Frontiers in Additive Manufacturing

The Shape of Things to Come

Additive manufacturing technologies—machines that can automatically fabricate arbitrarily shaped parts, pixel by pixel, layer by layer, from almost any material—have evolved over the last three decades from a limited number of expensive prototypes to widely available, small-scale commodity production tools. But this trend is only the tip of a technological iceberg, a burgeoning “second industrial revolution” that is expected to transform every aspect of our lives. It is easy to predict that, like any other new manufacturing technology, additive manufacturing will lead to more material options, better resolution, faster production, easier and more reliable operation, and lower costs. But it is less obvious where this technology will go next. We can look at the evolution of additive manufacturing technologies past, present, and future as a series of milestones in human control over physical matter.

Printing Forms: Programming the Shape of Matter

The first milestone, which is maturing today, has been unprecedented control over the shape of objects. Machines can now fabricate objects of almost any material—from nylon to glass, from chocolate to titanium—and with any complex geometry. This capability is transforming not only engineering, but also many other fields, from biology to archaeology to education to the culinary arts.

Complexity for Free

Perhaps the most dramatic impact of 3D printing is that it can manufacture objects without factoring their complexity. Fabricating a solid block costs almost the same as fabricating an oddly shaped object with curved surfaces and notches. In either case, the printer scans back and forth, depositing material one layer at a time. The only difference is in the pattern of material deposition. Just as printing a picture of a circle takes no longer than printing a picture of the map of the world, printing a mousetrap takes no longer and requires the same skills and resources as printing a paperweight.

Regardless of how we measure it—in production time, material weight, energy waste, or production planning—the addition of features hardly changes cost. In some cases, added complexity can even reduce cost. For example, printing a block with a hole in it is cheaper and faster than printing a solid block. Therefore, in stark contrast to conventional manufacturing where every additional hole, surface, protrusion, and corner requires more planning, takes longer to produce, and consumes more energy and possibly more raw material, the marginal cost of added complexity with additive manufacturing is near zero.

Why should we care about the marginal cost of complexity? Lowering the cost of manufacturing complex products is obviously a good thing, but the consequences have profound implications. Industrial revolutions are triggered when a fundamental cost associated with production drops to zero, essentially taking that factor out of the cost equation.

The industrial revolution of the nineteenth century occurred when steam engines replaced horses and waterwheels, dramatically reducing the cost of power to near zero. Not only were traditional power sources replaced, but the types of work that machines could perform led to a cascade of innovation, such as railroads and factory automation. Similarly, the Internet reduced the cost of disseminating information to near zero. As a consequence, it also expanded the range and types of media that could be distributed—not just online newspapers, but also wikis, blogs, and user-generated content.

One could argue that additive manufacturing has drastically reduced the cost of complexity, bringing it to near zero. Initially, this simply implies that new technology will gradually replace older, more expensive ways of making things. But in the long term, the range and types of objects that can be manufactured will also increase dramatically.

Personalized Manufacturing

In addition to vast new design possibilities, manufacturing will be personalized. This trend has profound economic implications for how future products will be designed and used, who will design them, and where they will be made (Lipson and Kurman, 2010). Most important, the opportunity for anyone to design and produce complex products without the need for the resources and skills of traditional manufacturing will democratize innovation and unleash the long tail of human creativity. Look online today, and you will see thousands of objects for sale ready to be printed on demand, from custom-shaped hearing aids to flapping, hovering micro air vehicles (Figure 1f), to authentic-looking replicas of ancient cuneiform tablets (Figure 1e).

Figure 1

Printing Composition: Shaping the Internal Structure of Materials

The second milestone, which has just appeared on the horizon, is controlling the composition of matter—not just shaping external geometry, but also shaping the internal structure of materials with unprecedented fidelity. Using multi-material additive manufacturing technologies, we can make materials within materials, embed and weave multiple materials into complex patterns, and co-fabricate entangled components. For example, we can print hard and soft materials in patterns that create materials with bizarre new structural behaviors, such as materials that expand laterally when pulled longitudinally.

We are eliminating traditional limitations imposed by conventional manufacturing when each part had to be made of a single material. Instead, microstructure can now be specified with micron-scale precision. This will make it feasible for you or me to print a custom tennis racket to enhance our backhands or for a doctor to produce a replacement spinal disc implant tailored for a specific patient.
The number of possibilities is enormous, but so far few theories can predict the properties of these new materials, and few designers are prepared to exploit the new design space. Clearly, we will need new design tools to augment human creativity.

From Bio-printing to Food Printing

Every discipline will be affected by the unprecedented control over the shape and composition of matter. Imagine the implications for any field that involves the design and fabrication of physical objects—from mechanical engineering to art and architecture. But additive manufacturing technology can also change fields with no immediate connection to engineering or manufacturing. Take, for example, the emerging field of 3D printing for medical applications.

Custom-Shaped Prostheses. One of the earliest applications of health-related 3D printing was the fabrication of custom-shaped, complex prosthetic limbs and devices, particularly to address the key challenge of the interface point with the body. For example, a good match between the socket of a limb prosthesis and a patient’s bony prominences or muscular tissue can greatly improve both functionality and comfort.

When the aesthetic appearance of a prosthesis is a factor, the exterior can be custom shaped to blend with its surroundings or to match symmetrical features. In addition, the ability to shape internal cavities, add strengthening girders, and shave material off non-load-bearing components can all improve the weight-to-strength ratio of the device and make it both stronger and lighter. With sophisticated, multi-material fabrication, the mechanical performance of prosthetic devices could be tailored even further to improve elasticity, shock-absorbance, and energetic performance.

Another type of shaped prosthesis is the hearing aid. A major factor in the comfort and effectiveness of hearing aids is how they fit in the aural canal. With 3D printing, the patient’s canal can be optically scanned, and a tailored soft prosthesis can be fabricated almost instantly.

Custom-Shaped Implants. One-size-fits-all is not a good compromise when it comes to hip replacements. The idea of using 3D printing to fabricate custom-shaped implants dates back to the earliest days of freeform fabrication. Today, custom-shaped titanium or platinum implants can be fabricated with exactly the right size and shape, either parametrically scaled to fit the patient or produced directly from CT scan geometry of the original bone or a symmetrical healthy bone. Because complexity does not increase cost, printed implants can not only match the standard bone shape, but can also include various cavities and connection points to make bonding with existing tissue more compatible, reliable, and effective.

Bio-printing. Instead of using titanium and other engineering materials, implants (and other constructs) can be fabricated directly from biological materials. Early experiments involved 3D printing of bio-compatible scaffolds, which were later infused with live cells and incubated before implantation. The live cells gradually replaced the scaffold, resulting in a custom-shaped live tissue-engineered implant.

Printed scaffolds are still prevalent, but printing with biological cells directly, with no scaffold at all, is now possible. In this case, cells are first immersed in a bio-compatible hydrogel ink—a bio-ink—and then printed into their target form. The bio-ink has two key, contradictory properties that make it tricky to develop. On one hand, it must be fluid enough to be printed—to flow through the nozzle without damaging cells that are subjected to severe shear forces as they come out of the print-head. On the other hand, bio-ink must be stiff enough to hold its shape after printing to prevent the material from oozing into a shapeless mass. To address this double challenge in fabricating cartilage implants in the shape of a meniscus directly from CT data (Figure 1d), we used a variety of chemical and optical cross-linking agents.

Unlike scaffold-infusion techniques, printing with live cells directly makes it possible to fabricate heterogeneous tissue implants. Imagine the fabrication of a form as complex as a spinal disk or a heart valve, which involves multiple cell types in a complex spatial arrangement that is critical to its proper functioning (Cohen et al., 2006).

Drug-Screening Models. An exciting opportunity for fabricating complex, multi-cell heterogeneous tissue arrangements in three dimensions is the fabrication of models for drug screening. Traditionally, drugs and other treatments are tested in petri-dish-like environments that are meant to replicate the anticipated target environment in which the drug or treatment will be used. However, these petri-dishes or test-tubes are relatively simple compared to the real environment, both in the range of cell types and their spatial distribution. This mismatch often necessitates the use of animal models and other sophisticated, high-throughput testing procedures.

With 3D bio-printing, however, 3D spatially heterogeneous tissue models can be fabricated to more closely resemble the target application of a drug. For example, if cancer cells can be fabricated directly in the shape and distribution of a tumor, the effectiveness of treatments could be tested in vitro. Such experiments could provide a more realistic prediction of drug effectiveness and shorten the cycle of drug development.

Surgical Planning. A less obvious, but equally important application of 3D printing is preparation for surgery. Non-routine, complex surgical procedures often involve manipulation of tissue and tools through an intertwined, unknown environment. Some operations also require the preparation and attachment of permanent or temporary plates or other devices in the patient’s body. Just as it is easier to put a puzzle together the second time around, surgeons can substantially reduce the time and improve the reliability of an operation if they practice it beforehand. This is especially important for complex, non-routine procedures being attempted for the first time.

For example, the veterinary school often asks us to bio-print a 3D set of shattered or deformed bones from a CT scan of an injured animal that needs emergency surgery. With the printed bones, surgeons can practice the operation—learning to identify fragments, optimize reconstruction, and prepare plates and jigs in advance. Informally, surgeons have reported that surgery time can be reduced by half and that the quality of care is greatly improved.

Surgical Training. Surgeons in training often have access to state-of-the-art surgical equipment and tools, but they rarely have a chance to practice on realistic cases. For example, a surgeon in training is unlikely to be able to practice removing a brain tumor on an animal model, a human cadaver with this condition, or a synthetic training model. With 3D printing, however, relevant, typical cases from real patients can be recorded and reproduced on demand for practice.

With some modeling, it is even possible to combine cases and adjust their severity on demand to challenge surgeons-in-training at the appropriate level for the best learning experience. With multi-material bio-printing, training models could be fabricated with biological materials to provide realistic experience that also provides the feel and responsiveness of real wet tissue.

Custom Medications. An excellent example of an application of 3D printing is the fabrication of medical pills on demand. A growing challenge for both doctors and patients is the administration of multiple medications simultaneously. Instead of keeping track of a dozen pills daily, a printer can now fabricate a single, custom-made pill for each patient. The pill could have an identification marking, which would eliminate the confusion and uncertainty associated with conventional delivery methods.

Food Printing. Food printing is to 3D printing what video gaming is to computers. Based on the same technology as bio-printing, food printing is the fabrication of edible items from raw edible inks. “Edible inks” include chocolate and peanut butter, cookie dough and frosting, as well as organic pesto and locally made goat cheese. It doesn’t really matter: Download the recipe, load in the frozen food cartridges, and hit print.

The recipe dictates which material goes where and which in-line-cooking procedure is applied during deposition and after. We have printed foods with all of the materials mentioned above and more, creating chocolate confections with frosted decorations and vanilla cookies with chocolate vertical text lettering inside (Figure 1a). Imagine a cookie printer with sliders to adjust crispiness, flavor, color, and texture. Imagination is the only limiting factor.

Although some may consider printed foods the epitome of processed foods, others are fascinated by the opportunities for almost unlimited innovation. You can download and share recipes, tailor variants as you wish, and produce freshly made dishes to your exact liking.

Printing Function: Programming the Behavior of Active Materials

The third and final milestone, of which we are just beginning to see early signs, is control of behavior. At this point, we will not only control the shape of matter and its composition, we will also be able to program materials to function in arbitrary ways—to sense and react, to compute and behave. This last step entails a blurring of the line between material and code (Gershenfeld, 2005), leading to what is essentially programmable matter, including everything from controlling the mechanical functionality of an object to controlling how it processes information and energy.

When we reach this milestone, we will be able to print virtually anything—from a cell phone to a robot that walks out of the printer, batteries included. But the robot will not look at all like today’s robots, because it will not be limited by the constraints imposed by conventional manufacturing, and it won’t be designed directly by humans (Figure 1c). The ability to manufacture arbitrary active systems comprising both passive and active substructures will have opened the door to a new space for designs and a new paradigm of engineering—one that is not unlike biology.

The New Computer-Aided Design

For the last four decades, computer-aided design (CAD) tools have played a critical role in product design. But the role and format of CAD tools have remained relatively unchanged. Although user interfaces have improved, geometric manipulations have become faster and more reliable, and graphics have become three-dimensional and photo-realistic, conceptually CAD software remains a passive 3D drawing board that records intentions but offers little insight or ideas of its own.

As 3D printing technologies become more abundant, traditional barriers of resources and skills are all but vanishing. The limit is now our imaginations, but, unfortunately, our imaginations are limited. Over and over again, I have seen students faced with the blank page of CAD and the unlimited capability of a 3D printer design rectangular objects with a few linear notches.

To a large extent, this cultural blindness is the result of years of observing mass-produced objects subject to traditional manufacturing constraints. However, it also reflects design thinking imposed by conventional CAD tools and the lack of new tools that can take advantage of the vast new design space offered by 3D printing capabilities. Clearly, the classical CAD paradigm will be dominant for the foreseeable future, but new paradigms for design tools are beginning to emerge.

Function Representations. As our ability to control the shape, composition, and behavior of materials advances, it becomes more appropriate to think about geometry and material specification as programming rather than drawing. For example, say you want to fabricate a spherical surface with periodic patterned protrusions (Figure 2b). Fabricating such an object using traditional manufacturing techniques would be a complicated, expensive nightmare. As a result, such a capability is buried far down the advanced CAD options menu, if it can be found at all.

The spherical surface might be more easily described using algorithmic rather than descriptive geometry, a procedural construction process rather than a target geometry (Pasko et al., 2011)—much the way a biologist describes phenotypes using a developmental process or a software engineer describes the appearance of a dynamic web page.

In fact, there are many more crazy shapes than regular shapes (Figure 2a), but they are currently difficult to explore. If designers were unencumbered by traditional manufacturing constraints, a growing number of them would be anxious to explore these kinds of geometries.

Figure 2

Matter Compilers. An alternative approach to design is to specify what the design should accomplish rather than what it should look like and then let the machine compile the design to meet your specifications. Some products, especially products with purely functional roles, would fit well into such a design process.

Consider, for example, designing a supporting bracket. We know the geometric constraints of the load and support contact points, the weight to be carried, and the material properties. Specify those requirements, hit the design button, and watch the optimal design emerge automatically. I guarantee that it will not be a block with rectangular notches and holes, but an organic-looking optimal structure with beautifully shaped cavities, a design that would take a human designer years to come up with manually.

Now imagine that the bracket can’t be installed because of a protruding pipe. No problem—just add the pipe protrusion constraint and recompile (Figure 2c). Programming by specifying target behavior and constraints, then compiling it into functional geometry, can address complex requirements, while simultaneously exploiting the new manufacturing capabilities afforded by multi-material 3D printing as they become available. In a way, the designer becomes the customer, and CAD software becomes the designer.

Interactive Evolution. What happens when a desired design goal can’t be described quantitatively, such as an object that has an aesthetic component that defies quantification? Imagine, for example, designing a perfume bottle. Some aspects of the design, such as volume and size, can be specified quantitatively, but others, such as the look and feel, are more difficult to describe. Moreover, people skilled in the art of assessing perfume bottles may have no inclination to dabble in “CAD speak.”

Instead, a new type of CAD software can display a range of initial design concepts and allow an expert to indicate the ones he or she likes best or dislikes least. With this information, the machine can automatically infer a sense of the aesthetic the designer has in mind and generate new solutions that meet the quantitative requirements but are also closer to the designer’s aesthetic preferences. By repeating this process through a sequence of solutions and selections, a designer with little CAD expertise might design a complex perfume bottle.

In addition, the designer might come across new ideas and be provoked into exploring corners of the design space where no one else has been. Using this same process, visitors to the EndlessForms website (Clune et al., 2011) have collaboratively designed a variety of objects, from furniture to faces and from bottles to butterflies.

FabApps. What if I want to design a toothbrush to print on my 3D printer today? It is unlikely that I will be able to design a good, ergonomic, safe toothbrush. Even though a toothbrush seems like a simple product, it takes years of experience and know-how to design a successful one. Nevertheless, with 3D printers, people with almost no experience and little patience to learn are likely to want to design their own.

The solution may be simple CAD applications for a specific product that encapsulate all relevant knowledge, yet provide just the right level of flexibility to the user. Such FabApps (a term coined by Daniel Cohen and Jeffrey Lipton), similar to the iPhone apps you can download for 99 cents, could guide you through the design process and make you look like a pro. A toothbrush app could ask for the dimensions of your hand and mouth, process pictures of your face and palm, walk you through 50 different options, ask 20 more questions, and then produce a toothbrush that fits your unique needs perfectly and is guaranteed to be a success.

Conclusion

Tool-making is one of the characteristics that distinguishes modern humans from their evolutionary ancestors. Additive manufacturing, the ultimate tool, has the potential to change human culture forever in ways we can hardly anticipate.

References

Clune, J., and H. Lipson. 2011. Evolving Three-Dimensional Objects with a Generative Encoding Inspired by Developmental Biology. Proceedings of the European Conference on Artificial Life. See http://EndlessForms.com.

Cohen, D.L., E. Malone, H. Lipson, and L. Bonassar. 2006. 3D direct printing of heterogeneous tissue implants. Tissue Engineering 12(5): 1325–1335.

Gershenfeld, N. 2005. FAB: The Coming Revolution on Your Desktop—From Personal Computers to Personal Fabrication. New York: Basic Books.

Hiller, J., and H. Lipson. 2009. Design Automation for Multi-Material Printing. Solid Freeform Fabrication Symposium (SFF’09), August 3–5, 2009, Austin, Tex. Available online at http://creativemachines.cornell.edu/sites/default/files/ SFF09_Hiller2.pdf.

Knapp, M., R. Wolff, and H. Lipson. 2008. Developing Printable Content: A Repository for Printable Teaching Models. Proceedings of the 19th Annual Solid Freeform Fabrication Symposium, Austin, Tex., August 2008. Available online at http://creativemachines.cornell.edu/papers/SFF08_Knapp. pdf.

Lipson, H., and M. Kurman. 2010. Factory@Home: The Emerging Economy of Personal Fabrication. Washington, D.C.: Office of Science & Technology Policy. Available online at http://web.mae.cornell.edu/lipson/FactoryAtHome.pdf.

Lipson, H., and J.B. Pollack. 2000. Automatic design and Manufacture of Robotic Lifeforms. Nature 406(6799): 974–978. Available online at http://creativemachines.cornell.edu/papers/Nature00_Lipson. pdf.

Lipton, J.I., D. Arnold, F. Nigl, N. Lopez, D.L. Cohen, N. Noren, and H. Lipson. 2010. Multi-Material Food Printing with Complex Internal Structure Suitable for Conventional Post-Processing. 21st Solid Freeform Fabrication Symposium (SFF’10), Austin, Tex.

Luft, K. Titanium Footware, See L. Carrabine, (2009) Explore the Range of DMLS of Titanium, Make Parts Fast. Available online at http://www.makepartsfast.com/2009/12/828/explore-the-range- of-dmls-of-titanium/.

Pasko, A., O. Fryazinov, T. Vilbrandt, P-A. Fayolle, and V. Adzhiev. 2011. Procedural function-based modelling of volumetric microstructures. Graphical Models 73(5): 165–181.

Richter, C., and H. Lipson. 2010. Untethered Hovering Flapping Flight of a 3D-Printed Mechanical Insect. Pp. 797–803 in 12th International Conference on Artificial Life (Alife XII), Odense, Denmark, August 2010. Available online at http://creativemachines.cornell.edu/sites/default/files/ Alife10_Richter.pdf.