Functional and histological assessment of 3D-printed helmet and hearing protection devices in preventing blast-induced auditory injury in Chinchillas.

Blast-induced auditory injury remains a persistent concern in many high-risk occupational environments, especially military service members and veterans. Although helmets and hearing protection devices (HPDs) have independently demonstrated efficacy in mitigating acoustic injuries, the synergistic protective effect of their combined use against blast overpressure (BOP) has not been comprehensively evaluated. The present study investigated the auditory protective performance of a custom-engineered 3D-printed helmet, both independently and in conjunction with HPDs, in a chinchilla model subjected to repeated high-intensity blast exposures. Finite element analysis (FEA) was harnessed to optimize the design and the material selections of the three-layered, stiffness gradient helmet, leading to 47% reduction of the peak stress in the chinchilla's skull. Four cases were tested during blast experiments including unprotected, earplugged, helmeted, and dual-protection. Animals were exposed to three consecutive blasts with a peak amplitude up to 25 psi (175 kPa, equivalent to 199 dB SPL) under anesthesia. Auditory function test of auditory brainstem responses (ABR), middle latency responses (MLR), and distortion product otoacoustic emissions (DPOAE) were performed. The dual-protection group demonstrated the smallest ABR threshold shifts with reductions of up to 40 dB compared with the unprotected group, along with the greatest preservation of ABR Wave I amplitudes and stable MLR amplitudes. The cochleae were harvested for histological analysis, involving immunostaining to assess hair cell survival and quantify hair cell loss across cochlear regions. DPOAE amplitudes showed pronounced recovery following blast exposures, corresponding with significant less losses of cochlear hair cells and improved preservation of both inner and outer hair cells.