We have presented an alternative method for fabrication of microfluidic devices—referred to as toner-transfer masking (TTM)—that should make microfluidic tools more accessible to the non-expert. Several key advantages separate this method from typical photolithographic techniques. First, the metal masters are more robust than photoresist masters (e.g. SU-8 or AZ photoresists) on silicon wafers, thus they are more resistant to wear, and they can feasibly be reused indefinitely if care is taken. Second, turnaround times for master fabrication (as little as 1 h, depending on channel depth) are shown to be comparable to the PDMS curing time, thus providing a means for rapid iterations of design, fabrication, and testing. Third, since the technique does not require cleanroom facilities and utilizes a standard laser-jet printer with over-the-counter materials, fabrication consumable costs (estimated < $1 per master, see Supplementary Information
) are reduced by over an order of magnitude compared to photolithography (estimated $13 per master). Supplies and equipment needed for this method can be cheaply and easily obtained through non-scientific commercial sources. This aspect alone should allow those with little or no expertise in microfluidic device fabrication to begin utilizing powerful microfluidic tools in their own research. Finally, the TTM technique allows accurate control of channel depths (see Equation 1
), and the method can be easily extrapolated to fabrication of multi-depth devices (). These benefits, together, have not been achieved using other rapid fabrication techniques noted above 14, 15
. In light of these advantages, it can be argued that the TTM method provides the best combination of fabrication flexibility, accessibility, speed, and cost reduction compared to any alternative to microfluidic device fabrication that has been reported to date.
In fact, the brass etching time could feasibly be extended to produce channels as deep as the brass sheet being etched, although this would eventually move the channel volumes outside the realm of microfluidics. Of course, for applications requiring line width resolution better than ≥ 100 μm, the TTM technique described here will not suffice () without a higher-quality printer. As commonly performed in electronics fabrication, however, it should be possible to reduce the achievable channel width (and volume) by under-etching thin toner line widths.
Perhaps more important are the demonstrations that TTM is a flexible fabrication technique capable of producing elastomeric valving structures (), three-dimensional patterns with through-membrane vias (), droplet-generating devices (), and devices capable of cell trapping, stimulation, imaging, and staining (). These results were obtained without the use of any specialized equipment or photolithography rooms. The TTM method merely requires a standard office laser printer and a clothing iron to print toner onto photographic paper and transfer it to brass substrates. Although brass was characterized in this work, the method could be extrapolated to other etchable metals. Furthermore, since Soper and coworkers 34
have shown that milled brass masters provide excellent reproducibility for hot embossing of poly(methylmethacrylate) (PMMA) devices, the brass features produced in this work could be used as masters for various polymeric molding or hot embossing approaches. By comparison, the “Shrinky-Dink microfluidics” approach presented by Khine and coworkers 14, 35
allowed polystyrene sheets to be stacked and bonded to directly serve as the microfluidic channels in three dimensions, rather than creating a master for soft lithography. An advantage of their three-dimensional microfluidic devices is that they could be designed and fabricated to full functionality in a matter of minutes. It was possible to achieve variable height channels using this method, although the channel width, spacing, and depth resolutions were not characterized. However, it is important to note that, since the polystyrene is pre-stressed in the plane of the sheet (x-y plane), both of these methods achieve their reduction in lateral resolution at the expense of expanding the channel in the axial direction (z). Thus the volume of the shrunken channels will be essentially equal to the volume of the channels before shrinking. Moreover, for those who require reproducibility in channel depths or volumes, the toner shrinkage 14
and manual scribing 35
techniques may be limited.
Multilayer soft lithography 21
was achieved using TTM masters (). Interestingly, the rounded trapezoidal cross section shown here () has recently been proposed as the optimal geometry for elastomeric valving, requiring very low actuation pressures 25
. Future work should be carried out to determine the required actuation pressures of the valves shown in . Additionally, the three dimensional channels shown in this work were connected through vias of minimal dead volume (), with an average volume of 2.5 ± 0.6 nL. By comparison to other rapid fabrication techniques, the manually-punched vias shown by Chen et al. 35
were approximately an order of magnitude larger in volume (2-3 μL), which is much larger than the total volume of typical microfluidic channel networks (100's of nL). Although these relatively large vias were shown capable of vortex-based mixing 35
, the large dead volumes would be disadvantageous with respect to transit times and additional use of expensive reagents used for biological assays 36
Finally, the TTM method was shown to provide a novel and rapid approach for reliable identification of intracellular metabolism of rare pancreatic α-cells within intact, live murine islets of Langerhans (). Handheld, disposable microfluidic devices were fabricated, and the devices were utilized for trapping islets, fluorescence imaging of oscillations in intracellular free calcium during glucose stimulation, and finally for alpha and beta cell-specific staining of these islets. Due to the superior fluidic control and cell manipulation provided by the microfluidic platform, these islets remained stationary throughout the imaging, stimulation, and staining procedures. Therefore, images of stained islets could be spatially correlated with the calcium oscillation data without performing tedious islet flattening techniques 23
that require overnight culture on extracellular matrix. Furthermore, the cost of expensive staining reagents, namely antibodies, could be reduced by an order of magnitude owing to the small volume of the device (~1 μL total). These results demonstrate that a rapid and inexpensive method for fabricating microfluidic devices can provide novel tools for cellular imaging while simultaneously reducing reagent costs and analysis time.