Numerous problems that occur during orthopedic interventions and implantation has led to increasing research interests in new hard tissue implant biomaterials, aimed to alleviate many issues associated with operative and post-operative complications in orthopedics. Titanium (Ti) is by far the most widely used metal for hard tissue implants due to its mechanical strength, stiffness, toughness, and low corrosion susceptibility. One of the most frequent issues is poor bio- and osseointegration of Ti, which is why its surface is usually modified by applying new, innovative bioactive coatings that provide strong support, improving the biomineralization process and reducing post-operation pain and infection risk. Hydroxyapatite (HAP,) a calcium phosphate mineral that has a structure similar to the natural bone usually represents a base of such materials. HAP-based coatings can be easily obtained on the titanium surface by powerful electrophoretic deposition (EPD). This method exhibits numerous advantages over the other existing coating methods: room temperature processing, single-step deposition of composite coatings from multicomponent deposition bath including HAP along with polymer components and antibacterial agents, coating deposition on inner or complex shape surfaces, control of coating thickness and morphology by changing the deposition parameters (applied voltage, deposition time) [1,2].

Lately, great attention has been paid to the antibacterial activity of these materials. Although much progress has been made in this field, bacterial infections remain a major problem to be addressed. An effective approach for the prevention of bacterial infections and biofilm formation can be achieved by the addition of various antibacterial agents, which would act locally at the site of implantation, without causing a cytotoxic effect. Also, in this way, a long-term controlled release of antibacterial agents at the desired location would be achieved.

Using single-step EPD at a constant voltage (Figure 1a) we have produced various HAP-based composite bioceramic coatings in combination with polymers (lignin, chitosan), graphene, and antibacterial agents (silver, gentamicin): HAP [3,5], HAP/graphene (HAP/Gr) [3], silver/HAP (Ag/HAP) [4], silver/HAP/graphene (Ag/HAP/Gr) [4], HAP/lignin (HAP/Lig) [5], silver/HAP/lignin (Ag/HAP/Lig) [5,6], HAP/chitosan (HAP/CS) [7], HAP/chitosan/graphene (HAP/CS/Gr) [7] coatings, deposited on Ti from ethanolic suspensions, and HAP/chitosan (HAP/CS) [8,10], HAP/chitosan/graphene (HAP/CS/Gr) [9], HAP/chitosan/gentamicin (HAP/CS/Gent) [8,10] and HAP/chitosan/graphene/gentamicin (HAP/CS/Gr/Gent) [9] coatings, deposited  on Ti from aqueous suspensions.

Detailed physicochemical characterization (X-ray diffraction, infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis) and electrochemical measurements (electrochemical impedance spectroscopy and potentiodynamic sweep measurements) of obtained composite coatings were performed to gain an insight into coatings’ morphology and structure, as well as their bioactive properties. High biomineralization ability was obtained for all the investigated coatings [4-7,10] except the pure HAP coating that lossеs its corrosion stability after just 24h [3]. Graphene addition reduced surface cracks and improved the thermal stability of HAP/Gr, Ag/HAP/Gr, and HAP/CS/Gr with respect to the graphene-free coatings, as a consequence of the toughening action of graphene [3,4]. The addition of lignin in composite Ag/HAP/Lig improved the coatings’ biocompatibility and served at the same time as a carrier for active component providing favorable slow and controlled silver release [5,6]. Chitosan addition improved the coatings’ adhesion to the substrate and provided a suitable platform for gentamicin loading [8-10]. Gentamicin release studies from HAP/CS/Gent and HAP/CS/Gr/Gent coatings exhibited a favorable “burst” release effect in the initial period that should prevent biofilm formation and ensure long-term protection from bacterial infections [8-10].

Strong antibacterial activity against Staphylococcus aureus (Figure 1b) and Escherichia coli bacterial strains was confirmed for Ag/HAP, Ag/HAP/Gr, Ag/HAP/Lig, HAP/CS/Gent and HAP/CS/Gr/Gent coatings. The biocompatibility of all investigated coatings towards human fibroblast cell line (MRC-5) and peripheral blood mononuclear cells (PBMC), as well as mouse  fibroblast cell line (L929), was proved using MTT test of cytotoxicity (Figure 1c).