Far-infrared (FIR) therapy has been extensively studied because of its numerous benefits in improving human health. A planar heating element device was designed to assess the in vitro efficacy of FIR therapy by using a flexible carbon fiber composite sheet (CFS) with versatile designs and shapes. The physical properties were optimized for in vitro assays, and low hygroscopicity and stable thermal performance were obtained through thermoplastic polyurethane (TPU) lamination. The CFS evenly released FIR at temperatures ranging from 25 to 80 °C. There was no significant effect on cell viability under most conditions. However, the FIR emitted from the CFS at 70 and 80 °C for 2 h increased apoptotic cell death. The FIR irradiation between 60 and 80 °C for 1 h or less effectively induced osteogenesis of the tonsil-derived mesenchymal stem cells (TMSCs) without inducing cytotoxicity. The most significant osteogenic induction occurred at 60 and 70 °C, as evidenced by increased calcium mineralization and osteogenic markers. This study demonstrated that CFS efficiently generates the FIR, which induces the osteogenicity of TMSCs at a non-cytotoxic level, suggesting the potential applicability of the CFS device for treating bone diseases such as osteoporosis.
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