One obstacle to delivering drugs into cells is that many types of nanoparticles can’t get through the membranes surrounding cells to deliver drugs. One way to get drugs through cell membranes is to enclose drug molecules in artificially created spherical nanoparticles called liposomes or lipid nanoparticles. By placing these molecules with their hydrophilic head and hydrophobic tails in water that contains drug molecules you can encapsulate them.
To understand why that’s so and how nano can provide a solution for breaking through cell membranes, you have to understand a bit about the nature of cells. Cell membranes in our bodies are composed of molecules called phospholipids. One end of these phospholipids, called the head, is hydrophilic, meaning that it mixes well with water. On the other end of the cell are two tails that are hydrophobic, meaning that they don’t mix well with water.
The hydrophilic end of the phospholipid mixes well with water because it’s polar, which means that it’s composed of atoms with different levels of attraction to electrons. This measure of attraction is called electronegativity.
The hydrophobic end of the phospholipid is composed of atoms that have similar electronegativity, therefore the electrons are evenly distributed, making that end of the molecule nonpolar. Nonpolar molecules don’t mix well with water, something you can witness when you try to mix oil with water.
The membrane of a cell is composed of many of these molecules in a two-layer film. In this film hydrophilic ends of the outer layer of the molecules form the outside of the membrane, and the hydrophobic tails of the molecules meet in the middle. This structure has a couple of purposes: the hydrophilic outer layer lets the cell mix with the water-containing fluids in our bodies, while the hydrophobic layer in the middle of the membrane stops the water-containing fluids inside the cell from leaving.
The result of this structure is that the cell membrane blocks the entry of many therapeutic drugs into the interior of the cell. That motivated researchers to develop methods to deliver these molecules through the cell membrane. The hydrophilic heads in lipid nanoparticles line up to form an outer shell facing the water solution, while another set of hydrophilic heads lines up to form an inner shell that contains a solution of drug molecules, providing that delivery method.
The shells of liposomes can fuse with cell membranes. If a therapeutic drug is encased in a liposome, the liposome membrane creates an opening as it fuses with the cell membrane, and the drug inside the liposome can be delivered into a cell