Bioprinting: From concept to reality.

Organovo's NovoGen MMX Bioprinter. Image: OrganovoThe human cell represents the smallest functional unit of life. All tissues in the body are composed of multiple cell types, typically arranged in a 3-D architecture that is relevant to the functions they carry out. Since cells were first isolated and grown in the laboratory environment, biologists and engineers have pursued the utilization of these tiny building blocks in the reconstruction and regeneration of functional tissue. Whether used in a controlled laboratory setting to model specific diseases and test the effects of drugs, or delivered into the body as therapeutics for the treatment of disease, the common goal is to establish or re-establish in vivo-like function.

The field of tissue engineering has deployed several fabrication strategies aimed at bringing cells and structure together to generate tissue. Biomaterial scaffolding—which provides structural support and can be formed into biologically relevant shapes—has been combined with cells to generate hybrid 3-D structures for use as tissue surrogates in vitro and in vivo. Protocols have been developed that enable removal of living cells from native tissues, leaving only a natural scaffolding of extracellular matrix, which can then be re-seeded with cells to reconstruct or partially reconstruct 3-D tissues. Another approach to soft tissue reconstruction has been the development of cell-laden hydrogels, which are often cast into a specific shape and placed into a permissive environment in vitro or in vivo that allows maturation and establishment of tissue-specific characteristics. In recent years, with the advancement of 3-D printing technologies for the on-demand fabrication of complex polymer-based objects, efforts have been underway to adapt 3-D printing technologies and engineer bioprinting instruments that can leverage similar 3-D replication concepts and accommodate the incorporation of living cells.

First-generation 3-D prototyping techniques relied on subtractive processes—the removal of material from a solid block using filing, milling, drilling, cutting and grinding methods. Advanced 3-D prototyping technologies utilize additive processes in which the desired part is built up—or “printed” layer-by-layer. Objects of virtually any shape can now be fabricated from a wide range of non-biological materials using additive technologies.

The power and utility of 3-D printing in the non-biological materials area has sparked the imaginations of biologists and engineers alike and fueled R&D activities aimed at producing intricate biological 3-D structures. Consequently, precise, automated, layer-by-layer fabrication of tissue (bioprinting) is now possible using only living cells as building blocks. This is resulting in simultaneous achievement of unique features such as true 3-D, tissue-like cellular densities and reproduction of native tissue architecture through the spatially directed placement of distinct cell types.

Bioprinting hardware requires unique features that ensure success at the interface of engineering and biology. Low-shear deposition mechanisms are essential to maintaining viability and function of living cells. Fabrication speed must be rapid enough to prevent destruction of the bioprinted tissue due to nutrient or oxygen deprivation. All components that contact living cells must be non-toxic and either disposable or sterilizable to prevent cross-contamination between runs. And tissues must be generated in a format that enables them to be manipulated and used in application(s) after fabrication. Bioprinting combines the synergistic potential of engineering and biology to create living human tissues that mimic the form of native tissue and, therefore, achieve unique tissue-specific metabolic functions.

Organovo’s NovoGen MMX Bioprinter precisely dispenses “bio-ink”—tiny building blocks composed of living cells—generating tissues layer-by-layer according to user-defined designs. Built for flexibility, the bioprinter enables fabrication of tissues with a wide array of cellular compositions and geometries; side-by-side comparison of multiple tissue prototypes facilitates optimization and selection of specific designs geared toward a particular application. Working within the confines of an object library, bio-ink building blocks of various shapes, sizes and compositions are assembled into architectures that recapitulate the form of native tissue. Tubes, layered sheets and patterned structures have been bioprinted, yielding 3-D tissues that are free of biomaterial scaffolding and characterized by tissue-like microarchitecture, including the development of intercellular junctions and endothelial networks.

In the short-term, 3-D human tissues are being deployed in the laboratory setting as models of human physiology and pathology; cell-based assays are a mainstay of the drug discovery and development process, and multicellular/multitissue systems may serve as more predictive indicators of clinical outcomes. Longer-term applications of 3-D tissue technologies will extend our knowledge of how to build the smallest functional units of a tissue to the fabrication of larger-scale tissues useful for surgical grafts to repair or replace damaged tissues and organs in the body. What are the next steps in the evolution of bioprinting? The first step is scaling up and down—increasing the resolution of specific features while advancing fabrication hardware and techniques to produce larger-scale tissues. The next, enhancing the complexity of designs—building the tool set that enables conceptual or visual inputs to be translated rapidly to executable bioprinting programs that select from a library of bio-ink building blocks to translate the vision into reality.