Biomedical Engineering Reference
In-Depth Information
Recently, it has been reported that several functional cells, e.g., embryonic stem
(ES) cells and induced pluripotent stem (iPS) cells, were established and produced
in culture [
8
-
10
]. These cells are expected to be used as a tool in regenerative
medicine and cell engineering. The ES cells are generally preserved by classic
cryopreservation after
in vitro
cell culture. However, it is well known that the
recovery rate after thawing is quite low, and that some organic solvents contained in
the general cryopreservation medium cause serious damage and the cells lose their
function. Thus, developing a cytocompatible hydrogel that can preserve the cell
functions without any adverse effects on bioactivity is extremely important in the
field of cell engineering and tissue engineering. Polymeric hydrogels can be used
for biomedical applications because these soft biomaterials can provide a three-
dimensional (3D) network that supports biological materials (Table
1
). The water
permeability and gas permeability of hydrogels are important to living cells in the
network. Further, the cytocompatibility of hydrogels depends on the chemical
structure of the polymer chains and network size of the gel structure.
In order to replace or restore physiological functions lost in diseased or damaged
organs, tissue engineering typically involves fabrication of tissue structures using
cells and polymer scaffolds. The polymer scaffolds are designed to provide
mechanical support for the cells, which can then perform the appropriate tissue
functions; however, in practice, the simple addition of cells to porous polymer
scaffolds is often inadequate for reproducing sufficient tissue function. One
approach for increasing the functionality of these tissue-engineered constructs
relies on attempts to mimic both the architecture of tissues and the environment
around cells within the living organism.
Tissues consist of smaller repeating units on the scale of hundreds of micro-
meters
in vivo
. The 3D architecture of these repeating tissue units underlies the
coordination of multicellular processes, emergent mechanical properties, and inte-
gration with other organ systems via the microcirculation [
11
]. Furthermore, the
local cellular environment presents biochemical, cellular, and physical stimuli that
orchestrate cellular fate processes such as proliferation, differentiation, migration,
and apoptosis. Thus, successful fabrication of a fully functional tissue must include
both an appropriate environment for cell viability and function at the microscale
Table 1 Advantages and disadvantages of hydrogels as tissue engineering matrices
Advantages
Aqueous environment can protect cells and fragile drugs (peptides,
proteins, oligonucleotides, DNA)
Good transport of nutrient to cells and products from cells
Can be easily modified with cell adhesion ligands
Can be injected in vivo as a liquid that gets at body temperature
Usually biocompatible
Disadvantages
Can be hard to handle
Usually mechanically weak
Can be difficult to load drugs and cells and then crosslink in vitro
as a prefabricated matrix
Can be difficult to sterilize
