This story was originally published by Knowable Magazine.
What you see in the image above is a lobe of a liver, times two
— on the right, a flesh-and-blood one, removed from a transplant
donor, and on the left, one created from plastic to represent bile
ducts, arteries and veins, laid down, layer by layer, by a 3-D
printer. The goal of such technology: to help surgeons plan and
practice complex procedures and train new surgeons with simulators
that respond as a patient would.
Surgeons navigate complex anatomical terrain as they manipulate
scalpel and suture to cut and stitch precisely and quickly. Their
job is made harder by the fact that human anatomy is far from
uniform. To properly prepare, they routinely use two-dimensional
images from computerized tomography (CT) scans or magnetic
resonance imaging (MRI) to plan.
But increasingly they are turning to realistic 3-D models that
are specific for individual patients.
Such models are already used to educate patients, to do general
training and to plan and practice especially difficult procedures.
But in the future, 3-D models, be they physical or virtual, could
become routine tools for training surgeons or mapping procedures in
From 2-D to 3-D
To 3-D-print, researchers first combine the many successive
digital 2-D slices from CT or MRI scans into a topographic map that
highlights the complex structures at different levels of the organ.
The printers then build the models, layer by layer — sometimes
using an inkjet to deposit droplets of resin that are fixed into
place by shining UV light, or by extruding polymer ribbons that
harden once they’re released.
The technology was first developed in the 1980s, when such
printers were expensive and temperamental and materials were
limited. But in the last few years, advances have made it
affordable even for home users, and improvements in software and
printing methods have enabled scientists and engineers to print
complex mixtures of colors and textures with high accuracy, thus
creating far more accurate and realistic organ models, as outlined
in an overview in the 2018 Annual Review of
Cleveland Clinic gastroenterologist Nizar Zein first considered
the idea of printing organ models in 2012 after reading about
people constructing houses with 3-D printers and the technology’s
potential uses in space exploration. He wondered if the method
could make liver transplants from living donors safer.
Each liver has a unique, complex web of arteries, veins and bile
ducts, and a misplaced cut can lead to complications, even death,
in donor or recipient. So Zein assembled a team of clinicians,
imaging experts, engineers and software designers to produce a
patient-specific printed liver from resin to guide the surgical
The first prototype was crude — “really like a child playing
with Play-Doh,” Zein recalls. Less than three-quarters life size,
it wasn’t fully transparent and lacked color codes for different
tissue types. But it was promising enough to show to surgical
colleagues as they discussed a high-profile case where a liver
donor suffered a dangerous complication on the operating table. One
of those surgeons, Zein recalls, said that such a model might have
saved the donor.
Zein refined the model, and in 2013 started studying how a
life-size 3-D liver model, in addition to 2-D scans, would change
how surgeons planned operations. An initial small study showed that
their models matched the live organs in both
anatomy and shape. Zein and his team’s models have been used in
more than 20 surgeries. In many cases, looking at the model caused
surgeons to change how they cut into the organ, Zein says, and in
one case led the surgeons to conclude that the donor wasn’t
Zein’s team has gone on to print 3-D models of complex liver
tumors to understand how they are connected to the organ, and thus
inform surgical planning. “The more we know in advance about the
patient’s anatomy and structures, the better off the surgery would
be,” Zein says.
Soft models that bleed
Organ models can also help train doctors. Urologist Ahmed Ghazi
of the University of Rochester was inspired to build realistic
kidney models that would provide an immersive way to simulate
surgeries. “I just wanted something that looks like a kidney that
bleeds,” he says. Kidney surgeons are often faced with removing
tumors from an organ riddled with blood vessels, and may have just
30 minutes to complete their work before kidney tissue, clamped off
from circulating blood, starts to die.
