Imitation of Life
By Dan Harvey
Radiology Today
Vol. 21 No. 8 P. 18

3D bioprinting efforts aim to create implantable living tissue.

For more than five years, health care professionals have realized a place for 3D printing in medicine. Once mainly entrenched in the lay community—the engineering sector, in particular—3D-printed models are being rapidly embraced.

Currently, the “3D printing laboratory” in radiology is emerging. In fact, it is “exploding,” driven by work being done by an RSNA Special Interest Group (SIG) dedicated to 3D printing and its implications for radiology, according to Scott Drikakis, MD, health care segment leader for Stratasys.

The exponential expansion has been driven by specific needs such as surgical preparation, patient-specific simulations, and education. Further growth is being made possible by interested and involved companies.

Stages of 3D Printing
Adam E. Jakus, PhD, is cofounder and chief technology officer of Dimension Inx, a company that provides 3D printing for tissue and organ repair and regeneration. He explains how printing and radiology intersect. “The term ‘bioprinting,’” Jakus says, “is often used to refer to the whole field of medical 3D printing,” in other words, any 3D printing done with a medically related intent. “However, this isn’t accurate. Bioprinting is a subcategory within medical 3D printing.” He describes four primary groups.

Category 1 describes 3D printing of structures not intended to go into the body but to assist medical activities, eg, surgical guides, surgical models, educational tools, and external prosthetics. This category includes objects made of plastics and resins that are nonbiocompatible from an implant perspective.

Category 2 3D printing involves structures that are intended to go inside the body but not substantially change with time, ie, permanent implants. These structures include 3D-printed titanium alloys or PEEK/PEKK plastics.

“Many off-the-shelf medical implant products are made from these materials using 3D printing,” Jakus points out. “Additionally, custom patient-matched implants are typically made from these materials. These materials and implants are primarily intended to serve a structural purpose—ie, replace bone—and are not intended to change over time. For example, the titanium implant on day one should, ideally, be the same at year 10. This type of 3D printing is not widely done in hospitals, due the types of materials and 3D printing processes involved. However, there are an increasing number of cases where this is being done, and it will become more common over the next few years.” Radiology is typically involved in this subcategory, as the printing is based on patient imaging data.

Category 3 3D printing produces advanced “biomaterials,” without cells, that are intended to transform into functional biological tissues after implantation in the body. “In this case, the cells and tissue of the body recognize the materials/structures in such a way as to transform them into natural target tissues,” Jakus explains. “Thus, objects in this category are intended to change with time. This is the category most people are unfamiliar with, as it is not widely known that completely new materials can be designed and 3D printed, rather than just designing geometries for 3D printing. This is primarily what my company and I focus on. We have designed new 3D printing processes—3D Painting—and advanced biomaterials like Hyperelastic Bone, 3D Graphene, Fluffy-X, and others. This is still an emerging area and not used in hospitals or approved medical products yet, but this will change in the next few years. It will likely be five to 10 years, however, before this is being used by radiologists in the hospital.”

Category 4 3D printing uses living cells and tissues. “This is bioprinting, by definition,” Jakus says. “We also do this at Dimension Inx, and there is a lot of academic research activity in this area.” He adds that printing with living cells presents many technical and logistical challenges. For one, unlike medical models or anything from the other three categories, the object that is printed can die or become infected. Additionally, the 3D processes themselves can kill or damage living cells or tissues.

These categories represent a progression, Jakus says. The first category was once an area of select research but is now being widely used in hospitals. Categories 3 and 4 are, for the most part, still in the research realm.

Jakus notes that bioprinting has come a long way in the past 20 years, but it is still a long way from hospital usage. Eventually, as bioprinting progresses, becomes more standardized, receives regulatory approvals, and develops its workforce, it will become a useful tool for radiologists and their teams. However, “this is still likely more than 15 to 20 years away, if not longer,” Jakus says. When it happens, implications for patient care will be huge, he adds. “I believe it will also transform the discipline and/or result in radiology subdisciplines that are focused on ‘biodesign’ and ‘biofabrication.’”

