Which technique has been known to develop print impressions on human skin?

The technique of using iodine to develop print impressions on human skin is based on the ability of iodine vapors to react with lipids and moisture present in the ridge patterns of fingerprints. When iodine is applied to the skin, it can visualize the latent prints by forming a temporary brownish stain. This method can be particularly useful in forensic applications, especially in cases where traditional methods may not be suitable due to the unique challenges posed by skin as a substrate.

While other techniques mentioned—such as ninhydrin, super glue fuming, and magnetic powder—are effective for developing fingerprints on various surfaces, they are not optimal or practical for use directly on human skin. For example, ninhydrin is primarily used on porous surfaces and requires a chemical reaction that doesn't readily occur on skin. Super glue fuming (cyanoacrylate) is typically applied in a controlled environment to non-porous surfaces and may not adhere well to skin. Magnetic powder is used to visualize prints on surfaces like glass or metal but won't interact successfully with skin.

The unique physical and chemical properties of iodine allow it to effectively adhere to the oils and amino acids present in latent prints, making it a suitable technique for developing fingerprints on human skin.