Exploiting the Oxygen Inhibitory Effect on UV Curing in Microfabrication: A Modified Lithography Technique

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.

Impact of oxygen-permeable mold on fabrication of microchannels with ultraviolet curable materials

“…If upper surface of resin is covered by an impermeable or very low permeability material, a continuous inhibition takes place only by diffusion of oxygen from PDMS mold. The cross-linked PUMA in contact with the resin has no inhibitory effect related to oxygen diffusion (Alvankarian and Majlis 2015).…”

“…The chemical compositions of UV-curable materials have ability to overcome the dissolved oxygen during cross-linking to a limited level and further diffusion of O 2 from the external and internal boundaries which have higher oxygen concentration than the resin itself delays the crosslinking or stops it from complete curing (Jeong 2008;Alvankarian and Majlis 2015). Therefore, in using PDMS mold and PUMA as the main substrate in micromolding, the high concentration of molecules of oxygen in resin has inhibitory effect on polymerization process.…”

“…1a schematically shows, diffusion of oxygen due to its gradient direction into resin is possible by two ways of diffusion from its top surface that is in direction contact with air and from its interface with internal surfaces of permeable micromold that contains a higher oxygen concentration than the resin itself. Both ways of inhibitions can be prevented by using an impermeable mold and covering the upper surface of resin with an UV-transparent and impermeable sheet (Alvankarian and Majlis 2015). The exposure process is conducted on PUMA resin molded with physical boundaries as it is demonstrated in Fig.…”

“…The polymerization rate in a layer of UV-curable resin is changed by air because the molecules of oxygen scavenge the radical species needed for crosslinking initialization (Dendukuri et al 2008;Alvankarian and Majlis 2015). Diffusion of oxygen from a high concentration zone into a pre-polymer resin during UV curing requires an additional amount of UV energy to consume the dissolved and diffused oxygen (Jeong 2008).…”

“…polymerization can be totally stopped by isolation of resin from ambient air using techniques such as covering it with a sheet of impermeable or with very low permeability to oxygen. Other techniques of reducing the inhibition effect are by covering the sample using nitrogen gas or using a high vacuum condition (Jeong et al 2009;Alvankarian and Majlis 2015).…”