To build a kidney model, Ghazi’s team layers simulated fat,
intestines and other tissues in an abdomen-like cavity, just as
they would be situated within a patient. “The surgeon is able to do
the procedure from the beginning to the very end,” Ghazi says. He
and his colleagues have made general models for teaching as well as
ones from scans of individual patients for simulating specific
surgeries. Ghazi has tested this system with five expert and ten novice
surgeons on a common but challenging procedure for breaking up
large kidney stones. Experts found the model very realistic, and
the novices unanimously agreed that it would help to practice with
these models before surgery.
Biomedical imaging expert Nicole Wake, now at the Albert
Einstein College of Medicine in New York City, also has looked at
how kidney models affect surgery planning. In a 2017 study, she and
her colleagues asked three experienced surgeons to review ten
different complex kidney surgeries. They first reviewed 2-D
images from the patients and described their surgical plan. A week
or more later, they repeated the exercise with a 3-D model. In all
cases, at least one of the surgeons then altered their strategy —
changing how they would access or clamp the organ, for example —
and reported greater confidence in their plan.
Materials make the model
In building 3-D organ models, the choice of material depends on
its intended use, Zein says. Hard plastic is cheaper and adequate
for simple 3-D visualization where clinicians want to zero in on
visual features — a tumor’s shape, say, or the curving paths of
blood vessels or ducts.
But spongier, more flexible materials including silicones, soft
plastics and hydrogels are more realistic. Their springiness can
mimic the mechanical properties of living tissue, providing a
practice organ that surgeons may slice open, gauging width and
depth of the necessary cuts to clear out tumors and guide repairs.
Softer models can also incorporate other features, such as pressure
sensors, that give surgeons more information as they prepare.
Ghazi and colleagues have created a range of tissue-textured
models. Instead of printing them directly, the team uses the 3-D
printer to create a series of detailed molds. They then inject
specialized hydrogels — jiggly plastics that in their recipe
consist of 70 percent water — that they tune to respond like
muscle, fat or blood vessels. They even incorporate fluids so that
slicing blood vessels or other ducts makes the organs bleed or seep
as they would in an actual operation.
Mechanical engineer Michael McAlpine of the University of
Minnesota has produced organ models of prostates that mimic the
tissue’s mechanical properties, as training tools for surgeons. His
team used samples of prostates removed from cancer patients to test
properties such as firmness and flexibility. They replicated these
features by adjusting the silicone ink’s chemical components to
change how many links the polymers make between chains, rendering
the models softer or stiffer.
They’ve even outfitted the prostate models with printed,
pressure-sensitive sensors made from hydrogels and
rubbery silicone. The sensors can measure the force applied by
endoscopes or surgical scissors, providing doctors with useful
information before they enter the operating room.
From physical to virtual
It’s unclear how many US hospitals are printing 3-D organ
models. “I suspect most medium to large hospitals are at least
trying it out,” says William Weadock, a University of Michigan
radiologist and chair of the Radiological Society of North
America’s 3-D printing special interest group. Medical centers with
the equipment and expertise can now print some types of organ
models for no more than a few hundred dollars for a hard plastic
model. And companies such as Lazarus 3D and Materialise now offer
to produce organ models from imaging data.
But regulatory agencies and health insurance companies are still
catching up with this technology. Just one software package,
produced by Materialise, is approved by the US Food and Drug
Administration for constructing the printing files for diagnosis in
patients, and insurance doesn’t pay for such patient-specific
Printed organ models might turn out to be a stepping-stone
toward immersive computer-based models that use augmented reality
wherein surgeons use headsets and other tools to observe and
manipulate 3-D representations. Originally, Ghazi had wanted to
start his work this way, but he soon moved to physical models: It’s
not yet possible to cut something virtually with the right “feel,”
or to know how much an organ might bleed if cut in one spot or
another. For now he is working with virtual reality companies —
offering physical models as realistic tools for developing
programming software for virtual surgeries.
In time, practice surgeries using 3-D imaging — be it augmented
reality or printed, physical models — could become the norm rather
than the exception, McAlpine says. “I think that will become