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Push print: Pioneering the 3D-printing techniques of tomorrow
This feature appears in the winter 2022 edition of Cornell Engineering Magazine.
In his early years at Cornell, Larry Bonassar, the Daljit S. and Elaine Sarkaria Professor in Biomedical Engineering, and his colleagues engineered a process for making printable materials that can also carry living cells. “If you do it right, the material is literally alive when it comes off the printer,” he says. That innovation opened the door for a pioneering career in biomedical research that, for two decades and counting, has largely been centered around 3D printing.
The Bonassar group initially printed cartilage, an a-vascular tissue that does not normally have blood vessels. In 2017, the researchers received some attention for 3D printing human ears, which are essentially pure cartilage. “The idea that you could grow cartilage in the right shape and use it to replace a body part was first brought up in the mid ’90s,” Bonassar says. “But the technology that’s necessary to do it, the engineering to do it, requires the precision and the reproducibility of something like a printing process. That also makes it customizable and scalable.”
3D printing is one of the great engineering innovations of the early 21st century. It has been applied to everything from biomedicine to architecture, and scientists continue to explore the full range of possibilities enabled by the process. Much has been achieved, but — if predictions hold true — there is much more to come. As researchers devise processes and techniques to push 3D printing technology to the next level for a range of materials, many next-generation breakthroughs are likely to emerge from Cornell Engineering.
Bonassar and his colleagues, for example, have applied their expertise to printing increasingly complicated cartilage. Once they had practiced on the ear, they turned to intervertebral discs—the soft tissues between vertebrae in the spine—which have different types of cartilage on the outside and inside of the disc. The overall goal is to eventually replace diseased or injured cartilage in human beings with 3D printed implants. “Living 3D-printed body parts like these have not yet been implanted in humans,” Bonassar says. “But I expect clinical trials to start in the very near future.”
In the meantime, the Bonassar group is developing a new generation tissue printer that can monitor itself by taking measurements on the material as it is depositing it. “The real challenge is quality control,” Bonassar explains. “We’re working on closed-loop control: a printer that is able to count cells and determine their viability while we’re depositing them and that can confirm the implant is sterile as we’re making it, so that in the end, we make exactly what we wanted to make.”
Monitoring and controlling the printing process is an issue across the spectrum of 3D printing, also known as additive manufacturing. It is central to the research of Atieh Moridi, assistant professor in the Sibley School of Mechanical and Aerospace Engineering. In an effort to better understand the complex physics of the 3D printing process, Moridi uses the Cornell High-Energy Synchrotron Source (CHESS), a high-intensity X-ray research facility, to watch while a 3D printer lays down deposits of metallic materials. She is able to follow the fabrication process in more detail than ever before and to identify defects as they occur.
That work led to a National Science Foundation CAREER Award. But even while Moridi is making waves with her CHESS-assisted monitoring techniques, she is well aware that a sophisticated research facility like CHESS is not available in industry settings. To boost the adoption of 3D printing by manufacturers, she sought to create a cheaper, more accessible monitoring technique. As part of that endeavor, she now serves as mentor to a NASA University Student Research Challenge project that seeks to combine the results of CHESS X-rays with those of acoustic emission sensors to identify errors during the additive manufacturing process.
“We want to acquire signals from both of these sources and correlate them, what I call concurrent watching and listening to the additive manufacturing process” Moridi says. “We watch with the synchrotron, acquiring hard-proof physics, and we listen with acoustic emission sensors. If there’s a defect, we will hear that in the acoustic signal, and we will know what that means because we are watching the process with X-rays.”
The overall project goal is to build up a database of signals correlated to specific types of defects. “We can essentially train people so when they listen to the additive manufacturing process, they will know if this is a good print or a bad print, or if this is a good layer or a defective layer,” Moridi says. “If there is a defective layer, in the future we hope to be able to add some corrective measures during the printing process, maybe remove the defective layer and redo it.”
Moridi is also researching 3D printing at lower temperatures, known as cold-spray printing. This technique avoids the residual stresses that can weaken a part printed with the current melt-based processes used for metal additive manufacturing. “Normally we rely on melting to fuse powder particles together,” she explains. “In this project, we use kinetic energy for bonding, rather than thermal energy; essentially we’re printing at supersonic speeds that smash the materials together and cause fusing of the particles.”
