The aim of this study was to assess the effects of (a) auto-injector form factor on maximum applied force capability and (b) auto-injector design and instructions on force production and orientation.

Effective delivery of epinephrine through an auto-injector is the result of a multitude of design factors. At minimum, the design needs to allow the user to apply sufficient force for the needle to penetrate clothing and tissue.

Trainer devices for three commercially available epinephrine auto-injectors with different form factors (cylindrical, elliptical, prismatic) were tested in a laboratory-based repeated-measures experiment with 20 adults. Participants applied their maximum force onto a force plate positioned over their thigh and practiced an injection using the trainer device after viewing training videos. Participants also rated force confidence and preference.

The maximum force varied significantly across devices. The greatest force observed was 64 newtons with the elliptical device, and the lowest force was 61 newtons with the cylindrical device. Participants reported the highest force confidence when using the elliptical and cylindrical devices, ranking the elliptical as their preferred device.

Force capability results for the elliptical device suggest that it may be more successful in achieving the necessary force for drug delivery in a larger set of adult users.

Results suggest that the auto-injector with the elliptical form may enable more successful drug delivery among a larger set of users.