## 3 Results
### Challenges and Future Directions  
#### 3.6.1 Toxicity, Biocompatibility, and Immune Clearance  
Despite significant progress in nanomedicine, nanoparticle toxicity and biocompatibility remain key concerns. Due to their high surface area and energy, NPs are thermodynamically unstable and prone to aggregation or dissolution in biological environments. These changes--driven by factors such as protein concentration, ionic strength, and pH--can alter their physicochemical properties and biological interactions [89].  
Gold nanoparticles exhibit favorable biocompatibility, largely due to their chemical inertness and stability in physiological media. In contrast, silver nanoparticles, though valued for their antimicrobial properties, may cause oxidative stress and cellular damage at elevated concentrations, resulting in non-specific cytotoxicity [90].  
NP toxicity is influenced by their ability to interact with cell membranes, generate reactive oxygen species (ROS), and penetrate biological barriers like the blood-brain barrier. Titanium dioxide and zinc oxide NPs, for instance, have been associated with inflammation and DNA damage under certain conditions. Systemic toxicity also arises from NP accumulation in organs such as the liver, kidneys, and lungs, with severity depending on plasma half-life, administration route, and biodegradability [91].  
Surface modifications can mitigate toxicity. Coating NPs with polyethylene glycol or bioactive ligands reduces immune recognition and enhances biocompatibility. PEGylation improves colloidal stability and prolongs circulation by evading phagocytosis via the reticuloendothelial system (RES) [90].  
Nevertheless, rapid immune clearance remains a major hurdle. Unmodified NPs are often eliminated by macrophages, particularly Kupffer cells in the liver, through opsonin-mediated phagocytosis. These cells evaluate size, surface charge, and protein corona composition, favoring uptake of larger, negatively charged, or hydrophobic particles. Liver sinusoidal endothelial cells also contribute via clathrin-mediated endocytosis [92; 93].  
Hepatic architecture and blood flow significantly influence NP sequestration. Upon entering the sinusoidal space, NPs experience a marked reduction in velocity, which increases their interaction time with hepatic cells. Studies show that Kupffer cells, hepatic B cells, and LSECs readily internalize NPs, while hepatocyte uptake is minimal. For example, over 80% of Kupffer and B cells internalized quantum dots within 12 h of injection, with uptake varying by lobular location and protein adsorption [94].  
Some NPs, particularly metallic ones like silver or carbon nanotubes, also exhibit immunotoxicity. They can induce oxidative stress, inflammation, and cell injury [92], and they may activate inflammosomes such as NLRP3, promoting cytokine release and pyproptosis in phagocytes.  
To reduce hepatic clearance, researchers are optimizing NP design--adjusting size, surface charge, and employing PEGylation or decoy particles to block Kupffer cell receptors [93]. However, modifying NP characteristics alone is not always sufficient. Altering the biological environment can also be effective. For instance, increasing hepatic blood flow in microfluidic models reduced Kupffer cell uptake by over 80%, while immunomododulation with cytokine cocktails decreased gold NP internalization by about 37% [94].  
Still, clinical translation is hindered by individual variability in immune responses. A deeper understanding of immune recognition mechanisms, protein corona dynamics, and long-term tissue interactions remains essential for ensuring the safe and effective application of NPs in medicine [92].