Microtransfer Molding - Marissa Burgess, Ruptanu Banerjee, Vaishali Malik

Mictrotransfer molding (μTM) is a form of soft lithography - a set of pattern transfer techniques that employs a patterned elastomer as a stamp, mask, or mold.

The most commonly used elastic polymer in microtransfer molding is polydimethylsiloxane (PDMS). PDMS generally comes in two components including the prepolymer and the curing agent. The hardening of PDMS is achieved through the cross-linking of its polymer chains via thermal curing.

General Procedure

In microtransfer molding, a PDMS-based mold called a "stamp" is utilized. The PDMS stamp contains recessed regions, or microchannels, that are manually filled with prepolymer or ceramic precursor. The PDMS stamp, filled with prepolymer, is then inverted and brought into contact with the substrate. Next, the polymer is cured, and the PDMS stamp is peeled away. This results in polymer microstructures on the substrate arranged based on the design of the channels in the PDMS stamp.

Weibel et al., Nat. Rev. Microbiology, 2007, 5, 209.

Advantages

Several significant benefits can be noted regarding the process of microtransfer molding. The entire process is rapid with, the capability of replicating microstructures in five minutes or less. Both isolated and interconnected structures can be generated as well. In contrast to other patterning techniques, microtransfer molding can form structures on contoured surfaces in addition to flat surfaces. This trait is necessary for the microfabrication of 3-Dimensional structures, as the polymer can be deposited and cured layer by layer into the desired 3-D shape. [1]

Microtransfer molding also has low capital and operating costs, and it is cheap to create which results in the ability to rapidly prototype.

Limitations

Stamp or mold deformation is one of the main challenges that can be encountered during microtransfer molding. For example, the precursors used to create ceramics are generally liquid-based sol or colloidal suspensions, largely consisting of organic and aqueous material that must be removed when the liquid is converted into a solid. During the drying and sintering process, the thin film may diverge from the original shape from shrinking. [2]

Recent Advances

Microtransfer molding holds great promise in the design of microfluidic devices. An example is the development of a chip for capillary electrophoresis separation. [3]

Electrically modulated microtransfer molding is a newer technique closely related to microtransfer molding. The ability to create electrically modulated micropillar arrays over a large area has opened up new frontiers in surface engineering, biotechnology, and microfluidics. [4]

References

[1] Zhao, X.-M.; Xia, Y.; Whitesides, G. M. Advanced Materials 1996, 8 (10), 837–840.

[2] Martin, C. R.; Aksay, I. A. Journal of Materials Research 2005, 20 (08), 1995–2003.

[3] Duffy et al., Anal. Chem., 1998, 70, 4974.

[4] Li, X.; Tian, H.; Shao, J.; Ding, Y.; Liu, H. Langmuir 2013, 29 (5), 1351–1355

[5] Weibel et al. Nat. Rev. Microbiology, 2007, 5, 209.