text: Finally, biological and bio-hybrid drug carriers have shown potential for chemotactic applications. In general, these systems are inspired by microorganisms or cells to help design drug delivery systems that mimic their surface, shape, texture, and movement. One phenomenon that has become increasingly popular in improving the movement and release of bio-hybrid drug carriers is that of chemotaxis. Indeed, thanks to their natural chemotactic sensing property, bacteria can be used to locate a tumor, carry a therapeutic payload to the site, and release that drug in a controlled manner. Researchers can also genetically modify these bacteria to produce a specific protein like anti-tumor cytotoxins for cancer treatment.
  Yet, this is not to say that they dont come with their own set of challenges and limitations. For one, the genetic modifications of the bacteria used can be manipulated by recent or unforeseen mutations, leading to a decrease in the efficacy of the drug and drug carrier. Moreover, the therapeutic proteins produced may have incomplete protein folding, decreasing the drugs effectiveness or causing unforeseen side effects. Generally speaking, using bacteria may provide some advantages, but further research and development are still needed to address their limitations.
  Another example of bio-hybrid drug carriers is human cells, like macrophages, which offer compatibility with the human immune system and a simple way to load drugs as a bio-hybrid drug carrier. Leukocytes demonstrate great promise because Tumor cells secrete large amounts of chemoattractants when the cell undergoes inflammation. This secretion of chemoattractants naturally attracts leukocytes, such as macrophages, to the T cell location. Thus, with their well-known chemotactic homing behavior to inflammation or pathogens' sites in mind, researchers can manipulate leukocytes to carry and deliver a therapeutic payload to the tumor site. However, this is not to say that Biological and bio-hybrid drug carriers do not have challenges and limitations of their own. For example, Leukocytes cannot penetrate deeply into the tumors, have a low capacity for carrying drugs, and slow down when the tumor size reduces. Thus, similar to bacteria drug carriers, further research and development are still needed to address their limitations and improve the overall drug delivery system.