The major impediment to the delivery of therapeutics to the brain is the presence of the blood-brain barrier (BBB). The BBB allows for the entrance of essential nutrients while excluding harmful substances, including most therapeutic agents; hence, brain disorders, especially tumours, are very difficult to treat. Chitosan is a well-researched polymer that offers advantageous biological and chemical properties, such as mucoadhesion and the ease of functionalisation. Chitosan-based nanocarriers (CsNCs) establish ionic interactions with the endothelial cells, facilitating the crossing of drugs through the BBB by adsorptive mediated transcytosis. This process is further enhanced by modifications of the structure of chitosan, owing to the presence of reactive amino and hydroxyl groups. Finally, by permanently binding ligands or molecules, such as antibodies or lipids, CsNCs have showed a boosted passage through the BBB, in both in vivo and in vitro studies which will be discussed in this review.

中文翻译：

---

克服血脑屏障：功能化的壳聚糖纳米载体

药物向大脑输送的主要障碍是血脑屏障（BBB）的存在。BBB可以吸收必需营养素，同时排除有害物质，包括大多数治疗药物；因此，脑部疾病，特别是肿瘤，很难治疗。壳聚糖是一种经过充分研究的聚合物，可提供有利的生物学和化学特性，例如粘膜粘附和易于功能化。基于壳聚糖的纳米载体（CsNCs）与内皮细胞建立离子相互作用，从而通过吸附介导的胞吞作用促进药物穿过BBB的穿过。由于存在反应性氨基和羟基，通过修饰壳聚糖的结构进一步增强了该过程。最后，通过永久性结合配体或分子，