Aspiration Assisted Bioprinting: A New Method for Precise Printing of Tissue Spheroids

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In the fields of tissue engineering and regenerative medicine, an increased interest and use of bioprinting technology is changing the way researchers are conducting investigations. To satisfy particular requirements of their research, many labs around the globe are creating customized bioprinters, developing new bioprinting techniques, and even producing their own bioinks. In the past few years, we have been following news from the Pennsylvania State University (Penn State) biomedical engineering experts as their projects advance bioprinting and fabrication of tissue scaffolding to create living tissue. In their latest development, researchers at Penn State’s College of Engineering have hatched a new method of bioprinting that uses aspiration of tiny biologics – like spheroids, cells, and tissue strands – to precisely place them in 3D patterns, either on scaffolding or without, to create artificial tissues with natural properties.

The method, called aspiration-assisted bioprinting (AAB), enables picking and bioprinting biologics in 3D through harnessing the power of aspiration forces. When coupled with microvalve bioprinting, it facilitated different biofabrication schemes, including scaffold-based or scaffold-free bioprinting at an unprecedented placement precision.

Penn State researchers published an article on their tissue engineering work, titled, “Aspiration-assisted Bioprinting for Precise Positioning of Biologics,” in the journal Science Advances, whereby Ibrahim T. Özbolat, co-author and Hartz Family Career Development Associate Professor of Engineering Science and Mechanics, and his team used their new AAB method along with conventional micro-valve printing to create homogeneous tissues and tissues containing a variety of cells.

“Tissue spheroids have been increasingly used as building blocks for the fabrication of tissues, but their precise bioprinting has been a major limitation,” said Özbolat. “In addition, these spheroids have been primarily bioprinted in a scaffold-free manner and could not be applied for fabrication with a scaffold.”

Using scaffolding is necessary for many applications in regenerative medicine and tissue engineering and also in the fabrication of microphysiological systems for disease modeling or drug screening. Scaffolds play an important role in creating a 3D environment to induce tissue formation; in fact, the application of scaffolding materials together with stem cell technologies is believed to hold enormous potential for tissue regeneration.

 

According to Penn State, AAB uses the power of suction to move tiny microscopic spheroids. Suggesting that, just as one could pick up a pea by placing a drinking straw on it and sucking through the straw, AAB picks up the tissue spheroid, holds the suction on the spheroid until it is placed in exactly the proper location, and then releases it.

“Of course, we have to gently aspirate the spheroids according to their viscoelastic properties so no damage occurs in transferring the spheroids to the gel substrate. The spheroids need to be structurally intact and biologically viable,” Özbolat explained. “We demonstrated for the first time that by controlling the location and distance between spheroids we can mediate collective capillary sprouting.”

It appears that by controlling the exact placement and type of spheroid, the researchers were able to create samples of heterocellular tissues – those containing different types of cells – as well as create a matrix of spheroids with capillary sprouting in the desired directions. Furthermore, they describe capillaries as necessary for the creation of tissues that can grow and continue to live, so they are a means of delivering oxygen and nutrients to the cells, without which, cells would die. Without capillaries, only the outermost cells will receive oxygen and nutrients.

Penn State specified that the precise placement of spheroids also allows the creation of heterocellular tissues like bone and that by starting their investigation with human mesenchymal stem cells, the researchers found that the cells differentiated and self-assembled bone tissue.

In the paper, the investigators describe how they modified a MakerBot Replicator 1 (which cost them less than 1000 dollars) to develop the new AAB platform, which operates a custom-made glass pipette that is used to “pick up” biologics and 3D bioprint them into or onto a gel substrate. The extrusion head of the MakerBot was removed and a holder for a pipette and two microvalve heads were 3D printed using an Ultimaker 2 3D printer. The AAB was coupled with microvalves for droplet-based bioprinting of functional or sacrificial hydrogels. And to bioprint spheroids onto a sacrificial hydrogel (such as alginate), microvalve bioprinting and aerosol cross-linking processes were used. To control the 3D motion stage, an Uberlock smoothie board was integrated.

The AAB platform, where micro-valves and a customized pipette head were integrated on the print head (Image: Bugra Ayan, Penn State)

In addition to spheroids, Özbolat suggested that there may also be other uses for this system. In fact, other living cells and tissue building blocks could benefit from this method, including electrocytes from electric eel and tissue strands that can be bioprinted for a wide variety of applications, such as tissue engineering, regenerative medicine, drug testing and pharmaceutics, disease modeling, microphysiological systems, biophysics, and biocomputing.

Actually, the ability to produce artificial living tissues is valuable in areas outside of regenerative medicine. Frequently, tissue samples are necessary to test drugs or screen other chemical products; other disciplines include: microfluidics, in vitro human disease models, organoid engineering, biofabrication and tissue engineering, biocomputing, and biophysics.

This approach is presented as the first bioprinting method that allows high-precision bioprinting of spheroids in both a scaffold-free and a scaffold-based manner. With this custom-made bioprinter, the researchers claimed to be able to exploit it for multiple applications, since they consider that the precise positioning of spheroids can also be critical for applications such as building organ-on-a-chip devices where the proximity of spheroids to each other or a perfusion channel can be crucial for the viability and function of spheroids as well as the robustness of the developed system.

The paper proposes that “organoids or spheroids are currently loaded using manual approaches, which may reduce the repeatability of system outcome measurements (such as measured insulin in circulating perfusion media in a pancreas-on-a-chip model).” While AAB proposes “a more effective strategy, taking advantage of a simple-to-use, cost-effective and reproducible tissue bioprinting platform.”

According to the researchers of the project, which was supported by the National Science Foundation, the National Institutes of Health and Penn State’s Materials Research Institute, the system still needs improvement to print spheroids in high-throughput to create larger tissues in a shorter time. Actually, scalable tissue fabrication is one of the most important aspects of many bioprinting research initiatives. To create scalable, structurally‐stable tissue constructs in the future, customized bioprinting methods like this one can help to eliminate certain roadblocks in the field. There are some remarkable advances in bioprinting and we are ecstatic to hear more about them, as well as the challenges that researchers are facing to make this new field one of the most relevant and necessary to advance ideal medical treatments as well as so many other disciplines.

A CAD model of the AAB platform developed using Autodesk (Image: Penn State)

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