An automatic fluid injection device having a body fastened to a support for coming into contact with an injection zone, the body containing one or more fluid reservoirs each containing an injection piston； a needle assembly including an injection needle for penetrating into the injection zone； and an injection mechanism. The injection mechanism having an actuator cap containing an expandable material, and a heater mechanism for heating the expandable material, thus causing it to expand so as to move the piston in the reservoir, and thus inject the fluid into the injection zone through the injection needle. The expandable material has thermally expandable microspheres.
