Tissue Bioprinting: A granular approach

The field of bioprinting aims to fabricate tissues and organs that can be used to model drug efficacy and disease mechanisms, repair and ultimate replace dysfunctional parts in living beings. It utilizes computer aided design to create a construct that mimics native tissue, by adding step-by-step every detail of the complex multiscale hierarchical structure. This approach, demands the ability to be able to a) design a-priori the features of the tissue, and b) have the ability to actually create the features of that resolution. In extrusion bioprinting, hydrogels containing cells are used as the bioink, and then the desired structure is extruded through a nozzle forming the desired pattern. The hydrogel matrix attempts to mimic the extracellular matrix (ECM), and the cells attempt to grow within that matrix. However, creating a hydrogel that can mimic the dynamic nature of ECM, which changes in response to cellular behavior is non-trivial. Moreover, the resolution necessary is in the order of 10-100μm which leads to the challenge of fabricating very small nozzles, and ensuring that the shear stress during extrusion remains minimal.
This thesis describes i) an approach where particle suspensions are 3D printed in an
embedding bath and result in self assembled complex tissue fibers that are a fraction of a nozzle, utilizing a first time measured phenomenon of diffusive packing, ii) the invention of a platform where controlling the volume fraction of the particle suspensions allows them to transition reversibly between granular bioinks and embedding baths, as well as a newly observed intermediate state of freestanding embedding baths, iii) the 3D printing of bioactive microgel inks into bioactive embedding baths that cells can populate the granular interstitial space iv) the
invention of a freeze-dying method that allows those granular architectures to form
modular blocks that can be used as tissue blueprints, and have an extended shelf life, a necessary component for clinical translation. Together, this thesis presents how to 3D print cells and particles that can self-assemble into tissues, how to make blueprints for tissues, surpassing current customization and resolution limits and how to preserve them into a shelf steady state so that one day they can be translated into a clinical application.