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Magnesium and Biological Hydrogel Degradation within a Polycaprolactone (PCL) Conduit for Peripheral Nerve Repair
Danielle M. Minteer, BS1, Andrew Villasenor, BS2, Ricardo Andres-Nieves, BS1, Wesley Sivak, MD/PhD1, Sarah Pixley, PhD3, Kacey G. Marra, PhD1.
1University of Pittsburgh, Pittsburgh, PA, USA, 2Summer Premedical Academic Enrichment Program, Pittsburgh, PA, USA, 3University of Cincinnati, Cincinnati, OH, USA.
PURPOSE: The objectives of the study were to 1) observe Mg degradation, 2) assess hydrogel scaffold behavior within a PCL conduit over a period of 7 days at 37 °C, and 3) to evaluate the tissue engineered device as a potential solution to long-gap peripheral nerve repair. Peripheral nerve damage results from trauma, infection, or surgical procedures. While collagen nerve guides are commercially available, current solutions do not provide adequate mechanical support required by the growing axon nor provide promotion of axonal growth for defects larger than three centimeters.
Exposure to magnesium (Mg) ions has been shown to increase neural stem cell proliferation in vitro and in vivo testing has indicated replenishing Mg ions via blood delivery improves functional recovery following nerve damage. Keratin, after degrading into peptides, promotes Schwann cell migration and improves axonal elongation. Poloxamer-407 is a copolymer used in several biological applications, including as a drug delivery system.
Keratin was isolated from freshly cut human hair, rinsed, extracted and freeze-dried and reconstituted with PBS to achieve hydrogel scaffold. Poloxamer-407 solution was reconstituted with PBS in order to achieve hydrogel scaffold. Salt-leeching fabrication of PCL conduits followed a previously established protocol. Next, 0.5 centimeter-Mg wire was placed within PCL conduits and keratin hydrogel, poloxamer-407, or phosphate buffered saline (PBS) was introduced to the lumen of the conduit via a 18G syringe and stored at 37°C. As a control for the PCL conduit, a Mg wire was also embedded within keratin, poloxamer-407, or PBS within a plastic cryotube, with no PCL conduit and stored at 37°C. To assess magnesium degradation in vitro after 7 days, scanning electron micrographs (SEM) were obtained.
RESULTS: SEM indicated that Mg degradation occurred over 7 days at 37°C with exposure to keratin and Poloxamer-407 hydrogels and, interestingly, as well as with PBS, either when incorporated within a PCL conduit, when embedded within hydrogels or with PBS only (no PCL). Magnesium embedded within the keratin scaffold appeared to have surface erosion, while Mg within PBS and poloxamer-407 appeared to have undergone bulk erosion, with less degradation of the Mg in PBS
CONCLUSION: To address the clinical need of a tissue engineered solution to long-gap peripheral nerve repair, degradation of metal magnesium wire within keratin and poloxamer-407 hydrogels was assessed in vitro over a 7-day period. Keratin and poloxamer-407 both contain potential to serve as scaffolds to maintain orientation of a Mg wire within a PCL conduit for peripheral nerve repair. Future directions include applying a coating to the Mg wire to tailor degradation, cytotoxicity studies in vitro and in vivo studies within a large defect rat sciatic nerve hind limb model.
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