**1. Introduction**

Rheumatoid arthritis (RA) is an autoimmune disease that affects primarily the synovial cavity tissues on the small diarthrodial joints of the hands and feet [1], leading to persistent synovial inflammation [2] and progressive erosion of the articular structures [3,4]. The triggers for the disease susceptibility and the pathological cascade of events encompass environmental, genetic and stochastic

factors [3,4]. For example, RA has special incidence on females [5], which is related with genetic [5–7] and hormonal factors [5,8–10], but also presents a heterogeneous geographic distribution, being less common in developing countries [11,12], which confirms the involvement of environmental and socio-cultural factors [12].

The disease pathophysiology involves several interconnected mechanisms. Specific antibodies for immunoglobulin G (IgG) mediate the autoimmune process, as well as the imbalanced expression of the pro-inflammatory cytokine's profile and its functionality [1,12–14]. Consequently, this leads to inflammatory processes, autoimmunity enhancement, long-term inflammatory synovitis and joint damage [13,15]. Additionally, locally expressed degradative enzymes digest the cartilaginous matrix and destroy the articular surfaces [1]. Infiltration of B cells, CD4+ T cells and macrophages into the synovium, which in normal conditions is relatively acellular [1,14,16], leads to soft tissue oedema and stiffness [2]. Moreover, other inflammatory cells such as neutrophils, natural killer cells and mast cells play key roles in the disease's progression [13].

From the autoimmune disease (AD) subsets that entail the multiple inflammatory cascades, one of the most significant players is the TNF-α, particularly the over-expression of the TNF-α [11,13,17]. TNF-α, a 233 amino acid protein [18], is a key signaling cytokine in the immune system mainly produced by monocytes, macrophages [19], and B and T cells [2,13,17]. TNF-α stimulates the production of other inflammatory mediators, namely IL's, as well as the recruitment of immune and inflammatory cells into the joint [20,21]. As a regulatory cytokine that manages communication between immune cells and controls many of their functions when deregulated, TNF-α plays a key role in the pathogenesis of chronic inflammatory diseases, such as RA [22,23]. TNF deregulation in RA is linked to TNF-α converting enzyme (TACE or ADAM17), a metalloproteinase that cleaves trans-membrane TNF, releasing the soluble segment [18,24]; and to TNF receptors (mostly TNFr-I) [18,25,26], which are likely to be related with proinflammatory, cytotoxic and apoptotic responses [2].

The increasing knowledge on RA pathogenesis stimulated the development of different therapeutic modalities aiming to avoid joint destruction, minimize the symptomatic profile, and enhance physical function [27]. These options comprise analgesics, symptomatic management and inflammatory drugs [11,28]. Analgesics enclose the non-steroidal anti-inflammatory drugs (NSAIDs) that are commonly used to treat RA. The inflammatory suppressive drugs include glucocorticoids and disease-modifying anti-rheumatic drugs (DMARDs) both non-biologic and biologic. Due to the disease heterogeneity, therapeutic strategies must be tailored to the individual patient in order to achieve a low level of disease activity within a limited period of time [29]. Thus, the combination of two treatment modalities has been proposed to achieve such a purpose [30].

Considering the significant involvement of TNF-α in RA, as well as the rising evidences that this cytokine heads the pro-inflammatory cytokine cascade, it becomes a significant therapeutic target [31,32]. Specifically, these evidences led to the development of TNF inhibitors for the treatment of ADs [33–37], as they became the first class of biologic agents to be used in RA treatment [38]. A specific high affinity monoclonal antibody is used to recognize and neutralize selectively its antigen, i.e., TNF-α [18,27,39]. Five different types of TNF-α inhibitors are currently licensed for human clinical use in RA treatment, namely Infliximab, Entarnecept, Adalimumab, Certalizumab Pegol and Golimumab. With the exception of Infliximab, which is administrated by intravenous (IV) infusion, all other medicines are administered subcutaneously [13]. For long-term control of RA, a continuous treatment is required because of disease flares' risk when the therapy is discontinued [1]. Although these therapeutic modalities have been used for quite some time, they present many shortcomings such as lack of specificity, limited antibody half-life, high cost, response variability, or even lack of response to treatments [11,40]. Due to the systemic character of these treatments, not only target tissues but also healthy tissues are exposed to a significant dose of drug, leading to adverse side effects [41]. These effects are common to many tissues, organs and systems, such as cardiovascular, renal, dermatological, and neurological, or risk of severe infection [11,27,28,40].

Furthermore, some attention has been given to drug carriers, in order to maintain effective concentration levels in plasma for extended periods [41]. Examples of these systems are the encapsulation of Infliximab in polylactide-co-glycolide microspheres [42] or porous silicon 3D structures [43], as well as long-term release of Etanercept by polyelectrolyte complex formulated particles [44]. Despite the promising preliminary results, these systems are still poorly developed.

Considering the shortcomings of these treatments, there is a need for innovative approaches that circumvent the aforementioned adverse side effects. Therefore, the main objective of this work was to develop an implantable system capable of capturing excessive TNF-alpha present in intra-articular cavities of an RA patient. For that, we immobilized a neutralizing TNF-α antibody at the surface of a polymeric substrate (i.e., electrospun nanofibers). This strategy takes advantage of specific interactions between the antibody and the antigen, where the antibody (TNF-α antibody) binds to the antigen (TNF-α) avoiding/limiting its harmful pro-inflammatory effects. After the system´s assembly and proved ability to capture TNF-α secreted by activated monocyte-derived macrophages, the cytotoxicity of the biofunctionalized system was tested with human articular chondrocytes. Indeed, the cytocompatibility and chondrogenic differentiation potential of bare electrospun nanofibers were previously reported by us [45–47], corroborating their potential use in cartilage regeneration approaches.
