Ion-Exchange Polymer Nanofibers for Enhanced Osteogenic Differentiation of Stem Cells and Ectopic Bone Formation

Iman Shabani,† Vahid Haddadi-Asl,*,† Masoud Soleimani,‡ Ehsan Seyedjafari,⊥ and Seyed Mahmoud Hashemi

†Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran
‡Department of Hematology, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran
§Department of Stem Cell Biology, Stem Cell Technology Research Center, Tehran, Iran
⊥Department of Biotechnology, College of Science, University of Tehran, Tehran, Iran

ABSTRACT:

Nanofibrous scaffolds with specific modifica- tions have shown promising potential for bone tissue engineering applications. In the present study, poly(ether sulfone) (PES) and sulfonated PES (SPES) nanofibers were fabricated via electrospinning. Calcium ions were then incorporated in SPES by immersion in a Ca(OH)2 solution. The calcium-ion-exchanged SPES (Ca-SPES), PES, and SPES nanofibers were characterized and then evaluated for their osteogenic capacity: both in vitro using stem cell culture and in
vivo after subcutaneous implantation in mice. After 7 days of immersion in simulated body fluid, the formation of an apatite layer was only observed on Ca-SPES nanofibers. According to the MTT results, an increasing stem cell population was detected on all scaffolds during the period of study. Using real-time reverse transcriptase−polymerase chain reaction, alkaline phosphatase activity, and calcium content assays, it was demonstrated that the osteogenic differentiation of stem cells was higher on Ca-SPES scaffolds in comparison with PES and SPES nanofibers. Interestingly, Ca-SPES scaffolds were shown to induce ectopic bone formation after 12 weeks of subcutaneous implantation in mice. This was confirmed by mineralization and the production of collagen fibers using van Kossa and Masson’s trichrome staining, respectively. Taken together, it was demonstrated that the incorporation of calcium ions into the ion-exchange nanofibrous scaffolds not only gives them the ability to enhance osteogenic differentiation of stem cells in vitro but also to induce ectopic bone formation in vivo.
KEYWORDS: nanofiber, ion-exchange polymer, electrospinning, stem cell, osteogenic differentiation, calcium ion

Introduction

Recently, an increasing trend is arising for the utilization of bone graft substitutes (BGSs). It is due to both the requirement of more than two million grafting procedures for the repair of bone damages worldwide and the well-known challenges concerning the use of autografts and allografts. An ideal BGS is considered to have the following properties: (1) osteoconductivity, which is defined as the potential of BGSs to provide a matrix or scaffold for bone formation, (2) osteoinductivity, which is the ability of a material to induce bone formation in nonosseous tissues, (3) osteogenic cells such as stem cells, progenitors, or mature osteoblasts, which produce the new bone matrix, and (4) osteointegration, which is defined as the ability of a material to bond chemically to the host tissue without formation of an intervening fibrous tissue.1−3

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