Currently, Jakus’ company is involved in research, using its 3D-painted biomaterials for a variety of hard and soft tissue and organ repair and regenerations. As far as the bioprinting future, Jakus says standards, regulatory development, and manufacturing logistics are in development. Base technologies are established, but they are just beginning to become commercially viable.

Long Road to Tissue Mimics
Prachi Bogetto, segment leader for diagnostics and global marketing for cell analysis at Cytiva, formerly GE Life Sciences, agrees with Jakus about his assessment of certain bioprinting capabilities being a long way off—perhaps even longer, as it enters a realm once considered only as science fiction. Bogetto believes, however, that researchers should be able to use bioprinting to accomplish drug discovery and develop effective therapies faster.

“But, if you look well down the road—as we are only in the very early stages of doing anything like this—it would be incredible to print replacement bones,” Bogetto says. She points to knee replacement as an example. “Typically, you perform clinical imaging, a knee arrives for the patient, but there are about five different ones, as it’s difficult to know which will work best,” she says.

In this scenario, the surgeon determines which knee fits best during surgery. One is used; the rest are discarded. “In the distant future—we are a long way from this—the goal will be to image and bioprint for perfect fit,” she says.

An even more fascinating challenge is the shortage of transplant organs. “Many years down the road, when we come to understand how to print in a way that mimics the human body, we could find a way to create compatible organ replacements—the kidney, for instance. Again, that’s far into the future, as a lot of work needs to be done that involves complete understanding of vascularization and tissue differentiation,” she explains. The necessary scientific detail is daunting, but the promise is there, she asserts.

To help fulfill that promise, Cytiva formed a partnership with Advanced Solutions Life Sciences (ASLS) to create new opportunities for the manufacture of regenerative tissues. When the partnership was announced, Cytiva was still known as GE Life Sciences. It changed its name when it became part of the Danaher Corporation Life Sciences platform on April 1, 2020. Bogetto describes the reason the companies partnered.

“Both realized the large promise of 3D bioprinting for various applications. ASLS has a revolutionary system for performing bioprinting that mimics behaviors that happen within the human body,” Bogetto says. “Cytiva possesses imaging expertise—specifically, live cell imaging—such that you can bioprint something that is alive and changing. If you can see this in great detail, you can better understand the structure that’s printed, not just what it looks like but how it behaves.”

The companies combined Cytiva’s IN Cell Analyzer 6500HS with ASLS’ BioAssemblyBot. The strategic research and development and distribution partnership seeks to individualize regeneration of tissue, eg, bone, soft tissue, and organ replacements. The combined technologies will insert assessments—at the cellular level—into 3D bioprinting workflow that create human tissue models. The endeavor may sound straightforward when stated, but significant challenges are involved. Bogetto says the biggest challenge is coaxing bioprinted cells and structures to organize themselves and mimic biological growth.

“We thought that if we worked together with the combined ASLS technology and our live cell know-how, we could solve this fundamental problem,” Bogetto says. “Printed models have been used for an exceptionally long time for things like drug safety testing. It is certainly faster and easier to test drugs on self-growing tissues than any other way. For that reason, for cells printed outside of the body, it can be very much like cells inside the body. It is one thing to have a group of cells, but can you get those cells to vascularize and behave and organize like tissue? And can you get those tissues to organize into more biological systems? Right now, we can get individual cells and individual groups of cells to behave very much like they behave inside the body, in a very isolated way in isolated applications. That part, we got. The next part is the more complex structures, outside of the body.”

Also, bioprinted tissues are small and quickly die, due to an inability to create small blood vessels. Now, the ASLS Angiomics technology enables the self-assembly of bioprinted microvessels into functional capillary beds. These vessels supply nutrients, oxygen, and hormones to the 3D tissue model and remove waste. The partnership is aiming to make this happen in a single process. What helps make it possible is that the platforms from the two companies offer higher levels of speed, efficiency, and quality. This sets them apart from traditional bioprinters.

