Comparison between electrospun membrane and solvent cast film

There are many applications where electrospun membrane can be used to replace solvent cast film for better performance depending on the desired properties. Although both are generally flat, electrospun membrane is made of fibers from the micron level down to the nanometer dimension with interconnected pores while cast film are fully filled solid. This has significant influence on their properties such as diffusion, mechanical and dissolution.

Mechanical Properties

The fibers that made up the electrospun membrane has very high aspect ratio and this made it much more malleable than the solvent cast film form. Ghosal et al (2018) compared the mechanical properties of solvent cast poly(ε-caprolactone) (PCL) versus electrospun membrane. Solvent cast PCL was so brittle that it cannot be handled without cracking. However, electrospun PCL membranes exhibit good elongation property and appreciable tensile strength of up to 2.6 MPa. Investigation into its crystallinity found that solvent cast films exhibited presence of crystalline domains while electrospun membrane was amorphous. This may be one of the reasons why electrospun membrane is much more flexible than solvent cast membrane. Kwak et al (2017) also showed that repeated folding of electrospun caffeine loaded fish gelatin (FG) membrane did not form any cracks but solvent cast FG film cracks after only folding 5 times. However, solvent cast generally has a higher tensile strength than electrospun membrane. Feng et al (2019) showed that polylactic acid/TiO₂ composite electrospun membrane and cast film has tensile strength of 2.71 MPa and 14.5 MPa respectively. This can be expected as in cast film form, there are more material mass to take on the load compared to electrospun membrane which is porous and whose fibers are usually not interconnected.

Dissolution

High surface area and interconnected pores of electrospun membranes encourages fast penetration of its solvent and dissolution of the material. Kwak et al (2019) showed that electrospun fish gelatin (FG) /caffeine membrane took only 1.5s for total disintegration in water but took 40 s in solvent cast film form. On wetted sponge, total disintegration of electrospun FG/caffeine membrane took 5 s but solvent cast film maintained in gel state until 3 minutes due to inadequate water absorption for total disintegration.

Water Contact Angle

Fibrous topography of electrospun membrane has been shown to significantly increase the water contact angle of already hydrophobic material compared to its film form. For thermoresponsive hydrophobic material, Poly(N-isopropylacrylamide)/Polystyrene composite nanofiber membrane, the increment in the water contact angle goes from less than 20° to more than 150° in electrospun membrane compared to an increase from 76° to 94° for smooth film at the temperature transitions [Wang et al 2008]. Electrospun core-shell membrane with polycaprolactone as the core and Teflon as the shell demonstrated superhydrophobicity while Teflon film is just hydrophobic [Han et al 2009]. Electrospun gelatin/zein nanofibrous membrane had a hydrophobic surface with water contact angle of 118° but the casted gelatin/zein film had a hydrophilic surface water contact angle of 53.5° [Deng et al 2017].

In terms of hydrophilic property, electrospun membrane has been shown to be more responsive to treatment to make it hydrophilic. Huang et al (2014) coated electrospun polysulfone (PSU) with hydrophilic polydopamine (PDA) by polymerization of dopamine. The electrospun PSU membrane with an initial contact angle of 146° drops to zero with the PDA coating. For cast PSU film, the reduction in contact angle is less significant from 79° to 71°.

Drug release

Large surface area of nanofibers membrane have some advantages over cast film in drug release. Ngawhirunpat et al (2009) used electrospun polyvinyl alcohol (PVA) fibers for encapsulation of meloxicam. High porosity and surface area of PVA fibers meant that the release of meloxicam is faster than as-cast PVA films. Drug release rate from PVA fibers may also be modified by the addition of water soluble polyvinyl pyrrolidone (PVP). Drug loaded in electrospun fibers may also be maintained in an amorphous state which will facilitate diffusion and adsorption through the skin. Shi et al (2013) showed that ibuprofen (IBU)-loaded in cellulose acetate/poly(vinyl pyrrolidone) (CA/PVP) blends was uniformly distributed in an amorphous state. IBU diffusion and ex vivo skin permeation was also found to be faster in electrospun nanofibers membrane compared to cast membrane. For membrane and film loaded with antibacterial agent, Feng et al (2019) showed that the zone of inhibition of gram-negative (E. coli) and gram-positive (S. aureus) bacteria is significantly greater for nanofiber membrane compared to cast film made of the same material, polylactic acid, and loaded with the same amount of TiO₂ nanoparticles. Inhibition of bacterial by TiO₂ was attributed to the generation of reactive oxygen species (ROS) which potentially disrupts microbial functions leading to its death. Thus, these generated ROS was better able to diffuse from the porous electrospun membrane and exhibit anti-microbial effect.

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