Universal pre-mixing dry-film stickers capable of retrofitting existing microfluidics

The roll-based silicone microfluidics were first drafted using an Adobe illustrator. In preparation for laser cutting, a non-silicone release liner (3M 5053) was layered onto the dry adhesive exposed side of 0.13 mm thick roll-based silicone dry-adhesives on a polyethylene terephthalate (PET) carrier (3M 96042). Any air bubbles between the two layers that are formed were removed by scaping the top of the release liner with a squeegee. The specific characteristics of the silicone dry-adhesive were important in that it is designed to stick to silicone surfaces unlike more commonly available double-sided tape, as acrylic dry-adhesive would not hold the same adhesive strength to PDMS. Selecting the silicone dry-adhesive allows the sticker to have a flexible PDMS roof and optionally have an existing PDMS microfluidic-based bottom layer. The combination was then used to manufacture the final pieces of the tape with a laser cutter (Universal Laser Systems VLS 4.60) to create the tape layers as shown in Figs. 2(a) and 2(b). The settings used were 0.13 mm height and −50% quality. These settings on the laser cutter prevented excessive scorching on the tape for easier cleaning and resulted in a channel width of 200 μm. Particulates were removed from the laser cut pieces of the tape via brushing off with the blunt edge of a razor blade and simple tape cleaning. A PDMS interface layer was manufactured from Sylgard 184 (Dow Inc.) at a standard 10:1 mixture of elastomer base: curing agent. After vigorously mixing for 1–2 min, the PDMS solution was degassed for 40 min and 10 g was poured into a 100 × 15 mm Petri dish. The PDMS interface layer was cured in a 65 °C oven for 3–4 h or until use. After curing, the interface layer was peeled out of the Petri dish and manually cut to the dimensions of the final sticker device. PDMS can be made in bulk beforehand to save additional time when assembling the microfluidic sticker. A catheter punch (Syneo) was used to create a 0.75 mm⌀ hole for the inlets and outlets. Once the sticker is complete, the user can select the bottom layer based off of their applications. The configurations for the linear and spiral mixers shown in Fig. 2 are for the linear mixer to be used as a traditional standalone device and the spiral mixer to be retrofit. An additional configuration for the linear F-mixer to be compatible with being adhered to an existing microfluidic is found in Fig. 1 in the supplementary material.The stickers are built layer by layer from the top down [Fig. 3(a)], starting with the PDMS interface layer. As shown in Fig. 3(b) for the linear F-mixer, a clear backing must be removed from the silicone tape layer before adhering it to the PDMS fluidic interface layer. Alignment was done easily by eye, as each layer was designed with tolerances to allow for minor human errors during assembly. To account for misalignment during assembly, we included tolerances that allow for a maximum misalignment of about ∼1 mm for the x- and y-translational misalignment and ∼3° in rotational misalignment. If a greater translational error or rotational error occurs, the channels no longer intersect with one another and the device is unable to function properly. In later designs, we also included an alignment marker consisting of a square cutout in one corner of the device (Fig. 2 in the supplementary material). Each layer is the exact same width and length as the previous layer, enabling users to align edges if preferred, akin to a deck of cards. We tested whether these tolerances were appropriate by asking five users to build three devices each. On average, the misalignment on the x-translation, y-translation, and rotational translation was 0.29 mm, 0.4 mm, and 1.88°, respectively (Table 1 in the supplementary material). In addition, all 15 devices were successfully built microfluidic, and all of the channels were able to properly intersect and allow for fluid to flow. Because each piece has an adhesive backing, only gentle pressure is needed to attach the first tape layer to the PDMS fluidic interface layer [Fig. 3(c)]. Before attaching the second silicone tape layer, the white backing must be removed from the first tape layer [Fig. 3(d)]. The second tape layer is attached, by following similar steps, to the first one [Figs. 3(e) and 3(f)]. When adhered to the first tape layer and the PDMS layer, the linear F-mixer sticker is ready to be adhered to the user's floor layer of choice [Fig. 3(g)]. The bottom layers could consist of a traditional glass slide, PDMS, a pre-existing microfluidic or any other mixing surface of interest. The spiral F-mixer is manufactured in a similar method; however, it requires more than two pieces of silicone tape layers and a vacuum step to reduce the prevalence of bubbles blocking the channels. The F-layer of the spiral fluidic has been designed to be able to be rotated 90° and create as many layers as necessary to mix the fluid. In this paper, eight F layers for the spiral mixer were chosen for optimal mixing performance. Immediately prior to each experiment, the lone spiral mixer was placed in a vacuum chamber for at least an hour to eliminate bubbles prior to being adhered to a pre-existing microfluidic. Within seconds of removing the tape from the vacuum chamber, the spiral mixer was attached to the floor layer and 1× PBS was loaded into the channels to prime and aid in the removal of bubbles from the channels. 20 μl droplets of 1× PBS were placed at each inlet and outlet of the completed microfluidic. The adhesive bond strength for each layer is significant and difficult to pull apart manually. At the Reynolds number of 11.53, the microfluidic tape held firm for 1 h and 40 min without showing any signs of leakage. At this flow rate, we would expect any signs of failure to become apparent during this time frame, such as slight delamination. As no signs of failure were present, we anticipate that the mixer can withstand this Reynolds number for long periods of time. It was not until the mixer was brought up to the Reynolds numbers up to ∼23 that the tape started to show immediate damage and exhibit the signs of leakage.

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