Pain during injection of a medical needle is greatly influenced by the surface roughness and geometry of its cutting edges. An ionplasma sputtering treatment is a promising candidate as a clean and precise polishing method for the painless needles. In this paper, the mechanism of the reduction of inserting force of the plasma-treated needles is investigated by microscopically observing artificial skins and needles during insertion. It was found that the time distribution of insertion force was

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... nfluenced by the roughness at the cutting edges. Plasma-treated needles resulted in force reduction by approximate half in comparison of untreated ones. Furthermore, the microscopic observation found that the sharp dropping of force was occurring at the corners between cutting faces. Using the plasma-treated needle, the sharp dropping was significantly decreased. The plasma-etching treatment will lead to a clean finishing process for painless medical needles.