*3.3. Physical Filters against Solar Radiations*

Sunlight is essential for our well-being; it is responsible for regulating our internal clock, metabolism, immune systems, and for vitamin D production, essential for healthy bones. Nevertheless, it is well-known that excessive exposure to the solar radiations can cause serious damage to human health [102]. In particular, UVA (320–400 nm) and UVB (290–320 nm) radiations are the main radiations to interact with the human body, and their hazard relies in their ability to generate reactive oxygen species (ROS), which cause skin photo-aging, sunburn, dermatitis, and can also evoke long-term health effects, such as malignant tumors [103,104]. In this regard, the use of sunscreens composed of effective UV filters as protective barriers, absorbing harmful UVA and UVB radiation, has become a very important topic on which research is paying an increasing attention. UV filters can be divided in two main classes, chemical or physical filters, but nowadays physical filters are considered more attractive for sunscreens. Indeed chemical filters, despite having different advantages [105], were proven to increase environment pollution [106], and resulted to be harmful for human health [107]. For these reasons, physical filters, especially titanium dioxide (TiO2) and zinc oxide (ZnO) [104], being able to shield the skin from both UVA and UVB radiation, are the most commonly used. However, these also have some important limitations, especially regarding their size and photocatalytic properties (Table 2). Indeed, to decrease the difficult spreading and whitening effect on the skin, the particle size is reduced to the nano-size range [108]. This entails that these particles are able to penetrate deep layers of skin, causing phototoxic reactions [109]. Furthermore, TiO2 is known for its high photocatalytic activity, generating reactive oxygen species (ROS), which can oxidize and degrade other ingredients in the formulation, raising safety concerns [110]. Finally, the main problem, relating all common UV-filters, is that, being mainly used on the beach, the components of the cream are often released in water, causing damage to the marine environment, coral blenching, and bioaccumulation in the fauna [111]. Considering all these issues, attention is shifting towards the development of effective and safer UV-physical filters for both humans and the environment.

Recently, Battistin and co-workers [112] reported a new class of UV-physical filters through the combination of a common physical filter, TiO2, and dihydroxyphenyl benzimidazole carboxylic acid (Oxisol) [113], an antioxidant molecule with booster effect. Boosters can be small molecules, polymers, or other particles, that act on the rheological properties of the formulation, but can also synergistically interact with the UV filters through antioxidant mechanisms or interfere with the electronic processes of UV radiations absorption. In particular, this work reported that Oxisol, functionalizing the surface of TiO2, is able to increase the UV-protection, and also to stabilize TiO2 nanoparticles, preventing their penetration to deeper skin layers. Furthermore, its booster activity, by means of antioxidant effects, allows a reduction in physical filter content in sunscreen formulation and a significant lowering of photocatalytic effect, typical of TiO2. Nevertheless, Oxisol is considered as a low eco-sustainable sunscreen product. Thus, alternative, safer, and eco-sustainable sunscreen products are currently under investigation.

In the last years, the sector of sunscreens shifted attention towards formulations containing calcium phosphates (CaPs), especially hydroxyapatite (HA), the main component of animal bones, due to their excellent biocompatibility, non-toxicity, and ability to partially absorb UV radiation (Table 3) [114]. In the literature, there are some works related to hydroxyapatite as physical filter; for instance, Rehab and collaborators [115] reported the synthesis of ascorbic acid-modified, nanosized HA, stabilized with polyvinylpyrrolidone (PVP), to act as a potential biocompatible and safe constituent of sunscreens. In detail, the incorporation of the antioxidant ascorbic acid (vitamin C) [116] in HA particles maximizes photoprotection against UV damage and removes reactive oxygen species (ROS), while PVP prevent nanoparticles aggregation avoiding their skin permeation.

