Peripheral nerve scaffold assembly

Structural organization of nerve guidance conduit (H S Koh et al. J. Neural Eng. 2010; 7: 046003. © Copyright (2010) IOP Publishing Ltd. http://www.iop.org.)

To mimick peripheral nerve, a nanofibrous assembly consisting of a nanofibrous tube and nanofibrous yarn in the lumen of the tube was constructed by Koh et al (2010). The tubular scaffold was fabricated by depositing nanofibers on a rotating rod followed by the removal of the tube from the rod. Nanofibrous yarn was constructed using a liquid flow method although other methods that can produced aligned fibers yarn can be used. A bundle of yarns were gathered and thread through the lumen of the tube and this can be done using a metal hook.

Clements et al (2009) used the contact guidance ability of nanofibers to construct a hybrid setup comprising of aligned nanofiber membrane and commercially available nerve tube [Clements et al 2009]. The tube was first cut into segments to allow placement of the membrane inside before gluing back together. The membrane with nanofibers aligned in the longitudinal axis of the tube was glued into the tube. Two different configurations were constructed as shown in the diagrams below.

Construction of nerve tube with guidance channel. A single membrane consisting of longitudinally aligned nanofibers were glued to lumen of the tube [click image to enlarge].

Nerve tube with three layers of nanofibrous membrane inserted into the lumen

For the nerve guidance conduit described by Koh et al 2010, it is also possible to use the nanofibrous yarn with commercially available tube since both components (yarn and tube) are manufactured separately before combining. As seen in the picture below, a simple hook design can be used to push the bundle of yarns into a tube. The density of the yarn can be varied to achieve a balance between contact guidance and obstruction of nerve axon.

Nanofibrous yarn for insertion into nerve conduits to function as contact guidance.

A common drawback of guidance channel or film is that the materials used for guiding are themselves a barrier to axon growth or neurite. In a scaffold developed by Brown (2012) to be used for spinal cord treatment, its architecture introduces guidance function while minimizes neurite obstruction. Instead of fibrous yarn or film which creates a large signature, longitudinally aligned electrospun monofilaments were assembled in the lumen and encapsulated with an outer nanofibrous covering. A clear advantage of monofilaments is that the individual cross sectional diameter is so small that neurite may easily get pass it. The challenge is to create spacings between the filaments that is sufficiently large for the neurite to get through. The study has already demonstrated alignment of endothelial cells at its core and it is likely that neurite will be guided through as well.

Construction of this scaffold is based on two electrospinning setup concepts. To form the longitudinally aligned monofilament core, a parallel electrodes setup was used. The electrodes may be made out of small diameter rods with their ends facing one another. This way, fibers will bridge across the gap while charges on each fiber will cause them to repel one another, thereby creating the space for neurite growth. Next, without removing the fibers from the rods, the fibers (while still attached to the rod) are placed against a flat plate. A schematic of the process is shown in the figure below. Electrospinning is then carried out to depost more fibers over the longitudinally aligned fibers. Due to the presence of the repulsive longitudinally aligned fibers, the subsequent electrospinning jet tends to deposit the fibers across the longitudinally aligned fibers. The deposited fibers are used to wrap the longitudinally aligned fibers.

Two steps process to construct longitudinally aligned monofilament with an outer sheath.

A closer mimicry of the peripheral nerve extracellular matrix (ECM) is to reconstruct the endoneural tubes within a sheath. Dinis et al (2014) used a parallel electrodes collector system to collect highly aligned nanofiber membrane. A small diameter Teflon stick (0.3 mm diameter) is used to roll the membrane with the fiber alignment along the long axis of the rod to reconstruct the endoneural tubes. Several of these "channels" are then rolled into a larger sheath which is also comprised of fibers aligned along the length of the tube. While in vitro studies of cell alignment on aligned fibrous membrane is promising, it remains to be seen whether such a structure would do well in vivo.

Fibroin scaffold is collected at the end of the electrospinning process and rolled up on teflon sticks (A), higher magnification of the designed fibroin tube (B), SEM observation of a cross section of the fibroin tube (C), higher magnification highlighting the alignment of nanofibers inside the tube (D) and nanofiber-based tube sutured to a rat sciatic nerve (E). Scale bar: 1 cm A), 1 mm B and E), 200µm C) and 30µm D). [Dinis et al PLoS ONE 2014; 9: e109770. doi:10.1371/journal.pone.0109770. This work is licensed under a Creative Commons Attribution 4.0 International.]

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Content [3D nanofibrous structures]