New Technique Uses Hydrogel Particles to Build 3D Structures
Minecraft is one of the world’s best-selling video games, with 126 million active players across the globe.
In the 3D-mosaic world of Minecraft, everything – animals, houses, even the sun and the moon – is made of small cubes or voxels, the basic building blocks for 3D structures. Players customize the voxels with various functions and colors to construct their own art works. The only limitation is the players’ imagination.
An interdisciplinary team of researchers in the University of Virginia’s School of Engineering and Applied Science and School of Medicine adopted Minecraft’s voxelated approach to advance the field of 3D bioprinting, where the goal is to engineer 3D structures that mimic the geometry, texture, and function of human tissues and organs. Liheng Cai, an assistant professor of materials science and engineering, chemical engineering and biomedical engineering, leads the team.
Their paper, Digital Assembly of Spherical Viscoelastic Bio-ink Particles, termed DASP, is featured as a cover article in Advanced Functional Materials. The team has earned funding from the National Science Foundation Division of Materials Research-Polymers, the American Chemical Society Petroleum Research Fund, UVA LaunchPad for Diabetes, the Virginia Commonwealth Health Research Board and the UVA Center for Advanced Biomanufacturing to support their research.
Their technology uses small spherical bio-ink particles (i.e., voxels) as the basic unit to create 3D structures. DASP offers an alternative to available bioprinting techniques in which researchers mix living cells and biopolymers to create bio-inks, shape the bio-inks into filaments, and then assemble the one-dimensional filaments layer by layer.
Because voxels are effectively zero-dimensional compared to filaments, they offer greater versatility, opening new possibilities for biomedical engineering.
“It is very challenging to print bio-ink particles because they are very sticky,” Cai said. “Because the bio-inks have the consistency of honey, it is hard to control when and how they detach from a printer nozzle. This becomes even more challenging when the bio-ink particles are as small as the diameter of a strand of human hair. A second challenge is to manipulate each particle into place to build a 3D structure.”
To solve the first challenge, instead of creating droplets of bio-inks in air, they shape a droplet in a slurry of gelatin microparticles. The slurry is solid-like without stress, but turns to a liquid when being pressed. They use this slurry to trap a bio-ink droplet when it is extruded from the 3D printer nozzle, then detach the nozzle quickly to liquefy the supporting matrix, leaving a bio-ink particle behind.
A good bio-ink is both elastic and semi-fluid, a characteristic of soft materials referred to as viscoelasticity.
“You need to make particles that maintain a round shape as they detach from the printer nozzle,” said Jinchang Zhu, lead author and Ph.D. student in Cai’s Soft Biomatter Laboratory. “If the particles are too elastic, they will be deformed into a long thin strand instead of being a ball.”
The team uses a mixture of short and long polymers, in which the long ones can entangle with each other like spaghetti, whereas the short ones can be crosslinked to form a network. They control the amount and the length of the long polymers to control the viscoelasticity.
The team uses these bio-inks to precisely deposit each droplet in a 3D space without perturbing the surrounding printed structures. Then, they swell the droplets to ensure the neighboring ones gradually come into contact, forming a whole structure composed of interconnected yet distinct particles.
“Because the particles are distinguishable, the printed 3D structure is porous,” Zhu said. Porosity allows efficient nutrient and oxygen exchange that is critical to biomedical research and its related applications.
Collaborating with Drs. Yong Wang and Jose Oberholzer, professors in the Department of Surgery at UVA’s School of Medicine, the team is using DASP to develop new treatments for type 1 diabetes. Zhu and Wang’s Ph.D. student Yi He, who co-first authored the team’s paper, applied the 3D printed porous structures to encapsulate human islets. They found that the voxelated 3D structures show fast, sustained insulin secretion in response to glucose stimulation.
“This is very exciting, as this technology has the potential for human islet transplantation by providing cell-based therapy to treat type 1 diabetes,” Dr. Oberholzer said. “We are optimizing this technology to encapsulate islets to test their capability in reversing type 1 diabetes in mice.”
There is still a lot more to do. “We cannot yet precisely define the properties of each particle as Minecraft does for each voxel,” Cai said. “But this technology is the first step toward 3D printing tissue with the complexity and organization needed for biomedical engineering, drug screening and disease modeling.”