Another type of HA-based sunscreen has been shown by Morsy and co-authors [117], who developed a multifunctional hydroxyapatite-chitosan (HA-chitosan) gel that works as a natural antibacterial sunscreen agent for skin care. Through the simple coprecipitation

method, thus avoiding use of toxic or high-cost materials, nanosized HA particles trapped within the chitosan matrix were obtained. HA acts as a physical filter against solar radiation, while chitosan acts as polymer matrix, able to avoid the agglomeration of particles and to prevent skin penetration. Additionally, chitosan acts as a natural antimicrobial agent, preventing skin wound infections caused by excessive sun exposure. Both works [115,117] focused on the intrinsic photoprotective capability of hydroxyapatite, combining it with other compounds to improve its absorption range in the UV region and to bypass the main drawbacks related to this kind of material, such as nano-size and whitening effect.

However, several studies on HA [118,119] showed its lattice has the particular ability to be modified through the doping with ions (such as Mg2+, Sr2+, CO3, Fe2+/3+, Zn2+, and Ti4+), thus making it a multifunctional product, adaptable according to the requests. Due to this, in the specific case of sunscreens, some works have reported that doping HA with appropriate ions can lead to an increase in the value of protection factor without necessarily having to combine other external components. In 2010, de Araujo et al. [120,121] developed, through a chemical precipitation process, four different hydroxyapatites doped with Cr3+, Fe3+, Zn2+, and Mn2+ ions, having better absorption properties than pure HA in the UV region. Mostly, iron and manganese-doped HA showed the best absorption features in the UV range, necessary to be an effective sunscreen, without creating problems of toxicity or photocatalytic effect. Inspired by the previously published results [120,121], another work [122] has reported an iron-doped HA-based material containing both Fe ions (Fe2+/Fe3+) substituted into the hydroxyapatite lattice and iron oxide in hematite (α-Fe2O3) form, successfully developed from waste fish bones with a simple treatment. This was the first time an HA-based sunscreen has been synthetized, formulated in cream, and validated as proof of concept. In detail, the introduction of iron ions in the HA lattice allowed an increase in the absorption range in the UV-region, creating an effective physical filter, no photoreactivity, and a potential safe option for cream formulation, starting from waste by-products with several environmental benefits. Although iron is able to improve the photoprotective abilities of hydroxyapatite, several studies reported that titanium has a greater shielding power [123,124].

On the other hand, taking into account the photocatalytic problems associated with the use of titanium dioxide within sunscreens, some recent works shifted attention towards titanium as Ti4+ ions, developing titanium-doped hydroxyapatite. Yasukawa and Tamura [125] were the first to demonstrate the effective protection from solar radiations of titanium-hydroxyapatite suspensions combined with cerium ions (TiCeHA). In particular, it was revealed that the Ti4+ ions and Ce3+ ions absorbed another range of UV: UVB and UVA, respectively. Therefore, the simultaneous use of these ions further enhances the UV absorptive ability and by changing their contents in TiCeHA it is possible to create a physical filter suitable for shielding from UVA and/or UVB. Given the potential of these compounds, it would be interesting to evaluate the development of a new UV-physical filter composed of titanium-doped hydroxyapatite and biopolymers obtained by a nature inspired calcium-based biomineralization process [126]. Considering the problems associated with "classic" commercial sunscreen, having a physical filer not only able to shield solar radiations, but also safe for the human body and eco-friendly, could be the solution to overcome the main UV-filters drawbacks.


**Table 2.** Advantages and drawbacks of some physical filters.

**Table 3.** Differences between chemical, physical and hydroxyapatite (HA)-based physical filters.


#### *3.4. Nano and Micro Drug Delivery Systems*

Bioceramics are widely used as components of implants for bone and teeth restoration. Nowadays the advanced processing techniques and the new synthesis strategies allow the incorporation of drugs, bioactive molecules, or cells within them or on their functionalized surfaces. In this regard, bioceramics and biomineralized materials can be exploited as drug delivery or controlled release in several applications, such as nanomedicine, wound healing, and bone regeneration [128,129].

