As Michael McAlpine weaves through aisles of tables holding 3-D printers of various sizes, he picks up one of the projects he’s most excited about: a mannequin’s hand with electronic wiring printed on its back. The printing on the plastic hand is a representation of what his team recently printed onto the back of a researcher’s hand. 

“The applications for this are limitless,” he says. For instance, McAlpine envisions a future when soldiers in the field carry 3-D printers in their backpacks that can “tattoo” a biochemical sensor on their skin that alerts them to danger.

Printing on the human body is just one example of the way McAlpine, the U’s Benjamin Mayhugh Associate Professor in Mechanical Engineering, is rethinking 3-D printing. 

As he shows off his lab and stops to credit the post-docs and grad students who work with him, he quietly admits he’s an early adopter of the current wave—“the big wave,” as he calls it—of innovations. 

In addition to experimenting with printing on skin, he’s combining living cells with electronic “inks”—a departure from the hard plastics traditionally associated with 3-D printing. And his group recently received a patent for 3-D-printing active electronic devices (think semiconductor chips)—an invention that could turn ordinary things like catheters and contact lenses into “smart” devices that can monitor medical conditions. 

IT STARTED WITH AN EAR

3-D printing is a product of the 1980s, when it was first used for prototyping. Sometimes called additive manufacturing, it involves a computer-controlled set of nozzles that lay down various materials. The nozzles make repeated passes over a work platform, precisely adding material—or many materials—to create a three-dimensional object of almost any shape.

McAlpine first learned of 3-D printing at an engineering conference in 2011, where a professor explained how he “printed” an edible muffin from flour “ink.” Inside the muffin was a letter printed with food coloring—all made in a single operation using a 3-D printer.

“I was amazed at the whole thing,” says McAlpine, then an assistant professor of mechanical and aerospace engineering at Princeton University in New Jersey. “I went back and told my students we had to try something with this 3-D printing.”

Because of his interest in the intersection of technology and medicine, his research group 3-D-printed a “bionic ear”—an ear-shaped cellular construct with an embedded electronic antenna that gave it the ability to hear. “We showed that 3-D-printing biological materials like cells could be combined with printing electronics,” he says. “I realized then that this technology had greater potential than what it was being used for.”  

Since then, 3-D printing has taken off, finding uses in everything from electronics to medicine to food. McAlpine’s career has taken off as well. His long list of academic awards includes the National Institutes of Health Director’s New Innovator Award and the Presidential Early Career Award for Scientists and Engineers. His inventions have been recognized by CNN, Time, and the New York Times Magazine, among others. Most recently, he received the U’s George W. Taylor Award for Distinguished Research, funded by an endowment from the late George and Edna May Taylor. “It’s the U’s way of saying ‘We like you,’” McAlpine says as if he’s not quite believing it yet.

The award recognizes what he’s accomplished since he joined the U in July 2015 and began working with researchers from the U of M Medical School on ventures that bridge medicine and engineering. 

TISSUES AND TOUCH

Among McAlpine’s medical projects is a 3-D-printed silicone tube that guides the regrowth of nerves in complex patterns. The printing process leaves tiny grooves running along the length of branching tubes. “We actually use those to our advantage,” he says. “They direct the way the nerves are supposed to grow.” 

With further experimentation, McAlpine found he could print “capsules of biomolecules” such as proteins into the microgrooves to direct how nerve regrowth occurs. “We may even be able to direct motor nerves to go one way and sensory nerves to go down the other branch,” he says.

McAlpine has used these guides to regenerate severed sciatic nerves in laboratory rats, enabling them to walk normally again. He is now working with Ann Parr, director of spinal neurosurgery at the Medical School, to develop similar guides to repair damage to the central nervous system—a much more difficult task since the spinal cord generally doesn’t regenerate. “Your peripheral nerves are like weeds. They just grow,” he says. “But the spinal cord is different. The nerves don’t like to regrow.”

In a different project with Brenda Ogle, associate professor of biomedical engineering, McAlpine has printed a fine-grained collagen mesh embedded with pluripotent stem cells (undifferentiated adult cells that can become any kind of specialized cell) to create a living bandage for tissue damaged by a heart attack.

To this, he added an element of time, a twist he calls “4-D printing,” embedding the mesh with 3-D-printed polymer capsules containing signaling molecules that direct some stem cells to turn into cardiac muscle and others into cardiac blood vessels. The molecules are released when the capsules and the gold nanorods they contain are heated with laser light. 

McAlpine also created stretchable bionic “skin” with touch sensors. He hopes it will give robotic surgical devices a sense of feel to help surgeons—who must look into a camera, then use hand and foot controls to remotely move robotic arms attached to tiny instruments—perform complex procedures. 

“I don’t know how they do it,” says McAlpine, who says he gave up after two seconds when trying a demo with the system. “You can’t feel anything. If you could put some of these touch sensors onto those tools and provide touch feedback, then that would help them actually have some tactile sense.”

Another recent creation is a 3-D-printed prostate with lifelike feel and embedded electronic sensors that can be used to train surgeons. 3-D-printed models are already used for training and surgery “rehearsals,” but they’re made from hard plastic. “It may replicate the 3-D geometry of the organ, but it doesn’t feel anything like the organ,” McAlpine says of those models. 

By sampling actual cancerous prostates that had been removed, McAlpine was able to formulate silicone “inks” to mimic the feel and resilience of the real organs. The electronic sensors provide feedback to would-be surgeons as they palpate or cut into the model. “It’s sort of like a game of Operation,” he says. “If you touch the side, you get a zap.” In this case, the sensor provides a digital readout of the pressure being applied by the hand or scalpel to the model organ.

INSPIRING ENVIRONMENT

McAlpine says being next door to a major medical institution inspires him. “At Princeton I was sort of my own island, which allowed us to be creative and make bionic ears,” he says. “But we didn’t know what the real-world biomedical problems were.” Now, researchers come to him saying, “Here’s a problem. How can we solve it?” In fact, it was urologist Robert Sweet (now at the University of Washington) who asked McAlpine’s group to print a lifelike prostate. 

He has also benefited from his position as Benjamin Mayhugh Associate Professor in Mechanical Engineering, an appointment for incoming faculty that comes with startup funds. The position has given him money, time, and freedom—as well as renovated laboratory space for his five 3-D printers. 

When asked about the key to his success, McAlpine credits his willingness to take on big projects that can lead to other things, rather than focusing on many small ones. “Spinal cords and nerves, that’s a big thing. Printing electronics on the body, that’s a big thing. Understanding cancer, that’s a big thing,” he says. “We have so many ideas. We look forward to spending the next several decades at the University of Minnesota pushing the limits of what 3-D printing can do.”

Greg Breining is a St. Paul writer.

Some of the pieces created by Michael McAlpine and his research team