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3D bioprinting for tissue engineering: Stem cells in hydrogels

The introduction of cytotoxic crosslinking agents stem cells which can be considered for 3D bioprinting:
should be avoided and as cells are only able to survive embryonic, somatic and iPSC.
in a narrow temperature range, the list of gel candi- 6.1.1 Embryonic Stem Cells (ESCs)
dates is substantially reduced. However, by combining With the ability to form any cell type and indefinite
materials, the list of printable gels could once again be self-renewal [99] , embryonic stem cells (ESCs) are the
expanded. ideal cell type for tissue engineering. One of the chal-
Furthermore, the time required for gelation is of lenges in using ESCs for regenerating or repairing
importance. The longer it takes for the material to be tissue is identifying the conditions needed to drive the
printed and form the structure, the more likely the cells towards a specific lineage. As cell differentiation
layers printed at the start of the process will lose via- is influenced by both chemical and physical cues, the
bility, thus limiting the use of the construct. Other identification of ideal culture conditions adds another
factors include the introduction of stress on cells thr- level of complexity to an already difficult task.
ough changes in the viscosity of the gel. While control The biggest drawback of using ESCs is that they
of viscosity would make the gel more printable, slight are derived from a blastocyst. In some countries,
changes could lead to low cell viability rates. Stress ESCs research is prohibited or severely restricted due
could also be introduced by methods of extruding the to the ethical issues this raises. Furthermore, where
bioink [36] and changes in temperature during the prin- research in the field is allowed, the number of cells
ting process, although the latter is dependent on how derived from an embryonic source is low and, unless
long high temperatures are maintained. In their study, expanded significantly in vitro, is unlikely to meet
Cui et al. [16] reported a rise in temperature during clinical demand.
printing from 22°C to 46°C. However, as the drops 6.1.2 Adult Stem Cells
produced cooled within seconds, no significant apop- Adult stem cells cover any postnatal somatic cell that
tosis was observed. is undifferentiated and can self-renew [100] . These cells
6. Using Stem Cells for 3D Bioprinting can be derived from a number of sources including
brain, liver and bone marrow [101] . Mesenchymal stem
The ideal cell type for bioprinting is dependent on the cells (MSCs) are readily available from bone marrow,
accessibility and availability of the cells, the self-ren- adipose tissue, amniotic fluid, the synovium and pe-
ewal and expansion capacity, differentiation profile riosteum and are known to be less tumorigenic than
and cellular tumorigenicity as well as viability fol- their embryonic or fetal counterparts [98] . MSCs are
lowing encapsulation and printing. Stem cells are a non-haematopoietic, are relatively straightforward to
particularly attractive cell type as they are pluripotent obtain via bone marrow harvesting methods [102] and
and able to differentiate into other cell types upon ex- interact well with a range of materials that may be
posure to the correct physical and chemical guidance used for cellular encapsulation to produce viable bio-
cues [93] . Within the human body, there are a number of inks. Table 1 features the types of adult MSCs which
viable sources of stem cells, such as the bone marrow, have been used for bioprinting applications.
periosteum and adipose tissue [94–96] . Although MSCs can be harvested from the patient’s
own tissue, and therefore reduce the risk of rejection,
6.1 Stem Cells Selection
only 0.001%–0.01% of total nucleated cells in bone
Stem cell differentiation can be guided through the marrow are MSCs [102] . A possible alternative source
incorporation of tissue-specific chemical signals in the which could be used is adipose derived MSCs
scaffold, although some researchers suggest that this (ADMSCs). Adipose tissue is abundant and many re-
may not be necessary to promote differentiation and searchers have used ADMSCs successfully towards
subsequent tissue regeneration [97] . While the advan- tissue engineering [94,103,104] .
tages of using pluripotent cells in bioprinting are clear, 6.1.3 Induced Pluripotent Stem Cells (iPSC)
there are ethical considerations which must be taken The discovery that stem cells can be generated directly
into account when using stem cells. Furthermore, the from adult cells by the introduction of four transcription
generation of pluripotent stem cells from adult cells factors has revolutionised biomedical research [105–108] .
(induced pluripotent stem cells, iPSC) pose the risk of By using the patient’s own cells, the ethical issues
tumorigenicity which must also be considered [98] . Eth- related to stem cell research and the concern sur-
ical issues aside, there are three main categories of rounding tissue rejection can be avoided. Furthermore,

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