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Nazia Mehrban, Gui Zhen Teoh and Martin Anthony Birchall

Figure 1. Biodegradable polymers used for bioprinting applications.

easily and are therefore easier to handle during manu- cell viabilities; typically between 40% and 90%.
facture. However, natural materials often lack the
mechanical integrity required whilst synthetic mate- 3.2 Biocompatibility
rials are often not biocompatible [50] . Some researchers Original expectations of material biocompatibility
have sought to overcome these issues by combining centered on minimising inflammation and creating
favourable elements from both categories to create materials that would not produce cytotoxic side-eff-
hybrid materials [51,52] . Even so, not all of these mate- ects. Today, however, biocompatibility can include the
rials are suited to 3D printing. While the high temper- incorporation of biochemical functionality, i.e. growth
atures and solvents used in the initial 3D printing factors or growth factor mimics, and nanoscale scaf-
techniques are not employed for bioprinting, there are fold morphology to improve and enhance the interac-
still certain criteria, which need to be met when se- tion of cells with the scaffold, and therefore engi-
lecting suitable bioprinting materials. neered tissue with the in vivo environment [13] . It is
vital to select a material which can be modified
3.1 Printability
through the printing process such that there is the op-
It is important to be able to both deposit the material tion of building complexity into the system.
accurately and retain spatial resolution in order to
control the overall scaffold geometry. Some bioprint- 3.3 Degradation
ing techniques cannot print viscous materials (such as Degradation of a material into smaller chemical units
inkjet methods) while others shear-thin the material due to material chemistry, oxidising agents, enzymes
and therefore affect its formation (such as microextru- or ionising radiation and ultrasound occurs via two
sion). Temporal resolution is another aspect which mechanisms: surface (materials loss layer by layer) or
needs to be addressed, as materials that take too long bulk (fragmentation of the whole material) [53] . Figure
to ‘set’ will affect the spatial resolution of the scaffold, 2 shows both mechanisms.
whilst materials that set too quickly will be in danger The main indicators of degradation are reduction in
of blocking the nozzle. Other factors to consider are sample mass, loss of mechanical strength and changes
whether the cells or biomolecules will encounter shear in chemical bonds and groups. Controlled degradation
stress or high temperatures during printing. Current is vital as material loss and a reduction in mechanical
cell-printing technologies report a high variation in integrity of the overall scaffold [54] can alter the

International Journal of Bioprinting (2016)–Volume 2, Issue 1 9