Direct ink writing of silicone elastomers to fabricate microfluidic devices and soft robots

This article reviews the recent progress in fabricating microfluidic devices and soft robots using direct ink writing (DIW) three-dimensional (3D) printing with silicone elastomers. Additive manufacturing, especially 3D printing, has become an alternative method to traditional soft lithography for producing microchannels, establishing a new standard in the field of microfluidics. This approach offers unprecedented opportunities for digital control, automation, and the elimination of manual assembly. Among different 3D printing technologies, DIW 3D printing facilitates the accurate deposition of liquid silicone precursors on various substrates in the air or liquid media, enabling the fabrication of microfluidic structures using a one-part room-temperature-vulcanizing (RTV) silicone sealant and two-part addition-curing silicone elastomers. The effectiveness of DIW 3D printing is demonstrated through (1) creating microchannels on various substrates, (2) printing interconnected, multilayer microchannels without the need for sacrificial support materials or extensive post-processing steps, and (3) integrating electronic components into microchannels during the printing process. In this article, overviews of the fabrication of microfluidic devices using 3D printing are provided first, followed by a discussion of different criteria and approaches for DIW 3D printing of silicone-based elastomeric structures in open-air and embedded media. Next, the structure–property relations of silicone-based microfluidic devices are discussed. Then, examples of DIW-fabricated silicone microfluidic devices and soft robotics are showcased, highlighting the unique benefits and opportunities of the methods. Finally, current challenges and future directions in DIW  3D printing of microfluidic systems are discussed.