Professor of Biomedical Engineering Columbia University, United States
Introduction: Bone and cartilage have known piezoelectric properties; wherein electrical activity can be generated in response to mechanical deformation. Piezoelectric, smart materials are promising as scaffolds for cartilage repair applications due to their self-powered capability for electrical stimulation to promote tissue formation. Yet, the use of natural piezoelectric materials has been unexplored for promoting cartilage repair, which are advantageous compared to synthetic piezoelectric polymers because of biodegradability and environmental sustainability. Bone marrow-derived mesenchymal stem cells (MSCs) may be a promising cell source, however developing scaffolds that promote MSC chondrogenesis and reduce hypertrophy may be beneficial for the formation of stable hyaline cartilage. In this study, we evaluated piezoelectric gelatin scaffolds for growth, chondrogenesis and hypertrophic differentiation of human MSCs as a potential scaffold to treat cartilage lesions/osteoarthritis.
Materials and
Methods: Gelatin (bovine skin) were electrospun using standard conditions to form random fibrous mats and crosslinked. The scaffolds were poled by exposure to a high electric field to render the scaffold piezoelectric. The following scaffolds were studied: negatively poled (Poled(-)) and positively poled (Poled(+)), which are based on the direction of the applied electric field, and the unpoled scaffold, which serves as a control. The piezoelectric properties of scaffolds were tuned by poling using different electric fields and voltage output was measured using cyclic compression at 10% deformation. MSCs at passage 3, were cultured on scaffolds in complete chondrogenic media containing 10 ng/ml of TGF-β3 over 28 days. Cell growth was evaluated using the picogreen assay. Production of collagen type II, collagen type I and aggrecan was determined by ELISA and immunostaining. Production of sulfated glycosaminoglycan (GAG) was measured by the DMMB assay. Gene expression for chondrogenic and hypertrophic markers was determined by qRT-PCR. Statistical analysis was performed using one-way and two-way ANOVA with comparisons by Tukey’s posthoc test (p < 0.05).
Results, Conclusions, and Discussions: Results and
Discussion: We demonstrated a tunable piezoelectric property for gelatin scaffolds. The piezoelectric coefficients, d33, of low piezo (LP) scaffolds were -0.14±0.07 for Poled(-), and 0.12±0.09 pC/N for Poled(+). The high piezo (HP) group had significantly higher d33 values of -0.33±0.21 and 0.30±0.20 pC/N for Poled(-) and Poled(+), respectively (Fig. 1). A clear voltage output with the peak voltage of 29 and 44 mV for LP and HP poled groups were measured, while a consistent waveform was not detected for the unpoled group. LP scaffolds were used for subsequent cell studies. Poled(-) gelatin scaffolds showed a significantly higher cell growth at days 7 and 14 compared to the unpoled (UP) scaffold (Fig. 2). A significant increase in GAG production was observed from days 14 to 28 for all groups, while cells on Poled scaffolds had increased GAG production as compared to UP group at day 28. Production of collagen II, collagen I and aggrecan was detected (Fig. 3). The gene expression for collagen II significantly increased from days 14 to 28 only for Poled(-) group, while no statistical differences were detected between Poled(+) and unpoled groups. The expression of hypertrophic markers (Fig. 4), collagen type X, vascular endothelial growth factor (VEGF) and Runt-related transcription factor 2 (RUNX2) were significantly lower for Poled(-) as compared to Poled(+) and UP, suggesting that negatively charged scaffold reduces hypertrophy while enhancing growth and chondrogenesis. Poled(-) groups showed significantly lower expression of matrix metallopeptidase 2 (MMP2) as compared to Poled(+), which could potentially promote less matrix degradation and support extracellular matrix homeostasis.
Conclusions: This study demonstrated the potential of degradable piezoelectric gelatin scaffolds and the importance of the charge of the scaffold in enhancing growth, chondrogenesis and reducing hypertrophy. Future directions will investigate the chondrogenesis on these scaffolds when subjected to mechanical loading to evaluate the piezoelectric effect.
Acknowledgements (Optional): We would like to thank the support from National Science Foundation (NSF)-Center for Engineering Mechanobiology (CEMB)-1548571.
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