Biomaterials are advanced materials that provide unique and valuable functions in vivo and/or in vitro. The production of biomaterials has continuously been increasing in step with progress in biomaterials engineering and related fields of research and technology, especially in regenerative medicine (Chen et al., 2016). Currently, biomaterials are frequently used in diverse medical devices, such as implants, scaffolds, dental products, prostheses, stents, catheters, and artificial organs. Once these biomaterials have served their purpose, they are replaced or withdrawn. As biomaterials come to play an increasingly more prominent role in medicine, their collection and management in medical waste is becoming a more significant problem. Currently, waste biomaterials are most often landfilled or incinerated (Joseph et al., 2021). However, as they are potentially infectious waste, biomaterials discarded in landfills may present environmental hazards, including soil and water pollution resulting from waste degradation. On the other hand, inadequate incineration of biomaterials may lead to the release of toxic chemical compounds into the atmosphere (Joseph et al., 2021). Therefore, biomaterials recycling is an urgent issue to be addressed. Table 1 provides the most pertinent information extracted from published review articles relevant to biomaterials recycling.

Considering all available reports, the recommended first step to successful recycling of biomaterials is segregation by the type of material (polymer, metal, or ceramic) (Kheirabadi and Sheikhi, 2022; Yadav et al., 2020), specific composition (Lee et al., 2002; Joseph et al., 2021), waste source (Attrah et al., 2022; Lee et al., 2002), and infection chance (Kheirabadi and Sheikhi, 2022; Yadav et al., 2020). Figure 1 illustrates an exemplary composition of biomaterials waste, highlighting the chemical diversity of the materials.

One study estimated that direct collection and appropriate recycling of metal alloys used in dental restorations and silver amalgam alloys, from 300 dental colleges, could recover approximately 500 kg of Co-Cr and Ni-Cr alloys, 500 kg of silver, and 850 kg of mercury (Thopegowda et al., 2013). In another study, Yadav et al. (2020) proposed effective ways to recycle orthopedic implants. They claimed that extrusion can be used for recycling of polymeric implants, while powder metallurgy or liquid resin recycling methods would be suitable for metallic and ceramic-based implants. According to the authors, following these paths would allow these recycled products to be reused in the manufacturing of subsequent implants or other biomaterials. Moreover, they highlighted that the crucial role in the fate of the waste implant is played by the doctor who, based on the condition of the implant, may decide to reuse, remanufacture, or recycle it. Regarding practical research, Wang et al. (2020) showed that there is no significant difference in the interfacial morphology of Pd-Cu-Ga and Au-Pt metal-ceramic dental alloys after recasting up to 3 times. Nevertheless, it is worth noting that in some cases, even collection at source and recycling of a specific type of biomaterial may not ensure similar characteristics to the original material. For example, recasting of some metallic alloys could potentially change the corrosion properties and thus the cytotoxicity of these alloys. Tripuraneni and Namburi (2008) demonstrated that Ni-Cr dental casting alloys prepared from their recycled form induced considerable genotoxicity, especially those with a higher content of recycled material. However, if recycling is not taken into consideration during the removal of biomaterials from the human body, managing them becomes significantly more challenging. For instance, the recycling of mixed polymeric biomaterials becomes complicated due to the wide range of components used in their production and their diverse behavior during processing.

According to Kheirabadi and Sheikhi (2022), the recycling of medical plastic waste, including polymeric biomaterials, can theoretically undergo mechanical (secondary) and chemical (tertiary) recycling methods commonly used for typical polymers. However, it is essential to sterilize waste biomaterials before including them with municipal plastic waste (Joseph, et al., 2021). In practical terms, limitations in waste sorting may result in materials made from recycled polymers having weak interfacial adhesion and low mechanical properties. The utilization of advanced tools such as machine learning to enhance sorting efficiency may help to overcome this challenge (Kheirabadi and Sheikhi, 2022). Although this approach represents a conventional solution to tackle the problem, there are other possible strategies for biomaterials recycling, as depicted in Figure 2.

Based on the conclusions drawn by other scientists (Attrah et al., 2022; Kheirabadi and Sheikhi, 2022; Yadav et al., 2020) and our own experience, greater attention should be given to the reuse of biomaterials after adequate treatment and sterilization. Several studies have reported successful application of this strategy, demonstrating that material properties can be maintained without compromise. For example, Estelita et al. (2014) compared the mechanical properties of unused mini-implants with mini-implants inserted and removed from pig iliac bone with and without additional treatment (ultrasonic cleaning, autoclave sterilization, and/or sandblasting). It was found that regardless of the treatment/sterilization technique used, the torsional strength of screws was unchanged. On the other hand, it was revealed that for some cases, the sterilization process may affect the final behavior of the biomaterial. For instance, Sayed et al. (2018) found that orthodontic archwires composed of Ni-Ti and Cu-Ni-Ti alloys experienced deterioration in ultimate tensile strength after two cycles of autoclave sterilization (134°C, 18 min) or chemical disinfection (1% hydrogen peroxide solution, 30 min). In addition, after just one cycle, some surface irregularities were observed on the archwires. Similarly, it was reported that flaming or flaming with ultrasonic cleaning resulted in noticeable reduction in shear bond strength of stainless steel orthodontic brackets (Khanal et al., 2021). However, applying flame treatment along with sandblasting provided satisfactory strength for clinical use, which was almost indistinguishable from the original product. Therefore, the field urgently needs novel, environmentally friendly disinfection technologies that would allow greater reuse of biomaterials without the fear of deterioration of properties (Attrah et al., 2022).

Although the above recycling strategies shed light on the proper management of waste biomaterials, such approaches require major changes in the biomaterial waste collection system and the awareness of healthcare professionals. For instance, each type of biomaterial should be segregated and categorized at the source and, whenever possible, reused after sterilization. This will maximize the effectiveness of recycling and allow the production of valuable secondary products in accordance with the concept of sustainable development. However, the most promising alternative for achieving a sustainable, circular economy of biomaterials is the shift towards the development of biodegradable or bioresorbable biomaterials. This approach has been referenced multiple times in the literature (Attrah et al., 2022; Joseph et al., 2021; Yadav et al., 2020; Zhu et al., 2021). Such materials do not pose any burden to the environment, as they dissolve or degrade inside the body after a predefined period. Nevertheless, it is believed that low ultimate tensile strength is a key parameter that may limit their use to soft bone implants, among other applications (Yadav et al., 2020).