3.2 Electrospinning Hydrogel Fiber Mats for Potential Tissue
Scaffolds
Hydrogels are ideal synthetic materials for a variety of biomedical applications, from
tissue engineering to wound healing, because of their similarities with extracellular
matrix and excellent biological compatibility and performance [73]. Nature, however, uses porosity to provide transport pathways, space for cell growth, and
improved strength-to-weight ratios. A number of synthetic pathways for creating
porous hydrogel biomimetics have been proposed [74], but the electrospinning
manufacturing process conveniently provides a direct route for generating
interconnected porous hydrogels with controlled dimensions. Electrospun templates
have garnered significant interest for tissue engineering [75]. The solubility of the
copolymer precursors of the microphase-separated, amphiphilic hydrogels in a
variety of organic solvents facilitates the use of electrospinning for fabricating highly
stretchable, tough, porous hydrogel micro- or nanofiber mats [18].
An example of a DFm10 microfiber mat (area ~100 cm
2 ; thickness ~125 μm) that
was electrospun from a 25 wt% solution of the DFm10 copolymer in isopropanol is
shown in Fig. 16. The details of the electrospinning are given in Ref. [18]. The dry
microfiber mat, shown in the photograph on the left, was opaque due to light
scattering from the microfibers. The morphology of the microfibers varied from
2.5-μm-diameter cylindrical fibers at the mat surface to 3.8-μm-wide ribbonlike
fibers in the interior of the mat. The ribbonlike morphology was similar to that
reported when high viscosity polymer solutions were electrospun with rapid solidification [76]. Cylindrical-shaped fibers were achieved by adding ethylene glycol as
a cosolvent with the isopropanol, but in that case, the fibers fused when the mats
were swollen with water.
In general, the fiber structure in the mats produced by electrospinning from
isopropanol was stable when swollen with water. The distinct fiber structure at the
surface of the microfiber mat persisted for nearly 1 week, after which the fibers at the
surface showed some coalescence, even though the equilibrium swelling of the fibers
was achieved within 12 h. The dense coating at the surface due to fiber coalescence,
however, was discontinuous, and the fiber structure was still visible through the
holes in the skin layer of the mat surface (see the photo for 168 h in Fig. 17a). The
cross-sectional images of the DFm10 fiber mats, Fig. 17b, show that the interior
structure of the electrospun DFm9 fiber mat remained as anisotropic ribbonlike
fibers, even after soaking in water for 7 days.
The density of the DFm10 microfiber mat was about 20% that of the density of a
hydrogel prepared from compression-molded DFm10, see Table 4, which indicates
that the porosity of the fiber mat was about 80%. The swelling ratio for the DFm10
microfiber mat was about 30% higher than that of the dense film, Table 4, which is
due to water trapped in the interstitial regions of the fibrous mat. Note that water
swells the fibers and reduces the spacing between them, so that the porosity of the
hydrogel mat is actually lower than that of the dry fiber mat.
Hydrophobically Associating Hydrogels with Microphase-Separated Morphologies
191
Scaffolds
Hydrogels are ideal synthetic materials for a variety of biomedical applications, from
tissue engineering to wound healing, because of their similarities with extracellular
matrix and excellent biological compatibility and performance [73]. Nature, however, uses porosity to provide transport pathways, space for cell growth, and
improved strength-to-weight ratios. A number of synthetic pathways for creating
porous hydrogel biomimetics have been proposed [74], but the electrospinning
manufacturing process conveniently provides a direct route for generating
interconnected porous hydrogels with controlled dimensions. Electrospun templates
have garnered significant interest for tissue engineering [75]. The solubility of the
copolymer precursors of the microphase-separated, amphiphilic hydrogels in a
variety of organic solvents facilitates the use of electrospinning for fabricating highly
stretchable, tough, porous hydrogel micro- or nanofiber mats [18].
An example of a DFm10 microfiber mat (area ~100 cm
2 ; thickness ~125 μm) that
was electrospun from a 25 wt% solution of the DFm10 copolymer in isopropanol is
shown in Fig. 16. The details of the electrospinning are given in Ref. [18]. The dry
microfiber mat, shown in the photograph on the left, was opaque due to light
scattering from the microfibers. The morphology of the microfibers varied from
2.5-μm-diameter cylindrical fibers at the mat surface to 3.8-μm-wide ribbonlike
fibers in the interior of the mat. The ribbonlike morphology was similar to that
reported when high viscosity polymer solutions were electrospun with rapid solidification [76]. Cylindrical-shaped fibers were achieved by adding ethylene glycol as
a cosolvent with the isopropanol, but in that case, the fibers fused when the mats
were swollen with water.
In general, the fiber structure in the mats produced by electrospinning from
isopropanol was stable when swollen with water. The distinct fiber structure at the
surface of the microfiber mat persisted for nearly 1 week, after which the fibers at the
surface showed some coalescence, even though the equilibrium swelling of the fibers
was achieved within 12 h. The dense coating at the surface due to fiber coalescence,
however, was discontinuous, and the fiber structure was still visible through the
holes in the skin layer of the mat surface (see the photo for 168 h in Fig. 17a). The
cross-sectional images of the DFm10 fiber mats, Fig. 17b, show that the interior
structure of the electrospun DFm9 fiber mat remained as anisotropic ribbonlike
fibers, even after soaking in water for 7 days.
The density of the DFm10 microfiber mat was about 20% that of the density of a
hydrogel prepared from compression-molded DFm10, see Table 4, which indicates
that the porosity of the fiber mat was about 80%. The swelling ratio for the DFm10
microfiber mat was about 30% higher than that of the dense film, Table 4, which is
due to water trapped in the interstitial regions of the fibrous mat. Note that water
swells the fibers and reduces the spacing between them, so that the porosity of the
hydrogel mat is actually lower than that of the dry fiber mat.
Hydrophobically Associating Hydrogels with Microphase-Separated Morphologies
191