 In between each etching the silicon is coated with a polymer in order to preserve the integrity of the channel walls.The focal spot size is important in x-ray microprobe instrumentation where x-rays are focused onto a biological sample to investigate phenomena such as the bystander effect.To target a specific cell the focal spot size of the system must be around 10 micrometers, whereas to target specific areas of a cell such as the cytoplasm or the cell nucleus it should be no more than a few micrometers.