*5.3. Medical Imaging*

The structural stability and optical properties of BP make it an ideal candidate for therapeutic and diagnostic applications in oncology. The nanolayers of BP are used as carriers of targeted drugs. In photothermic therapy, BP has been demonstrated to have excellent properties related to the absorption of light: when it accumulates on the tumor mass, thanks to the use of surgical lasers, the warmth will quickly destroy the tumor mass. Even in medical imaging, the behavior of BP is very promising; in fact, when it is absorbed by the tumor mass, it generates a fluorescence which may serve to define the morphology of cancer with extraordinary precision. Finally, it may also be used for photoacoustic imaging [32]. Through liquid exfoliation, we can obtain nanosheets of BPQDs that, as previously mentioned, maintain BP's performance in diagnostic applications [33]. During synthesis by exfoliation, BPQDs can be combined with TiL4, at a specific temperature and time (approximately 15 h). This combination generates TiL4@BPQDs, interesting nanosheets which may be exploited as a contrast agen<sup>t</sup> for photoacoustic imaging (PAI), a technique adopted in vivo, which has shown remarkable success in terms of PA response under near-infra-red (NIR) stress. TiL4@ BPQDs has excellent potential in diagnostic applications because of its excellent sensitivity and high spatial resolution in detecting tumor masses [34].

### *5.4. Sca*ff*olds and Coatings*

BP is an ideal candidate both as a coating and as a transporter. Wei Tao et al. highlighted the e fficacy of BP nanolayers coated with PEG (polyethylene glycol) to administrate Doxorubicin (DOX) in oncological chemotherapy. More in detail, BP nanosheets loaded with DOX can selectively degrade in the target area with better e fficacy. Moreover, thanks to the photodynamic, photoacoustic, and photothermal properties of BP, in addition to the therapeutic aspects previously described, some optimal diagnostic aspects can be simultaneously obtained [7].

Bone regeneration is fundamental in the field of tissue engineering, and many studies have focused on technologies such as 3D bioprinting for the realization of increasingly precise and biocompatible sca ffolds. In 2014, Inzana et al. focused their studies on the production of calcium phosphate sca ffolds using low-temperature 3D printing. These 3D sca ffolds provided excellent results in terms of cytocompatibility and osteoconductive, paving the way for the composition of bio link with substances that are present in bone tissue. BP is present, as previously mentioned, in very low percentages in bones; this fact can be exploited in the composition of bio links with other substances that are favorable to the osteoconduction [35]. Recently, researchers are trying to combine the proliferation and regeneration of damaged or surgically removed tissues. In bone cancer, the challenge is to promote regeneration around the prosthetic device, ensuring the osteogenic and antibacterial properties of the implant surface. Bone regeneration is often supported by sca ffolds, made from both organic and inorganic materials that mimic the extracellular matrix of bone. In this context, BP nanosheets contain phosphorus, which is already naturally present in bone. Yang et al. focused their studies on the production of sca ffolds made from BP nanosheets combined with BioGlass (BG) by 3D printing. Bio-printing makes it possible to produce sca ffolds with complex shapes, sizes, and compositions [36]. In the therapeutic protocol against osteosarcoma, 3D printing with biomaterials doped or coated with BP could be a useful strategy to improve the therapeutic e ffects. The experimental sca ffolds made by Yang et al. were designed with a reticular trauma, similarly to the medullary bone, to promote and improve cell adhesion and colonization. These sca ffolds were coated with a BP nanosheet (200–400 nm) that was shown to bind the sca ffold structure safely and strongly. The coated sca ffolds were tested in vitro; they showed an exceptional ability to improve bone formation, after an improved cell proliferation on their surface, probably due to the peculiar sca ffold geometry. Further in vivo studies on a mice model a ffected by osteosarcoma revealed that the BP-BG sca ffolds worked more e ffectively on post-oncological bone defects [36]. The BP coating was also applied to hydrogels; specifically, a hydrogel was obtained through photo-reticulation with UV of methacrylamide gelatin (GelMA). GelMA was coated with arginine and poly (ester amide) (U-Arg-PEAs), and BP. The functionalized hydrogel showed improved bone formation. In vitro, the mechanical characteristics of the hydrogel were assessed in the presence of BP immersed in substances that simulate body fluids, obtaining a good response in terms of compression modulus and biodegradability time. Furthermore, in the osteogenic di fferentiation of stem cells from human dental pulp (hDPSC), BP-based hydrogel improved the proliferation of hDPSCs. BP coating appears to be an ideal environment for the growth of hDPSCs, thus demonstrating a potential use of BP-coated hydrogels in the dental field [9].

The possibility of using 2D materials for biomedicine has led researchers to verify their possible use in the therapeutic, diagnostic field. Among the transition-metal dichalcogenides (TMDs), covalent–organic frameworks (COFs), hexagonal boron nitrides (h-BN), metal-organic frameworks (MOFs), layered double hydroxides (LDHs), Wei Tao et al. focused research on BP multifunctionality for Cancer Theranostics. They specifically considered the PEGylated BP Theranostic delivery platform with three di fferent configurations of agents that are capable of drug delivery, photodetection in bio-imaging, and targeting during the photothermal therapy [7]. In vivo tests on mice with platforms composed of nanosheets of BP for DOX therapy were performed, confirming that BP has a greater drug loading capacity than other 2D materials, i.e., MoS2, and graphene. It also responds more quickly to laser radiation with immediate drug release, and shows good photostability and biocompatibility within the body [37]. In photothermal therapy, photothermal conversion and biocompatibility are essential factors, but mechanical performance also plays an important role. PVA hydrogels compounded with BP nanosheets modified with polydopamine, pBP, through freezing and thawing, showed peculiarities in mechanical performance. pBP shows good cellular interactions and e ffective response to controlled NIR radiation, which is able to dissolve the pBP envelope and release the drug [38]. Currently, nanomedicine drug administration is one of the fundamental areas on which to concentrate studies, and BP appears to be a pioneer. DDS (drug delivery system) are among the most promising techniques in cancer therapy, and BP within hydrogel structures, as seen, provides good results. This feature is interesting for researchers because it is possible to control the biodegradability of the BP envelope based on its composition with the hydrogel and the transmitted NIR radiation. This would make it possible to act more precisely on the area under study [39]. The composition and structural conformation of hydrogels also favor or inhibit the properties of BP. One of the solutions to the easy degradation and oxidation of BP is the use of a hydrogel based on BP nanosheets and cellulose (BPNS). The 3D structure presents nanometric irregularities and pores that yield greater stability, flexibility and e ffective response to PTT, even in in vivo experiments [40]. Researchers taking advantage of the liquid-base synthesis technique made three di fferent samples of nanosheets, i.e., small S-BP, medium M-BP, and large L-BP. They reported four di fferent types of behavior based on the size of the sample. This suggests that depending on the type of field of application (photothermal therapy, bio-imaging, drug-delivery), BP nanosheets must be correctly sized to be exploited to the fullest of their potential [41]. BP also works as a coating for electrodes or biofilms for bone implants and wearable devices. For example, Xiong J et al. proposed the realization of a tactile triboelectric nanogenerator to be worn on the skin, which is capable of accumulating mechanical energy and transforming it into power [42]. This voltage can be used as a precise self-supply device and for monitoring vital real-time parameters, for the input of mechatronic prostheses and drug release.

**Figure 3.** Strategic applications of Black Phosphorene (BP) in biomedical fields. Adapted from Reference [9].
