Post-electrospinning patterning

Electrospinning typically creates a nonwoven mesh of fibers with nanometer or micrometer diameter. For some applications, it may be desirable to introduce patterns on its membrane. Macro-patterns in the form of millimeters scale islands have been created on electrospun membranes for the purpose of isolating individual samples of cancer detection markers. This is carried out with the use of UV curable resin and a module to create rings on the membranes with the UV curable resin forming the ring barrier [Wang et al 2013]. In cases where the modifications or patterns are in the micron dimension, standard techniques such as mechanical cutting or shearing is not appropriate as they are likely to damage the fiber and the precision is also insufficient. Appropriate methods for physical and structural modification of electrospun fibers include lasers, photolithography and chemical methods.

(A) Schematic diagram of UV curable resin penetrating through the porous structure, and bonding with the glass substrate and the surrounding fibers (cross section). (B) and (C) Digital camera pictures of the top view of real products. (D) Micro-chambers (islands) filled with different color inks. [Wang et al. PLoS ONE 2013; 8(12): e82888. doi:10.1371/journal.pone.0082888.g003. This work is licensed under a Creative Commons Attribution 4.0 International.]

Laser cutting uses heat generated from the laser beam to cut the material. In electrospun membrane, it is important to demonstrate that the heat from the laser beam is highly localized and does not cause extensive collateral damage to the neighboring fibers. Femtosecond laser pulses have been shown to be capable of creating holes of various sizes quickly [Lee et al 2012, Wu et al 2011, Rebollar et al 2011]. Collateral damage by the heat used to create the holes is sufficiently low to preserve the fiber structure at the wall of the cavity [Rebollar et al 2011]. Advances in the use of lasers on electrospun membrane have moved beyond producing through-hole to controlled depth ablation. Jenness et al [2012] used phase modulation of femtosecond laser beams to control ablation with resolution of width 1 to 15 µm and depth 15 to 110 µm on the electrospun membrane. Holes of various shapes and depths have been constructed using this method.

Another method of hole creation is to use short wave length ultra violet radiation with masking. In this method, short wave length ultra violet radiation is used to degrade exposed part of the membrane. This technique has been successfully used to create holes of 100 µm diameter on poly(D,L-lactic-co-glycolic) acid and poly(L-lactide-co-epsilon-caprolactone) electrospun fiber membranes [Dong et al 2008]. This method may not be useful for polymers that are resistant to photo-degradation from ultra violet radiation.

Chemical methods used in photolithography and selective local dissolution of fiber membrane has been used for creating simple to more complicated patterns. Surface chemical patterns technique comprising of microcontact printing, lithography and pattern-transfer has been successfully tested on electrospun membrane to create patterns on a substrate with nanofibers. Shi et al (2009) used electrospinning to deposit polymethylglutarimide (PMGI) fibers on a polydimethlysiloxane (PDMS) substrate. The fibers were subsequently transferred onto a glass substrate through heat treatment. Finally, photolithography technique and reactive ion etching was used to remove unprotected fibers to create a patterned fibrous structure on the substrate. Short strands of electrospun fibers may also be produced using this technique. High surface area of electrospun nanofibers in a form of relatively thin membrane ensures quick dissolution upon contact with suitable solvent. Jia et al (2014) made use of this property to form patterns on nanofibrous membrane by using ink-jet printer for selective local dissolution. Instead of filling "ink", suitable solvents are loaded and various micro-patterns can be easily printed onto the electrospun nanofiber mesh.