Drug delivery by lateral tail-vein injection in mice is widely used in preclinical research, but the technique is laborious to perform because the tail vein is hardly visible and too small to be cannulated. Misinjections of test components can lead to defective or even false experiment results. We present a simple but useful injection-assistant device to visualize the tail vein of mice. The device consists of a light-emitting diode (LED) circuit and a finger component. The finger component consists of an open-looped ring to slide on the finger, a slot to accommodate the mouse's tail and a lamp cage in which to set the LED lamp. Once the mouse's tail has been illuminated, the tail vein can be clearly seen as a dark line along the bright background of the tail, which facilitates venipuncture and improves the success rate of tail-vein injection. If the protocol provided has been followed correctly, a robust tail-vein injection-assistant device can be set up in 3 h with low-cost components.
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