Researchers at Carnegie Mellon University in Pittsburgh have developed a new 3D bioprinting method that can produce any part of the heart, from tiny capillaries through to full-scale heart components.Using human cardiac muscle cells, the team could even make a structure that contracts and functions similarly to a real heart.
"What we've shown is that we can print pieces of the heart out of cells and collagen into parts that truly function, like a heart valve or a small beating ventricle," said Adam Feinberg, the Biomedical Engineer whose lab conducted the research.
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Collagen a major protein that maintains structural integrity
The specialized cells that make up various body organs are held together by the a network of proteins called the extracellular matrix.
Collagen, a major protein that plays a key role in this structural integrity, is a highly desirable biomaterial for engineers working on 3D bioprinters. However, it has proved difficult print because it starts out as a fluid, so printing it in the air only results in a puddle forming on the printing platform.
The technique that Andrew Hudson and colleagues have developed employs a supporting hydrogel that stops the collagen deforming.
“Indeed, 3D bioprinting up to now has generally been limited to very thin and tiny scaffolds. In our work we have solved this problem by 3D bioprinting inside a ‘support’ gel that prevents the cells and hydrogels (similar to gelatine) from collapsing,” says Feinberg.
Using the new technique, the researchers say that, for the first time, it is possible to 3D print collagen with the same ease with which metals, plastics, and other materials can be printed.
“Being able to do this means that we can now print scaffolds and tissues that actually have biological function,” says Feinberg.
A FRESH technique
The technique, called FRESH (for freeform reversible embedding of suspended hydrogels), was first proposed in 2015 but has now been improved to use changes in pH to assemble collagen and force it to solidify.
The technique works by extruding bioinks that contain unmodified collagen from a needle in the printer, which contains a bath made up of gelatine microparticles that serve as a support during printing. The needle extrudes thin layers of collagen and deposits them on top of each to form a 3D solid structure.
Once the printing is complete, the researchers remove the supporting hydrogel by simply heating it to room temperature, which gently melts the support and leaves the printed collagen structure in place.
Creating functioning cardiac blood vessels, valves and ventricles
The researchers say the technique can produce complex, proof-of-concept, functional architectures that can be embedded with living cells or vascular-like networks at resolutions of up to 20 microns.
Using cardiac muscle cells (cardiomyocytes) and heart imaging data, they were able to recreate blood vessels, heart valves that open and close and ventricles that contracted synchronously and propagated action potentials in a specific direction – just like real cardiac ventricles do.
The wall of the printed heart also thickens by as much as 14% during peak systole, again similar to what happens in the real organ.
Patient-specific anatomical structures
Feinberg and team say they can also use the imaging data to accurately reproduce patient-specific anatomical structures:
…We start with a magnetic resonance imaging (MRI) or micro-computed tomography (CT) image of a patient’s entire heart or just a component, such as a heart valve. This allows us to create a 3D computer model matched to the patient. Next, we use the computer to convert the 3D model of the heart valve, for example, into instructions that tell the 3D printer what to do.”
Adam Feinberg, Biomedical Engineer
He adds that currently, no other techniques are available that can 3D bioprint collagen with the resolution and fidelity achieved with FRESH.
Furthermore, the technique can also use unmodified collagen, meaning it can produce pure collagen and use changes in pH to transform the liquid collagen into a gel. “Other methods that have been able to print collagen typically needed to chemically modify the collagen or mix it with other materials to improve printability.”
Potential applications are “really exciting”
For the current study, which was recently published in the journal Science, the team focused on reproducing the heart and its component. However, what is really exciting, says Feinberg, is that since collagen is the major protein component in almost every tissue and organ in the body, it has broad applications across many areas of tissue engineering and regenerative medicine:
“We have also developed custom bioprinters for FRESH and released the designs as open-source hardware that we hope the research community will widely adopt. This will hopefully lead to new research and ultimately treatments for a range of diseases in the years to come.”
The team says that as well as working on other parts of the body including collagen scaffolds to repair damaged skeletal muscle and regeneration of the trachea, they are now focusing on building more complex cellularized models of the heart.
This will help them to better understand how the heart muscle forms and functions and possibly how to replace damaged regions of the heart in the long term.
However, although the current work represents another promising step forward in 3D bioprinting, there remains a huge gap between printing these heart components and building a complete, fully functioning 3D-printed heart that could help to address the worldwide shortage of donor organs.
It is important to understand that there are many years of research yet to be done. But there should still be excitement that we're making real progress towards engineering functional human tissues and organs, and this paper is one step along that path.”
Adam Feinberg, Biomedical Engineer
Dasgupta, D. & Black III, L. D. (2019) A FRESH SLATE for 3D bioprinting. Science. DOI: 10.1126/science.aay0478