Local antibiotic release is a promising and effective procedure for delivering drugs at the implantation site. With this strategy, antibiotic was loaded on a scaffold in order to both promote bone regeneration and to prevent common bacterial infections happening after surgery. In this way, scaffolds act as carriers for local antibiotic release to avoid following implant removal due to osteomyelitis (Figure 6) [130]. Different drugs can be loaded into the scaffolds, including anticancer drugs. For instance antitumoral drug-loaded scaffolds can be used to restore large bone defects after tumor extirpation, resulting in tumor inhibition with low levels of systemic toxicity [131–134].

**Figure 6.** Schematic illustration of the loading of antibiotics within hybrid scaffolds based on collagen and magnesium-doped hydroxyapatite. Two different antibiotics, namely gentamicin and vancomycin, were introduced during biomineralization of collagen. Antibiotics proved to be tightly associated within the biomineralized scaffolds. These scaffolds were able to provide a piecemeal release of antibiotics, avoiding microbial colonization (and therefore avoiding infections) and simultaneously promoting bone tissue regeneration. The figure is reproduced from *Medicated Hydroxyapatite/Collagen Hybrid Scaffolds for Bone Regeneration and Local Antimicrobial Therapy to Prevent Bone Infections* by M. Mulazzi et al., 2021, Pharmaceutics, 13 (7), 1090 (doi:10.3390/pharmaceutics13071090) (Under a Creative Commons Attribution 4.0 International License).

Often, in orthopedic, maxillofacial, and dental surgery, whether the defect size is complex or irregular, bioceramic beads are used to induce bone tissue regeneration [135–137]. In the last decades, some research has been focused on the possibility, not only to promote tissue regeneration due to bioceramics beads, but also to modify the functionalization of them with several active molecules such as antibiotics, anticancer agents, and osteogenic agents to act themselves as drug delivery vehicles [129,138–140].

Ceramic component contributes to the mechanical stability and bioactivity of the structure; however, its adsorption of drugs often is featured by weak bonds leading to an initial burst release [140–142]. To overcome this issue, polymers can be added, forming a composite material endowed with a fine chemical and physical control of the drug adsorption and release [143,144]. These polymeric and bioceramic phases can be used as separated phases or as a single mixed phase. For example, hydrogel/bioceramic coreshell beads can be developed, by means of concentric nozzles or microfluidics exploiting both advantages of the two phases; polymers can preserve the drug, avoiding an initial excessive release, whereas ceramics contribute to the mechanical stability and bioactivity of the structure for a synergic and effective loading and sustained release of proteins [145], or drugs [146] (Figure 7), as well as cells [147].

For example, Raja and co-workers fabricated a multifunctional core-shell bead structure featured by a hydrogel shell composed of alginate including cells around a ceramic core made of α-tricalcium phosphate (α-TCP) loaded with Quercitin dihydrate, a wellknown phytochemical used for the treatment of osteoporosis [129]. The core-shell beads, immersed in PBS, lead to the formation of bone-like low-crystalline apatite from α-TCP that provides structural integrity to the bead, along with a surface for the growth of cells embedded in the hydrogel shell. Researchers demonstrated a slow release of quercetin throughout the entire 120 days testing period, together with the formation of a homogenous cell layer on the ceramics structure, due to cells loaded into the hydrogel. Finally, they showed that in the region in which hydrogel and ceramics are strictly in contact, cell growth was specifically increased, highlighting the potential of the core-shell model for further

material–cells interaction study [129]. This kind of composite belongs to promising class of materials able to load different types of drugs and cells to produce highly biofunctional beads, which provide an effective bone substitute for both drug delivery and bone tissue regeneration [8,147,148].

**Figure 7.** Schematic illustration of the two-stage synthesis approach of hybrid microparticles based on bioactive glass and poly(ester amide) (PEA). The singular components were mixed with an active agent. Subsequently the two mixtures were mixed together, and gelation was promoted, forming hybrid microparticles loaded with two different drugs. The figure is reproduced with permission from *Intrinsically fluorescent bioactive glass-poly(ester amide) hybrid microparticles for dual drug delivery and bone repair* by Aslankoohia and Mequanint et al., 2021, Materials Science and Engineering: C, 128, 112288 (doi:10.1016/j.msec.2021.112288). Copyright 2021 by Elsevier.

