Optical absorbance of the tympanic membrane in rat and human samples

Abstract

Chronic infections are often connected to biofilm formation. In presence of implants, this can lead to loss of the implant. Systemic or local application of drugs is relatively ineffective in case of biofilm formation. One technique to provide antibacterial properties on demand is the antibacterial photodynamic therapy (aPDT). Using this technique, these properties can be “switched on” by light illumination. In the middle ear with the semitransparent tympanic membrane, it might be possible in future to activate the antibacterial effect without opening the membrane. Therefore, we investigated the optical absorbance spectra of the tympanic membrane. Optical absorbance spectra were measured in ex vivo preparations from neonatal and adult rats with the membrane still being attached to the surrounding bony ring and four human samples. After performing area scans, the spot with the lowest absorbance being surrounded by a ring like structure with higher absorbance was chosen as region of interest for scanning wavelengths between 300 and 900 nm. Absorbance is generally higher at lower wavelengths with a local absorbance maximum at 420 nm and a weak second maximum with two neighbouring peaks at 540 / 580 nm and is significantly higher in adult rats compared to neonatal rats where about 10% of light was transmitted. The human samples show similar characteristics with a little higher absorbance. For activation of aPDT through the tympanic membrane, larger wavelengths are more promising. Whether the amount of light transmitted through the membrane would be sufficient to induce aPDT remains to be tested in further experiments.

Safety margin evaluations for transtympanic laser stimulation of the cochlea from the outer ear in Mongolian gerbils (Meriones unguiculatus)

“…This indicates that the laser irradiation of 27.9 W/cm 2 or above at 532 nm could cause thermal damage. Although these results are not directly comparable to our data because of the difference in wavelength-dependent absorbance in the tympanic membrane and auditory ossicles, as some studies described (Carr, Valdez, Bruns, & Bawendi, 2016; Goblet, Matin, Lenarz, & Paasche, 2021; Heimann et al, 2021), thermal damage to the tympanic membrane and auditory ossicles might be one reason why continuous laser exposure affects the auditory-evoked response more than the laser-evoked response.…”

Feasibility evaluation of transtympanic laser stimulation of the cochlea from the outer ear

“…The tympanic membrane on the glass slide was irradiated using the pulsed laser (duration, 100 µs; repetition rate, 4,000 Hz) at 10.1 mJ/cm 2 . The original radiant energy (I0) and transmitted energy of the tympanic membrane (I) was recorded to calculate the absorbance (A) as previously described (Goblet et al, 2021;Sorg et al, 2021) using the following equation:…”