Ultimately, the partners hope to create a new product that integrates the IN Cell Analyzer confocal imaging platform, IN Carta cell analysis software, and ASLS’ BioAssemblyBot 3D bioprinter with TSIM design software.

Live vs Synthetic Tissues
Currently, Stratasys’ digital printers are not configured to print live tissue, Drikakis says, but the technology is used for synthetic twin simulation of different anatomies and pathologies. Drikakis says there are three components to the digital anatomy solutions: printer, materials, and software.

“The software is probably the most critical piece,” he says. “It allows us to select different anatomies and pathologies and automatically assign material durometers that replicate the selected anatomy, as opposed to just selecting soft or rigid materials, for example.”

Three materials are involved. Tissue matrix is designed to replicate myocardial tissue with clinically validated, favorable comparisons to native tissue. Gel matrix is material that can be used to replicate lubricity in vasculature and allow postprocessing of small/fine inner diameter anatomy. Bone matrix is for orthopedic applications.

“But, today, the technology is not used for actual digital twin manufacturing, such as bioprinting,” Drikakis says. “Rather, it is more the synthetic simulation for education or training.”

Stratasys’ latest technology is its Digital Anatomy 3D printer material, which was released in October 2019. It was recently involved in a study that compared myocardium—tissue comprised of heart muscle cells located between the outer layer of the heart wall (epicardium) and the inner layer (endocardium)—made from Stratasys’ printer with actual porcine myocardium. The researchers determined that Stratasys myocardium performed similarly to porcine myocardium—and better than any other similar 3D-printed materials currently marketed.

The company’s Digital Anatomy solutions combine the aforementioned materials with Stratasys’ J750 printer and custom software inside GrabCAD. This enables users to turn segmented 3D scans into 3D-printed anatomical parts for training and surgical planning purposes. The Digital Anatomy materials very closely mimic the mechanical properties of human tissue. Medtronic, a medical device company, conducted the research, seeking to evaluate materials that mimicked myocardial tissue.

A paper detailing the research described the attempt to quantify and compare application-specific mechanical properties of the Digital Anatomy materials with equivalent porcine tissue. Researchers elected to compare human with porcine myocardial tissue due to porcine tissue’s availability, similarity to human tissue, and previous use for preclinical testing of cardiac devices.

“The study wasn’t just about what it could do; it was a baseline assessment of what it couldn’t do, as well,” Drikakis explains, adding that this was the first study of its kind and the first step in providing model comparison data to the 3D-printed materials market.

“There is a lot of research coming from radiology departments throughout the United States. Much of it involves members of the RSNA Special Interest Group. They’re collecting data that quantify how 3D printing benefits clinical outcomes,” Drikakis says. “So, this is really a radiology-driven effort in health care. When we use a 3D printer, the data come from an MRI or CT scan. The DICOM data are a source of truth. Radiology departments segment the data, and then they convert them into a format that allows them to 3D print it.”

Radiology departments work closely with surgeons through the entire process. The RSNA SIG developed clinical appropriateness guidelines for the use of 3D printing for surgical planning that help outline where 3D printing may or may not be appropriate. These are critical elements that need to be identified in order for 3D printing to receive reimbursement. Category III CPT codes were established in 2019 as an indication of the interest and positive outcomes associated with 3D printing. Expect to see more as the field develops.

“The goal of the digital model is to help with research in creating a digital biological twin that won’t affect a patient in a simulation environment,” Drikakis says. “This previously hasn’t existed.”

3D printing is also serving education and training with the creation of a digital library, he adds, and logistics have been simplified to positive effect, including costs, sourcing of cadavers with specific pathologies, and the ethical considerations of handling animals.

— Dan Harvey is a freelance writer based in Wilmington, Delaware.