3D scaffold from short strand electrospun fibers

Illustration of short electrospun fibers

Electrospinning generally forms a membrane comprising of a network of randomly organized fibers. The interconnecting long strands of fiber prevent any major movement in the fibers and maintain the 2D membrane form. To form a 3D block structure after collection of the fibers in the form of 2D membrane, one way is to break up the network of fibers by converting them into short strand fibers. This can be achieved by various means such as chemical treatment, chopping and grinding. The freed short strands of fibers may be recombined to form 3D block structure if they can be bounded together. Chen et al (2016) generated short strand nanofibers by cutting electrospun gelatin/poly(L-lactide) membrane into small pieces (1 cm by 1 cm) and dispersing in tert-butanol by homogenizing. The vibration by the homogenizer was able to break up the nanofibers into shorter strands (average length of 86 µm) and with uniform dispersion in tert-butanol. The mixture was poured into a mold and freeze dried to form a loose, unstable 3D scaffold. Cross linking by glutaraldehyde was carried out to form interconnected networks between the fibers to form a stable 3D scaffold.

The ability to create and control the size of the macropores within the resultant 3D structure using freeze drying allows a hierarchically organized structure to be constructed. The walls of the macropores are formed by collective short strand nanofibers and the walls contain micropores from the space between the nanofibers. Such highly porous structure is extremely light and has been demonstrated to float on cold air [Si et al 2014]. Compression tests showed that it has one of the lowest compressive strength against density [Duan et al 2015]. Using homogenized electrospun Pullulan/PVA nanofiber in 1,4 dioxane solvent and freeze drying, Deuber et al (2016) were able to introduce macro-pores in the 3D sponge while the nanofibers formed micro-pores. Secondary pore sizes between 9.5 and 123 µm may be generated within the electrospun sponge. The resultant 3D sponge following freeze drying was thermally cross-linked to improve mechanical properties. Although the sponge would almost certainly lose its macrostructure arrangement if it is used in wet condition, it may be used in air filtration or as catalytic support under dry environment.

Hydrogel may be used as binding agent to keep the short strand fibers together. Rivet et al (2015) fabricate individual short strand nanofibers by collecting aligned poly(L-lactide) (PLLA) fibers on a substrate with a thin film of polyvinyl alcohol (PVA) as release agent. The substrate with the deposited aligned PLLA fibers was cut into short segments of about 1 mm width, perpendicular to the fiber orientation. The PVA was subsequently removed from the fibers by dissolving in water. The dispersed chopped PLLA fibers were then mixed in agarose/methylcellulose hydrogel. Other materials such as gelatin and fibrin glue may also be used as binding agents. John et al (2023) used short strands electrospun poly(glycolide-co-lactide) (PGLA 90:10)/gelatin and poly-p-dioxanone (PDO)/gelatin fibers in a gelatin matrix for the construction of diabetic wound healing scaffold. To form macro-pores in the matrix, a 3D-printed alginate meshes was used as a sacrificial template and this was immersed in the gel containing the short strand fibers before freeze drying. Cross-linking was then carried out using glutaraldehyde (GA) vapor to stabilize the scaffold followed by the removal of the sacrificial template using ethylenediaminetetraacetic acid (EDTA) to create macro channels. The presence of the macro channels would facilitate cell migration and hence wound healing.

Chopped and dispersed electrospun fibers may be mixed with binding agents and used with 3D printer to build up a defined structure. Chen et al (2019) first cut an electrospun gelatin/poly (lactic-co-glycolic acid) (PLGA) membrane into 0.5 x 0.5 cm pieces before using a homogenizer to disperse the fibers. The fiber strands are dried and mixed with HA and polyethylene oxide solution and then kneaded like dough to form a stable semifluid. This paste is then used with a 3D plotter to create an organised 3D structure. The plotted scaffold is freeze dried. Cross-linking may be carried out to improve the stability of the scaffold. Using this process, scaffolds with fixed macropore sizes can be constructed. The walls in the scaffold will be made of interconnected pores from the electrospun fibers.

3D printed scaffolds using chopped electrospun fibers [Chen et al 2019]

For some materials, binding of short strand fibers need not require an external binding agent. Short strand fibers made of materials with relatively low melting point may be fused together with the application of heat. Xu et al (2015) demonstrated this using electrospun polycaprolactone (PCL) fiber. The collected PCL membrane was grounded into short strands of nanofibers with the aid of liquid nitrogen to make the fiber brittle and isolated using sieve. The shortened fibers were subsequently suspended in a mixture of water, ethanol and gelatin to form a uniform dispersion. The suspension was heated to 55°C which is close to the melting point of PCL at 60°C which caused agglomeration of the fibers. This is probably due to the fusion of the fibers at random contact points.

[+]

[-] comments powered by Disqus

Content [3D nanofibrous structures]