Pen microfluidics: rapid desktop manufacturing of sealed thermoplastic microchannels

Abstract

A unique technique for the rapid fabrication of thermoplastic microfluidic chips is described. The method enables the realization of fully-sealed microchannels in around one hour while requiring only minimal infrastructure by taking advantage of a solvent swelling mechanism that allows raised features to be patterned on the surface of homogeneous thermoplastic materials. Patterning is achieved without photolithography by simply drawing the desired microchannel pattern onto the polymer surface using a suitable ink as a masking layer, either manually or under robotic control, followed by timed exposure to solvent vapor to yield a desired depth for the masked channel features. The channels are then permanently sealed through solvent bonding of the microchannel chip to a mating thermoplastic substrate. The process is demonstrated using cyclic olefin copolymer as a thermoplastic material, with fully operational microfluidic devices fabricated following a true desktop manufacturing model suitable for rapid prototyping.

Fig. 1

Overview of the pen microfluidics fabrication process. (a) An ink mask is drawn on a COC chip surface. (b) Vapor-phase solvent exposure results in patterned growth of the COC surface by solvent swelling. (b) Bonding is realized by bringing the patterned surface into contact with a sealing layer, followed by solvent bonding using a desktop laminator. (d) The water-soluble ink masking layer remaining within the sealed microchannel is removed by pumping aqueous buffer through the channel.

Fig. 2

(a) Manual writing with a wet-erase pen onto a COC chip, followed by 15 min orogenic growth and solvent bonding, and sequential sequential injection of (b) water and (c) red food coloring through the resulting microchannel network. (d) SEM image revealing the cross-section of the sealed microchannel.

Fig. 3

Bright field images of a microchannel formed in a COC chip by orogenic growth with a manually-drawn ink mask (a) immediately after microchannel sealing and (b) following buffer rinsing to remove the water-soluble ink. (c) Cross-sectional SEM image of a typical microchannel, 188 µm wide and 22 µm tall.

Fig. 4

Cross-sectional views of channels fabricated using (a) 8 min, (b) 30 min, and (c) 60 min solvent exposure times, resulting in channel heights of 8 µm, 29 µm, and 61 µm, respectively. In case (a), chip bonding was performed using a hot press, while in cases (b) and (c) a low-pressure lamination process was used for bonding to minimize channel height reduction.

Fig. 5

Example of a 1.2 µm tall microchannel with fabricated using a 3 min solvent exposure time. While submicron channel features have been fabricated, solvent exposure times below 3 min result in compromised bond strength for the sealed chips.

Fig. 6

Spiral diffusive micromixer fabricated using computer-controlled mask definition. (a) Image of the fully-bonded chip before flushing ink from the enclosed microchannel, and images of dye solutions (b) at the confluence of the injected dye streams and (c) within an arm of the spiral showing formation of a smooth dye gradient due to diffusive mixing.