Actually, there are even thinner ones.
A mosquito's proboscis has a diameter of approximately 40-100 μm. Mosquitoes use this needle-like structure to suck blood from defenseless animals.
Since a single red blood cell is approximately 8 μm wide, about 5 to 15 red blood cells can fit within its inner width.
However, the needles used in intracytoplasmic sperm injection (ICSI), a type of in vitro fertilization, have a much smaller diameter. In ICSI, a tiny needle is used to inject a single sperm into the center of the egg.
This needle is extremely thin and can only hold a single sperm. Its width is 5-10 μm.
Look at the red blood cells next to the needle used for intracytoplasmic sperm injection (ICSI).