Researchers at Penn State have developed a granular hydrogel that contains both hydrogel microparticles and self-assembling nanoparticles, and which could be highly suited for bioprinting purposes. The concept involves the nanoparticles becoming adsorbed onto the hydrogel microparticles and reversibly adhering the microparticles together, providing a printed gel structure that is porous enough to permit cell viability, but which maintains a desired shape and mechanical properties. Unlike conventional hydrogels, which consist of long polymer strands that are interlinked and surrounded by water, and which require a substantial trade-off between their porosity and their ability to maintain shape, the new bioink finds a sweet spot, allowing both porosity and shape-fidelity. Bioprinting offers great opportunities in addressing the transplant shortage. Simply printing a new organ to order could revolutionize how we deliver medicine. However, printing viable tissues or organs is no small task, and a significant amount of research is devoted to fine tuning the properties of such printed materials so that they are best suited for their purpose. Many bioinks trialed to date consist of bulk hydrogels. These conventional materials typically consist of an interlinked network of long polymer strands that is infused with significant amounts of water. While the material can be tuned to maintain its shape after printing, typically this results in diminished porosity, which limits the influx of biological fluids carrying nutrients and oxygen for cells within the gel. This essentially means that there is a trade-off between the mechanical properties of conventional hydrogel bioinks and their ability to support living cells. “The main limitation of 3D bioprinting using conventional bulk hydrogel bioinks is the trade-off between shape fidelity and cell viability, which is regulated by hydrogel stiffness and porosity,” said Amir Sheikhi, a researcher involved in the study. “Increasing the hydrogel stiffness improves the construct shape fidelity, but it also reduces porosity, compromising cell viability.” To address this, these researchers have turned to another type of hydrogel that consists of small granules that are packed together. Their hydrogel consists of gel microspheres that are mixed with self-assembling nanoparticles. The microspheres are sticky and will pack together when printed, and the self-assembling nanoparticles also help them to bind together into a cohesive shape. This mixture forms a porous gel, but still maintains its shape. “Our work is based on the premise that nanoparticles can adsorb onto polymeric microgel surfaces and reversibly adhere the microgels to each other, while not filling the pores among the microgels,” said Sheikhi. “The reversible adhesion mechanism is based on heterogeneously charged nanoparticles that can impart dynamic bonding to loosely packed microgels. Such dynamic bonds may form or break upon release or exertion of shear force, enabling the 3D bioprintability of microgel suspensions without densely packing them.” Source