Moreover, calcium-based biomineralization paves the way to promising and very interesting materials where ceramics composites are nucleated onto the organic phase, as previously described, creating a single and very reactive phase that combines the advantages of the two phases. Furthermore, thanks to the possibility of introducing doping ions into apatite lattice, the resulting phase will be featured by different and new functionalities, in addition to those normally occurring in vivo, such as high bioactivity, biocompatibility, and biodegradability. Doping with magnesium or Zn ions, for example, is possible to confer antimicrobic properties essential to prevent bacterial infection or in wound healing [149,150].

The encapsulation of antibiotics in nanocarriers such as bioceramics allows the elimination of microorganisms by releasing a high antibiotic dose at a target site before the development of resistance [151]. Furthermore, many researchers demonstrated that ions present in hydroxyapatite can promote the antibacterial activity of the device. For instance, Ain and co-workers demonstrated that vancomycin-loaded HA had a slower release in comparison with pure vancomycin and also an enhanced antibacterial activity due to the presence of ions in the HA structure [140].

On the other hand, doping with Fe ions results in an interesting superparamagnetic apatite phase, able to be exploited in diagnostic field as a contrast agent or therapeutic field, due to the possibility to move it by an external magnetic field or to release drugs by means of magneto-shaking [4,139,152,153].

Concerning that point, Patricio and collaborators have developed a bio-hybrid microsphere obtained through the biomineralization of iron-doped hydroxyapatite (FeHA) within an organic matrix. In this case, the organic matrix is an animal-free recombinant peptide based on human type I collagen (RCP) enriched with RGD motif. The resulting material is bioresorbable, biocompatible, and can enhance cell adhesion. Through the fine tuning of the emulsification process, the resulting hybrid microsphere is endowed with a monomodal size dispersion, low crystallinity, and superparamagnetic properties typical of FeHA [4,23,154–156].

The resulting microspheres displayed excellent osteogenic ability with human mesenchymal stem cells, and were able to provide a slow release of recombinant human bone

morphogenetic protein-2 (rhBMP-2) within 14 days. Furthermore, the release profile can be finely tuned by application of pulsed electromagnetic field, thus highlighting the potential of remote controlling the bioactivity of the new micro-devices, an interesting feature for their application in precisely designed and personalized therapies.

To conclude, the administration of therapeutic agents is still a major concern of medicine, as the systemic dose prescribed needs to be high to ensure the suitable dose in the target area, causing several collateral effects. The synergy between bioceramics and drugs therapy has paved the way to several possibilities, especially in bone pathologies, anticancer therapy, and heart diseases [157–159].

#### **4. Conclusions and Future Perspectives**

Biomimetic approaches are very promising for the design of advanced and multifunctional materials. The application of self-organization has wide potential for the tailoring structure, composition, properties, and function of materials from nano- to macroscale. Additionally, the calcium-based biomineralization process can be finely tuned by changing the environmental conditions (e.g., pH), doping ions, and organic network. Biomineralized materials can be tailored to address specific issues, including devising of materials for regenerative medicine, as well as 3D predictive models and development of drug delivery systems. Furthermore, these hybrid materials display an excellent resource to devise physical filters able to prevent UV-light-induced danger.

We believe this review will point out the future development of calcium-based biomineralization process for the creation of materials in several applications. Indeed, some issues need to be addressed, including the industrial production scale up and the sustainability both economic and environmental—of the production.

**Author Contributions:** Writing—original draft preparation and editing, E.C., M.M. (Margherita Montanari), C.A., G.B. and F.F.; writing—review, M.M. (Monica Montesi), S.P., M.S. and A.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

