3D Bioprinting of Hydrogel Constructs for Integration with Islet Organ-on-Chip System

One of the biggest medical problems the world faces today is the lack of viable organs available for transplantation. As recently as February 2021, in the US alone over 107,000 patients were waiting on a transplant¹. It is becoming increasingly evident that while the need for organs steadily increases the supply remains stagnant.One way to bridge this gap between organ supply and demand is by 3D bioprinting of tissues from a regenerative cell source. The advantage of this technology is that layers of cell-embedded biomaterials are printed into structures designed to mimic tissue constructs. By utilizing the biomaterial in the form of a printable bio-ink, the cells embedded can be precisely spatiotemporally placed, encouraging the subsequent growth of tissue. In parallel to organ replication, 3D bioprinting can also be integrated with disease modeling in a micro physiological system, by providing a structure that can replicate a human organ on a smaller scale. There are several challenges however, when working with 3D bioprinting, one of them being the need for a stable bio-ink that can support cell growth and function, while providing the desired resolution and structure. Achieving a compatible bio-ink often requires extensive trial and error testing, even before the addition of cells to the bio-ink, or the usage of crosslinking agents to stabilize the printed structure.

The current aim of this project is to print a bio-ink scaffold with embedded islets using conditions recommended by a predictive resolution model, and subsequently integrate it with a micro physiology system towards disease modeling of diabetes. By using an analytical model to predict the printability of a cell-laden bio-ink composition using bio-ink factors, printer settings, and printed structure environment conditions, it is possible to narrow down the parameter space to find compatible properties of bio-ink before the addition of cells. This will enable a scaffold to be printed onto an organ-on-chip membrane permitting increased perfusion for a more compatible islet environment.

The initial analytical model inputs the bio-ink properties of temperature, component concentrations, and viscosity, the printing properties of nozzle size, extrusion speed and pressure, and the post printing properties of spreading, crosslinking concentration, and type of substrate to be printed on. From these variables, a structure size and resolution can be determined that would most efficiently be perfused in the islet on chip device, while ensuring steady flow of media and adequate oxygen supply. Several variations of an alginate/gelatin bio-ink were tested, as this mixture ensures initial compatibility with pancreatic islets, and are self-supporting once deposited and crosslinked. All inks were printed using the Cellink BioX printer with CaCl₂ as the crosslinking agent. Following polymerization of the printed structure, it was placed on a membrane and loaded into a Micronit lab-on-chip. The printed structure was then monitored over an extended period of time, in order to ensure that successful media diffusion and oxygen perfusion within the printed structure could be achieved. Currently, these findings are being furthered by printing the structures with embedded islets, and monitoring islet viability and oxygen perfusion within the printed structure on the membrane. It is expected that 3D printing islets on the microfluidic device will significantly reduce the variability associated with manual loading of islets on the device – a necessary step towards disease modeling and drug testing. While our goal is todemonstrate success in showing cell viability and proliferation for islets in printed organ-on-chip structures, this novel experimental setup can easily be applied for a variety of other tissue types.

¹â€œOrgan Donation Statistics.” OrganDonor.gov, Health Resources and Services Administration, 25 Feb. 2021, www.organdonor.gov/statistics-stories/statistics.html.