Christopher Herrmann (David Eigenberg) suffered a pretty serious injury on the job. The character disposed of an explosive device that was left at the doorstep of the 51, and was pretty close to the device when it went off.
Unfortunately, the blast led to a persistent ringing in Herrmann's ears, and when he finally went to the doctor, he discovered that he required the use of hearing aids. It was unexpected, albeit interesting arc for the character, but it takes on emotional resonance when one realizes why it was written into the show in the first place.
David Eigenberg relies on hearing aids in real life
According to The Hollywood Reporter, the decision to give Herrmann hearing aids is because the actor who plays him, David Eigenberg, uses hearing aids in real life. He's grown more reliant on them as he's gotten older, and he told the outlet that it became increasingly trickier to hide during filming:
"I need them in real life, so, it was written in the show because I’ve gotten to the point where I need them. I’ve been needing them for years."
Eigenberg, who has been on Chicago Fire since the pilot, explained that his reliance on hearing aids has become more common in the last few years, and the writing team was nice enough to work them into the show so that he could operate freely with them in his ears.
The actor noted that having a flourishing actor career can be tough with hearing aids, but that he's seen them become increasingly normalized in the industry, and hopes this is a trend that he can help to further with his portrayal of Herrmann in season 12.
Eigenberg's hearing aids were written into the show
Eigenberg told The Hollywood Reporter that the push to get Herrmann hearing aids was prompted by an email he sent to the show's writing team, and he was very pleased to find out that it was going to be worked in:
"They were kind enough to incorporate it, and I think it’s a wonderful thing in a lot of ways. I’m really glad that we’re using it."
Eigenberg assured fans that he plans to stick around for a long time on Chicago Fire, so we don't have to worry about sudden exits for his character.