The influence of autoinjector applied force and shield size on abdominal skin and subcutaneous tissue deformation - An in-vivo ultrasound and digital image correlation study.

BACKGROUND

Shield-triggered autoinjectors (AIs) aim to reduce needle phobia and accidental needlestick injuries and improve usability. However, they may cause deeper injections due to tissue compression. This study investigates the mechanical response of AI application into abdominal tissue in-vivo.

METHODS

Tissue thickness/compression level, skin strain, and indentation depth were measured in 10 healthy participants. Digital image correlation (DIC) captured the skin surface, while an ultrasound (US) probe attached to a 3D-printed shield recorded the underlying skin and subcutaneous tissue (SCT) during application of forces up to 20 N using two indenter sizes (Ø15 mm and Ø30 mm).

RESULTS

Increased force correlated with reduced skin + SCT layer thickness; however, a non-linear tissue lockup/plateau around 8-10 N was evident. The Ø30 indenter caused a 45 % mean compression, while the Ø15 resulted in 55 %. ANOVA revealed statistical differences between shield sizes for indentation depth and skin + SCT thickness reduction (p < 0.05). ANOVA further revealed a statistical difference between mean indentation depth for Ø15 (50.39 mm) versus Ø30 (34.84 mm) (p < 0.05). Average strain rates were higher (10.5 % versus 15 %) and skin surface deflection curves were steeper for Ø15 compared to Ø30.

CONCLUSIONS

Skin + SCT compression increases until it reaches a lock-up state, but indentation depth responds linearly to increases in force. The high level of tissue compression will increase the risk of intramuscular injections when using shield-triggered AIs, depending on needle length, activation force, and shield size. Therefore, increasing shield sizes to reduce tissue compression and indentation depth can enhance user experience and the probability of reaching the subcutaneous layer. Conclusions are drawn from a small, homogenous cohort, and although the trends are clear, variation between participants highlight the complex nature of human skin tissue. More user groups and injection sites should be investigated.