Direct Electrospinning of Degradable Hydrogel Nanofibres

Xu, Fei (McMaster University)
Sheardown, Heather (McMaster University)
Hoare, Todd (McMaster University)

Introduction

Hydrogels have attracted significant interest as matrices for tissue engineering due to their many advantages such as low protein adsorption, high water content, tunable chemical properties and high cell compatibility. [1] Considering the micro- and nanofibrous structures that exist in natural extracellular matrix (ECM), hydrogels with micro- or nanostructures may offer unique advantages for creating more biomimetic synthetic ECMs. [2] Electrospinning has been widely demonstrated to offer a versatile and effective method to prepare nanofibres. In our study, electrospinning was used to directly prepare hydrogel nanofibres to mimic ECM. The hydrogel nanofibres could swell in phosphate buffered saline and degrade at physiological conditions.

Materials and Methods

1. Materials Poly(oligoethylene glycol methacrylate) (POEGMA) polymers were synthesized via chain transfer polymerization as previously reported. [3] POEGMA polymers were functionalized with hydrazide groups via carbodiimide-mediated conjugation of a large excess of adipic acid dihydrazide, while aldehyde groups were incorporated via copolymerization with a diacetal-containing comonomer. The resulting hydrazide and aldehyde-functionalized POEGMA can crosslink to rapidly form a hydrogel in situ at room temperature. [3] Poly (ethylene oxide) (PEO, Mw = 600,000 g/mol, 5% w/v in deionized water) was used as an electrospinning modifier to increase the chain entanglement of the whole system. 2. Electrospinning Hydrazide and aldehyde functionalized POEGMA (15% w/v in deionized water) were loaded into separate barrels of a double barrel syringe, as shown in Figure 1. Each POEGMA polymer was mixed with PEO solutions before loading into the double barrel syringe. A static mixer was attached to the end of the syringe to allow for intimate mixing between the two reactive POEGMA precursors. Electrospinning was performed using a voltage of 8.5 kV and a falling distance of 10 cm. 3. Characterization Scanning electron microscopy (SEM) was used to investigate morphology of hydrogel nanofibres. Swelling ratios and degradation kinetics were both assessed gravimetrically. Confocal laser scanning microscope (CLSM) and AT-FTIR were used to investigate the co-localization and distribution of hydrazide and aldehyde functional groups.

Results

The diameters of electrospun hydrogel nanofibres were between 200-300 nm, as shown in Figure 2. The optimized falling distance was 10 cm. To prove that a hydrogel scaffold was indeed formed, the electrospun nanofibres were placed in deionized water and were observed to swell but not dissolve, with the fibrous structure maintained over at least two weeks in phosphate buffered saline (Figure 2E-H). The diameter of fibers increased after exposure to PBS due to swelling of the hydrogel network comprising the fibres, with equilibrium achieved in 10 minutes (significantly faster than that achieved with non-fibrous hydrogels of the same composition). AT-FTIR result showed that hydrogel scaffolds exhibited a narrow peak at 1721 cm-1 related to C=O from aldehyde groups and a broad peak at 1662-1670 cm-1 related to C=O from hydrazide gruops. Fluorescein isothiocyanate (FITC) was used to label hydrazide groups and rhodamine was used to label aldehyde groups to assess the quantity of residual (uncross-linked) reactive functional groups and track the distribution of each polymer within the fibrous matrix. CLSM images (Figure 3) of the scaffold showed co-localization of the green (FITC, 488 nm) and red (rhodamine, 543 nm) signals, confirming the observed nanofibers consisted of hydrazone-crosslinked hydrogel.

Discussion and Conclusion

Hydrogel nanofibres were prepared for the first time directly via electrospinning, using hydrazide and aldehyde-functionalized POEGMA as the gelation agents and a small fraction of high molecular weight PEO as an electrospinning aid. The morphologies of electrospun matrix could be adjusted from smooth nanofibres to bead-in-fibres by changing the PEO concentration as well as humidity. SEM and CLSM confirmed that the electrospun nanofibres consisted of crosslinked hydrogel. The hydrogel nanofibres could swell in PBS and reach to equilibrium in 10 minutes but degraded in 0.1 M HCl within 40 hours via hydrazone bond hydrolysis. These matrices are anticipated to use as cell-encapsulating scaffolds, with other application also now being explored.

Figure 1. Scheme of electrospinning hydrogel nanofibres.

Figure 2. SEM images of electrospun hydrogel nanofibres with different falling distances (A) 5 cm, (B) 10 cm, (C) 15 cm, (D) 20 c; and SEM images of scaffolds (E) not exposed to water, (F) after exposure to water, (G,H) center of scaffold between exposed and not exposed to water sections.

Figure 3. CLSM images of electrospun hydrogel with FITC at 488nm (A), rhodamine at 543 nm (B), bright field image (C) and merged image (D).

References

[1] Ahmed EM. J. Adv. Res. 2013; 18: 1-17. [2] He CL, et al. J. Mater. Chem. B. 2014; 2: 7828-7848. [3] Smeets NMB, et al. Chem. Commun. 2014; 50: 3306-3309.

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