**1. Introduction**

Tissue regeneration is the result of a complex process, which involves the synergistic contributes of cells, biomaterials, and bioactive factors (e.g., drugs, ions, growth factors). In most cases the complete regeneration of tissues fails due to a lack of vascularization; hence insufficient angiogenesis is one of the major current pathological concerns. In addition, the occurrence of drug-resistant infections may lead to several complications and greatly enhance the number of non-healing cases [1]. In order to overcome these issues, much attention has focused on the design of multifunctional systems which are able to simultaneously promote all the processes involved in the tissue/organ healing cascade. The incorporation of drugs, growth factors or inorganic nanomaterials into hydrogels has been extensively investigated [2,3], with the aim of combining in a single formulation several therapeutic abilities (such as pro-angiogenesis, anti-bacterial, anti-inflammatory). Among the approaches already

explored, the combination of hydrogels with inorganic nanocarriers, which are able to incorporate and release several agents, represents a promising therapeutic strategy with a high potential for clinical translation. With this perspective, Zhu et al. prepared chitosan/mesoporous silica nanoparticle (MSN) composite hydrogels for localized co-delivery of drugs and macromolecules [4]. Similar hydrogel embedding ibuprofen-loaded MSN was proposed for surface coating of titanium implants [5] and poly(vinyl alcohol) hydrogels carrying oil-loaded MSNs containing curcumin were studied for dermal application [6]. More recently, a similar strategy has been explored to design thermosensitive hydrogels based on a poly(N-isopropylacrylamide) (PNIPAM) copolymer carrying mesoporous silica for dual-responsive (i.e., temperature and light) drug release [7] and for bone tissue regeneration [8].

More recently, in the field of tissue regeneration, considerable attention has been addressed towards the use of mesoporous bioactive glasses (MBGs) enriched with specific metallic ions (i.e., Sr2+, Cu2+, Ag<sup>+</sup>, Ce3+), as multifunctional therapeutic platforms for advanced medical devices [9,10]. Among the elements which can exert therapeutic properties, copper has been extensively investigated, due to the well documented pro-angiogenic effect alongside its antibacterial potential [11,12]. Several works have evidenced the antibacterial potential of Cu-containing MBGs against both planktonic bacteria and biofilms. In particular, Wu et al. [11] tested the successful antibacterial potential of Cu-containing MBG scaffolds against *Escherichia coli bacteria* and, accordingly with this, the authors recently demonstrated that Cu-containing MBG nanoparticles and their ionic extracts exert promising antibacterial activity against both Gram positive and Gram negative bacteria (*Staphylococcus aureus*, *Staphylococcus epidermidis* and *E. coli*) and are effective in inhibiting and preventing biofilm produced by *S. epidermidis* [13]. In addition to antibacterial abilities, Cu-substituted bioactive glasses were also demonstrated to stimulate angiogenesis and promote revascularization of soft and hard tissues [14,15].

Based on these promising results, we have recently reported the development of hybrid formulations able to combine the injectability of thermosensitive poly(ether urethane)-based (PEU) hydrogels and the capability of MBG particles to release functional copper ions [16]. This system was developed for a prolonged and sustained release of copper ions, with the final aim to promote simultaneously angiogenesis and anti-microbial effect [13,17–19]. However, no effects against persistent inflammation could be achieved with the formulated system. Hence, in order to expand the potentialities of the previously developed hybrid platform, Cu-substituted MBGs, in the form of micro- and nano-particles, were loaded with ibuprofen (Ibu), chosen as an anti-inflammatory agent, and embedded into PEU hydrogels. The resulting hybrid formulations were evaluated in terms of ion/drug co-delivery kinetics, gelation behavior and stability in an aqueous environment.

Since the main goal of this contribution was the definition and the characterization of a novel and adaptable drug/ion release platform, our investigation mainly focused to identify the maximum amount of particles which could be incorporated within the hydrogel and to assess the related effect on the gelation behavior as well as the ion/drug co-release capacity that the final combined formulation could provide. The high potentiality of the proposed strategy lies in its wide versatility, as by tailoring the composition and cargo of MBGs, different ions and bioactive factors can be co-released according to the therapeutic effects required by the final targeted application. Moreover, their release timing and dosages can be finally optimized according to the requirements of each specific application, to make the released payload effective (i.e., releasing kinetics within the therapeutic window), while avoiding cytotoxic concentrations. In addition, the use of a custom-made PEU hydrogel as the vehicle phase of the MBGs provides the final system with a further degree of freedom. The characteristic LEGO-like chemical structure of PEUs allows polymer properties to be finely tuned by changing their constituting building blocks. This versatility opens the way to the design of stimuli-responsive hydrogels with an enhanced control over payload release and/or hydrogel dissolution/degradation, thus improving the release properties compared to those observed for MBGs alone. To the best of our knowledge, this is the first attempt to design a hybrid injectable formulation based on a thermo-sensitive hydrogel embedding MBGs able to co-release therapeutic ions and drugs.
