US Army and Researchers 3D Print Microfluidic Channels on Curved Surface in an Open Lab

The field of microfluidics continues to grow as it’s used in a variety of applications: tissue engineering, drug screening and delivery, sensors, bioprinting, and more. Microfluidics involves manipulating and controlling fluid flows at the micron scale; they are essentially a tiny plumbing system with an increasing overlap with 3D printing. The traditional way to make microfluidic devices is through a complex photolithography technique, which requires several steps and takes place in a cleanroom with a controlled environment. A silicone liquid is sent flowing over a patterned surface, then cured so the patterns will form channels in the solidified silicone.

But a team of researchers from the University of Minnesota, together with the U.S. Army Combat Capabilities Development Command Soldier Center, figured out a way to 3D print fluidic microscale channels that could be used to help automate the fabrication of sensors, diagnostics, and assays for medical testing… no cleanroom necessary.

“This new effort opens up numerous future possibilities for microfluidic devices. Being able to 3D print these devices without a cleanroom means that diagnostic tools could be printed by a doctor right in their office or printed remotely by soldiers in the field,” explained Michael McAlpine, a UMN mechanical engineering professor and leader of the UMN McAlpine Research Group.

McAlpine, who holds the Kuhrmeyer Family Chair Professorship in the Department of Mechanical Engineering, is also the senior researcher for the team, which published a study about their work, “3D printed self-supporting elastomeric structures for multifunctional microfluidics,” in the peer-reviewed Science Advances journal. Other co-authors of the study are UMN mechanical engineering graduate student Ruitao Su; UMN electrical and computer engineering researchers and PhD candidates Jiaxuan Wen and Qun Su; US Army CCDC Soldier Center researcher Dr. Michael S. Wiederoder; UMN’s Louis John Schnell Professor in Electrical and Computer Engineering Steven Koester; and Dr. Joshua R. Uzarski, also with the Army’s CCDC Soldier Center.

“Microfluidic devices fabricated via soft lithography have demonstrated compelling applications such as lab-on-a-chip diagnostics, DNA microarrays, and cell-based assays. These technologies could be further developed by directly integrating microfluidics with electronic sensors and curvilinear substrates as well as improved automation for higher throughput. Current additive manufacturing methods, such as stereolithography and multi-jet printing, tend to contaminate substrates with uncured resins or supporting materials during printing,” the abstract states.

3D printed self-supporting microfluidic structures. (A) Top: Schematic of 3D printing a microfluidic channel. Bottom: 3D models of self-supporting structures including triangular & circular channels, hexagonal & conical domes. (B) Left: Bending moment analysis of self-supporting wall printed with straight profile. Right: (a) Composite cross-sectional images of silicone walls of varying incline angles and an overhang length of 700 μm. The boundary of each image is distinguished by the edge of the wall. (b) 37° was found to be the smallest incline angle that can be printed. (c) A silicone wall printed at an incline angle below 37° collapsed at the root. Scale bars, 200 μm. (C) Photos of 3D printed microfluidic channels and chambers with walls cut open to display cross-sectional profiles. Scale bars, 1 mm. (D) SEM images of triangular and circular channels with a width of ca. 100 μm. Scale bars, 100 μm. (E) Plot of burst pressure and wall thickness of the triangular channels with respect to printing speed (N = 3). Inset shows one specimen under test with a length of 5 mm and wall thickness of ca. 150 μm. (Photos courtesy of Ruitao Su, UMN)

What’s really exciting here, according to the researchers, is that this is the first time we’ve seen microfluidic structures 3D printed directly onto a curved surface. This is one important step closer to printing them right on a person’s skin in order to sense bodily fluids in real time.

“The self-supporting microfluidic structures enable the automatable fabrication of multifunctional devices, including multimaterial mixers, microfluidic-integrated sensors, automation components, and 3D microfluidics,” the researchers wrote.

The team used a custom 3D printer to print microfluidic channels—three times the size of a human hair—directly on a surface, in just one step, in an open lab, not in a cleanroom setting. They used a series of valves to control, pump, and re-direct fluid flow through the tiny channels. All devices used in the research act as a proof of concept for their hypothesis.

“Here, we introduce an automatable extrusion-based printing methodology that can directly align and print elastomeric microfluidic structures onto planar and curvilinear substrates with minimal involvement of postprocessing. By selecting inks of proper yield strength and controlling the profiles of printed overhung structures, self-supporting walls can be realized and further enclosed to form hollow structures such as channels and chambers. Since the microfluidic spanning distance is in the submillimeter regime, a sufficiently small bending moment results that the as-printed walls can withstand, rendering this method suitable for printing microfluidic structures. Printing toolpaths can then be designed to create leakage-free transitions between channels and chambers, T-shaped intersections, and overlapping channels,” the researchers wrote.

3D printed microfluidic valve, pump, and spherical microfluidic network. (A) Schematic displaying configuration of the 3D printed microfluidic valve. (B) Photos displaying open and closed states of the 3D printed microfluidic valve. The valve was closed with a pressure of 100 kPa. Scale bar, 3 mm. (C) Closing pressure test of 3D printed microfluidic valve under varying flow pressures. (D) Flow rate test of microfluidic pump, which was actuated with a standard peristaltic code: 001, 100, and 010, where 1 and 0 denote the open and closed state, respectively. Inset displays a 3D printed microfluidic pump with two liquid reservoirs. Scale bar, 5 mm. (E) 3D printed spherical converging and serpentine microfluidic channels with integrated valves. The images show three combinational operation states of valves 1 and 2. Scale bars, 10 mm. (F) Filament stacking schemes of spherical microfluidic channels. (a) to (c) demonstrate the designed and printed profiles of three channel cross sections. Spacer filaments were added to prevent the collapse of asymmetric channels distal to the sphere center. Scale bars. 1 mm. (Photos courtesy of Ruitao Su, UMN)

The microfluidics that the team 3D printed onto a curved surface were also integrated with electronic sensors for lab-on-a-chip capabilities.

McAlpine reiterated, “Being able to print on a curved surface also opens up many new possibilities and uses for the devices, including printing microfluidics directly on the skin for real-time sensing of bodily fluids and functions.”

While 3D printing the microfluidic channels onto a curved surface certainly makes them unique, another important aspect of this research is the fact that they could be fabricated outside of a cleanroom setting. This means there is a future where the devices can be produced, with repeatable results, with robotic-based automation.

Researchers at the University of Minnesota are the first to 3D print microfluidic channels on a curved surface, providing the initial step for someday printing them directly on the skin for real-time sensing of bodily fluids. Photo courtesy of McAlpine Group.

Parts of this research were completed in the Minnesota Nano Center, which is supported by the National Science Foundation through the National Nanotechnology Coordinated Infrastructure (NNCI) Network. The work was funded by the US Army Research Office via the CCDC Soldier Center; the National Institute of Health’s (NIH) National Institute of Biomedical Imaging and Bioengineering; and the Minnesota Discovery, Research, and InnoVation Economy (MnDRIVE) Initiative.

(Source: University of Minnesota)