Customizing Mechanical properties of electrospun membrane

Wide selections of materials that can be electrospun have seen its membrane used in a variety of applications. Unlike solvent cast film, the same polymer when electrospun to nanofiber membrane exhibit much greater flexibility without the need for addition of plasticizers. 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. For optimal performance from the electrospun membrane, understanding the possible modifications to tailor its mechanical properties enables the engineer to adopt the most appropriate strategy. Such modifications may include varying fiber arrangement, having additives, chemical modifications, heat treatment or their combinations.

Fiber orientation and alignment

The mechanical properties of the electrospun tube are dependent on its fiber orientation. Completely randomly distributed nanofibers will give rise to isotropic mechanical properties while oriented fibers will give rise to anisotropic mechanical properties. The strength of the scaffold is highest in the direction of the fiber alignment. Electrospinning generally give rise to non-oriented nanofibers distribution. Since electrospun tubes are commonly constructed using a rotating small diameter rod, increasing its rotation is able to encourage fiber alignment along the circumference of the rod. Shang et al (2010) constructed a small diameter tubular scaffold by depositing PLGA nanofibers on a water bathe followed by winding of the nanofibers as yarn onto a rotating rod. As the yarn was drawn onto rotating rod, it was traversed such that the tube was made of cross aligned fibers bundles. At slow traverse rate, a tubular scaffold made of parallel aligned fibers that wound around the circumference of the rod was constructed. Comparing the strength of the scaffold wall made of randomly oriented fibers, cross-aligned fibers and parallel aligned fibers, scaffold wall made of randomly oriented fibers showed the lowest strength at less than 6 MPa. With loading of parallel aligned fibers scaffold wall in the direction of fiber alignment, the scaffold showed the highest strength at more than 12 MPa. The cross aligned fibers scaffold with loading applied to the bisector between the two principal fiber axes showed intermediate strength at about 10 MPa.

Additives

Blending is a commonly used method of altering the mechanical properties of electrospun membrane. Plasticizers may be added to the polymer to improve flexibility and durability of the electrospun fibers. A common plasticizer is low molecular weight poly(ethylene glycol) (PEG). Salmani and Nouri (2016) added PEG to silk fibroin solution for electrospinning. The resultant silk/PEG electrospun membrane showed improved stress and strain. With just 10% PEG, the strain of the silk membrane more than 300% while the stress almost doubled. However when the amount of PEG added reached 30%, the stress and strain of the composite was reduced to close to that of pure silk membrane.

Composites

One of the simplest methods of improving the mechanical strength of an electrospun membrane is to use additives. Collagen is a natural protein which facilitates cell adhesion and proliferation. Generally, electrospun membrane made of randomly organized fibers from collagen alone is mechanically weaker compared to man-made polymers. Even with aligned collagen fibers, the ultimate tensile stress in the direction of fiber orientation is relatively weak at about 1.5 MPa [Matthews et al 2002]. Therefore, it is common to incorporate man-made biodegradable polymers to improve the mechanical strength of the resultant scaffold [Jose et al 2009]. Inorganic nanotubes and other nanoparticles are often used as reinforcement in electrospun nanofibrous matrix. Carbon nanotubes have been shown to be effective in strengthening nanofibers [Sen et al 2004].

Electrospun fiber mixture

Similar to blending, however, instead of having the two polymers in the same solution, the two different polymers may be electrospun through separate nozzles so that there is a mixture of stronger and weaker fibers in the membrane. Nabzdyk et al (2015) showed that the mechanical strength of fiber mixture is superior to blended fibers based on nondegradable poly(ethylene terephthalate) (PET) and biodegradable poly(glycolic acid) (PGA ) nanofibers. The faster reduction in the mechanical strength of the blended system was due to compromised structure of the fibers. However, in the fiber mixture, while the faster degrading PGA fiber may lose its strength at a faster rate, the retention of PET fiber integrity provides the necessary support for the membrane.

Evaluation of tensile strength of the electrospun materials: electrospun materials were evaluated for tensile strength. ePET is the strongest material, while ePGA is the weakest. ePET/ePGA-d (fiber mixture) had significantly higher strength as compared to ePET/ePGA-s (blended fiber). [Nabzdyk et al Journal of Nanomaterials, vol. 2015, Article ID 340981, 7 pages, 2015. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Fiber morphology

Fiber morphology plays an important role in determining the mechanical strength of the membrane. It is known that fibers with beads tend to exhibit a lower mechanical property [Gaharwar et al 2014] compared to smooth fibers. This is due to stress concentration as a result of large dimensional variation between the beads and fiber region. The presence of beads may also reduce the number of contact points between electrospun fibers. It has been hypothesised that electrospun membrane breaks due to disruption of cohesion points instead of individual fiber breakage therefore a membrane with fewer contact points will be weaker [Tarus et al 2020]. Apart from beads, there are other fiber morphology such as porous fibers and wrinkled surface fibers just to name a few. Zaarour et al (2019) showed that electrospun poly(vinylidene fluoride) (PVDF) fiber membrane comprising of smooth fibers have the highest strength (3.8 MPa), followed by wrinkled surface fibers (3.3 MPa) and porous fibers have the least strength (1.4 MPa). For strain, membrane with wrinkled surface fibers was the highest (125%) followed by smooth fibers (93%) and porous fibers have the least strain (78%). Membrane comprising of aligned PVDF fibers showed the same trend.

Representative pictures of samples fabricated by electrospinning of PVDF solutions with different morphologies. (a-c) Randomly oriented fibers, (a) wrinkled, (b) smooth, and (c) porous [Zaarour et al 2019].

Inter-fiber bonding

Electrospun fibers in the membrane are typically separated from one another. When a tension is applied, the fibers would slide over one another except at points of entanglements. Tarus et al (2020) suggested that the initial failure of electrospun membrane is due to breakage of inter-fiber points instead of fiber breakage. Using polyvinyl chloride (PVC) as the model polymer, tensile application on the electrospun PVC membrane showed an initial high modulus followed by a long low modulus region. The initial high modulus comes from the breaking of inter-fiber points and the long low modulus region comes from sliding of nanofibers after all the inter-fiber points are broken. This contrasts with the much higher modulus of PVC cast film. To improve membrane stiffness and strength, an often used method is by inducing inter-fiber bonding. There are several ways to encourage bonding at contact junctions between fibers. Chemical cross-linking may be used for this purpose. Heat treatment with and without applied pressure may also be used to encourage bonding at contact points. For some polymer solutions, simply reducing the electrospinning distance between the tip and collector is sufficient for the semi-dried fibers to fuse at the junctions.

Stress-strain curves for nanofiber mat and cast micro-thickness film [Tarus et al 2020].

Mathematical modeling has been used to predict the effect of inter-fiber bonding on electrospun membranes. Chavoshnejad and Razavi (2020) showed several interesting relationships between the changes to membrane stiffness due to bonding and the membrane characteristics. With inter-fiber bonding, stiffness of the membrane increases by 60% regardless of the mechanical properties of the individual fibers. Increasing elastic modulus of individual fibers led to a linear increase in membrane stiffness while increasing fiber diameter led to a parabolic increase in membrane stiffness. Comparing the stiffness ratio of 100% bonding over unbonded membrane, membranes with lower porosity or greater fiber density showed greater stiffening effect with inter-fiber bonding. For other membrane characteristics such as fiber elastic modulus and diameter, the stiffness ratio did not change significantly.

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