微流控器件制造的双铸造方法比较

Abstract: We compare two methods for creating a microfluidic device from an initial positive master with the same surface relief as the final device. The first method uses urethane or epoxy resin to create an intermediate master for PDMS casting. This method has a very low failure rate but a longer processing time. The second method utilizes PDMS double casting where the intermediate master is passivated PDMS. This method has a higher failure rate but a significantly shorter processing time. Detailed methodology for both methods is included along with a comparison of the strengths and weaknesses of each method. The resin intermediate is a simpler method while PDMS double casting is faster and more scalable. Ultimately it is found that PDMS double casting is preferable in most situations.

1. Introduction 

Microfluidic devices are increasingly common in many scientific fields. The typical manufacturing process for microfluidic devices is soft lithography where the silicone polymer polydimethylsiloxane (PDMS) is cast onto a silicon wafer with surface relief created by photolithography. In this process cured photoresist creates surface relief which is the inverse of the final device and for that reason is called a “negative master” using photographic terminology. However, in some situations it is preferable to create a “positive master” with surface relief which matches the final device via wet etching,plasma etching, or micromilling to reduce fabrication time, cost, or create channels with shapes or curvature which may be impractical to create in a negative master. 

In these cases a two stage casting method is required to first invert the original positive master into an intermediate negative master which can then be used to cast the final PDMS device. Common methods of creating an intermediate master include thermal imprinting polymethylmethacrylate (PMMA) sheets for mass production and using epoxy resin or polyurethane resin for rapid prototyping. Another method is PDMS “double casting” where the intermediate master is also PDMS, but this method requires careful separation of the PDMS layers.The separation is typically facilitated by the application of a release agent to the intermediate or the chemical passivation of the intermediate to prevent bonding between the PDMS layers.Early release agents used for PDMS double casting include silanization using tetraethylorthosilicate and hydroxypropylmethylcellulose (HPMC).Later work passivated the intermediate using thermal aging or plasma/alcohol treatment.

2. Materials and Methods

The first step for any microfluidic device is to design the model in 3-D modeling software. The software used for the work presented here is Fusion 360, but most CAD 3-D modeling applications have the same tools and operate similarly. Using the sketch tool (or similar analog), make a rectangular outline of a suitable size for the glass slides being used, then extrude (apply height/z-axis dimension) to a height greater than the maximum drill depth to allow for material to remain under the milled channels. Next, sketch the outline of the desired channel (a curved Y-channel for the work presented here, see Figure 1) on top of the extruded rectangular base. Ensure that the injection sites and output site have a diameter of 1 mm to accommodate external tubing connections and that the channel depth and width are within the specifications of the mill-end being used. The range used in this paper were between 0.2 mm and 0.4 mm for the depth and width of the device channels and the length of the longer channel being between 0.8 mm and 1.0 mm. Once you have your finished model with the desired dimensions switch from design to manufacture mode. In manufacture mode, the g-code for the CNC machine is created for the milling of the device. This code should have negative values for the z position to mill into the dish.

Figure 1. The milled positive master before (left) and after (right) PDMS cleaning. In the before image milling residue is obvious while in the after image the positive master is ready for casting. The flow channel is 0.4 mm wide and 0.4 mm deep

3. Results and Discussion 

The two different methods described above both have their own strengths and weaknesses. Using a resin intermediate requires nearly 30 hours to go from initial master to final device while PDMS double casting requires only about 9 hours. The timing of each method is compared in Table 1. Using a resin intermediate will more than likely cause the initial master dish to break while the intermediate is being removed. Thus, even when the intermediate cast is successful only one intermediate master can be created from each initial master. However, the intermediate resin mold is highly reusable with the failure rate for final PDMS casting extremely low. The PDMS double casting method poses little risk to the initial master but is heavily reliant on the successful passivation of the intermediate. Failure of the PDMS intermediate to become distinct from the final PDMS device will result in the loss of both the device and the intermediate. However, even in this situation the initial master is unaffected and is reusable. Additionally, PDMS double casting avoids the lengthy 24 hour curing duration required by the resin. From initial master to final device using a passivated PDMS intermediate requires only 30% of the time compared to using a resin intermediate. This provides the potential for a one day process from design to a finished device, a significant boon for rapid prototyping.

Table 1. Comparison of the timing for double casting using a resin intermediate and a passivated PDMS intermediate.

4. Conclusions 

From the comparative study here we come to the conclusion that both resin intermediate casting and PDMS double casting have utility but PDMS double casting is preferable in most situations. With PDMS double casting the process time is shorter, the risks to the initial master are far lower, and multiple intermediates can be created from the same initial master. The possibility for failure of the passivation step is higher than the possibility of failure for any step using a resin intermediate, but the failure is not catastrophic as the initial master remains intact. The possibility of failure is lower using a resin intermediate, near zero after the successful creation of the intermediate, but the production of multiple intermediates from one initial master is nearly impossible and the process time is significantly longer. From this we conclude that PDMS double casting is the preferable method both for rapid prototyping due to the shorter process time and for scalable production due to the ability to create multiple intermediates from one initial master. The resin intermediate method could remain a preference for research groups with scarce resources where the loss of material due to failure is a larger concern than the increased processing time.