In the course of that research, the Moridi group also explored the ramifications of intentionally printing defects, causing the resulting structure to be porous. They then tested the feasibility of using the porous structures, made of titanium alloy, as biomedical implants specifically for encouraging new bone cells to grow inside them. “We showed that cells actually like these kinds of structures” she says. “The porous networks give enough space for bone to grow inside the pores and integrate the implant so there is less incidence of implant loosening.”
Also, unlike solid metallic implants—which are many times stiffer than bone and therefore bear more of the load, weakening the surrounding bone in the process—the porous implants can be printed to match bone strength, Moridi says. Both the bone and the implant then carry equal amounts of the load.
Scaling up the size of 3D printed materials — from the dimensions of a human ear to the size of, say, an office building — is another challenge of the technology. Sriramya Duddukuri Nair, assistant professor of civil and environmental engineering, is looking into the potential of layer-by-layer printing to solve some of the most pressing 21st century construction issues. Nair, who specializes in novel cementitious materials, is attracted to the technique for its flexibility as well as its potential for real-time quality control.
The traditional process for constructing large-scale structures from concrete depends on building a wooden formwork first and then filling it in with concrete. These frameworks limit the shapes that structural members can assume. They also often end up in landfills after one use, especially if they are created to hold a specialized shape specific to a particular architectural or structural component.
“But with 3D printing, you’re not limited in the shape the form can take because it removes the need to use formwork,” she says. “It gives you more flexibility on the shape, and it increases ease of construction. 3D printing is here to revolutionize our way of thinking about structures and to open up new possibilities.”
Like Bonassar in biomedical engineering, Nair has identified the need for better printing materials. In Nair’s case, the desired material is steel-fiber-reinforced concrete, which is able to withstand heavier loads. Unlike Bonassar’s focus on perfecting bio ink, however, Nair’s conundrum is how to engineer the printing head, or extruder, so that it can handle the thick material.
The problem came to light when the Nair group began to shop around for a 3D concrete printing system for Cornell’s Bovay Civil Infrastructure Laboratory Complex. “The printing heads available can only print with cement, sand and flexible polymeric fibers,” Nair says. “You can’t print with steel fibers. So, there are a lot of challenges on what can be printed because of the pumping systems and the extruder head capabilities.”
The Nair group has taken on the challenge of engineering a new extruder head capable of printing with steel-fiber-reinforced concrete. At the same time, Nair will be joining with Greg McLaskey, assistant professor of civil and environmental engineering, who specializes in work with ultrasonic sensors, to develop a method for real-time evaluation of 3D printed concrete so that errors can be identified and fixed as they happen.
Further down the line, autonomous robotic 3D printers may be employed to build housing and infrastructure, Nair says, which could result in construction jobs becoming safer and less stressful on the human body. “There’s predicted to be a shortage of construction workers in the future because these types of jobs are physically hard and don’t pay well,” she says. “With autonomous 3D printing, construction jobs may become more skilled and higher paying because workers will be handling robots and using data to analyze if everything is going to plan.”
Advances in 3D printing are expected to continue accelerating at scales both large and small. For Bonassar, the big frontier in 3D tissue printing is figuring out how to print vasculated tissues like kidney, liver or brain. Breakthroughs in that area will probably incorporate advances in cell-based therapy—the infusion of stem cells and other types of reprogrammed cells directly into the body, he says.
“With direct infusion of cells, the challenge is how to be sure that the cells are going where you want them to, that they’re staying where you want them to engraft, and that they are organizing relative to each other in the way you want,” Bonassar says. “The beauty of 3D printing is that you can address all of those issues at once. You can arrange them in the material the way you want, and you can use materials that will engraft in the target area of the body and encourage blood vessels to grow so that the cells stay alive and connect with the rest of the system.”
Meanwhile, others are thinking about what the future of 3D printing means for the production of larger structures. “For example, do we need all those dense solids that make up a bridge? Probably not,” Moridi says. “They are like that because we can’t affordably make these structures hollow. There are endless opportunities to exploit 3D printing processes, and we are just starting to scratch the surface of what’s possible.”