	December 31, 2020
	Estimated Useful Lives	Gross Carrying Value		Accumulated Amortization			Impairment		Net Book Value
Internally developed patents	6 – 10 years	$	884,787	$	(479,908	)	$	—	$	404,879
Acquired licenses	16 – 20 years		285,000		(285,000	)		—		—
Total intangible assets subject to amortization			1,169,787		(764,908	)		—		404,879
IPR&D assets	Indefinite		12,418,967		—			—		12,418,967
Total		$	13,588,754	$	(764,908	)	$	—	$	12,823,846

	December 31, 2019
	Estimated Useful Lives	Gross Carrying Value		Accumulated Amortization			Impairment			Net Book Value
Internally developed patents	6 – 10 years	$	746,323	$	(448,874	)	$	—		$	297,449
Acquired licenses	16 – 20 years		285,000		(269,221	)		—			15,779
Total intangible assets subject to amortization			1,031,323		(718,095	)		—			313,228
IPR&D assets	Indefinite		13,418,967		—			(1,000,000	)		12,418,967
Total		$	14,450,290	$	(718,095	)	$	(1,000,000	)	$	12,732,195

Amortization expense of intangible assets subject to amortization totaled $46,813 and $147,500 for the years ended December 31, 2020 and 2019, respectively, and was classified as research and development expenses in the accompanying consolidated statements of operations and comprehensive loss.

The above future estimated amortization expense does not include potential amortization charges related to the remaining carrying value of IPR&D assets as of December 31, 2020. Those assets, which represent incomplete technologies, will be amortized to expense once the underlying technologies are substantially complete over their estimated useful lives, expected to be 15 to 18 years. In the event that in the future the Company ceases the development of these assets, the remaining carrying value would be written off at that time. IPR&D assets are periodically assessed for impairment by considering the state of completion of the projects, the remaining activities required to complete development, the anticipated market for the completed products, and anticipated future cash required to complete development.

There was no impairment loss during the year ended December 31, 2020. During 2019, as a result of the SparVax-L NIAID contract completion and the U.S. government’s funding prioritization of only single dose anthrax vaccine candidates, the Company abandoned the project and concluded that the full remaining net book value of the SparVax-L IPR&D asset was impaired. As a result, $1.0 million was written off as an impairment charge which was classified as a component of operating expenses during the year ended December 31, 2019.