3. Intangible assets

During the year ended December 31, 2008, the Company acquired Comfort Life Medical, LLC (Comfort Life). The acquisition resulted in recording an intangible asset in the amount of $92 related to the Medicare license held by the acquired company. The Company amortizes this intangible asset over its estimated useful life of ten years. As of December 31, 2014 and 2013, there were no impairments recorded related to this intangible asset.

On April 1, 2009, Comfort Life Medical, LLC merged with Inogen, Inc., and was simultaneously dissolved.

During the year ended December 31, 2009, the Company was assigned four patents previously held as an exclusive license from Air Products & Chemicals (APC) in exchange for an increase in a long term liability due to APC of $250. The acquisition of these patents resulted in an intangible asset of $250. During the year ended December 31, 2011, the Company purchased additional patents from APC for a total value of $650. The Company amortizes these intangible assets over an estimated useful life of five years. As of December 31, 2014 and 2013, there were no impairments recorded related to these intangible assets. The Company in 2013 calculated imputed interest on these patents and associated debt over the term of the contractual agreement, and reduced the underlying asset, debt by $177. The company recalculated interest and amortization of the period based on adjusted asset and debt.

During the year ended December 31, 2011, the Company acquired Breathe Oxygen Services, LLC. The acquisition resulted in recording an intangible asset in the amount of $66 related to the Medicare license held by the acquired company that allowed them to operate in the state of Tennessee as well as assets of the company. The Company amortizes this intangible asset over its estimated useful life of ten years. As of December 31, 2014 and 2013, there were no impairments recorded related to this intangible asset.

On August 29, 2011, Breathe Oxygen Services, LLC merged with Inogen, Inc., and was simultaneously dissolved.

The Company also capitalizes costs incurred for the production of direct response advertising commercials and amortizes these intangible assets over a useful life of two years. During the year ended December 31, 2011, the Company paid $95 for its G2 commercial and during the year ended December 31, 2012, the Company paid $63 for its G3 commercial.

Amortization expense for intangible assets for the years ended December 31, 2014, 2013 and 2012 was $150 and $203 and $298, respectively.

The following tables represent the changes in net carrying values of the intangibles as of the respective dates:

Annual estimated amortization expense for each of the succeeding fiscal years is as follows: