Bioprinting is closely associated with the production of 3D-printed transplant organs, but the impact of the technology may reach far beyond tissue engineering. One biomaterials engineer at Northwestern University, Assistant Professor of Materials Science and Engineering and Surgery, Dr. Ramille Shah, and her postdoc, Dr. Adam Jakus have achieved a number of breakthroughs not just in 3D printing living tissue, but in applying the same technology to such functional materials as ceramics, metal oxides and metals.
In an interview with ENGINEERING.com, Shah relayed some of the exciting work that she and the Shah Tissue Engineering and Additive Manufacturing (TEAM) Lab are performing.
The Birth of Bioprinting at Northwestern
With a background in materials science and engineering and a concentration in biomaterials, Shah first began her exploration of 3D printing in 2011 while preparing for a class on new fabrication methods for creating scaffolds for her tissue engineering course. At the time, bioprinting was still in its nascent stages, and Shah decided to use her faculty funds to purchase a bioprinter.
Interestingly, one of the first and few manufacturers of bioprinters both then and now is actually better known for its digital light processing 3D printers. EnvisionTEC introduced its 3D-Bioplotter line in 2000, and has made regular improvements to its technology as it continues to lead in the bioprinting space.
By 2011, when Shah was looking to purchase her lab’s first bioprinter, there were different commercially available 3D printers for bioprinting applications. These platforms, however, did not have the straightforward user interface of the 3D-Bioplotter Manufacturer Series, according to Shah.
“Our inks are really compatible with any extrusion 3D printer that has the ability to print in cartridge form, meaning liquids that can be loaded into a syringe,” Shah explained. “But there's a big difference in the 3D-Bioplotter's software, in that other extrusion-based printers are not as user friendly. The Bioplotter gives us the capacity to do more with the printing aspect and have better control over the different parameters that we can change during the printing process. It makes our lives easier and makes material development a lot easier.”
Shah said that she recently discovered that she may have actually been one of the first customers to purchase the 3D-Bioplotter Manufacturer Series, which uses a syringe to extrude viscous materials in three dimensions. The 4th Generation of the machine can print up to five different materials within a single build job and includes such features as semiautomatic calibration, automatic needle cleaning and a variety of temperature control mechanisms.
Despite the friendly user interface, at the time of the printer’s purchase, there were very few inks available for 3D printing, Shah said. For that reason, she and her team began work to develop materials that could be printed in her lab. As a result, the Shah TEAM Lab has developed some of the most remarkable bioinks in 3D printing.
3D Printing Flexible Bones
“Our lab mainly focuses on developing new materials that are compatible with room temperature, extrusion-based 3D printing,” Shah said. “It's a very simple platform and we’re trying to develop a number of different new functional materials that are compatible with this platform.”
So far, the Shah TEAM Lab has developed two families of inks: hydrogels, which are used mainly for tissue engineering and regenerative medicine, and particle-laden inks, in which functional particles are suspended within a polymer solution. While the particle-laden inks were initially developed for 3D printing synthetic bone, Shah's group discovered that this material platform could be opened up for a wide variety of functional materials.
“The very first particle-laden ink was made up of hydroxyapatite, because it is the main mineral component of bone,” Shah explained. “What makes our particle-laden inks unique is that the resulting printed structures have a significantly higher number of particles compared to those fabricated using other additive manufacturing platforms.”
Shah added, “A lot of the platforms that deal with creating composites of ceramic and polymers usually can't get that level of loading. We can get upwards of 90 percent loading within the 3D-printed structure. With most other types of composites with polymers, it's hard to get past 50 percent by volume. Usually they're in the range of 20 percent.”
In the case of the artificial bones, Shah's lab was able to 3D print a hyperelastic hydroxyapatite structure that can be implanted directly after printing (after washing and sterilization), and which can quickly encourage bone growth within the body. Once the polymer component of the print is degraded by the body, all that's left is a bone-like implant that can be integrated by the surrounding bone. These osteogenerative properties are possible due to two important features of Shah’s particle-laden ink platform.
Because Shah's group is able to tune the ratio of particles to biodegradable polymer, the particle density and porosity of a part can be altered. On the one hand, Shah’s group is able to achieve high levels of particle density, giving migrating cells a suitable environment in which to thrive and start synthesizing new bone. On the other hand, the porosity of the material allows for native bone and blood vessels to grow within the implant.
Additionally, the material porosity makes the implant flexible by allowing room for the particles and polymer to move into upon deformation, presenting new mechanical properties and surgical handing capabilities previously not available to surgeons.
From the Shah TEAM Lab's experiments with hydroxyapatite, the researchers discovered that tuning the properties of particle-laden inks could be carried over to other materials as well.
From Bone to Metal
Upon printing with hydroxyapatite, Shah's postdoc, Adam Jakus, also began researching the 3D printing of particle-laden inks with such materials as iron oxide, nickel and even graphene. The process of printing metal is similar to the lab's work with artificial bone, except that, once the object is printed, this “green body” is placed in a furnace, where the polymer binder is burned out, metal oxide gets converted into metal, and the particles get sintered together, leaving a dense metal part.
With this process, Shah's group can alter the sintering conditions, such as the temperature of the furnace or the composition of the inks, to bring about desired qualities in the final part. For instance, her researchers can actually reduce iron oxide into iron or create highly flexible copper-based materials that can be bent into new shapes without breaking before they are sintered in the furnace.
Iron oxide, or rust, is more affordable than iron: this process opens up the ability to recycle wasted metal at a low cost. And the ability to print flexible metals could enable the 3D printing of large objects that are then folded into new structures before sintering.
“At the end of the day, we have an object that is completely metallic,” Shah said. “It's a new way to have a printed structure made of metal but not requiring the use of lasers or high temperatures during the printing process. It's a lot more scalable and it also gives us a lot more versatility when it comes to different types of metals and metal oxides that we can print.”
Shah and her team are currently exploring the applications of the technology, but examples include new energy devices and lightweight, yet strong metal structures.
3D-Printed Graphene
As it stands, some metals like copper have not been 3D printed very widely. Graphene is a completely different story, with only a handful of researchers capable of 3D printing the wonder material. At one carbon atom thick, graphene is most often printed as an aerogel with ultra-strong, lightweight graphene atoms making up only about 20 percent of the actual printed object.
Shah’s lab, however, is able to obtain remarkable densities of 60 to 70 percent using their particle-laden ink process, as well as making it possible to print larger structures than obtained by other labs.“Their morphology started to change drastically compared to, say, if we cultured them on hyperelastic bone,” Shah relayed. “The cells started to elongate and had these very long cellular processes that spanned long distances reminiscent of a neuron.”
Gene expression analysis validated their observation, revealing the expression of neuron-specific genes. The group is still trying to understand the mechanism behind this behavior, but Shah believes that the inherent electrical conductivity in graphene, made possible by the high loading capacity of their 3D printing technique, acted as a signal to the stem cells to become neuron-like cells.
The possibilities for this technology range from 3D printing biocompatible sensors to inducing tissue regeneration (e.g. for cardiac, nerve, or muscle tissue); however, Shahs lab will have to optimize the printing platform for a given application.
From Tissues to Organs
Shah's research with particle-laden inks are so far the most applicable to traditional engineering applications, but her work with hydrogels will be key for a future in which patient-tailored, bioartificial organs can be 3D printed on demand.
In the United States alone, approximately 119,788 people are currently on a waiting list for a donor organ. Although a number of these transplants could be procured through an increase in willing donors, or even an opt-out organ donation system, the ability to create custom organs specifically tailored to the immune system of a given patient would greatly reduce the number of people on this list.
Shah is currently engaged in a number of projects related to 3D printing complex human tissues, including an endeavor with Northwestern's Woodruff Lab to create bioprosthetic ovaries. By 3D printing uniquely-designed gelatin scaffolds and seeding them with ovarian follicles, the researchers were able to successfully create bioprosthetic ovaries that, when implanted in sterilized mice, allowed the test subjects to give birth to new litters of pups.
The next step, Shah explained, is to extrapolate the process to larger animal models before ultimately performing clinical trials. Funding, however, is a limiting factor in speeding the process along.
“We have done research when it comes to testing these materials within smaller animal models,” Shah said. “Now we are planning on doing some larger animal studies so that we can validate the efficacy. Those will probably take awhile and they’re also very dependent on funding and support to carry out these in vivo studies. Hopefully, we will be lucky in getting the funding we need that can change the timeline. If we had all the money in the world, I think we can get 3D-printed organs and tissues faster to the market.”
Shah believes that, due to the initial results of her work with hydroxyapatite, artificial bones may be closer to undergoing human clinical trials, potentially in 5- 10 years. Outside of tissue engineering, her ink platforms may be even closer to market, as Shah is in the midst of launching a start-up to translate and commercialize the research performed in her lab.
In other words, not only are bioartificial tissues and organs on the horizon, but the ability to create 3D printed iron objects from rust and to 3D print flexible copper and graphene sensors may be just right around the corner, as well.