1. Toxic Materials: Substances that exhibit cytotoxicity, genotoxicity, or other harmful biological interactions are often excluded. 2. Persistent Materials: Materials that do not degrade easily and can accumulate in the environment or within biological systems. 3. Unstable Nanomaterials: Those that can easily agglomerate, react unpredictably, or degrade under standard conditions. 4. Ethically Controversial Substances: Materials sourced unethically or those that raise significant ethical concerns.
