Conventional electrospinning typically forms flat, 2D membrane with small inter-fiber pores. As the individual fiber is weak, post-electrospinning methods of fluffing up the membrane to form a 3D scaffold is challenging and run the risk of breaking the fibers in the process. Gas foaming provides a viable alternative to open up the pores between the fibers without excessive application of force. In this technique, the 2D electrospun membrane is soaked in a solution that bubbles when agitated. Since the membrane is totally immersed in the solution, bubbles are formed on the fiber surfaces that are located outside and within the membrane. Formation and coalescence of the bubbles will force the fibers apart which create a more open scaffold. Jiang et al (2015) demonstrated the feasibility of this process using electrospun polycaprolactone (PCL) fiber membrane and NaBH4 as the foaming agent. Higher concentration of foaming agent was found to significantly increase the thickness of the membrane over a shorter duration.
McCarthy et al (2021) also used NaBH4 as a foaming agent for expansion of electrospun PCL nanofiber mats. Prior to submersion into NaBH4 solution, the nanofiber mat was first submerged in liquid nitrogen so that the mat is fully wetted to facilitate penetration of NaBH4 solution into the pores. The mat was left in NaBH4 solution for 24h for expansion. This is followed by washing in distilled water and vacuum lyophilization until frozen and dry. The resultant PCL matrices were subsequently gelatin-coated and crosslinked to ensure elasticity and matrix shape memory. The foaming agent was so effective that the nanofiber mat with an initial mean thickness of 0.105 cm was expanded to a thickness of 7.33 cm.
This technique for expanding 2D membrane is very versatile as demonstrated by the expansion of the membrane forming a tubular scaffold. With a tubular scaffold as the starting structure, after 24 hours of foam forming expansion, its lumen was completely covered with fibers and the end form is a solid rod instead of a tube [Jiang et al 2015]. This is in contrast with the cuboid and random form constructed from flat 2D membrane.
The expanded membrane with increased pore size allows better infiltration of the scaffold by mammalian cells. Comparison of NIH3T3 fibroblasts cultured on 2D electrospun PCL membrane and the foam-expanded electrospun PCL membrane showed great contrast in the cell distribution. On 2D electrospun PCL membrane, cells are only found on the surface of the scaffold. On the expanded scaffold, cells are found throughout the thickness even for scaffold with less thickness increment [Jiang et al 2015].
In a modified version of using gas foaming to transform 2D electrospun membrane into 3D structure, Chen et al (2020) added F-127 into poly(ε-caprolactone) (PCL) solution before electrospinning into nanofibrous membranes. The surfactant F-127 enhances the hydrophilicity of the PCl nanofiber. This allows rapid water penetration into the fiber matrix. NaBH4 was used as the bubbling agent for rapid evolution of H2 gas in the presence of water. For the formation of 3D structure, F-127/PCL nanofiber membrane was placed in a mold and immersed in NaBH4 solution. It took less than a minute for the bubbling to expand the membrane to fill up the mold. To fix the 3D structure, the expanded sample was immersed in gelatin solution before freeze drying.
Chen et al (2020) et al has demonstrated that this is a fast and easy method to transform 2D electrospun membranes into 3D structures that conforms to the shape of the mold. However, there are several questions that need to be addressed. During bubbling, the gas may enter the nanofiber matrix and disrupt the nanofiber morphology. From the SEM images, it can be seen that the 3D structure is made of separate fibrous walls. However, it is not clear whether the nanofibers on the wall retain a distinct and separated fiber morphology or have mostly fused together. While it is obvious that in its dry form, the 3D structure was able to take the shape of the mold. It is likely that the structure will collapse when wetted and it may not recover to the original shape. This will need to be verified with subsequent reports.
Published date: 27 October 2015
Last updated: 28 December 2021
▼ Reference
-
Chen S, John J V, McCarthy A, Carlson M A, Li X, Xie J. Fast transformation of 2D nanofiber membranes into pre-molded 3D scaffolds with biomimetic and oriented porous structure for biomedical applications. Appl. Phys. Rev. 2020; 7: 021406.
Open Access
- Jiang J, Carlson M A, Teusink M J, Wang H, MacEwan M R, Xie J. Expanding Two-Dimensional Electrospun Nanofiber Membranes in the Third Dimension By a Modified Gas-Foaming Technique. ACS Biomater. Sci. Eng. 2015 Article in press.
-
McCarthy A, Saldana, L, McGoldrick D, John J V, Kuss M, Chen S, Duan B, Carlson M A, Xie J. Large-scale synthesis of compressible and re-expandable three-dimensional nanofiber matrices. Nano Select. 2021; 1- 14.
Open Access
▲ Close list
ElectrospinTech
