Ceramic biomaterials are widely used in medicine, mainly in orthopaedic and dentistry applications [1, 2]. This is due to their high hardness, resistance to tribological wear, chemical inertness, lack of inflammatory reaction for the host organisms, their ability to form complex shapes, and aesthetic effects of prostheses, these are important for dental prosthetics [2–4]. Ceramics such as alumina and cubic zirconia are used for acetabulum joints, auditory ossicles, bone scaffolds, dental prostheses (such as crowns), bridges and prosthetic implants, and abutments [5, 6]. Ceramics are popular in biomedical applications and allows for the elimination of problems with the use of metal implants such as metallosis and corrosion which can cause inflammation, colonization by pathogenic microorganisms, rejection of the implants by the human body, and in extreme cases can lead to cancer [7, 8]. The adverse effects of metallic implants is associated with the accumulation of harmful metallic ions in detoxification organs [7, 9]. Particularly important problems are associated with inflammation caused by pathogenic microorganisms, which are the main cause of inflammation and rejection of the implants [7]. The main pathogens, which cause implant rejection, are from the fungal family Candida albicans, while within bacteria, the main culprits include bacteria from the Typhylococcus aureus group [10]. Various other organisms also can cause complications, these include; Aggregatibacter actinomycetemcomitans, Eikenella corrodens, fusobacterium nucleatum, and Tannerella Fforsythia [11]. These pathogens are well-known symbiotes that are frequently isolated in oral diseases, and because of their nature, varying antimicrobial treatments are required to treat them [11 − 3. Based on the published data, it is assumed that over 65% of all human infections have been estimated to be biofilm-related [14]. Bacterial and fungal inflammations, and their related issues, are the most prominent in metal biomaterials such as 316 LVM SS and Ti6Al4V alloy [14–16]. Inflammations, bone lose, are problems can be avoided with the use of ceramic biomaterials.
Despite the numerous advantages, the application of ceramics brings some issues that arise from the physicochemical nature of this group of materials. This is analogous to the zirconium drawbacks connected with metastable nature of this materials, which a significant amount of work has been dedicated to, such as the work of Guo, Gremillard and Chevalier [17, 18]. In the work of Chevalier and Gremillard, the problem of the influence of the body environment on the structure is addressed, i.e. the structural changes caused by the interaction between the zirconium material with saliva and moisture [19]. These structural changes are described with the term low thermal degradation (LTD) and are a well-known mechanism that destructively affects dentures and prostheses made from ZrO2 [19, 20]. The main problem that results from LTD is swelling of zirconia grains, falling out of the structure and, as a result, the formation of critical fractures of the dentures and prostheses [17]. The mechanisms associated with the formation of these adverse phenomena have been described by Guo, who presents kinetic models which are confirmed experimentally by other authors [21, 22]. Other disadvantages of ceramics are their high fragility and a relatively time-consuming and expensive production process [23, 24]. The proper selection of process parameters when milling dental crowns and bridges creates many problems, for the case of zirconia, special attention needs to be paid to the cooling of the material during milling to prevent overheating of the structure, which negatively affects the durability of the dentures [23, 25]. Besides, to the best of our knowledge, there is no clear position as to whether water or oils used during cooling initiate the uncontrolled phase transformation and in consequence LTD which cause premature ageing and destruction of dentures [17, 18]. These and other problems i.e. metastable phases stabilisation, dopant selection have been previously addressed in the literature [26–28]. In summary, biomedical ceramics are an interesting class of materials but require a lot of attention and further development to aforementioned deficiencies.
Polymer-ceramic composites i.e. ZrO2-PEEK (polyether ether ketone), SiO2-PMMA (poly(methyl0 methacrylate) as biomaterials are an interesting alternative to ceramics and metals. These materials aim to combine the required properties of the ceramic and polymer groups and minimize occurrence of structural defects, as much as possible. Particular interesting to this type of composite is connected with durability and tribological resistance of ceramics, and low costs and the resistance to brittle fracture of polymers. Previous literature has focused on oxide ceramics combined with PMMA and PEEK [29, 30]. The main problem associated with obtaining valuable polymer-ceramic composites is achieving a secure connection between the ceramic and the polymer [31, 32]. Depending on the surface treatment of the ceramic grains, a chemical or mechanical bond can be formed which is the adhesive between the ceramic grains and the polymer resin. Therefore, surface modification of ceramics is one of the most important issues in these composites. An important way to improve the bond between the ceramic and the polymer is to obtain a chemical bond between the chemically inert ceramic and the polymer. It is possible through the functionalization of the ceramic surface with functional groups and coupling agents i.e. -OH, -CH2HN2, -NH2, -CF3, -COOH, H3PO4, aminosilanes and fluorosilanes [33, 34]. The main reason of application of coupling agents is to prevent to the high differences between the surface energy of the hydrophilic ceramic and that of the hydrophobic polymer matrix, which leads to agglomeration and poor dispersion of the ceramic filler particles and in the results preventing in the formation of voids and interfacial defects [34]. There are many ways to create a sufficiently large surface area development on the ceramic surface: oxidation, application of characteristic functional groups, silanization, thermal treatment in a gas atmosphere, melt infiltration, ionic liquid etching, sol-gel process, and co-precipitation [35–39]. For example, the chemical etching processes have a large amount of literature focused on chemical etching concerns the preparation of the ZrO2 surface in a way that enables permanent bonding to hard dental tissues or composite materials [39]. One of the important aspects required for the success of ZrO2 ceramics is the establishment of proper adhesion between substrate and adherent [39, 40]. The gold-standard protocol for resin bonding to glass-ceramics is etching with hydrofluoric acid followed by the application of a silane coupling agent (chemical and mechanical adhesion) [39, 40]. Acid etching (various concentration and times) has been shown to change the surface micro-morphology of glass and oxide ceramics (many surface defects) [39], the resin adhesion, the increase in HF acid concentration, and the etching time associated with an increase of the surface area available to adhesion with resin [39].
Here, we report a simple and low-cost surface modification process for medical-grade ZrO2 and Al2O3 ceramics using chemical acid in the form of an acid-perchlorate bath. This treatment was carried out in order to prepare ceramic fillers for mixing with selected polymers in future, and in consequence preparation of polymer-ceramic composites for biomedical applications. For etching process we use sulphuric acid, nitric acid, perchlorate, and hydrofluoric acid, for comparison, baths. This performed research concerns the assessment of the impact of the prepared baths on surface development, chemical and phase composition, as well as the occurrence of characteristic functional groups, i.e. hydroxyl, which positively affect the chemical connection with the polymer species in composite.