Improving Alignment of Kinematically Coupled Polymer Microfluidic Modules by Modularization of Coupling Features




Miller, Joseph E.

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Mass production of affordable microfluidic devices is a subject of importance to the biological and medical communities. If devices are cheap enough to be disposable, tests can be run at a higher rate and lower cost without concern for contamination. Achieving this cost-effectiveness requires devices to be of polymer material, made by injection molding or hot embossing. Though these manufacturing processes provide high throughput, accuracy of the features is a concern. Features of particular interest are those used for alignment of device modules in the event devices are stacked to perform a series of functions. A kinematic coupling was previously introduced as a method of passive alignment of microfluidic device modules. The coupling involves a set of three hemisphere-tipped posts and three v-grooves to provide exact constraint of the modules using six contact points, two for each post-groove connection. The objective this research is to provide guidelines for improving the alignment of kinematically coupled devices and to explore a method of doing so on microfluidic device modules. Sensitivity analysis was performed on the kinematic coupling dimensions of injection molded microfluidic modules to identify the main causes of misalignment. The results indicate that the height and angularity of the hemisphere-tipped posts are the dimensions with the greatest effect on the alignment of two modules. Therefore, the proposed method of improving the alignment is to manufacture the posts separately by additive manufacturing, and then connect them to an injection molded device module. Misalignment of functional features such as aligned through-holes was used to characterize the effect of variation in the six degrees of freedom. Monte Carlo simulations were used to show the reduction in functional feature misalignment depending on the reduction in component dimension tolerances. The maximum reduction in worst-case x-y plane misalignment possible by the proposed method is 22 percent. Maximum reduction in z-direction misalignment when decreasing only post dimension tolerances is 51 percent. However, simulations showed significant benefit to also decreasing groove dimension tolerances. When both are considered and set to ±2 µm, the maximum z-direction reduction rises to 80 percent. Hemisphere-tipped posts were additively manufactured by Boston Micro Fabrication using projection micro-stereolithography. The measurements of the post dimensions produced a maximum tolerance of ±1.3 µm, which is a 91 percent reduction from the tolerance of the same dimension on injection molded modules. If precision can be maintained throughout connection of the posts to the modules, the worst-case z-direction functional feature misalignment can be reduced by at least 51 percent and the x-y plane misalignment by 22 percent.



Kinematic, Coupling, Tolerance, Sensitivity


Miller, J. E. (2020). <i>Improving alignment of kinematically coupled polymer microfluidic modules by modularization of coupling features</i> (Unpublished thesis). Texas State University, San Marcos, Texas.


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