What are the properties of an ideal bioink?
There is growing recognition that printable and biocompatible materials which can be used for additive manufacturing applications are poised to make a significant impact in several clinically-relevant fields of research. These materials, known as ‘bioinks’, offer the potential to print precisely controlled structures using a cell-laden ‘ink’ which can support cell culture in 3D. There are many material properties which must be considered when developing a novel bioink, and the final application of the material will ultimately dictate which of these properties are prioritised.
In the latest volume of the journal Bioprinting, the biocompatibility and material properties of an ideal bioink are discussed in an excellent review which has a specific focus on the application of bioprinting to address challenges in tissue engineering.
In this article, the authors outline key parameters which must be optimised to produce a successful bioink, while rationalising that it is unlikely that a single material can meet the needs of every research project. However, underlying each bioink application is a common requirement for control of mechanical and biological properties of the printable material.
In terms of mechanical control, it is imperative that the bioink forms a micro-structure which mimics that of the cell’s native environment. As well as a familiar architecture, the gel stiffness and porosity should be matched to that found in vivo so as to support cell growth, signaling and proliferation. Ideally, the bioink will exhibit shear-thinning behaviour, as this will reduce the stress exerted on the cells during the printing process, which most commonly involves extrusion of the bioink through a narrow print-head.
In order to assure biocompatibility, the raw materials used for the production of the bioink should not be cytotoxic to the cells in question, nor elicit an immune or inflammatory response.
Hydrogels of natural or synthetic origin are commonly used as bioinks, as these can match the high water content of the ECM, while allowing tunability of the mechanical strength and biochemical functionalisation of the base material. Often, these materials are optimised for use in bioprinting by increasing the viscosity of the hydrogel through the addition of thickening agents, or by partial cross-linking of the nanofibers which form the ECM-like scaffold.
The paper highlighted here presents an expert review of the history, continuing work and future prospects of 3D bioprinting as applied to the field of tissue engineering, and establishes the guidelines which should be followed in the development of a novel bioink. Several unmet needs are discussed in this publication, specifically regarding the limited number of materials available which exhibit the necessary printability for use as bioinks. However, in an expanding research field such as 3D bioprinting, the increasing demand for new bioinks presents a key opportunity for the development of novel materials to meet these needs.
D. Williams et. al. “A perspective on the physical, mechanical and biological specifications of bioinks and the development of functional tissues in 3D bioprinting.” Journal of Bioprinting (2018)