Electrospun fibers as absorbent material

Water absorbent material used in baby diapers

There are many absorbent products found in healthcare, household and hygiene market. These products provide a clean environment by taking away the excessive or unwanted fluid. Commonly known products are diapers, sanitary pads and wound dressings. Electrospun fibers have the potential to significantly improve existing absorbent products with its high surface area and high membrane porosity.

There are a few basic requirements for a good absorbent material. First, it must be hydrophilic to quickly draw away the fluid. Secondly, it must have the ability to hold onto the fluid and in some applications, even when it is subjected to load. Thirdly, it should have sufficient absorption capacity. While meeting these requirements generally come from the materials property, the structure and form of electrospun fibers membrane will enhance performance in particular, the rate at which the fluid is being absorbed and the maximising the holding capacity. Hansen et al (2005) tested the performance of electrospun membrane in water absorption made from Tecophilic polymer loaded with Waterlock superabsorbent polymer. The resultant hydrogel fibers absorbed 400-5000% in water and 500-1250% in synthetic urine. At loading of less 50% Waterlock, the fiber was able to achieve maximum water absorption within 5s. At higher loading of 85% Waterlock, it takes only 30s to reached 96% of equilibrium absorption.

Materials based on acrylic acid and acryl amide compounds are commonly used in super-absorbent products but there are concerns about their impact on health and environment. Other more benign materials have been tested as super-absorbent product by making them into the form of nanofibers through electrospinning. Islam et al (2015) constructed a composite nanofiber comprising of natural polysaccharide pullulan(PULL) with polyvinyl alcohol (PVA) and montmorillonite (MMT) clay using electrospinning. Heat treatment was carried out to stabilize the nanofibrous membrane. The resultant membrane showed a water absorbency of 143 gg^-1 in distilled water and 40 gg^-1 in 0.9 wt% NaCl solution.

Given the potential of electrospun membrane as a fast absorbent material, tests have been carried to determine its performance for various applications. Yadav et al (2016) investigated the use of cellulose acetate electrospun nanofibers for sanitary napkins and compared its performance against commercial sanitary napkins. Their results showed that cellulose acetate (CA) electrospun nanofibers membrane performed better than commercial sanitary napkins when tested using synthetic urine and saline solution. Under compression, the absorbency in saline solution for CA was measured to be 962% which is higher than commercial sanitary napkins. Interestingly, the addition of sodium polyacrylate into electrospun nanofibers did not improve the performance of the membrane but instead reduced it.

The good absorbency of electrospun fibers comes from both the material used and the high surface area of the nanofibers. The ability to form a three-dimensional (3D) structure made of electrospun fibers will increase its absorbent capacity and open up new areas of applications. McCarthy et al (2021) also used NaBH₄ as a foaming agent for expansion of electrospun polycaprolactone (PCL) nanofiber mats. 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. The gelatin-coated PCL nanofiber matrices showed super absorptive properties demonstrating the ability to absorb more than 6000% of its initial weight in simulated body fluid (SBF). This is in contrast to 4521% of its weight for uncoated matrices.

Morphology of nanofiber mats produced using the Fluidnatek LE-100 with 3D printed caps on a 20-emitter array before and after expansion and 0.5% gelatin-coating. A, SEM image of LE-100-fabricated nanofiber mat. B, SEM image of the cross section of 3D nanofiber matrices expanded from a LE-100-fabricated nanofiber mat followed by 0.5% gelatin coating. C, High magnification of (B). D, A side (i. and front) view of a LE-100-fabricated nanofiber mat. E, A side view of a 3D nanofiber matrice expanded from a LE-100-fabricated nanofiber mat followed by 0.5% gelatin coating. F, Thicknesses of the nanofiber mats before and after expansion. (****P <.0001) [McCarthy et al 2021]

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