Carbon nanotubes (CNTs) have emerged as promising candidates for biomedical applications due to their unique structural, mechanical, and electrical properties. Their high aspect ratio, large surface area, and ability to be functionalized make them suitable for drug delivery, neural interfaces, and biosensors. However, concerns regarding toxicity and biocompatibility must be carefully addressed to ensure safe clinical translation.

In drug delivery, CNTs serve as efficient carriers for therapeutic agents. Their hollow cylindrical structure allows for the encapsulation of drugs, while their surface can be chemically modified to enhance solubility and targeting. Functionalization is critical to improve biocompatibility and reduce potential toxicity. Covalent modifications, such as the attachment of carboxyl or amine groups, enable better dispersion in aqueous solutions and provide anchor points for further conjugation with targeting ligands. Non-covalent modifications, including polymer wrapping or adsorption of biomolecules, preserve the intrinsic properties of CNTs while improving their interaction with biological systems. For example, CNTs functionalized with folic acid can selectively target cancer cells overexpressing folate receptors, enhancing drug delivery specificity. The large surface area of CNTs allows for high drug-loading capacity, and their ability to penetrate cell membranes facilitates efficient intracellular delivery. Studies have demonstrated the successful use of CNTs to deliver chemotherapeutic agents, such as doxorubicin, with improved efficacy and reduced systemic toxicity compared to conventional methods.

Neural interfaces benefit from the electrical conductivity and mechanical flexibility of CNTs. Their ability to form intimate contacts with neuronal tissues makes them ideal for recording and stimulating neural activity. CNT-based electrodes exhibit lower impedance and higher charge transfer capacity compared to traditional metal electrodes, enabling more sensitive detection of neural signals. Functionalized CNTs can also promote neuronal growth and adhesion, making them suitable for neural regeneration applications. For instance, CNTs coated with bioactive molecules, such as laminin or nerve growth factor, have been shown to enhance neurite outgrowth and synapse formation in vitro. In vivo studies have demonstrated the potential of CNT-based scaffolds to bridge neural tissue defects and restore function in animal models of spinal cord injury. The flexibility of CNT arrays reduces mechanical mismatch with soft neural tissues, minimizing inflammatory responses and improving long-term stability.

Biosensors incorporating CNTs leverage their high electrical conductivity and surface-to-volume ratio for sensitive and selective detection of biomolecules. CNTs can be functionalized with antibodies, enzymes, or nucleic acids to recognize specific targets, such as proteins, pathogens, or DNA sequences. Their conductive properties enable direct electron transfer between the biomolecule and the electrode, facilitating real-time monitoring of biochemical interactions. For example, CNT-based biosensors have been developed for the detection of glucose, cholesterol, and cancer biomarkers with high sensitivity and low detection limits. The integration of CNTs with flexible substrates further enables wearable biosensors for continuous health monitoring. The ability to detect minute changes in electrical signals makes CNTs particularly useful for early diagnosis and point-of-care testing.

Despite their potential, the toxicity of CNTs remains a significant concern. The aspect ratio of CNTs plays a critical role in their biological interactions. Long, rigid CNTs can resemble asbestos fibers, leading to frustration of phagocytosis and chronic inflammation. Studies have shown that high-aspect-ratio CNTs can induce granuloma formation and fibrosis in the lungs, similar to pathogenic fibers. In contrast, shorter or tangled CNTs are more readily cleared by immune cells and exhibit reduced toxicity. Surface functionalization also influences biocompatibility. Pristine CNTs, which are hydrophobic, tend to aggregate and induce oxidative stress, while functionalized CNTs with hydrophilic groups demonstrate improved dispersibility and reduced cytotoxicity. Biocompatibility assessments must consider the route of administration, as intravenous injection, inhalation, or implantation can lead to different biological responses. Rigorous in vitro and in vivo studies are necessary to evaluate the long-term safety of CNTs for medical applications.

The potential for CNTs to accumulate in organs, such as the liver, spleen, or kidneys, raises concerns about chronic exposure effects. Research has indicated that functionalized CNTs can be excreted through renal or biliary pathways, depending on their size and surface chemistry. However, incomplete clearance may result in prolonged tissue retention, necessitating further investigation into degradation pathways and elimination kinetics. Immunogenicity is another consideration, as certain functional groups or contaminants may trigger immune responses. Comprehensive characterization of CNT batches is essential to ensure consistency and reproducibility in biomedical applications.

In summary, carbon nanotubes offer significant advantages for drug delivery, neural interfaces, and biosensors due to their unique properties and versatility in functionalization. However, their biomedical use must be carefully evaluated to address toxicity concerns related to aspect ratio, surface chemistry, and long-term biocompatibility. Ongoing research aims to optimize CNT design and functionalization strategies to maximize therapeutic benefits while minimizing adverse effects. The continued development of standardized protocols for toxicity assessment and regulatory frameworks will be crucial for the successful translation of CNT-based technologies into clinical practice.