An automatic fluid injection device having a body fastened to a support. The body containing one or more fluid reservoirs, each containing an injection piston； a needle assembly including an injection needle； and an injection mechanism. The needle assembly including an injection needle and an insertion actuator made of a shape memory alloy. The injection mechanism having a retraction ring； a needle support； an insertion ring； a retraction spring； an actuator ring； and an insertion spring. The insertion spring and the retraction spring retained, at rest, in their compressed state, activation of the actuator by heating to about 50% of the available contraction causing the needle support to be released, and activation of the actuator by heating to reach 100% of the available contraction causing the insertion ring and the assembly formed of the needle support and of the injection needle to be released.
