K9 is a novel ionizable lipid designed by the multi-objective AI model MOLEA. Its core function is to enable tissue-selective mRNA delivery. When formulated into Lipid Nanoparticles (LNPs), K9 can preferentially deliver mRNA efficiently to articular chondrocytes while significantly reducing delivery to hepatocytes. This facilitates targeted drug delivery for conditions like osteoarthritis and lowers the risk of off-target effects in the liver. In vitro experiments confirm that K9-LNPs promote endosomal escape and efficient expression of mRNA in chondrocytes. Its targeted delivery efficiency is further enhanced by optimizing the LNP formulation (e.g., lipid composition ratios) using a Design of Experiment (DoE) approach. In vivo studies show that after intra-articular injection, K9-LNPs exhibit high biodistribution and gene expression in the knee joint, with limited distribution in major organs like the liver. Therefore, K9 represents an advanced, rationally designed LNP component for achieving joint tissue-specific therapy, providing a key tool for developing targeted mRNA therapeutics.