New 3D printing technique integrates electronics with microchannels to create flexible, stretchable microfluidic devices

The transition from traditional 2D to 3D microfluidic structures represents a significant advancement in the field of microfluidics, offering advantages in scientific and industrial applications. These 3D systems improve throughput through parallel operation and soft elastomeric networks when filled with conductive materials such as liquid metal, enabling the integration of microfluidics and electronics.

However, traditional methods such as soft lithography fabrication, which require cleanrooms, have limitations in achieving fully automated, interconnected 3D microchannels. The manual procedures used in these methods, including polydimethylsiloxane (PDMS) molding and layer alignment, hamper the potential of automating the fabrication of microfluidic devices.

3D printing is a promising alternative to traditional microfluidic manufacturing methods. Photopolymerization techniques such as stereolithography (SLA) and digital light processing (DLP) enable the creation of complex microchannels.

Although photopolymerization allows for the creation of flexible devices, there are still challenges in integrating external components such as electronic components during light-based printing.

Extrusion-based methods such as fused deposition modeling (FDM) and direct ink writing (DIW) offer automated production but face difficulties in printing hollow elastomeric structures. The key challenge is to find an ink that balances soft component deposition and strength for structural integrity to achieve fully printed, interconnected microfluidic devices with built-in functionality.

So far, existing 3D printing technologies do not simultaneously realize 1) direct printing of interconnected multilayer microchannels without auxiliary materials and post-processing and 2) integration of electronic components during the printing process.

Scientists from the Soft Fluidics Lab at the Singapore University of Technology and Design (SUTD) addressed these two significant challenges in a study published in the journal: Advanced functional materials: :

1. Direct printing of interconnected multilayer microchannels

DIW 3D printing settings have been optimized to create hollow structures with no supports for the silicone sealant, ensuring that the extruded structure does not collapse. The research team further extended this demonstration to produce interconnected multilayer microchannels with through holes between the layers; such microchannel (and electrical wire) geometries are often required for electronic devices such as antennas for wireless communications.

2. Integration of electronic components

Another challenge is the integration of electronic components with microchannels during the 3D printing process. This is difficult to achieve with resins that cure immediately.

The research team used gradually curing resins to embed and immobilize small electronic components (such as RFID tags and LED chips). Self-centering of these elements using micro-channels enabled the self-assembly of elements with electrical wiring after passing liquid metal through the channel.

Why is this technology important?

Although many electronic devices require 3D configuration of conductive wires, such as a jumper wire in a coil, achieving this using conventional 3D printing methods is difficult.

The SUTD research team proposed a simple solution enabling the implementation of devices with such complex configurations. Injecting liquid metal into a multilayer 3D microchannel containing embedded electronic components facilitates self-assembly of conductive wires with these elements, enabling streamlined fabrication of flexible and stretchable liquid metal coils.

To illustrate the practical benefits of this technology, the team created a skin-attachable radio-frequency identification (RFID) tag using a commercially available skin adhesive patch as the substrate and a free-standing, flexible, wireless light-emitting device of small size (21.4 mm × 15 mm).

The first demonstration highlights the solution’s ability to automate the production of stretchable printed circuit boards on a widely accepted, medically approved platform. The fabricated RFID tag showed a high Q factor (~70) even after 1000 cycles of tensile stress (50% strain), demonstrating stability in the face of repeated deformation and adhesion to the skin. Alternatively, the research team envisages the use of small, flexible wireless optoelectronics for photodynamic therapy in the form of medical implants on biological surfaces and lights.

“Our technology will provide new possibilities for the automated production of stretchable printed circuits with 3D configuration of electrical circuits composed of liquid metals,” says the paper’s lead author, Dr. Kento Yamagishi, SUTD.

“3D DIW printing of elastomeric multilayer microchannels will enable the automated fabrication of flow devices with a three-dimensional channel array, including multifunctional sensors, multi-material mixers, and 3D tissue engineering scaffolds,” says Associate Professor Michinao Hashimoto, principal investigator of SUTD.