Singapore University of Technology and Design‘s (SUTD) Soft Fluidics Lab has developed a simple method to 3D print microfluidic devices integrated with fluid handling and functional components. The lab’s direct ink writing 3D printer dispenses a fast-curing flexible silicone resin on various substrates to form microchannels. Microchannels fabricated by SUTD have tunable dimensions and a wider range of available materials than previous 3D printing methods. This direct ink writing technique enables rapid prototyping of microfluidics for lab-on-a-chip applications in chemical testing and cell analysis.

Fabrication of microfluidic devices

Microfluidics is the manipulation and study of sub-microscopic liters of fluids. Roughly the size of a dollar coin, microfluidic devices allow experiments to be conducted precisely at microscale levels. Control over reaction conditions are improved while reaction times are reduced. In addition, the small size of devices reduces the amount of reagents used, waste produced and overall cost of experiments. Ranging from engineering to biology, microfluidic are found in many multidisciplinary fields. Microfluidic chips and microfluidic control instruments are examples of microfluidics application in drug testing and single cell analysis.

For the fabrication of microfluidic devices, soft lithography sets the current standard. Soft lithography is a manual process where elastomeric materials are casted on a mold fabricated in a cleanroom. Although this technique has multiple desirable characteristics to fabricate microfluidic channels, its design-to-prototype cycle is typically a few days. The fabrication process is hard to automate as well.

Able to turn design into working prototypes in the order of hours, 3D printing has emerged as an attractive alternative to soft lithography. Processes such as “in-air microfluidics” 3D bioprinting and FFF 3D printing are then developed for the fabrication of microfluidic devices. However, 3D printing of microfluidics suffers from a few limitations. First is the limited materials available for 3D printing in terms of optical transparency, flexibility and biocompatibility. Secondly, the commercial 3D printers used restrict the achievable dimensions of microchannels. The last difficulty is integrating 3D printed microfluidics with functional materials and substrates.

3D print microchannels by direct ink writing

Identifying the shortcomings of 3D printing, the SUTD researchers adopted a different route to apply 3D printing for fabricating microchannels. Using direct ink writing (DIW) of fast-curing silicone sealant, the team managed to rapidly 3D print microfluidic devices on various substrates such as glass, plastic and membranes.

The patterned silicone sealant determines the design of the fluidic channels. Channel dimensions are controlled simply by adjusting the distance between the top and bottom substrates that serves to seal the channel. Hence, this method permits the fabrication of microfluidic channels that are dynamically tunable in dimensions. With transparent substrates, the researchers can image the channel using a microscope.

In this experiment, the SUTD method has achieved channel dimensions as small as 32 μm in width and 30 μm height. Basic microfluidic modalities (e.g. straight and branched channels, mixers and droplet generators) and functional modalities (e.g. valves, variable flow resistors and gradient generators) are 3D printed on an optically transparent substrate.

After tackling the problems of limited material choices and achievable dimensions, the team applied their DIW method to integrate 3D printed microfluidics with functional materials. Engineering and biology applications are the focuses of this study. SUTD’s DIW approached managed to pattern silicone barriers directly on an unmodified printed circuit board at ease. It is also capable of immediately integrating electrodes into the microchannels that would function as real-time flow sensors. The team also performed air-liquid human keratinocyte cell culture by integrating microporous membranes to microchannels.

“Our approach to apply DIW 3D printing allows direct patterning of microchannels essentially on any flat substrate” said Assistant Professor Michinao Hashimoto, the principal investigator of the project. SUTD researchers’ method has successfully demonstrated rapid prototyping of microfluidic devices integrated with functional components, meeting the requirements for lab-on-a-chip applications.

Fabrication of integrated microfluidic devices by direct ink writing (DIW) 3D printing is published in Sensors and Actuators B: Chemical Volume 297. It is co-authored by Terry Ching, Yingying Li, Rahul Karyappa, Akihiro Ohno, Yi-Chin Toh, Michinao Hashimoto.

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Featured image shows multi-layer microchannels of DIW microfluidic device. Image via SUTD.