## 1 Introduction  
Accidents resulting in bone trauma, degenerative diseases at the bone level and failed implantation of bone substitutes are generating the highest demand in terms of artificial materials with suitable properties for hard tissue engineering [1]. As a consequence, modern medicine must respond to the challenges imposed by the need to increase the quality of life for the patients involved in such scenarios [2, 3]. Beyond the basic features of an implant, namely size, shape, mechanical strength, lack of cytotoxicity [4], current concerns are directed towards the design and fabrication of devices with superior characteristics, among which bioactivity and osseointegration potential are the most important [5]. By combining well-known and long-tested materials with bioactive coatings [6, 7], antibacterial agents [8, 9], antioxidant compounds [10] or biologically active molecules [11], novel systems that mimic the means of healing proper to the living body can be proposed, subsequently leading to bone regeneration and remodelling. If, to date, the focus has been on the quality and durability up to 15 years of medical implants, the real and actual states are defined in the activity of metallic substitutes, namely absence of bioactivityand low chemical stability [12]. Poor bioactivity leads to an incorrect fixation of the substitute, requiring a later replacement through surgery, while rapid corrosion favours the spread of toxic ions in the body, endangering patient's health [13]. Surface engineering is absolutely necessary to contribute to the optimization of the implant-tissue interface and overall physical condition. In other words, a successful replacement operation implies the emergence of a strong bond between the implantable entity and host tissue, followed by an active bioconnection at the surface level. The identified solution consists in covering the orthopaedic implants with ceramics [14], glasses [15] or vitroceramics [16; 17] in order to functionally modify their external side and provide bioactivity and increased biocompatibility, as well as a specific framework for performant integration and osseointegration [18; 19]. Among the multitude of artificial materials that enhance the efficiency of metallic implants (demineralized bone matrix, coralling hydroxyapatite, calcium phosphate-based ceramics, etc. ), bioglasses are the only ones which favour osteoinduction, besides osteoconduction and osseointegration. Contrariwise, their disadvantages begin with a short lifespan and continue with low mechanical properties [20]. However, the process of ion doping allowed the improvement in bioglass performances: magnesium, strontium, iron, zinc and silver are just a few of the tackled metallic cations [21; 22]. Moreover, doping with targeted elements can stimulate bone regeneration, cells differentiation and finally healing [23]. In conclusion, the symbiosis between a brittle bioactive glass and a resistant bioinert metal, in the form of a coating attached to a support, means a winning approach that ensures a high level of bioactivity and excellent mechanical strength. Bioactive glasses are inorganic materials with a high ability to react to surface stimuli, triggering biological effects in contact with physiological fluids or living cells. The ionic exchange and further apatite formation on their surface represent mechanisms that are extensively studied at present [24]. In addition, the unlimited compositional possibilities make a little explored field available to the researchers, where exceptional findings can arise based on synergistic implications in complex systems [25]. Zinc (Zn) is an element with great potential for cellular growth, proliferation and differentiation, also having a particularly important role in DNA replication and enzyme production; additionally, it is a well-known antibacterial agent [26]. [PERSON] et al.