Biomedical Engineering Reference
In-Depth Information
Stimuli-responsive hydrogels that undergo abrupt changes in volume in response
to external stimuli such as pH, temperature, and solvent composition have potential
applications in cell engineering. Temperature-responsive hydrogels are some of the
most widely studied stimuli-responsive hydrogel systems. These systems, which
are mostly based on PNIPAAm, undergo a reversible volume phase transition with
a change in the temperature of the environmental conditions [ 30 ].
Alginate is a linear polysaccharide that is widely used for drug delivery and in
cell engineering. Alginate forms a hydrogel under benign conditions, which makes
it attractive for cell encapsulation. When divalent cations such as Ca 2+ ,Mg 2+ ,Ba 2+ ,
and Sr 2+ are added into alginate aqueous solution, the hydrogel is obtained [ 27 ].
The hydrogels have also been used for transplantation of cells such as chondrocytes,
hepatocytes, and Langerhans islets to treat diabetes.
The molecular design of polymers for spontaneously forming hydrogels has
been carried out using a MPC unit. Watanabe et al . reported a MPC polymer porous
scaffold prepared through stereocomplex formation between poly( D -lactic acid)
and PLA [ 21 - 25 ]. Kimura et al . and Nam et al . reported a spontaneously forming
hydrogel by mixing two kinds of MPC polymer solutions [ 35 - 38 ]. Hydrophobic
domains were formed by water-soluble MPC polymer containing a hydrophobic
unit, poly(MPC- co - n -butyl methacrylate) (PMB), in aqueous solution. Another
water-soluble MPC polymer containing methacrylic acid, poly(MPC- co -methacrylic
acid) (PMA) was introduced into the hydrophobic domain. After being gently shaken,
the gelation occurred due to the formation of dimers by hydrogen bonding, which acts
as a physical crosslinking of the polymer chains. The formed hydrogel (PMA/PMB
hydrogel) is expected to be useful as a drug reservoir for a pH-responsive drug
delivery system. Also, a PMA/PMB hydrogel has been applied for preventing
peritendinos adhesion after physical operation [ 39 ].
3 Cytocompatible Polymer Hydrogels Composed
of Water-Soluble Phospholipid Polymers
3.1 Polymer Design and Fundamental Properties
Similarly to the phospholipid polymers, the MPC polymers show excellent biocom-
patibility and blood compatibility [ 43 - 48 ]. These properties are based on the bioinert
character of the MPC polymers, i.e., inhibition of specific interaction with
biomolecules [ 49 , 50 ]. Recently, the MPC polymers have been applied to various
medical and pharmaceutical applications [ 44 - 47 , 51 - 55 ]. The crosslinked MPC
polymers provide good hydrogels and they have been used in the manufacture
of soft contact lenses. We have applied the MPC polymer hydrogel as a cell-encapsu-
lation matrix due to its excellent cytocompatibility. At the same time, to prepare a
spontaneously forming reversible hydrogel, we focused on the reversible covalent
bonding formed between phenylboronic acid and polyol in an aqueous system.
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