsiloxane (PDMS) by soft lithography/molding is a common research approach[1]. Another fabrication method is direct ultraviolet (UV) which is applicable in UV-curable materials such as polyurethane methacrylate (PUMA) and involves the following process steps: resin deposition, UV-exposure, and post-exposure development. The composition of a typical UV-curable material contains a photo-initiator for absorbing the UV light and generating the radical species for curing initialization. The polymerization rate is inhibited by air because the oxygen molecules scavenge the radical species needed for crosslinking initialization. Dendukuri et al have reported a general modeling of the radical UV curing under the inhibitory effect of oxygen and the resulting loss of optical transparency [10]
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