## 1 Introduction
The proposed studies have been inspired by in vivo lineage tracing experiments in mice where the preponderance of evidence (labeled cells from three independent mouse strains including postnatal tamoxifen induction, proliferative capacity of this cell lineage, no migration of adjacent articular chondrocytes) suggests that cells from the synovium are the source for healing [23; 24]. These studies suggest repair cells migrating into the full thickness cartilage defects are primarily derived from the synovium rather than bone marrow or adjacent cartilage [23; 24]. Therefore, strategies aimed at targeting synovial cells could offer novel and effective therapies to boost repair and regeneration of joint tissues [23]. The poor intrinsic healing ability of articular cartilage is predominantly attributed to scarce migration of native cells that have the potential to repair the defect site [25]. Electrotherapeutics possess potential as low-cost, low-risk, and non-invasive adjunctive therapies for improving the rate of articular cartilage repair [26; 27]. Electrical stimulation of native cells within larger tissue and organ systems is promising as endogenous electric field (EF) gradients have been shown to guide cellular behavior during wound healing [28; 29]. Pulsed electromagnetic fields (PEMFs) and applied direct current (DC) EFs via galvanotaxis are two potential electrotherapeutic strategies that have yet to be fully explored in the context of promoting cartilage repair by stimulating the migration of native FLS within the wound or defect site. PEMF treatment has been clinically used to stimulate bone repair and alleviate joint pain [30]. Studies have shown that PEMF stimulation is an adjuvant, anti-inflammatory therapy and pain management tool that contributes to improvements in joint function as well as pain relief following arthroscopy, including microfracture, chondrograsion, collagen scaffold seeded with bone marrow-derived cells (BMDCs), matrix-assisted chondrocyte implantation (MACI), osteochondral allograft (OCA) transplantation, and knee arthroplasty [31; 32; 33; 34]. However, the use of PEMF treatment for articular cartilage repair has not been thoroughly investigated yet, with limited mechanistic studies characterizing the application of this electrotherapeutic technique in initiating an intrinsic healing response [35; 36]. Previous work in our laboratory demonstrated the positive impact of PEMF on articular cartilage defect repair in vivo with engineered cartilage constructs [37]. The current study develops an in vitro model, where experimental boundary conditions are well defined, to gain a better understanding of how PEMFs can be used to directly augment natural cartilage repair strategies. DC EFs is another electrotherapeutic strategy that may stimulate the intrinsic repair process of synovium and articular cartilage. EFs of strengths from 1 to 10 V/cm have been shown to induce directed movement (galvanotaxis) and shape change (galvanotropism) in several musculoskeletal cell types, including chondrocytes, osteoblasts, and meniscal fibrochondrocytes [38; 39; 40; 41]. Endogenously generated gradients within this range of EF strengths have also been shown to guide cell migration at the cut surface of wounds [29; 42]. Clinically, the placement of electrodes for galvanotaxis may involve slightly invasive interventions compared to PEMF systems. Nonetheless, similar to PEMFs, DC EF galvanotaxis may also serve as a useful tool for promoting articular cartilage repair via directed FLS migration toward the defect region. FLS migration in 2D is yet to be fully explored in the context of promoting intrinsic cartilage repair under electrotherapeutic stimulation. Study 1 characterized the migration of bovine FLS using an in vitro scratch assay under PEMF exposure to assess the rate of wound closure following cruciform injury.