Ultrasonic Vibration

The conventional use of ultrasonic vibration is to encourage dispersion or for loosening stains from a surface through high frequency vibration of the suspension or object. The same concept has been shown to be applicable for loosening the entangled non-woven electrospun fiber membrane to create a more open and porous structure [Lee et al 2011]. For optimal transmission of the vibrating force into the membrane, it is preferred that the membrane is suspended in water during the treatment. Ultra-sonication of just 45 s has been shown to increase the thickness of electrospun chitin membrane from 120 µm to more than 2.5 mm. The graph of membrane thickness vs ultra-sonication duration shows an exponential increase before plateauing at about 45 s which represents maximum unraveling of the tangled fibers and corresponding maximum pore size [Jung et al 2012].

The energy and duration used in the sonication process needs to be optimized to generate a 3D scaffold. Gu et al (2013) found that for electrospun chitosan membrane, a higher energy of 225 W is necessary compared to 125 W [Lee et al 2011] for opening electrospun PLLA membrane. Such differences may be due to bonding strength between inter-fibers contact points as chitosan is likely to exert strong intermolecular hydrogen bonding between molecules. Maximum thickness was obtained for sonication at 4 minute which yielded a 300% increase in thickness over non-sonicated samples [Gu et al 2013].

There are a few potential points to look out for using this method. One is that the membrane should be fully infiltrated by water especially for hydrophobic polymers. Poor infiltration of water will result in areas of differing fiber density after sonication. To achieve good infiltration, one way is to pre-wet the fiber in ethanol (low surface tension) before putting the membrane into the ultrasonication bath. In electrospinning over a large area, the fiber density within the membrane is likely to be non-uniform. Any difference in fiber density will be magnified by the sonication process resulting in a non-uniform membrane thickness. Similarly for electrospun thick membrane, the membrane may be made out of layers of nanofiber with an interface between the layers which may not be apparent when the membrane is relatively thinner but are magnified during the ultrasonication process [Jung et al 2012, Gu et al 2013]. It is also important to note that the mechanical integrity of the fibers may be adversely affected by the ultrasonication to the extend of breaking into shorter strands especially for more brittle polymers such as poly(methyl methacrylate) (PMMA) and polystyrene [Sawawi et al 2013]. This is known as shear-induced scission where the cavitation bubble collapsing on fibril wall causing shortening of fibrils [Huang et al 2009]. Nevertheless, this technique is a simple and straightforward to increase the volume and porosity of electrospun membrane. Cell cultured on electrospun chitin scaffold following this treatment has been shown to allow cell infiltration [Jung et al 2012].