## 1 Introduction  
Mesenchymal stem cells (MSCs) are one of the promising stem cell types due to their availability and relatively simple requirements for in vitro expansion and genetic manipulation [1]. In addition to the well-characterized MSCs derived from bone marrow, increasing evidence suggests that human dental pulp contains a substantial amount of stem cells, i.e., human dental pulp stem cells (hDPSCs) [2]. These cells demonstrate proliferation and differentiation properties similar to those of MSCs [3]. Unlike bone marrow stem cells, the harvest of hDPSCs does not require additional clinical procedures, making them a promising source of stem cells for tissue regeneration [4]. In addition to the application of generating dentin-like structures [5], hDPSCs also exhibit proliferative ability in vitro and can be induced to differentiate into numerous cell types, such as neurons, osteoblasts, and adipocytes [6; 7; 8]. Therefore, hDPSCs have been applied in several regenerative studies including ischemia [9], muscular dystrophy [10], neurological diseases [11], and diseases of bone and cartilages [12; 13]. Bone is a specialized connective tissue that develops through the differentiation of osteo-progenitor cells, primarily osteoblasts, towards gradual ossification, i.e., osteogenesis [14]. Osteoblasts produce amorphously fibrous tissue that gradually becomes densely packed to form core bone matrix through adhesion between the secreted extracellular matrices (ECM) followed by calcium phosphate crystal deposition, which is known as bone mineralization [15]. When the stem cells were cultured in vitro, they could be osteo-induced by chemicals, including dexamethasone, 2-phospho-L-ascorbic acid, and \(\beta\)-glycerophosphate [16]. Because osteogenic growth factors, such as bone morphogenetic proteins ( BMPs) and their receptors, can modulate the proliferation and differentiation of implanted osteogenic cells [17], another induction method is to transect cells with certain kind of BMPs genes to increase their osteogenic capability [18]. Due to multiple functions of BMPs in postnatal bone growth and bone homeostasis [19], they are highly required for osteoblast differentiation and bone formation during embryonic development. Therefore, BMPs are broadly applied for bone regeneration to attract precursor cells from the host to invade scaffolds and induce osteoblastic differentiation. In addition to chemical and biological inductions, physical cues are also applied for bone tissue engineering. Electrical stimulation (ES) has been proven to influence numerous cellular processes, including migration (via TORC2/PI3K), cell cycle, cell proliferation, and angiogenesis [20; 21; 22]. Therefore, different tissues, such as nerves, muscles, and cartilage, have been guided by ES to promote their development and regeneration [23]. Actually, hDPSCs have been administrated in vivo combing pulsed electrical magnetic field (PEMF) treatment for healing injured nerves, however, there was no difference when comparing to the PEMF only group [24]. In contrast, electrodes have been inserted to medium to directly stimulate hDPSCs, which significantly improved the expression of osteocalcin [25], suggesting that direct-current electric field (DCEF) may facilitate the differentiation of hDPSCs compared to the PEMF treatment. Regarding the bone repair, [PERSON]'s law indicates that bone regeneration always adapts to the loading. Because collagen in bone tissue demonstrates piezoelectricity, it has been hypothesized that mechanical signals delivered to cells may be mediated by electrical current generated by bone matrix [26]. Therefore, ES may be a potential treatment to promote differentiation of stem cells. Although the insertion of electrodes to culture medium may easily provide DCEF treatment, this method may elicit unwanted redox reactions of the medium ingredients as well as the faradaic reaction and corrosion of the electrodes [27]. Therefore, we have previously fabricated a conductive polypyrrole (PPy) film to construct an ES device [28].