Researchers and students have demonstrated that inks can be used instead of thermoplastic filaments to 3D print functional biomedical devices. Michael McAlpine, Benjamin Mayhugh Associate Professor of Mechanical Engineering at the University of Minnesota, described such advances in the ESC Minneapolis keynote, “3D Printing Functional Materials & Devices.” A lot of the inks McAlpine’s group uses are nanometer-scale particle inks printed at a line-width scale of 10 microns and above for printing devices at the macro level. They have developed software as well as a high-precision robotic stage whose motion is coordinated with dispensing. Multiple inks, in a variety of viscosities, have been 3D printed on the same platform. Among the breakthroughs McAlpine described are 3D-printed patient-specific organ models, 3D-printed sensors and LEDs, and 3D-printed neural regeneration devices for peripheral and central nerve repair.
Organ models have been used to aid doctors and educate patients before surgery, but those made of hard plastic or rubber-like materials have had some limitations. After a request for a prostate model that “felt like a prostate,” McAlpine’s group set out to 3D-print patient-specific models based on MRI scans using customizable soft inks. Silicone is typically difficult to print with a typical 3D printer, he said, but for use on the team’s platform they devised ink formations with silicone that retain a soft structure, he told the audience. “The inks match the mechanical behavior of the prostate itself,” he said, and they can match natural organ color.
“We got amazing feedback” from doctors on campus, he added. “The 3D structure features a hollow structure and allows doctors to look inside the model. Surgeons can suture these models . . . and they retain mechanical integrity and don’t fall apart.”
Next steps entail integrating “complex functionality such as the ability to electronically sense the application of surgical tools to give quantitative feedback to give the surgeon not just qualitative response but also quantitative mapping about what is going on with these models as they are performing procedures.”
The team also set out to develop capacitive as well as piezoresistive tactile sensors. For instance, they have 3D-printed coils that can respond to applications of pressure and give electronic feedback. They have also developed inks with silver flakes curable at room temperature. Potential applications could include 3D-printed electronic tattoos on human skin. “A soldier in the middle of nowhere” could print any device directly on a wrist like a solar cell, he said.
Such work raises the question as to whether semiconducting devices could be fully 3D printed. “Can every component of a device come out of a 3D printer?” McAlpine wondered aloud to the audience. “The point is that we are using the 3D printing platform to print liquid metals, solid metals, polymers, inorganics, organics, semiconductors, and elastomers, all on one platform.” The group has already printed LEDs and is now printing light-receiving materials or photo-detectors.
Perhaps the most exciting developments McAlpine shared were 3D-printed neural regeneration devices for peripheral and central nerve repair. Peripheral nerve injuries (PNI) from accidents or disease leads to “200,000 repairs in the United States annually,” he explained. Injuries often involve loss of both motor and sensory function. “The gold standard is an autograft,” he said, “which needs a second surgery and sacrifice of that healthy nerve. . . . You could have donor tissue from cadaver, but that could involve an immune response.” Nerve-guide conduits consisting of polymer tubes have been developed, “but those are typically short tubes.”
The team set out to “solve complex nerve injuries,” such as those involving the sciatic nerve, he explained, which are often bifurcating nerve injuries associated with loss of both motor and sensory function branches.
During studies on a rat with nerve injury, they 3D printed a silicone nerve guide conduit that matched the original structure of the branching bifurcating nerve injury. Silicone printed line by line and welded together in the 3D printing process provided physical cues that help direct the regrowing axons. “Geometry replicates what goes on in the body. When the distal end nerve is regenerated, it leaves behind band-like structures, and our guide replicates these structures and replicates the body’s natural process of providing physical cues . . . that help the regeneration process.”
The studies involving the rat implanted with the guide were successful—“the rat could walk again,” he said.
Spinal cord injuries are much more complex injuries, and there are about 12,000 cases every year, he said. “They are irreversible and devasting,” he said. “Spinal cord axons do not spontaneously regenerate after injury the way peripheral nerves do.” The nerves are still there, but bruises act as barriers to regrowth, he explained.
He said people have implanted scaffolds similar to treating peripheral nerve injuries and injecting random collections of stem cells. His team took a different approach. “The nice thing about 3D printing is that we can do a combination therapy. We can print scaffolds and print cells within the scaffolds so they aren’t random.”
McAlpine’s group worked with a collaborator at the university who took skin cells, reverse engineered them back to stem cells, and turned them into a type of stem cell that can differentiate into neurons. His team was then able to 3D print the cells in the printed scaffold with up to 75% viability, and then axons propagated in the scaffold. The team used the same printer that is also used for printing LEDs.
The group has much more planned. “Right now we have no shortage of ideas.” He credits their success to “tight integration of engineering program with the biomedical program at the University of Minnesota, the building of our own instruments, an understanding of materials science, and the willingness to work with the interface in various sciences and technologies that is allowing us to push this forward. ”