**1. Introduction**

Bone tissue continuously undergoes shape remodelling and repair at the microscale through processes of local regeneration, which are regulated by growth factors, hormones and the action of mechanical stresses. In reconstructive surgeries, the regeneration of bone and cartilage by autologous cell transplantation after an injury or tumour removal is one of the most promising strategies in order to reduce issues related to immunocompatibility and consequent immune rejection, as well as to avoid potential pathogen transfer [1]. However, since autologous grafts are often poorly available, an appealing approach is represented by the development of scaffold-based tissue engineering approaches based on bioactive materials for restoring bone morphology and function [2].

Therefore, enormous efforts are being made to create engineered constructs that mimic the physiological environment with its structural, topographical and mechanical properties. To this aim, various biomaterials, metals, natural or synthetic polymers and ceramics, have been investigated but no single one was proved to show all the crucial features required for an optimal scaffold [3,4]. In this framework, a promising approach relies on composite biomaterials with osteoconductive and osteoinductive capabilities, which might allow for osteogenesis stimulation while mimicking the extracellular matrix (ECM) morphology [4,5]. Several studies have been focused on the addition of inorganic and bioactive fillers, such as bioactive glasses, bioceramics or hydroxyapatite [6] in polymeric constructs, with the aim to promote chemical links to bone tissue by forming hydroxycarbonate apatite (HCA) layers as a result of ion leaching, in case of bioactive glass components, into the surrounding physiological fluids. In this process, the precipitation of microcrystalline HCA onto the scaffold surface [7] is due to a well-defined ion exchange mechanism between modifier ions (Na+ and Ca2+) in the glass and hydronium ions (H3O+) in the surrounding fluid, thereby causing dissolution of the glass network [8,9]. Compared to other bioactive materials, silica-based bioactive glasses are available in different compositions, which exhibit remarkable osteoinductive behaviour since they feature ionic dissolution products (Si4+, Mg2+, Ca2+) able to stimulate osteogenesis and angiogenesis [8,9]. In fact, the potential angiogenic effects of silica-based bioactive glass has been recently highlighted, through increased secretion of vascular endothelial growth factor involved in vascularization processes [10]. For these reasons, these materials are not only useful for bone tissue engineering but they might also promote the regeneration of soft tissues as needed in wound healing [11].

In fact, 45S5 Bioglass®has been largely used as inorganic phase in polymer foams and matrices to realize porous composites [12–16]. Silicate [17], borate [18] and phosphate-based glasses [19,20] have been recently tested in bulky bioresorbable polymeric sponges or polymer-coated scaffolds. A number of nano- and micro-fabrication technologies allow these systems to be processed as biocompatible and biodegradable fillers in polymers retaining higher surface-to-volume ratio and interconnected porous networks to better support tissue ingrowth and vascularization. In the last years, the electrospinning technology has been largely developed and notable progress has been made to realize biomimetic porous scaffolds designed for tissue engineering and for drug delivery [21–23]. Electrospinning is a versatile technique, which allows for the fabrication of polymer, ceramic or nanocomposite fibres with diameter ranging from a few tens of nanometres to a few micrometres, which strongly resemble the morphology of the native ECM and provide a networked architecture suitable for cell attachment [22–25]. Various different structures, morphologies and compositions can be achieved in fibres, to make them suitable for different tissue applications including vascular, bone, neural and tendon or ligament [21,25,26]. Particularly, fibres with nanocomposite materials and complex internal [27] or surface [28,29] nanostructures can be electrospun by blends of polymers or from colloidal solutions [30,31].

In this work we introduce nanocomposite electrospun fibres embedding silica-based bioactive glass (SBA2). SBA2 belongs to the SiO2–Na2O–CaO–P2O5–B2O3–Al2O3 class of systems, previously investigated as component of antibacterial and bioactive, bulky bone cements [7,32,33]. The FDA-approved polymer, poly(lactic acid) (PLLA), is chosen as matrix because of its excellent biocompatibility and biodegradability, already assessed in clinical treatments [1], as well as for its excellent processability with electrospinning [34]. The obtained PLLA-SBA2 fibres are characterized in their morphology and in their chemical and mechanical properties. The addition of inorganic particles in a polymeric matrix [35–38] leads to composites with varied mechanical properties, depending on the filler size and on their dispersion in the organic phase [35,36,38], as well as on the fabrication parameters, including the solvent used for electrospinning [38]. Additionally, acellular, in vitro bioactivity and cell viability are investigated, evidencing the biocompatibility of the PLLA-SBA2 fibrous composites. Overall, dispersed silica-based bioactive glass in resorbable polymeric composites with microscale texturing are highly promising systems for supporting cell cultures as well as for the development of biomedical applications. The novelty of this work is represented by the successful incorporation of SBA2 in electrospun PLLA fibres, not reported previously in literature and also on the correlation of the nanopores on the fibres surface with fibres degradation and bioactive glass particles release.
